(674a) Mechanistic and Structural Transformations of Pt/Al2O3 Catalysts during NH3 oxidation for NH3 Slip Applications

NH₃ emissions are predominantly caused by denitrification units of coal-fired power plants and diesel engine vehicles. To meet stringent nitrogen oxide (NO_x) emission regulations from diesel engine vehicles, NH₃ is introduced into the 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 NO_x SCR catalyst (e.g., Cu-CHA) and a bottom-layer NH₃ oxidation catalyst comprising platinum supported on gamma alumina (Pt/γ-Al₂O₃). During operation, ASCs experience varying operating conditions that influence Pt oxidation state, surface coverage, and structure, arising in differences in observed NH₃ oxidation kinetics and selectivities. When heating or cooling the ASC under NH₃ oxidation reaction conditions, rates differ during ignition and extinction cycles resulting in a hysteresis phenomenon, with higher NH₃ oxidation rates observed for the extinction branch. Such hysteresis phenomena have been observed for other oxidation reactions (e.g., CO, CH_4, and NO oxidation) over Pt/γ-Al₂O₃; however, the mechanistic origins and structural changes underlying this behavior remains debated. Here, we combine steady-state and transient kinetic measurements with theoretical calculations to elucidate the kinetic and mechanistic details of NH₃ oxidation on model Pt/γ-Al₂O₃ powder samples of different Pt particle size and pretreatment history. A suite of characterization techniques including H₂ temperature-programmed reduction (TPR) and spectroscopic techniques (in situ X-ray absorption and ex situ XPS) were used to determine the Pt oxidation state and structure across a range of reaction-relevant conditions. The understanding of the kinetic, mechanistic, and structural factors that give rise to a hysteresis in oxidation behavior from these techniques can provide guidance on how to design and operate robust ASC catalysts in real-world applications.