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

Abstract

The energy of adsorbed hydroxyl on Pt(111) was measured by dosing D 2 O gas onto oxygen precovered Pt(111) at 150 K while following the heat of reaction with single-crystal adsorption calorimetry. The adsorption of D 2 O on oxygen precovered Pt(111) is known to produce surface OD (hydroxyl) coadsorbed with molecular D 2 O in a well-defined structure. The heat of reaction and sticking probability of D 2 O on Pt(111) were measured as a function of oxygen precoverage and D 2 O 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 O ad precoverages of 0.18 and 0.07 ML gave lower saturation D 2 O 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 D 2 O coverage where step sites may play a role. From the integral heat of D 2 O adsorption, the standard enthalpy of formation of the (D 2 O···OD) ad complex is estimated to be −527 kJ/mol. Assuming that the adsorbed D 2 O molecules in this (D 2 O···OD) ad complex have the same enthalpy as when adsorbed as a pure D 2 O 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 D 2 O ad → OD ad + D ad is endothermic by 29–39 kJ/mol. At D 2 O coverages below 0.4 ML, the sticking probability of D 2 O on Pt(111) at 150 K is ∼0.9 for oxygen precoverages between 0.14 and 0.25 ML.