Theoretical insights of nitrate reduction to ammonia on partially reduced In2O3 surfaces

Abstract

The electrocatalytic nitrate reduction reaction (NO₃RR) has attracted much attention due to the formation of value-added ammonia as well as being an environmentally benign process. However, there is still a lack of high-performance electrocatalysts and an in-depth understanding of the electrocatalytic reaction mechanism for NO₃RR. Based on first-principles calculations, the partially reduced In₂O_3-x catalysts with different monolayers (MLs) of oxygen vacancies were examined for the electrochemical conversion of nitrate to ammonia. The thermodynamically favourable pathways were identified for the screened candidates with various MLs of oxygen vacancies from 0 to 3, and the Gibbs free energy evolution of the 2 MLs of oxygen vacancies was downhill. The catalysts’ performance is highly associated with the oxygen vacancy layers, and In₂O₃ with 2 ML of oxygen vacancies exhibits the highest NO₃RR activity. The introduction of oxygen vacancies can enhance the interfacial charge density around In active sites. The PDOS comparison between 0 ML and 2 MLs unravelled that the oxygen vacancies can downshift the overall orbitals and make the defective In₂O₃ metallic, thus promoting the electron transfer for improved performance of NO₃RR. Meanwhile, the competing hydrogen evolution reaction (HER) is effectively inhibited. This work not only proposes a high-performance electrocatalyst for ammonia production but also reveals the relationship between the layer number of oxygen vacancies and the NO₃RR activity, thus highlighting vacancy engineering and providing novel insights into the design of NO₃RR catalysts.