DFT and microkinetic comparison of Pt, Pd and Rh-catalyzed ammonia oxidation.

• Consistent DFT calculations and microkinetic modeling of ammonia oxidation on Pt, Pd and Rh. • Predicted kinetic trends on stepped surface correspond well with experiments. • Reaction flux analysis reveals that NH 3 is activated by O and OH. • Relative product formation barriers and relative coverages influence selectivities. Ammonia oxidation is the heart of the Ostwald process and is important in emissions control. Catalytic behaviors are a function of conditions and are observed to vary across the platinum group metals (PGMs) Pt, Pd, Rh. Here, we combine density functional theory computations and microkinetic modeling to rationalize these dependencies. We compute reactions over model (2 1 1) and (1 1 1) surfaces of PGMs. Binding energies are similar on Pd and Pt and generally greater on Rh, while activation energies vary across all metals. Rates on (2 1 1) surfaces are greater than (1 1 1) surfaces. The stepped Pt is most active and stepped Rh most selective to N 2 at ammonia slip conditions, while at Ostwald process conditions, stepped Pd is most active and stepped Pt most selective to NO. Degree of rate and selectivity control analysis provides insights into the reactions limiting performance of PGMs. Both activation barriers and surface coverages influence rates and selectivities. [ABSTRACT FROM AUTHOR]