The adsorption and desorption of oxygen on the Pt(110) surface; A thermal desorption and LEED/AES study

The interaction of oxygen with a Pt(110) crystal surface has been investigated by thermal desorption mass spectroscopy, LEED and AES. Adsorption at room temperature produces a β-state which desorbs at ~800 K. Complete isotopic mixing occurs in desorption from this state and it populates with a sticking probability which varies as (1 − θ)2, both observations consistent with dissociative adsorption. The desorption is second order at low coverage but becomes first order at high coverage. The saturationcoverage is 3.5 × 1014 mol cm−2. The spectra have been computer analysed to determine the fraction desorbing by first (β1) and second (β2) order kinetics as a function of total fractional coverage θ using this fraction as the only adjustable parameter. The β1 desorption commences at θ ∼ 0.25 and β1 and β2 contribute equally to the desorption at saturation. The kinetic parameters for β1 desorption were calculated from the variation of peak temperature with heating rate as ν1 = 1.7 × 109 s−1 and E≠1 = 32 kcal mole−1 whereas two different methods of analysis gave consistent parameters ν2 = 6.5 × 10−7 cm2 mol−1 s−1 and E≠2 = 29 and 30 kcal mole−1 for β2 desorption. The kinetics of desorptior are discussed in terms of the statistics for occupation of near neighbour sites. While many fea tures of the results are consistent with this picture, it is concluded that simple models considering either completely mobile or immobile adlayers with either strong or zero adatom repulsion are not completely satisfactory. The thermal desorption surface coverage has been correlated with the AES measurements and it has been possible to use the AES data for PtO as an internal standard for calibration of the AES oxygen coverage determination. At low temperature (170 K) oxygen populates an additional molecular α-state. Adsorption into the α- and β-states is competitive for the same sites and pre-saturation of the β-state at 300 K excludes the α-state. This, together with the AES observation that the adsorption is enhanced and faster at 450 than 325 K suggests a low activation energy for adsorption into the β-state.