Π-bonded State Research Articles

H–D Exchange Mechanism of Butene on a D-Absorbed Pd–Au Alloy Surface

Complete H–D exchange of butene was reported on a D-absorbed Pd–Au alloy surface. However, its reaction mechanism has not been clarified yet. Here we investigate the reaction of cis-2- and 1-butene on Pd70Au30(110) by means of high-resolution electron energy loss spectroscopy. We demonstrate that both butene isomers are in the π-bonded state and that 1-butene irreversibly converts to cis-2-butene around 250 K. This isomerization accounts for previously observed similar product distributions in the H–D exchange reaction of cis-2- and 1-butene on the D-absorbed Pd70Au30(110) surface. Since the reverse change from cis-2- to 1-butene is not observed at any temperature, we conclude that the H–D exchange proceeds by the dissociative mechanism via vinylic and π-allylic intermediates rather than the Horiuti–Polanyi mechanism.

Theoretical study of adsorption and dehydrogenation of C2H4 on Cu(410)

Adsorption and dehydrogenation of ethylene on Cu(410) surface are investigated with first-principles calculations and micro-kinetics analysis. Ethylene dehydrogenation is found to start from the most stable π-bonded state instead of the previously proposed di-σ-bonded state. Our vibrational frequencies calculations verify the π-bonded adsorption at step sites at low coverage and low surface temperature and di-σ-bonded ethylene on C−C dimer (C2H4-CC) is proposed to be the species contributing to the vibrational peaks experimentally observed at high coverage at 193 K. The presence of C2H4-CC indicates that the dehydrogenation of ethylene on Cu(410) can proceed at temperature as low as 193 K.

Self-Activated Catalyst Layer for Partial Hydrogenation of 1,3-Butadiene on a Hydrogen-Precovered Pd(110) Surface

We have studied partial hydrogenation of 1,3-butadiene to butene on a hydrogen-precovered Pd(110) surface, focusing on how reactants were catalytically activated on a metal surface. Techniques of surface analysis, including temperature-programmed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS), and scanning tunneling microscopy (STM), were utilized to identify chemisorbed states of the active species involved in hydrogenation. For 1,3-butadiene coverage of lower than 0.25 ML, 1,3-butadiene was in a strongly π-bonded state on H/Pd(110) and caused some of the precovered hydrogen atoms to move into the subsurface site. Under this condition, a hydrogen exchange reaction between the adsorbed 1,3-butadiene molecules and precovered hydrogen atoms occurred. At 1,3-butadiene coverage of 0.25 ML, the 1,3-butadiene molecules, which formed a c(4 × 2) layer accompanied with subsurface hydrogen atoms, were less active in hydrogen exchange and hydrogenation. In contrast, when 1,3-butadiene c...

Coverage dependence of the sticking probability of ethylene on Ag(4 1 0)

The interaction of ethylene with Ag(4 1 0), a vicinal surface of Ag(1 0 0) characterised by a high density of open steps, is investigated. Energy and angle resolved measurements of the sticking coefficient are performed with the retarded reflector method of King and Wells using a supersonic molecular beam. We find that open steps remove the translational barrier for adsorption into the π-bonded state present for Ag(1 0 0). Steering removes the angle dependence of the initial sticking probability. The coverage dependence of the adsorption probability indicates that a precursor mechanism is present and that adsorbate assisted adsorption is important at hyperthermal energies. With increasing coverage steering becomes ineffective and rotations inhibit adsorption. Admolecules with second nearest neighbours are destabilised and these sites can be populated only by gas-phase molecules impinging along trajectories for which the step heights are exposed.

Selective Partial Hydrogenation of 1,3-Butadiene to Butene on Pd(110): Specification of Reactant Adsorption States and Product Stability

Selective partial hydrogenation of 1,3-butadiene (C4H6) to butene (C4H8) on hydrogen-precovered Pd(110) has been investigated using temperature-programmed desorption (TPD), scanning tunneling microscopy (STM), and high-resolution electron energy loss spectroscopy (HREELS). Below room temperature, C4H6 was molecularly chemisorbed on a hydrogen-precovered Pd(110) surface and was in the π-bonded chemisorption state. The hydrogenation of C4H6 did not occur in the sparse domains of less than 0.25 ML. With increasing C4H6 coverage, C4H6 formed dense domains, and the π bond was weakened. Only the weakly π bonded C4H6 was hydrogenated to C4H8 at about 200 K. Because the stability of resultant C4H8 was low on the surface at the reaction temperature, C4H8 desorbed immediately after being produced or dehydrogenated to C4H6 if even a part of the C4H8 remained on the surface. We concluded that the balance between the hydrogenation reactivity of C4H6 and the stability of C4H8 leads to the partial hydrogenation on the Pd surface.

The dynamics of ethylene adsorption on Pt(111) into di-σ and π-bonded states

The dynamics of ethylene adsorption on Pt(111) into both the di-σ- and π-bonded states were investigated at 95 and 40 K, respectively, using supersonic molecular beam techniques. The angular dependence of ethylene adsorption into both states is similar to the angular dependence for ethane adsorption, which has a much weaker bond to the surface in its final state. In contrast to ethane, high adsorption probabilities for ethylene prevail to high incident kinetic energies, suggesting that the strong interaction of ethylene with the surface influences adsorption. The initial adsorption probability of ethylene is approximately independent of surface temperature between 40 and 450 K, suggesting that there is no reversible, thermalized intrinsic precursor to adsorption. At 40 K, the adsorption probability increases with coverage (in the π-bonded state). However, at 95 K, the adsorption probability of ethylene remains constant with increasing self-coverage (in the di-σ- bonded state) for trajectories incident with low parallel momentum, but decreases with coverage at high parallel momentum. High parallel momentum may contribute to an increased overall scattering probability from a “hot” extrinsic precursor, resulting in a decreased net adsorption probability at higher ethylene coverage in the rigidly bound di-σ state.

Adsorption and reaction of ethylene and hydrogen on Rh(100) surface

Adsorption and reaction of ethylene on clean and H-presaturated Rh(100) have been investigated using HREELS. Adsorption of ethylene on H-presaturated Rh(100) surface at 110 K leads to a π-bonded state, as indicated by a πσ parameter of 0.39 equivalent to that of Zeise's salt. The formation of an ethyl intermediate from the π-bonded ethylene by insertion of preadsorbed H atom already takes place even at 110 K, leading to the evolution of ethane below 200 K. Upon heating, CC bond breakage is promoted on the H-preadsorbed surface, where the strength of metal to H in the adsorbed ethylene is considered to be reduced by preadsorbed hydrogen.

Quantitative studies of benzonitrile adsorption at the low-index gold single crystal electrodes

The adsorption of benzonitrile (BN) on the Au(111), Au(100), and Au(110) electrodes in aqueous solutions was investigated. The chronocoulometric technique was used to measure the charge density at the metal surface as a function of the electrode potential for BN concentrations ranging from 0 to 0.030 M. The adsorption parameters such as the film pressure, the surface concentration of the organic adsorbate, the standard Gibbs energy of adsorption (Δ G ads 0), and the charge flowing to the interface per one adsorbed molecule, were determined from the thermodynamic analysis of the charge density data. The analysis was carried out at constant electrode potential and at constant charge density. In addition, subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) was employed to determine the orientation of BN molecules at the Au(111) surfaces. The thermodynamic and spectroscopic data indicate that BN molecules assume a flat (π-bonded) state at the negatively charged surface and progressively reorient from the flat to the tilted (N-bonded) state as the charge at the metal surface becomes more positive. At very positive potentials the character of BN adsorption becomes reactive and the adsorbed molecules partially hydrolyze to form benzamide (BA).

Origin of vinylic CH bond activation in the combustion of alkenes on palladium: an HREELS study of propene on Pd(100)-p(2 × 2)-O

While the mechanism of many reactions with oxygen on Ag(110) can be successfully predicted according to the gas-phase acidity of the reacting molecule, recent temperature-programmed reaction spectroscopic study of propene reactions with oxygen on Pd(100) reveals that the most reactive CH bond is the vinylic rather than the most acidic, allylic, CH bond. Using vibrational spectroscopy it is found that two forms of propene species exist on the surface at 130 K: a π-bonded state and a di-σ-bonded state. The former desorbs molecularly upon heating, while the latter undergoes combustion. In the di-σ-bonded state, the sp 3 rehybridization strongly reduces the acidity from its gas-phase value, rendering the methyl hydrogens unreactive. The mechanism for the combustion of propene is proposed.

Adsorption of some substituted ethylene molecules on Pt(111) at 95 K Part 1: NEXAFS, XPS and UPS studies

Abstract The present work examines the interactions of propanoic acid, acrylic acid, acrolein and methylmethacrylate (MMA) with Pt(111) at 95 K to identify the nature of the interactions on this surface. The investigations are carried out by XPS, UPS and NEXAFS on monolayer and multilayer. Theoretical molecular orbital calculations are firstly performed to determine the nature of the bonding and antibonding orbitals of these molecules. The NEXAFS results show that the condensed multilayers of acrylic acid and acrolein are almost oriented parallel to the surface when propanoic acid and MMA are randomly oriented. The monolayer formed at 95 K for all these molecules are also oriented flat on Pt(111). However two different interaction processes are observed depending on the chemical structure of the compound: acrolein and propanoic acid are physisorbed when MMA and acrylic acid are in strong interaction with the metal but with an uncertainty on the chemisorption mode between a π-bonded state or a “di-σ like” state.

Coverage dependence of CO dissociation on clean and hydrogen presaturated Fe(100) surface

A detailed thermal desorption spectroscopy (TDS) study of very low and intermediate exposures of CO on clean and H 2 presaturated Fe(100) surfaces was conducted. At extremely low CO exposures, CO adsorbs only in the π-bonded CO state at low temperature. Molecules in this state exhibit an extremely weak carbon-oxygen bond and exclusively undergo dissociation when the temperature is raised above 440 K. At higher coverages, both dissociation and desorption is observed around 440 K even though all the molecules are still bound in the π-bonded CO state at lower temperatures. A model describing the partitioning between desorption and dissociation is proposed in which the dissociation fragments displace the strongly bound CO in the π-bonded state. The stoichiometry of this reaction allows a maximum of one half of π-bonded CO molecules to dissociate. Preadsorbed hydrogen strongly influences the ratio of dissociation to desorption as a function of coverage and suggests that hydrogen may induce island formation in the CO adlayer.

Fluorescence yield near edge spectroscopy of π-bonded CO on Fe(100)

Near edge X-ray absorption fine structure (NEXAFS) spectra of CO adsorbed on the Fe(100) surface are reported. Spectra, obtained by fluorescence yield (FYNES), are presented for each of the four individual CO adsorption configurations observed on this surface. The π-bonded state exhibits an unusual FYNES spectrum and polarization dependence which indicates that the molecule is either extensively rehybridized or tilted with respect to the surface normal. The FYNES spectra of each of the adsorption states directly reflect the perturbation of the carbon-oxygen bond by the surface and track systematically with the heat of adsorption.

Raman spectroscopic studies df chemisorbed unsaturated hydrdcarbons on nickel and silver films

Raman spectroscopy have been used to study chemisorption of ethylene, acetylene and benzene on thin evaporated Ni and Ag films at 80 K. In the spectra of C2H4 and C2H2 were observed Π-d bonds structures, the latters are characterized by a low-frequency shift of double and triple bond frequencies of about 100 cm−1. The hybridization change of the carbon atoms from sp toward sp2 with acetylene chemisorption and from sp2 toward sp3 with ethylene chemisorption is also observed at 80 K. The Raman spectra provide evidence that benzene is chemisorbed predominantly in Π-bonded state to the Ni and Ag surface respectively.