Theoretical Explanation of Hydrogen Spillover in Metal-Organic Frameworks

The thermodynamics and kinetics of hydrogen spillover in the IRMOF-1 (MOF-5) structure were studied via density functional theory. Addition of six H atoms is thermodynamically favored with respect to molecular H-2, which is close to the maximum capacity that has been observed experimentally by spillover in IRMOF-1 (similar to 2.6 wt % excess). Through modeling, mechanistic details of the spillover process are proposed: During hydrogen spillover, atomic hydrogen cannot migrate in the chemisorbed state in the pore network but rather diffuses in the gas phase. During diffusion, atomic hydrogen can stick on all available binding sites on the linkers with negligible energy barriers. However, diffusing hydrogen atoms can also abstract adsorbed hydrogen atoms via Eley-Rideal pathways to form H-2. Diffusion of atomic H in the MOP is greatly assisted by pore defects, and spillover would be plugged in ideal MOP structures. Experimental observations for spillover in MOFs are discussed in view of these results, and possible ways of enhancing spillover capacities are presented.