Mechanism of ammonia oxidation over PGM (Pt, Pd, Rh) wires by temporal analysis of products and density functional theory

The mechanism of ammonia oxidation over Pt, Pd, and Rh wires has been investigated in the Temporal Analysis of Products (TAP) reactor at relevant temperatures in industrial ammonia burners. The results of primary (NH₃+O₂) and secondary (NH₃+NO) interactions with isotopically labeled ammonia at 1073 K enable to conclude that the overall reaction pathways to NO, N₂O, and N₂ are equivalent on the three noble metals. NO is a primary reaction product, while N₂ and N₂O originate from consecutive NO transformations. The extent of the secondary reactions determines the net NO selectivity. Rhodium is the most active catalyst for the unwanted reduction of NO by NH₃, while platinum shows the lowest activity. This explains the superior NO selectivity attained over Pt and, therefore, its industrial application. The TAP-derived selectivity ranking was substantiated by Density Functional Theory calculations on the (100) facets of the noble metals. We proved experimentally that NO selectivity approaching 100% at complete NH₃ conversion can be equivalently attained over Pt, Pd, and Rh by increasing the oxygen content in the feed. For a feed of O₂/NH₃=10, both N₂O and N₂ production are suppressed due to the impeded NO dissociation and favored NO desorption at high oxygen coverage.