Propene Oxidation over Cu 2O Single-Crystal Surfaces: A Surface Science Study of Propene Activation at 1 atm and 300 K

Abstract

Propene oxidation was studied on Cu 2O single-crystal surfaces using XPS and TDS in ultrahigh vacuum (UHV) following propene exposures at 300 K and atmospheric pressure. Three different Cu 2O surfaces were examined: a CW-terminated (100) surface, an oxygen-terminated (100) surface, and a (111) surface with accessible copper cations and lattice oxygen. Selective oxidation to acrolein is promoted by coordinately unsaturated surface lattice oxygen. No clear correlation between the formation of partial oxidation products and oxygen coordination was observed. Both selective and nonselective oxidation products are formed from lattice oxygen. The propene oxidation pathway proceeds through an allyloxy (CH 2CHCH 2O-) species where oxygen insertion occurs prior to the second hydrogen abstraction over Cu 2O. This conclusion was reached by comparing the propene TDS results from atmospheric pressure exposures with UHV allyl alcohol and acrolein thermal desorption data. The selective oxidation process over Cu 2O is subject to a "pressure gap" at 300 K with oxygen insertion occurring at higher pressures, presumably because of translational and collisional activation. However, the subsequent steps in the selective oxidation pathway can be effectively modeled by dissociative adsorption of oxygenates under UHV to simulate the oxygenated surface intermediates.