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.