3.1. Overview

Catalyst deactivation, the loss of catalytic activity and/or selectivity over time, is of crucial importance in three-way catalysis where catalytic materials are exposed to high temperatures under fluctuating conditions. In the literature, there are several definitions for deactivation, but none of them, as yet found by the author, is exclusively broad enough as a definition of the deactivation of a three-way catalyst. The precise definition of deactivation is rather difficult to present, because deactivation is a complex phenomenon, as will be indicated in this Chapter, and the type of deactivation changes depending on the application area. In this thesis, catalyst deactivation is defined as a phenomenon in which the structure and state of the catalyst change, leading to the loss of active sites on the catalyst’s surface and thus causing a decrease in the catalyst’s performance. Furthermore, another important term in this thesis is ageing, which is commonly used to describe high temperature-induced deactivation.

Catalyst deactivation is a result of a number of unwanted chemical and physical changes. The causes of deactivation are classically divided to three categories: chemical, thermal and mechanical (Butt & Petersen 1988, Bartholomew 2001). In this thesis, mechanisms of thermal and chemical deactivation are mostly considered. Mechanical deactivation as a result of physical breakage, attrition or crushing is also an important deactivation phenomenon. However, for the current catalytic converters, deactivation during the normal vehicle operation is typically a result of chemical and thermal mechanisms, rather than fouling and mechanical factors (Koltsakis & Stamatelos 1997), one reason why mechanical deactivation has been excluded from this thesis.

Recently, several papers have reviewed the causes of deactivation (Forzatti & Lietti 1999, Bartholomew 2001, Moulijn et al. 2001). Deactivation of a catalyst is usually an inevitable and slow phenomenon. Despite its inevitable nature, some immediate consequences of deactivation may at least be partly avoided, or even reversed. Deactivation of a three-way catalyst can result from various processes (deactivation mechanisms) as summarized in Table 4. The three major categories of deactivation mechanisms are sintering, poisoning, and coke formation or fouling (Forzatti & Lietti 1999). They may occur separately or in combination, but the net effect is always the removal of active sites from the catalytic surface. Some other deactivation mechanisms, such as pore blockage, volatilization of active component, destruction of the active surface and incorporation of the active component into the washcoat in an inactive form can also cause decline in the catalyst’s activity. (Butt & Petersen 1988, Bartholomew 2001)

Deactivation of a three-way catalyst is a complex phenomenon since the purification performance of a three-way catalytic converter is affected by many factors, such as changes in the exhaust gas velocity and composition, temperature, precious metal loading and the catalyst’s age. Three-way catalysts are designed to withstand momentarily high operation temperatures, but the long-lasting exposure to high thermal loading increases the risk of thermal deactivation. (Beck et al. 1997a, Beck et al. 1997b, Koltsakis & Stamatelos 1997)

Thermal, or thermo-chemical degradation, is probably the main cause for the deactivation of automotive exhaust gas catalysts. Three-way catalysts are known to loose their activity, especially under oxidizing conditions at temperatures higher than 900°C (Ihara et al. 1987, Härkönen et al. 1991). Exposure to high operation temperatures enhances the reduction of the alumina surface area and sintering of the precious metals, resulting in a loss of effective catalytic area. The thermal degradation of three-way catalysts is caused not only by high temperature but also by sudden temperature changes in the catalytic converter. Catalysts may also be poisoned in the presence of some pollutants, such as sulfur or phosphorus. These components contaminate the washcoat and precious metals and reduce the active catalytic area by blocking the active sites. On the other hand, deactivation by fouling or coke formation is not regarded as a major problem in the current high-temperature catalytic purification systems. (Koltsakis & Stamatelos 1997, Sideris 1998)