Role of surface hydration state on the nature and reactivity of copper ions in Cu-ZrO2 catalysts:: N2O decomposition

The characteristics of Cu ions loaded in different amounts on ZrO2 have been studied as a function of the pretreatment conditions (in vacuo, in air, or pure O-2, 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 Cu-II centres at the surface of oxidized samples; (ii) the easy reducibility of Cu-II ions to Cu-I ions, and, eventually, to Cu-0, especially in the presence of some surface contaminants and/or when starting from highly hydrated materials; (iii) Cu-I centres are easily reoxidized to Cu-II by the room temperature action of H2O, whereas the reoxidation of Cu-0 centres requires the action of O-2 at high temperatures; (iv) the surface situation typical of the starting Cu-ZrO2 catalysts can be obtained again from reduced catalysts only by the joint action of O-2 at high temperatures (leading to CuO nanoparticles) and H2O at ambient temperature (redispersing the Cu-II species). The stationary and nonstationary catalytic behaviour of Cu-ZrO2 systems 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. (C) 1998 Academic Press