A work function change study of oxygen adsorption on Pt(111) and Pt(100)

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Abstract
Oxygen adsorption on Pt(111) and Pt(100) was investigated using a Kelvin probe to measure adsorbate induced work function changes. The variation of Δφ with θ was linear (p=0.103 D/atom) on Pt(111) even up to θmax∼7×1014 atoms cm−2. On Pt(100), there was an abrupt increase in Δφ at low coverage (14 atom cm−2) which was presumed to be adsorption at defect sites (p=0.62 D/atom). Sticking coefficients were calculated from the rate of change of the work function with time. For room temperature adsorption on Pt(111), the sticking coefficient followed the functional form S0(1−ϑ)2, ϑ=θ/θmax, and S0=0.038. On Pt(100) the functional dependence was more complex with an initial increase in S (from S0=0.042) to a ϑ of ∼0.1, then an approximately quadratic decrease as the coverage increased to saturation. The maximum coverage observed in room temperatue dosing of either surface was ∼3×1014. However, dosing at 200–300 °C at 10−5–10−4 Torr induced coverages as high as (7–9)×1014 on both surfaces. The isosteric heat of adsorption was measured from equilibrium isotherms and found to be 232±36 kJ mol−1 with a coverage dependence less than the experimental precision. For Pt(111), it is postulated that population of the high coverage state proceeds via the activated process of direct dissociation, whereas the unactivated room temperature process is via a molecularly adsorbed precursor. For Pt(100), population of the high coverage state involves activated transitions in the Pt surface structure which require further study.