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

Rh-Cu/SiO2 catalysts prepared by the sol-gel method by mixing solutions of Rh(acac)(3) in acetone and/or of Cu(NO3)(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 wa s found that the addition of Cu to Rh/SiO2 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) s pecies remain trapped in the support matrix, changing the metal-support int eraction. Support for these interactions was provided by testing their reac tivity in the gas-phase hydrogenation of crotonaldehyde at 373 K. A signifi cant enhancement in both activity and selectivity to crotyl alcohol was obs erved depending on these interactions. Selectivity to the unsaturated alcoh ol higher than 60% was obtained for Rh-Cu catalysts having a rhodium atom r atio close to 35%.