Effect of metal catalyst on the mechanism of hydrogen spillover in three-dimensional covalent-organic frameworks

Abstract

Hydrogen spillover mechanism of metal-supported covalent-organic frameworks COF-105 is investigated by means of the density functional theory, and the effects of metal catalysts M-4 (Pt-4, Pd-4, and Ni-4) on the whole spillover process are systematically analyzed. These three metal catalysts exhibit several similar phenomena: (i) they prefer to deposit on the tetra (4-dihydroxyborylphenyl) silane (TBPS) cluster with surface-contacted configuration; (ii) only the H atoms at the bridge site can migrate to 2,3,6,7,10,11-hexahydroxy triphenylene (HHTP) and TBPS surfaces, and the migration process is an endothermic reaction and not stable; (iii) the introduction of M-4 catalyst can greatly reduce the diffusion energy barrier of H atoms, which makes it easier for the H atoms to diffuse on the substrate surface. Differently, all of the H-2 molecules spontaneously dissociate into H atoms onto Pt-4 and Pd-4 clusters. However, the adsorbed H-2 molecules on Ni-4 cluster show two types of adsorption states: one activated state with stretched H-H bond length of 0.88 angstrom via the Kubas interaction and five dissociated states with separated hydrogen atoms. Among all the M-4 catalysts, the orders of the binding energy of M-4 deposited on the substrate and average chemisorption energy per H-2 molecule are Pt-4 > Ni-4 > Pd-4. On the contrary, the orders of the migration and diffusion barriers of H atoms are Pt-4 < Ni-4 < Pd-4, which indicates that Pt-4 is the most promising catalyst for the hydrogen spillover with the lowest migration and diffusion energy barriers. However, the migration of H atoms from Pt-4 toward the substrate is still endothermic. Thus direct migration of H atom from metal catalyst toward the substrate is thermodynamically unfavorable.