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%.