Development of active Pd-only three-way catalysts: The balance between surface composition and oxygen vacancies of ceria-zirconia support

Abstract

CeO₂-based catalysts have attracted widespread attention in gasoline vehicle exhaust purification, and their catalytic performance is often intimately associated with the supports used. In this work, a series CeO₂-ZrO₂ (CZ) supports with varying composition and phase structure (with or without partial κ-Ce₂Zr₂O₈ structure) have been employed for Pd-only three-way catalysts (TWCs), and the corresponding activities were evaluated. It was found that for catalysts without κ-Ce₂Zr₂O₈ structure, the activity is positively related with the content of CeO₂ in CZ due to the increased Pd-O-Ce interfaces. However, as the formation of κ-Ce₂Zr₂O₈ structure is accompanied by the increased surface oxygen vacancies but decreased surface Ce-O_x species, for catalysts with κ-Ce₂Zr₂O₈ structure, there exists a balance between the reasonable concentration of oxygen vacancies and surface Ce-O_x species in determining the corresponding activities. Among these catalysts, only the Pd-only TWC with medium content of CeO₂ and partial κ-Ce₂Zr₂O₈ structure (Pd/κMC) exhibits superior activity than that in the same composition but without κ-Ce₂Zr₂O₈ structure (Pd/MC). This may be associated with the obviously increased surface oxygen vacancies introduced in κMC, which could compensate the slight decrease of its surface Ce-O_x species to stabilize the supported Pd species in smaller size and higher oxidation states. This finding enhances the understanding of support effects to rationally design efficient heterogeneous catalysts.