(674c) Adsorbate Induced Restructuring of Pt Nanoparticle and Resultant Kinetics for Catalytic Ammonia Oxidation

Catalytic ammonia (NH₃) oxidation has been applied in the Ostwald process to produce Nitric Oxide (NO) and in ammonia slip control to produce nitrogen (N₂). Nitrous oxide (N₂O), a greenhouse gas, is an undesired product in both of these applications. Platinum has been shown to be an effective catalyst for this reaction. Previous computational studies have shown activity and selectivity of Pt-100, Pt-111 and Pt edge sites as a function of temperature. They also show that these surfaces can be covered with different amounts of O and NO at different temperatures. As these adsorbates bind strongly with the Pt surface, they have an effect on its surface energy. In this work, we quantify the effect of O and NO on the surface energy of Pt-100 and Pt-111 using Density Functional Theory (DFT) calculations. We use Wulff construction to calculate change in distribution of Pt-111, Pt-100 and edge sites on a Pt nanoparticle in presence of O and NO under different reaction conditions. We find that Pt-100 sites are more abundant in the conditions that favor high NO coverage and this effect is more significant on smaller nanoparticles. Finally, we use a mixed site microkinetic model to calculate expected activity and selectivity for Pt nanoparticles of different sizes and show how they explain experimental observations of the same quantities on different particle sizes.