5.8. Volatilization and encapsulation of metal particles

The most important and relevant ageing-induced changes in the catalyst are presented in sections 5.1 to 5.7. Additionally, the deactivation of the Pd/Rh catalysts may be due to some other reasons, e.g. the encapsulation of metal particles in the sintered washcoat, or the volatilization of active metals at high ageing temperatures, which will be discussed in this section. The effect of these two as possible deactivation mechanisms is, however, of secondary importance compared to the ageing-induced collapse in surface area, to the sintering of active metals and to ageing-induced solid-solid phase transitions in the catalyst’s bulk washcoat.

According to the thermodynamics, when heating the catalyst above a critical temperature, the vapour pressure of metals or metal oxides rises so that significant amounts of material can be transported either to react with the substrate, or into the gas phase where they are lost in the effluent gas streams. This property of volatilization limits the use of active metals as catalysts, and potentially represents the limiting trade-off between high activity and a long, active lifetime independent of the design and structure of the catalyst. (McCarty et al. 1999)

First, the volatilization of active metals as a possible deactivation mechanism is considered. Active metals, palladium and rhodium, were added onto the different washcoats, as stated previously. In the fresh catalyst, palladium was not observed on the catalyst’s surface, as shown by XPS measurements. However, after high temperature ageings, palladium is found to be present on the catalyst’s surface, and as the ageing temperature increased, the amount of palladium on the surface of the catalyst increased. This is clearly observed in the XPS and XRD measurements. Furthermore, according to the XPS results, the amount of Pd on the surface of the catalyst increased in the reductive gas atmosphere as the ageing temperature increased (see Fig. 41). However, based on XRD results, this kind of conclusion could not be drawn. (Lassi et al. 2002b, Lassi et al. 2002c)

There are two possible interpretations for the observation of Pd on the surface of the catalyst after the ageings. First, the surface of the catalyst may be eroded during the ageing, revealing the Pd metal particles present in the lower areas of the washcoat. However, this does not explain why palladium is concentrated on the catalyst’s surface after the reductive ageing, but not after the oxidative ageing. Second, at high ageing temperatures, palladium may become mobile and may be transported to the surface through the washcoat and volatilized into the surrounding gas phase. Therefore, it may be possible that the total amount of palladium in the catalyst diminishes during ageing (Lassi et al. 2002b). The transport of Pd to the surface over a distance of couple of tens of microns is quite likely to take place at least partially via the gas phase. The vapour pressure of Pd at 1000°C is ~10^-6 Torr and at 1100°C ~10^-5 Torr (Margrave 1967), whereas the vapour pressure of Rh is several orders of magnitude lower. Furthermore, as indicated earlier in Chapter 3 (Figures 9A and 9B), according to thermodynamic equilibrium calculations, Pd is more volatile than Rh in a 5 vol-% O₂/N₂ atmosphere. In addition, the vapour pressure of metallic palladium is higher than that of PdO, whereas in the case of Rh the behaviour is the reverse. This explains why the amount of Rh remained unchanged during the ageing treatments. On the other hand, it is reasonable to assume that at high ageing temperatures, Pd may be lost via volatilization into the gas phase, as will be discussed in the following paragraphs. (Lassi et al. 2002b)

One role of the wet analysis (see section 4.3.8) was to provide information on the significance of the volatilization of active metals as a possible deactivation mechanism. According to the ICP-AES results, the Rh content in the washcoat of the fresh catalyst was close to rhodium loading. After the thermal and hydrothermal ageings in the temperature range of 800°–1200°C, equal amounts of rhodium within the measurement accuracy were observed in the catalyst, indicating that Rh has not been volatilized during the ageing treatments. Similarly, the Rh content in the washcoat after engine and vehicle ageings was close to that of the fresh catalyst. Therefore, it can be said that Rh metal particles are sintered in the washcoat, as shown earlier, but Rh is not lost as a result of its volatilization into the gas phase. This is also consistent with the results of other characterization methods where no Rh loss was observed as a result of ageing.

The effect of ageing on Pd metal particles is rather different. The wet analysis confirmed, namely, that the amount of palladium in the aged catalysts differed from that of the fresh catalyst. It can, therefore, be assumed that high temperature ageing-induced changes in the washcoat decreased the amount of palladium in the catalyst. According to the ICP-AES analysis, the amount of Pd in the washcoat of the fresh catalyst was close to palladium loading. The Pd content in the aged catalysts was smaller than that of the fresh catalyst, but no systematically increased loss in Pd content was observed as a function of ageing temperature. The loss of Pd after the ageings was at a maximum of 15%. The only exception was the air/1200°C/3h-aged catalyst in which the loss of Pd was significantly larger compared to other aged catalysts. It was also rather difficult to discover differences in the Pd content of the catalysts between the reductive and oxidative ageings as a function of ageing temperature and therefore, no support for the earlier XPS observations is obtained. Furthermore, after engine bench and vehicle ageings, no significant Pd loss was observed. Therefore, it can be concluded that Pd is most likely lost via volatilization into the gas phase as a result of ageing under extreme conditions, as is also expected by thermodynamics, but under the vehicle operating conditions, this is not a significant mechanism of deactivation. It is also rather difficult to draw any reliable conclusions if volatilization of palladium is more pronounced in the reductive ageing atmosphere (as assumed earlier based on XPS results) or not. Therefore, further studies of volatilization; for example, with simplified model catalysts, are needed, and the analytical methods used to determine Pd content in the complex matrix should also be improved.

The vapour phase transport of Pd was also recently discussed by Graham et al. (1999), where they considered the role of vapour phase transport within the pore structure of the ceria-zirconia being significant relative to transport via surface diffusion. High temperature ageing of ceria-zirconia-supported Pd may result in deep encapsulation of sintered Pd metal particles, affecting the catalytic activity. This phenomenon is also observed on Rh catalysts but not on Pt catalysts. The encapsulated particles cannot participate in catalysis since they are inaccessible to gas-phase molecules. The encapsulation and incorporation of metal particles in the sintered washcoat after the ageings were also studied by SEM-EDS. Ageing-induced encapsulation of metal particles was observed in a SEM image as an increase in the metal particle size. After the ageing treatments, metal particles are relatively large and probably even more susceptible to encapsulation because of their limited mobility. In a SEM-EDS analysis, palladium was also observed clearly to be concentrated to ceria-zirconia mixed oxide (e.g. in the fresh catalyst) and, therefore, this was considered as a preparation-induced phenomenon rather than as an ageing-induced phenomenon.

In order to understand the microstructural evolution of catalysts upon ageing, especially the incorporation and encapsulation of Rh and Pd metal particles, as presented by Graham et al. (1999) and Weng-Sieh et al. (1997), more sophisticated characterization methods would be needed. For instance, transmission electron microscopy (TEM) would provide detailed information on metal particle size and shape of active metals after the ageings. However, in this thesis it was not utilized as a characterization tool because the use of TEM technique requires a specific sample preparation. Usually, TEM samples are prepared by scraping the washcoat of the catalyst and crushing the sample to a very fine and thin layer in order to allow the electron beams to penetrate through the sample. This sample preparation destroys the macrostructure of the catalyst and, therefore, in the author’s opinion, does not give a representative view of ageing-induced changes in the catalyst. Currently, methods for TEM sample preparation, which do not destroy the macrostructure of the catalyst, are under development. (Polvinen 2002)