The Energy of Adsorbed Hydroxyl on Pt(111) by Microcalorimetry

The energy of adsorbed hydroxyl on Pt(111) was measured by dosing D2O gas onto oxygen precovered Pt(111) at 150 K while following the heat of reaction with single-crystal adsorption calorimetry. The adsorption of D2O on oxygen precovered Pt(111) is known to produce surface OD (hydroxyl) coadsorbed with molecular D2O in a well-defined structure. The heat of reaction and sticking probability of D2O on Pt(111) were measured as a function of oxygen precoverage and D2O dose. With 0.25 monolayers (ML) of oxygen atoms, the differential heat of adsorption is nearly constant at 61.3 kJ/mol for the first similar to 1/3 ML but drops to 57.9 kJ/mol by 0.50 ML and 50.5 kJ/mol by saturation (0.62 ML). Similar experiments with O-ad precoverages of 0.18 and 0.07 ML gave lower saturation D2O coverages (0.55 and 0.22 ML, respectively) and lower heats of reaction by similar to 3.4 and similar to 4.6 kJ/mol, respectively, except at very low D2O coverage where step sites may play a role. From the integral heat of D2O adsorption, the standard enthalpy of formation of the (D2O center dot center dot center dot OD)(ad) complex is estimated to be -527 kJ/mol. Assuming that the adsorbed D2O molecules in this (D2O center dot center dot center dot OD)(ad) complex have the same enthalpy as when adsorbed as a pure D2O film on Pt(111), the enthalpy of formation of adsorbed OD is found to be -226 kJ/mol, and the Pt-OD bond enthalpy to be 263-274 kJ/mol. From these values, we estimate that the reaction D2Oad -> ODad + D-ad is endothermic by 29-39 kJ/mol. At D2O coverages below 0.4 ML, the sticking probability of D2O on Pt(111) at 150 K is similar to 0.9 for oxygen precoverages between 0.14 and 0.25 ML.