N2O selectivity in industrial NH3 oxidation on Pt-gauze is determined by interaction of local flow and surface chemistry: A simulation study using mechanistic kinetics

Despite significant effort spent on the investigation of catalytic ammonia oxidation at the molecular scale, there is surprisingly little work that investigates the behavior of the established reaction mechanisms under industrial conditions in the presence of mass transfer limitations. This paper presents reactive flow simulations of ammonia oxidation on platinum gauzes under industrial operating conditions, combining a mechanistic description of the surface chemistry with the computation of the flow-, temperature and concentration fields around the platinum wires. Overall, the simulations yield temperature- and concentration fields, as well as integral N2O selectivity in line with industrial experience and (limited available) experimental data. In particular, the simulations predict the experimentally observed increase of the integral N2O selectivity with increasing flow velocity, decreasing wire diameter and wire-to-wire distance, and increased surface area due to surface reconstruction. The main result of the paper is that the local interaction of flow and surface chemistry leads to a variation in the local N2O selectivity across the gauze:The N2O and N2 selectivity is higher on the front side of a wire than on the rear side. A reduced N2O selectivity is observed where one wire is shadowed by another wire. Increased N2O selectivity is observed at stagnation points where upstream wires direct the flow so that it hits a downstream wire with higher velocity. These examples show that - through the flow directing effect of the upstream wires- the selectivity on an individual wire is influenced by the presence of other wires. This observation provides a mechanistic explanation for the industrial observation that optimized gauze geometries can lead to reduced N2O formation.