First-Principles Study of Molecular Hydrogen Activation by Defects in Boron Nitride

Abstract

We used density functional theory simulations in combination with ab initio thermodynamics to determine the H₂ partial pressure (p_H2)-dependent energetics associated with H₂ activation and recovery at various defect sites in hexagonal boron nitride (h-BN). We found that some defects are very reactive with hydrogen, thereby definitely trapping hydrogen in defective h-BN. However, depending on hydrogen partial pressure, less reactive defect sites can be populated. Because of the lower binding capability of these sites, they would allow hydrogen to be recycled and recovered. For small defect sizes, we found that hydrogen preferentially binds to nitrogen sites by forming N–H bonds, and if no N sites are available then boron sites would be the next to bind hydrogen. Hydrogen dissociation via frustrated Lewis pair is found to be more favorable than forming only N–H bonds but only if the defect size is large enough to accommodate steric effects. For specific conditions such as T = 400 K, p_H2 = 1 bar, and only considering one molecular H₂ per defect, three defects, namely, the N monovacancy, 3V(1B2N), and hexagonal 6V(3B3N) could play a role in both the activation and recycling of H₂ as they would be reacting enough to allow a favorable splitting of H₂ while not binding too strongly to allow its recovery. More broadly, a range of p_H2 and hydrogen loading conditions were investigated for different types of defects and the finding suggests that p_H2 could be used to fine-tune the Gibbs free energy of hydrogenation, thereby allowing several types of defects at different hydrogen loading contents to play a role in the activation/recovery process of H₂ in defective h-BN.