Showing papers on "Chemisorption published in 1985"

The surfaces of metal oxides

Abstract: The author reviews the present understanding of the electronic, geometric and chemisorption properties of metal oxide surfaces. It is restricted to experimental and theoretical studies of single-crystal oxide surfaces since only for those systems has it been possible to correlate surface properties with specific site geometry, ligand coordination, defect structure, etc. The geometric structures of the oxide surfaces that have been investigated to date are described in relation to bulk crystal structure and cation ligand coordination. The electronic structure of both perfect and defect surfaces is discussed for the various classes of metal oxides, and similarities and differences in their behaviour are correlated with surface geometry and cation electronic configuration. The chemisorption of several types of atoms and molecules on single-crystal oxide surfaces, both nearly perfect and containing point defects is reviewed and interpreted in terms of surface electronic and geometric structure. Some recent electron- and photon-stimulated desorption results on oxides are also reviewed, as the measurements of surface phonon and plasmon modes.

Surface reactions of metal clusters. II. Reactivity surveys with D2, N2, and CO

Abstract: Reactions on the surface of a variety of transition metal clusters have been studied in the gas phase at near room temperature using a newly developed fast-flow reaction device. Initial examples of the use of this device are provided by survey studies of the reactivity of iron, cobalt, nickel, copper, and niobium clusters in contact with low concentrations of D2, N2 and CO. Dissociative chemisorption of D2 is found to occur with dramatic sensitivity to cluster size in the cases of iron, cobalt, and niobium clusters, the detailed pattern of reactivity differing markedly for each metal. The corresponding reaction is also observed with nickel clusters, but here the reactivity shows only a slow, steady increase with cluster size. Copper clusters are found to be completely unreactive to H2 chemisorption under these conditions. Molecular nitrogen is found to chemisorb readily to clusters of cobalt and niobium, with a reactivity pattern very similar to that observed with D2. Iron clusters are found to show slight reactivity with N2; only a small amount of chemisorption is observed on the most reactive clusters at high N2 concentration, but the pattern of this reactivity with cluster size is consistent with that observed in D2 chemisorption. In contrast to these highly structured reactivity patterns of D2 and N2, carbon monoxide is found to show only a slow, monotonic increase in reactivity with cluster size. It is suggested that these dramatic reactivity patterns for chemisorption on metal clusters provide stringent tests for future theories as to the nature of chemisorption on metal surfaces at a detailed, molecular level.

Structure of rhodium in an ultradispersed Rh/Al2O3 catalyst as studied by EXAFS and other techniques

Abstract: The structure of rhodium in an ultradispersed 0.57 wt % Rh/y-Al,O, catalyst before and after CO adsorption was studied with extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR), temperature programmed reduction (TPR), CO infrared spectroscopy, and H2 and CO chemisorption. With the aid of these complementary techniques, it could be established that the structure of the rhodium catalyst was completely different before and after CO adsorption. Before CO adsorption and after reduction of the catalyst at 593 K, all the rhodium was reduced and in the form of three-dimensional metallic crystallites. CO adsorption disrupted the metal-metal bonds in the crystallites, leading to isolated rhodium geminal dicarbonyl species in which the rhodium was present as Rh'. Each rhodium ion was surrounded by two carbon monoxide molecules and three oxygen anions of the support.

Correspondence between electron binding energy and chemisorption reactivity of iron clusters.

Abstract: New experiments in a fast-flow reactor have uncovered a strong correlation between the reactivity of free iron clusters and cluster ionization thresholds: Clusters with low thresholds efficiently add molecular hydrogen, and the relative rates of this reaction closely follow variations in cluster-electron binding energy. This correspondence can be understood in terms of a requirement for metal-to-hydrogen charge transfer in the activation of the ${\mathrm{H}}_{2}$ bond.

IR Studies of NH3, Pyridine, CO, and NO Adsorbed on Transition Metal Oxides

Abstract: Chemisorption of small molecules is often used as a probe for By probing the surface properties of transition metal oxides the interaction of molecules with the surface, information is often obtained on the oxidation state, the coordination symmetry, the degree of coordination unsaturation of the surface cations, the acid-base properties of the surface hydroxyl groups, and the presence and the nature of surface Lewis acid and B rosnsted acid sites This information is deduced from experimental measurements of the adsorption isotherms, the heats of adsorption, the thermal desorption spectra, and the vibrational spectra of the adsorbate Until recently, when high resolution electron energy loss spectroscopy became available, vibrational spectra were obtained with infrared spectroscopy Laser Raman spectroscopy has seldom been used because of the low Raman scattering cross section of most molecules

Hydrogen chemisorption on transition metal clusters

Abstract: The dissociative chemisorption of H2 on supersonic beams of Nb, Co, and Cu clusters is studied. (AIP)

Catalytic promotion and poisoning: All-electron local-density-functional theory of CO on Ni(001) surfaces coadsorbed with K or S.

Abstract: The adsorption of CO on Ni(001) and its coadsorption with K or S as studied with our all-electron full-potential linearized-augmented-plane-wave method show that (i) K modifies the electrostatic potential in the surface layer, produces an increased occupation of the CO antibonding $2{\ensuremath{\pi}}^{*}$ states, and facilitates the dissociation of CO; (ii) in contrast, S interacts more covalently with the Ni surface and neighboring CO molecules thereby deactivating the surface and inhibiting CO dissociation.

An Introduction to Chemisorption and Catalysis by Metals

Abstract: The fundamentals Primary process during collisions Adsorption and desorption Metallic structures and bonding Low energy electron diffraction Electron emission Surface potentials and vibrational spectra General background to heterogeneous catalysis and its application to the reaction o hydrogen isotopes The catalytic oxidation of carbon monoxide on palladium and platinum.

Claus catalysis. 1. Adsorption of sulfur dioxide on the alumina catalyst studied by FTIR and EPR spectroscopy

Abstract: The absorption of SO/sub 2/ on ..gamma..-alumina activated at 400 and 700/sup 0/C has been studied by FTIR and EPR spectroscopy. Four different types of absorbed species have been identified on alumina activated at 400/sup 0/C, and an additional fifth species has been detected on the sample activated at 700/sup 0/C. The most strongly held species appears to be a surface sulfite bonded in a unidentate fashion to sites involving metal ions. The other four remaining species probably involve SO/sub 2/-like structures, the strengths of adsorption of which are dependent on the nature of the adsorption sites. Two of the more strongly held species can be described as SO/sub 2/ bonded to metal atoms through the sulfur in either pyramidal or planar form. Of the remaining two, the species which is more strongly adsorbed appears to be SO/sub 2/ attached to an anionic oxygen site, whereas the weakly held species are adsorbed on surface hydroxyls. The EPR studies demonstrate the formation of SO/sub 2//sup -/ radicals upon adsorption of SO/sub 2/ on alumina at 25/sup 0/C. The decrease in the concentration of radicals upon heating to 200/sup 0/C parallels the disappearance, at this temperature, of the hitherto unobserved bands atmore » 1255 and 1189 cm-/sup 1/. However, the increase in the radical signal upon further heating to 400 /sup 0/C did not correlate with the behavior of any of the infrared bands. 21 refs., 11 figs.« less

Adsorption and decomposition of dimethyl methylphosphonate on an aluminum oxide surface

Abstract: Etudes, par spectroscopie electronique tunnel inelastique, de l'adsorption de dimethyl methylphosphonate sur des couches minces d'alumine

Microcalorimetric and Fourier transform infrared spectroscopic studies of methanol adsorption on alumina

Abstract: Adsorption microcalorimetry and infrared measurements using CH/sub 3/OH, CD/sub 3/OH, and CHD/sub 2/OH show the existence of at least three steps during the adsorption of methanol on alumina activated at 773 K. At very low coverages, molecules coordinated on the strongest Lewis sites are evident, which easily transform into bridged methoxide species. With increasing coverage, another form, irreversibly adsorbed at room temperature but desorbed by evacuation at 373-473 K, becomes predominant. It is identified as undissociated methanol strongly hydrogen bonded on a cation-anion couple having a strong basic character. Finally, at high coverages a reversible form hydrogen bonded to basic sites is detected. 33 references, 7 figures, 1 table.

Physicochemical properties of rare earth perovskite oxides used as gas sensor material

Abstract: The change of the conductivity in the rare earth perovskite oxide took place after the chemisorption of flammable gases. The sensitivity for methanol was highest. From the conductivity change of these perovskite oxides after the injection of methanol, the energy needed to promote an electron from a conducting to a nonconducting state, ΔE=E c-E t, could be derived from the equationσ=A σ 0 exp (−ΔE/kT). LnCoO3 had the smallest ΔE and ΔH (metal-O), which is the binding energy of oxygen coordinating to the metal ions, but exhibited the highest activity for gas sensing. The gas sensing mechanism was also considered.

Precursor-mediated molecular chemisorption and thermal desorption: The interrelationships among energetics, kinetics, and adsorbate lattice structures.

Abstract: Precursor-mediated molecular chemisorption and thermal desorption are examined by means of a lattice-gas model using a combination of time-dependent Monte Carlo simulations and deterministic rate equations. Lateral interactions between coadsorbates are assumed to be both pairwise additive and limited in range to nearest and next-nearest neighbors. The interrelationships among kinetics, energetics, and adsorbate overlayer structures are analyzed in detail. The method is applied to the interaction of molecular nitrogen with the Ru(001) surface.

Chemisorption and catalysis on LaMO3 oxides

Abstract: The adsorption of oxygen and isobutene and the catalytic activity for propene and isobutene oxidation have been studied on a series of LaMO3(M = Cr, Mn, Fe, Co and Ni) perovskite oxides. Coadsorption results point to the non-competitive adsorption of oxygen and isobutene; i.e. these molecules adsorb on different centres. Oxygen adsorption underwent a remarkable increase after isobutene had been preadsorbed on these oxides (enhanced adsorption). Activation energies for complete oxidation ranged between 16 kcal mol–1(LaMnO3, LaCoO3 and LaNiO3) and 31 kcal mol–1(LaFeO3). LaCrO3 showed some activity for methacrolein formation. Adsorption and catalytic-activity profiles showed maxima for LaMnO3 and LaCoO3. These results are discussed within the framework of the ideas of Dowden and Wells on the local symmetry of surface cations and its influence on chemisorption and catalysis and show the importance of localized interactions in the processes studied.