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

Abstract

Calorimetric measurements of the adsorption enthalpy of O 2,g to make 2 O ad 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 (Campbell et al. 2 and Parker et al.3 ). Differences arise above 0.15 ML, but we attribute these to the very low sticking probability of O 2,g on Pt(111) (< 0.05) above 0.15ML, 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 ¼ 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 O 2 below ¼ ML, where θ is the O ad coverage in ML (i.e., O ad per Pt surface atom). We also report calculations of coverage-dependent adsorption energies for oxygen on Pt(111) from density functional theory (DFT) and find the results to be an average of 21 kJ/mol higher than the integral heats measured by TPD. We further use these corrected adsorption enthalpies to amend the energetics of hydroxyl species on Pt(111) that we previously measured 4 . This gives revised values for the standard enthalpies of formation of the coadsobed (D 2 O-OD) ad complex 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 (H 2 O-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 compared to DFT.

Density functional theory with modified dispersion correction for metals applied to molecular adsorption on Pt(111)

“…Atomic O and H bind in the three-fold hollow sites, while CO is known to bind in the top site [27][28][29] . The adsorption energies for the range of molecules investigated here are presented in Table 1 along with the experimental values taken from 21 (which in turn were obtained from [30][31][32][33][34][35][36][37][38][39][40] ). Our DFT predictions compare quite favorably to experiments (Table 1): the mean deviation is -0.11 eV, the mean absolute deviation is 0.19 eV and the maximum deviation is 0.37 eV, all across a range of adsorption energies from 0 to 3 eV.…”

A benchmark database for adsorption bond energies to transition metal surfaces and comparison to selected DFT functionals

“…It was originally proposed [23], and later corroborated [24], that the reported heats of adsorption for Pt surfaces from the review by Brown et al [7] require a correction factor of 0.71 due to the use of an incorrect optical reflectivity for Pt (which had been reported incorrectly in the prior literature) during data analysis of the reference heats used for calibration. For a more detailed discussion, see reference [24].…”

“…It was originally proposed [23], and later corroborated [24], that the reported heats of adsorption for Pt surfaces from the review by Brown et al [7] require a correction factor of 0.71 due to the use of an incorrect optical reflectivity for Pt (which had been reported incorrectly in the prior literature) during data analysis of the reference heats used for calibration. For a more detailed discussion, see reference [24]. For the Pd(100) surface, we found that the reported heats of adsorption for CO from SCAC were 26 kJ/mol lower than very reliable experimental studies by other methods, which we believe resulted from the same type of error in optical reflectivity (taken from the same prior literature source).…”

“…The error in chemisorption energies could be due to a poor description of surfaceadsorbate bonds, the adsorbate itself in the gas-phase or both. It has been found that rather simple gas-phase molecules such as O2 and CO2 are poorly described by some functionals [24,[81][82][83]. The problem seems to be located in the description of double and triple bonds of gas-phase molecules while fully hydrogenated derivatives of these molecules are usually described rather well.…”

“…For some reaction energies, e.g. Reaction 3g involving O2, the error is so large that corrections to the O2 molecule became a standard way of addressing this issue for the RPBE [81] and BEEF-vdW [24] functional. If one assumes that the majority of the error in these reaction energies lies in the description of the gaseous molecules having double and triple bonds, an interesting question arises.…”