Mechanistic Insights into pH-Controlled Nitrite Reduction to Ammonia and Hydrazine over Rhodium

An unintended consequence of industrial nitrogen fixation through the Haber–Bosch process is nitrate (NO3–) and nitrite (NO2–) contamination of ocean, ground, and surface waters from fertilizer runoff. Transition-metal catalysts, particularly those based on Pd, are effective in removing NO3–/NO2– through reduction to N2 or NH4+. Pd is regarded as the most effective metal for NO3–/NO2– reduction, and as such, few studies have thoroughly explored the performance of other transition metals as a function of varying reaction conditions. In this work, we investigated the NO2– reduction properties of alumina-supported Rh using Pd as a benchmark, where we varied the bulk solution pH to probe the effect of reaction conditions on the catalytic chemistry. Pd expectedly showed a high reduction activity (289 L/g-surface-metal/min) and a high N2 selectivity (>99% at 20% conversion) at low pH and near inactivity at high pH. Surprisingly, the Rh catalyst, while inactive at low pH, showed moderate activity (22 L/g-surface-metal/min) and high NH4+ selectivity (>90% at 20% conversion) at high pH. Hydrazine (N2H4) was also detected as a reaction intermediate when NH4+ was formed. Microkinetic models built with energetics from density functional theory reveal that Rh catalysts are poisoned by NO* at low pH because of the rapid dissociative adsorption of protonated nitrite (HNO2) under acidic conditions, which was confirmed by in aqua surface-enhanced Raman spectroscopy. NO* poisoning of the Rh surface lessens at increased solution pH because NO2– does not dissociate as readily compared to HNO2, which explains why Rh exhibits higher activity in basic solutions. The microkinetic models further elucidate the competition between N2H4 and NH3/NH4+ formation as a function of pH, where we find that hydrogen surface coverage dictates product selectivity. These results update the common view that only Pd-based catalysts are effective for NO2– reduction and suggest unexplored avenues for nitrogen chemistry.