Surface structures of Rh-Cu sol-gel catalysts and performance for crotonaldehyde hydrogenation

Rh-Cu/SiO(2) catalysts prepared by the sol-gel method by mixing solutions of Rh(acac)(3) in acetone and/or of Cu(NO(3))(2) in water with tetraethoxysilane (TEOS) in ethanol at 318 K at pH 3. The influence has been studied of the copper content on the surface and catalytic properties were analyzed. It was found that the addition of Cu to Rh/SiO(2) catalysts increases the rhodium dispersion and modifies the rhodium clusters, either by alloy formation and/or by electron transfer from copper to rhodium metal clusters in intimate contact. X-ray photoelectron spectroscopy results indicate that copper(I) species remain trapped in the support matrix, changing the metal-support interaction. Support for these interactions was provided by testing their reactivity in the gas-phase hydrogenation of crotonaldehyde at 373 K. A significant enhancement in both activity and selectivity to crotyl alcohol was observed depending on these interactions. Selectivity to the unsaturated alcohol higher than 60% was obtained for Rh-Cu catalysts having a rhodium atom ratio close to 35%.