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

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 M₄ (Pt₄, Pd₄, and Ni₄) 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 M₄ 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 H₂ molecules spontaneously dissociate into H atoms onto Pt₄ and Pd₄ clusters. However, the adsorbed H₂ molecules on Ni₄ cluster show two types of adsorption states: one activated state with stretched H-H bond length of 0.88 Å via the Kubas interaction and five dissociated states with separated hydrogen atoms. Among all the M₄ catalysts, the orders of the binding energy of M₄ deposited on the substrate and average chemisorption energy per H₂ molecule are Pt₄ > Ni₄ > Pd₄. On the contrary, the orders of the migration and diffusion barriers of H atoms are Pt₄ < Ni₄ < Pd₄, which indicates that Pt₄ is the most promising catalyst for the hydrogen spillover with the lowest migration and diffusion energy barriers. However, the migration of H atoms from Pt₄ toward the substrate is still endothermic. Thus direct migration of H atom from metal catalyst toward the substrate is thermodynamically unfavorable.