Probing the reactivity of C6-hydrocarbons on Au surfaces: cyclohexane, cyclohexyl and cyclohexene on Au(1 1 1)

Au is often added to reduce the activity of Group-VIII transition metals like Ni, Pd and Pt in dehydrogenation catalysts. In order to improve understanding of the fundamental chemistry of hydrocarbons at Au sites, further studies of stable hydrocarbon molecules and reactive intermediates on Au surfaces are needed. The adsorption and thermal chemistry of cyclohexane (C6H12), cyclohexyl groups (C6H11) produced by electron-induced dissociation (EID) of cyclohexane, and cyclohexene (C6H10) on Au(1 1 1) was studied by using primarily temperature programmed desorption (TPD) and Fourier-Transform Infrared Reflection-Absorption spectroscopy (IRAS). Both cyclohexane and cyclohexene are reversibly adsorbed on Au(1 1 1) surfaces, with desorption peak temperatures of 198 and 213 K from the adsorbed monolayer, respectively, and 143 K from both multilayer films. EID with an incident electron energy of 30 eV on a monolayer film of cyclohexane selectively breaks one C–H bond in weakly bound cyclohexane molecules to form adsorbed C6H11 groups on the Au(1 1 1) surface, as established by TPD and IRAS. The Au(1 1 1) surface can readily and selectively dehydrogenate adsorbed cyclohexyl groups to produce cyclohexene at or below 216 K. Cyclohexyl groups also undergo disproportionation reactions at 273 K to produce cyclohexane and cyclohexene. We suggest that surface Au atoms may play a more important, direct role in the hydrogenation/dehydrogenation selectivity found for transition metal–Au alloys than is commonly discussed.