First principles study of molecular hydrogen activation by defects in boron nitride

We used density functional theory simulations in combination with ab initio thermodynamics to determine the H2 partial pressure (p_("H" _"2" )) 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 for hydrogen to be recycled and recovered. For small defect sizes, we found that hydrogen preferentially bind 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 (FLP) 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_("H" _"2" )=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_("H" _"2" ) and hydrogen loading conditions were investigated for different types of defects and the finding suggests that p_("H" _"2" ) could be used to fine tune the Gibbs free energy of hydrogenation, thereby allowing several types of defects at different hydrogen loading content to play a role in the activation/recovery process of H2 in defective h-BN.