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Selective Oxidation of Ammonia on RuO2(110): a combined DFT and KMC study

We have used a combination of density functional theory (DFT) and kinetic Monte Carlo (KMC) simulations to calculate the reaction rates for the selective oxidation of ammonia on RuO2(110). We find that the overall energy barrier for NH3 + O -> NH + H2O is 0.56 eV, while that for N + N -> N2, and N + O -> NO to be 0.27, and 0.14 eV, respectively. Our KMC calculations enable us to examine the kinetics not only of these mainline processes but also of additional 15 reactions among several intermediate species that illuminate the reactivity and selectivity of the former. In agreement with previous studies, we find that the key role in high reactivity of RuO2(110) in ammonia decomposition is played by the way in which, by promoting easier H abstraction, H bonding between ammonia and its intermediates with the adsorbate and substrate O converts NH decomposition into a non-activated (spontaneous) process. Further, our findings confirm in detail what experiment and previous studies have suggested about NO and N2 formation rates, namely, that NO desorption is a rate-limiting process in that surface NO blocks reaction sites at lower temperatures. Our calculations concerning selectivity (93% in favor of NO over N2) are in very close agreement with experiment (95%). They also explain why NO formation is so favored over N2 formation even at low O2 pressure. The significantly reduced N diffusion caused by various intermediates present on the RuO2(110) surface severely inhibits the recombination site for N+N -> N2, but interferes with that for N + O -> NO far less owing to the nearby availability of O from dissociation of O2.