The chemistry of methanol on gold and gold based bimetallic clusters on titanium dioxide (110) single crystal.

摘要

The activity of methanol oxidation over gold clusters deposited by physical vapor deposition on a TiO2 (110) single crystal was studied by temperature programmed desorption. These studies were performed under ultra high vacuum conditions to elucidate the mechanism of gold catalyzed oxidation reactions at the active interfacial site between the gold cluster and the titania support. It was found that when oxidizing a 0.25ML coverage of gold with oxygen the hydroxyl bond scission to make adsorbed methoxy intermediates occurred more efficiently. This effect transposed into a shift to lower production temperatures and increased activity for the proposed disproportionation of two methoxy intermediates to produce methanol and formaldehyde at 580K. It was also discovered through the substitution of CD3OH for methanol that the rate limiting step in the reaction was not C-H bond scission in these intermediates. A primary kinetic isotope effect for the production of formaldehyde could not be observed.;Modifying the active site by the use of nickel as a second metal in dosed clusters has shown little benefit without encapsulating Ni. When Ni-Au bimetallic clusters are formed they form core shell structures as observed by LEIS. When these clusters are exposed to methanol the Ni core migrates to the surface to bind methanol and produce carbon monoxide and hydrogen as major products. When annealing the nickel clusters, the CO production activity is diminished, but not eliminated, which implies only partial encapsulation. Dosing gold on these partially encapsulated structures only improves the activity for methanol oxidation when the surface is oxidized after encapsulating nickel.