Hydrogen adsorption and diffusion in amorphous, metal-decorated nanoporous silica

Amorphous, nanoporous adsorbents composed of spherosilicate building blocks and incorporating isolated metal sites were investigated for their ability to adsorb and desorb hydrogen. This novel adsorbent contains cubic silicate building blocks (spherosilicate units: Si8O20), which are cross-linked by SiCl2O2 bridges and decorated with either -OTiCl3 or -OSiMe3 groups. The models for the structures were generated to describe experimentally synthesized materials, based on physical properties including density, surface area, and accessible volume. Adsorption isotherms and energies at 77 K and 300 K for pressures up to 100 bar were generated via molecular simulation describing physisorption only. The maximum gravimetric capacity of these materials is 5.8 wt% H2, occurring at 77 K and 89.8 bar. A low density (high accessible volume) material with no -OTiCl3 groups proved to be the best performing adsorbent. The presence of -OTiCl3 did not enhance physisorption even on a volumetric basis, while the high molecular weight of Ti provided a strong penalty on a gravimetric basis. Pair correlation functions illustrate that the most favorable adsorption sites for hydrogen are located in front of the faces of the spherosilicate cubes. The self-diffusivity of hydrogen was reported and found to be highly correlated with accessible volume.