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 N-2 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.