1.1. Background

Recently, environmental concern and demand for a catalyst’s high performance have increased the research activity focused on the operation of a three-way catalyst (TWC) at the end of its lifetime. Currently-used three-way catalysts are exposed to high operation temperatures due to the use of closed-loop coupled catalysts near the engine. Three-way catalysts have a lifetime requirement of at least 100 000 kilometres or five years (Directive 98/69/EC). Catalytic materials used in TWC applications have also changed, and the new materials have to be thermally stable under the fluctuating exhaust gas conditions. The new emission limits (Euro IV level for 2005) are becoming tighter and new restrictions are set for the fuel consumption and quality, e.g. low sulfur content of gasoline (Directive 98/70/EC). Further demands are being placed on discovering other techniques to minimise emissions during cold start to obtain a fast light-off of a catalyst. In addition, there are new requirements regarding the catalyst’s monitoring techniques, e.g. On-Board Diagnostics (OBD), which is used to monitor the degree of the catalyst’s deterioration. All these requirements correlate to the catalyst’s deactivation phenomenon. The attainment of these requirements demands highly active and thermally stable catalysts as well as an optimised design of the exhaust system and engine control. (Heck & Farrauto 1997, Farrauto & Heck 2000, Shelef & McCabe 2000, Bosteels & Searles 2002)

The requirement for high thermal stability and activity, which the current three-way catalysts have to fulfill, is one of the most crucial demands for a successful commercial application. Therefore, the understanding of deactivation phenomena and deactivation correlations is an important issue in the design and preparation of a catalytic system. Deactivation of a three-way catalyst can be due to many ageing phenomena, such as thermal, chemical and/or mechanical. Thermal or thermo-chemical degradation is reported to be one of the main causes for the deactivation of three-way catalysts (Ihara et al. 1987, Funabiki & Yamada 1988, Härkönen et al. 1991, Usmen et al. 1992, Jobson et al. 1993). High temperature and temperature gradients, the presence of poisons and other impurities, as well as the fluctuating gas phase composition and flow rate in the catalytic converter increase the possibility of deactivation. Furthermore, some other factors, such as the chemical composition of the catalyst and pressure changes in the converter affect the catalytic activity. Thus, it is essential to study the role of different ageing factors in order to understand the relevant physical and chemical mechanisms of deactivation and deactivation correlations of a three-way catalyst.

As indicated in the previous paragraphs, deactivation of a three-way catalyst is a complex phenomenon and it extensively affects the catalyst’s preparation and use. When preparing catalysts for the purification of exhaust gases of gasoline engines, catalyst manufacturers have to guarantee the required lifetime for the catalyst. It is, therefore, essential to know which are the most important factors during the deactivation. Furthermore, it is also important to understand these factors’s effect i.e., what are the ageing-induced chemical and physical changes in the catalysts in order to arrive at the deactivation correlations. It is reasonable to assume that currently-used laboratory scale ageing cycles are not consistent with the ageing phenomena in an engine bench or on-road. Therefore, there is a need for a novel laboratory scale ageing cycle, which better corresponds to the physical and chemical changes where the catalyst is exposed during real driving conditions.