Data: Mechanistic elucidation of selective catalytic ammonia oxidation over Rh by spatially resolved profiling

The use of ammonia as a hydrogen carrier and sustainable fuel necessitates efficient control of ammonia slip. In this study, the selective catalytic oxidation (SCO) of ammonia over a Rh/Al2O3-coated honeycomb monolith was investigated using spatially resolved capillary sampling. Dry NH3/O2/N2 feeds were examined at equivalence ratios φ = 0.1, 0.5, 1, and 2; an additional NO co-feed experiment was conducted to probe the formation pathway of N2O. Light-off temperatures increase with decreasing φ, consistent with competitive oxygen adsorption on Rh that inhibits NH3 activation in excess O2. Under φ = 1, N2 was the dominant product across the entire temperature window subject to this study, NO2 was undetected, and N2O appeared only at low–intermediate temperatures before vanishing at higher temperatures. The NO co-feed experiment indicates that the coupling of NH3-derived species with NO contributes significantly to N2O formation. NO showed a three-stage evolution (increase–decrease–increase), with the mid-temperature decline owing to an internal selective catalytic reduction (i-SCR) pathway. Spatial profiles corroborated this mechanism: NH3 is first oxidized to NO near the monolith inlet, and the nascent NO is subsequently reduced downstream by residual NH3 to N2, thereby continuously removing NO along the catalyst. ... mehrUnder fuel-rich conditions, NH3 decomposition becomes a significant conversion pathway at elevated temperatures, and N2 selectivity reaches 100% with negligible NOx formation above 225 °C. Under lean feeds, excess O2 suppresses the i-SCR pathway, yielding oxidation-dominated chemistry in which NO accumulates and NO2 forms at high temperature; the N2O peak shifts to higher temperatures and overall N2 selectivity declines. These findings provide a mechanistic basis for operation as well as for the design/optimization of Rh-based NH3-slip control systems