NH3 emissions are predominantly caused by diesel engine vehicles and denitrification units of coal-fired power plants. For diesel engine vehicles to meet stringent nitrogen oxide (NOx) emission regulations, NH3 is introduced into diesel engine aftertreatment systems containing selective catalytic reduction (SCR) units, with downstream ammonia slip catalysts (ASC) responsible for removing residual NH3 from the exhaust stream. ASCs are dual-layer materials consisting of a top-layer NOx SCR catalyst (e.g., Cu-chabazite zeolites; Cu-CHA) and a bottom-layer NH3 oxidation catalyst comprising platinum supported on gamma alumina (Pt/γ-Al2O3). During vehicle operation, ASCs experience wide operating conditions that influence Pt oxidation state and structure, and cause differences in observed NH3 oxidation kinetics and selectivities. During heating and cooling of the ASC under NH3 oxidation conditions, rates differ during ignition and extinction cycles to show a hysteresis in oxidation behavior, with higher NH3 oxidation rates observed for the extinction branch. This hysteresis phenomena has been observed for other oxidation reactions of other compounds (e.g., NO, CO, CH4) over Pt/γ-Al2O3; however, the mechanistic origins underlying this phenomena remains debated. In this work, we combine steady-state and transient kinetic measurements and theoretical calculations to elucidate the kinetic and mechanistic details of NH3 oxidation on Pt/γ-Al2O3 powder samples of different Pt particle size and pretreatment history. Supplemental characterization including H2 temperature-programmed reduction (TPR) and in situ spectroscopic techniques (X-ray absorption, CO DRIFTS) were used to determine the Pt oxidation state and structure in a range of reaction-relevant conditions. Experimental data are connected to DFT predicted states of the catalyst surface and expected reaction kinetics as a function of catalyst particle size. The understanding of the kinetic and mechanistic factors that give rise to a hysteresis in oxidation behavior from these techniques can provide guidance on how to design ASC catalysts for real-world applications.

Mechanistic insights into the hysteresis phenomena during NH3 oxidation on Pt/Al2O3 catalysts for NH3 slip applications

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