Surface States Governing the Activity and Selectivity of Pt-Based Ammonia Slip Catalysts for Selective Ammonia Oxidation

Abstract

Selective oxidation of ammonia to nitrogen over Pt/Al2O3 was studied in order to determine active Pt species for the activity and selectivity of Pt under conditions close to those of realistic emission control applications. For this purpose, reaction rates and apparent activation energies were measured at different compositions of the reaction feed. Additionally, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and operando X-ray absorption spectroscopy (XAS) including its interpretation based on theoretical XAS calculations were applied. Three main chemically different states of Pt were detected. The predominance of each of them correlated with the different performance of the catalyst at distinct temperature ranges. At low temperatures (<150 °C), the Pt surface was covered by oxygen species, which poisoned the catalyst. They needed to be removed by heating in the reaction mixture to start with a light off. At 150-300 °C, Pt was covered with NHx species, which provided the maximal selectivity to N2. At higher temperatures, when full ammonia conversion was reached, the Pt surface again became available for oxidation by the O species, resulting in both surface chemisorbed and subsurface O. This high-temperature state possessed high oxidation activity and high selectivity to undesired N2O and NOx