Insight Investigation of Active Palladium Surface Sites in Palladium-Ceria Catalysts for NO + CO Reaction.

The palladium species in ceria-based catalysts have a significant influence on their catalytic performance. In this work, the structure evolution of palladium species induced by various calcination rate was investigated and then these calcined catalysts were applied to NO + CO catalytic reaction. Systematic investigations by various measurements demonstrate that the calcination rate and catalytic process play crucial roles on the formation ways of palladium species and identify the forms of active palladium surface sites for NO + CO reaction. Results indicate that the calcination process resulted in the formation of three types of palladium components: PdO interacted with ceria supports (PdO x/Pd-O-Ce cluster), PdO nanoparticles on the surface, and Pd2+ ions incorporated into the subsurface lattice (Pd-O-Ce solid solution). It is also proven that the state and distribution of palladium components are dependent on the calcination rate: rapid calcination rate is beneficial for the generation of PdO species (PdO x/Pd-O-Ce and PdO), while slow calcination rate makes contribution to the formation of Pd-O-Ce. Furthermore, the subsequent catalytic process could induce the decomposition of PdO x/Pd-O-Ce and formation of more fractions of active Pd species in PdO oxide phase. On the basis of the catalytic performances, we found the superior catalytic properties are detected for catalysts containing 0.5% Pd (0.5% is corresponding to the palladium content in molar ratio) with fast calcination rate. This is due to the synergistic effect of active Pd in PdO decomposed form PdO x/Pd-O-Ce in the catalytic process and the palladium ions in Pd-O-Ce solid solution.