Dynamics of OH desorption from single crystal Pt(111) and polycrystalline Pt foil surfaces

The laser-induced fluorescence technique in conjunction with an UHV apparatus was used to probe the desorption dynamics of OH radicals formed in the association reaction of chemisorbed O and H atoms on single crystal Pt(111) and polycrystalline Pt foil surfaces. The apparent activation energies of desorption and the rotational energy distributions of OH radicals were measured in these experiments. The apparent OH desorption energies for both surfaces were observed to increase by about 10 kcal/mol as the O/H reactant gas mixture ratio is decreased by a factor of 100, with the values for the Pt(111) single crystal at 5-10 kcal/mol below those for the Pt foil. Computer kinetic modeling of the temperature dependence of OH desorption rate as a function of O/H ratio showed similar trends as those observed in the experiments. Boltzmann-like OH rotational energy distributions with relatively high rotational to surface temperature ratios, T_r/T_s=0.85, were measured over the temperature range 1227-1479 K for the Pt(111) single crystal and over 1283-1475 K for the polycrystalline Pt foil. For both surfaces the OH spin-orbit temperatures were essentially the same as the rotational temperatures and no preference for any lambda-doublet state was observed. The T_r/T_s ratio does not seem to depend on the O/H reactant gas mixture ratio for each surface, suggesting that the rotational energy accommodation is insensitive to the apparent Pt-OH bond strength.