4.5.2 Thermal effects
Exhaust catalysts usually operate in the temperature range 150–600 °C, but they can experience temperatures of up to 1000 °C. The conditions that can produce such high temperatures include repeated misfire (resulting in the oxidation of large amounts of unburned fuel over the catalyst) and high driving speeds. In addition, very high temperatures may be experienced if the catalyst is ‘close coupled’ to the engine, which is one of the possible solutions to the cold-start problem. Although commercial catalysts are designed to withstand occasional high-temperature operation prolonged and repeated exposure to temperatures in excess of 800 °C, especially under oxidising conditions, have a number of serious effects.
High temperatures may affect all the components of the catalyst. The noble metal particles may sinter (recall Figure 21 in section 4.6), resulting in a decrease in the fraction of the metal available for catalytic reactions. Such sintering particularly affects the low-temperature activity of the catalyst. It can be countered to some extent by the addition of ceria as a structural promoter, which also stabilises the alumina support against sintering. However, the ceria may itself undergo crystallite growth at elevated temperatures. This can be inhibited by the addition of barium and zirconium, as shown by the comparison in Figure 22: it is apparent that barium and zirconium help to stabilise the catalytic activity.
At very high temperatures the support may itself sinter or even undergo a phase change, affecting the total surface area. Mechanical loss of catalyst support material may also occur in cases where the washcoat shrinks and cracks, causing it to separate from the monolith (Figure 23a). Again, the problem can be overcome by incorporating so called ‘phase stabilisers’ – examples include barium and lanthanum – into the washcoat (Figure 23b). At excessively high temperatures the ceramic monolith may even melt, forming additional channels that may allow the exhaust gases to pass through the converter without contacting the catalyst.
High temperatures can also promote damaging interactions between the noble metals, resulting in the formation of a less active alloy, or between a noble metal and base metals in the washcoat support. In particular, it has been established that Rh begins to penetrate the surface of γ-Al2O3 at temperatures greater than 600° C by a solid state reaction between Rh2O3 and γ-Al2O3. This subsurface penetration and loss of active Rh can be slowed down if the reactivity of the support is minimised by first supporting the Rh on zirconia, ZrO2, and then incorporating the resulting powder into the γ-Al2O3 washcoat. Figure 24 shows the dramatic effect this can have on the catalytic activity of a Rh/γ-Al2O3 catalyst. Unfortunately, the incorporation of Rh/ZrO2 into three-way catalysts requires complex manufacturing methods, which are not yet suitable for high-speed production. An alternative approach has been suggested by work that indicates that the Rh/Al2O3 interaction may occur preferentially at the grain boundaries of the support. Ceria can be incorporated as a stabilizer into the alumina in an attempt to preferentially block this interaction.