Role of Surface Hydration State on the Nature and Reactivity of Copper Ions in Cu-ZrO2Catalysts: N2O Decomposition

Abstract

The characteristics of Cu ions loaded in different amounts on ZrO2have been studied as a function of the pretreatment conditions (in vacuo, in air, or pure O2, and after room temperature hydration) by EPR, FTIR of CO adsorption (at 77 K and 300 K), and UV-Vis-NIR diffuse reflectance spectroscopy. The present results indicate : (i) two families of CuIIcentres at the surface of oxidized samples; (ii) the easy reducibility of CuIIions to CuIions, and, eventually, to Cu0, especially in the presence of some surface contaminants and/or when starting from highly hydrated materials; (iii) CuIcentres are easily reoxidized to CuIIby the room temperature action of H2O, whereas the reoxidation of Cu0centres requires the action of O2at high temperatures; (iv) the surface situation typical of the starting Cu-ZrO2catalysts can be obtained again from reduced catalysts only by the joint action of O2at high temperatures (leading to CuO nanoparticles) and H2O at ambient temperature (redispersing the CuIIspecies). The stationary and nonstationary catalytic behaviour of Cu-ZrO2systems in the decomposition of N2O at 723 K depends strongly on the pretreatment conditions and can be correlated to the dispersion, oxidation, and hydration state of Cu surface species.