benchmark database for adsorption bond energies to transition metal surfaces and comparison to selected DFT functionals, Surface Science (2015), ABSTRACTWe present a literature collection of experimental adsorption energies over late transition metal surfaces for systems where we believe the energy measurements are particularly accurate, and the atomic-scale adsorption geometries are particularly well established. We propose that this could become useful for benchmarking theoretical methods for calculating adsorption processes. We compare the experimental results to six commonly used electron density functionals, including some (RPBE, BEEF-vdW) which were specifically developed to treat adsorption processes. The comparison shows that there is ample room for improvements in the theoretical descriptions.

The energies of adsorbates containing H, N, C, O, and halogens that are of interest as intermediates, poisons, and promoters in catalytic reactions have been measured on well-defined single-crystal surfaces by equilibrium adsorption isotherms, temperature-programmed desorption (TPD), and single-crystal adsorption calorimetry (SCAC). Here we tabulate a large collection of those experimental adsorption energies which we consider to be particularly reliable based on reproducibility by other groups, comparisons to results for closely related systems, and/or reliability of other results reported by the same group. Specifically, we list the enthalpies and energies of 81 molecular and dissociative adsorption reactions that were measured on 26 different metal single-crystal faces of 12 different late transition metals, and we extract from these the standard enthalpies of formation of the adsorbates thus produced. These can serve as benchmarks for validating computational methods for estimating surface reaction energies.

The heat of adsorption and sticking probability of methanol were measured on clean Pt(111) at 100, 150, and 210 K and on oxygen-precovered Pt(111) at 150 K by single-crystal adsorption calorimetry (SCAC). On clean Pt(111) at 100 K, the heat of methanol adsorption was found to be 60.5 ± 0.8 kJ/mol in the limit of low coverage, resulting in a standard enthalpy of formation (ΔH(f)°) of CH(3)OH(ad) of -263 ± 0.8 kJ/mol. The results at 150 and 210 K on clean Pt(111) were indistinguishable from the energetics measured at 100 K in the same coverage range. Calorimetry of methanol on oxygen-precovered Pt(111) at 150 K yielded the energetics of adsorbed methoxy, giving ΔH(f)°[CH(3)O(ad)] = -170 ± 10 kJ/mol and a CH(3)O-Pt(111) bond enthalpy of 187 ± 11 kJ/mol. By use of these enthalpies, the dissociation of adsorbed methanol on Pt(111) to form methoxy and a hydrogen adatom is found to be uphill by +57 kJ/mol. At coverages below 0.2 monolayer (ML), the sticking probability for methanol on both surfaces at or below 150 K was >0.95. At 210 K, ∼80% of the methanol beam pulse transiently adsorbs to clean Pt(111) with a surface residence time of 238 ms and heat of adsorption of 61.2 ± 2.0 kJ/mol, giving a prefactor for methanol desorption of 4 × 10(15±0.5) s(-1). These measured energetics for methoxy and methanol were compared to density functional theory (DFT) calculations from previous literature, showing DFT to routinely underestimate the bond energy of both adsorbed methanol and methoxy by 15-52 kJ/mol.

Carboxylates adsorbed on solid surfaces are important in many technological applications, ranging from heterogeneous catalysis and surface organo-functionalization to medical implants. We report here the first experimentally determined enthalpy of formation of any surface bound carboxylate on any surface, formate on Pt(111). This was accomplished by studying the dissociative adsorption of formic acid on oxygen-presaturated (O-sat) Pt(111) to make adsorbed monodentate and bidentate formates using single-crystal adsorption calorimetry. The integral heat of molecular adsorption of formic acid on clean Pt(111) at 100 K is 62.5 kJ/mol at 0.25 monolayer (ML). On O-sat Pt(111), the integral heat of the dissociative adsorption of formic acid to make monodentate formate (HCOOmon,ad) plus the water-hydroxyl complex ((H2O-OH)ad) was found to be 76 kJ/mol at 3/8 ML and 100-150 K. Similarly, its integral heat of dissociative adsorption to make bidentate formate (HCOObi,ad) plus (H2O-OH)ad was 106 kJ/mol at 3/8 ML and 150 K. These heats give the standard enthalpies of formation of adsorbed monodentate and bidentate formate on Pt(111) to be -354 ± 5 and -384 ± 5 kJ/mol, respectively, and their net bond enthalpies to the Pt(111) surface to be 224 ± 13 and 254 ± 13 kJ/mol, respectively. Coverage-dependent enthalpies of formation were used to estimate the enthalpy of the elementary reaction HCOOHad → HCOObi,ad + Had to be -4 kJ/mol at zero coverage and +24 kJ/mol at 3/8 ML.

One important aid in understanding catalysis by gold nanoparticles would be to understand the strength with which they bond to different support materials and the strength with which they bond adsorbed intermediates, and how these strengths depend on nanoparticle size. We present here new measurements of adsorption energies by single crystal adsorption calorimetry, and new analyses of other recent measurements by this technique in our lab, which imply that: (1) small nanoparticles of metals like Au bind much more strongly to supports like titania and iron oxide which are generally observed to be effective in making Au nanoparticles active in catalysis than to supports like MgO which are considered less effective, (2) the thermodynamic stability of adsorbed intermediates for catalytic reactions can either increase strongly or decrease strongly with decreasing metal nanoparticle size below 8 nm, depending on the system, and (3) the reaction to insert O2 into the Au-H bond of adsorbed H on the Au(111) surface to make Au-OOH (O2,g + H(ad) --> OOH(ad)) is exothermic by -80 kJ mol(-1). This adsorbed hydroperoxy species is thought to be a key intermediate in selective oxidation reactions over Au nanoparticle catalysts, but its production by this reaction may also provide a route for O2 activation in less demanding reactions (like CO oxidation) as well. Its stability would be even higher on Au nanoparticles below 3 nm in diameter, but even there it is too unstable to be formed by combining adsorbed OH with an O adatom (OH(ad) + O(ad) --> OOH(ad)), which is estimated to be endothermic by 175 kJ mol(-1). The implications of the stability of metal nanoparticles versus particle size on different supports and of the stability and potential reactions of OOH(ad) in Au catalysis will be discussed.