Buffer Assists Electrocatalytic Nitrite Reduction by a Cobalt Macrocycle Complex

This work reports a combined experimental and computational
study
of the activation of an otherwise catalytically inactive cobalt complex,
[Co­(TIM)­Br2]+, for aqueous nitrite reduction.
The presence of phosphate buffer leads to efficient electrocatalysis,
with rapid reduction to ammonium occurring close to the thermodynamic
potential and with high Faradaic efficiency. At neutral pH, increasing
buffer concentrations increase catalytic current while simultaneously
decreasing overpotential, although high concentrations have an inhibitory
effect. Controlled potential electrolysis and rotating ring-disk electrode
experiments indicate that ammonium is directly produced from nitrite
by [Co­(TIM)­Br2]+, along with hydroxylamine.
Mechanistic investigations implicate a vital role for the phosphate
buffer, specifically as a proton shuttle, although high buffer concentrations
inhibit catalysis. These results indicate a role for buffer in the
design of electrocatalysts for nitrogen oxide conversion.