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systems prepared by anionic exchange of anionic rhodium chloro complexes contain signiÐcant Rh/CeO 2 amounts of chloride. They are referred to as catalysts. Reduction at 1173 K does not decrease the Rh(Cl)/CeO 2 chloride content of these catalysts but calcination in air at the same temperature leads to total elimination of Cl~ions. A temperature-programmed reduction (TPR) study evidenced the promoting e †ect of rhodium on the reducibility of and showed that quasi-stoichiometric incorporation of chloride ions into ceria CeO 2 occurred at a relatively low reduction temperature (323È523 K), simultaneously with rhodium reduction to Rh(0). It is proposed that chloride incorporation occurs through the migration of surface Cl~ions into oxygen vacancies of resulting from the reduction of surface oxygen by dissociated hydrogen spilled over from CeO 2 Rh(0) to the surface. The incorporation of chloride was found to inhibit progressively the process of CeO 2 hydrogen spillover below 773 K. No CeOCl phase could be detected at the corresponding low reduction temperatures. However, catalysts containing higher chloride loadings were used to show that CeOCl was formed at higher reduction temperatures than that necessary for chloride incorporation into supporting CeO 2 , the hypothesis that Cl~ions are incorporated into the ceria lattice at low temperatures. CeOCl was found to be unstable in air but the phase could be restored upon additional reducing treatment, thus showing the reversibility of the Cl~ion migration into the oxygen vacancies of ceria.

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