Johnson Matthey has over 200 years of history, creating sustainable technologies, shaped around customers’ needs. Our ambition is to research, develop and innovate solutions to make the world cleaner and healthier, today and for future generations. Much of the underpinning science behind these technologies relies on a knowledge of chemistry and its application. Like most successful organisations, Johnson Matthey reflects on the scientific capabilities that are key to developing these solutions today but also looks to the future to plan which capabilities will be required to meet future challenges and opportunities. Much of this learning comes from external insight by looking at what is happening both within the markets and scientific disciplines we are familiar with but also in parallel disciplines. Today, our core scientific capabilities can be grouped into nine key areas covering catalysis, characterisation and modelling, chemical synthesis, materials design and engineering, electrochemistry, platinum group metal and specialist metallurgy, process optimisation, product formulation, surface chemistry and coatings. Pulling these together forms a powerful toolbox to develop solutions for our customers’ needs.

When looking beyond these capabilities, one useful ‘lens’ to look through is the overlap between scientific disciplines. For Johnson Matthey this might be to look at the interface between chemistry, one of our key underpinning strengths, and other sciences. For example, the interface between chemistry and physics; chemistry and biology or with other enablers such as the digital transformation that is enabling different ways of exploring science. Following this premise, Johnson Matthey Technology Review has devoted this issue to focus on physics and a future edition will look at biology.

Figure 1 shows how Johnson Matthey’s core science capabilities today may overlap with physics and biology. Further insights can then be drawn by mapping how we apply these capabilities to provide customer solutions into our existing markets (pink text) or where they may be aligned with global drivers and world challenges. Such techniques can be a valuable tool to help discuss and identify opportunities and needs for an organisation.

Fig. 1.

Two areas increasingly dependent on capabilities bridging chemistry and physics are characterisation and modelling of materials and processes and the development of functional surfaces and coatings. These topics feature heavily in this edition of the Johnson Matthey Technology Review. For Johnson Matthey, characterisation and modelling are key capabilities to help develop new technology.

Characterisation provides insights into composition, structure and property-performance relationships at all length scales. The latter includes in situ and operando analysis, which is important to understanding how materials may respond in their intended application.

Modelling also encompasses all length scales and includes statistical, empirical and physical models. Modelling has been used for a long time in chemical engineering to design reactors, systems and processes. Examples include designing a new reactor for a chemical reaction, an aftertreatment system for a vehicle or a process flow sheet for recycling waste materials. More recently, advances in modelling are permitting chemists to be more predictive, to be able to design materials, reactions and their performance with far fewer experiments. For example, in this edition, the need for computational modelling methods to replace incremental experimental development to meet the need to design complex new advanced materials is explained (1).

The application of nuclear magnetic resonance (NMR) to characterising activated carbons leads to insights into kinetic exchange of solvent molecules (2). The technique makes use of the magnetic shielding properties of the carbon structure to give insights into molecular level mechanisms which can give information to the chemist about where adsorbed species are in the material’s structure. These techniques enable the industrial chemist to gain a better understanding of the materials being used which leads to faster development and better understood technology. Equally important is the fundamental understanding of new materials and their properties both at the atomic and molecular scales which in time can lead to advances in existing or new technology.

Coatings and surface properties is another area at the interface of chemistry and physics. Johnson Matthey has many examples of products which rely on the functionality of particles deposited onto a surface. Examples include precious and base metal catalysts, advanced energy materials and medical components. As the coating thickness reduces from micron to atomic, the chemist’s traditional toolbox to deposit layers of formulated slurries, pastes and inks changes towards different deposition techniques such as chemical vapour deposition (CVD) and physical vapour deposition (PVD). The ability to design and deposit functional particles of a controlled size and shape onto a surface can find application in many disciplines such as transparent or reflective coatings, semiconductor devices, energy harvesting and sensing. Typically, these applications harness a combination of electronic, optical and chemical functionality. Further examples of applications in areas such as sensing, electronics and renewable energy are explored within this edition (3, 4).

Looking forward, global drivers such as climate change, the energy transition, population growth and longevity and resource challenges will drive the need for new technologies in areas such as more sustainable products, low carbon operations, clean energy and improved health and medical care. To meet these challenges chemists will increasingly need to reach out to adjacent disciplines to develop innovative solutions. In this edition of Johnson Matthey Technology Review, we welcome you to look at some of the advances in physics and explore how they are being used to drive forward R&D.

“Solid-State NMR in Zeolite Catalysis” was written by four professors from the Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, who have tremendous expertise in the fields of solid-state nuclear magnetic resonance (solid-state NMR) and heterogeneous catalysis. This book is Volume 103 in the series ‘Lecture Notes in Chemistry’ published by Springer. It can be organised into three sections: (a) principles of NMR theory (Chapter 1); (b) solid-state NMR studies of zeolites and related materials (Chapters 2 and 3); and (c) surface chemistry and reaction chemistry over zeolites (Chapters 4–6). Each chapter offers a sufficient introduction to move smoothly into the main content. The book guides readers to familiarise themselves with solid‐state NMR and gives sufficient practical examples, detail and illustration. However, visually supported explanations by the authors were included only after Chapter 1. Adding to this could help deepen the understanding of pre-graduate readers. A few abbreviations were not explained, which could confuse readers. Advantages and disadvantages of solid-state NMR for each purpose are addressed in comparison with other techniques in each chapter. The discussion in this book further applies to materials for heterogeneous catalysts, glasses, ceramics, superconductors, lithium-ion batteries and biological systems.

In Chapter 1, the authors provide a snapshot of NMR theory with the current state of methodology development, including strategies to enhance NMR sensitivity. This chapter gives sufficient background to understand the subsequent chapters without overwhelming.

Highlights and lowlights of solid-state NMR for material characterisation are addressed at the beginning of Chapter 2. The chapter goes on to explain the local framework structure characterised by different chemical environments in zeolites. NMR spectroscopy provides detailed information on synthesis mechanisms in the solid and liquid phases, which may be complementary to X-ray diffraction and electron microscopy methods that are often used to gain structural information.

The application of NMR spectroscopy in zeolite crystallisation, in situ and ex situ methods is also introduced to investigate the kinetics of crystallisation for the convenience of a general audience. As an example, the structural changes in the aluminosilicate gels of zeolite faujasite (FAU) were confirmed in addition to the changes in silicon:aluminium ratios during the ageing and crystallising processes. In addition to zeolite analysis, xenon-129 chemical shifts were used to examine the pore geometry and chemical surroundings. This also fits the discussion of the guest-host interaction study. Additional visual aids could improve understanding in this chapter. The basics of characterisation of Brønsted acid sites may also be complemented by a review by Hunger (1).

NMR spectroscopy has the advantage of providing detailed information on local bonding and solid-state interactions. Investigations on host-guest interactions between organic molecules, such as aromatics to surfactants and inorganic moieties of zeolites, are well summarised in Chapter 4 with sufficient examples and explanation of local structures and dynamic behaviours between adsorbed molecules and active sites. Although several informative figures were cited from references, additional visual supports to explain the host-guest interaction would further elucidate this interaction. The content of Chapter 4 reintroduced and extended this reviewer’s understanding of Li‐ion battery and Li-ion ultracapacitor materials (2).

Chapters 5 and 6 elucidate interfacial chemistry by solid-state NMR approaches. Several representative reactions are given including in situ studies with a focus on how surface and interfacial phenomena significantly influence the catalytic performances (activity and selectivity) of heterogeneous catalysts. Many essential aspects of interfacial chemistry can be emphasised on the chemical and electrochemical reactions at interfaces. The following readings may bring additional value (3, 4), and this is also applicable for an understanding of Li-ion battery and Li-ion ultracapacitor materials (5).

The reviewer would like to recommend this book for anyone who would like to quickly grasp the capability of solid-state NMR as well as researchers working on nanoporous materials, nanocrystals, nanomaterials, Li-ion batteries and capacitors with surface and interfacial phenomena in addition to zeolites and zeolite related materials. This is exciting reading and reminded the reviewer of the ability of solid-state NMR to assess the mechanisms of intermolecular interactions, chemical reactions and transport phenomena. Additional visual supporting materials should be added to deepen the readers’ understanding of the authors’ explanations, particularly in Chapter 4.

Manufacturing technology for medical implants undergoes constant improvement in order to accelerate the patient’s recovery and increase the service life of the implant. Following this trend, special attention is paid to applying biocompatible coatings on the surface of implants (1–7). There is a huge clinical need for advanced biomaterials with enhanced functionality to improve the quality of life of patients and reduce the burden of health care for the world’s ageing population. In recent years, Ti and HA have been widely used in medical devices due to their favourable biocompatibility (1–8).

The research presented here offers robot assisted MPS of Ti wires and HA powder onto Ti substrates (9–12). Currently, MPS technology is highly specialised with few research groups working on its development. Yet it has shown promising results in delivery of high quality and well-designed coatings (1–3). Among existing plasma spraying processes, MPS is particularly characterised by low plasma power (up to 4 kW), small diameter of the spray deposition spot on the surface (up to 15 mm) and the possibility of forming a laminar jet. This laminar jet can be up to 150 mm in length. It heats the refractory material in a stream of argon plasma and provides low heat input into the substrate (2). The process provides deposition of Ti or HA on small size parts and components, including those with complex geometry. This is normally unachievable with any other method. The MPS generally provides a micro-rough surface and a higher degree of porosity (~20%) that in the case of biocompatible coatings facilitates bony tissues ingrowth; in most cases the bond strength of MPS coatings with substrates is acceptable (2). However, there are still a number of challenges remaining to be addressed. The most important issue is the formation of coatings with specified structure and properties.

Currently, endoprosthesis medical practice widely uses metal implants coated with HA (1–8, 13, 14). HA is the calcium phosphate mineral Ca₁₀(PO₄)₆(OH)₂ of the apatite group, which is chemically similar to the apatite of the host bone and is a source of Ca and P for the bone-HA interface (3, 4, 6, 8). HA coatings improve osseointegration and can significantly reduce the duration of implantation of the endoprosthesis. It provides a reliable connection with the bone and increases the reliability of the implant (1–8, 13, 14). In the case of thermal spraying of HA powder, the chemical composition of the final HA coating is dependent on the thermal decomposition occurring during spraying. The high temperatures experienced by HA powder particles in the plasma spraying process lead to the dihydroxylation and decomposition of the particles. At temperatures above 1050°C HA decomposes to tricalcium phosphate (β-TCP, Ca₃(PO₄)₂) and tetracalcium phosphate (TTCP, Ca₄(PO₄)₂O), and above 1120°C β-TCP is converted to α-tricalcium phosphate (α-TCP, Ca₃(PO₄)₂) (13, 14). Thus, the resultant coating phase composition depends on the thermal history of the powder particles. The higher the plasma jet temperature and the longer the exposure of the particles to plasma, the greater the degree of phase transformation. According to the British Standards Institution standard specification BS ISO 13779-2:2000 (15), the maximum allowable content of non-HA phases in a HA coating is 5%, and the minimum allowable percentage of crystallinity is 50%. The degree of crystallinity of the HA coating affects the process of osseointegration (16). Amorphous calcium phosphate (ACP) has a higher rate of dissolution, which reduces the recovery time of the patient, but at the same time some reduction of the reliability of fixation of the endoprosthesis in the bone is also possible. Thus, increased crystallinity provides reliable fixation of the implant in the bone (2, 3).

It is assumed that to increase the biocompatibility of the implants and rapid accretion with the bone, the implant surface should be covered with a biocompatible coating with an extensive surface morphology, with recommended pore sizes in the coating from 20 μm to 200 μm, and closed porosity of at least 30% (2–4, 16, 17). At the same time, the coating must be firmly connected to the implant, without transversal porosity, so that the implant material does not interact directly with the human body. Therefore in this research, the proposed composition and thickness of the coating is designed to meet these requirements. In order to characterise the coated layer structures, SEM, TEM and XRD are used. Optimum modes of plasma spraying are chosen on the basis of the results of structural characterisation.

The aim of this study is to select the modes of MPS of both Ti wire and HA powder to obtain a sufficiently thick (up to 300 μm) multilayer Ti/HA coating. The dense Ti sublayer is supposed to provide good adhesion to the substrate and the porous Ti middle layer and HA top layer can accelerate the bone ingrowth. Another objective is to clarify the relationship between various plasma spray process parameters and the resultant coating structure. This is achieved through the development of process models that relate process parameters to various coating structures.

The HA was synthesised in the laboratories of East Kazakhstan State Technical University. The process of synthesis of HA powder with the ratio Ca:P of 1.65 by a chemical precipitation method was described in our previous paper (10). The purity of HA powder was 99.5%, which met the purity requirement (not less than 95%) set out in the ASTM International (USA) standard ASTM F1185-03(2014) (18). The purity of the synthesised HA is determined using the XRD results which are further explained below. Before spraying, the powder was dried at a temperature of 120°C for 6 h, in order to avoid clogging during powder feeding. HA powder was used as a sprayed coating material. The particles had irregular shape with smooth edges (Figure 1).

Fig. 1.

The shapes of the particles are particularly designed with smooth edges to make sure that they do not stick together. Moreover the HA particles should not cling to each other in order to provide the necessary flowability of the powder. The flowability is essential in gradual supply of the powder to the plasma jet.

According to previous studies (9, 10), the particle size of the HA powder for MPS should be in the range of 40 μm to 90 μm. Before MPS, the dried and milled HA powder was sieved through mesh diameters 40 μm and 90 μm to obtain only the powder fraction within the desired range. The flow rate of the HA powder is in the range of 120 s 50 g^–1.

Samples of medical Ti alloy of Grade 5 extra low interstitials (ELI) (Table I) were used as substrates for MPS. For deposition of Ti coatings, wires of VT1‐00 commercially pure Ti with a diameter of 0.3 mm were used (Table I). The samples of medical Ti alloy of Grade 5 ELI (Table I) were cut with thicknesses of 3 mm from rods with a diameter of 50 mm and 30 mm on CTX 510 ecoline computer numerical control (CNC) machine (DMG MORI AG, Germany). Plates of size 15 mm × 15 mm × 2 mm were cut from large sheet of Grade 5 ELI alloy.

MPS of the HA powders and Ti wires was carried out by MPN-004 microplasmatron (produced by E.O.Paton Institute of Electric Welding, Kiev, Ukraine) (22). The microplasmatron was mounted on an industrial robot arm (RS010L, Kawasaki Heavy Industries, Japan). It is able to move horizontally along a computed trajectory at set speed. The thickness of the coatings was varied from 80 μm to 300 μm by changing the MPS parameters. The speed of linear movement of the plasmatron along the substrate was chosen to be 50 mm s^–1. The choice of speed of the plasmatron was based on preliminary estimates of the temperature of the substrate when exposed to a plasma jet (12). This was to make sure the temperatures remain well below the melting temperature. Ar served as a plasma-forming and transporting gas for MPS; additional heating of the substrate was not carried out. Using a robotic arm allows precise spraying of coatings with a uniform speed of movement of the plasmatron along the surface of the implant, as well as moving the plasmatron along a predetermined path.

Before MPS, the surfaces of the samples were degreased with acetone and subjected to ultrasonic cleaning. To ensure proper adhesion of the coatings, it was important to pre-treat the surfaces of the substrates to increase their roughness. For surface activation, gas abrasive surface treatment was carried out on a Contracor^® ECO abrasive blasting machine (Comprag Group GmbH, Germany) using normal grade A14 electrocorundum. The chemical reactivity of the substrate’s surface rapidly falls due to oxidation and the adsorption of chemical gases from the atmosphere. Therefore it is important that the time interval between the gas abrasive treatment and the coating on the surface does not exceed 2 h. Before coating, samples were stored in a tightly closed container.

To evaluate the porosity of biocompatible coatings, the images obtained by the scanning electron microscope JSM-6390LV (JEOL Ltd, Japan) were processed using MicroCapture (MustCam, Hong Kong) and ATLAS.ti (ATLAS.ti Scientific Software Development GmbH) computer-aided programs. The measurements were carried out on the polished cross-section of the coatings according to ASTM E2109-01(2014) standard (23). The surface roughness of the substrates and the as-sprayed coatings was measured in accordance with ISO 4287:1997 (24) using a MarSurf PS 10 mobile roughness measuring instrument (Mahr, Germany). Four measurements were taken for each sample and the average was determined. The adhesion strength of coatings to the substrates was measured in tension using a AG-X universal testing machine (Shimadzu, Japan) in a static experiment according to ASTM C633-13(2017) standard (25).

The crystallinity and the structure-phase compositions of HA powders and plasma sprayed HA coatings were measured by X’Pert PRO diffractometer (PANalytical, The Netherlands). The interpretation of the XRD patterns was carried out using Rietveld method and powder diffraction data from the database of the International Centre for Diffraction Data (ICDD, USA, 2003), the ASTM card file and X’Pert HighScore Plus software (Malvern Panalytical, UK). The percentage of crystallinity of the HA powder was calculated using the area of crystalline peaks in the region of 15° to 45° 2θ and the area of the amorphous diffuse background in this region. Diffraction scans of the HA powder and coatings were carried out in accordance with ASTM F2024-10(2016) (26). The purity of HA powder and HA coatings was evaluated by calculating the areas of all non‐HA peaks found in the diffraction pattern. The impurity area was determined by calculating the area in the region where the highest impurity phase peaks were present. The impurity peaks that would be expected to be present in HA powders and HA coatings were those of TTCP, α-TCP and β-TCP. The Rietveld method was used to quantitatively determine the percentage of various phases of impurity in HA coatings. X’Pert HighScore Plus software was used to calculate the impurity. Three purity measurements were carried out for each of the XRD patterns.

Electron diffraction patterns of samples of HA powder were obtained by TEM on JEM-2100 (JEOL). The structural-phase composition data obtained using TEM were compared with the data obtained using XRD analysis. TEM sample preparation techniques are described in detail in a previous publication (11).

The particles of the sprayed Ti wire after collision with the substrate were studied using splat tests (2). MPS of Ti wire onto the plates of polished Ti alloy was performed in the plane perpendicular to the axis of the plasma jet. Speed of the linear movement of plasma jet was set at 50 mm s^–1. As a result, single particles of the sprayed material (splats) were fixed on the substrate and deformed upon contact with the substrate surface. The splats’ visual analysis was carried out by SEM; the splats were classified according to their appearance and their spraying modes (Table II).

The effect of parameters of MPS such as electric arc current (I, A), plasma gas flow rate in standard litre per minute (V_pg, slpm), spraying distance (H, mm) and wire flow rate (V_w, m min^–1) or powder consumption (P_powder, g min^–1) on the surface morphology, porosity and structural‐phase transformations of the coatings have been studied. The coating experiments for MPS were accomplished in a two level fractional factorial design (2^4–1). The experimental conditions in fractional factorial designs have been selected to provide balanced design (27). The maximum and minimum values of the parameters for feasible processing of high-quality coatings were chosen empirically.

MPS of Ti wires on Ti alloy gas-abrasive treated substrates was carried out in various modes as shown in Table II. The key characteristics of the resultant coatings such as their porosity and roughness were examined. The data in Table II and Table III represent the averaged values for three experimental runs.

Examination of the Ti particles splats obtained in Modes 1, 4 and 8 (Table II) showed that the samples are completely melted and have formed a disk (Figures 2(a) and 2(b)). The thicker splats are formed in Mode 8 (Table II, Figure 2(c)). The beginning of the process of solidification of the particles upon impact with the substrate is shown in Figure 2(c). The increase in the thickness of the splats in Mode 8 (Table II) is due to the minimum velocity of the particles during their interaction with the substrate.

Fig. 2.

As can be seen in Figure 2, the splats obtained in different modes have similar area but different thickness. The increase in the thickness of the splats in Mode 8 (Table II) is due to the minimum velocity of the particles during their interaction with the substrate. The minimum particle velocity in Mode 8 is caused by the large size of the sprayed particles, low gas flow rate and low spraying distance. This also leads to a decrease in the speed of the sprayed particles. Particles of Ti wire melted by a plasma jet move towards the substrate during plasma spraying. On the one hand, the size of these particles depends on the set spraying parameters. On the other hand, both the set spraying parameters and the size of the particles affect the speed and the degree of heating of the particles in the plasma jet. Before interacting with the substrate, the particles of molten metal can either heat up, being in the high-temperature zone of the plasma jet (the initial section of the plasma), or, more likely, cool down by going to the low-temperature zone (the end of the plasma jet). The size of the sprayed particles and the degree of their melting in the plasma jet can be varied by spraying parameters. A porous coating with a high surface roughness can be achieved by large particles moving with low speed, as in Mode 8. A dense coating with a relatively low surface roughness can be obtained by high speed small and completely melted particles, as in Mode 1 or in Mode 4 (Table II).

The appearance and structure of the splats varies depending on the interactions of the sprayed particles with the substrate. The interaction is generally defined by the velocity of the particles on impact and also their degree of melting. It is possible to correlate the temperature and velocity of the particles before the collision to the substrate with the resultant structure of the coating. The profile and cracks on the surface of the splat correlate with the stress state of the coating (2, 28).

The substrate average roughness (Ra) after gas abrasive treatment was Ra = 7.0 ± 0.35 μm. The analysis of the surface morphology of Ti coatings showed the possibility of obtaining dense coatings with relatively low roughness (Ra is about 12.0 μm) using Mode 1 and Mode 4 (Table II), the remaining modes provide a high surface roughness (above 30 μm). The maximum size of open pores (up to 300 μm) is observed on the surface of coatings obtained in Mode 8 (Figure 3).

Fig. 3.

The analysis of the cross-sections of the Ti coatings showed that the coatings sprayed in Mode 8 have the highest average porosity of 31.0%, while Mode 4 makes it possible to obtain dense coatings with an average porosity of 5.7% (Figure 3 and Table II). The tensile strength test established the average adhesion strength of the coating with a thickness of 100 μm sprayed in Mode 4 (Table II) to be 38.7 MPa. This meets the requirements of ISO 13179‐1:2014 (29). According to ISO 13179‐1:2014 (29), the average static tensile strength of a Ti coating should be more than 22 MPa.

As can be seen from the results presented in Table II, the arithmetic Ra of the coatings surface correlates with the porosity of the sprayed coatings. The condition of the presence of large, incompletely molten particles with a low speed in the plasma jet leads to a high surface roughness and increased porosity of the coatings. Thus, the roughness of the coating is affected by the size of the particles involved in the formation of the coating. The main purpose of plasma spraying of the Ti layers at the initial stage of the coating is to form a porous surface with high roughness in order to spray fully molten HA particles onto it. It was previously shown (30) that partially melted HA particles were not able to form dense coatings. This leads to poor adhesion of the HA coating to the substrate. A desirable HA coating shows good adhesion and high surface roughness. The surface roughness of the HA coating affects osteoblast cell attachment and thus accelerates bone growth into the implant. Whereas fibroblasts and epithelial cells prefer smoother surfaces, osteoblasts attach and proliferate better on rough surfaces (31, 32). A relatively thin HA layer (not more than 100 μm) sprayed on a porous surface with a high roughness (above 50 μm) can ensure the reliable adhesion of HA coating to the surface, while maintaining high surface roughness and porosity. XRD analysis confirmed that the phase composition of the initial HA powder was fully crystalline Ca₁₀(PO₄)₆(OH)₂.

The TEM images (Figures 4(a) and 4(b)) of the HA powder particle and the corresponding microelectron diffraction pattern (Figure 4(c)) are shown in Figure 4. The results of TEM analysis are in good agreement with the results of XRD analysis (Figure 4(d)). X-ray phase analysis showed that the main phase (99.5%) is HA with the hexagonal crystal system P6₃/m. The electron diffraction pattern corresponds to the hexagonal phase of HA with unit cell parameters a = 0.94 nm, c = 0.68 nm.

The plasma spraying of HA powder was carried out using nine different modes (Table III). The key criteria were the phase composition, the degree of crystallinity (the proportion of the amorphous phase (A_ph), the proportion of the crystalline phase (HA_cryst)) and the coating transfer efficiency (CTE). In powder coating, transfer efficiency is the ratio of the quantity of powder deposited on the part to the quantity of powder directed at the part. Transfer efficiency is provided as a percentage, with 100% being most desirable. An experiment was conducted to determine the impact of process parameters such as I, V_pg, H and P_powder on the CTE.

Fig. 4.

The application of well-known methods of fractional factorial design (27) and the design of experiment method (33) for the analysis of plasma spraying of HA powder are described elsewhere (34, 35) and in our previous paper (10). A factorial experimental design to investigate the relationship between plasma spray parameters and the microstructure of HA coatings was first used by Dyshlovenko et al. (35). Three responses were examined (35). This included the fraction of HA, the fraction of decomposition phases and the amorphous content of the coatings. In this study, the next three responses were examined: the fraction of HA, the amorphous content of the coatings and CTE. The first two responses were chosen to ensure purity and crystallinity of the HA coating. CTE was selected in order to increase the efficiency of the plasma spraying process. Moreover it allows interpretation by a linear regression model which could quite easily and reliably be measured in experimental runs. The linear regression model was chosen to estimate CTE. The coefficients in the regression Equation (i) were calculated by assigning the corresponding units of measure: 2.575 A^–1; –0.246 slpm^–1; –0.203 mm^–1; 4.06 min g^–1; –0.825.

(i)

The comparison of calculated and experimental results indicates good agreement (Table III). Therefore, Equation (i) can be used for preliminary estimation of CTE when selecting MPS modes. A more complex regression model that takes into account the mutual influence of factors should be applied to determine the dependency of other values presented in Table II and Table III on the spraying parameters. This requires further investigation using the obtained experimental data.

The results of XRD analysis presented in Table III show that the phase compositions of all coatings comply with ISO 13779-2:2000 (15). However, Mode 3 provides the highest CTE. Thus, we consider Mode 3 to be the most cost-effective. This mode allows a desired HA coating thickness (about 100 μm) to be obtained in one pass of a plasma jet.

The modes of application of multilayer coatings of Ti/HA were selected on the basis of the analysis of the tests of MPS of Ti wire and HA powders with the measurement of thickness and porosity of the deposited layers. The porosity and surface roughness of Ti coatings and the high CTE value, purity and crystallinity of the HA coating indicate the optimum coating composition and MPS modes. The coating thickness and the expected adhesion to the substrate are other parameters indicating the quality of the coating. The first relatively thin (up to 80 μm) and dense layer of Ti wire coating is applied in Mode 4 (Table II), then another layer of Ti wire up to 100 μm thick is sprayed in Mode 8 (Table II) to form a porous coating with a rough surface, then the top layer of HA powder coating is applied with thickness up to 100 μm in Mode 3 (Table III). The microstructure of microplasma sprayed multilayer Ti/HÀ coatings under the above modes is shown in Figures 5(a) and 5(b). The XRD pattern for microplasma-sprayed HA coating is presented in Figure 5(c).

Fig. 5.

The desired porosity was achieved in the Ti lower layer (30 vol%) (Figure 5(b)). Pore sizes in both the Ti middle layer and the HA top layer are in the range 20–50 μm (Figure 5(b)). HA coating porosity is about 20% (Figure 5(b)). It should be noted that for biocompatible coatings, open porosity is essential: that is the egress of the pore on the surface of the coating, where the bone grows. Therefore, measuring the diameters of the pore craters on the surface of the coating is an appropriate way to indicate the surface morphology and profile. In our experiment, the maximum pore diameter on the surface of the HA coating was about 150 μm (Figure 5(a)).

It was established by XRD that the mode specified for HA powder provides the required structure-phase composition in the HA coating: 93% by weight of the HA_cryst, 5% by weight of β-TCP phase, and 2% by weight of the A_ph. The coating purity was determined using the procedure outlined in Materials and Methods section above. The highest peaks of HA and β phases for HA coatings were located at 32° 2θ and 31° 2θ respectively. The purity of coatings was found to be 95.1%. This shows that the purity meets the 95% purity requirements of ISO 13779-2:2000 (15). The measurement error was 0.06.

The loci of A_ph have been found on the XRD patterns between 18° and 38° 2θ (Figure 5(c)). All the diffraction patterns in the range of 37.3° 2θ were thoroughly investigated, but even weak peaks of calcium oxide (CaO) were not found (Figure 5(c)). It confirms that no harmful CaO compound is formed through MPS coating of HA powder.

For a multilayer coating, the porosity and adhesion of the top HA layer depends on the characteristics of the lower Ti layers such as the roughness and open porosity of the middle Ti layer. To determine the dependency of the porosity and adhesion of the HA coating on the parameters of MPS, further research is needed.

This study proves that it is possible to obtain coatings from biocompatible materials with the desired level of porosity and satisfactory adhesion to the substrate using MPS. A robot assisted MPS of coatings from biocompatible materials of Ti and HA onto Ti implants has been implemented. Also the composition and modes of microplasma deposition of multilayer coatings for Ti implants have been identified. The next stage of the research includes the study of the biocompatibility of microplasma-sprayed coatings (in vitro tests) and MPS of different materials such as tantalum and zirconium.

Among the number of works that have demonstrated the advantages of thermal spraying of biocompatible coatings for use in medical applications, three recent papers (36–38) have shown promising directions for further development of the research presented here. Cizek et al. (36) have reviewed the patents concerning thermal spraying for biomedical applications for the period 2005 to 2018. They have also reported recent research and development trends in this field. Among the materials recommended for bio-applications, they have mentioned Ta. It can be noted that MPS of Ta wire onto Ti alloys using the technology presented in our paper is highly feasible. Our trials with Ta have indicated great potential. This could open the potential to apply the developed technology for other materials. Fotovvati et al. (37) have compared the results of obtaining biocompatible coatings by cold and thermal spraying in favour of thermal spraying. Fousova et al. (38) have shown the benefits of using thermal plasma spray to prepare bulk Ti for bone enlargements. However, despite the advantages and relative cost effectiveness of thermal plasma spraying, its use for the manufacture of medical implants has not yet become widespread. This is mainly due to the high temperatures of the bulk resulting from the thermal spraying process. MPS avoids the issue of overheating. It allows coatings to be obtained from materials with a high melting point, such as Ti and Ta, by a microplasma jet while introducing a very small thermal impact into the substrate.

The use of robotic MPS could be considered promising for the production of patient specific implants. Three-dimensional scanning and rapid prototyping technologies facilitate the manufacture of specifically designed complex geometry implants and robot assisted plasma coating is used for coating. This is more advantageous for the production of small endoprostheses with biocompatible coatings, such as vertebral cages and dental implants (39, 40).

It has been established that the main parameters controlling the porosity of microplasma sprayed coatings are I and V_pg. MPS parameters for the formation of porous coatings of Ti wire and HA powders with rough surfaces have been determined and are reported here. The advantages of applying SEM, TEM and XRD to analyse the structure of sprayed Ti and HA coatings to substantiate the choice of plasma spraying modes of the coatings were demonstrated. It is also proven that by using the appropriate MPS process parameters, a layer of HA with a high degree of crystallinity (93%) can be obtained, controlled by changing the deposition mode. The small size of the spraying spot (up to 8 mm) provides a significant reduction in P_powder when depositing on implants of small size compared to conventional plasma spraying.

The composition and modes of microplasma deposition of multilayer coatings for Ti implants, including a dense Ti sublayer, porous Ti middle layer and HA top layer have been established. The total thickness of such coatings is about 300 μm. The porous middle coating layer has a porosity of 30% and a pore size varying from 20 μm to 50 μm. Moreover the upper layer of HA indicates a thickness up to 100 μm with a 95% level of HA phases and 93% crystallinity. The results of this research are of significance for a wide range of researchers developing plasma spray technologies for biocompatible coatings manufacture.

The 14th European Congress on Catalysis (EuropaCat 2019), themed ‘Catalysis without Borders’, was held on the 18th–23rd August 2019 at the Eurogress conference centre in Aachen, Germany. The conference hosted over 1500 participants from academia and industry across the world, with around 400 lectures and 800 posters presented throughout the week. There were six parallel sessions; this review is a small selection of the talks attended by the reviewers.

Plenary lectures of an hour were intended to showcase the significant breadth and depth of scientific knowledge acquired by established academics over the course of their careers. Summaries of key themes presented in the lectures are provided below.

Two plenary lectures were delivered on the subject of using molecular oxygen as a reagent, one by Professor Karen Goldberg of the University of Pennsylvania, USA and another by Professor Shannon Stahl from the University of Wisconsin-Madison, USA. Professor Goldberg opened by highlighting how molecular O₂ represents the ideal oxidant for chemical transformations, with the ‘jackpot’ being to achieve benign and facile routes to chemicals with atmospheric O₂, avoiding O₂ separation from air. Professor Stahl provided examples of work studying the control of molecular O₂ reactivity including maximising the energy efficiency of O₂ reduction to water in fuel cells and achieving selective oxidation of organic molecules without overoxidation to carbon dioxide or other undesirable byproducts (1–3).

A high-impact plenary was presented by Professor Ib Chorkendorff from the Technical University of Denmark. The talk started with a few headline data to emphasise that sectors such as aviation, long haul transport and the chemical industry will always have to rely on the same chemical building blocks we use today rather than full electrification. The speaker predicted that the chemical demands of the future should be met by ‘solar’ fuels in the form of photovoltaics providing the energy for electrochemistry. Several areas of focus within the research team were summarised including a collaboration with Haldor Topsøe, Denmark, in electrified steam reforming (4), green hydrogen generation (5) and electrochemical hydrogenation of CO₂ (6).

Professor Valentin Parmon of the Boreskov Institute of Catalysis SB RAS in Novosibirsk, Russia, presented an overview of some non-traditional approaches to achieve various endothermic thermocatalytic transformations. An example was the application of nuclear radiation to supply heat for steam reforming in an integrated chemical-nuclear reactor. A steam reforming catalyst was developed for the system; a porous uranium oxide support with nickel on the surface and titanium dioxide to protect against nucleotides (7). Solar energy was also considered to provide heating for steam reforming, temperatures >1000 K can be achieved by concentrating solar radiation (8). More recent work in the Parmon group focused on the use of microwaves to supply energy to reactions such as H₂ evolution from alkanes though the cost of the electrical energy required is prohibitive for large scale application (9).

A range of fundamental structure-activity relationships were described in the lecture of Professor Christophe Copéret from ETH Zürich, Switzerland. Work in the group is focused on the understanding and control of chemistry on surfaces, with the ultimate goal to generate isolated metal sites with defined chemical environment to elucidate structure-activity relationships. The approach in the team has led to highly active and selective single-site catalysts that out-performed their homogeneous counterparts, but that also provided useful information to understand industrial catalysts. Several examples were provided in the lecture including details of research into chromium polymerisation catalysts to generate a Cr(II) species directly rather than the need to reduce Cr(VI) in situ (10). An example was also provided for copper-particle catalysed methanol synthesis from methane, investigating the effect of the support; copper on alumina displayed higher activity than copper on silica (11). Figure 1 summarises work undertaken in the Copéret team and acknowledges the ‘pioneering’ work of Denis Ballard, Imperial Chemical Industries (ICI), UK.

Fig. 1.

The industrial forum sessions of the conference are to present work undertaken by industrial corporations or of direct industrial application. Over the course of the conference there were 21 talks in the industrial forum by both academics and speakers from commercial organisations.

3.1 Growth of Chemicals

3.2 European Federation of Catalysis Societies Applied Catalysis Award

3.3 SunCarbon – Creating a New Value Chain from Forest to Refineries

An ongoing theme of EuropaCat 2019 was the focus of many research groups on utilisation of CO₂. Many talks were presented on addressing various aspects of the Sabatier reaction for CO₂ hydrogenation, on the dry reforming of methane (DRM) for H₂ production or for the direct synthesis of methanol from CO₂.

Matteo Monai from Utrecht University, The Netherlands, presented a short talk on work done tuning metal-support interactions on supported Ni catalysts for the Sabatier reaction hydrogenating CO₂ to CH₄ (Equation (i)):

(i)

Both a reaction mechanism with carbide and formate intermediates are observed on Ni catalysts and both mechanisms were reported to be sensitive to particle size and local structure. Using strong metal-support interactions and a reducible support, it was possible to ‘embed’ Ni particles into the support material and suppress sintering. Enhanced C–C coupling was also observed on these materials when utilising the embedded metal particles, the mechanism for which was still subject to investigation (20).

An experimental programme investigating the mechanism for carbon formation over rhodium on alumina catalysts in the DRM was shared by Gianluca Moroni from the Politecnico di Milano, Italy (Equation (ii)):

(ii)

A variety of CH₄:CO₂ ratios were flowed over the catalyst at temperatures from 300–700°C in an operando-Raman annular reactor with gas analysis by Micro GC Natural Gas Analyzer (Agilent, USA). An adverse dependence between CH₄ concentration and activity was observed and a presented microkinetic model was said to predict experimentally observed activity with catalyst surface area as an input. Catalyst activity was also observed to reduce with increased carbon monoxide levels, and C deposition was said to begin on the Rh sites before migration and deposition on adjacent sites on the support (21).

Shohei Tada from The University of Tokyo, Japan, gave a talk on work done investigating catalysts for the conversion of CO₂ to methanol. Zirconia supported Cu catalysts were discussed and work was presented showing Cu species being metallic during reaction and methanol synthesis being performed at Cu-ZrO₂ interface sites (22). Tada went on to discuss the importance of finding a good support material to suppress methanol decomposition to CO, with the methanol-support interaction being said to be key. Methanol weakly adsorbs on an amorphous surface (a-ZrO₂) and strongly adsorbs on a monoclinic surface (m-ZrO₂), with the stronger interaction yielding more unfavourable decomposition of methanol to CO (23, 24).

EuropaCat 2019 was a large conference with wide-ranging themes delivered in multiple sessions over six days. It has not been possible to summarise the conference in its entirety. Major topics included: alternative energy inputs (plasma, electrochemistry, light); ammonia synthesis; biomass valorisation; CO₂ to chemicals; dry reforming; Fischer-Tropsch catalysis; industrial forum; mechanistic insights; nitrogen oxides (NOx) reduction; organometallic catalysis; and zeolite catalysis. More information can be found on the conference website.

To directly observe the impact of particle size on 1D NMR spectra, two activated PDC samples were reduced from approximately 100 μm particle size to approximately 15–20 μm by sieving (see Methods in SI). The samples were named xBO_y, where x is the percentage of BO and y the median particle size D₅₀, measured by dynamic light scattering. The samples were in the first instance wetted using a microsyringe with a defined volume of deionised water less than the PV, then with a volume greater than PV, to observe the ¹H NMR spectrum before and after saturation. We have previously shown that this method allows us to compare the NICS averaged over the whole pore network without, and then including, perturbations related to in-pore–ex-pore exchange. This is because the in-pore peak before sample saturation corresponds to water located in completely filled particles that are not yet surrounded by water. Figures 5(a) and 5(b) show the spectra of sample 54BO_80 and 54BO_21, respectively. The in-pore peak shifts by approximately 0.2 ppm upon saturation, regardless of the particle size. This means that in this range of particle size, diffusion of water out of the pores has a negligible impact on the NICS for most of the adsorbed water and the width of the in-pore peak is solely due to the distribution of average NICS. However, exchange averaging has a measurable impact on the ex-pore peaks. In both samples they were fitted with a narrow and a broad component, the intensity and full width at half maximum (FWHM) of which vary. The broad component is assigned to ex-pore water having experienced the pores for a period of time and is therefore also partially homogeneously broadened by exchange-averaging; the longer the residence time in the pores, the broader and the more shifted the broad component is, i.e. the bigger the ratio V_in^exch/V_ex^exch as per Figure 4. In big particles, the broad component amounts to 30% of the ex-pore peak with a FWHM = 0.30 ppm and is located within 0.10 ppm of bulk water chemical shift. On the other hand, with small particles the broad component represents 82% of the total ex-pore water with a much bigger FWHM = 0.70 ppm and is shifted by 0.20 ppm relative to bulk water. This indicates a slight increase of V_in^exch/V_ex^exch when the particles are reduced; in other words, a bigger proportion of ex-pore water is able to experience the in-pore environment for a longer time.

Fig. 5.

To explain the broadening of the ex-pore peak, we must adopt the point of view of water molecules that spend the majority of their time in the ex-pore environment, where we believe the packing of the particles plays an important role. A simple model consisting of spherical particles (Figure S3 in the SI) allows exchange rates to be calculated in function of the particle diameter (Figure S4). With 80 μm particles (similar to 54BO_80) we find exchange rates of 2.5 Hz, and of 36.7 Hz with 54BO_21 particles (see SI for more details on the calculations). The NICS being around 3000 Hz, the exchange regime would be slow in both cases, which is consistent with our observations, and increases by a factor of 15 when reducing the particle size from 80 μm to 21 μm.

Examples of 2D EXSY NMR spectra are shown in Figures 6(a) and 6(b) corresponding to sample 54BO_80 and 54BO_21 saturated with water. The mixing time (t_mix) was 20 ms for both spectra, and it can be seen that the cross-peaks are more pronounced with small particles, giving a first indication that the exchange is faster. Figures 6(c) and 6(d) show the build-up curves of the intensity ratio of cross-peaks over diagonal peaks. Visually, we can see that the curve for big particles reaches the maximum after long t_mix intervals, whereas for small particles the build-up is complete within 0.1 s.

Fig. 6.

In principle, the build-up of the ratio of cross- and diagonal peak intensity (I_cross/I_diag) in EXSY spectra can be described by a single dependence on tanh(kt_mix), where k is the exchange rate constant. However, Fulik et al., have shown that better agreement is observed if two processes with different rates are assumed, i.e. a slow process attributed to diffusion from the centre of the particle to the surface and a much faster process attributed to effective exchange between ex-pore and in-pore (15). However, over the course of our experiments, it was observed that the ex-pore resonance reduced in intensity due to evaporation from the NMR rotor leading to a global reduction in I_cross/I_diag for EXSY spectra recorded at the end of the series with long mixing times. Although the exact kinetics of the solvent evaporation are complex and were not studied in detail, we found that this could be accounted for with sufficient accuracy through the incorporation of an exponential term with a characteristic decay constant T_evap (see SI). Rate constants were therefore extracted from the EXSY data using Equation (i):

(i)

where I₀ is an additional constant introduced to account for t₁ noise giving rise to spurious off-diagonal low-intensity signal at zero mixing time. The fits were optimised by minimising the root mean squared deviation (RMSD) between calculated and experimental points, to converged values around 10^–2. The errors on the data points were calculated from the signal-to-noise ratio of each peak for a selection of 2D spectra and propagated to I_cross/I_diag, and were found to be smaller than 0.02%. The best fit for 54BO_80 was obtained with k₁ = 57 Hz and k₂ = 4 Hz, and for 54BO_21 with k₁ = 597 Hz and k₂ = 50 Hz. The carbon particles with D₅₀ = 21 μm showed around 10 times faster in-pore–ex-pore diffusion versus the D₅₀ = 80 μm particles. This is close to the factor of 15 which we obtained from our simple calculations based on spherical particles (see Figure S4) and shows the impact of the particle size on the rate of the exchange processes, with larger particles significantly reducing the exchange kinetics between the in-pore and ex-pore environments. Therefore, particle size is an important factor to take into account when comparing in-pore–ex-pore exchange phenomena in porous carbon samples.

Regarding the in-pore resonances, the shape and position in the NMR spectrum is largely unaffected by in-pore–ex-pore exchange despite the difference in particle size. We can therefore assume that the diffusion path out of the particle from any point of the pore network, apart from the very surface, is simply too long and exchange-averaging of the in-pore environment is in the slow regime. This would mean that if the particle size is decreased further, at some point the in-pore peak should also become affected by faster diffusion of in-pore water into the ex-pore environment. To test this hypothesis, 6 μm-sized YP50 particles were wetted with deionised water. YP50 is a commercial activated carbon, and similar to our PDCs with respect to composition, pore size distribution and average pore size (see Figure S13). The ¹H NMR spectra before and after saturation are shown in Figure 7. As expected due to the small particle size, an ex-pore peak with sharp and broad components is observed. Relative to 54BO_21, the broad component is shifted five times more, consistent with smaller inter-particle voids that facilitate adsorption of ex-pore water in close proximity. The in-pore peak shifts from a NICS of 7.3 ppm to 7.0 ppm upon saturation, which is similar to 54BO_21, however the intensity decreases significantly, and we notice that the peak exhibits a tail towards the ex-pore peak. This means that after saturation, a significant proportion of adsorbed water is able to quickly diffuse into the ex-pore environment and therefore does not appear at the purely in-pore chemical shift, but rather intermediate between the ex-pore and the in-pore chemical shifts. The exchange rate constants were estimated to be k₁ = 1117 Hz and k₂= 165 Hz. Assuming that k₁ relates to the ex-pore–in-pore exchange process that we calculate as a function of the particle size (Figure S4), we find that 4 μm particles give a similar exchange rate. This is good agreement considering that 20% of YP50 particles are smaller than 2.5 μm (Figure S14), especially because for particles smaller than 10 μm the exchange rate increases sharply. This explains why YP50 presents a relatively flat valley between the two broad peaks: even a narrow particle size distribution in the few-micrometre range gives a very broad distribution of exchange rates, therefore generating exchange-averaged peaks at a continuum of shifts.

Fig. 7.

One point worth addressing is whether the inhomogeneous broadening of the in-pore peak is responsible for broadening the exchange-averaged ex-pore peak, which can give an idea of the reliability of the exchange rate constants obtained. The spectrum of Figure 7 was successfully reproduced in Express software (22) in the case of a homogeneous as well as a inhomogeneous broadening of the broad ex-pore peak, so the simulation does not allow it to be determined whether the exchange-averaged peak is homogeneously or inhomogeneously broadened. Refer to SI for further detail. However in 2D EXSY spectra, for any t_mix, the ex-pore peak shows symmetrical off-diagonal broadening meaning the broadening is homogeneous. This suggests that one component of the in-pore peak is much more exposed to the ex-pore environment than the other components. In consequence the values of exchange rate constants may be more reliable than if the broadening were inhomogeneous. This result is consistent with a radial distribution of NICS in the particle, in agreement with our previous observations where the centre of particles of diameter greater than 100 μm is poorly affected by steam activation. The particles employed here were smaller than 100 μm but it seems that a small gradient of activation still remains. It is possible that the valley between ex-pore and in-pore is the result of in-pore–ex-pore exchange broadening, in the fast or slow regime, of the other components of the in-pore peak, which are located further and further away from the surface, and therefore have access to smaller and smaller volumes V_ex^exch.

In summary, from these observations it could be deduced that particles smaller than 10 μm offer a diffusion path short enough for water to diffuse out of the pores at a rate that affects the in-pore peak as well. However, given that the diffusion coefficient of adsorbed water depends on the pore size, we cannot compare 54BO_21 and YP50 because they have different average pore sizes. Using the equation provided previously (6), these can be estimated using the NICS before saturation. We obtain for 54BO_21 and YP50, 1.17 nm and 1.10 nm respectively. Therefore, it is necessary to investigate the effect of average pore size, which can be tuned by controlling the BO, on the diffusion of water and the appearance of the spectra.

The effect of BO, and thus average pore size, on the dynamics of water in PDCs can be described by comparing 54BO samples (high BO) with 20BO samples (low BO). Figure 8(a) shows samples 20BO_83 and Figure 8(b) shows 20BO_15 injected with water. The NICS of samples 20BO are 8.4–9.0 ppm, giving an average pore size less than 1.0 nm based on the previous equation (6), which is smaller than sample 54BO, in agreement with the linear dependence of the average pore size on the BO (18, 19, 23). The in-pore peaks appear to contain two components as is sometimes observed for samples with low BO. Possibly, the degree of activation was not homogeneous from the surface to the centre of the particles. In sample 20BO_83, the in-pore peak is constant regardless of the injected volume (shift < 0.10 ppm). Half the ex-pore peak was fitted with a sharp component of FWHM <0.1 ppm, and half with a broad component of FWHM = 0.16 ppm with a difference in chemical shift < 0.10 ppm. This suggests in the first instance that the in-pore–ex-pore exchange process is in a slower regime than in 54BO_80, where the broad component was broader and the in-pore peaks were shifted slightly, in agreement with the study mentioned earlier (24). With small particles 20BO_15, 90% of the ex-pore peak is broad, FWHM = 0.35 ppm but within 0.10 ppm of the bulk water, suggesting again limited exchange with negligible V_in^exch.

Fig. 8.

The exchange rate constants provide a more quantitative description of the impact of average pore size. For the big particles 20BO_83, k₁= 54 Hz and k₂ = 4 Hz, which are comparable to the values for 54BO_80 (57 Hz and 4 Hz). This was perhaps not expected from the 1D spectra, where 20BO_83 appeared much less affected by diffusion. However, one must keep in mind that the exchange regime depends not only on the actual rates, but also on the chemical shift difference of the two environments involved. The NICS of samples 20BO is higher than in samples 54BO, so even identical exchange rates still situate 20BO in a slower regime than 54BO.

For the small particles 20BO_15, k₁ = 162 Hz and k₂ = 6 Hz, which are three times as fast when compared to 20BO_83. However, the calculated factor was 31, which is a clear discrepancy. Given that sample 20BO_15 contains particles smaller than sample 54BO_21, we can attribute the discrepancy to a BO effect and not to a particle size effect. Besides the average pore sizes, other structural parameters that could perhaps vary with BO are oxygen content and tortuosity due to the smaller amount of mesopores. These observations also raise the question of homogeneity of the diffusion coefficients within the particle: it is likely that at low BOs a bigger gradient of activation within the particle is present.

Overall, these results show that a smaller average pore size hinders exchange as opposed to a smaller particle size, which promotes it. Since YP50 has a smaller average pore size than 54BO_21 but higher exchange rate constants, we can safely deduce that in YP50, the particles are truly small enough to allow for in-pore–ex-pore exchange to affect the in-pore peak. These conclusions are in agreement with simulations (25), and also an experimental study focusing on diffusion measurements in hydrophobic slit pores by neutron-scattering (23). The material contained approximately 95% carbon with 5% oxygen atoms on the surface of the pores and average pore sizes of 1.2 nm and 1.8 nm, which is similar to our PDCs. It was found that the diffusion coefficients of water in the 1.2 nm and 1.8 nm pores are 40% and 30% smaller than that of bulk water, respectively. Furthermore, on the very surface of pore, water diffuses at 0.035 × 10^–5 cm² s^–1 and 0.014 × 10^–5 cm² s^–1 respectively, which is two orders of magnitude slower than in the bulk. Counterintuitively, the value on the surface of the bigger pores was found to be nearly half that in the smaller pores, which was attributed to the promotion of slightly faster concerted motion in extreme confinement, although the centre of the pore followed an overall slower regime.

Many applications of porous carbons (such as electric double-layer capacitors) commonly employ electrolytes in organic solvents as an alternative to aqueous electrolytes. To extend the knowledge presented here to such systems, it is desirable to be able to predict how the NICS may be affected. The viscosity and the polarity are the two parameters that will be considered here as tools to predict the diffusion regime adopted by any solvent.

The effect of polarity can be estimated by comparing water and cyclohexane which have the same viscosity, but different dipole moments. YP50 is chosen for this comparison because the exchange rates are high enough to significantly impact the NMR spectra. The exchange rate constants were measured to be 1117 Hz and 165 Hz for water, and 257 Hz and 23 Hz for cyclohexane, which means that cyclohexane diffuses roughly four times slower. On one hand, this could be related to increased van der Waals interactions with the hydrophobic surface of the pores, as was also found to be the case in a xerogel using the same solvents (24). On the other hand, the bulk self-diffusion coefficient of cyclohexane is smaller than for water (1.42 × 10^–5 cm² s^–1vs. 2.3 × 10^–5 cm² s^–1) (26), which could also contribute. At this stage it is unclear whether other parameters such as the different molecular sizes of water and cyclohexane play a role. Figure 9 shows the ¹H NMR spectrum of YP50 wetted with cyclohexane. As expected with such high exchange rate constants, we see a narrow and a broad ex-pore peak, and an in-pore peak that decreases and shifts upon saturation, much like in Figure 7 with water in YP50. With water and cyclohexane, the in-pore peaks are similar; they shift by 0.30 ppm and are broadened by a factor two upon saturation. Interestingly, we note that the NICS of cyclohexane (and hexane) is smaller than water by 0.2–0.3 ppm, which means that the apolar solvents are on average located in slightly bigger pores than water. The diffusion being faster in big pores, this is in contrast with the slower exchange kinetics measured, showing that solvent parameters are prevalent over pore size effects. The broad ex-pore peak is narrower and less shifted in cyclohexane (FWHM = 0.64 ppm and shift = 0.27 ppm) than in water (FWHM = 1.69 ppm and shift = 0.98 ppm), consistent with smaller exchange rate constants.

Fig. 9.

The effect of viscosity can be observed by comparing cyclohexane (0.89 cP) and hexane (0.30 cP) which have the same very low polarity. Figure 10 shows the ¹H NMR spectrum of YP50 wetted with hexane. Unlike water and cyclohexane, hexane shows two ex-pore peaks, which are simply due to the two visible proton environments on the molecule; 1.28 ppm are the methylene (CH₂) and 0.88 ppm are the terminal methyl (CH₃) protons. The in-pore peak before saturation can be fitted with two broad peaks 0.30 ppm apart, which is similar to the difference between the CH₂ and CH₃ chemical shifts (refer to SI for details on the fits). This suggests that all protons of adsorbed hexane molecules roughly experience the same NICS, therefore the molecule is either tumbling isotropically or aligned with the pore wall. The exchange rate constants are k₁ = 784 Hz and k₂ = 104 Hz for hexane in YP50, which is approximately three times the rates seen for cyclohexane. Therefore in this case, the exchange rate was roughly proportional to the viscosity, as hexane is three times less viscous than cyclohexane.

Fig. 10.

More interestingly, the broad ex-pore peak is shifted by 0.61 ppm relative to the average between the two narrow peaks, and the FWHM = 1.30 ppm. These values are between the values for cyclohexane and water. This is consistent with the shift and width being proportional to the exchange rate constants as they are even higher for water in YP50. Overall, pronounced solvent exchange affects the ex-pore and in-pore peaks and the valley between the two peaks, but the ex-pore peak seems to be the most reliable indicator to estimate at a glance perturbations due to exchange effects.

Further discussion is required about parameters that could not be assessed with this set of experiments, for example the kinetic diameter of the solvent molecules. This factor was not considered separately because it was first assumed to be reflected in the viscosity parameter, although in pores of similar size, viscosity and diameter may have independent contributions to the exchange kinetics. In addition, to identify the impact of this factor alone, two solvents with similar viscosity and polarity but different molecular sizes should be compared. The kinetic diameter of water and cyclohexane perhaps contributes to the exchange rate difference.

The behaviour of cyclohexane was peculiar in the case of sample 20BO_83 and 20BO_15. In the big particles, exchange was too slow to be observed, and by decreasing the particle size, the exchange became visible but the rate was higher than that of hexane and also of cyclohexane in the other PDC samples, which goes against the trends. Another interesting observation was that there were two in-pore peaks in 20BO before saturation and both peaks shifted to the right upon saturation, meaning exchange-averaging takes place within different pores but not with ex-pore, while in 54BO there was only one in-pore peak (see Figures S5 and S6). We believe these observations are all related and hint towards the ability of cyclohexane to form organised structures in slit-like micropores, which affects its diffusion coefficient. The diffusion coefficient of confined cyclohexane was shown by Fomin et al. to drop to zero in slit pores smaller than 2.1 nm, and its density to increase two-fold from 2.6 nm to 1.2 nm pores (27). Another study showed that cyclohexane forms a monolayer in 0.8 nm pores and a bilayer in 1.0 nm pores with a denser hexagonal packing structure resembling the solid phase (28). Similar behaviour has also been observed for propylene carbonate which was observed to form an ordered structure upon nanoconfinement (29). In a disordered structure the packing is expected to be less efficient, nonetheless the diffusion coefficient may still be significantly lower. The exchange rate constants determined take into account the diffusion of species in all types of pores located in V_in^exch. When cyclohexane forms an immobile structure in the smallest pores, only the diffusion coefficient in the biggest pores where it is still liquid contributes to the exchange rate constant, explaining why in 20BO_15 the exchange rates are much higher than in the other PDC samples. The amount of mesopore is smaller than in 54BO and YP50, so the long-range diffusion within the particles is probably significantly slower. In that regard, the gradient of NICS will be less well averaged, and even more so with slow diffusing solvents like cyclohexane. The 3.3 ppm NICS of the small peak gives a pore size of 2.2 nm and the 7.7 ppm NICS of the main peak 1.0 nm. The small in-pore peak is therefore likely to come from few isolated mesopores while the other from micropores and mesopores in contact.

One area where physics and chemistry come together across the disciplines is in the field of plasma catalysis. A plasma can be described as a ‘soup’ of species including molecules and atoms that are charged or excited and free electrons. Depending on the energy of the plasma it can be fully ionised and have a bulk temperature of tens of thousands of kelvin (or in the case of nuclear fusion millions of kelvin) or it can be in a low temperature non-equilibrium state where only a small portion of the gas phase is energised. At the lower end of the energy spectrum a non-thermal or cold plasma will have a bulk temperature a few tens of degrees above ambient and yet still have some exceedingly high energy species present. Even though the majority of the species in the plasma are not in equilibrium, there can exist some partial equilibria among species with similar kinetic temperatures existing at localised sites (1).

This energy can be used to initiate chemical reactions in the gas phase and on the surface of solids. The term plasma catalysis can be used to describe both the use of plasma to initiate a reaction directly and the use of plasma in combination with a catalytic material. The catalyst can be positioned after a plasma zone as post plasma catalysis (PPC) or within the plasma zone as in-plasma catalysis (IPC). The PPC configuration allows the longer-lived excited species and the products from the plasma reaction to interact with the catalyst and the IPC configuration allows a greater opportunity for the catalytic material to influence the nature of the chemistry by directly interacting with the plasma excitation as well as the products from the plasma-initiated reactions.

With the introduction of a plasma to a catalytic reactor the standard models of the chemistry and the traditional understanding of the mechanisms by which reactions take place start to become less relevant due to the non-equilibrium concentration of excited species such as free atoms, electrons and radicals (2).

Within the universe around us there are many examples of plasma (3) including the sun, stars, auroras, lightning, welding arcs and fluorescent lighting tubes. A plasma can be generated in different ways, but all require energy to be applied, either as heat in the case of a thermal ionisation or through the generation of an intense electric field via the use of electrodes, radio frequency (rf) or microwave (MW).

Electrical excitation is one of the most feasible ways for producing well controlled plasma discharges at industrial scale. This type of excitation is controlled by three main parameters: (a) the applied voltage amplitude, (b) the applied frequency and (c) the waveform shape. The combination of the aforementioned parameters defines different operating regimes (4). The main types are:

From the regimes described above, sinusoidal and pulsed power sources present the advantage of producing non-thermal plasma without the need for noble gasses and low pressure. Moreover, the use of dielectric barrier material protects the electrodes from erosion in chemical processes and limits the power needed for initiating and maintaining plasma discharge.

It is also possible to influence the nature and stability of the plasma through external forces such as the introduction of an applied magnetic field, a vortex flow or by reducing the pressure within the system.

In scientific and industrial applications, average power delivered in the plasma discharge is critical. Accurately measuring and monitoring the power consumption in plasma discharges is not a straightforward task. Average power consumption is calculated through the measurement of voltage and current waveforms. Although the voltage signals are monitored with high accuracy through capacitive voltage dividers, current waveforms as shown in Figure 1 and Figure 2 present significant measuring difficulties. Current spikes with very short (nanosecond) duration and high amplitude and frequency are imposed over the sinusoidal low frequency and amplitude signal. Monitoring those high dynamic range waveforms can be erroneous and special care needs to be taken in the method of acquiring those signals. In the literature (5), three district ways are described for measuring directly the current or the charge in plasma.

Fig. 1.

Fig. 2.

Although the above measurements have comparable accuracy in scaled plasma reactors, in industrial environments it is preferable that in situ non-invasive techniques are used. Rogowski coils have an intrinsically safe way of operation given that they are galvanically isolated from the main circuit without compromising the accuracy.

Accurate power measurement and delivery in combination with optimised mechanical and electrical design can lead to improvements in energy efficiency. This is a critical parameter that must be considered, especially when scaling up plasma systems. It is unavoidable that some part of the initial energy is lost in the electrical transformation to high voltage and in the plasma reaction as heat. By carefully selecting the electric field and reactor characteristics those losses can be minimised. Modelling can significantly enhance the understanding of the plasma transitions in short time and space frameworks (6). The next improvement step involves the synergistic effects of plasma catalyst interaction. By introducing a catalyst in the plasma region or next to it, different works have shown considerable improvement in conversion and in total efficiency (7). All these optimisations allow energy consumption of the plasma system to be decreased and make plasma technologies feasible from the view point of economic and life cycle assessment. For example, a recent paper by Rooij et al. (8) shows that the combination of plasma with renewable energy sources is an economical method even for such an expensive process as carbon dioxide (CO₂) reduction. The technoeconomic solution will of course be different for each market application based upon comparison to current scaled state of the art techniques.

Even the simplest of plasma generation arrangements takes in aspects of a wide range of physics disciplines. A schematic of a simple plasma tube is shown in Figure 3. Gas is passed through the cell, where a high voltage is applied across a central cylindrical electrode and a cylindrical outer electrode. The generation of the plasma in the cell leads to a shift in chemical make-up across the cell, hence the potential to use such cells in chemical processing. Even prior to the excitation of any plasma, the gas flow through the cell has the potential to become a complex fluid mechanics problem. Once a plasma is generated the equations of magnetohydrodynamics become applicable. This apparently innocuous point massively increases the experimental phase space that needs to be controlled and understood. A general introduction into the topic of magnetohydrodynamics can be gained from reading “An Introduction to Magnetohydrodynamics” by P. A. Davidson (9). The bulk physical continuous control parameters in a traditional reactor vessel can be broadly listed as temperatures, pressures and flow rates. In a plasma cell we have in addition voltages, currents and frequencies which must be controlled and monitored. This opens huge opportunities in chemical processing as significant parts of this phase space remain largely unmapped.

Fig. 3.

One of the oldest applications of plasma is in fluorescent lighting. This fact means that there has been a significant amount of study into the behaviour of plasma cells where there is no flow. The graph shown in Figure 4 is a schematic illustration of the characteristic (current vs. voltage) curve of a typical gas discharge in neon gas at a pressure of 1 torr, between two planar electrodes separated by 50 cm. This figure has been recreated from the information presented by Gallo (10). There are broadly speaking three classes of behaviour. A dark discharge region, a glow region and an arc region. In the dark discharge region, a voltage lower than the breakdown voltage of the gas is applied to the tube. External radiation such as gamma photons and beta particles then trigger a Townsend cascade within the tube. This behaviour is utilised in a number of simple nucleonic detection devices, notably Geiger counters (11). In this region the current is limited to remain low and the discharge is reliant on external excitation. In the glow region, the current is no longer limited, and the accelerated electrons excite further electrons and the voltage then drops. The gas emits light causing a glow, an effect utilised in fluorescent lighting. If the current is high enough the neutral gas in the tube becomes heated and arcs start to form which is the final region.

Fig. 4.

The graph shown in Figure 4 illustrates that the behaviour of plasmas is complex and many transitions occur in a cell where all of the standard chemical processing variables are fixed i.e. pressure is constant, temperature is fixed and the flow rate is zero. In this example there was no time dependence to the applied voltage and current.

There are an increasing number of references in the scientific literature giving examples of laboratory scale plasma catalysis (12, 13) or industrial processes being investigated at semi-industrial level (14). However, there are currently no known large-scale industrial applications that combine plasma with a catalytic material. The following two examples show the scaled production of chemicals using plasma excitation.

Plasma is used to generate ozone (O₃) industrially for applications including cleaning, disinfection, deodorisation and sanitisation. Commercially this is done using ultraviolet (UV) light at a wavelength of 185 nm, electrolytically or via a DBD plasma depicted in Figure 5. Siemens, Germany, were the first to use plasma for an industrial application in 1857 to produce O₃ (15). This plasma is generated as a large number of statistically distributed micro-discharges between electrodes where the potential is insufficiently high to create an arc. Diatomic oxygen is broken down through interaction with the electrons produced within these micro-discharges that have sufficient energy to split the O₂ double bond. These newly separated O atoms then combine with other diatomic O₂ molecules to produce O₃. The micro-discharges are individually only present for a few nanoseconds each and the number of micro-discharges generated is dependent upon the gap between the electrodes, the humidity and pressure of the air, the properties of the dielectric barrier and the characteristics of the electrical supply (16). O₃ production can be as high as 100 kg h^–1 from a horizontal honeycomb reactor and is closely related to the specific voltage and frequency applied, with typical voltages being 7–30 kV and frequencies between 50–1000s Hz.

Fig. 5.

Thermal plasma has been used to produce acetylene (C₂H₂) since the 1940s in the Huels process. The original Huels plant used the low-boiling components of the motor fuel industry as raw material; however, a wide range of hydrocarbons including natural gas were shown to be suitable as process feed stocks. The equilibrium formation of C₂H₂ is characterised by the requirement of very high temperatures, around 3000°C. Therefore, the reaction gas should be rapidly cooled by liquid water spray injection downstream of the plasma reaction zone to avoid formation of solid carbon that is a thermodynamically preferred product between about 1000°C and 2500°C. This fast quenching prevents decomposition of the C₂H₂ formed in the plasma. Because the formation of C₂H₂ from methane (CH₄) is strongly endothermic, relatively large amounts of energy are required. In industry the best energy performance was shown by the DuPont process (a modification of the Huels reaction) with the specific energy consumption 8.8 kWh kg^–1 of C₂H₂ (17).

More recently different methods of C₂H₂ formation in plasma were compared (18) and it was found that pulsed spark discharges gave the highest C₂H₂ yield (54%) with 69% of CH₄ conversion in a pure CH₄ system. It was suggested that the main disadvantage of the plasma method of C₂H₂ production was the fact that excited species reacted with formed C₂H₂ and decomposed it to undesirable byproducts. As a result, the energy costs for CH₄ conversion and C₂H₂ formation increased with CH₄ conversion percentage and were found to be best in pulsed spark discharges (highest CH₄ conversions 18–69%). A Korean group reported the energy cost in its systems as 9 kWh kg^–1 C₂H₂. They calculated their theoretical minimum energy requirement as 4.03 kWh kg^–1 C₂H₂ based on heat of reaction (19). In this publication, it was also found that hydrogen in the plasma counterintuitively increased the selectivity to C₂H₂ in the process, from a respectable 70% to well above 90%. The paper claims that it is possible to decrease the specific energy consumption to 6 kWh kg^–1 C₂H₂ in the low-temperature arc with the application of argon as the recycling reactant. It is close to the theoretical limit of 4.03 kWh kg^–1 of C₂H₂, which is 50% of the energy required for the DuPont process. These numbers show that there is space to improve energy efficiency of the industrial production of C₂H₂ by plasma methods.

It is possible to introduce a catalyst after a plasma zone and transform some of the still excited species present in the gas phase into products at the surface of the catalyst, known as PPC (20).

Plasma can also generate activated species at a surface as well as in the gas phase. There are many terms used to describe the use of a catalyst and a plasma together, including plasma catalysis, plasma enhanced catalysis, plasma assisted catalysis, plasma driven catalysis (21); this IPC uses the excitation of both the catalyst surface and the gas phase reactants to effect chemistry through reactions between:

These plasma and catalyst interactions can be thought of as occurring between one or more excited states and can be representative of both the surface and the gas being excited. An electrically induced surface potential of a material is equally defined as a catalyst as a material that satisfies more traditional thermal catalysis ideals of adsorption and reaction, such as the use of glass beads for CH₄ conversion (22).

In addition to the above interactions, further thought should be given to the interdependency of the plasma formed on the properties of the catalytic material present, and vice versa where the plasma will have an impact on the properties of the catalyst surface, including even impacting upon the physical morphology of the material. These synergistic interactions have been proven to offer a different mechanism for chemical reaction when the excitation comes from electrical plasma rather than thermal means (23). A study of plasma activated catalytic (palladium/aluminium oxide (Pd/ Al₂O₃)) CH₄ oxidation, conducted in a synchrotron beamline, concluded that the Pd nanoparticles are heated within the plasma but the temperature of the nanoparticles remains lower than that required to initiate the thermal CH₄ oxidation reaction. Thus, an alternative reaction mechanism with a lower activation barrier must be taking place (24).

There have been many studies looking at the application of plasma catalysis for chemical synthesis. In particular, small molecules that are difficult to activate using more traditional thermal methods, such as CO₂ and CH₄ lend themselves towards activation using plasma techniques (13). A special issue of the journal Catalysts titled ‘Plasma Catalysis’ was recently published including papers covering the application of plasma catalysis for CO₂ splitting (25), ammonia (NH₃) synthesis (26) and CH₄ reforming (27).

A key challenge for plasma catalysis is to design a reactor that is suitable to house a plasma and a catalyst that has low backpressure, but retain good catalyst and gas-plasma interaction similar to the ceramic monolith widely used in automotive emission treatment. Uytdenhouwen et al. (1) identify power, pressure and gap size in a reactor as key process parameters for utilisation in design of plasma reactor and then go on to discuss their effect on CO₂ disassociation in a DBD microreactor.

Mizuno (28) describes multiple approaches to tackle this problem in his review: including (a) micro-discharge plasma with the process gas flowing through a catalyst coated metal plate with very narrow (micron) gaps to improve the catalyst-gas interaction, (b) a metal mesh DC powered electrode in front of a ceramic monolith and a packed bed alternating current (AC) electrode at the rear of the ceramic monolith in order to introduce surface streamers along the monolith channels and (c) a sliding three-electrode DBD system combining a negative AC electrode, a DC electrode and a ground to create more homogeneous and widely dispersed surface plasma (29).

Plasma catalysis has been studied for automotive emission control in both the PPC and IPC configurations. Some of the first studies were conducted using a packed bed for nitrogen oxides (NOx) control (30, 31) and proposed a two-stage process whereby the nitric oxide (NO) was oxidised to nitrogen dioxide (NO₂) which was then subsequently selectively reduced over the catalyst by the hydrocarbons present. This system has also been proposed as a pre-particulate filter plasma reactor to attain additional particulate matter oxidation benefits from the increased NO₂ generated by the plasma (32). A successful demonstration of a combined plasma and catalytic system has also taken place for CH₄ removal from dual fuel engines at low temperatures (33).

If the plasma can generate excited species at a surface, then it follows that it should be possible to change the surface by plasma treatment of a material. One area where plasma surface treatment has garnered significant interest is in the treatment of plastics and polymers. Plasma treatment of plastics and polymers can have a significant effect on the chemical and physical properties of the materials, these changes occur rapidly, often within seconds (34), through the following proposed mechanisms (35):

Examples of surface functionalisation include the incorporation of hydroxyl groups (OH) from humidity present in the plasma or N fixation using an NH₃ plasma. The introduction of polar groups such as hydroxyls allows for a significant improvement in the wettability of polymers and therefore has significant advantages for the printing and adhesives markets. This is a complex area and large bodies of work have been produced documenting the effects of different types of plasma on different polymeric materials and summarised elsewhere (34, 36).

Increasingly plasma has been used for vapour deposition (VD) processes applying a uniform thin film coating to a material (37). These films are usually within the nanometre thickness range and used for modification of optical, chemical, electronic, physical and decorative properties of the materials. The methods of plasma application for physical VD include sputtering, ion plating and cathodic arc deposition. Sputter deposition involves deposition onto a substrate of a molecule previously vaporised from a target. The target is vaporised through the mechanism of momentum transfer from gaseous ions accelerated from a plasma. The plasma ion plating process uses the material vaporised from a target (by whichever method is suitable) and bombards the depositing film with molecules produced from a reactive gas plasma as the film is deposited in order to change the properties of the depositing film. Arc VD occurs when an electrode is vaporised through the application of high current across a biased cell with the vaporised molecules being accelerated towards, and deposited on, the polarised substrate.

These treatment and deposition techniques require an understanding of physics and electrical engineering to generate, measure and optimise a plasma in order to effect the chemical change on, or within, a surface.

It has been reported that using plasma as a preparative technique can improve catalyst dispersion, increase metal-support interactions and change metal particle morphology, which in turn can lead to improved catalytic activity and stability (38). An example of improved activity from plasma preparation is the supported nickel catalysts used for steam reforming; using a DBD plasma reactor to decompose the precursors such as nickel nitrate, for catalyst preparation it is possible to increase the proportion of the (111) Ni facets which show enhanced performance and coke resistance.

Fig. 6.

Another example of plasma used for catalyst preparation is the use of DBD plasma instead of the standard thermal calcination for silicon dioxide (SiO₂) supported cobalt materials for Fischer Tropsch synthesis. Li et al. found that plasma prepared materials had enhanced activity and a greater yield of heavy hydrocarbons when compared to the thermally calcined materials. This performance was attributed to the measured increase in Co dispersion, smaller Co(II,III) oxide (Co₃O₄) cluster size and more even Co distribution. A byproduct of the plasma preparation is the claim that this route can be a ‘greener’ method of preparing materials: using a low temperature electron reduction instead of using H₂ as a reductant removes the need for dealing with H₂ in the process (39).

Another advantage of the low bulk temperature plasma treatment as a preparative technique in comparison to a standard thermal treatment is the ability to remove precursors without inducing the detrimental changes that are associated with the temperatures normally required to oxidise the precursor molecules. An example of where this is useful is in the preparation of zeolite materials through plasma template removal. Liu et al. have conducted the removal of zeolite templates at around 125°C using a DBD plasma technique with O₂. They identified the two major reactions taking place as the dissociation of template molecules by active species such as electrons or excited O₂ and oxidation by excited O₂ or O₃ molecules (40). When directly comparing template removal from ZSM-5 zeolite by thermal and plasma techniques Liu et al. found that the rate of removal was approximately eight times higher using the DBD plasma method (41).

Using a DBD system AZO Materials, UK, reported differences in the temperature and in the intensity of the peaks resulting from temperature programmed reduction of magnesium oxide (MgO) supported Ni catalysts compared to non-plasma treated materials (42). The differences were attributed to Ni particle morphology and dispersion.

Zhu et al. also treated supported Ni catalysts with DBD plasma and found an increase in the catalytic activity and stability for the partial oxidation of CH₄ (43). The scanning electron microscopy (SEM) images support a case for enhanced dispersion and increased interaction between the 10% Ni and the Al₂O₃ support. As well as a measurable increase in catalytic activity (3–5%) they also report a reduction in the formation of C around the Ni. This is consistent with a change in the Ni particle morphology towards having more (111) facets, as observed by others during the plasma preparation of catalysts (38).

The literature related to the plasma application for material regeneration is limited. The current state and perspectives of plasma applications for catalyst regeneration was discussed in a recent review (44). Plasma regeneration was successfully applied for the reduction of oxidised catalysts and removal of poisons and C deposits. The largest advantage of plasma is that it allows catalyst regeneration to be performed at temperatures lower than those of typical thermal regeneration. The supply of gaseous reactive species and alteration of the surface structure to a more energetic state were identified as prerequisites of successful low-temperature regeneration and it was also shown that plasma can supply heat in a more cost-effective way than conventional thermal treatment. The energetic species produced in non-thermal plasma can initiate diverse reactions and open up or enhance reaction pathways other than those expected for equilibrium chemistry. As thermal regeneration can result in catalyst sintering and thus a reduced number of active sites, plasma is a viable alternative to thermal treatment.

The advantage of plasma regeneration was shown in the recent work that has been undertaken at the University of Central Lancashire, UK, studying deactivated coked zeolite regenerated with the application of different techniques including thermal, MW plasma and DBD discharge plasma (45). This work showed that plasma not only removes C from the deactivated catalyst but increases the activity of the catalyst significantly. Toluene disproportionation was used as the probe reaction in this study. Unlike thermally regenerated catalysts the material regenerated by plasma shows improved catalyst performance and the activity of the regenerated catalyst is even higher than that of the virgin material. Characterisation methods including pyridine and collidine infrared studies, NH₃ temperature programmed desorption and solid state nuclear magnetic resonance were used to explain the changes in catalytic activity. Results showed MW plasma regeneration extended the catalytic life of zeolite due to the destruction of Brönsted acid sites caused by dealumination, without loss of crystal structure. In the toluene disproportionation reaction, this reduces the amount of cracking which occurs, subsequently leading to less coke deposition and therefore an extended catalytic life.

A key challenge for the field of plasma catalysis is to assemble teams with the relevant complementary skills in electronics, physics, engineering and chemistry together to gain an understanding of the system in order to produce the desired technological progress. A subset of this challenge is the communication between the different disciplines; involving not just the different language used, but also the models derived to express the concepts and understanding of processes which are often not ideally accessible to other branches of science and engineering.

As well as the broader topic of reducing barriers for communication, there are key challenges remaining within each of the disciplines required, including but not limited to:

It is necessary to understand physics to be able to correctly use electronic engineering to generate the requisite plasma to react with the designed catalyst in order to affect the desired chemistry. Whether this chemistry change is in the gas phase, at the surface of a catalyst or within the surface, there are clearly a large number of challenges to be faced when considering the large experimental space brought by the additional variables from non-chemistry fields. The general challenges for plasma catalysis can be expressed in broad terms of, for instance, increased efficiency, targeted catalysts or improved scaling of reactors; however within each of these targets underlying incremental improvements are required from additional scientific and engineering disciplines to achieve this.

Therefore, a key challenge for the field of plasma catalysis is to assemble teams with the relevant knowledge and skills in their own area of expertise that can work together and communicate ideas to initiate and progress the technology. Within Johnson Matthey there are teams with expertise across all the relevant disciples who can overcome these barriers and work successfully together to realise the potential of plasma catalysis on a commercial scale.

Particulate filters, comprising so-called wall-flow filter substrates, are of increasing importance in reducing pollutant emissions from vehicles to the levels required by legislation. Early legislation addressed the emissions of carbon monoxide, hydrocarbons and nitrogen oxides and the removal of these pollutants was achieved using FTM catalysts. Later, when PM emissions from diesel vehicles came under scrutiny, wall-flow filter devices were added to the emissions control system: often these were uncatalysed extruded cordierite or silicon carbide filter monoliths, though sometimes a catalyst was incorporated on the filter to widen their operating window. Wall-flow filters differ in their operation from FTMs, since adjacent channels are alternately blocked meaning exhaust gas must pass through the porous monolith wall to flow from inlet to outlet; in this way PM is deposited on the inlet channel walls. Previous PM legislation was based on particulate mass emissions, however, more recently the legislation, such as in Euro 6 (1), has turned to address particulate number emissions. Due to this change in emphasis both diesel and gasoline vehicles now require filter systems to achieve compliance. In gasoline applications, a GPF is typically a wall-flow filter, similar to that used for diesel, with a catalyst coating applied: by combining the catalyst and filter devices, multifunctional emissions control systems have also been developed resulting in reduced packaging volume.

In the manufacture of catalysed filter devices, first a catalyst coating is applied to the monolith substrate in the form of a slurry. The subsequent coated monolith is then dried and finally calcined to fix the catalyst coating. The drying process is both energy intensive and known to influence the final metal distribution within the catalyst throughout the catalysed filter and therefore also influences filter performance on a vehicle. Indeed, it has previously been shown (2, 3) that non-ideal drying in FTMs can result in the macroscopic redistribution of the catalyst. In the work of Vergunst et al. (3) and Wahlberg et al. (4) it was observed that the metal phase, when not bound to the catalyst support, will migrate to the surface where evaporation occurs. This results in varying degrees of inhomogeneous catalyst distribution depending on the method of drying used. Finally, enrichment of the catalyst in the washcoat layer of ceramic monoliths during drying has been observed using MRI (5). Traditionally, drying is studied using gravimetric methods, humidity and temperature measurements from which the drying kinetics can be determined (6). Although these techniques are well established in both industry and research, they are somewhat limited in that they are only able to provide macroscopic measurements and the process itself must be treated as a ‘black box’. Spatially resolved information has most often been obtained by sectioning the sample and then weighing the individual components. To gain a greater understanding of the intrinsic water migration characteristics during drying, in the first part of this work we use MRI to image the time-resolved water distribution during the drying. The data for the filter are compared with the same data acquired during drying of the related FTM.

The second part of this work uses MRI methods to measure the gas velocity in the channels of a clean and particulate loaded filter. During the operation of an automobile, PM-laden exhaust gas passes through the particulate filter and the particulate or soot is deposited inside. However, the micro- and macroscopic distribution of this soot deposition impacts the subsequent filtration behaviour, pressure drop, regeneration behaviour and ultimately the useful lifetime of the filter. Hence, a complete understanding of the filtration process is needed in order to optimise the function of particulate filters. The regeneration behaviour of particulate filters is coupled with the gas fluid dynamics. Both heat transfer and the mass transfer of oxidative species (oxygen for active regeneration and nitrogen dioxide for passive regeneration) are affected by the gas flow fields and will impact the efficacy and safety of the regeneration process. Large thermal gradients within the filter can be formed if the soot distribution is non-uniform that may damage the filter. While modelling and macroscopic measurements of these effects have been performed by many authors, there is a lack of experimental work focusing on the relationship between the filter structure, the gas transport and the perturbations of these by the loaded soot. As with drying, the range of techniques available to non-invasively measure flow in opaque filter systems is very limited, and most studies have used models (7–10). Magnetic resonance (MR) can provide spatial information on the effect of the PM on the filter operation.

MR techniques have gained prominence in chemical engineering research as they provide a non-invasive method of studying the chemistry and dynamics of a range of opaque systems (11). They are particularly useful for the study of porous media such as catalysts (12–16), construction materials (17–19) and pharmaceuticals (20, 21). Such techniques have also been used to provide information on the drying mechanism in a range of other applications such as detergent powders (22, 23), dehydration and preparation of foodstuffs (24–26), evaporation from contaminated surfaces (27–29) and fired-clay brick at elevated temperature (30). Applications of MR to drying and sorption have been covered in the review by Koptyug (31). While MR has previously been used to study drying and active component distribution in FTMs by Koptyug and coworkers (5, 15), no studies have looked at the drying process in particulate filters to date. This study investigates how the structure of the filter substrate influences the water migration behaviour during drying. MR techniques are used to characterise water migration behaviour in a wall-flow filter and FTM substrate under identical drying conditions. While MRI of liquids and their transport is common, imaging studies of nuclear magnetic resonance (NMR) active gas flows are relatively few. This is mainly due to challenges associated with the low signal-to-noise ratio (SNR) presented to the experimentalist. While hyperpolarised gases (for example, xenon-129) offer a significant boost to the SNR, they are costly and unsuitable for studying porous materials. The first application of imaging thermally polarised gases was demonstrated by Koptyug et al. (32–34) who acquired two-dimensional (2D) velocity images of hydrocarbon gases at atmospheric pressure flowing through a cylindrical pipe and alumina monoliths of different channel geometries. In the monolith studies (32), from the spatially resolved profiles of the axial component of the velocity vector on the individual channel scale, information pertaining to shear rates and entry lengths were obtained which enabled useful insights into the mass transfer between the bulk gas flow and the porous channel walls in the monolith to be made. Codd and Altobelli (35) have also shown the application of using thermally polarised gases as a probe of the structure of porous materials. Ramskill et al. (36) performed an early study using sulfur hexafluoride gas to image flow profiles inside a clean emissions control filter. They implemented compressed sensing (CS) techniques, which enable spectra and images to be reconstructed with sufficient accuracy from relatively few data points and allow a reduction in data acquisition times (37–41). Here we show a continuation of this study with more commercially relevant samples and with particulate loading.

This study highlights two ways in which MRI can be used to gain insight into automotive particulate filters. First a comparison of water migration during drying within a wall-flow particulate filter is reported and compared with the analogous process occurring within a FTM. The mechanism of water migration during drying will influence catalyst distribution in the manufactured filter and valuable insights are gained by being able to image this process in more than one dimension. Second, MRI is used to image gas fluid dynamics within a GPF both in the clean state and following two stages of soot loading. From these flow profiles, permeabilities as a function of space and time are predicted. Using MRI to gain a greater understanding of automotive particulate filters could eventually lead to improved catalyst properties, filtration efficiency and more efficient and improved manufacturing.

This section provides the reader with a brief introduction to the principles of MRI but the interested reader is directed to the texts by Callaghan (42) and Haacke (43) and review articles by Mantle and Sederman (44) and Caprihan and Fukushima (45) for further detail.

When NMR active nuclei (such as ¹H or ¹⁹F) are placed in an external magnetic field (B₀), the nuclei will precess at a characteristic frequency known as the Larmor frequency (ω₀) as given by Equation (i):

(i)

where γ is the gyromagnetic ratio which is characteristic of the nuclei under observation. Considering the system in the rotating frame at reference frequency, ω₀, spatial dependence of the precession frequency is achieved by applying a spatially-dependent magnetic field gradient (G) and can be expressed as follows (Equation (ii)):

(ii)

where ω(r) is the precession frequency at position vector r. The precessing nuclei in a volume element induce a voltage in the receiver coil and the complex NMR signal, S(t), is detected in the time domain. The time-dependent signal is then Fourier transformed into the frequency domain where it is represented by the spin-density function, ρ(r). The Fourier conjugate relationship between the time and frequency domain of the NMR signal is shown below in Equations (iii) and (iv):

(iii)

(iv)

From this, a spin-density map, i.e. an image, is obtained by taking the modulus of the complex function, ρ(r). In the case of the drying experiments the spin-density map provides a spatially-resolved measurement of the water content within the monolith sample.

Magnetic field gradients can be used to make the NMR signal sensitive to nuclei displacements in addition to position. This is achieved by first encoding the nuclei positions by applying a given magnetic field gradient (G) for time (δ), then decoding by applying the same magnetic field gradient in the opposing direction after an evolution time (Δ). Any static nuclei will be unchanged but any moving nuclei will create a phase shift in the NMR signal, ΔΦ proportional to their displacement Δr (Equation (v)):

(v)

Hence the gas velocity can be measured by consideration of the signal phase at each spatial location.

Comparison of Drying in a Particulate Filter and a Flow Through Monolith

Effect of Soot Loading on Gas Fluid Dynamics in a Gasoline Particulate Filter

For the drying experiments, the MR experiments were performed using a 2 Tesla (85 MHz for ¹H) horizontal bore magnet controlled by an AV spectrometer (Bruker Corporation, USA). An 85 mm radio frequency (rf) coil tuned to a frequency of 85.1 MHz was used for excitation and signal detection and spatial resolution was achieved with magnetic field gradients with a maximum strength of 10.7 gauss cm^–1. Three MR techniques were used as follows.

The MRI method used to measure gas velocity in the filter samples is described fully in Ramskill et al. (36). In the present study SF₆ has been chosen as the NMR active gas to be used for velocity imaging due to its favourable MR properties in comparison with other potential candidates such as the hydrocarbon gases (48). Eleven images were acquired along the length of the GPF samples, each with a slice width of 6 mm. An SF₆ gas pressure of 5.0 barg ± 0.1 barg and mass flow rate of 16 g min−¹ was used for each sample. Axial velocity profiles were acquired for the GPF samples after all three soot loading protocols. The mean volume flow for each sample agreed with the value calculated from the mass flow rate to within 8.5%. The through-wall velocities were calculated for each based on the gas mass balance. Velocity profiles inside the inlet channels were extracted from the MR velocity images through the mid-point of the channels parallel to the filter wall.

Comparison of Drying in a Particulate Filter and a Flow Through Monolith

Effect of Soot Loading on Gas Fluid Dynamics in a Gasoline Particulate Filter

A range of MR and traditional techniques, namely RH and temperature measurement, have been applied to study wall-flow filter substrates used for PM vehicle emissions control. Drying of the filter material has been compared with an FTM using 2D RARE imaging to investigate the effect of the structure on the drying kinetics. Since little deviation in the radial drying profile was seen the problem was reduced to 1D in the axial (z) direction. This allowed increased temporal resolution that revealed different drying mechanisms are associated with the wall-flow and FTM substrates, attributable to differences in the physical structures of the two autocatalyst substrates. MRI and velocimetry has also been used to investigate the effect of PM on filter channel fluid dynamics. In the gasoline system studied, any inhomogeneities in the filter wall permeability are ‘self-corrected’ by the particulate loading over the initial hour of filter operation. MRI is a method that can indirectly visualise soot location in the filter, while also providing important information on the flow characteristics and substrate properties. The application of MRI to study filters used in vehicle emissions control has provided new insight into their manufacturing and operation. The greater understanding of the drying process could ultimately result in an improved and more efficient drying process, while great understanding of their operation can lead to improved final product performance, for example higher filtration efficiency.

Nanosurfaces 2019 took place on 15th October 2019 at the Institute of Materials, Minerals & Mining (IOM3), London, UK. This event included commercialised techniques featuring plasma, surface coatings and graphene. Applications ranged from anticorrosion and waterproofing of electronics to battery materials and biomedical applications. Scalability, reproducibility, economics and sustainability were considered by many of the speakers.

The Keynote ‘Water Protection Technologies for Smartphones and Beyond’ was presented by Angeliki Elina Siokou, P2i Ltd, UK. P2i originated as a spinoff from Durham University, UK. The original product was an oil repellent coating for textiles created in 2004. They then developed a hydrophobic coating for electronics. Initial applications were in hearing aids then, around 2011, the company moved into water protection for smartphones. They are now coating half a billion smartphones per year. Customers include Samsung, Apple, Bose, Huawei and ASUS. Around 18% of smartphones are thought to be damaged by water ingress every year. P2i’s technology uses a radio frequency (rf) plasma attachment and polymerisation of surface molecules to provide a very thin hydrophobic conformal coating. This is suitable for mass production with good quality control. If a direct current (DC) plasma is used, the molecular structure is disrupted whereas pulsed DC retains coherent molecular structure and the best liquid repellence. Photoelectron spectroscopy analysis was used to analyse bond structure. The original product coating was splash proof but not completely waterproof, so a new coating was developed which is thicker (~100 nm) and has increased crosslinking. Thickness alone does not determine the level of water repellence (standards from IPX2 to IPX8), the process parameters affect porosity and film defects (1, 2). The coating was further improved by ensuring all product pieces are placed close to a plasma electrode. The coating was claimed to be durable and is tested with methods including the adhesive tape test, thermal, chemical and abrasion. It is economic for high-value items. The maximum temperature for the coating is 200ºC, determined by the maximum temperature for the items to be coated.

‘Enhancement in the Reactive Deposition Process Using Remote Plasma Spluttering’ by James Dutson, Plasma Quest Ltd, UK, presented the use of a remote plasma to generate ions whose path when accelerated is curved by magnets and focussed onto the sputter target. Key advantages of this technology are: high target usage, control over energy of sputter ions, low temperature at the substrate. They have had good success with aluminium doped zinc oxide as an indium tin oxide (ITO) replacement. Substrate heating is not required to control the phase or crystallinity of deposited material as this can be done via sputter ions energy control. Deposition rates are typically 1–6 μm h^–1, and 1 μm h^–1 for the ZnO:Al materials. Plasma Quest is working with CSEM, Neuchâtel, Switzerland, on photovoltaics and transistors. The process can be carried out at ambient temperature so is suitable for coating plastics or sensitive substrates. Reactive deposition is possible, i.e. to incorporate an oxide or nitride from a metal target. The main driver is replacement of ITO for transparent conductive coatings and there is a Horizon 2020 European Union (EU) project: INREP, Towards Indium Free TCOs. The company is currently scaling up to large surface areas ≥50 cm². The process does not require a clean room.

‘Molecular Plasma Technology’ was presented by Britta Kleinsorge, Molecular Plasma Group, Luxembourg. This company offers atmospheric plasma (50 kHz) processes for surface modification. It is lower temperature compared to corona like arc plasma processes. Organic precursors could be functionalised and adhere to a surface at low temperature. Some interesting work on biological non-specific binding and antibody immobilisation was carried out with KU Leuven, Belgium, in which the process time was reduced from 24–72 h to 10 s. Superhydrophobic barriers have been developed for anticorrosion applications, waterproofing and adhesion. They presented a case study of a process they developed for a German automotive original equipment manufacturer that could be automated with reduced chemical usage.

‘A Radically New Antimicrobial Nanosurface Formed by Plasma Processing’ was presented by Alistair Kean, NikaWorks Ltd, Watlington, UK. There is a need for more environmentally friendly coatings, for example crisp packets which currently use 30–40 nm Al on plastic film. Tungsten carbide (WC) physical vapour deposition (PVD) coatings for surgical scissors need to be durable for decontamination cycles. The BeBionic prosthetic hand includes a titanium alloy coating on one surface. Three dimensional (3D) nanomaterials (metamaterials) are inspired by nature and nanosurfaces can have surface areas of the order of ~1000 m² g^–1. SOLAMON was a 7th Framework Programme for ‘large’ NP 20 nm, 30 nm or 40 nm scale. Gencoa Ltd is a PVD company in Liverpool, UK, which is developing antimicrobial coatings with NikaWorks.

‘Virtual Cathode Deposition’ was presented by Dmitry Yarmolich, Plasma App Ltd, UK. Plasma App is based at Harwell Oxford Science and Innovation Campus, UK. The speaker described a thin film battery project with the University of Cambridge, UK, which can deposit carbon as a combination of graphite and graphene immediately followed by the lithium cobalt oxide (LCO) cathode. It is a platform technology that can deposit a thick or thin film of almost anything from nanometre scale to 50 μm. Superior adhesion and solvent-free scalability were claimed. The speaker claimed that the technique can use two source virtual cathode deposition (VCD) to design materials in minutes, not hours.

‘Manufacture and Applications of Nanoclusters’ was presented by Richard Palmer, Swansea University, UK. He set up Grove Nanomaterials to commercialise matrix assembly cluster source (MACS) technology (3) for the solvent-free synthesis of nanoparticles (NP) for catalysis, sensors and electrodes. Atomic nanoclusters can be created to mimic enzymes with formation of grooves and pockets. Size selected cluster beam deposition (CBD) of gold or molybdenum sulfide, which can be nickel or cobalt doped to replace platinum in water splitting, was carried out. The present plan is to scale up the research system to be able to produce 1 g h^–1 and the technology has the potential to be scaled to mass production (tonne scale).

‘Silica Nanoparticles for Super-Hydrophobic Ice-Repellant Coatings’ was presented by Simon Haas, Promethean Particles Ltd, UK. Promethean Particles Ltd is a spinout from the University of Nottingham, UK. Using a continuous hydrothermal process, copper, silver, ZnO, barium titanate and silica NP can be produced as dispersion in liquid with no dry powders for safer handling. The liquids can be used in further processes and agglomeration is prevented. The dispersions are highly stable and the process is scalable. A production plant has been built which can produce 1000 tonnes per year based on dry weight equivalent. Conductive inks and printed electronics are currently being developed with partners. The speaker presented an Innovate UK funded programme, ICEMART, looking at SiO₂ NP to prevent icing of plane wings. The Welding Institute, Great Abington, UK, was part of this project and provided a process to functionalise the SiO₂ particles to render them hydrophobic. This technology is currently being commercialised via Sharc^® Matter, an Opus Materials Technology Company, UK.

‘Graphene Enhanced Products’ was presented by Thanuja Galhena, Versarien plc, UK. Versarien acquired Cambridge Graphene Ltd, UK, in 2017 and supplies proprietary grades of two-dimensional materials. There are two main products: Nanene^TM (graphene) and Hexotene^TM (boron nitride). Both can be manufactured at a 3 tonnes per annum scale. The company has certification from the Graphene Council, New Bern, USA and the National Physical Laboratory (NPL), Teddington, UK, European Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) registration in the EU and is applying for similar certifications in China. Potential applications are composites, fire retardants, barrier coatings, energy storage, filtration and heat dissipation. A fairly unusual product application was a railway arch supporting sensors, claiming that lighter weight reduces transport, installation time and personnel costs. Graphene based inks known as GRAPHINKS^® have been developed (4). Another example is a graphene coated diaphragm for some earphones – apparently bass is clearer. FLEXIBAT is an Innovate UK funded project. The company is working on flexible batteries with Zinergy UK Ltd, Cambridge, UK, making graphene coated electrodes for corrosion resistance. There are also supercapacitors using graphene plus metal oxide NP. The speaker noted that the cost of graphene has reduced considerably, making it cost effective for many applications.

‘Enhanced Surface-Analysis Capability’ was presented by Mike Petty, Loughborough Surface Analysis Ltd, UK. Petty focussed on the new secondary ion mass spectrometry (SIMS) system (Cameca, France) which can do quite sophisticated analysis mainly on semiconductor materials. Data cubes are formed so one can go back and analyse specific coordinates within a scanned cube (Figure 1). This technique is sensitive to isotope identification.

Fig. 1.

‘Photocatalytic Antimicrobial Surfaces’ by Jeremy Ramsden, University of Buckingham, UK. Ramsden discussed the antimicrobial application of photocatalytic powders coated via spray from titanium dioxide powders in a water solution. He described the problem of hospital acquired infections, especially resistant infections such as methicillin-resistant Staphylococcus aureus (MRSA), and presented studies of deaths and disability-adjusted life year (DALY) statistics in Europe, USA and elsewhere dating from 1930s to the present. There are many vectors of microbial transfer in and around hospital environments, these are surprisingly little studied and there is much debate about the relative importance of air, walls, floors, shoes or wheels and ceilings in transmission of pathogens. In particular there are few or no controlled studies on hospital cleaning efficacy. Hand hygiene compliance seems to have peaked at ~40%. He studies non-sacrificial catalytic coatings, i.e. TiO₂ which can kill microbes even under ambient lighting with continuous effectiveness. Estimated 10 colony forming units (CFU) m^–2 s^–1 of bacteria arriving and 6000 s^–1 oxidising equivalents forming. He carried out a trial in which a coating was applied after a hospital deep clean on the high-touch surfaces, for example bed rails, table and tubing. The surfaces were tested using Agar pads (selective for MRSA and general). The coating used a commercial sol of TiO₂ NP in a sol-gel process to form a hard coating within 20–30 mins. The coating is invisible (1 μm) and can be applied to mirrors and glass. Sampling was carried out for three months, the coating was durable in this timeframe. Evidence suggested the coating resulted in lower microbial growth (Figure 2) (5). Microbial resistance is unlikely as the microbial cell has no mechanism for defence against peroxide. The half-life of bacteria was ~6 h.

Fig. 2.

Martin Kemp, Chairman of the IOM3 Nanomaterials Conference organising committee, remarked how many of the presentations featured plasma techniques and was excited by the many examples of commercialised techniques. There will be another meeting in early 2020.