Energetics of Oxygen Adatoms, Hydroxyl Species and Water Dissociation on Pt(111)

Calorimetric measurements of the adsorption enthalpy of gaseous oxygen (O2,g) to make two oxygen adatoms (2 Oad) on Pt(111) were performed by Fiorin et al.(1) However, we show that they used a calibration value for the optical reflectivity of Pt(111) that was incorrectly reported in the literature. This error in reflectivity led to a 40% error in the adsorption energies originally reported.

We use our more accurate reflectivity of 76% to recalibrate their oxygen adsorption enthalpy data and show that it gives nearly identical results below 0.15 ML to the heats of adsorption determined from the temperature programmed desorption (TPD) experiments of two separate groups.(2, 3) Differences arise above 0.15 ML, but we attribute these to the very low sticking probability of O2,g on Pt(111) (<0.05) above 0.15 ML, which can lead to large errors in the adsorption energies measured by calorimetry.

Given this, we propose that the most reliable values for the adsorption enthalpy of oxygen on Pt(111) up to 1/4 ML are those derived from TPD experiments rather than the more recent calorimetry data. The best values are well described by (−217 + 151θ) kJ/mol O2 below 1/4 ML, where θ is the Oad coverage in ML (where 1 ML is defined as 1 adsorbate per Pt surface atom).

We also report calculations of coverage-dependent adsorption energies for oxygen on Pt(111) from density functional theory (DFT-RPBE) and find the results to be within ca. 20 kJ/mol of the integral heats measured by TPD. We further use these corrected adsorption enthalpies to amend the energetics of deuterated hydroxyl species on Pt(111) that we previously measured.(4) This gives revised values for the standard enthalpies of formation of the coadsorbed water–hydroxyl complex ((D2O–OD)ad) of −511 ± 7 kJ/mol and a Pt–OD bond energy of 248 ± 7 kJ/mol for the OD species within this complex.

DFT compares reasonably well, calculating an enthalpy of formation for the (H2O–OH)ad complex of −456 kJ/mol and an O–Pt bond energy of 217 kJ/mol for the OH species within this complex. These revised values are used to estimate reaction enthalpies for the dissociation of adsorbed water and hydroxyl on Pt(111) and are compared to DFT.