  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.
