Zirconia-supported copper catalysts for NO[ndash ]CO reactions Surface copper species on zirconia

Abstract

Cu/ZrO₂ catalysts show high activity for the NO–CO reaction, even at low temperature (100–200°C). The structure and reduction behaviour of ZrO₂-supported Cu species have been characterized by means of X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), FTIR and temperature-programmed reduction (TPR). It was found that highly dispersed Cu²⁺ species in an octahedral symmetry dominated at a Cu content lower than 1 wt.% and that the amount of the Cu²⁺ species was saturated at 1 wt.% Cu. On the other hand, Cu oxide clusters were exclusively formed upon additional Cu incorporation after 1 wt.%. The highly dispersed Cu²⁺ species were produced by reaction with the terminal OH groups of the ZrO₂ surface. The highly dispersed Cu²⁺ species were reduced by CO at a much lower temperature than Cu oxide clusters and produced highly dispersed Cu⁺ species at 100°C, which subsequently formed C species at 170–180°C. It is concluded that the high catalytic activity of Cu/ZrO₂ for NO–CO reactions at low temperature is brought about by the formation of highly active C species on low-temperature reduction of the Cu²⁺ species interacting with the ZrO₂ surface, and that the activity of the C species is lost by agglomeration into Cu metal particles.