title: Modeling of industrial NH3 oxidation on Pt catalysts: The effect of local mass transfer on N2O selectivity
Creators: name: Haas, Michael
Publisher: name: UNSPECIFIED
Year: 2022

description: Despite the enormous effort spent on the investigation of heterogeneously catalized ammonia oxidation at the molecular scale, there is surprisingly little work that studies the behavior of established reaction mechanisms under industrial conditions with mass transfer limitations being present. In this work, the industrial ammonia oxidation on platinum catalyst gauzes is investigated by means of reactive flow simulations. To this end, a mechanistic description of the surface chemistry is coupled with the computation of flow-, temperature- and concentration fields around the Pt wires. The simulations predict integral N2O selectivities, as well as temperature and concentration fields in line with the industrial experience and (limited available) experimental data. It is demonstrated that the axial temperature gradient inside the gauze is controlled by the Lewis number of the reacting gas and the operating pressure. The experimentally observed decrease in integral N2O selectivity with decreasing flow velocity, increasing wire diameter, increasing wire-to-wire distance, as well as increased surface area, due to catalyst surface reconstruction, is reproduced by the models. In particular it is found that the local interaction of flow-field and surface chemistry leads to a variation in the local selectivity towards the by-products N2O and N2 across the gauze: The selectivity of side products is higher on the front side of the wire than on the rear side. A reduced selectivity is observed, where one wire is shadowed by another. At the stagnation points, where upstream wires direct the flow so that it hits a downstream wire with higher velocity, an increased side product selectivity is observed. These examples show that, through the flow-directing effect of upstream wires, the mass transfer intensity and thus the selectivity on an individual wire is influenced by the presence of other wires. The degree of mass transfer control, i.e. the sensitivity of product selectivity with respect to changes in mass transfer intensity, provides a mathematical formalism allowing to quantify the flow-induced effect of local mass transfer on the product selectivity. Combined with flow simulations of catalyst structures, yielding the distribution of local mass transfer coefficients throughout the catalyst, this type of sensitivity analysis constitutes a novel and universal instrument to investigate and design structured catalysts for chemical reactions being controlled by external mass transfer. With the industrial ammonia oxidation as a test case, the capabilities of this tool are demonstrated for gauze catalysts with different levels of geometric complexity. This concept also finds applications at the catalyst microscale, which is demonstrated using a Pt wire with a restructured surface as an example. Finally, the influence of upstream turbulence on the catalytic performance of a single Pt cylinder as well as a stack of multiple wires is studied, combining large eddy simulations with detailed surface chemistry. Although significant time-dependent fluctuations of the local Sherwood number and N2O selectivity can be observed along the wires, in both cases the integral N2O selectivity only slightly deviates from the results found in case of laminar flow.