Ceria lattice oxygen ion substitution by Cl- during the reduction of Rh(Cl)/CeO2 catalysts.: Formation and stability of CeOCl

Rh/CeO2 systems prepared by anionic exchange of anionic rhodium chloro complexes contain significant amounts of chloride. They are referred to as Rh(Cl)/CeO2 catalysts. Reduction at 1173 K does not decrease the 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 effect of rhodium on the reducibility of CeO2 and showed that quasi-stoichiometric incorporation of chloride ions into ceria occurred at a relatively low reduction temperature (323-523 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 CeO2 resulting from the reduction of surface oxygen by dissociated hydrogen spilled over from Rh(0) to the CeO2 surface. The incorporation of chloride was found to inhibit progressively the process of 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 CeO2, supporting 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.