Kinetics of water formation on Pt(111)

Measurements using modulated molecular beam relaxation spectroscopy identify ODa consumption as the rate‐limiting step for D2O formation on Pt(111). In the limit of low surface coverage, D2O is produced via two parallel pathways, Da+ODa→D2Oa with activation energy Ea≊16 kcal/mol, and 2ODa→D2Oa+Oa with Ea≊18 kcal/mol. From these results the reactions Da+Oa →ODa, Da+ODa→D2Oa, and D2Oa+Oa→2ODa are found to be exothermic with enthalpy changes ΔH≊−16, −9, and −8 kcal/mol, respectively, and ΔHOHa≊−61 kcal/mol is obtained for the heat of adsorption of hydroxyl on platinum. The general features of the reaction mechanism cause complex kinetic behavior, manifest as changes in the apparent order of the rate function and the effective activation energy as surface temperature and reactant pressures are varied. The implications for transient and steady‐state rate measurements are discussed.