Surface Oxygen and Chemical Reactivity
The probe-molecule or coadsorption approach has emphasised some unique aspects of the chemistry of dioxygen at metal surfaces at low temperatures. Models have been developed making use of XP and HREEL spectroscopies for the chemisorption of dioxygen where the chemical reactivity is dominated by the transients Qδ-(s) and Oδ-(s) - which of these controls the chemistry depending on the metal. By comparison the oxide overlayer is unreactive under the same conditions.
Dynamic or real time studies provided the evidence for the transient or precursor states and estimates have been made of their surface life-times and concentrations with support for the concepts coming from recent scanning tunnelling microscope results. The charge on the surface oxygen and whether it exists as isolated atoms or as a component of an ‘oxide’ island - even at low coverage - are suggested to be the crucial factors in determining its chemistry. Theoretical calculations support this view and analogies drawn with ‘defective’ oxide surfaces where both O- and O2- -like species can exist.
KeywordsDioxide Hydroxyl Enthalpy Recombination Catalysis
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