Model catalyst studies with single crystals and epitaxial thin oxide films

The paper describes recent results from our relatively new program to perform detailed studies of the catalytic properties of metal-oxide materials; in particular, to effect a determination of the active catalytic site(s) and the mechanism for reactions over this especially important class of heterogeneous catalysts. Issues of structure-sensitivity, poisoning and promotion, and competing reaction mechanisms are critical questions that need to be addressed in a detailed manner for catalysis by oxides. As just one important example, both surface (Langmui–Hinshelwood) and direct (Eley–Rideal) reaction mechanisms have been proposed for the selective catalytic reduction (SCR) reaction of nitrogen oxides (NO_x) over vanadia/titania catalysts. For this program, we are using a number of unique, state-of-the-art capabilities available in the Environmental Molecular Sciences Laboratory (EMSL) at Pacific Northwest National Laboratory; for example, the first molecular beam epitaxy (MBE) system dedicated to the growth of model metal-oxide films, and a unique moderate-pressure catalytic reactor/surface science apparatus. We describe the growth, characterization, and water adsorption properties of a thin Fe₃O₄(0 0 1) film grown on a lattice-matched MgO(0 0 1) substrate. Because our moderate pressure catalysis studies are preliminary at this point, we instead describe our previous results on the CO oxidation reaction over a Ru(0 0 0 1) model catalyst to demonstrate the utility of the experimental approach. We specifically discuss the possibility that this reaction occurs by an Eley–Rideal mechanism.