aOH

An adsorbed hydroxyl group, abbreviated aOH, refers to a hydroxyl species that is chemically bound to a solid surface, typically on oxide materials, rather than existing as a free hydroxide ion in solution. aOH differs from OH− in the surrounding liquid phase and from lattice hydroxyls that are part of the crystal structure; it represents a surface-bound, often protonated, moiety formed by interactions with water or hydroxyl-bearing precursors on the surface.

Formation and types

aOH on metal oxide surfaces commonly forms by dissociative adsorption or deprotonation of water: H2O on a coordinatively unsaturated site can yield MOH (terminal aOH) and a neighboring site bound to H. On many surfaces, two main configurations are observed: terminal (μ1-OH) attached to a single metal center, and bridging (μ2-OH) connected to two adjacent metal centers. The distribution and stability of these species depend on surface structure, defect density, temperature, and the solution environment, including pH and solvent.

Detection and properties

aOH can be characterized by vibrational spectroscopy, with O–H stretching bands typically found in the region of 3600–3700 cm−1 for terminal hydroxyls and lower frequencies for bridging species. Their acidity and basicity can vary with the local coordination environment, making them capable of donating or accepting protons and participating in surface redox reactions. aOH participate in proton transfer, adsorption of molecules, and can act as precursors to reactive oxygen species under photoexcitation or oxidative conditions.

Roles and applications

In catalysis and photocatalysis, aOH influence the adsorption of organics, water dissociation, and the formation of reactive species such as hydroxyl radicals. Coverage and reactivity of aOH on surfaces like TiO2, Al2O3, Fe2O3, and CeO2 affect reaction pathways in environmental remediation, fuel cells, and solar-to-chemical conversions. Their behavior is often linked to surface facets, defect sites, and operating pH.

See also

Hydroxyl group, surface chemistry of metal oxides, photocatalysis, adsorption phenomena.