Showing papers on "Chemisorption published in 1988"

Cooperative growth phenomena in silicon/germanium low-temperature epitaxy

Abstract: A series of Si:Ge alloys and structures has been prepared by ultrahigh‐vacuum chemical vapor deposition. Alloys of composition 0≤Ge/Si≤0.20 are readily deposited at T=550 °C. Commensurate, defect‐free strained layers are deposited up to a critical thickness, whereupon the accumulated stress in the films is accommodated by the formation of dislocation networks in the substrate wafers. A cooperative growth phenomenon is observed where the addition of 10% germane to the gaseous deposition source accelerates silane’s heterogeneous reaction rate by a factor of 25. A model is proposed where Ge acts as a desorption center for mobile hydrogen adatoms on the Si[100] surface, accelerating heterogeneous silane pyrolysis by the enhanced availability of chemisorption sites.

Vibrational Spectroscopic Studies of the Structure of Species Derived From the Chemisorption of Hydrocarbons on Metal Single-Crystal Surfaces

Abstract: This review is primarily concerned with the identification of the structures of chemisorbed complexes formed between hydrocarbons and metal sur­ faces through the use of various forms of vibrational spectroscopy (1) . Thc identification of the chemisorbed species and the study of their reactivities by spectroscopic methods provides an essential basis for the understanding of many classes of metal-catalyzed reactions of hydrocarbons such as hydrogenation/dehydrogenation, hydrogenolysis, isomerization, and metathesis. The particular emphasis of this article is on the study of species formed on metal single crystals that are cut so that specific surfaces of known atomic arrangements, e.g. the [1 1 1], [100], and [1 10] surfaces of face­ centered cubic (fcc) metals, are exposed to the hydrocarbon adsorbate. Such experiments have become feasible during the past 15 years or so. The principal vibrational spectroscopic techniques used for this purpose are high resolution electron energy loss spectroscopy (EELS) (2) and reflec­ tion/absorption infrared spectroscopy (RAIRS) (3). In recent years i t has been shown that results obtained with these simplified single-crystal systems are of great value III interpreting

Surface barrier for electrons in metals

Abstract: Density-functional calculations using the full-potential linearized augmented-plane-wave (FLAPW) film method have been used to compute the effective potential for electrons at different surfaces of Al, Ni, Cu, Ag, and W. In each case, the average of the potential parallel to the surface is reproduced well by a simple model barrier with three adjustable parameters, with relatively small variations between different metals and surfaces. FLAPW calculations have also been used to examine the effects of chemisorption on the surface barrier. The results of this work provide a basis for the analysis of low-energy-electron-diffraction fine-structure and inverse-photoemission spectra.

Fourier transform ion cyclotron resonance studies of H2 chemisorption on niobium cluster cations

Abstract: Reaction rates and saturation values were determined for H2 dissociative chemisorption on positive niobium cluster ions in an FT‐ICR apparatus. Clusters with 8,10,12, and 16 atoms were found to be particularly unreactive, in remarkable agreement with the reactivity patterns observed previously for neutral niobium clusters. Saturation coverage for most clusters was found to occur near a hydrogen/niobium ratio of 1.3, although some clusters (8–12,16, and 19) reached effectively inert compositions at considerably lower coverages. Several examples were found of clusters having two isomeric forms with different reactivities. One form of Nb+19 was found to readily react with H2, whereas a second form representing one‐third of the original sample of 19 atom clusters was completely inert to H2 chemisorption under the same FT‐ICR conditions. The geometrical shape of these niobium clusters must therefore have a critical effect on reactivity.

Adsorption and decomposition of aliphatic alcohols on titania

Abstract: Etude, par thermodesorption, de l'adsorption puis de la decomposition chimique du methanol, ethanol, propanol-1 et propanol-2 sur le dioxyde de titane

Coadsorption of ethylene and potassium on platinum (111). 1. Formation of a .pi.-bonded state of ethylene

Abstract: High resolution electron energy spectroscopy (HREELS), temperature-programmed desorption (TPD), and ultraviolet photoelectron spectroscopy (UPS) have been combined to study the effect of preadsorbed potassium on the adsorption of ethylene on the platinum(111) single-crystal surface. Addition of potassium increases the relative amount of reversible ethylene adsorption at 100 K/. Upon coadsorption with potassium, the authors also observe a new low-temperature ethylene desorption state at 150 K, with E/sub d/ = 5-9 kcal mol/sup -1/, which is about one-half the binding energy of ethylene adsorbed on clean Pt(111). The origin of this new desorption state is the formation of a weakly interacting ethylene species, which is only slightly distorted from the gas-phase hybridization, as determined primarily by vibrational spectroscopy. An explanation is given for these effects in which the increased charge density at the platinum surface due to coadsorbed potassium inhibits the strong sigma-donation of adsorbed ethylene to platinum and greatly reduces the chemisorption bond strength.

Dependence of metal cluster reaction kinetics on charge state. I. Reaction of neutral (Nbx) and ionic (Nb+x, Nb−x) niobium clusters with D2

Abstract: The effect of charge state on niobium cluster chemisorption kinetics is explored via measurement of the relative rates of D2 activation by Nb−x, Nbx, and Nb+x containing up to 28 atoms. The presence of the + or − charge is found to have only a minor effect on rate for the majority of the clusters, with the reactivity of the ions being generally within a factor of 2.5 of the corresponding neutrals. The excess charge does, however, have a profund influence on reaction rate for a number of clusters in the 7≤x≤16 size range, which may be indicative of the importance of cluster electronic structure in the chemisorption process. Kinetic data for Nb9, Nb12, and Nb+12 are found to deviate significantly from the expected pseudo‐first‐order behavior, suggesting the existence of structural isomers for these species. The anomalous behavior for Nb9 and Nb12 was not observed in previous neutral Nbx chemisorption studies. The maximum uptake of D2 by niobium clusters is found to be essentially independent of charge state...

Surface diffusion of hydrogen and CO on Rh(111): Laser‐induced thermal desorption studies

Abstract: Surface diffusion of hydrogen, deuterium, and CO on Rh(111) has been investigated by laser‐induced thermal desorption (LITD) and compared with previous results for these species on Pt(111) and on other metals. As the coverage θ of deuterium increases from 0.02 to 0.33, the preexponential factor D0 remains constant at 8×10−2 cm2/s, but the diffusion activation energy Ediff rises from 3.7 to 4.3 kcal/mol. Ediff for hydrogen is 0.6 kcal/mol lower than for deuterium, consistent with the difference in zero‐point energy. For CO, Ediff =7 kcal/mol at all coverages, but D0 rises from 10−3 to 10−2 cm2/s between θ=0.01 and 0.40. Values of Ediff for these adsorbates vary by several orders of magnitude for surfaces on which heats of adsorption are essentially identical. These differences appear to correlate with differences in heats of adsorption in different binding states which form saddle point configurations in surface diffusion. Ediff is found to be nearly identical to the reaction activation energies for the CO...

The reactive chemisorption of carbon dioxide at magnesium and copper surfaces at low temperature

Abstract: X-ray photoelectron spectroscopic studies reveal that the following species are present when carbon dioxides reactively chemisorbed at magnesium and copper surfaces: CO2(phys) CO3(a), CO 2 − (a) CO(a) and C(a). At Mg(0001) surfaces, it has been shown that there is a step-wise process, with a whole range of metastable species formed above 80 K. By contrast, at copper surfaces the chemistry is less complex, the most significant species being CO 2 − (a); these form preferentially under conditions which favour high surface concentrations of CO2(a) and therefore co-operative effects within the adlayer. The need to develop experimental methods for studying surface processes at high pressures is well recognised, however the present work shows that for some systems low temperatures (and pressures) offer a possible alternative strategy for delineating the step-wise nature of the reaction.

Effect of collision energy and incidence angle on the precursor‐mediated dissociative chemisorption of N2 on W(100)

Abstract: The dissociative chemisorption of N2 on W(100) is found to fall rapidly with increasing kinetic energy Ei in the range 26 to 450 meV For a surface temperature Ts of 300 K, the initial dissociative chemisorption probability S0 falls from ∼08 at Ei=26 meV to 015 at 450 meV Over this range of energies the dissociation probability is also found to fall rapidly with Ts, and to be relatively insensitive to surface coverage at low Ts, strongly suggesting that dissociation occurs in this system via a precursor under these conditions This picture is supported by angular distribution measurements of the scattered molecules which are consistent with an appreciable cosine component, which also becomes smaller as Ei increases Results are found to be surprisingly insensitive to the incidence angle over the range 0° to 70°, indicating that the trapping process scales quite closely with the total incidence energy

The role of diffusion in the kinetics of phosphate desorption: the relevance of the Elovich equation

Abstract: SUMMARY Various kinetic expressions were tested with phosphate desorption data from four contrasting soils and found inadequate, but the data supported a model recently proposed by Aharoni and Suzin. This model assumes that chemisorption reactions are kinetically controlled by diffusion and integration of Pick's equation, under appropriate boundary conditions, results in an expression for the time dependence of adsorption or desorption, that can be approximated by a sequence of equations-parabolic, Elovich and exponential–at small, intermediate and large times, respectively. Thus, full or even partial conformation of data to such a sequence would suggest diffusion control of the rate determining step, a suggestion usually relying on a time relationship alone. Since true rate constants could not be derived from the other expressions tested, calculation of physically meaningful activation energies was not possible. An approach to the calculation of activation energies at different stages of the process, based on the model of Aharoni and Suzin, and the difficulties encountered are also discussed.

Hydrogen on molybdenum disulfide: theory of its heterolytic and homolytic chemisorption

Abstract: An atom superposition and electron delocalization molecular orbital study has been made of hydrogen adsorption on MoS/sub 2/. The authors have calculated structures, binding energies, force constants, and frequencies. The most stable chemisorption form is heterolytic at edges of the crystal layers. Reductive homolytic adsorption on the sulfur anion basal planes is predicted to be less stable but is still predicted to occur up to a stoichiometry of H/sub X/MoS/sub 2/ where X approx. 1. For values of X greater than 1, the Mo conduction band is filled to such a level that further reduction by hydrogen becomes energetically unfavorable. H/sub X/MoS/sub 2/ should be a conducting bronze, according to their results. They propose that, under high H/sub 2/ pressures and at sufficiently high temperatures for H to diffuse over the anion surface (a 1.2-eV barrier is calculated), H can diffuse from edge sites out over the basal planes and the edges will be replenished by further heterolytic adsorption. They calculate a basal plane SH bending vibration at 431 cm/sup -1/ and values for edge and corner SH and one MoH edge bond in the 500-600-cm/sup -1/ range, thereby providing an interpretation for the time, temperature-, and pressure-dependent inelastic neutronmore » scattering vibrational spectra of Wright and co-workers.« less

A theoretical study of methyl chemisorption on Ni(111)

Abstract: Calculations including electron correlation have been performed for methyl adsorbed on nickel clusters mimicking the Ni(111) surface. The chemisorption energies are evaluated following a recently developed scheme, in which the cluster is prepared for bonding. In this way cluster dependent oscillations of the chemisorption energies are largely eliminated. By also using calculated vibrational frequencies of the adsorbed methyl an almost certain assignment of the preferred chemisorption site is obtained. Methyl is found to adsorb in the threefold hollow site with a chemisorption energy in the range 50–55 kcal/mol. The origin of the soft C–H frequencies observed experimentally is a charge transfer from the metal into the C–H antibonding orbitals. The only weak sign of a direct metal–carbon–hydrogen interaction in the calculations is that the C–H frequency is slightly lower for an eclipsed compared to a staggered orientation of methyl in the threefold hollow. The present results are compared to previous experi...

Reactive and kinetic properties of carbon monoxide and carbon dioxide on a graphite surface

Abstract: Temperature-programmed desorption (TPD) results after chemisorption of carbon monoxide (CO) and carbon dioxide (CO,) on polycrystalline graphite are presented. CO adsorbs onto graphite with a very low sticking coefficient. After CO Chemisorption, CO (mass 28 amu) desorbs in two temperature regions, between 400 and 700 K and between 1000 and 1300 K, and COz (mass 44 amu) desorbs below 950 K. The intensity of the COz signal is less than 1 order of magnitude lower than the CO intensity. After C02 adsorption the major desorption product is CO at high temperatures (1000 < T (K) < 1300), whereas a small amount of COz desorbs around 450 K. The adsorption of a CI6Oz and C'*Oz mixture leads to a nearly total oxygen scrambling of the C02 desorbed. A mechanism for CO and COz interconversion on the graphite surface is presented in terms of surface oxide species, mainly lactones and semiquinones, and their relative stability. Assignments of the TPD features are proposed accordingly. Reaction studies on the C02 gasification of clean graphite and the CO disproportionation (Boudouard reaction) have been performed. A good agreement is found between the activation energies obtained and the desorption energies calculated from the analysis of the TPD results.

Surface-enhanced Raman spectra of pyrazine, pyrimidine, and pyridazine adsorbed on silver sols

Abstract: Raman spectra of pyrazine, pyrimidine, and pyridazine adsorbed on silver sols have been obtained and compared with existing data from corresponding experiments on Ag electrode. These two techniques give similar results except for pyrazine for with strong bands, observed only in the SER spectrum on the electrode, may be due to reduction products coadsorbed on the Ag surface. Chemisorption plays a role in the absorption of diazines as evidence of Ag-N bond formation was found in the SERS of all three molecules. Both N atoms of pyridazine are bound to the substrate and this explains an enhancement, for the latter molecule, of 2 orders of magnitude larger than for the other two compounds. The presence of low-frequency Ag-N modes and predictions of surface selection rules support edge on, rather than flat, orientation of diazines on the colloidal particles.

Soft-x-ray photoemission study of chemisorption and Fermi-level pinning at the Cs/GaAs(110) and K/GaAs(110) interfaces.

Abstract: The room-temperature adsorption of Cs on n-type GaAs(110) has been studied with use of core-level photoelectron spectroscopy. The coverage-dependent splitting of the Cs 4d core level into two sets of doublets indicate site-dependent adsorption for submonolayer coverages up to about 0.5 monolayer (ML). The broadening of this core level close to the saturation coverage is consistent with the growth of an out of registry monolayer as suggested by low-energy electron diffraction (LEED) data. Analysis of the substrate core levels indicate the formation of a laterally uniform overlayer and lack of chemical reaction. The Schottky barrier of 0.7 eV obtained from the low-coverage band bending is similar to the values for many other metals with much larger work function (Al, Ni, or Ti). We report also data on the K overlayers. They indicate a similar value of band bending and presence of a chemical reaction at the K/GaAs interface. The band bending results for Cs and K indicate that Schottky barrier for GaAs interfaces do not depend on the metal work function in a very broad range of the work-function values.