By: Sarah Ackermann, MSc
IUPAC defines absorption as “The process of one material (absorbate) being retained by another (absorbent); this may be the physical solution of a gas, liquid, or solid in a liquid, attachment of molecules of a gas, vapour, liquid, or dissolved substance to a solid surface by physical forces, etc” It’s explicitly a very general term that includes the technical definition of adsorption. Adsorption is the subset of absorption that considers surface interactions, like when water droplets stick to glass.
In the context of metal hydrides, the sorption typically happens physically at the surface, where the hydrogen molecule interacts with the metal in a van der Waals type of interaction. Then, the chemical bond is formed with the metal and broken in the hydrogen simultaneously in a reduction reaction (organic chemists would refer to it as a “nucleophilic attack,” while inorganic chemists would say the metal is acting as a Lewis base – IMO this is different jargonese for the same chemical phenomenon).
The hydride is then transported further within the HSM due to Le Chatelier’s Principle (the high concentration of hydrogen outside encourages the migration of the hydrides further into the material to allow more reaction with the hydrogen). So, in the context of metal hydrides, the correct term can be adsorption or absorption – but you could also be more precise and refer to it as chemisorption.
However, it’d be absorption or adsorption if we were talking about molecular sieves, zeolites, or MOFs (also physisorption but not chemisorption), and absorption (specifically physisorption) but not adsorption if we were talking about hydrogen gas dissolving in a liquid solvent without a chemical reaction.
Then, to make things a bit more complicated – some chemistry fields have historically made a distinction between adsorption being gas-solid and absorption being liquid-solid or gas-liquid. IUPAC doesn’t support this, but this distinction is in some older texts and papers. This older-fashioned definition of absorption has since been depreciated in favor of the newer version.
TAKEAWAY
Both terms are acceptable in the context of hydrogen storage in metal hydrides – and you could also use chemisorption.
References
1. ‘absorption’ in IUPAC Compendium of Chemical Terminology, 3rd ed. International Union of Pure and Applied Chemistry; 2006. Online version 3.0.1, 2019. https://doi.org/10.1351/goldbook.A00036
2. ‘adsorption’ in IUPAC Compendium of Chemical Terminology, 3rd ed. International Union of Pure and Applied Chemistry; 2006. Online version 3.0.1, 2019. https://doi.org/10.1351/goldbook.A00155
3. ‘chemisorption’ in IUPAC Compendium of Chemical Terminology, 3rd ed. International Union of Pure and Applied Chemistry; 2006. Online version 3.0.1, 2019. https://doi.org/10.1351/goldbook.C01048
4. ‘physisorption’ in IUPAC Compendium of Chemical Terminology, 3rd ed. International Union of Pure and Applied Chemistry; 2006. Online version 3.0.1, 2019. https://doi.org/10.1351/goldbook.P04667
About the Author
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Sarah Ackermann is the Laboratory Services Manager of the Thermal Analysis Labs division. She has over a decade of experience working in thermal analysis on a diverse range of materials, from pyrophorics to phase change materials and nearly everything in between.
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