Steps in the reaction hydrogen + oxygen .dblharw. water on platinum(111): laser-induced thermal desorption at low temperature

Abstract

The low-temperature reactions of coadsorbed hydrogen and oxygen to form adsorbed water on Pt(111) have been characterized using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD). Hydrogen and oxygen adatoms were reacted in the temperature range 110-145 K, which is below the water desorption temperature (180 K), for varying initial hydrogen coverages and an initial saturation p(2x2)oxygen layers. LITD was used to monitor the coverages of product water molecules and reactant hydrogen adatoms by measuring the H{sub 2} and H{sub 2} LITD yields calibrated against known H, O, and H{sub 2}O coverages, both before the onset of the reaction when only hydrogen and oxygen adatoms were present and also after reacting the two species at higher temperatures where a mixture of hydrogen, oxygen, water and perhaps hydroxyl species were present. The coverage of hydrogen adatoms was found to decrease with an activation energy of E{sub a} = 2.6 kcal/mol, attributed to the step H + O {r_arrow} OH. Approximately the same activation energy was obtained from the temperature dependence of the increase in water coverage. However, the initial increase in water coverage was not sufficient to account for the decrease in the hydrogen adatom coverage and, furthermore, the watermore » coverage at long times actually decreased, implying that significant amounts of adsorbed hydroxyl were formed from the reaction H{sub 2}O + O {r_arrow} 2OH. 40 refs., 9 figs., 1 tab.« less