The Energy of Adsorbed Hydroxyl on Pt(111) by Microcalorimetry

Abstract

The energy of adsorbed hydroxyl on Pt(111) was measured by dosing D2O gas onto oxygen precovered Pt(111) at 150 K while following the heat of reaction with single-crystal adsorption calorimetry. The adsorption of D2O on oxygen precovered Pt(111) is known to produce surface OD (hydroxyl) coadsorbed with molecular D2O in a well-defined structure. The heat of reaction and sticking probability of D2O on Pt(111) were measured as a function of oxygen precoverage and D2O dose. With 0.25 monolayers (ML) of oxygen atoms, the differential heat of adsorption is nearly constant at 61.3 kJ/mol for the first ∼1/3 ML but drops to 57.9 kJ/mol by 0.50 ML and 50.5 kJ/mol by saturation (0.62 ML). Similar experiments with Oad precoverages of 0.18 and 0.07 ML gave lower saturation D2O coverages (0.55 and 0.22 ML, respectively) and lower heats of reaction by ∼3.4 and ∼4.6 kJ/mol, respectively, except at very low D2O coverage where step sites may play a role. From the integral heat of D2O adsorption, the standard enthalpy of formation of the (D2O···OD)ad complex is estimated to be −527 kJ/mol. Assuming that the adsorbed D2O molecules in this (D2O···OD)ad complex have the same enthalpy as when adsorbed as a pure D2O film on Pt(111), the enthalpy of formation of adsorbed OD is found to be −226 kJ/mol, and the Pt–OD bond enthalpy to be 263–274 kJ/mol. From these values, we estimate that the reaction D2Oad → ODad + Dad is endothermic by 29–39 kJ/mol. At D2O coverages below 0.4 ML, the sticking probability of D2O on Pt(111) at 150 K is ∼0.9 for oxygen precoverages between 0.14 and 0.25 ML.

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

“…The hexagonal network structure of D2O [26] and the (√3x√3)R30 of (D2O-OD) [27] on Pt(111) are well established in the literature. The structures of adsorbed saturated hydrocarbons (e.g.…”

“…It is quite revealing to observe the performance of the various functionals for these adsorption processes in more detail. Thus, comparison of the overall performance of the various density functionals with respect to the ZPE-corrected experimental reaction energies is shown in Tables 4a for chemisorbed systems (reactions 1-25) and 4b for systems where vdW interactions make a large contribution to the total adsorption energy (reactions [26][27][28][29][30][31][32][33][34][35][36][37][38][39]. Average surface reaction energy errors are computed in terms of the mean signed error (MSE), the mean absolute error (MAE), and the root-mean squared error (RMSE).…”

“…When considering energies that have large contributions from van der Waals interactions (Table 4b, reactions [26][27][28][29][30][31][32][33][34][35][36][37][38][39], it is noted that the strong overbinding of the PW91 functional leads also to a better description of the systems with large vdW contributions compared to RPBE, which does not overbind in general. RPBE performs quite well for all chemisorbed systems for which it was optimized, but fails badly for species with large vdW contributions.…”

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

“…1) and the thermodynamic cycle of Figure 5 within ref. 4 . Numbers in italics are the original values reported in ref.…”

Elucidating challenges of reactions with correlated reactant and product binding energies on an example of oxygen reduction reaction

“…It has been observed in experiments by Ertl et al 5,6 and Campbell et. al 7 . It was also shown that the reactant, O ad , dominates as the adsorbed intermediate above 0.7V, while OH ad dominates below 8,9 , making this reaction more relevant at desirable potentials.…”