Surface Segregation of Oxygen Vacancies and Metal Ions

Surface Segregation of Oxygen Vacancies and Metal Ions 

Automobile exhaust catalysts typically contain noble metals such as Pt, Pd and Rh with a ceria promoter supported on alumina. Traditionally, the principal function of the Rh is to control emissions of nitrogen oxides (NOx) by reaction with carbon monoxide, although the increasing use of Pd has been proposed. For example, recent X-ray absorption spectroscopy studies of Holies and Davis show that the average oxidation state of Pd was affected by gaseous environment with an average oxidation state between 0 and +2 for a stoichiometric mixture of NO and CO. Exposure of Pd particles to NO resulted in the formation of chemisorbed oxygen and/or a surface oxide layer. 

In this context, Sayle et al. have used atomistic simulation methods to investigate the interaction of ceria with impurities, particularly rhodium, palladium and platinum. The energetics of the most common valence states for the metal atoms were investigated, as well as the variation of the energy of the impurities with depth below the surface and the tendency of defects to segregate to the surfaces. Fig. 8.9. shows the substitutional defect formation energies as a function of the distance from the (111) and (110) surfaces of Ce02 for C e , PcP+ and Sayle etal. found that 

The results also suggest that substitutional formation is most favorable on the (110) surface. This supports the view that the (110) surface will be more catalytically active than the (111) surface, as impurities segregate preferentially to this surface. Sayle et al. note that the segregation energies (j.e. the differences between bulk and surface energies) are larger for the M + cations than for M + cations due to elec- 

This survey has aimed to demonstrate that computational techniques can play a valuable role in contemporary studies of ceria-based oxides, which complement related experimental work, and provide information that is relevant to catalytic and fuel cell applications. Materials that were investigated include pure Ce02, doped Ce02 (where, for example, M = Sc, Y, Gd) and the mixed Cei-xZrx02 system. 

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A, Surface Segregation of Oxygen Vacancies and Metal Ions... 

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