The Electrocatalytic Role of Oxygen Vacancy in Nitrate Reduction Reactions

Ammonia, with high energy density and easy transportation, holds significant potential to become an integral part of future energy systems. Among tremendous strategies, electrocatalytic ammonia production is no doubt an efficient and eco-friendly method. One particularly intriguing class of electrocatalysts for reducing nitrate to ammonia is transition metal oxides, which have been heavily researched. However, how these catalysts' oxygen vacancy (V_O) affects their performance remains elusive. To address this, taking titania (the most important catalyst) as an example, we carried out experimental investigations and simulations. Contrary to the prevailing belief that the concentrated V_O would increase the catalytic efficiency of nitrate reduction, it was found that a relatively low level of V_O is favorable for maximizing catalytic efficiency. At low cathodic voltages, titania with minimal V_O delivered both the highest reduction efficiency and the best selectivity among the different titania samples in this paper. In addition to outlining the merits of lower electron transfer resistance and accelerated reaction dynamics, we also put forth a previously unmentioned factor, the adsorption of hydrogen or the creation of an ordered hydrogen bond network, which put up a hydrogen-rich atmosphere for following nitrate reduction. Further simulation study revealed that within the hydrogen-rich atmosphere isolated V_O serves as the ideal active center to enable the lowest energy barriers for the reduction of nitrate into ammonia. These findings offer fresh insights into the working mechanism of oxide-based electrocatalysts for ammonia production.