Adsorbed Hydroxyl and Water on Ni(111): Heats of Formation by Calorimetry

Adsorbed hydroxyl is a common intermediate in many catalytic reactions on Ni catalysts. The heat of the dissociative adsorption of D2O onto an O-precovered Ni(111) surface producing adsorbed hydroxyls is measured here by single crystal adsorption calorimetry, with an average value of 67.3 kJ/mol up to 0.23 ML coverage. From this, the enthalpy of formation for adsorbed hydroxyl on Ni(111) and the bond energy of hydroxyl to Ni(111) were estimated to be -278.3 and 314.1 kJ/mol, respectively. The molecular adsorption of D2O on clean Ni(111) gave an integral heat of 53.6 kJ/mol at 0.5 ML. In comparison to Pt(111), adsorbed hydroxyl binds to Ni(111) 67 kJ/mol more strongly and adsorbed D2O binds to Ni(111) similar to 2 kJ/mol more strongly. These energetics help clarify the different catalytic properties of Ni versus Pt, and serve as benchmarks to evaluate the energy accuracy of quantum mechanical methods used in surface chemistry.