Dehydrogenation of ammonia on free-standing and epitaxial hexagonal boron nitride

Abstract

We report a thermodynamically feasible mechanism for producing H₂ from NH₃ using hBN as a catalyst. 2D catalysts have exceptional surface areas with unique thermal and electronic properties suited for catalysis. Metal-free, 2D catalysts, are highly desirable materials that can be more sustainable than the ubiquitously employed precious and transition metal-based catalysts. Here, using density functional theory (DFT) calculations, we demonstrate that metal-free hexagonal boron nitride (hBN) is a valid alternative to precious metal catalysts for producing H₂via reaction of ammonia with a boron and nitrogen divacancy (V_BN). Our results show that the decomposition of ammonia proceeds on monolayer hBN with an activation energy barrier of 0.52 eV. Furthermore, the reaction of ammonia with epitaxially grown hBN on a Ru(0001) substrate was investigated, and we observed similar NH₃ decomposition energy barriers (0.61 eV), but a much more facile H₂ associative desorption barrier (0.69 eV vs 5.89 eV). H₂ generation from the free-standing monolayer would instead occur through a diffusion process with an energy barrier of 3.36 eV. A detailed analysis of the electron density and charge distribution along the reaction pathways was carried out to rationalise the substrate effects on the catalytic reaction.