The adsorption rate for O2 on Rh(111) at 335 K exhibits Langmuir (1 - θ) kinetics rather than the expected (1 - θ)2 dependence. Adsorption occurs with a sticking coefficient near unity on empty sites, producing a mixture of ordered and disordered domains. At θ ≤ 0.5, the intensity of the(1, 1 2)overlayer beam increases in proportion to θ2 as expected for ordered oxygen island formation. Extensive ordering occurs only in the very last stages of monolayer formation. The reaction of the ordered-O layer with H2(g) to produce H2O takes place rapidly above 275 K in the 10-8-10-6 Torr range and can be followed quantitatively by observation of the(1, 1 2) LEED beam intensity. The reaction accurately exhibits kinetics which are first order in H2 pressure, implying an efficient trapping mechanism for adsorbed hydrogen on the ordered-O layer. The activation energy for reaction is5.3 ± 0.3 kcal mole; each reaction event leads to a loss of order from a ∼-300 Å2 domain. This suggests that near saturation coverage, extensive disordering occurs for small decreases in ordered oxygen coverage during reaction with Hz(g) and is consistent with the massive ordering behavior seen in the final stages of oxygen adsorption. The reaction of H2(g) with disordered oxygen species follows approximate half-order kinetics in PH2 as expected for a mechanism which does not involve hydrogen atom trapping. © 1979.
