Energy of Molecularly Adsorbed Water on Clean Pt(111) and Pt(111) with Coadsorbed Oxygen by Calorimetry

The heat of adsorption and sticking probability of D2O were measured on Pt(111) with and without preadsorbed oxygen adatoms as a function of D2O coverage using single-crystal adsorption calorimetry from 88 to 120 K. In this temperature range, water adsorbs molecularly on both surfaces, and, at multilayer coverages, forms amorphous solid water on Pt(111). The integral heat of adsorption at a D2O coverage of 0.5 ML on clean Pt(111) was found to be 51.3 +/- 1.6 kJ/mol at 120 K, 4.1 kJ/mol larger than in the multilayer. Its change with temperature gives a heat capacity for the adlayer islands of 130 +/- 83 J/(mol K). This heat of adsorption at 120 K provides a standard enthalpy of formation of adsorbed D2O on Pt(111) of -301 kJ/inol at 120 K and a D2O coverage of 0.5 ML, which we attribute to water islands in the (root 37 x root 37)R25.3 degrees structure based on prior structural studies. The integral heat of molecular adsorption of D2O at 120 K on Pt(111) predosed with 0.25 ML of oxygen adatoms was similar to 55.0 kJ/mol at 0.5 ML, 3.7 +/- 0.5 kJ/mol larger than with no O present. The heat of adsorption in the multilayer regime at 120 K (when growing amorphous solid water) was found to be 47.2 1.4 kJ/mol, consistent with previous measurements.