Enhanced performance of oxygen vacancy-rich In-TiO2 materials for electrocatalytic urea synthesis via a relay catalysis strategy

As an essential source of nitrogen fertilizer for agricultural production and an important raw material in the chemical industry, urea is of great importance to the sustainable development of our economy and society. Oxygen vacancy -rich In3+-doped TiO2, denoted as Vo-In-TiO2, exhibits high catalytic activity for the co -activated reduction of NO3- and CO2 to synthesize urea, with respectively Faraday efficiency (FE) and urea yield (Rurea) of 9.06 % and 759.8 mu g mg - 1h- 1 at an applied potential of -0.65 V versus reversible hydrogen electrode (vs. RHE), which can be attributed to the respective efficacy of oxygen vacancies (Vo) and In3+ and their synergistic effect through relay catalysis. Both experimental and theoretical investigations show that the doped In3+ endows the catalyst with a strong ability to capture CO2, while the Vo acts directly as active site to effectively facilitate the adsorption of *NO3 and further conversion of *NO2, and shows high activity for the NO3- reduction reaction (NO3- RR). That is, Vo reduces NO3- to *NH2, and In3+ plays a role in inducing the direct combination of *NH2 and *CO2 adsorbed on the surface of the catalyst to form the key intermediate *CO2NH2 and thus urea. This strategy of relay catalysis can make full use of the advantages of different parts of the catalyst to achieve a synergistic effect, providing a new approach to the design of highly efficient catalysts.