Structure- and Temperature-Dependence of Pt-Catalyzed Ammonia Oxidation Rates and Selectivities

Ammonia oxidation is operated at different temperatures over Pt catalysts of different structures to recover different products. In this work, we elucidate the dependency of ammonia oxidation rates and selectivities on both Pt structure and temperature. We perform density functional theory (DFT) computations to compare the reaction and activation energies of elementary reactions on Pt(211) and Pt(111). We develop a microkinetic model parametrized with the DFT results. We show that barriers to product formation are lower on stepped Pt than on terrace, leading to a much higher step rate at low temperature to selectively oxidize ammonia to nitrogen. At high temperature, however, both step and terrace perform comparably in rate to selectively produce nitric oxide. While N-2 is always the thermodynamic product, relative N and O coverages interact to make NO the kinetic product at high temperature. The predicted rate and selectivity are consistent with experiments. We further show rate controlling steps on the two Pt surfaces are different at low temperature but are the same at high temperature. The degrees of selectivity control for elementary reactions are comparable for the two surfaces. Finally, we demonstrate the flows of elementary reactions in the reaction network are also structure- and temperature-dependent.