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.     

A DFT study of H adsorption on Pt(111) and Pt-Ru(111) surfaces

In this work a comparative analysis between different Pt-Ru(111) surface models and pure Pt(111) surface is presented. Some aspects of the electronic structure of the surfaces and hydrogen adsorption are analysed based on density functional theory calculations. The hydrogen adsorption energy is significantly reduced when Ru is present on the surface. The substitution of Pt atoms by Ru atoms reinforce the Pt-H bond while the metal-metal bond is strongly modified, making the system less stable.     

Adsorption of acetylene and ethylene on a clean Ir(111) surface

The adsorption of C₂H₂ and C₂H₄ on Ir(111) is studied within the temperature range 180–500 K by the HREELS and XPS methods. The absolute concentration of hydrocarbon coverage is estimated by XPS. Data are obtained on the kinetics of adsorption of the two gases at different temperatures. It is established by HREELS studies that at 180 K C₂H₄ forms ethylidyne (CCH₃ whereas C₂H₂ is adsorbed as CCH and ethylidyne species. At 300 K both kinds of species are found on the Ir(111) surface after C₂H₂ or C₂H₄ exposures. The ethylidyne decomposes completely to CCH at 500 K, which can be accompanied by polymerization of adsorbed hydrocarbon species.     

Ethylene and acetylene adsorption on Cu(111) and Pt(111) studied by Auger spectroscopy

High resolution, electron impact excited, carbon Auger spectra of ethylene and acetylene adsorbed on Cu(111) and Pt(111) are compared. The spectra of ethylene on the two metals provide the first example of the sensitivity of AES to the nature of metal-adsorbate bonding for molecular adsorbates. The acetylene spectra are identical on the two metals. The changes in the carbon Auger spectra resulting from thermal decomposition of the two adsorbates on Pt(111) are discussed in the context of results from electron energy loss spectroscopy.     

Interaction of acetylene and ethylene with an Fe(111) surface

The chemisorption of C₂H₂ and C₂H₄ on a clean or partly C- or O-covered Fe(111) surface was investigated with AES, TDS and HREELS. On the clean surface, both molecules adsorb under strong rehybridization close to sp³. Above 230 K, C₂H₂ reacts to form CH and presumably CH₂ as the main products, which on further heating decompose to yield H₂ desorption maxima at 580 and 490 K, leaving two carbon species on the surface which correspond to two loss peaks at 400 and 1290 cm^?1 in the HREELS spectrum. C₂H₄ undergoes very rapid decomposition above 250 K; no intermediates have been detected. The presence of coadsorbed oxygen or carbon atoms only reduced the maximum uptake of C₂H₂, but led to the appearance of new molecular adsorption states of C₂H₄ and inhibited C₂H₄ decomposition.     

Phase diagram of molecular oxygen adsorption on the (111) platinum surface

Molecular oxygen adsorption on the Pt(111) surface is studied based on ab initio computations and thermodynamics. An O2 adsorption phase diagram is determined. There are two possible chemisorbed molecular states: one at a bridge site and another one at an fcc hollow site. While some population in the bridge sites persists at all coverages, the states coexist through the intermediate coverage phases. The relative coverage of the two species on the surface is determined by the competition between the Pt lattice distortion energy (that results from O2 adsorption) and the O2 repulsion energy. Our results give a reasonable explanation for the seemingly contradictory findings in previous experimental and theoretical work.     

First-principle study of Mg adsorption on Si（111） surfaces

We have carried out first-principle calculations of Mg adsorption on Si(111) surfaces. Different adsorption sites and coverage effects have been considered. We found that the threefold hollow adsorption is energy-favoured in each coverage considered, while for the clean Si(111) surface of metallic feature, we found that 0.25 and 0.5 ML Mg adsorption leads to a semiconducting surface. The results for the electronic behaviour suggest a polarized covalent bonding between the Mg adatom and Si(111) surface.     

The kinetics of dehydrogenated adsorption of ethylene on Ni(111)

The adsorption of ethylene on Ni(111) has been studied with Thermal Desorption and Auger Electron Spectroscopy. With these methods the coexistence of an acetylenic complex and hydrogen for low coverages, and the displacement of the hydrogen by the acetylenic complex for increasing coverage can be quantitatively described.     

CO adsorption studies on pure and Ni-covered Cu(111) surfaces

The adsorption of CO on pure and Ni-covered Cu(111) surfaces has been studied by means of LEED, TDS, UPS and work function measurements during adsorption and desorption. Different Ni-coverages between 0.1 and 2 monolayers were obtained by Ni-evaporation controlled by a quartz micro balance and by AES. Near room temperature Ni grows in a layer-by-layer mode on Cu(111). The island structure of the surfaces with submonolayer Ni-coverages is clearly demonstrated by TDS und LEED results obtained after CO adsorption. As with surfaces of bulk Cu-Ni alloys CO adsorption on Cu(111) with submonolayer Ni-coverage is dominated by a site effect. Cu-, Ni-, and mixed adsorption sites can be distinguished. The CO induced work function changes for Ni- and Cu-site adsorption show the same sign as observed with the pure metals. Mixed site adsorption has only a minor influence on the work function. A “ligand effect” observed only for the Ni-site adsorption, and only at small Ni-coverages is discussed in detail. Studies on the adsorption kinetics reveal that the Cu-sites may serve as precursor sites for Ni-site adsorption. Detailed UPS studies demonstrate that the CO-induced emission maxima observed on Cu surfaces with submonolayer Ni-coverages can be interpreted as a superposition of the respective adsorption features observed with the pure metals, roughly separated by their work function difference.     

The decomposition of ammonia on the flat (111) and stepped (557) platinum crystal surfaces

Ammonia adsorption, desorption and decomposition to H₂ and N₂ has been studied on the flat (111) and stepped (557) single crystal faces of platinum using molecular beam surface scattering techniques. Both surfaces show significant adsorption with sticking coefficients on the order of unity. The stepped (557) surface is 16 times more reactive for decomposition of ammonia to N₂ and H₂ than the flat (111) surface. Kinetic parameters have been determined for the ammonia desorption process from the Pt(111) surface. The mechanism of ammonia decomposition on the (557) face of platinum has been investigated.     

The adsorption of oxygen on silicon (111) surfaces. II

At a wavelength of 546 nm the change of the ellipsometric angles δΔ (relative phase change) and δψ (relative amplitude change) has been studied on clean cleaved silicon (111) surfaces during adsorption of oxygen. δψ increases linearly with exposure up to a saturation value. The saturation dose, i.e. also the sticking coefficient for the corresponding process depends exponentially on the mean step density of the surface. δΔ is nearly independent of step density. The interpretation in terms of a macroscopic theory (Drude model) gives evidence of two adsorption processes, the formation of a monolayer of oxygen controlled by step density and a subsequent further oxidation process.     

Phase transitions on clean and nickel containing vicinal Si(111) surfaces

Structure and reversible structural phase transitions on clean vicinal Si surfaces inclined from the (111) plane towards [2 ] and [ 11] poles and the effect of nickel on the structure and the structural transitions on these surfaces have been studied by LEED. The structures of clean surfaces inclined towards [2 ] and [ 11] are different. Phase transitions take place at about 870°C and 800° C, respectively. Above the transition temperatures these two surfaces consist of regular steps with heights equal to one interplanar distance d₁₁₁ (3.135 Å). The terrace width between steps is determined by the angle of inclination to the (111) plane. Below the transition temperatures the surfaces inclined to [2 ] contain steps with height 3d₁₁₁, those inclined towards the [ 11] pole consist of combinations of (111) facets and some other, apparently (133). The effect of nickel leads to the formation of regular steps with height 2d₁₁₁ on the surfaces inclined towards [2 ] and with height 2d₁₁₁ or 1d₁₁₁, depending on the nickel concentration, on the inclined towards [ 11]. On nickel-containing surfaces there also take place reversible structural transitions with varying temperature.     

An extended hückel study of adsorption of acetylene on Pt(111)

The parameterization of Extended Hückel MO Theory for the study of the interactions between hydrocarbons and Pt is analysed in detail. Single-ζ and double-ζ d functions of variable diffuseness are tested. The stability of acetylene and many molecular fragments and atoms on the surface is discussed in terms of binding energies and hydrocarbon—surface bond stretching force constants. Pt…H non-bonded repulsions are seen to be relevant, and a formula is proposed to describe these interactions. The metal is shown to offer a large catalytic effect in the reactions of rearrangement of acetylene, and, depending on the temperature and the availability of free hydrogen, H₃CC or H₃CCH are shown to be the most likely surface species. The geometries of adsorption compare rather favourably with LEED intensity studies. The use of EHT in the calculation of density of states curves and electronic spectra is discussed.     

A model calculation on adsorption of CO on the flat,stepped and kinked nickel surfaces

The adsorption of CO on Ni was investigated by quantum chemical calculations using the CNDO/2 tight binding method. The surfaces used as models are the (111), 4(111) × (111), 3(111) × (110) and 3(111) × (100) surfaces. The CO bond is weakened in this sequence of surfaces. The active sites for the CO bond fission are the trench regions of the step and kink structures. The Ni 3d orbitals play an important role for the weakening of the CO bond, though their contribution is small for the Ni-C bond formation.     

Crystal field surface orbital-bond energy bond order (CFSO-BEBO) calculations of adsorption: II. Carbon monoxide,oxygen and carbon dioxide on platinum (111) and oxygen on nickel (111)

The Crystal Field Surface Orbital-Bond Energy Bond Order (CFSO-BEBO) model of chemisorption is applied to the interaction of carbon monoxide, oxygen and carbon dioxide with a (111) platinum surface; and the interaction of oxygen with a (111) nickel surface. No activation energy for molecular adsorption of carbon monoxide on platinum is predicted; however a large activation energy for dissociative chemisorption is calculated. The molecular state has a binding energy of approximately 28 kcal/mole, and vibrational stretching frequencies of 1935 and 1975 cm^?1 are calculated by combining the CFSO-BEBO model with Badger's Rule. The adsorption of oxygen on (111) platinum and (111) nickel are predicted to be different in the following respects: (1) There is an activation energy of 2.1–4.5 kcal/mole on platinum, whereas adsorption on nickel is unactivated; and (2) The dissociative heat of chemisorption on platinum is 58–68 kcal/mole, whereas on nickel it is substantially larger, 112–116 kcal/mole. The adsorption of carbon dioxide on (111) platinum is predicted to be not only highly activated but also endothermic. All of the calculated results are essentially in quantitative agreement with available experimental data.