6.1. Integration of results

Based on the results presented in this thesis, the deactivation of Pd/Rh monoliths was caused by several mechanisms. The aged catalysts were clearly deactivated, which was observed in the activity measurements as significantly lower activities (higher light-off temperatures of CO and NO) compared to the fresh catalyst. However, all the aged catalysts were still active in the removal of major pollutants, which was observed as high final conversions (conversion values determined at 400°C), which were, in general, over 80% for CO and NO. Furthermore, the engine-aged and vehicle-aged catalyst systems reached the Euro IV emission level limits and, therefore, converted NO_X, HC and CO well in a European test driving cycle, as reported in Härkönen et al. (2001).

In Fig. 51, a short summary of ageing-induced phenomena, as found in this thesis, is presented. The main reasons for the deactivation of aged catalysts were the sintering of active metal particles in the washcoat and the collapse in the surface area and pore structure of the catalyst. The results of H₂ chemisorption gave information on the ageing- induced loss in the active metal surface area, which led to the increased metal particle size. The loss of metal surface area was associated with the sintering of active metals in the washcoat, in particular the Rh metal particles in the washcoat which were sintered at a high ageing temperature, and which was verified by XPS measurements. NO-TPD results showed the ageing-induced loss in the amount of active surface sites of the catalysts, which was consistent with chemisorption results. Furthermore, based on XPS and XRD results, evidence of the possible active metal loss via volatilization into the gas phase was obtained. In particular, this was one mechanism of deactivation for Pd metal particles which became mobile as the ageing temperature increased, diffused through the washcoat onto the surface and volatilized into the surrounding gas phase under extreme ageing conditions.

Ageing also induced structural and chemical changes in the bulk washcoat, as seen in Fig. 51. These changes included the collapse in pore structure and the loss in surface area, the increased pore size and the crystal growth, the sintering of the oxides in the washcoat, and the ageing-induced solid-solid phase transformations in the bulk washcoat. These phenomena were not separate reasons for deactivation, but closely associated with each other. The collapse in the pore structure and the loss in surface area were associated with the sintering of the high surface area metal oxides (γ -Al₂O₃, CeO₂) in the bulk washcoat. It should also be noted that these pure oxides formed a new crystalline phase, CeAlO₃, as a result of ageing. The formation of CeAlO₃ was more pronounced after the reductive and inert ageings and it prevented the phase transformation from γ -Al₂O₃ to low-surface area α-Al₂O₃. This also explained why the surface areas remained higher after the reductive and inert ageings compared to the oxidative ageing. Furthermore, Ce_XZr_1-XO₂ mixed oxides underwent the ageing induced decomposition, which led to the formation of new mixed oxides with the molar compositions 30 mol-%< Ce < 70 mol-%. In addition to ageing-induced changes in the active phase and bulk material, the vehicle-aged catalyst was also deactivated by poisoning. The accumulation of poisons was observed in a SEM-EDS analysis as a thin, contaminated overlayer, which was mostly composed of calcium, phosphorus and oxygen. All the deactivation phenomena led to a loss in the catalytic activity, as mentioned earlier.

Ageing has induced several changes in the catalyst, and this has been shown in this thesis. These changes were critical to the catalyst’s performance, since the changes are typically irreversible and the original activity of the catalyst is not restored after the ageings. As indicated, the reasons of deactivation of Pd/Rh monoliths found in this thesis were commonly known. Therefore, any new insight of deactivation in this respect is not provided. Instead, a new insight of deactivation correlation can be presented in this thesis. Although the ageing-induced phenomena in the catalyst are quite evident, it is rather difficult to clearly discriminate between the impact of some separate deactivation mechanisms.

Ageing temperature, atmosphere and time were all important variables in the deactivation of the catalysts. Deactivation is always a function of time, but in this study the role of ageing atmosphere and high ageing temperature was clearly pronounced compared to ageing time. Ageing temperature was an important variable since high ageing temperatures favour the sintering of active metals and washcoat materials that can be regarded as significant deactivation mechanisms in this case. Temperature also affected the catalytic activity and the ageing-induced structural changes observed in the bulk washcoat. Catalysts treated at high ageing temperatures were more deactivated than those aged at lower ageing temperatures, as expected. This was observed in the activity measurements, where the light-off temperatures increased systematically as a function of ageing temperature.

Based on NO-TPD results, ageing temperature had an effect on the desorption of NO. Thermal ageing at temperatures T>800°C at least partly destroyed the higher temperature desorption peak of NO. This peak originated from NO-active metal and NO-Al₂O₃ interactions and so this supports the ageing-induced sintering of active metals in the washcoat. Ageing temperature also affected the crystalline structure of catalysts. In the reductive and oxidative atmospheres, the crystal growth started after ageings at around 900°C. Ageing temperature was an important factor on the catalyst’s activity and stability. The increase in CO and NO light-off temperatures (decrease in catalytic activity) was proportional to ageing temperature. The activities of catalysts decreased as a function of ageing temperature. This was related to the loss of the active metal area.

In addition to ageing temperature, gas phase composition also affected the catalytic activity and structural properties. Ageings decreased the catalyst specific surface areas and pore volumes which correlated well with the changes observed by other characterization methods. According to BET results, ageing in the reductive atmosphere was not so strong compared to ageing in air. Thus pore sizes were clearly larger in the oxidative atmosphere. Furthermore, in the presence of water vapour, sintering phenomena were accelerated. Catalytic activities also remained high if ageing was carried out in the reductive or inert atmospheres, rather than in the presence of the oxidative gas phase. Ageing atmosphere either accelerated or inhibited phase transitions in the bulk material, as observed by XRD measurements, and it had an influence on the chemical states of active metals, as shown by XPS.