Integrating Pd(I) Atoms in Schottky Junctions for Visible-Light-Driven Urea Production from Ambient Nitrogenous Species and CO 2

Abstract

The mass production of urea, the most widely used agricultural fertilizer, usually relies on energy- and carbon-intensive processes. The light-driven synthesis path has great potential for more sustainable techniques to produce urea from abundant naturally occurring resources, but this method suffers from the use of pure CO₂ and high-energy-driven forces (high temperature or ultraviolet light). Herein, we present a mild photocatalytic pathway for urea production from diverse nitrogenous species (such as NO₃⁻, NH₃, and N₂) and diluted CO₂ using visible light. We designed a palladium (Pd)-doped Schottky heterojunction, composed of graphene and titanium dioxide, as an effective photocatalyst to achieve on-farm urea generation. With the injection of visible-light-generated hot electrons from graphene to TiO₂, the as-integrated Pd(I) centers with a special oxidation state of +1.36 in the TiO₂ lattice can initiate the universal reaction path of cascade reduction of NO₃⁻/N₂ to NH₃ and resulting C–N coupling of CO₂ with as-formed or added NH₃ to transform diverse nitrogenous species and CO₂ into urea. The urea yield over the at.-Pd@TiO₂/Gr photocatalyst is 1.62 mmol g⁻¹ h⁻¹ under visible-light irradiation with an apparent quantum yield of 1.05% at 400 nm and 0.39% even at 700 nm.