Oxidation of NH3 on polycrystalline copper and Cu(1 1 0): a combined FT-IRAS and kinetics investigation

The kinetics of the reaction of oxidation of ammonia on polycrystalline copper, has been investigated, in a re-circulating reactor, and correlated to a characterisation of the catalyst surface at different extent of conversions. The rapid formation of a nitride or oxynitride phase and its reactivity have been demonstrated. Under oxidising conditions, PNH3=PO2, and up to 650 K, dinitrogen is the only product of the reaction; N2O being formed when T or PO2 increases further. The correlation of these kinetics results to an in situ characterisation of the same reaction at RT by Fourier-transformed infrared reflection-absorption spectroscopy (FT-IRAS), on a well defined Cu(1 1 0) surface, led to the following conclusion: two reaction pathways contribute to the conversion of ammonia: (i) its decomposition on copper; (ii) the reaction between ammonia molecules and oxygen adsorbed from the gas phase. The major adsorbed species is oxygen; intermediate species are NH2, NH and possibly HNO formed when the oxygen surface concentration is sufficient. Increasing the pressure of oxygen induces, at high T, the formation of nitrous oxide; N2O results from an oxidation of the surface copper nitride or from the interaction of two surface HNO intermediates.