Reduction of NO on copper and its poisoning by SO2, a mechanistic study

A combination of catalytic tests and surface studies has been undertaken to better understand the poisoning effect of sulfur dioxide upon the reduction of NO by a hydrocarbon. The characterization of the copper surface by photoelectron spectroscopy, at various stages of the reaction, made clear an initial oxidation of the catalyst accompanied by the formation of an oxygenated intermediate; it is followed by a reduction of the surface and a rapid reaction of NO with this species. In situ surface infrared measurements have been made on a model copper surface, Cu(110), under 10(-5) Torr of SO2 or SO2 + O-2 revealing the formation of molecular adsorbed compounds when the surface is oxidized, slightly covered with oxygen or hydroxylated. SO, is a drastic poison of the catalytic NO reduction; it reacts with the oxidized surface leading to sulfite and sulfate groups, it interacts with water, leading to hydrogenosulfoxide adsorbates which can obviously block a number of surface sites. It also dissociates on the metallic copper and occupies surface sites. It does not block the hydrocarbon adsorption completely but severely reduces the formation of the active intermediate.