Showing papers on "Chemisorption published in 1987"

Fundamental Studies of the Chemisorption of Organosulfur Compounds on Au( 111). Implications for Molecular Self-Assembly on Gold Surfaces

Abstract: Studies of the adsorption of methanethiol and dimethyl disulfide on an Au(111) surface under UHV conditions are described. Both adsorbates bind strongly, with the bonding of the disulfide being greatly favored. It is found that, under these conditions, the disulfide bond is dissociated to give a stable surface thiolate. Adsorption of methanethiol does not involve cleavage of the S-H bond. The implications of these results for solution adsorption experiments and the thermodynamics characterizing monolayer formation are discussed.

Spontaneously organized molecular assemblies. 3. Preparation and properties of solution adsorbed monolayers of organic disulfides on gold surfaces

Abstract: This paper shows that stable, oriented, polyfunctional organic monolayers can be prepared by the spontaneous organization of structurally complex organic disulfides on polycrystalline gold substrates. Chemisorption proceeds to very high coverages, approaching that equivalent to the bulk-phase densities of the adsorbate molecules. The bonding to the surface is also highly specific, inasmuch as the chemisorption of the disulfide moiety is favored greatly over a wide range of other functionality. This latter feature allows the ready preparation of a broad variety of organic surfaces with well-defined microscopic and macroscopic characteristics. Several representative examples of monolayer films are described, their chemical and thermal properties explored, and their structures characterized by several techniques including infrared and photoelectron spectroscopies.

Dynamics of the activated dissociative chemisorption of CH4 and implication for the pressure gap in catalysis: A molecular beam–high resolution electron energy loss study

Abstract: The dynamics of the activated dissociative chemisorption of CH4 on Ni(111) are studied by molecular beam techniques coupled with high‐resolution electron energy loss spectroscopy. The probability of the dissociative chemisorption of CH4 increases exponentially with the normal component of the incident molecule’s translational energy and with vibrational excitation. The dissociative chemisorption probability of CD4 exhibits the same trends with a large kinetic isotope effect. High‐resolution electron energy loss spectroscopy identifies the nascent products of the dissociative chemisorption event as an adsorbed methyl radical and a hydrogen atom. These results, which have shown that there is a barrier to the dissociative chemisorption, are interpreted in terms of a deformation model for the role of translational and vibrational energy in promoting dissociative chemisorption. The barrier likely arises largely from the energy required to deform the molecule sufficiently to allow a strong attractive interaction between the carbon and the Ni surface atoms. Tunneling is suggested as the final process in the C–H bond cleavage. The presence of this barrier to dissociative chemisorption presents a plausible explanation for the pressure gap in heterogeneous catalysis.

Inelastic electron tunneling from a metal tip: The contribution from resonant processes.

Abstract: We present a simple model calculation for electron tunneling between a metal probe tip and a metal surface with a chemisorbed molecule. It is shown that under suitable conditions, resonant tunneling via interaction with a molecule vibration can give a decrease of 10% or more in the total tunneling conductance. Adsorbate systems are suggested where our predictions may be tested.

The adsorbed states of ethylene on Si(100)c(4×2), Si(100)(2×1), and vicinal Si(100) 9°: Electron energy loss spectroscopy and low‐energy electron diffraction studies

Abstract: The adsorbed states of ethylene on the Si(100)c(4×2), Si(100)(2×1), and the Si(100) 9° vicinal surfaces have been studied using high resolution electron energy loss spectroscopy (EELS) and low‐energy electron diffraction (LEED). Ethylene is nondissociatively chemisorbed on the Si(100) surface in the wide temperature range between 77 and ∼600 K, and is rehybridized to have a near sp3 hybridization state. The adsorbed structure is proposed in which ethylene is di‐σ bonded to two adjacent Si atoms of the dimer at the Si(100) surface. The thermal decomposition of chemisorbed ethylene and the influence of steps on the adsorbed states of ethylene are discussed.

Effect of translational energy on the chemisorption of N2 on Fe(111): Activated dissociation via a precursor state.

Abstract: Determination de la probabilite de chimisorption dissociative initiale en fonction de l'energie cinetique et de la temperature de surface

FT‐ICR probes of silicon cluster chemistry: The special behavior of Si+39

Abstract: FT‐ICR techniques were used to probe the surface chemistry of isolated silicon cluster ions in the 7–65 atom size range. Dissociative chemisorption reactions with NH3 were observed to proceed with rates which varied widely with cluster size. One particular cluster, Si+39, was found to be remarkably inert. Clusters with 20, 25, 33, and 45 atoms were found to be unreactive as well, while those with 18, 23, 30, 36, 43, or 46 atoms were quite reactive. Similarly oscillating reaction patterns were observed with CH3OH, whereas highly reactive free radical scavengers such as O2 and NO showed little selectivity. These results suggest the silicon clusters in this size range have well‐defined structures which vary in ability to catalyze dissociative chemisorption at the surface.

A detailed study of the reactions between size selected aluminum cluster ions, Al+n (n=3–26), and oxygen

Abstract: A detailed study of the reactions between size selected aluminum cluster ions and oxygen is presented. The experiments were performed using a low energy ion beam apparatus. Measurements of product distributions and total reaction cross sections at collision energies of 1.2 and 4.2 eV for aluminum cluster ions with between 3 and 26 atoms are reported. The total reaction cross sections increase with cluster size in a way which roughly correlates with the increase in the cluster’s physical size. The main products are Al+n−4, Al+n−5, Al+n−6, and Al+. Only a very small fraction of the product ions contain oxygen. We suggest that the reaction occurs by chemisorption of O2 onto the cluster followed by rapid loss of two Al2O molecules to give Al+n−4. If the Al+n−4 fragment contains sufficient energy it will undergo further dissociation by loss of one or more aluminum atoms to give Al+n−5, Al+n−6, and Al+. RRKM theory is used to estimate the amount of energy above the dissociation threshold required to cause dissociation of the bare clusters on the experimental time scale. For the larger clusters this excess energy is remarkably large. Using this data we are able to deduce some information about energy disposal in the reaction. It is likely that the Al2O molecules carry away a substantial fraction of the exothermicity arising from chemisorption of oxygen onto the clusters.

The chemisorption of hydrogen on the (111) and (110)‐(1×2) surfaces of iridium and platinum

Abstract: The chemisorption of hydrogen on both the Ir(111) and Pt(110)‐(1×2) surfaces has been examined under ultrahigh vacuum conditions with thermal desorption mass spectrometry, LEED, and contact potential difference measurements. No ordered adsorbate superstructures were observed on either surface at any fractional coverage and at surface temperatures from 100 to 700 K, and the (1×2) reconstruction of the Pt(110) surface was stable in all cases. Hydrogen adsorbs dissociatively on the Ir(111) surface, the adsorption reaction described by second‐order Langmuir kinetics with an initial probability of adsorption of 7×10−3. The rate parameters describing the second‐order desorption reaction of hydrogen from the Ir(111) surface are weakly dependent on coverage between fractional coverages of 0.1 and 0.3, and are given by Ed ≂12.6 kcal mol−1 and k(2)d ≂2×10−6 cm2 s−1. Beyond a fractional coverage of 0.3, however, both rate parameters decrease with increasing coverage. Hydrogen adsorbs dissociatively on the Pt(110)‐(1×2) surface into two distinct β2 and β1 adstates, and the ratio of the saturation densities of these two states, β2:β1, is 1:2. Adsorption into the higher binding energy β2 adstate is described by first‐order Langmuir kinetics with an initial probability of adsorption of 0.46, whereas adsorption into the β1 adstate is described by second‐order Langmuir kinetics with an ‘‘initial’’ probability of adsorption of 0.022. The rate parameters describing the desorption reaction of hydrogen fromthe Pt(110)‐(1×2) surface are strongly dependent on the coverage. In the coverage regime characteristic of the β2 adstate (θ≤0.32) the rate parameters are approximately symmetric about one‐half of saturation of this state. Specifically, from the values for the zero‐coverage limit of Ed ≂18 kcal mol−1 and k(2)d ≂10−4 cm2 s−1, the parameters first increase to maximum values of Ed ≂26.5 kcal mol−1 and k(2)d ≂0.3 cm2 s−1 at θ=0.15, and subsequently decrease approximately to the values for the zero‐coverage limit at θ=0.32 In the coverage regime characteristic of the β1 adstate (θ>0.32), the activation energy decreases continuously with increasing coverage from a value of Ed ≂17 kcal mol−1 at θ=0.35, whereas the preexponential factor remains essentially constant with a value of 3×10−4 cm2 s−1. The contact potential difference for hydrogen on Pt(110)‐(1×2) increases continuously with coverage to a value of 0.17 eV at θ=0.30. As the coverage increases further, however, it decreases continuously approaching a value of −0.50 eV at saturation. Probable binding states for the β2 and β1 adstates on the Pt(110)‐(1×2) surface are inferred from both the adsorption and desorption kinetics and the contact potential difference measurements. Comparisons of the results obtained on the (111) and (110)‐(1×2) surfaces of both iridium and platinum suggest strongly that local surface structure (e.g., ‘‘step’’ sites vs terrace sites) has a profound influence on the kinetics of adsorption of hydrogen on these surfaces. Surface structure apparently also has a profound influence on the desorption kinetics of hydrogen via the mediation of adatom–adatom interactions. Whereas both attractive and repulsive interactions are clearly manifest within the β2 adstates on the (110)‐(1×2) surfaces, only repulsive interactions are apparent on the (111) surfaces and for the β1 adstates on the (110)‐(1×2) surfaces.

Effect of vibrational energy on the dissociative chemisorption of N2 on Fe(111)

Abstract: Molecular beam techniques have been employed to determine the effect of vibrational energy on the dissociative chemisorption of N2 on Fe(111). We find that for kinetic energies in the range 0.3 to 0.6 eV that vibrational energy is an average only about half as effective as translational energy. Results are discussed in terms of a precursor mechanism, where access to the precursor state is blocked by a potential barrier that is more easily overcome with translational than with vibrational energy.

Model studies of the chemisorption of hydrogen and oxygen on Cu(100).

Abstract: Atomic chemisorption of hydrogen and oxygen on Cu(100) has been studied using up to 25 copper atoms as a model of the surface. The computational procedure used involves a reduction of the metal atoms to one-electron systems and extensive configuration-interaction calculations of the adsorbate and the cluster-adsorbate bonds. The calculations support the fourfold hollow site as the preferred chemisorption site for oxygen, with a barrier-to-surface migration of 25 kcal/mol. The calculated chemisorption energies for both hydrogen and oxygen, 51 and 90 kcal/mol, respectively, are in good agreement with experimental estimates (56 kcal/mol for hydrogen and 97 kcal/mol for oxygen). The effects of reducing the metal atoms to one-electron systems have been investigated through comparisons with all-electron calculations for Cu5H and Cu5O at the self-consistent-field level and by comparisons to previous calculations on Cu5Cl and Cu9Cl in which the 3d electrons were treated explicitly.

Effect of translational and vibrational energy on adsorption: The dynamics of molecular and dissociative chemisorption

Abstract: The dominant features of the molecule–surface potential energy surface which governs the dynamics of molecular and dissociative chemisorption are probed by molecular beam techniques coupled to electron spectroscopy. The collision energy and the vibrational energy of the incident adsorbate are the convenient probes of the interaction potential and high‐resolution electron energy‐loss spectroscopy is the sensitive and chemically specific detector of the result of the dissociative chemisorption event. The dissociative and molecular chemisorption of CH4 and CO on Ni (111) have been studied with these techniques. The results of these studies are summarized to illustrate the power of these techniques to provide information on the mechanism and dynamics of chemisorption.

Electronic promoters and semiconductor oxidation: Alkali metals on Si(111) surfaces.

Abstract: We examine the effect of thin (1--2 monolayers) overlayers of the low-electronegativity metals Cs and Na upon the oxidation of Si(111) surfaces. Synchrotron-radiation photoemission studies of oxygen chemisorption as a function of overlayer thickness and oxygen exposure indicate large oxidation-promotion effects and the formation of Si oxide phases where high Si oxidation states dominate. Comparison with promotion effects induced by transition-metal and noble-metal overlayers forces, for alkali metals, a reevaluation of the microscopic mechanisms proposed to explain overlayer-induced oxidation promotion.