Influence of the substrate structure on the bonding of chemisorbed acetylene to transition metal surfaces

The electronic spectrum of acetylene adsorbed on various transition metals has been measured by ultraviolet photoemission spectroscopy in various laboratories. At low temperatures (T < 150 K), all measurements concur in finding an electron spectrum that differs only moderately from the gas phase spectrum of acetylene. At room temperature, the electron spectrum of acetylene is reported to be similar to the low-temperature form on Ir(100) and Pt(100), but acetylene is reported to form an olefinic surface complex on Pd(111) and Pt(111) surfaces. In order to examine whether the surface structure of the substrate is responsible for the difference, we have measured the electronic spectrum of acetylene adsorbed on the Pd(100) and Ru(0001) surfaces. At 120 K, the spectrum of adsorbed acetylene is again a distorted gas phase spectrum on both surfaces. At 330 K, we find the acetylenic form (with a splitting of 2.5 eV of the σ-orbitals) on Pd(100) and an olefinic form on the basal plane of Ru. We conclude that the olefinic complex is proper to the threefold symmetry of the (111) and (0001) surfaces and the gas-like form is favored on the (100) surfaces of the fcc crystals.     

Propagation distances of surface electromagnetic waves on Ni,Pd, Pt,and W metal surfaces

The propagation distances, L_x, of surface electromagnetic waves (SEW) on Ni, Pd, Pt, and W were measured using the two-prism coupling technique at wavelengths from 9.3 to 10.6 microm. Metal samples consisted of films, deposited on glass substrates by electron beam deposition, and metal foils. The measured L_x values for the Pd and Pt firms and Pt foil agree to within 20% of the calculated values from the two-media dispersion relation. The measured values of the propagation distance for the Ni film and foil and the W foil agree to within 50% of the calculated L_x. Scanning electron microscope (SEM) photographs and Auger analysis indicate that film imperfections and foil surface roughness contributed to the lower propagation distances observed for Pt and Pd while imperfections, roughness, and oxidation contributed to the disagreement between measured and calculated propagation distance for Ni and W.     

Experimental study of the vibrations of acetylene chemisorbed on Ni(111)

High resolution electron energy-loss measurements of normal and deuterated acetylene chemisorbed on Ni(111) have been obtained. Observed vibrational modes are identified using the frequency shifts for the deuterated species and comparisons to the free molecule and a di-cobalt compound of acetylene. These vibrational frequencies indicate that chemisorbed acetylene is strongly rehybridized having a state of hybridization between ~sp^2.5 and sp³. Consideration of the types of modes observed, their assignments and the surface selection rule suggests a molecular orientation with the C-C bond axis slightly skewed relative to the surface and with the plane of the distorted molecule normal to the surface. A bonding geometry is proposed which has the carbon atoms residing above two adjacent 3 fold hollow sites of the Ni surface. This molecular geometry differs from that deduced previously by electron energy-loss spectroscopy for molecularly adsorbed acetylene on Pt(111).     

Low temperature electron energy loss spectra of acetylene chemisorbed on metal single-crystal surfaces; Cu(111), Ni(110) and Pd(110)

Vibrational spectra of acetylene chemisorbed on Cu(111), Ni(110) and Pd(110) at 110–120 K were measured using electron energy loss spectroscopy. Loss peaks were assigned to vibrational modes of the non-dissociatively adsorbed molecules with the aid of the corresponding C₂D₂ spectra. The spectra show that the molecules undergo significant rehybridisation on adsorption. Comparisons are made with the spectra of acetylene adsorbed on a range of other transition metal surfaces at low temperature. Taking into account these and earlier literature results, two distinct patterns of spectra are observed (Type A and Type B) for specular spectra. The Cu(111) spectrum is classified as Type A while the Ni(110) and Pd(110) spectra are classified as Type B. Suggestions are made for the structures of the surface species corresponding to the two spectral types.     

On the structure of chemisorbed sulfur overlayers on Ni(100)

A quarter of a monolayer of sulfur adsorbed on Ni(100) does not form a well-ordered p(2 × 2) overlayer as reported by other authors. In agreement with Hagstrum and Becker the diffraction pattern always showed weak streaks between the superstructure spots and indicates considerable amounts of (2 × 1) domains. Like for the c(2 × 2)S surface (θ_s = 0.5) the frequency of the dipole active sulfur vibration is 350 cm⁻¹ also for θ_s = 0.25.     

Studies on ethylene adsorption on Ni,Pd and Pt films and the reactivity of the selected forms of the adsorbate with hydrogen

A rapidly-recording, static-capacitor method was used to study surface potential changes that occur during ethylene adsorption on Ni, Pd, and Pt films at 298 K. A precise dosing system for the introduction of gases and a careful determination of the roughness factor allowed us to relate the surface potential to the concentration of the various adsorbed species. Using this information we propose a mechanism for ethylene selfhydrogenation. Ethylene admolecules adsorbed in the second layer react readily with hydrogen whereas those from the first layer do not. On platinum films, only the electropositively-polarised β⁺ hydrogen adatoms are reactive with ethylene, but with Ni and Pd films only the electronegatively-polarised β^? hydrogen adatoms are reactive.     

Thermal evolution of acetylene and ethylene on Pd(111)

The thermal evolution of acetylene and ethylene and their deuterated counterparts on a palladium (111) surface has been studied by high-resolution electron energy loss spectroscopy in the temperature range 150–500 K. Analysis of the vibrational spectra indicates that chemisorbed acetylene evolves at 300 K in the presence of surface hydrogen to mainly ethylidyne, CCH₃, and a small amount of residual acetylene. Spectra obtained with and without preadsorbed hydrogen provide evidence for a 〉C CH₂ intermediate in the reaction. Chemisorbed ethylene also evolves to ethylidyne after heating from 150 to 300 K but much of the ethylene desorbs. The high temperature (400–500 K) behavior of C₂H₂ and C₂H₄ involves formation of a CH species. Although a small amount of the CH species may be formed from the dehydrogenation of ethylidyne, it is found that carbon-carbon bond scission of acetylene near 400 K is the dominant mechanism in CH formation.     

Ni,Pd和Pt的表面应力

半经验的修正嵌入原子方法用于Ni，Pd和Pt的低指数面的表面应力计算，得到了与第一原理计算相符合的结果．给出了（110）表面［110］方向的应力是［001］方向应力的两倍左右；阐明了应力各向异性是所有fc金属（110）面的一般特性．预言了Pd和Pt（100）的表面应力的大小． 关键词：     

Highly active surfaces for CO oxidation on Rh, Pd, and Pt

Studies show that the rate of CO oxidation on Pt-group metals at temperatures between 450 and 600 K and pressures between 1 and 300 Torr increases markedly with an increase in the O₂/CO ratio above 0.5. The catalytic surfaces, formed at discrete O₂/CO ratios >0.5, exhibit rates 2-3 orders of magnitude greater than those rates observed for stoichiometric reaction conditions and similar reactant pressures or previously in ultrahigh vacuum studies at any reactant conditions and extrapolate to the collision limit of CO in the absence of mass transfer limitations. The O₂/CO ratios required to achieve these so-called “hyperactive” states (where the reaction probabilities of CO are thought to approach unity) for Rh, Pd, and Pt relate directly to the adsorption energies of oxygen, the heats of formation of the bulk oxides, and the metal particle sizes. Auger spectroscopy and X-ray photoemission spectroscopy reveal that the hyperactive surfaces consist of approximate 1 ML of surface oxygen. In situ polarization modulation reflectance absorption infrared spectroscopy measurements coupled with no detectable adsorbed CO. In contrast, under stoichiometric O₂/CO conditions and similar temperatures and pressures, Rh, Pd, and Pt are essentially saturated with chemisorbed CO and exhibit far less activity for CO oxidation.     

EELS analysis of the low temperature phase of ethylene chemisorbed on Pd(111)

The chemisorption of C₂H₄ and C₂D₄ on Pd(111) at 150 K has been studied by high resolution electron energy loss spectroscopy. Analysis of the vibrational spectra indicates that (i) C₂H₄ is more weakly bound on Pd(111) than on Ni(111) and Pt(111) and (ii) softened and broadened CH stretching frequencies suggest hydrogen bond-like interactions between the molecule and the metal surface.     

The interaction of acetylene with Ni(111), chemisorbed oxygen on Ni(111), and NiO(111); the formation of CH species on chemically modified Ni(111) surfaces

Ultraviolet photoelectron spectroscopy, temperature programmed thermal desorption and low-energy electron diffraction have been used to study the interaction of acetylene with a clean Ni(111) surface, with a Ni(111) surface having co-adsorbed oxygen and with an epitaxially grown NiO(111) surface produced by room temperature oxidation ofNi(111). The adsorption of a (2 × 2) overiayer of π-bonded acetylene or oxygen on the Ni(111) surface markedly alters the subsequent interaction and reaction of the surface with incident acetylene. We find that in the presence of either a (2 × 2) overiayer of oxygen or π-bonded acetylene, a new more strongly bound hydrocarbon phase forms at room temperature. We identify this new phase from its ionization levels as a CH species, and for saturation coverages we find approximately twice as many of these species as the number of π-bonded acetylene molecules in the (2 × 2) structure. Preadsorption of oxygen limits the adsorption of π-bonded acetylene but does not affect the subsequent formation of this CH species. Exposure of acetylene to NiO at room temperature produces only CH species. Based upon these results we propose idealized models for the bonding geometry of π-bonded acetylene and CH species on the Ni(111) surface. The conditions for the formation of CH species and the significance of CH species to surface reactions on Ni are also discussed.     

Theoretical study of the Ni growth on Pt stepped surfaces

We have investigated the growth of Ni on Pt stepped surfaces with (1 1 1) terraces by means of potentials derived from the second moment approximation in a tight-binding model. The activation energies associated to these processes are determined. The Schwoebel barriers of Ni atoms descending steps of Pt stepped surfaces are calculated for different kinds of straight steps (A and B steps) differing by the orientation of the ledge. In addition, we study the diffusion of Ni adatoms at fcc or hcp sites in the presence of small adislands on the terraces, in the vicinity of the A and B steps. We show that a good estimate of the potential wells and diffusion barriers could be given by introducing a lateral effective pair interaction model, the interactions extending up to the next nearest neighbors. Finally, we have carried out Kinetic Monte-Carlo simulations to investigate the Ni wire formation at Pt step edges and the influence of the exchange processes in the alloy formation.     

Carbon segregation to single crystal surfaces of Pt,Pd and Co

Results are reported on the surface segregation behaviour of carbon from dilute solid solutions in Pt, Pd and Co. With Pt(100) no preferential surface segregation was observed; this is similar to previous results for Pt(111). For Pd(lOO), Pd(111) and Co(0001) segregation was observed with evidence for a surface phase transition of the type previously reported for Ni(111). These observations suggest that the strong carbon-carbon interactions within a graphite monolayer are of more importance in producing the transition than a good epitaxial fit to the substrate. A comparison of the kinetics of carbon segregation to Co(0001) with those predicted by a simple diffusion model suggest that surface processes such as nucleation or lateral diffusion may play important roles.     

Dehydrogenation of acetylene and ethylene studied on clean and oxygen covered palladium surfaces

The interaction of acetylene and ethylene with a clean and oxygen covered Pd surface has been studied at a temperature of 473 K. The measurements were performed on a hydrogen sensitive Pd-MOS structure making it possible to obtain direct information on the dissociation of both hydrogen and oxygen containing species on a palladium surface. Desorption studies were also performed as well as ultraviolet photoelectron spectroscopy and work function measurements. The studies show that both acetylene and ethylene adsorb dissociatively at this temperature leaving mainly carbon on the surface. When an oxygen covered Pd surface is exposed to C₂H₂ or C₂H₄ carbon dioxide and water will be formed and desorb until the surface is oxygen free. In the case of acetylene the presence of preadsorbed oxygen does not block or prevent the C₂H₂ dissociation on the surface. For C₂H₄, a large preadsorbed oxygen coverage (⪆ 0.45) will have an impeding effect on the dissociation. The CO₂ desorption is oxygen coverage dependent contrary to the H₂O desorption. This is due to the fact that hydrogen has a large lateral mobility on the surface while carbon has not. Both the CO₂ and H₂O reactions are, however, due to the same type of mechanisms.     

Electronic structure of H chemisorbed on Si(111) surfaces

We study the electronic density of states of hydrogen chemisorbed (111) Si surface. We analyse two situations: one single chemisorbed hydrogen atom in an otherwise clean surface and a complete monolayer of hydrogen chemisorbed at the surface. The method of calculation is based on an extension of the cluster-Bethe lattice approximation developed by the authors to study surfaces. Our results for the monolayer are in good agreement with UPS data, as well as with other theoretical calculations.     

On the interpretation of the photoelectron spectrum of CO chemisorbed on Ni

A LCAO-SCF calculation on the NiCO molecule is reported. In addition to the ionization potentials the vibrational structure due to ionization is calculated. The computed ionization potentials are compared with those of recent semiempirical calculations on the CO — Ni system and with the photoelectron spectra of CO chemisorbed on nickel surfaces. We produce further evidence that the first peak in the photoelectron spectrum relates to the 5σ and 1π orbitals of CO and that the peak at higher ionization energy represents the 4σ orbital.