(460h) Alloying Induced Effects on Catalytic Ammonia Oxidation over Pt and Cu

Catalytic ammonia (NH₃) oxidation is important as part of the Ostwald process for fertilizer synthesis, and for NH₃ slip control from industrial processes as well as a diesel engine after-treatment system. Pt, a commonly utilized catalyst for this reaction shows good activity and major selectivity towards N₂ at low temperatures of ammonia slip control and NO at higher Ostwald process temperatures, but also gives N₂O, a greenhouse gas as a minor product. Coinage metals have been evaluated for the same reaction and among them, Cu has shown lower N₂O selectivity but inferior activity compared to Pt. Alloying is a common strategy to tune catalytic performance, and in some applications like diesel engine aftertreatment, alloying may be an unintended consequence of long-term use because of proximity between Cu and Pt containing catalysts. Preliminary evidence suggests that alloying between Cu and Pt can achieve activity comparable to Pt with lower N₂O selectivity. Here, we use Density Functional Theory to compute reaction pathways and parameterize microkinetic models with coverage dependent rate constants. We compare differential activity and selectivity, their sensitivity to inlet gas composition as well as their evolution down the reactor bed on metallic Pt, Cu, and Cu₃Pt, a thermodynamically stable alloy of the two. We then compare model results with experimental observations and identify origins of alloying-induced differences.