H2adsorption

H2 adsorption refers to the process by which hydrogen molecules adhere to a solid surface. It is distinct from absorption into the bulk and from chemical reaction, and it encompasses both physisorption and chemisorption of hydrogen on materials.

Physisorption involves weak, noncovalent interactions (van der Waals forces) between H2 and the surface. This type of adsorption is typically exothermic with relatively low enthalpies (on the order of a few kilojoules per mole) and is highly dependent on temperature and surface area. It is favored at low temperatures and on materials with high surface area and appropriate porosity, such as activated carbons, graphene-based materials, and certain metal-organic frameworks (MOFs).

Chemisorption involves the formation of chemical bonds between hydrogen and surface atoms, often in a dissociative manner in which H2 splits into H atoms that attach to sites on the surface. Chemisorption usually features higher adsorption enthalpies, can be slower, and is more likely to be less reversible than physisorption, depending on the system. Catalytic metal sites can promote dissociative adsorption, and spillover effects have been discussed in some contexts as facilitating hydrogen migration from metal sites to a support.

Materials used for H2 adsorption include porous solids such as MOFs, zeolites, covalent organic frameworks, and various carbon-based materials. These materials are studied to maximize hydrogen uptake under practical temperatures and pressures for storage and delivery in fuel-cell applications. Adsorption behavior is commonly analyzed with isotherm models (e.g., Langmuir, BET) to estimate surface area, capacity, and the nature of adsorption processes. Key challenges include achieving high reversible storage at ambient temperatures and understanding the balance between physisorption and chemisorption in different materials.