(697f) Modeling Acidic Nitrate Reduction on Early Transition Metal Surfaces

The electrochemical reduction of nitrate (NO₃R) to ammonia becomes in competition with the hydrogen evolution reaction (HER) when adsorbed H* compete for sites on late transition metals. In this work, we will show that for early transition metals like titanium, NO₃R can occur even with a full monolayer of hydrogen. We rationalize that the alternate hcp-threefold site is key to nitrate reduction on early transition metals. During nitrate reduction, these metals are capable of adsorbing NO₂ and NO in a prone state; for NO*, this enables breakage of the final N-O bond without resorting to under-coordinated sites. The primary cost is the additional overpotential required for reducing O* from a strongly bound state back into water. We use scaling relations to pinpoint regions where the metal hydride surfaces would have facile NO dissociation. We perform microkinetic modeling to describe the reduction of nitrate over hydride surfaces, to show that the optimal peak for these family of materials will vary depending on when electrochemical reactions actually takes place. Finally, we present a mechanism for NO₃R via a oxide covered Ti surface to show how even with a monolayer of strongly bound O*, the formation OH* on the surface can play a catalytic role in reactions such as HER and nitrate reduction at early applied potentials. The transformation of titanium to titanium hydride is also discussed.