Selective hydrogenation of citral to citronellol has been investigated over Ni supported catalysts. The aim was to compare different support materials and catalyst preparation methods. Selectivity to citronellol was dependent on conversion. The highest maximum selectivity towards citronellol for a Ni/Al2O3 (16.7wt.% Ni) catalyst at 2.3bar hydrogen increased from 70% in ethanol to 85% 2-pentanol, where acetalisation could be totally eliminated. In ethanol the highest maximum selectivities to citronellol were about 83% over a 8.8wt.% Ni/Al2O3 catalyst prepared by atomic layer epitaxy (ALE) method. The lower hydrogenation pressure (2.3bar) favoured the formation of citronellol compared to results obtained at 5.3bar hydrogen for all other catalysts, but not for Ni/Al2O3 (16.7wt.%). Both a bimetallic Ni-Cu/SiO2 (10wt.% Ni, 3wt.% Cu) catalyst and a Ni/SiO2 (17.6wt.% Ni) catalyst prepared by ALE method showed high selectivities to citronellol, 79 and 76%, respectively at 5.3bar hydrogen. An optimum hydrogenation rate and an optimum specific metal surface area in the catalyst gave the highest selectivities to the desired intermediate, citronellol. Ni supported Y zeolite catalysts were not selective towards formation of citronellol. Instead these catalysts favoured other reactions, like cyclisation, dehydrogenation and cracking.

The kinetics of hydrogenation of xylose to xylitol on a sponge nickel catalyst (commonly referred to as Raney Ni catalyst) and of catalyst deactivation were studied. Plausible explanations for the decrease in catalytic activity by means of surface studies, nitrogen adsorption and thermogravimetric analyses of the fresh and spent catalysts are presented. The kinetic parameters of the process were estimated by the use of a semi-competitive model, which allows full competition between the organic species and the hydrogen atoms for the adsorption sites on the catalyst surface (competitive case). In the model, a competitiveness factor (alpha) is introduced to take into account that even after complete coverage of the surface by the organic species, interstitial sites remain for the adsorption of the hydrogen atoms.

The hydrogenation kinetics of 2,2-dimethylol-1-butanal (TMP-aldol) and 2,2-dimethylol-1-propanal (TME-aldol) over a supported nickel catalyst were determined with experiments carried out in a batchwise operating autoclave at 50-90 degreesC and 40-80 bar hydrogen. Water was used as the solvent. TMP- and TME-aldol were hydrogenated with 100% selectivity tu the corresponding triols. The effects of the catalyst activation procedure and the formaldehyde concentration on the hydrogenation kinetics were studied with thermogravimetry, X-ray photoelectron spectroscopy, and hydrogenation experiments. Catalyst reduction at a high temperature (400 degreesC) under hydrogen flow was favorable because of a more effective reduction of nickel oxides. Formaldehyde had a considerable retarding effect on the aldol hydrogenation: the hydrogenation rate was low until all of the formaldehyde was hydrogenated to methanol. The hydrogenation rate of TME-aldol was found to be significantly lower than that of TMP-aldol at low temperatures and pressures (60 degreesC and 40 bar), whereas equally high rates for both aldol molecules were observed at the highest temperature and pressure studied. A kinetic model including the inhibitory effect of formaldehyde as well as real hydrogen solubility data was proposed for the aldol hydrogenation. The model is comprised of adsorption, desorption, and surface reaction steps. the rate equations based on the model were able to describe the experimentally recorded hydrogenation kinetics of TMP- and TME-aldol.

We present high resolution photoemission studies of Sn on vicinal Si(100)2×1 surface in submonolayer regime. Deposition of Sn on clean Si(100) surface produces an abrupt epitaxial interface having a large variety of surface reconstructions. At room temperature at low coverages Sn dimer rows are formed. After annealing above 500 °C following superstructures are known to exist with increasing Sn coverage: c(4×4), 2×6, c(4×8), and 1×5. The Sn4d core level spectra are now resolved in details displaying new components related to the different adsorption sites on the Si(100) surface. The spectra of the annealed phase reconstructions show components originating from dimer bonding, from bonding to the second layer, from Sn atoms in mixed Sn–Si dimers where Sn atoms have replaced one of the Si dimer pair atoms, and from adsorption onto defect sites. The photoemission results are in good agreement with the surface geometric structures.

Hydrothermally aged (1000/850°C, 12/16h) Pd-Ce-supported alumina catalysts with high and low Ce content were prepared and tested in conversion of gas mixtures simulating the emissions from natural gas (NG) driven vehicles and biofuel combustion. The test procedure contained lean and rich light-off activity tests, stationary and oscillating lambda sweeps, space velocity tests as well as runs with sulphur poisoning. The catalysts exhibited high conversion of the model pollutants. In the oscillating lambda sweep experiments, the Pd-Ce/Al2O3 with high Ce loading showed high activities in conversion of CH4 and CO at lean λ values up to 1.04. Ageing under the reactants flow as well as hydrothermal treatment of the catalysts resulted in improved catalytic activities in terms of light-off temperatures of the model pollutants. This activation was believed to be a result of both Cl-release from the surface as well as restructuring of the Pd-particles. Interaction of Pd-Ce or Al-Ce induced at high temperature was also believed to affect the activation. Addition of 5ppm of SO2 into the gas mixture of simulated emissions from biofuel combustion raised the T50% of CH4 by approximately 100°C. SO2-TPD experiments were used to correlate the catalytic activity to S-poisoning. The catalysts were characterised by H2-adsorption, XPS, FTIR, SO2-/NO-/O2-TPD, XRD, XRF and N2-physisorption.

This chapter describes the synthesis and characterization of mesoporous platinum (Pt)-MCM-41 and its application in the enantioselective hydrogenation of l-phenyl-l,2-propanedione. The maximum enantiomeric excess (ee) of (R)-l-hydroxy-l-phenylpropanone is 44% with the 15 wt.% Pt-MCM-41 modified with (-)-cinchonidine. The highest ee obtained with 15 wt.% Pt-MCM-41 is due to the slightly larger Pt particles. Further optimization of the reaction conditions is a very important task to utilize mesoporous materials in enantioselective hydrogenation.

A new knitted silica fiber was investigated as a support material in enantioselective hydrogenation of 1-phenyl-1,2-propanedione in a pressurized batch reactor. The active metal was platinum and the catalyst was modified in situ with (−)-cinchonidine. The catalysts were prepared by impregnation method using hexachloroplatinic acid as metal precursor. The main experimental parameters were the support calcination temperature and the metal loading of the catalyst. The optimum catalyst dispersion, BET specific surface area and mean Pt particle size for enantio-differentiation were measured. A comparison of alumina and silica supports revealed that the main parameters in the enantio-differentiation were the mean metal particle size and the metal dispersion. Optimal metal particle size and dispersion were detected which optimize the enantioselection. Experiments with continuously operated fixed bed reactor demonstrated that continuous hydrogenation, over the new knitted silica fiber catalyst, gives equally high enantiomeric excesses compared to the batch operation. Continuous operation can be used as tool to study catalyst deactivation and reaction mechanisms in enantioselective hydrogenation.

The activity of a Mo-promoted Raney nickel catalyst was studied in the hydrogenation of xylose to xylitol. Kinetic measurements carried out in a laboratory scale pressurised slurry reactor (40-70 bar H-2 and 80-130 degrees C) with recycled catalysts revealed that the catalyst deactivates during the use, but an asymptotic activity level is finally attained. Water and water-ethanol mixtures were used as solvents. The formation kinetics of the main product, xylitol as well as the by-products, xylulose, D-arabinitol, furfural and xylonic acid were registered quantitatively in the experiments.Catalyst characterisation studies carried out with nitrogen adsorption, XRD, ESCA-XPS and gravimetric reduction with hydrogen suggested that the main reasons for the deactivation is the decay of accessible active sites through collapse of the pore structure and leaching of the promoter metal, Mo and alumina. Also, accumulation of organic species in the pores may slightly contribute to the deactivation process. Catalyst deactivation was more rapid in aqueous milieu than in water-ethanol solutions. The deactivation rate was retarded, if the catalyst was treated with ethanol at elevated hydrogen pressure and temperature between the hydrogenation experiments.A rate model based on plausible surface reaction mechanisms was proposed for the generation of the main and by-products. The rate equations were based on a semi-competitive adsorption model for hydrogen and organic species. The catalyst deactivation kinetics was described with a reversible semi-empirical model, which lumped the physical and chemical reasons for deactivation to a simple two-parameter system. The deactivation model was combined with the rate equations and the model of the slurry reactor. The kinetic and deactivation parameters were determined with a sequential technique, by using non-linear regression analysis. The model was able to reproduce the hydrogenation behaviour of Raney nickel very well: it predicted the product distribution and the catalyst deactivation within a wide range of process parameters. (C) 2000 Elsevier Science B.V. All rights reserved.

The hydrogenation kinetics of 2,2-dimethylol-1-butanal. (TMP-aldol) over a supported nickel catalyst was determined with experiments carried out in a batchwise operating autoclave at 50-90 degrees C and 40-80 bar hydrogen. The reaction mixture was analyzed with gas and liquid chromatography. It was found that TMP-aldol can be hydrogenated with a 100% selectivity to the corresponding triol, trimethylolpropane. The effects of the catalyst; activation procedure and the formaldehyde concentration on the hydrogenation kinetics were studied. The hydrogenation experiments revealed that catalyst reduction at a high temperature (400 degrees C) under hydrogen flow was favorable for the catalyst performance. The reason was a more effective reduction of nickel oxides which was confirmed with thermogravimetry and X-ray photoelectron spectroscopy. The presence of formaldehyde had a considerable retarding effect on the aldol hydrogenation kinetics: the hydrogenation rate was low until all of the formaldehyde was hydrogenated to methanol. The retarding effect was more prominent at higher temperatures than at lower temperatures, which indicates that formaldehyde forms oligomers on the catalyst surface as the temperature increases. A kinetic model was proposed for the aldol hydrogenation. The model includes adsorption, desorption, and surface reaction steps as well as the inhibitory effect of formaldehyde on the aldol hydrogenation kinetics. The model was able to describe the experimentally recorded hydrogenation kinetics of TMP-aldol in the presence and in the absence of formaldehyde.

The activity of the Pd, Pt, Pd–Pt/HZSM-5 and Pd/HZSM-35 catalysts was investigated at temperature ranges of 120–600°C in the conversion of gas mixtures consisting of CH4, C2H4, CO, naphthalene (model polyaromatic hydrocarbon), O2, CO2, H2O and N2. The prepared catalysts exhibited high activities in the complete removal of the model pollutants. Increase in the Pd-content of the catalyst was found mostly to affect the low temperature activity of the catalyst in methane oxidation. The effect of S-poisoning was studied by subjecting the catalysts to 5ppm SO2 in the feed gas mixture. Catalyst de-activation due to catalyst ageing in the reactant flow was observed. The ageing of the catalyst only affected the methane oxidation. Addition of Zr and La improved the low-temperature activities of the Pd and Pd–Pt/HZSM-5 catalyst and stabilised the catalysts against de-activation. The XPS and O2-TPD data indicated that on the freshly prepared catalyst, palladium is present as Pd+ and Pd2+ species. The ageing of the catalysts decreased the number of the Pd2+ sites in favour of the PdOx species.

1-Phenyl-1,2-propanedione was hydrogenated at 5 bar and 25°C over Pt/Al2O3 catalysts in the presence of cinchonidine. The effect of four different solvents, ethyl acetate, toluene, ethanol and dichloromethane in the enantioselective hydrogenation of the dione was tested. The highest enantiomeric excess of (R)-1-hydroxy-1-phenyl-2-propanone, 65%, was obtained in dichloromethane. Also toluene and ethyl acetate gave relatively high enantiomeric excesses. In ethanol, which highly interacts with Pt surface, the enantiomeric excess was only 12%. Two different alumina supports and Pt-precursors were investigated using ethyl acetate as solvent. The highest enantiomeric excesses of (R)-1-hydroxy-1-phenyl-2-propanone were obtained with α-Al2O3 supported Pt catalyst prepared from hydrogen hexachloroplatinate(IV)hydrate presursor. The highest Pt loadings in the catalyst preferred enantiodifferentiation. Higher enantiomeric excesses were obtained by increasing the mean metal particle size of Pt.

Electrochemical oxidation of a self-assembled monolayer (SAM) of 4-aminothiophenol on polycrystalline gold electrodes leads to a complex voltammetric behavior characterized by an initial irreversible oxidation at similar to +0.77 V versus SSCE (sodium saturated calomel electrode) and the formation of a pseudostable surface redox couple at +0.53 V. The oxidized form of this couple is hydrolyzed in acidic solutions to another redox pair with the formal redox potential of similar to +0.3 V. We show that the oxidation leads to a radical-radical coupling reaction between two adjacent aminothiophenol molecules, yielding an electrode surface modified with 4'-mercapto-4-aminodiphenylamine, the thiol derivative of a head-to-tail aniline dimer. The oxidized form of the dimer, quinone diimine, undergoes hydrolysis to the corresponding quinone monoimine and, eventually, to the original surface-bound 4-aminothiophenol and benzoquinone; The mechanism of the monolayer oxidation reaction has been elucidated by a variety of electrochemical and spectroelectrochemical techniques together with electrochemical data obtained with a soluble model compound, 4-(methylthio)aniline. In addition, X-ray photoelectron spectroscopy (XPS) characterization of the 4-aminothiophenol (Au-SPhNH2), the 2-(4'-mercaptophenylamino)benzoquinone (Au-SPhNH-BQ), and the oxidized 4-aminothiophenol SAMs is reported. The formation of an electrode surface modified with aniline dimers explains the beneficial effect that 4-aminothiophenol SAM exhibits in the electrochemical polymerization of aniline. We suggest that it favors the direct addition of aniline monomers to the oligomer chains on the surface, which results in a more ordered structure compared with the deposition of oligomers from the solution. This result is very important for the preparation of highly ordered polyaniline films for advanced applications in molecular electronics and sensor technology. The results also show that after the initial dimerization step, aniline polymerization can proceed through coupling of the neutral monomer to the oxidized oligomer.

The k-resolved inverse photoemission spectra of the single crystal Nb(111) surface were measured with normally incident electrons in the energy range 21–52eV. Unoccupied electronic states were studied along the Γ–P–H symmetry direction in the ΓHPN symmetry plane. The experimental results are in a reasonable agreement with the energy-band calculations made by the linear muffin-tin orbital method with the atomic-sphere approximation (LMTO-ASA). Two characteristic features appeared in the spectra at the fixed emitted photon energy. First, we obtained an enhancement of the inverse photoemission intensity at the plasma frequency energy of 19.7eV, and second an enhanced emission at 31.0eV corresponding to the fluorescence decay of the Nb4p core holes.

Inverse photoemission spectra have been measured to study unoccupied electronic states from an FeS2(100) single crystal along the Γ-X symmetry line with normal incidence electrons at energies between 20 and 40 eV. Two main features of the spectra are found, corresponding to direct bulk transitions into unoccupied iron 3d(eg) and sulphur 3p(σ∗) states, respectively, followed by a prominent minimum and a onset of a band with a wide structure due to mainly transitions into Fe 4sp-S 3d bands. These peaks are located at 1.4 and 3.0 eV above the Fermi level for an initial electron energy of 19.8 eV. They disperse according to band-structure calculations. The onset of the Fe 4sp-S 3d bands is about 5.0 eV above the Fermi level. The experimental results obtained here are in a reasonable agreement with band-structure calculations for FeS2, and with previous results of optical, BIS and XAS measurements.

Here we report the experimental results of unoccupied electronic states of the clean V(100) surface along the FH symmetry line. The measurements were performed with incident electrons in the energy range 19–33 eV and the energies of UV photons were analysed by a spherical grating spectrometer equipped with the position sensitive detector. The vanadium surface was cleaned by several cycles of argon ion sputtering and annealing. The surface structure of V(100) determined by LEED showed a clear surface (1 × 1) geometry pattern. The spectra of the empty states near Fermi level are found to be of the two-dimensional rather than of the three-dimensional bulk character. The states locate at 0.4, 1.4 and 2.1 eV above Fermi level. We also observed states about 10 eV above Fermi level, which dispersed up to 15 eV at higher incident electron energy. The experimental results are compared with band structure calculations.

An ultraviolet k-resolved inverse photoemission spectrometer (KRIPES) is constructed for studies of the unoccupied electronic states of solids and surfaces. The spectrometer is based on the spherical grating in the off-Rowland geometry. The sample is bombarded by low-energy electrons from the electron source with the minimum energy distribution width of 0.25 eV. The UV photons emitted from the sample are dispersed by the grating onto the position-sensitive detector. The spectra can be measured in the photon energy range from 10 eV to 40 eV with parallel detection of the wavelength region 560 Å and with an optical resolution of about 8 Å. The overall resolution of the spectrometer is determined to be from 0.26 eV at 10.0 eV to 0.6 eV at 30 eV by measuring the Fermi edge of the polycrystalline gold sample. The typical collection time per spectrum is about an hour with total amount of 3000 c/ch at the Fermi edge of gold when the beam current and energy are 25 μA and 23 eV, respectively. The resolution of the optical system is obtained from the emission line of a uncleaned tantalum sample at the photon energy of 14.8 eV. In the k-resolved measurement of Cu(110) surface the spectra show the typical band dispersion.

The inverse photoemission spectra of lead sulphide have been investigated in the energy range 15 – 33 eV of normal incidence electrons. In addition the angle-resolved photoemission spectra have been studied using synchrotron radiation. The sample crystals have been cleaved in UHV and the crystal structure has been observed by LEED to correspond the (1×1) surface geometry. The band structure to the direction Γ - X has been determined below and above the Fermi-level from the photoemission and the inverse photoemission spectra. The prominent dispersion of one occupied band is clear but the others are markedly flat. The unoccupied bands show quite well behaving dispersion. There are no bands crossing the Fermi-level in the Γ - X direction displaying the band gap 3.6 eV. General shapes of the experimental band dispersions agree satisfactorily with those predicted by theoretical calculations, but the binding energies of the bands show clearly some disagreement, especially for the lowest bands.