Active Sites on Titanium and Hydrogen-Covered Transition Metal Surfaces for Nitrate Reduction

The electrochemical reduction of nitrate (NO3RR) to ammonia on late transition metals is in competition with the hydrogen evolution reaction (HER), where adsorbed H* competes with nitrate and other intermediates for sites on the catalytic surface. As an early transition metal, titanium is also known to be capable of reducing nitrate to ammonia, whilst under conditions that favor the formation of a surface metal hydride. In this work, we rationalize a mechanism for the NO3RR via a pristine Ti surface. We show how strongly bound O* and OH* may play a role in catalytic reactions such as HER and nitrate reduction at early applied potentials and how the transformation of titanium-to-titanium hydride can elucidate selectivity trends in nitrate reduction across applied potentials. During nitrate reduction, titanium is capable of adsorbing NO2 and NO in a prone state; for NO*, this enables breakage of the final N–O bond without resorting to undercoordinated sites. The primary cost is the additional overpotential required for reducing O* from a strongly bound state back into water. The ability to break N–O bonds allows titanium hydride to selectively reduce O* and N* into water and ammonia even with a full monolayer of hydrogen. This leads us to rationalize that the fcc 3-fold site is key to nitrate reduction on early transition metals. We use scaling relations to form voltage-dependent selectivity maps and pinpoint adsorption regimes where metals would have facile NO dissociation.