Energetics of cyclohexene adsorption and reaction on Pt(111) by low-temperature microcalorimetry

The heat of adsorption and sticking probability of cyclohexene on Pt(111) were measured as a function of coverage using single-crystal adsorption calorimetry in the temperature range from 100 to 300 K. At 100 K, cyclohexene adsorbs as intact di-sigma bonded cyclohexene on Pt(111), and the heat of adsorption is well described by a second-order polynomial (130 - 47 theta - 1250 theta(2)) kJ/mol, yielding a standard enthalpy of formation of di-sigma bonded cyclohexene on Pt(111) at low coverages of -135 kJ/mol and a C-Pt sigma bond strength of 205 kJ/mol. At 281 K, cyclohexene dehydrogenates upon adsorption, forming adsorbed 2-cyclohexenyl (c-C6H(9,a)) and adsorbed hydrogen, and the heat of adsorption is well described by another second-order polynomial (174 - 700 theta + 761 theta(2)) kJ/mol. This yields a standard enthalpy of formation of adsorbed 2-cyclohexenyl on Pt(111) at a low coverage of -143 kJ/mol. At coverages below 0.10 ML, the sticking probability of cyclohexene on Pt(111) is close to unity (>0.95), independent of temperature.     

Thermodynamic analysis of the temperature dependence of OH adsorption on Pt(111) and Pt(100) electrodes in acidic media in the absence of specific anion adsorption

The effect of temperature on the voltammetric OH adsorption on Pt(111) and Pt(100) electrodes in perchloric acid media has been studied. From a thermodynamic analysis based on a generalized adsorption isotherm, DeltaG degrees , DeltaH degrees , and DeltaS degrees values for the adsorption of OH have been determined. On Pt(111), the adsorption enthalpy ranges between -265 and -235 kJ mol(-1), becoming less exothermic as the OH coverage increases. These values are in reasonable agreement with experimental data and calculated values for the same reaction in gas phase. The adsorption entropy for OH adsorption on Pt(111) ranges from -200 J mol(-1) K(-1) (low coverage) to -110 J mol(-1) K(-1) (high coverage). On the other hand, the enthalpy and entropy of hydroxyl adsorption on Pt(100) are less sensitive to coverage variations, with values ca. DeltaH degrees = -280 kJ mol(-1) and DeltaS degrees = -180 J mol(-1) K(-1). The different dependence of DeltaS degrees with coverage on both electrode surfaces stresses the important effect of the substrate symmetry on the mobility of adsorbed OH species within the water network directly attached to the metal surface.     

Calcium adsorption on MgO(100): energetics, structure, and role of defects

The adsorption of Ca on the MgO(100) surface at 300 K has been studied using microcalorimetry, in combination with LEED, AES, ISS, work function, sticking probability measurements, and density functional theory (DFT) calculations. The MgO(100) thin films (approximately 4 nm thick) were grown epitaxially on a 1 microm thick Mo(100) single-crystal. The sticking probability of Ca on MgO(100) at 300 K is unity. On the basis of AES and ISS measurements, it was determined that Ca grows mainly as 3D particles on the MgO(100) surface with a density of approximately 1 x 10(12) islands/cm2. Ca adsorbs initially at defect sites with a very high heat of adsorption (approximately 410 kJ/mol). DFT calculations attribute this high initial heat to Ca binding to kink sites (376 kJ/mol), step sites (205 kJ/mol), and lower concentrations of stronger binding sites. The heat of adsorption decreases rapidly with coverage, reaching a minimum of 162 kJ/mol at approximately 0.3 ML, where Ca is mainly adding to small 3D Ca clusters. Afterward, it increases to the value of bulk Ca heat of sublimation (178 kJ/mol) at approximately 1.2 ML, attributed to the increase in stability with increasing Ca particle size. A 1.0 eV decrease of the work function with Ca coverage from 0 to 0.3 ML indicates that Ca adsorbed at defects is cationic, in agreement with calculations showing that Ca donates electron density to the MgO. Light ion sputtering of the MgO(100) surface generates point defects, but these do not change the heat of adsorption versus coverage, implying that they do not nucleate Ca particles. Oxygen vacancies are a likely candidate; DFT calculations show that F and F+ center vacancies bind Ca more weakly than terrace sites. More extensive sputtering creates extended defects (such as steps and kinks) that adsorb Ca with heats of adsorption up to approximately 400 kJ/mol, similar to that at the intrinsic defect sites.     

Microcalorimetry of oxygen adsorption on fcc Co{110}

The coverage dependent heats of adsorption and sticking probabilities for oxygen on fcc Co{110} have been measured at 300 K using single crystal adsorption calorimetry (SCAC). Initial adsorption is consistent with dissociative chemisorption at low coverage followed by oxide formation above 0.6 ML coverage. The initial heat of adsorption of 633 kJ mol(-1) is similar to heat values calorimetrically measured on other ferromagnetic metal surfaces, such as nickel and iron. As the coverage increases, the heat of adsorption and sticking probability drop very rapidly up to the onset of oxidation. As already observed for other oxygen-metal surface systems, strong lateral adatom repulsions are responsible for the transition from the chemisorption regime to oxide film formation at higher coverage. The heat of oxide formation at the onset is 475 kJ mol(-1), which is consistent with the formation of CoO crystallites. The oxide film formation is discussed in terms of nucleation and island growth, and the Mott-Cabrera mechanisms, the latter being evidenced by the relatively constant heat of adsorption and sticking probability in contrast to the nickel and iron oxidation cases.     

Studies of the roles of Sn or Fe on γ-Al2O3-supported Pt catalysts by CO adsorption microcalorimetry and dehydrogenation reaction of C4 alkanes

CO adsorption microcalorimetry was employed in the study of γ-Al₂O₃-supported Pt, Pt-Sn and Pt-Fe catalysts. The results indicated that the initial differential heat of CO adsorption of the Pt/γ-Al₂O₃ catalyst was 125 kJ/mol. As CO coverage increased, the differential heat of adsorption decreased. At higher coverages, the differential heat of adsorption decreased significantly. 60% of the differential heat of CO adsorption on the Pt/γ-N₂O₃ catalyst was higher than 100 kJ/mol. No significant effect on the initial differential heat was found after adding Sn and Fe to the Pt/γ-Al₂O₃ catalyst. The amount of strong CO adsorption sites decreased, while the portion of CO adsorption sites with differential heat of 60–110 kJ/mol increased after increasing the Sn or Fe content. This indicates that the surface adsorption energy was changed by adding Sn or Fe to Pt/γ-N₂O₃. The distribution of differential heat of CO adsorption on the Pt-Sn(C)/γ-Al₂O₃ catalyst was broad and homogeneous. Comparison of the dehydrogenation performance of C₄ alkanes with the number of CO adsorption sites with differential heat of 60–110 kJ/mol showed a good correlation. These results indicate that the surface Pt centers with differential heats of 60–110 kJ/mol for CO adsorption possess superior activity for the dehydrogenation of alkanes. Project supported by FORD and the National Natural Science Foundation of China (Grant No. 09412302) and the Transcentury Training Program Foundation for the Talents by The State Education Commission of China.     

Underpotential deposition of hydrogen on benzene-modified Pt(111) in aqueous H2SO4

The Pt(111) electrode is modified by an overlayer of C6H6 (ads) upon its cycling in the 0.05-0.80 V range in aq H2SO4 + 1 mM C6H6. The C6H6 (ads) overlayer significantly changes the underpotential-deposited H (H(UPD)) and anion adsorption, and cyclic-voltammetry (CV) profiles show a sharp cathodic peak and an asymmetric anodic one in the 0.05-0.80 V potential range. The C6H6 (ads) layer blocks the (bi)sulfate adsorption but facilitates the adsorption of one monolayer of H(UPD). Cycling of the benzene-modified Pt(111) in benzene-free aq 0.05 H2SO4 from 0.05 to 0.80 V results in a partial desorption of C6H6 (ads) and in a partial recovery of the CV profile characteristic of an unmodified Pt(111). The peak potential of the cathodic and anodic feature is independent of the scan rate, s (10 < or = s < or = 100 mV s(-1)), and the peak current density increases linearly with an increase of the scan rate. Temperature variation modifies the peak potential and current density but does not affect the charge density of the cathodic or anodic feature. Temperature-dependent studies allow us to determine the thermodynamic state function for the H(UPD) adsorption and desorption. Delta G degrees(ads)(H(UPD))assumes values from -4 to -12 kJ mol(-1), while has values from 9 to 14 kJ mol(-1). The values of delta Delta G degrees (delta Delta G degrees = delat Delta G degrees(ads) + delta Delta D degrees(des)) decrease almost linearly from 6 kJ mol(-1) at theta(H(UPD) --> 0 to 0 kJ mol(-1) at theta(H(UPD) --> 1. The nonzero values of delta Delta G degrees testify that the adsorbing and desorbing H(UPD) adatoms interact with an energetically different substrate. The lateral interactions changed from repulsive (omega = 29 kJ mol(-1) at theta(H(UPD) --> 0) to attractive (omega = -28 kJ mol(-1) at theta(H(UPD) --> 1) as the H(UPD) coverage increases. The values of delta S degrees(ads)(H(UPD)) increase from 19 to 56 J K(-1) mol(-1), while those of delta S degrees(des)(H(UPD)) decrease from 45 to -30 J K(-1) mol(-1) with an increase of H(UPD) coverage. The values of delta H degrees(des)(H(UPD)) and delta H degrees(des)(H(UPD)) vary from 0 to 27 kJ mol(-1). The Pt(111)-H(UPD) surface bond energy at the benzene-modified Pt(111) electrode falls in the 191-218 kJ mol(-1) range and is weaker than in the case of the unmodified Pt(111) electrode in the same electrolyte.     

Methylthiolate on Au(111): adsorption and desorption kinetics

Low energy electron diffraction, Auger electron spectroscopy, X-ray photoelectron spectroscopy and line of sight mass spectrometry have been used to study the adsorption and desorption of dimethyldisulfide (DMDS) on Au(111). At 300 K adsorption is dissociative, forming a chemisorbed adlayer of methylthiolate with a 1/3 ML, (sq rt 3 x sq rt 3)R30 degrees, structure. At 100 K adsorption is molecular, with dissociation to form the 1/3 ML (sq rt 3 x sq rt 3)R30 degrees methylthiolate structure occurring at 138-160 K. A physisorbed DMDS layer, with a coverage of 1/6 ML of DMDS, forms on top of the (sq rt 3 x sq rt 3)R30 degrees chemisorbed MT surface for T < or = 180 K, with multilayers forming for T < or = 150 K. In temperature programmed desorption, multilayers of DMDS desorbed with zero order kinetics and an activation energy of 41 kJ mol(-1); the physisorbed layer desorbed with first order kinetics, exhibiting repulsive lateral interactions with an activation energy which varied from 63 kJ mol(-1) (theta = 0) to 51 kJ mol(-1) (theta = 1); the chemisorbed methylthiolate layer desorbed associatively as DMDS via the physisorbed layer, the activation energy for the reaction, 2 methylthiolate --> physisorbed DMDS, exhibiting repulsive lateral interactions with an activation energy which varied from 65 kJ mol(-1) (theta = 0) to 61 kJ mol(-1) (theta = 1). The physisorbed disulfide layer explains the pre-cursor state adsorption kinetics observed in sticking probability measurement, while its relatively facile formation provides a mechanism by which thiolate self-assembled monolayers can become mobile at room temperature.     

Thermodynamic and neutron scattering study of hydrogen adsorption in two mesoporous ordered carbons

Two mesoporous ordered carbon materials (MOCs) have been synthesized from silica templates by using sucrose as the carbon precursor. The textural characterization using Ar, N2, and CO2 adsorption combined with neutron diffraction showed that the two samples exhibit a significant microporous volume close to 0.5 cm3/g and an ordered network of mesopores. For both MCM48 and SBA15 templated carbons, adsorption first proceeds with the filling of micropores and then by the filling of mesopores with an adsorption energy close to the enthalpy of vaporization of bulk hydrogen. The hydrogen isosteric heat of adsorption in the micropores (6-8 kJ/mol) is significantly larger than that on the graphite surface (approximately 4 kJ/mol) but still too small for a reasonable use of these MOCs as hydrogen adsorbents for storage at room temperature. The neutron scattering study showed that the structure at 10 K of the adsorbed deuterium phase is poorly organized; it exhibits short and medium range orders of about 13 angstroms in micropores and about 20 angstroms in mesopores, respectively. The average distance between adsorbed molecules decreases with coverage by about 10%. In the mesopores, the diffracted line is consistent with a pseudohexagonal packing.     

Experimental and DFT studies of the conversion of ethanol and acetic acid on PtSn-based catalysts

Reaction kinetics studies were conducted for the conversions of ethanol and acetic acid over silica-supported Pt and Pt/Sn catalysts at temperatures from 500 to 600 K. Addition of Sn to Pt catalysts inhibits the decomposition of ethanol to CO, CH4, and C2H6, such that PtSn-based catalysts are active for dehydrogenation of ethanol to acetaldehyde. Furthermore, PtSn-based catalysts are selective for the conversion of acetic acid to ethanol, acetaldehyde, and ethyl acetate, whereas Pt catalysts lead mainly to decomposition products such as CH4 and CO. These results are interpreted using density functional theory (DFT) calculations for various adsorbed species and transition states on Pt(111) and Pt3Sn(111) surfaces. The Pt3Sn alloy slab was selected for DFT studies because results from in situ (119)Sn M?ssbauer spectroscopy and CO adsorption microcalorimetry of silica-supported Pt/Sn catalysts indicate that Pt-Sn alloy is the major phase present. Accordingly, results from DFT calculations show that transition-state energies for C-O and C-C bond cleavage in ethanol-derived species increase by 25-60 kJ/mol on Pt3Sn(111) compared to Pt(111), whereas energies of transition states for dehydrogenation reactions increase by only 5-10 kJ/mol. Results from DFT calculations show that transition-state energies for CH3CO-OH bond cleavage increase by only 12 kJ/mol on Pt3Sn(111) compared to Pt(111). The suppression of C-C bond cleavage in ethanol and acetic acid upon addition of Sn to Pt is also confirmed by microcalorimetric and infrared spectroscopic measurements at 300 K of the interactions of ethanol and acetic acid with Pt and PtSn on a silica support that had been silylated to remove silanol groups.     

On the dynamics of H2 adsorption on the Pt(111) surface

The dynamics and kinetics of the dissociation of hydrogen over the hexagonal close packed platinum (Pt(111)) surface are investigated using Car–Parrinello molecular dynamics and static density functional theory calculations of the potential energy surfaces. The calculations model the reference energy‐resolved molecular beam experiments, considering the degrees of freedom of the catalytic surface. Two‐dimensional potential energy surfaces above the main sites on Pt(111) are determined. Combined with Car–Parrinello trajectories, they confirm the dissociative adsorption of H₂ as the only adsorption pathway on this surface at H₂ incindence energies above 5 kJ/mol. A direct determination of energy‐resolved sticking coefficients from molecular dynamics is also performed, showing an excellent agreement with the experimental data at incidence energies in the 5–30 kJ/mol range. Application of dispersion corrections does not lead to an improvement in the prediction of the H₂ sticking coefficient. The adsorption reaction rate obtained from the calculated sticking coefficients is consistent with experimentally derived literature values.     

Mechanisms of oxygen adsorption and desorption on polycrystalline palladium

The adsorption and desorption of oxygen on a polycrystalline palladium (Pd(poly)) surface (10-to 100-μm crystallites; ～32% (100), ～18% (111), ～34% (311), and ～15% (331)) at P _O2 ≤ 1.3 × 10^?5 Pa and T = 500–1300 K have been studied by TPD and mathematical modeling. The kinetics of O₂ adsorption and desorption on Pd(poly) are primarily governed by the formation and decomposition of oxygen adsorption structures on the (100) and (111) crystallite faces. The O₂ adsorption rate is constant at ? ≤ 0.15–0.25 owing to the formation of the p(2 × 2) structure with an O_ads-surface bonding energy of D(Pd-O) = 364 kJ/mol on the (100) and (111) faces. The adsorption rate decreases with increasing coverage at ? ≥ 0.15–0.25 because of the growth, on the (100) face, of the c(2 × 2) structure, in which D(Pd-O) is reduced to 324 kJ/mol by lateral interactions in the adsorption layer. A high-temperature (～800 K) O₂ desorption peak is observed for ? ≤ 0.25, which is due to O₂ desorption from a disordered adsorption layer according to a second-order rate law with an activation energy of E _des = 230 kJ/mol. A lower temperature (～700 K) O₂ desorption peak is observed for ? ≥ 0.25, which is due to O₂ released by the c(2 × 2) structure according to a first-order rate law with E _des = 150 kJ/mol. At ? ≥ 0.25, there are repulsive interactions between O_ads atoms on Pd(poly) (ε_aa = 5–10 kJ/mol).     

State-resolved reactivity of CH4(2nu3) on Pt(111) and Ni(111): effects of barrier height and transition state location

Quantum state-resolved sticking coefficients on Pt(111) and Ni(111) surfaces have been measured for CH4 excited to the first overtone of the antisymmetric C-H stretch (2nu3) at well-defined kinetic energies in the range of 10-90 kJ/mol. The ground-state reactivity of CH4 is approximately 3 orders of magnitude lower on Ni(111) than on Pt(111) for kinetic energies in the range of 10-64 kJ/mol, reflecting a difference in barrier height of 28+/-6 kJ/mol. 2nu3 excitation of CH4 increases its reactivity by more than 4 orders of magnitude on Ni(111), whereas on Pt(111) the reactivity increase is lower by 2 orders of magnitude. We discuss the observed differences in the state-resolved reactivity for the ground state and 2nu3 excited state of methane in terms of a difference in barrier height and transition state location for the dissociation reaction on the two metal surfaces.     

Thermodynamic study of water adsorption in high-silica zeolites

The joint use of microcalorimetric and computational approaches has been adopted to describe H2O interaction with cus Al(III) Lewis and Si(OH)+ Al- Br?nsted acidic sites within H-BEA and H-MFI zeolites (both with approximately 6 Al/unit cell). Adsorption data obtained at 303 K were compared to experimental model systems, such as all-silica zeolites, amorphous silica, and silico-alumina, transition alumina. In parallel, ab initio molecular modeling was carried out to mimic, in a cluster approach, Lewis and Br?nsted acidic sites, as well as a variety of Si-OH species either with H-bonding interacting (nests and pairs) or isolated. H-BEA and H-MFI water affinity values were found to be almost equivalent, in both quantitative and energetic terms, in that dominated by Al-containing sites population, more than by nanocavity topology or by acidic site nature. Both H-zeolites, saturated with approximately 5 Torr of H2O vapor, bind approximately 4 H2O per Al site, almost one of which is tightly bound and not eliminated by RT pumping-off. A 160 < q(diff) < 80 kJ/mol interval was measured for the adsorption up to 1H2O/Al. The zero-coverage heat of adsorption (q0 approximately 160 kJ/mol, for both H-zeolites) was assigned to H2O/Lewis complex formation, which dominates the early stage of the process, in agreement with the ab initio computed H2O/Lewis sites binding energy. The rather broad q(diff) interval was interpreted as due to the simultaneous adsorption of H2O on both structural Br?nsted sites and strongly polarized H2O already adsorbed on Lewis sites. For this latter species, BE = 74 kJ/mol was computed, slightly higher than BE = 65 kJ/mol for H2O/Br?nsted sites interaction, showing that H2O coordinated on cus Al(III) Lewis sites behaves as a structural Br?nsted site. The investigated all-silica zeolites have been categorized as hydrophilic in that the measured heat of adsorption (100 < q(diff) < 44 kJ/mol) was larger than the heat of liquefaction of water (44 kJ/mol) in the whole coverage examined. Indeed, polar defects present in the hydrophobic Si-O-Si framework do form relatively stable H2O adducts. Crystalline versus amorphous aluminosilicate q(diff) versus n(ads) plots showed that the measured adsorption heat is lower than expected, due to the extraction work of Al atoms from the amorphous matrix to bring them in interaction with H2O. On the contrary, such an energy cost is not required for the crystalline material, in which acidic sites are already in place, as imposed by the rigidity of the framework. Modeling results supported the experimental data interpretation.     

Carbon incorporation in Pd(111) by adsorption and dehydrogenation of ethene

The decomposition of ethene on the Pd(111) surface was studied at effective pressures in the 10(-8) to 10(-7) mbar range and at sample temperatures between 300 and 700 K, using an effusive capillary array beam doser for directional adsorption, LEED, AES, temperature programmed reaction, and TDS. In the temperature range of 350-440 K increasingly stronger dehydrogenation of the ethene molecule is observed. Whereas at 350 K an ethylidyne adlayer is still present after adsorption, already at temperatures around 440 K complete coverage of the surface by carbon is attained, while the bulk still retains the properties of pure Pd. Beyond 440 K a steady-state surface C coverage is established, which decreases with temperature and is determined by detailed balancing between the ethene gas-phase adsorption rate and the migration rate of carbon into the Pd bulk. This process gives rise to the formation of a "partially carbon-covered Pd(x)C(y) surface". Above 540 K the surface-bulk diffusion of adsorbed carbon becomes fast, and in the UHV experiment the ethene adsorption rate becomes limited by the ethene gas-phase supply. The carbon bulk migration rate and the steady-state carbon surface coverage were determined as a function of the sample temperature and the ethene flux. An activation energy of 107 kJ mol(-1) for the process of C diffusion from surface adsorption sites into the subsurface region was derived in the temperature range of 400-650 K by modeling the C surface coverage as a function of temperature on the basis of steady-state reaction kinetics, assuming a first-order process for C surface-subsurface diffusion and a second-order process for C(ads) formation by dissociative C2H4 adsorption.     

Theoretical study of CCl(4) adsorption and hydrogenation on a Pt (111) surface

The adsorption and hydrogenation of carbon tetrachloride (CCl(4)) on a Pt (111) surface have been investigated using density functional theory (DFT). We have performed calculations on the adsorption energies and structures of CCl(4) on four different adsorption sites of a Pt (111) surface using the full adsorbate geometry optimization method. The results show that the adsorption energy of all of the potential sites is less than -17 kcal/mol, which indicates that CCl(4) is physiosorbed on a Pt (111) surface through van der Waals interactions. The dissociation and hydrogenation pathways were investigated by a transition state search. For the Pt(15), Pt(19), and Pt(25) cluster surfaces, the activation energies of dissociation obtained in this work are 15.69, 16.94, and 16.77 kcal/mol, respectively. The hydrogenation of CCl(3). was studied at the on-top site of the Pt(15) cluster, and the calculated activation energy is 5.06 kcal/mol. The small activation energies indicate that the Pt (111) surface has high catalytic activity for the CCl(4) hydrogenation reaction. In addition, the Hirshfeld population analysis reveals that the charge transfer from the Pt (111) surface to the adsorbates occurs in both the dissociation and hydrogenation pathways.     

MP2 study on adsorption of hydrated Na+ and Au+ cations on the Au(111) surface

The interactions of Na+ and Au+ cations with an Au(111) surface in the presence and absence of water molecules were investigated using Au18 and Au22 cluster models and the MP2 method with a triple-zeta valence basis set. The interactions between Na+ ions and the Au(111) surface were found to be primarily electrostatic, contrary to the much stronger Au+-Au(111) interactions that were dominated by orbital contributions. The largest CP-corrected MP2 adsorption energies were -156.9 kJ/mol for Na+ and -478.7 kJ/mol for Au+. When hydrated, Na+ prefers to be completely surrounded by water molecules rather than adsorbed to the surface, whereas Au+ remains adsorbed to the surface as water molecules bond with each other and with the Au surface. CP correction did not change the relative adsorption energy trends of Na+ or Au+ ions, but it had an effect on the interaction energy trends of the hydrated cations because of the weak water-surface and water-water interactions.     

Adsorption energy, growth mode, and sticking probability of Ca on poly(methyl methacrylate) surfaces with and without electron damage

The adsorption of Ca atoms on pristine and electron-irradiated poly(methyl methacrylate) (PMMA) surfaces at 300 K has been studied by adsorption microcalorimetry, atomic beam/surface scattering, and low-energy He+ ion scattering spectroscopy (ISS). On pristine PMMA, the initial sticking probability of Ca is 0.5, increasing quickly with Ca coverage. Below 0.5 ML, the heat of adsorption is 730-780 kJ/mol, much higher than Ca's sublimation energy (178 kJ/mol). The Ca here is invisible to ISS, which is attributed to Ca binding to ester groups below the CH3/CH2-terminated PMMA surface. The adsorption energy increases with coverage, suggesting attractions between neighboring Ca-ester complexes. Above 0.5 ML, Ca starts to grow as three-dimensional (3D) Ca clusters on top of the surface, which dominate growth after 2 ML. It is proposed that each Ca reacts with two esters to form the Ca carboxylate of PMMA, because this reaction's heat would be close to that observed. The total amount of Ca that binds to subsurface sites is estimated from the integral heat of adsorption to involve 4-6 layers of ester groups. Exposing the PMMA surface to electrons increases Ca's initial sticking probability but lowers its adsorption energy. This is attributed to electron-induced defects acting as nucleation sites for 3D Ca islands, whose growth now competes kinetically with Ca diffusing to subsurface esters. Consequently, only two layers of subsurface esters get populated at saturation. The heat eventually reaches Ca's bulk heat of sublimation on all PMMA surfaces, where pure, bulk-like Ca thin films form.     

The adsorption of triethylenediamine on Al(2)O(3)-II: hydrogen bonding to Al-OH groups

The hydrogen bonding of the triethylenediamine (TEDA) molecule to isolated Al-OH groups on partially dehydroxylated high area gamma-Al(2)O(3) powder has been studied using transmission IR spectroscopy. It has been found that TEDA adsorbs both singly and as multiple species to single Al-OH groups in clearly separable equilibrium stages of adsorption at 300 K. The reversible adsorption of a single TEDA molecule to Al-OH fits the Langmuir adsorption isotherm well, and the enthalpy of adsorption is found to be -15.6 +/- 0.5 kJ mol(-1) in the range of fractional coverage of 0.5-0.6. Red shifts of the Al-OH frequency from approximately -200 cm(-1) to approximately -1000 cm(-1) are observed as a result of -OH bonding to the N lone pair in the TEDA molecule.     

Systematic DFT-GGA study of hydrogen adsorption on transition metals

Computational study of hydrogen adsorption on (111) surface of transition metals with face centered cubic (fcc) lattice is reported and the results are compared with available experimental and theoretical data. In addition, dissociative adsorption of hydrogen on Pt(111), Pt(100) and Pt(110) is studied in the range of coverage from 0.25 to 1 monolayer. In the case of Pt(111) preferential adsorption site was found to be three-coordinated fcc-hollow site, while on Pt(100) and Pt(110) surface hydrogen settles on two-coordinated bridge and short bridge site, respectively. Hydrogen adsorption energy was found to decrease with the increasing coverage. Structural changes of studied Pt surfaces upon hydrogen adsorption have been compared with the experimental data existing in the literature and good qualitative agreement has been obtained.     

Ab initio study on the adsorption of hydrated Na+ and Ag+ cations on a Ag(111) surface

The interactions of Na(+) and Ag(+) cations with an Ag(111) surface in the presence and absence of water molecules were investigated with cluster models and ab initio methods. The Ag surface was described with two-layered Ag(10) and Ag(18) cluster models, and MP2/RECP/6-31+G(d) was used as the computational method. The effect of the basis set superposition error (BSSE) was taken into account with counterpoise (CP) correction. The interactions between Na(+) and Ag(111) surface were found to be primarily electrostatic, and the interaction energies and equilibrium distances at the different adsorption sites were closely similar. The largest CP-corrected MP2 adsorption energy for Na(+) was -190.2 kJ/mol. Owing to the electrostatic nature of the Na(+)-Ag(111) interaction, Na(+) prefers to be completely surrounded by water molecules rather than directly adsorbed to the surface. Ag(+)-Ag(111) interactions were much stronger than Na(+)-Ag(111) interactions because they were dominated by orbital contributions. The largest CP-corrected MP2 adsorption energy for Ag(+) was -358.9 kJ/mol. Ag(+) prefers to adsorb on sites where it can bind to several surface atoms, and in the presence of water molecules, it remains adsorbed to the surface while the water molecules form hydrogen bonds with one another. The CP correction had an effect on the interaction energies but did not change the relative trends.