(143h) Selectivity in Multiple Guises: DFT and Microkinetic Models of NH3 Catalytic Oxidation
Ammonia oxidation is a key step in the Ostwald process for production of nitric acid and, as practiced in ammonia slip catalysis, important to environmental protection.1 Depending on application, either NO or N2 is the desired product. N2 is always the thermodynamically preferred product. Here we report DFT and microkinetic models of NH3 catalytic oxidation over the Pt, Pd and Rh terraces and steps.2 We show that barriers to product formation are lower on metal steps than on terraces, leading to a much higher step rate at low temperature to selectively oxidize NH3 to N2. At high temperature, however, both step and terrace perform comparably in rate to selectively produce NO. Relative N and O coverages interact to make NO the kinetic product at high temperature. We highlight the presence of microscopic selectivity towards different oxidation pathways, how this microscopic selectivity influences macroscopic product selectivity, and how these factors vary with catalyst structure, a third type of selectivity. We show higher N2 selectivity but lower oxidation activity on Rh than Pt and Pd. We discuss implications for application of these materials to ammonia slip catalysis relevant to lean NOx aftertreatment.
(1) Chen, J. G., et al. Science 2018, 360, 6391.
(2) Ma, H.; Schneider, W. F. ACS Catal. 2019, 9, 2407.