(490c) Mechanistic Insights into pH-Controlled Nitrite Reduction to Ammonia over Rhodium

An unintended consequence of industrial nitrogen fixation is nitrate (NO₃^-) and nitrite (NO₂^-) contamination of ocean, ground, and surface waters from fertilizer runoff. Transition metal catalysts are effective for removing NO₃^-/NO₂^- through reduction to N₂ or NH₄⁺. Pd is regarded as the most effective metal for non-electrochemical NO₃^-/NO₂^- reduction, and as such few studies have thoroughly explored the performance of other transition metals as a function of varying reaction conditions. This presentation will discuss our recent work, wherein we apply density functional theory (DFT) and experiment together to determine the condition-dependent reaction mechanisms that govern nitrite hydrogenation over Rh-based catalysts [ACS Catalysis 2020, 10 (1), 494-509]. Experiments demonstrate that Rh-based catalysts, while inactive at low pH, show moderate activity (22 L/g-surface-metal/min) and high NH₃ selectivity (>90% at 20% conversion) at high pH, which is opposite to the observed behavior of Pd-based catalysts. Furthermore, hydrazine (N₂H₄) was also detected as a minority side product during nitrite reduction over Rh, suggesting that N-N coupling reactions to value-added products may be feasible over Rh-based catalysts. The activity and selectivity of the reaction are strongly dependent on pH, and the underlying cause of this dependence is not well understood. Microkinetic models built with energetics from DFT reveal that Rh catalysts are poisoned by NO* at low pH due to the rapid dissociative adsorption of protonated nitrite (HNO₂) under acidic conditions. This prediction was confirmed experimentally by in aqua surface-enhanced Raman spectroscopy (SERS). Both theory and experiment demonstrate that the NO* poisoning of Rh lessens at increased solution pH because NO₂^- does not dissociate as readily as protonated HNO₂, which explains why Rh exhibits higher activity in basic solutions. These results update the common view that only Pd-based catalysts are effective for NO₂^- reduction and suggest new directions for unlocking unexplored avenues in nitrogen chemistry.