Nitrates Salts Converting by N-doped Carbon–iron Heterointerfaces

Nitrate salts are salts formed by the reaction of HNO₃ with metals and composed of metal ions (or ammonium ions) and nitrate ions. Common nitrates include sodium nitrate, potassium nitrate, ammonium nitrate, calcium nitrate, lead nitrate, cerium nitrate, etc. The nitrate ion is composed of one nitrogen atom and three oxygen atoms, and is usually represented by the symbol NO^3-. The electrochemical conversion of nitrate to ammonia is an important topic in green chemistry. This provides a sustainable pathway to restore the balance of the global nitrogen cycle and provides technical support in terms of environmental and economic impacts of sustainable ammonia synthesis. However, developing electrode materials with the advantages of low cost, high activity, and selectivity is a key challenge for research in this field.

Recently, researchers have constructed catalysts with new active sites by doping nitrogen, which activates adjacent carbon atoms and enhances the electron transfer from metal to carbon. Thus resulting in high catalytic activity, a metal-organic framework (MOF) material-derived nitrogen-doped carbon-iron heterostructure (Fe@N₁₀-C) electrocatalyst was developed for the electrochemical conversion of nitrate to ammonia. . The ammonia selectivity in the study was close to 100% (99.7 ± 0.1%).

The study uses synchrotron radiation to analyze the valence states and coordination environments of Fe, N, and C sites in the Fe@N_x-C catalyst, which further proves that nitrogen-doped carbon structures are formed during the pyrolysis process. After pyrolysis, the nitrogen species in the Fe@N_x-C catalyst is mainly graphitic nitrogen. The main active site of Fe@N_x-C is the C active site (CN) adjacent to the N site. The enhanced activity of the Fe@N₁₀-C catalyst is attributed to the moderate nitrogen dopant in the carbon layer around the Fe NPs.

To delve into the role of nitrogen element in activating C atoms and enhancing the electrochemical nitrate reduction activity of Fe@N₁₀-C, density functional theory calculations were performed in this study. The active sites of three representative models, Fe@N, Fe@N₁₀-C, and Fe@N₂₀-C, were revealed by differential charge maps. It was also confirmed that the charge transfer occurred at the CN site.

Subsequently, the effect of N doping on the electronic structure was investigated. There is more obvious charge accumulation at CN sites, which indicates that N doping increases the charge density and thus promotes the adsorption of NO^3-. Bader charge analysis also demonstrated that the CN site showed greater charge transfer, further suggesting that the CN is the active site. To better understand the activation of C atoms and the origin of the activity of Fe@N₁₀-C catalysts, we investigated the interaction between Fe and N-doped carbon. The differential charge map shows that N doping affects the electron transfer from Fe NPs to N-doped carbon, leading to the activation of C atoms, which further affects the adsorption of reactant molecules and improves the reaction kinetics.

Reference

1. N-doped carbon–iron heterointerfaces for boosted electrocatalytic active and selective ammonia production.
   Proceedings of the National Academy of Sciences, 120(3), e2207080119.