(Invited) Probing the Reaction Mechanism of the Electrocatalytic Reduction of Nitrate

Abstract

The nitrogen cycle is extremely important for the environmental health of the earth. Anthropogenic fixation of nitrogen such as for ammonia production through the Haber-Bosch process is causing an imbalance in the nitrogen cycle as oxidized nitrogen species such as nitrate builds up. This nitrate pollution from agricultural and industrial waste 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, bypassing or minimizing the need for the challenging molecular nitrogen fixation reaction. 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 associated with providing ammonia. 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. We will discuss how X-ray absorption spectroscopy can allow us to probe the electrocatalyst surface under reaction conditions to identify the competing effects of nitrate and adsorption coverage and explain the potential-dependence of the reaction. We will also show that using adsorption energies as descriptors, we can use density functional theory to make predictions of new electrocatalysts with higher activity for nitrate reduction. By tuning the adsorption energies with alloying, as shown by alloying Pt with Ru to increase the nitrate adsorption energy, we experimentally validate our computational predictions, and show an increase in the rates of nitrate reduction at low Ru content. We also show that the nitrate reduction rate is decreased when the Ru content is too high, and nitrate binds to the catalyst too strongly, also confirmed by our computational predictions. Our measurements of the product distribution show that this alloying does not significantly change the high faradaic efficiency towards ammonia production (>90%). By comparing the electrocatalytic results 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. We will discuss how these may be used in the future to take advantage of electrocatalysis to produce ammonia from nitrate waste. Figure 1