Surface States Governing the Activity and Selectivity of Pt-Based Ammonia Slip Catalysts for Selective Ammonia Oxidation
Vasyl Marchuk, Dmitry Sharapa, Jan‐Dierk Grunwaldt, Dmitry E. Doronkin
Abstract
High Resolution Image Download MS PowerPoint Slide Selective oxidation of ammonia to nitrogen over Pt/Al 2 O 3 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 NH x species, which provided the maximal selectivity to N 2 . 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 N 2 O and NO x .