A Review of Measurement Techniques of Mechanical Properties of the Catalyst Layer in Catalytic Converters

Four methods are classified in Group 1. All of the methods in this group use mass loss of the sample as a way to represent the strength of the catalyst layer. The mass loss is triggered by an external effect applied to the sample, for instance an ultrasonic vibration in the ultrasonic vibration bath or kinetic energy in the drop test. Two problems are associated with the measurement techniques in Group 1.

The first problem is that the strength results obtained cannot be compared between different authors, although many authors have normalised their mass loss results and therefore report the strength results using percentage of mass loss (Equation (vi)).

This problem is represented in Figure 8 where a wide range of weight loss values reported for a strong catalyst layer in different publications can be seen. The strongest catalyst layer in one work (24) is said to have a weight loss of only 2 wt% however work elsewhere (21) quotes the strongest layer having a weight loss of 45 wt%. The varying maximum sliding distance of the catalyst layer shown in Table II also indicates the same problem. The maximum sliding distance refers to the sliding distance in the abrasive test before the wear rate starts to rise. According to Table II, the strongest catalyst layer can have a maximum sliding distance of either 120 m or 12 m, which are widely different from each other.

Fig. 8.

The first problem is due to the fact that these results are not independent of the testing environment. For example, in the case of using an ultrasonic vibration bath test to quantify the strength of the catalyst layer, if the experimental conditions (power, frequency, medium, duration) of the ultrasonic bath used are different, one can hardly make a reliable comparison in the layer strength obtained by different studies. As can be seen in Table III, the experimental conditions from research that applied the ultrasonic vibration test are different in most of the key testing parameters and in certain cases (8, 19, 20, 21) some of these parameters are not even given. One could also notice that a much higher frequency of ultrasound in Jiang et al. (40 kHz) (24) compared with Adegbite et al. (0.06 kHz) (15) did not result in a higher weight loss; this could be because of a lower power and shorter exposure time in Adegbite et al. (15). However it is unknown at the moment how much decrease in the frequency would correlate to the lower power and exposure duration employed in the study by Jiang et al. (24). Without the same testing environment, it would be hard to compare the strength of a catalyst layer across different studies and therefore conclude on the standards of a strong catalyst layer.

As can be seen in Table II, Table IV and Table V, similar problems as in the case of the ultrasonic vibration test exist in the thermal shock test, the simulated environment test and the abrasive test; the experimental conditions are different in most key testing conditions such as the hot and cold temperatures in the thermal shock test and the normal compression in the abrasive test. The difference in experimental conditions makes it difficult for different researchers to compare their results and agree on what is regarded as a strong catalyst layer.

In the case of simulated environment, it could be seen that the three publications (6, 14, 25) which applied this testing method employed the same testing environment. However, from the arguments that are presented for the other testing methods in Group 1, it could be expected that further publications employing a simulated environment would suffer from the difficulty of comparison between different authors if they do not apply the exact same testing conditions. As for the drop test, given the fact that there is currently only one publication which applied this test, it would be difficult to make further comments on the results of this test.

Due to the fact that the origin of the strength of the catalyst layer is bonding between particles in the catalyst layer (cohesive) and bonding between these particles and substrate (adhesive), any indirect measurement of these bonding strengths can be affected by external factors as seen above.

A second problem for Group 1 methods is that the design of the method does not contain a way to control the failure pattern of a catalyst layer. The meaning of this statement is that a catalyst layer sample under test could fail either by the cohesive or the adhesive mode (as seen in Figure 9), depending on the weakest point of bonding.

Fig. 9.

From the operation principle of the ultrasonic test as described in Figure 4, it could be expected that the catalyst layer could fail both in the cohesive and the adhesive mode. The solution medium that is used in the ultrasonic vibration test could either remove an upper portion of the catalyst layer or penetrate to the interface between the catalyst layer and the sample and detach the catalyst layer at this interface. Similar arguments could be applied for the rest of the methods in Group 1. As can be seen in the operation principles of these methods shown earlier, there is not a mechanism designed in the method to control the failure pattern. The external force aimed to test the strength of the catalyst layer could destruct the catalyst layer in any direction, therefore a mixed result between the cohesive strength and the adhesive strength may be obtained.

However, as seen in the introduction, a catalyst layer in operation could fail in both the cohesive and the adhesive mode, suggesting that both the cohesive and the adhesive strength are important for the durability of a catalyst layer; it is essential not to mix the cohesive and the adhesive strength in any strength measurement of a catalyst layer.