Photoenhanced Electrochemical Conversion of Nitrate to Ammonia Via Sulfur Vacancy-Rich Exfoliated MoS2

Abstract

Nitrate ion is a common pollutant in surface and groundwater. Hence, its catalytic conversion into ammonia at ambient conditions by electrochemical and photoelectrochemical pathways is an attractive alternative to current ammonia production from the energy-intensive and high-carbon-featuring Haber-Bosch process. As such, developing highly active and product-selective catalysts with good durability and cost-effectiveness is highly desired. In this work, exfoliated MoS_2-x is reported as a highly active and selective electrocatalyst and a photoelectrocatalyst for nitrate reduction to ammonia. Exfoliation via the acid treatment of bulk MoS₂ results in exfoliated MoS_2-x, which is only a few layers thick and has a high degree of sulfur vacancies (ca. 12−13%). Electrochemical studies and electrolysis product analysis reveal promising nitrate reduction activity, which is found to be highly enhanced by the application of visible light illumination. The exfoliated MoS_2-x achieves a Faradaic efficiency of 69% with an ammonia yield rate of 5.56 mmol g_cat^–1 h^–1 in the absence of a light source, which is enhanced to 80% with an ammonia yield of 7.48 mmol g_cat^–1 h^–1 upon visible light illumination. DFT calculations support the binding of nitrate and other NO_x species to the sulfur vacancies, resulting in the formation of *N, which is then reduced to ammonia.