(Invited) Understanding Trends for Electrocatalytic and Catalytic Nitrate Reduction

Abstract

Fixation of nitrogen such as for ammonia production through the Haber-Bosch process is imperative for feeding the world’s population, but is causing an imbalance in the nitrogen cycle as oxidized nitrogen species such as nitrate builds up. This build up of nitrate pollution from agricultural and industrial waste poses an immediate threat to environmental and human health. While separation of this nitrate is a method to produce clean water, it does not chemically address the issue of oxidized nitrate. In addition, it does not form nitrogen species in a valuable form. One method is to use nitrate waste as a feedstock for production of fertilizers such as ammonia or ammonium nitrate. 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 carbon dioxide emissions from current methods to produce ammonia. In this talk we will discuss (1) trends in electrocatalytic nitrate reduction, (2) similarities and differences in electrocatalytic nitrate reduction and catalytic nitrate reduction (i.e., using molecular H2 rather than an applied potential), and (3) practical challenges in systems to commercially convert nitrate. 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 are descriptors for nitrate reduction on metals and alloy surfaces. We will discuss the importance of operando spectroscopy to understand the catalyst under reaction conditions. We will show how alloying Pt with Ru to increase the nitrate adsorption energy shows an increase in the rates of nitrate reduction at low Ru content. We will also show that the nitrate reduction rate is decreased when the Ru content is too high, because nitrate binds to the catalyst too strongly. We will compare the electrocatalytic results to thermal nitrate catalytic and show that while there are numerous similarities between electrocatalysis and thermal catalysis, there are also factors unique to electrocatalysis that impact the rate of reaction. These differences motivate a greater understanding of electrochemical steps and the effect of the applied potential, solvent, and ions. We will discuss how the best performing catalysts in batch systems translate to flow reactors more amenable to commercialized processes and other challenges in realistic nitrate streams containing potential catalyst poisons such as chlorides.