(437b) Controlling Electrocatalytic Nitrate Reduction to Ammonia through Adsorption Energies

Nitrate pollution from agricultural and industrial waste is causing an imbalance in the nitrogen cycle and poses an immediate threat to environmental and human health. Although there are numerous methods to treat this nitrate, the cost of treatment is often prohibitive, which could potentially be addressed by converting the waste nitrate to a valuable product such as ammonia. In this work we discuss the use of electrocatalysis to convert nitrate to ammonia, where renewable electricity can be used to drive the reaction, minimizing the issue of emissions of carbon dioxide. A major challenge with electrocatalytic nitrate reduction is decreasing the overvoltage of the reaction and increasing the reaction rate without losing faradaic efficiency towards the desired product. Through the use of kinetic studies, in situ spectroscopy, microkinetic modeling, and density functional theory calculations, we show how the adsorption energetics of nitrate and hydrogen can be used to understand the rates of nitrate reduction on metals and metal sulfide surfaces. By tuning these adsorption energies with alloying, as shown below by alloying Pt with Ru, we can increase the rates of nitrate reduction. Our measurements of the product distribution show that this alloying does not significantly change the high faradaic efficiency towards ammonia production (>90%). By comparing to thermal nitrate catalytic reduction (i.e., using molecular H2 rather than an applied potential to reduce nitrate) we identify numerous similarities between electrocatalysis and thermal catalysis, but also specific factors unique to electrocatalysis that impact the rate of reaction.