3.3. Accelerated catalyst ageing

Deactivation of a three-way catalyst is typically a very slow phenomenon, a reason why a large number of catalyst ageing cycles have been developed for the relative fast testing of catalyst thermal stability. A number of non-vehicle test methods have been developed to achieve the accelerated catalyst ageing under laboratory-controlled conditions. These ageing methods typically involve the use of test engines, atmosphere controlled ageing furnaces and pulse-flame reactors. The ageing procedures can be divided into the following three categories: vehicle ageing cycles, engine bench ageing and laboratory ageing in furnace (Summers et al. 1990, Hannington 1991, Bartley et al. 1993, Kumar et al. 1994, Koltsakis & Stamatelos 1997). These ageing procedures are briefly described in the next paragraphs.

A vehicle ageing cycle represents, in the best possible way, the real driving conditions to which a given vehicle could be subjected to during a required catalyst lifetime. Automotive manufacturers use vehicle ageing test procedures to test the stability and durability of catalytic converters under real driving conditions. However, the use of such test procedures is normally limited due to the high operation costs. (Koltsakis & Stamatelos 1997)

The aim of engine bench ageing is to test the thermal and chemical stability of a catalyst. An engine bench cycle contains the subjection of the catalyst to thermal loading, high temperature oxidation and the presence of catalyst poisons. As mentioned earlier, the major damage to a catalyst results from the exposure to oxidizing atmosphere at high temperatures (Koltsakis & Stamatelos 1997). Such conditions are normally encountered during vehicle braking with fuel cut after running at moderate or high power. These conditions can be simulated in an engine bench. All the catalyst manufacturers have their own ageing cycles for engine ageing that can include the measurements of activity, oxygen storage capacity and emissions, as well as redox scanning with varying air-to fuel ratios. (Kumar et al. 1994, Heck & Farrauto 1997, Koltsakis & Stamatelos 1997)

The high temperature laboratory ageing procedures simulate the oxidation and the sintering of washcoat and precious metals in artificial atmospheres, temperatures and flow conditions. In order to simulate severe ageing conditions within the minimum time, the laboratory scale furnace ageing is usually performed at very high temperatures and under gas atmospheres (typically in air) that are far removed from real driving conditions. Laboratory scale ageing procedures may also involve cycled feedstreams or pulses. (Koltsakis & Stamatelos 1997)