| Deactivation Correlations of Pd/Rh Three-way Catalysts Designed for Euro IV Emission Limits: Effect of Ageing Atmosphere, Temperature and Time | ||
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Throughout the course of this work and the writing of this thesis, three principal goals have been dominant. These were, and still are: (1) The knowledge of the deactivation mechanisms of Pd/Rh monoliths under different ageing conditions, (2) The deactivation correlation between laboratory scale ageing and engine bench and vehicle ageings, and (3) The utilization and the evaluation of the results of the first two main goals and the critical evaluation of ageing cycles. The attainment of these goals enables, at least partly, the development of a new laboratory scale ageing cycle, which corresponds to the ageing-induced changes in the catalyst during vehicle operation.
As presented in this thesis, the deactivation of Pd/Rh monoliths after the ageings is caused by several mechanisms, such as the sintering of active metals, the collapse in pore structure and the solid-solid phase transformations in the bulk washcoat. As a result of high temperature ageing, Rh particles in the washcoat were sintered and the active metal surface area of catalysts was collapsed. The surface area of catalysts decreased and pore sizes increased as a result of ageing. These phenomena were clearly enhanced in the oxidative gas phase as well as in the presence of water vapour. High temperature ageings also induced solid-solid phase transformations in the bulk washcoat, which were observed as a formation of CeAlO3 and LaAlO3 in the washcoat. The solid-solid phase transformations prevented the formation of low surface area α-Al2O3, which was mostly responsible for the surface area loss. This explained why the specific surface areas of catalysts remained higher in the presence of hydrogen. Furthermore, ageings induced the decomposition of Ce- and Zr- rich CeXZr1-XO2 mixed oxides, which led to a formation of new mixed oxides with molar compositions of 30 wt-% < Ce < 70 wt-%. After the vehicle ageing, the front zone of the catalyst was also poisoned, but poisoning in this case contributed only to one fifth of the total deactivation, as shown in the activity measurements.
In this thesis, an approach has been introduced for the deactivation correlation between the laboratory scale ageings and the engine and vehicle ageings. As indicated, the laboratory scale ageing in the reductive ageing atmosphere corresponds best to the engine bench and vehicle ageings. This is reasonable since the engine ageing cycle consists of both stoichiometric and rich air-to-fuel ratios. During vehicle operation, the gas phase composition fluctuates between lean and rich conditions. A deactivation correlation after oxidative (air) ageing, as commonly used in a catalyst’s laboratory scale testing, is also possible to discover on the basis of the activity tests and BET measurements. However, this correlation does not correspond to the ageing-induced chemical and physical changes in the catalyst that were observed after engine bench and vehicle ageings. Therefore, the commonly-used laboratory scale ageing in air is not a sufficiently reliable ageing procedure in the evaluation of the deactivation of Pd/Rh monoliths, as substantiated in this thesis.
The conclusions that can be drawn from the results of the current work can be presented as the following theses in support of the empirical evidence:
The deactivation of a Pd/Rh monolith is a combination of several ageing phenomena.
Temperature, gas phase composition and exposure time are essential variables in the deactivation of a Pd/Rh three-way catalyst.
The accelerated laboratory scale ageing in air does not correspond to ageing-induced changes in the catalyst under vehicle operation.
A deactivation correlation between the laboratory scale ageing and the engine bench ageing can be presented as a function of ageing temperature and atmosphere and time. When considering the deactivation correlation between the laboratory scale ageing and the vehicle ageing, poisoning has a notable role, which should also be taken into account.
In addition to the scientific contribution, the results of the current work can be utilized in the further development of the laboratory scale ageing cycles, which correspond to real ageing conditions in an engine bench or during vehicle operation. This further enables fast testing procedures in the research and development stage of catalyst’s manufacturing and it would also assist in making significant cost reductions in catalyst testing.