Despite its immense practical importance in industrial production of nitric acid, the mechanisms of catalytic ammonia oxidation on platinum group metals remain controversial. In this work, we employ velocity-resolved kinetics to study ammonia oxidation on a model Pd(332) catalyst between 600 and 700 K. We obtain the temporal evolution of gas-phase reactants (NH3), products (NO, H2O) andwith the help of femtosecond laser-induced desorptionof a reaction intermediate, N*. The reaction exhibits the prompt appearance of H2O and the delayed formation of NO; the rate-determining step is the reaction N* + O* → N*O occurring at step sites. This means that N* is the longest-lived reaction intermediate, an insight that helps explain formation of byproducts like N2 and N2O. We present a mechanism that explains all experimental observations, based on transition-state theory calculations and using input from density functional theory. We also show that N*O desorption is accelerated by coadsorbed oxygen.
Keywords: NH3 oxidation; heterogeneous catalysis; intermediate detection; surface kinetics; velocity-resolved kinetics.
© 2025 The Authors. Published by American Chemical Society.