4.3.6 The role of CeO2
Figure 20 shows the effect on performance of adding CeO2 to a Pt catalyst for three-way catalytic conversion.
What is the effect of CeO2 on the conversion efficiencies for CO, hydrocarbons and NOx?
Substantial improvements in all three conversion efficiencies are seen, particularly at A/F ratios just below stoichiometry.
Thus, ceria, which is added with the alumina in the washcoat, is an essential ingredient of the three-way catalyst. It plays a number of roles:
1: Ceria is a structural promoter, stabilising the precious metals and alumina against sintering and particle growth. Figure 21 emphasises this point.
How does the addition of ceria to the catalyst (the coloured line in Figure 21) affect the Pt dispersion?
Clearly, in the absence of ceria, substantial sintering of Pt occurs between 500 °C and 600 °C, causing a sharp reduction in dispersion. Addition of CeO2 results in a significant stabilization of the Pt metal dispersion up to 700–800 °C.
Ceria also stabilises the γ-Al2O3 used in the support, inhibiting a phase change to -Al2O3, which has a lower surface area. (Lanthanum oxide and/or barium oxide are also often added as stabilisers to help maintain the surface area of γ-Al2O3.)
2: Ceria is known to be able to pick up and store oxygen from the gas phase under fuel-lean operating conditions (excess oxygen) – thus promoting the reduction of NO to N2 – and to release it under fuel-rich conditions (excess fuel), for reaction with CO, H2 or hydrocarbons. Thus, it effectively dampens the variations in the A/F ratio as the exhaust gas mixture cycles about stoichiometry, thereby helping to keep operation within the desired window for optimum conversion over the catalyst.
3: As we have seen in section 4.2, the ceria also enhances the water-gas shift activity of Pt–Rh three-way catalysts, and hence promotes CO removal via the following reaction under fuel-rich conditions:
The point is illustrated by the results shown in Table 1 for CO conversion under fuel-rich conditions. Increasing the ceria content of the catalyst in the absence of water has no effect, but when water is present the water-gas shift reaction becomes increasingly important. (Addition of ceria also leads to an improvement in activity for steam reforming.)
Table 1 CO conversion for a 1.08 mass % Pt-Rha catalyst on γ-Al2O3 with 1.5, 4.0 and 8.0 mass % Ce levels, under fuel-rich conditions, with and without water present. (aPt 0.9 mass % and Rh 0.18 mass %.)
|With H2O||Without H2O|
|1.5 mass % Ce||54||49|
|4.0 mass % Ce||64||49|
|8.0 mass % Ce||70||49|
4: Enhanced conversions of CO, C3H6 and NO at low temperatures have also been observed for Pt/CeO2 catalysts that have undergone a reducing pretreatment. This is believed to be due to an interaction between Pt and CeO2 induced by the reduction, causing an increase in the number and activity of the active sites.