Dissociative adsorption of alkanes on clean and sulfur-modified nickel surfaces

Recent studies of the dissociative adsorption of methane on clean Ni(111), Ni(100), Ni(110), and sulfur-modified Ni(100), as well as ethane, propane, and n-butane on Ni(100) have been carried out under the high incident flux conditions of 1.00 Torr methane, 0.10 Torr ethane, 0.01 Torr propane, and 0.001 Torr n-butane, respectively. It has been found that the activation energies for these processes range from 3.1±1.0 to 13.3±1.5 kcal mol^–1. A comparison with the results of corresponding molecular beam studies suggests that the effects of vibrational energy on sticking probabilities must be accounted for and the sticking probabilities of molecules with very low normal kinetic energies must be accurately known when attempting to model high pressure processes using molecular beam techniques. While dissociation of ethane, propane, and n-butane on Ni(100) is believed to proceed primarily via a trapped molecular precursor, the results on sulfur-modified Ni(100) surface indicate that the direct channel to methane dissociation likely dominates and the contribution from the trapped molecular precursor mechanism is likely relatively small, with the sulfur atoms poisoning this reaction by a simple site blocking mechanism.     

Oxygen vacancies at oxide surfaces: ab initio density functional theory studies on vanadium pentoxide

The local electronic structure at the V₂O₅ (010) surface is studied by ab initio density functional theory (DFT) methods using gradient-corrected functionals (RPBE) where embedded clusters as large as V₂₀O₆₂H₂₄, representing one or two crystal layers of the substrate, are used as models. Results of local binding and charging of differently coordinated surface-oxygen sites as well as densities of states allow a characterization of the detailed electronic structure of the surface. Electronic and geometric details of surface-oxygen vacancies as well as hydrogen adsorption are studied by appropriate clusters. A comparison of the data, concerning vacancy energies, charging, geometric relaxation, and diffusion, shows sizeable variations between different oxygen sites and can give further insight into possible mechanisms of surface relaxation and reconstruction. Hydrogen is found to stabilize at all surface-oxygen sites forming surface-OH and H₂O species. As a result, the binding of surface oxygen with its vanadium neighbors is weakened. Therefore, the presence of hydrogen at the oxide surface facilitates oxygen removal and can contribute to the enhanced yield of oxygenated products near vanadia-based surfaces. Received: 10 April 2000 / Accepted: 25 July 2000 / Published online: 7 March 2001     

In situ investigation of the catalytic reaction H2+< ![IGNORE[O2→H2 O with second-harmonic generation

2 +O₂→H₂ O in the pressure range 0.2 Torr≤p_tot≤10 Torr on Pt(111) surface. At a catalyst temperature of T=700 K the equilibrium oxygen coverage θ_o is determined as a function of hydrogen partial pressure α. The experimentally obtained θ_o is modelled in a two step process considering the mass transport in the gas phase as well as the catalytic reaction on the surface. In this pressure range the mass transport in the gas phase changes from molecular flow conditions to laminar flow, inducing a strong modification of the gas phase present at the catalyst through different diffusivities of the reactants as well as through desorbing reaction products from the catalyst. It is shown that these gas phase alterations have to be taken into account for a proper modelling of the surface mechanism. Simulation calculations allow one to identify the sequential hydrogen addition reaction as the main reaction path for water production in this parameter range. Excellent agreement with previous investigations is obtained for the determined activation energies of the water-producing reaction steps equal to E^f _H2O≥0.7 eV. Received: 20 September 1998 / Revised version: 15 December 1998     

STM investigation of the adsorption and temperature dependent reactions of ethylene on Pt(111)

The adsorption and reactions of ethylene adsorbed in UHV on Pt(111) have been studied as a function of temperature by STM. The STM images taken at 160K show an ordered structure of adsorbed ethylene. Annealing to 300 K produces ethylidyne (C-CH₃) irreversibly, as has been demonstrated by a wide variety of surface science techniques. The ethylidyne on Pt(111) is not visible to the STM at room temperature. Cooling the sample allows direct observation of the ethylidyne ordered structure by STM. Annealing above 430 K results in further dehydrogenation, eventually leaving only carbon on the surface. The decomposition products appear as small clusters which are localized and uniformly distributed over the surface. Further annealing to temperatures >800 K results in the growth of graphite islands on the Pt(111) surface. The annealed graphite islands exhibit several supersturctures with lattice parameters of up to 22 Å, which are thought to result from the higher order commensurability with the Pt(111) substrate at different relative rotations.     

Catalysis by basic carbons: Preparation of dihydropyridines

The condensation of benzaldehyde and different substituted benzaldehydes, such as 2-nitrobenzaldehyde, 3-nitrobenzaldehyde, and 2,4-dichlorobenzaldehyde, with ethyl cyanoacetate was carried out using two alkaline carbons (Na-Norit and Cs-Norit) as catalysts in the absence of solvent. The reaction products are precursors in the production of 1,4-dihydropyridine derivatives, which have expanding practical applications as pharmaceuticals in the line of calcium channel blockers. High values of activity and selectivity were obtained. The most active carbon (Cs-Norit), which contains basic sites with pK_b = 11.2, is more active than pyridine, and less than piperidine. The selectivity to the desired condensation product when using these activated carbons is, at least, as high as in the case of the homogeneous catalyst. This “green” and “clean” method (alkaline doped carbon catalyst in the absence of solvent) can be extended to the preparation of other intermediates with medical applications.     

Desorption of chemisorbed Carbon on Mo(1 0 0) by noble gas ion sputtering: Validation of ground test measurements of ion engine lifetimes

We report desorption cross section measurements for one monolayer of chemisorbed carbon on a Mo(1 0 0) surface induced by sputtering with noble gas ions (Ne⁺, Ar⁺, Xe⁺) at different incident angles, ion energies, and substrate temperatures. Desorption cross sections were determined by using low-energy ion scattering (LEIS) to monitor the increase of the signal from the Mo substrate. A monolayer of p(1 × 1) carbon adatoms on the Mo(1 0 0) surface was created by dosing ethylene (C₂H₄) to the substrate at 800 K, and characterized by Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). We find that the carbon desorption cross section increases with increasing mass and energy of the impinging ions, and there is a maximum value for the desorption cross section at an incident angle for the ions of 30° from the surface plane. The desorption cross section also increases up to a substrate temperature of 300 °C. Values for the carbon desorption cross section for carbon adatoms on Mo(1 0 0) by 400-eV Xe⁺ ion sputtering are about 2 × 10⁻¹⁵ cm², which is one order of magnitude higher than those for bulk carbon samples. This information is particularly important for evaluation of ion-engine lifetimes from ground-test measurements in which contaminant carbon is deposited on Mo accelerator grids, potentially altering the sputtering rate of the Mo. Our measurements show that monolayer amounts of carbon on Mo have desorption cross sections that are two orders of magnitude higher than estimates of what would be required to reduce the Mo erosion rate, and thus ground-test measurements can be used with confidence to predict ion-engine wear in space, from this perspective.     

Raman diagnostics of heterogeneous chemical processes

The suitability of cw Raman spectroscopy for characterization of mass transport phenomena in connection with heterogeneous chemical processes is demonstrated for the example of the catalytic hydrogenation of acetylene. Temperature and concentration profiles in a model reactor were derived simultaneously from H₂ pure rotational lines and from the most intenseQ branches of C₂H₂ and of C₂H₄. The temperature dependence of the band contours has been considered; by careful calibration the systematic error of the derived concentration values is limited to less than 2%. The measurements allow the separation of the effects of thermal diffusion and of the reaction.     

Sum frequency generation (SFG) – surface vibrational spectroscopy studies of buried interfaces: catalytic reaction intermediates on transition metal crystal surfaces at high reactant pressures; polymer surface structures at the solid–gas and solid–liquid interfaces

SFG spectra of polyethylene and polypropylene show monolayer sensitivity and reveal temperature-dependent changes of surface structure. For polymer blends, the hydrophobic component segregates to the solid–air interface, and the hydrophilic component segregates at the solid–water interface. Changes in SFG spectra of polymer blends as a function of bulk concentration correlate with changes of contact angle. SFG is an excellent probe of surface-structure and surface-composition changes as the polymer interface is altered. Received: 20 September 1998     

Identification of adsorbed species at metal surfaces by electron energy loss spectroscopy (EELS)

Electron energy loss spectroscopy (EELS) is a surface analysis method for measuring vibrational spectra of adsorbed species on metal surfaces. This paper summarizes recent work on the study of bonding of simple adsorbates on metal surfaces, and the identification of new chemical intermediates in reactions between two or more species in the adsorbed monolayer. The spectra of atomic oxygen, di-oxygen, water and ammonia adsorbed on platinum, copper and silver are discussed with emphasis on identification of the adsorbed species and their orientations relative to the surface plane. Surface reactions between atomic oxygen and water, methanol and formic acid yield the new surface intermediates hydroxyl (OH), methoxy (CH₃O) and formate (HCOO), respectively, on copper and silver surfaces. Each species was identified by comparison of surface spectra with known infrared spectra and through the use of deuterium isotopic shifts. The ability to identify and distinguish between chemical species at surfaces with high sensitivity will allow direct correlation of low pressure UHV surface experiments with high pressure surface reactions on catalysts and liquid-solid interfaces.     

Plasma-electrolytic formation, composition and catalytic activity of manganese oxide containing structures on titanium

In the present work, we report the data about formation of TiO₂-rutile or TiO₂ and Mn₂O₃, Mn₃O₄ containing oxide structures on titanium in aqueous electrolytes by means of plasma-electrolytic deposition. The layers formed are characterized by X-ray diffraction, electron probe microanalysis and scanning electron microscopy methods. The PEO coatings on titanium formed in sodium tetraborate solution contain the TiO₂ stabile rutile modification that is important when utilizing such a structure as a catalyst carrier. Manganese acetate adding into the electrolyte leads to formation of layers that contain Mn₂O₃, Mn₃O₄ and TiO₂-rutile in outer region. The manganese content in the surface layer depends on the formation conditions as well as on manganese acetate concentration in the electrolyte. Catalytic activity of the layers in CO → CO₂ reaction is studied in the static and flow conditions. The manganese-containing layers obtained possess the catalytic activity in CO → CO₂ oxidation reaction at the temperature range of 250-350 °C. The catalytic activity depends on the concentration and surface distribution of manganese as well as on the layers morphology.     

Time-resolved two-photon photoemission spectroscopy of HOPG and Ag nanoparticles on HOPG

Received: 6 November 1998     

Hydrogen dissociation on metal surfaces – a model system for reactions on surfaces

Received: 23 March 1998/Accepted: 21 August 1998     

Dynamics of photoinduced reactions at oxide surfaces

This report summarizes our work on UV-laser induced desorption of small molecules and atoms from transition metal oxides. The systems presented serve as examples for a simple photochemical reaction, the fission of the molecule surface bond. State resolved detection methods were used to record the final state distributions of the desorbing neutral molecules. Detailed results on the systems NO/NiO(1 1 1) and CO/Cr₂O₃ (0 0 0 1) are presented. The experiments include investigations on stereodynamic aspects like the angular distributions of the desorbing molecules and, in the case of CO desorption, the rotational alignment with respect to the surface normal. Large desorption cross sections of (6 ± 1) · 10^–17 cm² for NO and (3.5 ± 1) · 10^–17 cm² for CO have been found for the desorption at 6.4 eV. The wavelength dependence indicates that the primary excitation step is substrate induced. The final state distributions show a high degree of translational, rotational and vibrational excitation and are clearly nonthermal of origin. The results are consistent with the formation of a negative ion intermediate state of the adsorbate. This observation is supported from a comparison to former results on NO/NiO(1 0 0) for which extensive ab initio calculations including electronically excited states exist. A spin state dependence of the vibrational excitation of NO could only be observed for NO/NiO(1 1 1) and is absent for NO/NiO(1 0 0). We attribute this observation to a spin state dependent coupling of the desorbing molecule to the surface in case the spin lattice orientation of the surface shows a preferential orientation. In the (111) plane the spin orientation is parallel within neighbour nickel ions while it is alternating in the (1 0 0) plane. For both systems studied the velocity component parallel to the surface is constant leading to a strong peaking along the surface normal for the fast molecules. The change from a preferred helicopter rotation (angular momentum vector aligned parallel to the surface normal) to a cartwheel motion (angular momentum vector aligned perpendicular to the surface normal) with increasing rotational excitation for desorption of the flat lying CO is consistent with a change of bonding geometry during the desorption process.     

High-temperature desorption of benzene from zirconium surfaces

We report on the interaction of benzene with zirconium (0001) surfaces. Following adsorption at 150 or 170 K benzene desorbs near 715 K at exposures above one Langmuir. The high desorption temperature of benzene is indicative of the complicated kinetics that zirconium surfaces exhibit. For lower exposures benzene dissociates during heating and an increase of the oxygen content at the surface is detected. We propose that hydrogen from the dissociated layer(s) attracts subsurface oxygen and that an exchange of adsorbed carbon with this oxygen takes place.     

Investigations of the selective population of hydrogen subsurface sites on Pd(110) using He diffraction and thermal-desorption spectroscopy

Surface-structure models for the 2×1 and 1×2 hydrogen chemisorption phases formed on Pd(110) at 100 K have been derived from He-diffraction data. The respective coverages correspond to 1 and 1.5 monolayers (ML). Upon heating to 200 K, the 1×2 saturation phase transforms back into the 2×1, and 0.5 ML hydrogen moves subsurface. Based on structural arguments, we suggest that only the first available subsurface sites, i.e., the octahedral interstitials between topmost and second layers are populated by thermal activation. The subsurface movement is eased since H-chemisorption sites on top of the second Pd layer can be occupied in the 1×2 owing to substrate reconstruction. Structural considerations also explain that exactly 1 ML H can be accommodated subsurface by thermal cycling. New TDS measurements corroborate these notions: only the ₂ desorption state, probably associated with Hin subsurface sites between first and second Pdlayers, is selectively filled by the thermal-cycling processes. The ₁ state remains empty upon thermal cycling, and is very likely connected with hydrogen deeper in the bulk.     

Isotope effects in the interaction of hydrogen and deuterium with a rhodium (110) surface

The adsorption of H₂ and D₂ on a Rh (110) surface at 100 K leads to a sequence of ordered phases, among others 1×2 phases at _H =0.5 and at _H =1.5 which likely involve a partial surface reconstruction consisting of a small perpendicular displacement of Rh surface atoms. The structure of the adsorbate phases is strongly correlated with the binding energy of the adsorbed phases. Three H (D) binding states (₁,₂ and) are populated at saturation as determined by thermal desorption spectroscopy (TDS). Whereas the peak temperature of the state is invariant with the hydrogen isotope, the D ₁ state appears at a 8 Klower and theD ₂ state at a 5 Khigher temperature than the respective H states. Generally the D phases exhibit a better long-range order than the H phases. The rate of adsorption is identical for the first three adsorbed phases but D₂ adsorbs appreciably faster in the 1×2–3H and the final l×1–2H phases.Zero point energy effects as well as a H coverage dependent local interaction model could account for the observed effects.     

Promotors,poisons and surfactants: Electronic effects of surface doping on metals

The interaction of adsorbates with metal surfaces is discussed. It is shown that the evanescent charge density produced by occupied sp derived surface states yields a considerable contribution to the Pauli repulsion experienced by adsorbate particles with a closed-shell electronic structure, e.g. rare-gases or molecules such as H₂ or N₂. For rare-gases this results in a reduction of the binding energy in the presence of occupied surface states, for molecules this gives rise to an additional contribution to the dissociation barrier. Suitable surface dopants are able to depopulate surface states and thereby to reduce the dissociation barrier. Such dopants can substantially promote catalytic reactions in which the dissociation from the gas phase or a physisorbed precursor is the rate limiting step. In contrast to closed-shell systems the bonding interaction for metal adsorbates on metal substrates is enhanced by occupied surface states. This leads to an extra diffusion barrier at steps, because the surface state amplitude drops to zero at upper step edges. The additional step-edge barrier, which is a kinetic hindrance for layer-by-layer growth, can be reduced by surface dopants depopulating the corresponding surface state. Such dopants promote layer-by-layer growth and act therefore as surfactants. It is concluded that the effect of promoters in catalysis and of surfactants in metal epitaxy is in part due to the same basic mechanism, namely the depopulation of surface states.     

Characterization of Pd-CeOx interaction on α-Al2O3 support

The Pd-Ce interaction was studied over CeO₂ (0.3-2.5 wt.%)-Pd (1 wt.%)/α-Al₂O₃ catalysts used in the reforming reaction of CH₄ with CO₂. The samples were characterized by using high resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). The activity and selectivity behavior was in good agreement with that of other supported metal catalysts (Ni and Pd) modified with different promoters. The preliminary results of HRTEM would indicate that the CeO_x forms small crystallites around the Pd particle. The XPS analysis for the regions of Ce 3d and Pd 3d, gives an account of Ce being present mostly as Ce³⁺ and a high binding energy for Pd 3d_5/2 (335.3 eV), an evidence of Pd-Ce chemical interaction. The Pd/Al XPS intensity ratios vs. the Pd average particle size, determined by TEM, show an excellent correlation for fresh and used catalyst. These results indicate that the diminution of the Pd/Al ratios was due to Pd sintering. Consequently, the small amounts of CeO_x species do not cover the Pd particle, in agreement with the HRTEM results. The overall results stand for the promoter action mechanism of the CeO_x for the reforming reaction with CO₂.