How Surface Species Drive Product Distribution during Ammonia Oxidation: An STM and Operando APXPS Study

Abstract

Ammonia (NH3) is critical in the production of fertilizers and is one of the main alternatives to substitute fossil marine fuels, however it is a highly polluting gas. We investigated Pt-Rh nanoparticles and thin films as model catalysts for oxidation of NH3 to minimize emissions by converting it into harmless N2. The bimetallic Pt-Rh nanoparticles demonstrated improved catalytic activity over monometallic Rh and enhanced N2 selectivity compared to monometallic Pt nanoparticles. Supporting operando studies on Pt-Rh films at lower oxygen pressure revealed the importance of the pressure gap on product selectivities in such fundamental studies.\nAccess to well-defined nanoparticles is key in catalytic studies as it enables correlation between the nanoparticle characteristics and the resulting properties. We developed nanoparticle synthesis approaches controlling key factors like metal distribution, Pt-Rh chemical composition, and nanoparticle size, all crucial for the catalyst's performance.\nWe also used advanced in situ electron microscopy to study the nanoparticles' thermal stability in vacuum, finding that Pt and Rh mixing or segregating depends on temperature and size. While vacuum conditions offer insights into mixing or segregation behaviors, they are not necessarily representative of realistic operational conditions. Hence, we also studied the Pt-Rh nanoparticles and films in oxidizing environments, observing migration of Rh to the surface. This result helped us to conclude on the optimal configuration of the second-generation Pt-Rh nanoparticle catalyst for the NH3 oxidation reaction.