Carbon Adlayer Research Articles

Efficient CoMoS Catalysts Supported on Bio‐Inspired Polymer Coated Alumina for Hydrotreating Reactions

AbstractOrganic additives are widely used in order to improve the performance of hydrotreating catalysts. Yet, the use of polymers, in particular bio‐polymers, is less widespread. We present, herein, a strategy to coat alumina support with a bio‐polymer, namely polydopamine (Pdop). The Pdop coated supports provide a higher number of adsorption sites for Mo species compared to the bare support. The hydrotreating catalyst prepared on Pdop coated support showed improved performances for various hydrotreating reactions (aromatic hydrogenation, gas‐oil HDS and HDN). The increase in activity is ascribed to a better accessibility of active sites, and the presence of carbon adlayer between the active phase and alumina limiting detrimental direct metal support interactions. Modification of the alumina surfaces by Pdop creates new adsorption sites for metallic precursors, thereby allowing an improved accessibility of active sites especially at high surface densities.

Carbon adsorption on tungsten and electronic field emission

Electronic emission taking place at the electrodes of high voltage systems and responsible for detrimental breakdown processes is known to be strongly dependent on the cathode surface state and in particular on the presence of carbon contamination. To understand better the effect of carbon adsorption on cathode electronic emission, density functional theory calculations are reported for bulk bcc tungsten as well as for clean and carbon-covered W(100) surfaces for several coverages up to 2ML. Adsorption geometries and energies, work functions and electronic densities of states are analyzed to assess the effect of the presence of adlayers on surface electronic field emission properties. It is shown that flat carbon adlayer deposition on clean W(100) surfaces induces an increase of the surface work function and a decrease of electronic density near the Fermi level. Both factors contribute to reducing electronic field emission levels.

Origin of non-Faradayicity in electrochemical promotion of catalytic ethylene oxidation

The electrochemical promotion of catalytic C(2)H(4) oxidation has been investigated under low pressure conditions (p approximately 10(-6)-10(-4) mbar) with a Pt film on yttrium stabilized zirconia (YSZ) as catalyst. All measurements were conducted with a UHV system with a differentially pumped quadruple mass spectrometer (QMS) for rate measurements and a photoelectron emission microscope (PEEM) for spatially resolved measurements. A pronounced rate hysteresis upon cyclic variation of p(C(2)H(4)) was observed under open circuit conditions which is attributed to carbonaceous CH(x) adlayer inhibiting O(2) adsorption and hence poisoning the reaction. Application of a positive potential causes a partial removal of the inhibiting CH(x) adlayer by spillover oxygen thus triggering a transition from thus poisoned state of the surface to an active state with reduced carbon coverage. The ignition effect and therefore also the electrochemical promotion effect are linked to the presence of a carbon adlayer on the surface. The non-Faradayicity which is observed in this reaction system is explained as an ignition effect.

Tip‐enhanced Raman spectroscopy of 6H‐SiC with graphene adlayers: selective suppression of E1 modes

AbstractWe have characterized 6H‐SiC substrates with tip‐enhanced Raman spectroscopy in a broad spectral range between 100 and 3000 cm−1. A few adlayers of graphene previously grown on the semiconductor's C surface enabled us to approach the Au tip of a scanning tunneling microscope (STM) into tunnelling contact. In the recorded STM images of the substrate, we discern the height of the carbon adlayers in different sample regions. We compare tip‐enhanced Raman (TER) spectra of the sample to normal Raman spectra recorded with the field vector of the linearly polarized light oriented at 30° or 90° with respect to the surface normal. All spectra show the phonon fingerprint of SiC and of graphene. We find the E1 modes of 6H‐SiC selectively suppressed in the TER spectra whereas they are present in the normal Raman spectra excited at 30°. This effect is explained by a tip‐induced symmetry change. Copyright © 2009 John Wiley & Sons, Ltd.

Structure of epitaxial graphene on Ir(111)

A graphene monolayer has been prepared on an Ir(111) single crystal via pyrolytic cleavage of ethylene (C2H4). The resulting superstructure has been examined with scanning tunneling microscopy (STM) and low energy electron diffraction. It has been identified as a well aligned, incommensurate (9.32×9.32) pattern, which is described as a moiré. This pattern shows three distinct regions resulting from different local configurations of the carbon adlayer with respect to the Ir-substrate. These regions are imaged differently by STM and differ strongly in their ability to bind metal deposits.

Erosion of beryllium and deposition of carbon and oxygen due to bombardment with C + and CO + ions

The bombardment of clean, metallic beryllium with carbon and the co-bombardment with carbon and oxygen is investigated experimentally and by computer simulation. The experiments use C + and CO + ions with energies between 3 and 12 keV at normal incidence. The simulation is carried out with the Monte Carlo code TRIDYN. The results of carbon bombardment from experiment and simulation agree well and show the buildup of a carbon adlayer on the metal surface. After the carbon adlayer formation, no more Be is eroded. For C and O co-bombardment, carried out by CO + ion bombardment, simulation and experiment agree for low fluences. At higher fluences, another erosion channel besides kinematic sputtering is opened by the chemical interactions of C and O. This leads to a deposition/erosion equilibrium which is not predicted by the kinematic TRIDYN code. The formation of a ternary mixture layer on the Be target leads to a continuous erosion of the beryllium target.

Hydrogenation of isolated atoms and small clusters of carbon on Pt(111) surface: HREELS/TDS studies

The reaction of hydrogen with isolated atoms and small clusters of carbon adsorbed on Pt(111) surface was investigated by HREELS and TDS. The carbon adsorption layers were prepared by evaporation of carbon atoms onto the metal surface cooled down to 100 K. The surface carbon produced by this method reveals a high activity towards hydrogen: the reaction occurs at T ⩾ 170 K. The initial carbon concentration is found to determine the chemical content of products in the adlayer. At n c < 2 × 10 14 at/ cm 2, when isolated atoms C ads prevail in the initial adlayer, methine CH ads formation is chiefly observed revealing two bands in HREELS: δ(CH) at 800 and v( CH) at 2960 cm −1. The CH ads particles dissociate at 510 K leading to hydrogen evolution and formation of carbon islands with a graphite-like structure. At higher concentration, the carbon adlayer contains small clusters C x ads in addition to the isolated atoms leading to more complex reaction products. An ethylidyne species, ▪, is detected among the products with characteristic bands δ s(CH 3) at 1360 and v( CC) at 1130 cm −1. It is assumed that ethylidyne molecules are produced in the course of a consecutive hydrogenation of the C 2 ads cluster. Dehydrogenation of hydrocarbon surface species causes hydrogen evolution at T≈350, 410, 450, 510 and 600–700 K. Ethylidyne dissociation is associated with the desorption peak at T≈ 450 K; an ethynyl CCH ads being the product. It is shown that the highest temperature stage of the dehydrogenation in the adlayer occurring at T > 600 K is accompanied by an increase of the carbon content in the C x CH ads molecules, finally resulting in the formation of islands with a graphite-like structure.

Reaction analysis of potassium promotion of Ru-catalyzed CO hydrogenation using steady-state isotopic transients

The effects of alkali on catalyst activity and deactivation during CO hydrogenation have been studied in the past mostly based on an available surface-metal atoms approach. However, such an approach cannot easily distinguish to what extent the modifier brings about changes in surface concentrations of reaction intermediates or affects site activity during steady-state reaction. Steady-state isotopic transient analysis (SSITKA) with carbon tracing was used to decouple the effects of potassium on the methane-producing sites during steady-state CO hydrogenation over Ru/SiO 2 catalysts having modifier loadings of up to (K/Ru) atom = 0.2. The SSITKA results indicate that, during steady-state CO hydrogenation, carbidic carbon evolved into methane via a high-reactivity (C 1α) and a low-reactivity (C 1β) trajectory. With increasing amounts of K + the average “true” intrinsic turnover frequency ( k) of both of these carbidic pools decreased, as did their steady-state surface abundance. Relative to C 1β, the C 1α pool was affected to a slightly greater extent, both in terms of its reactivity and abundance. It is likely that potassium was able to strengthen the carbon-metal interaction which made hydrogenation of the carbon adlayer more difficult, resulting in smaller methane-destined pools of active surface carbon. With time-on-stream, deactivation by deposition of inactive carbon did not significantly affect the product distribution or the methane rate constant at the prevailing K + doping levels; instead, deactivation was due to a loss in the steady-state abundance of carbon-containing surface intermediates exiting as methane. Implications of the role of potassium during steady-state CO hydrogenation in influencing the active metal surface, the carbidic adlayer, and the latter's transformation into unreactive carbon are addressed.

Interpretation of the carbon Auger line shapes for the adsorption and decomposition of ethylene on Ni(100)

The CKVV Auger line shapes of ethylene chemisorbed on Ni(100) at 100, 250, 300 and 600 K are reported and consistently interpreted. These line shapes are representative of π-bonded ethylene (C2H4(a)) at 100 K, σ-bonded vinyl (CHCH2(a)) at 250 K, a mixture of ethynyl (CCH(a)), methylidyne (CH(a)) and di-σ-bonded acetylene (HCCH(a)) at 300 K, and carbidic carbon (C(a)) at 600 K on the surface. The Auger line shape of graphitic carbon adlayers on Ni is also interpreted. The line shapes are found to consist of adsorbate-adsorbate, adsorbate-substrate, and substrate-substrate components, which are denoted by the final location of the two holes created by the Auger process. The adsorbate-adsorbate component, for the C2 chemisorbed species, reflects primarily the valence density of states of ethylene and shows negligible hole-hole correlation effects due to electron transfer from the metal. This is in contrast to the graphitic adlayer where the adsorbate-adsorbate component has the same line shape as bulk graphite and exhibits significant correlation effects. The adsorbate-substrate and substrate-substrate components seen in all of the line shapes directly reflect the adsorbate-substrate π or σ bonding character, and the extent of screening via charge transfer from the metal to the core excited adsorbate.

Interpretation of the carbon Auger line shapes for the adsorption and decomposition of ethylene on Ni(100)

The CKVV Auger line shapes of ethylene chemisorbed on Ni(100) at 100, 250, 300 and 600 K are reported and consistently interpreted. These line shapes are representative of π-bonded ethylene (C 2H 4(a)) at 100 K, σ-bonded vinyl (CHCH 2(a)) at 250 K, a mixture of ethynyl (CCH (a)), methylidyne (CH (a)) and di-σ-bonded acetylene (HCCH (a)) at 300 K, and carbidic carbon (C (a)) at 600 K on the surface. The Auger line shape of graphitic carbon adlayers on Ni is also interpreted. The line shapes are found to consist of adsorbate-adsorbate, adsorbate-substrate, and substrate-substrate components, which are denoted by the final location of the two holes created by the Auger process. The adsorbate-adsorbate component, for the C 2 chemisorbed species, reflects primarily the valence density of states of ethylene and shows negligible hole-hole correlation effects due to electron transfer from the metal. This is in contrast to the graphitic adlayer where the adsorbate-adsorbate component has the same line shape as bulk graphite and exhibits significant correlation effects. The adsorbate-substrate and substrate-substrate components seen in all of the line shapes directly reflect the adsorbate-substrate π or σ bonding character, and the extent of screening via charge transfer from the metal to the core excited adsorbate.

Electronic structure of carbidic and graphitic carbon adlayers on transition metals

Two substantially different forms of carbon adlayers can be prepared on transition metals by dissociation of CO or hydrocarbons in appropriate temperature and exposure conditions, called carbidic and graphitic carbon. We report electronic structure calculations for both types of overlayer on Ni surfaces. In the particular case of the (2×2)C p4g carbidic phase on Ni{001} the chemisorbed carbon induces a reconstructin of the top layer of metal atoms; we show the effects of these atomic displacements on the dispersion of the surface energy-bands, and compare with recent angle-resolved photoemission data.

The reactivity of the graphite overlayer on Fe(110) towards oxygen

The gasification by oxygen of a carbon adlayer on Fe(110) was studied by AES and LEED. The carbon layer was obtained by thermal decomposition of ethylene, and LEED measurements revealed that graphite islands were formed, in agreement with former studies. It appeared that at temperatures below 573 K only part of the carbon layer could be removed by oxygen. This phenomenon is explained by the fact that certain parts of the graphite overlayer matrix are better matched to the substrate than other parts, which leads to a lower reactivity towards oxygen.

Gasification of the carbide layer on Fe(100) by oxygen

The gasification of surface carbon by oxygen on Fe(100) was studied by AES and LEED. The iron surface was carburized by thermal decomposition of ethylene. Quasi-simultaneous AES measurements of the carbon and oxygen coverages reveal that upon reaction of a carbon adlayer on Fe(100) with oxygen in the temperature range of 423 to 573 K, carbon atoms are replaced by oxygen atoms while CO desorbs. Further oxidation of the subsurface region was observed after the carbide layer on Fe(100) was almost completely removed by oxygen. At crystal temperatures between 298 and 423 K it was not possible to remove the total amount of carbon since the carbide layer on Fe(100) was simultaneously covered with an oxide layer.

A study of the carbon adlayer on iridium

A study of the carbon adlayer on iridium

A study of the carbon adlayer on iridium

A study of the phase composition, structure and properties of the carbon adlayer on iridium by Auger electron and thermal desorption spectroscopy and the surface ionization and CsCl dissociation techniques has been continued. It has been confirmed that the central valence-saturated part of the graphite islands is elevated by ≳ 4 A ̊ above the metal surface, the valence-active edges of the islands being bonded to it chemically. The chemisorbed carbon gas has been shown to occupy all of the iridium surface both beyond the graphite islands and under the raised islands. It is present also under the graphite monolayer. The critical degree of coverage θ c in the chemisorbed carbon gas at equilibrium with the graphite islands on the metal surface (representing an analog of the saturated vapor pressure in a liquid-vapor system) has been determined for the first time. This fundamental characteristic of the phase transition in the carbon adlayer on iridium can be described by the expression θ c = 2 × 10 5 exp(−2.3 eV/ kT). The obtained high resolution ( ΔE/E ⋍ 0.1%) differential Auger spectra of carbon from the graphite islands and chemisorbed gas are shown to be essentially different. It is shown that the temperature-controlled process of graphite island dissolution and carbon gas condensation can be visualized by monitoring the shape of the carbon Auger peak.

Characterization of oxygen, carbon, and sulfur adlayers on W(211)

Characterization of oxygen, carbon, and sulfur adlayers on W(211)

Cesium ion desorption from oxygen and carbon adlayers on platinum surfaces with nanosecond time resolution: variation of desorption parameters with time available for surface diffusion and degree of surface heterogeneity

Cesium ion desorption from oxygen and carbon adlayers on platinum surfaces with nanosecond time resolution: variation of desorption parameters with time available for surface diffusion and degree of surface heterogeneity

Cesium ion desorption from oxygen and carbon adlayers on platinum surfaces with nanosecond time resolution: Variation of desorption parameters with time available for surface diffusion and degree of surface heterogeneity

Measurements of ionic cesium desorption from surfaces at temperatures above 1100 K have been made with the so-called field reversal method. Time constants down to 30 ns have been measured with very low surface densities of Cs. The prexponential factors for desorption from clean platinum and graphite covered platinum surfaces are close to 10 12 s −1, in agreement with simple desorption theory. However, for slightly oxygen covered surfaces and for carbon layers formed at low temperatures the preexponential and the desorption energy become significantly larger, while a strict Arrhenius form is retained. This is proposed to be due to the influence of surface inhomogeneity and surface diffusion, in line with the theoretical description of simultaneous diffusion and desorption recently given by Nordholm. In the oxygen case, the adsorbed oxygen is considered to be the origin of the heterogeneity, while in the carbon layer case the surface, which has been formed at low temperature, is disordered and may contain many strongly binding sites. The influence of bulk diffusion on the desorption is also described.

Auger electron spectroscopy (AES) studies of argon ion induced desorption of carbon from tantalum

AES studies of argon ion induced desorption of carbon from tantalum were performed. The carbon adlayer was allowed to adsorb from a well characterized residual gas atmosphere, that was unvarying within 20%. The argon ions impact on the surface at an angle of 60° from the surface normal with energies between 0.2 to 1.0 keV. The total desorption cross section values measured under these conditions are 0.07–1.1 × 10 −15 cm 2.

Auger electron spectroscopy (AES) studies of argon ion induced desorption of carbon from tantalum

AES studies of argon ion induced desorption of carbon from tantalum were performed. The carbon adlayer was allowed to adsorb from a well characterized residual gas atmosphere, that was unvarying within 20%. The argon ions impact on the surface at an angle of 60° from the surface normal with energies between 0.2 to 1.0 keV. The total desorption cross section values measured under these conditions are 0.07 -1.1 × 10 -15 cm 2.