Reduction of NOx on metal-free hydrogenated hexagonal boron nitride

Abstract

Sustainable catalysts are essential for critical industrial and environmental processes. 2D materials have exceptional surface area and unique thermal and electronic properties, making them excellent candidates for catalytic applications. Moreover, 2D materials can be functionalised to create metal-free active sites, which provide sustainable alternatives to transition and precious metals. Among the pollutants emitted by combustion engines, NO_x stands out as one of the most detrimental gases, contributing to environmental pollution and posing risks to human health. We demonstrate that functionalised defects in hexagonal boron nitride (hBN) provide a thermodynamically viable route to removing NO_x by reaction with a hydrogenated boron vacancy (3HV_B). The decomposition of NO₂ proceeds by initially overcoming an activation energy barrier of 1.12 eV to transfer a hydrogen atom from the surface, forming a NO₂H species, followed by the elimination of a water molecule. A thermodynamically favourable product consisting of a surface-bound hydroxyl adjacent to a nitrogen antisite defect (where a nitrogen atom occupies a site typically occupied by a boron atom) forms after overcoming an energy barrier of 1.28 eV. NO can further decompose by overcoming an activation energy barrier of 2.23 eV to form a surface HNO species. A rearrangement of the HNO species takes place with an activation energy of 1.96 eV, followed by the elimination of water. The overall reactions reduce NO_x into defective hBN and H₂O.