Beautiful calorimetric measurements of the adsorption enthalpy of O2,g to make 2 Oad on Pt(111) were performed by Fiorin et al.1 However, we show here that they used a calibration that relied upon a 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 they originally reported. We use here our more accurate reflectivity of 76% to recalibrate their oxygen adsorption enthalpy data and show that it gives nearly identical results to the heats of adsorption below 0.15 ML determined from activation energies for desorption of oxygen from Pt(111) measured with temperature programmed desorption (TPD) by two separate groups (Campbell et al.2 and Parker et al.3). Differences arises above 0.15 ML, but we attribute these to the very low sticking probability of O2,g on Pt(111) (< 0.05) above 0.15ML, which we argue 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 ¼ ML are those derived from those TPD experiments, rather than the more recent calorimetry data, except below 0.15 ML where they agree after this calibration correction. The best values are well described by (217-151Θ ) kJ/mol below ¼ ML, where θ is the Oad coverage in ML (i.e., Oad per Pt surface atom). We further use these corrected adsorption enthalpy data to revise the energetics of hydroxyl species on Pt(111) that we previously measured4. This gives revised values for the standard enthalpies of formation of the coadsobed (DO-D2O)ad complex and of ODad on Pt(111) of -511 ± 7 and -210 ± 7 kJ/mol, respectively, and a revised O-Pt bond enthalpy in adsorbed hydroxyl of 248 ± 7 kJ/mol. These three corrected enthalpies are all 16 kJ/mol smaller in absolute value than originally reported.

 

(1) Fiorin, V.; Borthwick, D.; King, D. A., surf. sci. ,2009, 603, 1360.

(2) Campbell, C. T.; Ertl, G.; Kuipers, H.; Segner, J., surf. sci., 1981, 107, 220.

(3) Parker, D. H.; Bartram, M. E.; Koel, B. E., surf. sci., 1989, 217, 489.

(4) Lew, W.; Crowe, M. C.; Karp, E.; Lytken, O.; Farmer, J. A.; Arnadottir, L.; Schoenbaum, C.; Campbell, C. T., J. Phys Chem. C., 115, 11586.

Work supported by the National Science Foundation.