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

• Reaction mechanism of nitrate reduction on partially reduced In 2 O 3 was revealed. • The catalytic activities were compared with varied monolayers of oxygen vacancy. • Analysis conducted to study the activity difference of partially reduced In 2 O 3 . • Competing reactions were found suppressed on partially reduced In 2 O 3 . The electrocatalytic nitrate reduction reaction (NO 3 RR) 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 NO 3 RR. Based on first-principles calculations, the partially reduced In 2 O 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 In 2 O 3 with 2 ML of oxygen vacancies exhibits the highest NO 3 RR 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 In 2 O 3 metallic, thus promoting the electron transfer for improved performance of NO 3 RR. 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 NO 3 RR activity, thus highlighting vacancy engineering and providing novel insights into the design of NO 3 RR catalysts.