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

Abstract

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 ∼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 Oad 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 ∼3.4 and ∼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···OD)ad complex is estimated to be −527 kJ/mol. Assuming that the adsorbed D2O molecules in this (D2O···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 + Dad 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 ∼0.9 for oxygen precoverages between 0.14 and 0.25 ML.