Kinetics and mechanistic model for hydrogen spillover on bridged metal-organic frameworks

The kinetics of hydrogen adsorption on IRMOF-8 by hydrogen spillover from a Pt/AC catalyst (activated carbon support) was studied at various pressures (0-100 atm) and temperatures (273-348 K). The sorbent studied was a mixture of IRMOF-8 and Pt/AC with added carbon bridges. The overall isosteric heats of adsorption (-25 to -20 kJ/mol) and the apparent activation energy for surface diffusion (9.3 kJ/mol) were determined from the temperature dependence of respectively equilibrium isotherm and diffusion time constant. Ab initio molecular orbital calculations were performed for the bonding energies between the spillover hydrogen atom and various sites on the IRMOF structure. The overall heats of adsorption fell reasonably within the range of bond energies. Surface diffusion of the spillover hydrogen at near ambienl temperature was a slow process, which resulted in slow hydrogen uptake rates especially at higher pressures (higher surface concentrations). Desorption rates were relatively faster, and also decreased with increasing pressure or surface concentration. Desorption appeared to follow a reverse spillover process. Finally, a simple mechanistic model was formulated for the equilibrium isotherm for spillover hydrogen. The model is capable of interpreting various shapes of isotherms, i.e., concave, convex, of nearly linear isotherms. The declining diffusion lime constant (D/a²) with surface concentration provided strong evidence that the adsorbed hydrogen served as bridges between the catalyst and the IRMOF receptor thereby causing further spillover.