3-fold Sites Research Articles

Scanning Tunneling Microscopy and Theoretical Study of Competitive Reactions in the Dissociative Chemisorption of CCl4 on Iron Oxide Surfaces

Variable temperature scanning tunneling microscopy (VT STM) and theoretical ab initio computer simulations were used to study dissociative chemisorption and competitive surface chemistry of CCl4 on an iron-terminated Fe3O4 (111) 2 × 2 surface in an ultrahigh vacuum. The Fe3O4 (111) surface was exposed to CCl4 molecules at 224 K, slowly annealed to 500 K (0.2 K/s), and scanned at room temperature. Two different chlorine species were observed only on the iron-terminated Fe3O4 (111) 2 × 2 surface due to chemisorption of CCl4, one on top of surface-terminating iron atoms (Cl bound to surface irons) and the other at 3-fold oxygen vacancy sites (Cl bound to subsurface irons). The ratio of the number of chlorine species on top sites to the number at 3-fold oxygen vacancy sites is approximately 9:1, which is dramatically different from the 1:10 ratio observed when CCl4 is dosed at room temperature. The difference in the ratio of these two chlorine species can be explained with a competitive surface reaction picture in which phosgene evolution/surface oxygen atom abstraction, leading to chlorine species at 3-fold oxygen vacancy sites, can only favorably compete with recombination and association reactions, leading to chlorine atoms on top sites, near room temperature. Theoretical calculations were performed that predict an activation barrier of 0.16 eV for the production of phosgene from CCl4 reacting with the iron-terminated Fe3O4 (111) 2 × 2 surface.

Inelastic Neutron Scattering of Hydrogen and Butyronitrile Adsorbed on Raney-Co Catalysts

Inelastic Neutron Scattering (INS) spectroscopy was used to characterize the catalytic hydrogenation of butyronitrile on the surface of parent and LiOH-modified Raney-Co. At low pressure hydrogen is mainly adsorbed on 3-fold sites. Co-adsorbed butyronitrile and hydrogen react on the unmodified Raney-Co sample to several partially hydrogenated products that contain N-H bonds. The nature of the species suggests that bi-molecular condensation leading to N-Butylidene-1-butanamine is feasible.

Ordered Local Domain Structures of Decaneselenolate and Dodecaneselenolate Monolayers on Au{111}

Coexisting adsorbate phases in high-coverage decaneselenolate and dodecaneselenolate [CH3(CH2)nSe, n = 9 and 11] self-assembled monolayers on Au{111} have been characterized by scanning tunneling microscopy and consist of two types: a densely packed distorted hexagonal lattice incommensurate to the underlying gold substrate, as revealed by the observation of a moiré pattern, and a commensurate linear missing-row structure. Examination of the nearest neighbor distances in the tightly packed lattice reveal two distinct repeat distances of 4.90 and 5.20 Å, which complements previous surface X-ray data. The linear missing row structure manifests in several variants of the (√3 × 3√3)R30° unit cell differentiated by whether the molecules bind at 2- or 3-fold substrate sites. While the number of molecules within this unit cell is typically two, in some cases an additional alkaneselenolate molecule is located at a site one Au atom lower than the rest. The structural conclusions are supported by excellent agreement of experimental lattice parameters and those derived from molecular packing models. Comparison of the alkaneselenolate data with analogous structural phases reported for alkanethiolate monolayers on Au{111} shows that differences between the two systems can be understood on the basis that self-assembly is guided both by headgroup−headgroup as well as headgroup−substrate interactions.

Adsorption of Atomic H and O on the (111) Surface of Pt3Ni Alloys

To determine the optimal structure and size of a cluster suitable for modeling chemical processes on the (111) surface of Pt_3Ni alloys and to understand how alloying changes the electronic structure, we used the B3LYP flavor of density functional theory (DFT) to study systematically the composition and the characteristics of the Pt/Ni(111) alloy surface for clusters up to 44 atoms. We find that the bulk structure (Pt_3Ni crystal) has the atoms cubic closest packed as in bulk Pt or bulk Ni, but ordered such that each Ni has 12 bonds to Pt atoms and we find that this same pattern is preferred on the (111) surface. As a result, the ordered (111) surface has each Ni surrounded by six Pt atoms. After determining the optimal cluster, we examined the chemisorption of atomic hydrogen and atomic oxygen at all on-top (μ_1), bridge (μ_2), and 3-fold (μ_3) sites on the (111) alloy surface. We find that oxygen binds most strongly at a fcc site (two Pt and one Ni atom) with an adsorption energy of 3.50 eV, which is 0.22 eV stronger than binding to the pure Pt(111) surface. The barrier for the O to migrate about fcc sites shared by the same Ni is 0.23 eV, but to hop over to a site bordering a different Ni is 0.53 eV. In contrast, the barrier on Pt(111) is 0.56 eV. Thus oxygen is much more localized on the Pt/Ni alloy surface than on pure Pt. A similar behavior was observed for the chemisorption of hydrogen. Here we find that the best site is on-top of Pt with D_e = 2.67 eV, which is comparable to pure Pt(111). In contrast to Pt(111) where the barrier for migration through the bridge and fcc sites is only 0.06 eV, making H very mobile, we find that on Pt/Ni(111) the adsorption energies range from 1.78 to 2.67 eV. Here the migration of the H from one Pt site to the next goes through the bridging and fcc sites sharing a Ni with a barrier of 0.12 eV, twice that for Pt(111).

Theory of Methane Dehydrogenation on Pt{110}(1 × 2). Part I: Chemisorption of CHx(x= 0 −3)

The chemisorption of methane dissociation intermediates, CHx (x = 0−3), on Pt{110}(1 × 2) has been investigated using calculations based on density functional theory. For all species considered, the most stable adsorption site identified on the missing-row reconstructed (1 × 2) surface is the site that not only completes the tetravalency of the carbon atom but also involves the maximum number of ridge Pt atoms for a site of that type. Thus, methyl (CH3) preferentially occupies the ridge atop site; methylene (CH2), the ridge bridge site; methylidyne (CH), the fcc 3-fold site on the {111} microfacet; and carbon, the pseudosubsurface 4-fold site at the bottom of the trough. A comparison of the relative stability of the chemisorbed CHx (x = 0−3) species reveals that CH is the most stable dissociation intermediate on Pt{110}(1 × 2).

Atomic and Molecular Adsorption on Ir(111)

The chemisorption of atomic (H, O, N, S, C), molecular (N2, CO, NO, NH3), and radical (CH3, OH, NOH) species on Ir(111) has been systematically studied. Self-consistent, periodic, density functional theory (DFT-GGA) calculations, using PW91 and RPBE functionals, have been used to determine preferred binding sites, chemisorbed structures, binding energies, vibrational frequencies, and the effect of surface relaxation for the above species at 0.25 ML surface coverage. The following order in binding energies from least to most strongly bound was determined: N2 < NH3 < NO < CH3 < CO < OH < H < NOH < O < N < S < C. A preference for 3-fold sites for the atomic adsorbates was observed, with the exception of atomic H, which prefers top sites. Molecular species showed a preference for top sites with the exception of NOH; this species preferred fcc sites. Surface relaxation had only a small effect on energetics in most cases. Calculated vibrational frequencies, in general, were in good agreement with experimental frequencies and support experimentally proposed site preferences for all adsorbates where data are available. Finally, the thermochemistry of CO, NO, NOH, N2, NH3, and CH3 decomposition on Ir(111) was examined, leading to predictions for each species of the preference for either desorption or decomposition.

Understanding the influence of support alkalinity on the hydrogen and oxygen chemisorption properties of Pt particles

The influence of the alkalinity of the support on the chemisorption of oxygen on Pt particles has been studied using Pt L3 and L2 X-ray absorption near edge data from Pt/Al2O3 doped with F or Rb. The O binding site is determined by using a procedure similar to that previously utilised for H chemisorption. The Pt L3 X-ray absorption edge data of the sample in vacuum is subtracted from the data obtained after chemisorption of oxygen. The signature of this difference spectrum is compared with that resulting from a similar procedure applied to theoretical data, the latter obtained with the help of full multiple scattering ab initio calculations on small model Pt4 clusters using the FEFF8 code. After O2 exposure of the F doped alumina, oxygen is found in hcp/sub-surface binding sites, whereas in the Rb doped case oxygen is present in the bridged/fcc sites. These results imply that the properties of the support have a strong effect on the oxygen chemisorption properties of small Pt particles. Since O is known to bond in a bridged position near step edges or corners at low coverage, followed by 3-fold fcc sites on flat faces or terraces at higher coverage, and then finally at hcp sites and/or in the subsurface region, these data indicate that the Pt–O bond strength and hence coverage increases with support acidity. This is in contrast to that found previously for the Pt–H bond, which decreases in strength with support acidity. Consistent with these results, it is concluded from density functional theory (DFT) calculations on model Pt4 clusters that the isolated Pt–O bond strength is indeed significantly altered by the support and in the opposite direction to that reported previously for Pt–H bonding. The calculations also show that the Pt–O bond ionic character is dependent on the properties of the support. The increased Pt–O ionic character in a basic support gives rise to a stronger O–O lateral repulsion. This lateral interaction, along with the decreased Pt–O bond strength, decreases the O coverage for basic supports. A survey of the recent literature reporting DFT and experimental results on H and O adsorption on Pt(111) and Pt clusters is given.

Theoretical Study of the Adsorption of Bisulfate on Small Gold Clusters

The results of density functional theory energy calculations of bisulfate adsorbed to gold atom clusters are presented and discussed. The gold atom clusters are used to model a gold(111) surface. These calculations involve the Becke three-parameter functional to approximate the exchange energy and the Lee, Yang, and Parr approximation to include the electronic correlation contribution. The structure and stability of the bisulfate ion at different adsorption sites on the gold surface are examined. The bisulfate in its tripod orientation is found to be the most stable configuration due to the direct interaction of each oxygen of the tripod with a gold atom of the surface. The binding of the bisulfate is predominantly electrostatic and is strongly dependent on the local charge of the gold up to second-nearest neighbors from the adsorption site. Additionally, the bisulfate prefers the face-centered cubic 3-fold hollow adsorption site directly above a hollow in the second layer and a gold atom in the third layer.

Theory of CO adsorption on Co{ [formula omitted

We report on density functional calculations for CO adsorbed on the Co {1 0 1 ̄ 0} surface. Energetic and vibrational results for half-monolayer coverage are difficult to interpret, but nevertheless are most consistent with existing experimental evidence for adsorption at atop sites. At monolayer coverage, a clear preference for adsorption at either short-bridge or 3-fold sites emerges. Bond lengths are found to be largely independent of coverage, although increased packing density does lead to a marked molecular tilt, alternating along the surface ridges.

Adsorption and dissociation reaction of carbon dioxide on Ni(1 1 1) surface: molecular orbital study

Calculations based on the atom-superposition and electron-delocalization molecular orbital (ASED-MO) method show that at low coverage, CO 2 binds on the di-σ (1), Δ, μ, and π/μ sites in the bent configuration of lying-down, and on the 1-fold, 2-fold, and 3-fold sites in the perpendicular configuration. The surface adsorbed binding site of carbon dioxide is strongest at the di-σ (1) site and weakest at π/μ site. Chemisorbed CO 2 bends because of metal mixing with 2π μ→6a 1 CO 2 orbital. The dissociation reaction occurred in two steps, with an intermediate complex composed of atomic oxygen and π-bonded CO. The OCO (1) angle of reaction intermediate complex structure for the dissociation reaction is 100° on the Ni(1 1 1) surface. The activation energy was 1.27 eV.

In Situ Scanning Tunneling Microscopy of Underpotential Deposition of Copper at Pt(100) Electrodes Coated with an Iodine Monolayer

Reported here are in situ scanning tunneling microscopy (STM) results of underpotential deposition (UPD) of copper at an iodine-modified Pt(100) electrode. The cyclic voltammograms reveal that an iodine adlayer strongly adsorbed on Pt(100) resulted in a 350 mV delay of Cu deposition with respect to that of a bare Pt(100) electrode. High-quality in situ STM imaging reveals the atomic structures of the iodine adlayers before and after the deposition of Cu adatoms. Depending on the dosage of iodine vapor to an annealed Pt(100) electrode, two ordered structures of the iodine adatoms are formed and are characterized as ( × 5 )R45° and ( × 9 )R45° with coverage of 0.6 and 0.55, respectively. The predominant inter-adsorbate interaction results in adsorption of iodine at either symmetric 4-fold or asymmetric sites. Since the Cu deposit displaces the iodine adatoms on Pt(100), the iodine adatoms with weaker surface bonding are replaced first, leading to anisotropic deposition. Regardless of the initial iodine stru...

Interaction between Catalyst and Support. 2. Low Coverage of Co and Ni at the Alumina Surface

The electronic and geometric structure of α-Al2O3 (0001) surface with and without adsorbed Co and Ni atoms has been investigated using the full-potential linearized augmented plane-wave density-functional theory method. It has been found that the truncated α-Al2O3 (0001) surface undergoes a large surface reconstruction from its bulk structure, which is further changed upon the metal atom adsorption. Geometries, energies, and electronic properties of the partially optimized and the truncated undistorted α-Al2O3 slabs are compared. Electronic “surface state levels” due to the unsatisfied bonding of the Al atoms at both ends of the slab are identified. Among several geometries, the 3-fold oxygen site has been found to be the only stable adsorption site for both Co and Ni atoms. Several factors determine the metal−support interaction between the Co (or Ni) atom and the α-Al2O3 substrate. Among these factors, the “screened ligand field” effects of partially occupied 3d electrons and the further relaxation of t...

Modelling of Pd and Pt supported on the {111} and {011} surfaces of cubic-ZrO2

The interaction of the noble metals Pd and Pt with the {111} and {011} surfaces of cubic-ZrO2 was examined by employing pseudo-potential plane-wave calculations within two Hamiltonians: the LDA and Perdew–Wang91 (GGA). The most favourable adsorption sites, adsorption energies, density of states and electron-charge density distributions are discussed, where the latter properties provide information on the bonding character between the ad-layer and the support, which is directly related to the catalytic activity. Owing to surface relaxation on the Pd/ZrO2{111} interface, we found a competition between the 1-fold and 3-fold oxygen sites; i.e. the differences in adsorption energies between the various adsorption sites are ca. 20 kJ mol−1, which in addition suggests that the Pd-adlayer may have significant mobility. We anticipated the Pt-adlayer to be associated with lower surface mobility than the Pd-layer, due to the tendency to favour one particular adsorption site: ΔEads on top of Os is ∽100 kJ mol−1, while on top of the 3-fold oxygen sites the energy is ∽60 kJ mol−1. Also, on the {011} surface, Pt is favoured by 1-fold oxygen coordination, while Pd exhibits higher oxygen coordination, and is adsorbed on the terraces of the {011} surface. Our calculations show the metal–support interactions to be mainly of an electrostatic nature, introduced by a polarisation of the metal ad-layers.

Weak Ag +–Ag + bonding in zeolite X. Crystal structures of Ag 92Si 100Al 92O 384 hydrated and fully dehydrated in flowing oxygen

Two crystal structures of fully Ag +-exchanged zeolite X, one hydrated ( a=24.996(4) Å) and the other fully dehydrated ( a=25.200(4) Å), have been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd 3 ̄ at 21(1)°C. Each initial Na 92–X crystal was ion exchanged in a flowing stream of 0.05 M aqueous AgNO 3. The second crystal was dehydrated at 360°C for two days in a flowing stream of oxygen gas (790 Torr) followed by evacuation at 400°C and 2×10 −6 Torr for 2 h. Their structures were refined to the final error indexes R 1/ R 2=0.088/0.104 with 216 reflections, and R 1/ R 2=0.047/0.041 with 312 reflections, respectively, for which I>3 σ( I). Both structures show weakly attractive 3.0–3.3 Å Ag +–Ag + interactions. In the hydrated crystal, 92 Ag + ions were found at seven crystallographic sites: 16 fill site I at the centers of the double six-rings, 16 at site I ′ in the sodalite cavities opposite double six-rings bond weakly (3.045(3) Å) to those at site I, 32 fill site II in the supercages, and 28 occupy four different III ′ sites. Some H 2O molecules were found at two different 3-fold axis sites: 16 coordinate to site I ′ Ag + ions in the sodalite cavities, and 32 coordinate to site II Ag + ions in the supercage. In the dehydrated crystal, Ag ions or atoms were found at eight crystallographic sites: three Ag + ions are at site I, 26 Ag + ions and six Ag 0 atoms are at two I ′ sites in the sodalite cavities filling site I ′, 32 Ag + ions fill site II as in crystal 1, two Ag 0 atoms are on 2-fold axes in the sodalite cavities, and 23 Ag + ions occupy three different III ′ sites. The 26 Ag + ions at site I ′ bond weakly in pairs (3.224(3) Å). Three linear Ag 3 + clusters per unit cell with atoms at sites I ′, I, and I ′, respectively, lie along 3-fold axes, and two bent 168(2)° Ag 3 2+ clusters per unit cell are in the sodalite cavities. It remains possible, considering Ag–Ag and Ag–O distances, that no Ag 0 atoms have formed, that the product is (Ag +) 92–X, and that the bonding in the clusters, both of which would then be Ag 3 3+, is due to additional Ag +–Ag + interactions.

Investigation of the energetics of the decomposition of methyl iodide on Ni(111) surface by the method of BOC–MP

The decomposition of methyl iodide on Ni(111) surface alone or in presence of hydrogen gives rise to the desorption of methane which appears as two peaks at two distinctly different temperatures (150 and 250 K) in the course of a temperature programmed desorption study (TPD). The method of bond order conservation–Morse potential (BOC–MP) analysis has been employed to rationalize the experimental findings by calculating the energies associated with the envisaged routes of the reactions. It is concluded that methyl groups adsorbed on 3-fold sites react with adsorbed hydrogen atoms with an activation energy of 14 kcal/mol to form gaseous methane desorbing at 250 K. The methyl group made mobile upon the crowdedness of the surface and traversing over on-top sites and having higher energy than the methyl groups adsorbed on 3-fold sites react with hydrogen with zero activation energy and forms the methane peak at 150 K in the TPD regime. The cleavage of CI bond with an activation energy of 3 kcal/mol constitutes the rate determining step. The prediction capabilities of the method are further proven by briefly considering the process on Ni(100) and Ni(110) surfaces where in the former excellent agreement with the experimental findings is met and in the latter the theoretical findings are brought to the attention of the experimentalist.

Thermochemistry for Hydrocarbon Intermediates Chemisorbed on Metal Surfaces: CHn-m(CH3)m with n = 1, 2, 3 and m ≤ n on Pt, Ir, Os, Pd, Rh, and Ru

To provide insight and understanding of the thermochemistry underlying hydrocarbon rearrangements on transition metal surfaces, we report systematic studies of hydrocarbon radicals chemisorbed on metal clusters representing the closest packed surfaces of the six second and third row group VIII transition metals. Using first principles quantum mechanics [nonlocal density functional theory with exact HF exchange (B3LYP)], we find that (i) CH_(3-m)(CH_3)_m forms one bond to the surface, preferring the on-top site (η^1), (ii) CH_(2-m)(CH_3)_m forms two bonds to the surface, preferring the bridge site (η^2), and (iii) CH_(1-m)(CH_3)_m forms three bonds to the surface, preferring the 3-fold site (η^3). For all six metals, the adiabatic bond energy is nearly proportional to the number of bonds to the surface, but there are dramatic decreases in the bond energy with successive methyl substitution. Thus from CH_3 to CH_2CH_3, CH(CH_3)_2, and C(CH_3)_3, the binding energy decreases by 6, 14, and 23 kcal/mol, respectively (out of ∼50). From CH_2 to CHCH_3 and C(CH_3)_2, the binding energy decreases by 8 and 22 kcal/mol, respectively (out of ∼100). These decreases due to methyl substitution can be understood in terms of steric repulsion with the electrons of the metal surface. For CH to C(CH_3), the bond energy decreases by 13 kcal/mol (out of ∼160), which is due to electronic promotion energies. These results are cast in terms of a thermochemical group additivity framework for hydrocarbons on metal surfaces similar to the Benson scheme so useful for gas-phase hydrocarbons. This is used to predict the chemisorption energies of more complex adsorbates.

Molecular structure of ternary Ge–As–Te glasses studied by Mössbauer spectroscopy

Local environments in As–Ge–Te glasses are studied using 125Te and 119Sn Mössbauer spectroscopy. Complementary to previous works on the binary As–Te and Ge–Te systems, the influence of Te content is reported in the ternary As–Ge–Te system. Particular attention is made in fitting the observed lineshapes, first using a `classical' method and second using a quadrupole splitting distribution. The results show evidence of three Te local environments, a 3-fold co-ordinated site and a pair of 2-fold co-ordinated sites. The concentration of the 3-fold co-ordinated Te site is found to increase with As content of glasses, suggesting specific microscopic structures of the observed sites.

Towards understanding the mechanism for the selective hydrogenation of maleic anhydride to tetrahydrofuran over palladium

First-principles density functional (DFT) quantum chemical calculations were carried out to understand the overall energetics for the hydrogenation of maleic anhydride to tetrahydrofuran (THF), over a Pd(1 1 1) cluster model. The calculated vapor phase structures and vibrational frequencies for maleic anhydride, succinic anhydride, γ-butyrolactone and THF compare well with the reported experimental X-ray crystal structure data and infrared (IR) frequency measurements. The overall reaction energies for vapor phase maleic anhydride hydrogenation to THF, determined using DFT, are within 5 kcal/mol of the enthalpies of reaction, based on standard heats of formation. The adsorption structures for maleic anhydride, succinic anhydride, γ-butyrolactone, THF, water and atomic hydrogen were completely optimized on a fixed Pd(12,7) cluster model of the Pd(1 1 1) surface. The binding energies for maleic anhydride on the Pd 19 cluster in the di-σ, π and η 1 adsorption modes were −83, −34 and −28 kJ/mol, respectively. The computed adsorption energy and vibrational frequencies for di-σ bound maleic anhydride on Pd(1 1 1) are in good agreement with temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS) measurements of Xu and Goodman [Langmuir 12 (1996) 1807-1816]. Preliminary calculations indicate that the most favorable adsorption mode for succinic anhydride, γ-butyrolactone, THF and water on Pd(1 1 1) is η 1, with binding energies of −28, −38, −45 and −30 kJ/mol, respectively. The energetically most stable adsorption site for atomic hydrogen on the Pd(1 1 1) surface is the 3-fold fcc site, with a binding energy of −257 kJ/mol. Towards understanding the detailed reaction mechanism on Pd(1 1 1), we have postulated elementary reaction pathways for C–H bond formation in maleic anhydride hydrogenation and the ring opening reaction of maleic anhydride on Pd(1 1 1). The DFT-computed activation barrier for C–H bond formation in maleic anhydride hydrogenation to maleic anhydryl on a Pd 19 cluster is +82 kJ/mol and the energy of reaction is −9 kJ/mol. The ring opening reaction of maleic anhydride on Pd(1 1 1) has an activation barrier of +163 kJ/mol and is endothermic by 90 kJ/mol. The activation barrier on Re(0 0 0 1), however, is only +80 kJ/mol, whereas the reaction is exothermic by −96 kJ/mol. These results are consistent with the observations from UHV experiments.

First-principles analysis of the hydrogenation of carbon monoxide over palladium

The reaction paths for the hydrogenation of CO to methanol over Pdx (x = 1–4 and 19) cluster models were examined using first-principle density functional quantum chemical calculations. The predicted adsorption energies for the most favorable binding modes for CO, H2, HCO, H3CO, CH3OH, C, O and H on a Pd19 model Pd(111) clusters were -147, -62, -340, -51, -195, -33, -610, -349 and -251 kJ/mol, respectively. The most favorable modes for CO, CH3O, H, C and O on Pd(111) were all found to be the 3-fold fcc site. The most favorable modes for the formyl and formaldehyde surface intermediates at low coverage were the 3-fold (ζ2µ3), and the di-σ sites, respectively. At higher surface coverages, however, the atop ζ1 (C) and the π modes for the formyl and formaldehyde intermediates were more likely. The computed adsorption energies were subsequently used to compute overall reaction energies for the hydrogenation of CO to methanol. The initial hydrogenation of CO to the ζ1 (C) HCO intermediate was found to be +52 kJ/mol endothermic and has been speculated as a possible rate-limiting step. The remaining surface hydrogenation steps become increasingly more exothermic as more hydrogen was added. The elementary steps of formyl to formaldehyde, formaldehyde to methoxide and methoxide to methanol were computed to be -9, -26 and -33 kJ/mol, respectively. The overall energy for CO dissociation was found to be highly unlikely at +260 kJ/mol and a clear indication that methanation and chain growth chemistry is not very likely over Pd. The most favorable reaction coordinate for the hydrogenation of CO to the ζ1 (C) formyl intermediate was that which proceeds over a single Pd site where there is a migratory insertion of the CO into a Pd–H bond. The barrier for this path was computed to be +78 kJ/mol on the Pd19 cluster. There was a very weak dependence on cluster size. This is a likely indication that this reaction is structure insensitive. A second path which involved the coupling of H and CO over a bridge site was found to be +130 kJ/mol which is less likely, but may also occur under different conditions.

Chemisorption of Organics on Platinum. 2. Chemisorption of C2Hx and CHx on Pt(111)

Using the interstitial electron surface model (IESM) developed in the accompanying part, we examined the structures and energetics of a number of organic fragments on Pt surfaces. Using nonlocal density functional methods (B3LYP) we find that organics covalently bond to the Pt(111) surface with localized σ bonds to the surface Pt atoms, leading to tetrahedral hybridization of each carbon bonded to the surface. Thus, (i) CH_3 prefers an on-top site (a bond energy of ∼54 kcal/mol), (ii) CH_2 prefers a 2-fold bridge site (a bond energy of ∼104 kcal/mol), and (iii) CH prefers the fcc 3-fold bridge site (a bond energy of ∼167 kcal/mol). Similarly, C_2H_4 forms a strong (36 kcal/mol) di-σ bond (the π bond is BE = 8.5 kcal/mol), while CHCH_2 forms a tri-σ bond. The results for C_2H_x/Pt_8 are in good agreement with available experimental results on Pt(111) (π- and di-σ-bonded ethylene and ethylidyne). These results are used to obtain heats of formation (ΔH_f) for chemisorbed intermediates useful in estimating the energetics of various hydrocarbon intermediates on Pt surfaces. The application of these ΔH_f values is illustrated by considering ethylene hydrogenation and the decomposition of C_2H_4 on Pt(111).