Structure sensitivity of ammonia oxidation over platinum

Ammonia oxidation over different Pt single crystal orientations and a Pt foil has been studied in the 10−5 to 10−2 mbar range with rate measurements and LEED. The reaction exhibits only a moderate structure sensitivity with the activity varying in the sequence Pt foil:Pt(533):Pt(865):Pt(443):Pt(100) = 6:4:4:2:1. The activity is apparently mainly determined by the oxygen sticking coefficient which reaches 0.14 on the Pt foil. Broad hystereses in the reaction rates upon cyclic variation of the temperature are associated with reversible structural changes. The extent of reaction-induced structural changes depends strongly on the orientation, the total pressure and also on the ratio of the reactants NH3/O2. On Pt(533) and Pt(443) the extent of restructuring and the hysteretic behavior was found to depend in a non-monotonic way on the total pressure. Exposure to reaction conditions at 10−3 mbar creates a disordered surface as seen in LEED with no LEED spots visible; but after exposure to reaction conditions at 10−2 mbar the integral order beams in LEED are again visible.