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

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


Ma, H., University of Notre Dame
Daya, R., Cummins Inc.
Trandal, D., Cummins Inc.
Gunugunuri, K., Cummins
Kamasamudram, K., Cummins Inc.
Schneider, W., University of Notre Dame
Gounder, R., Purdue University
NH3 emissions are predominantly caused by denitrification units of coal-fired power plants and diesel engine vehicles. To meet stringent nitrogen oxide (NOx) 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 NOx 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, CH4, 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.