Showing papers on "Chemisorption published in 1997"

Dynamic Processes in Electrochemical Reactions Studied by Surface-Enhanced Infrared Absorption Spectroscopy (SEIRAS)

Abstract: Molecules adsorbed on evaporated thin metal films exhibit enormously strong infrared absorption. The thin metal films that exhibit the surface-enhanced infrared absorption (SEIRA) consist of metal particles much smaller than the wavelength of light. Electric field associated with the incident infrared radiation is enhanced via the excitation of localized plasmon of the particles, yielding the absorption enhancement. Preferential orientation and the change in absorption coefficient of molecules caused by chemisorption onto the metal surface provide additional enhancement. Most characteristic observations in SEIRA experiments are well explained by a simple electromagnetic theory. The infrared spectroscopy utilizing the SEIRA effect is promising as a new surface analytical tool. In particular, it is very useful for in situ studies of electrode/electrolyte interfaces. By the combined use of the attenuated-total-reflection technique, reactions and adsorption/desorption of molecules at the interfaces can be inv...

On the nature of spilt-over hydrogen

Abstract: Various experimental results on hydrogen spillover which have been obtained since the first evidence for this phenomenon are discussed concerning the nature of the activated hydrogen species. It will be demonstrated that the physical nature of the spilt-over species, especially their charge, can only be described by considering their interaction with the solid. A new model is proposed which is based on the description of spilt-over hydrogen as electron donor located at the surface. Consequently, H+ ions and H atoms coexist on the surface of the catalyst as also stated experimentally. Their ratio is determined by the electronic properties of the adsorbate/solid system. Considering atomic and recombinative desorption of spilt-over hydrogen, the adsorption isotherms are calculated exemplary for titania. The model is applied to interpret some experimental results related to hydrogen spillover, especially the partial electron transfer from the hydrogen species to the solid.

Vanadium pentoxide I. Structures and properties

Abstract: Copyright (c) 1997 Elsevier Science B.V. All rights reserved. Structure and properties of V 2 O 5 are described and the role of three types of oxygen atoms present in the lattice: vanadyl oxygen atoms O(1) coordinated only to one vanadium atom, and bridging oxygen atoms O(2) and O(3) coordinated to two and three vanadium atoms, respectively, is discussed. Equilibration of gas phase oxygen with vanadium oxides results in the formation of the intrinsic defect structure of V 2 O 5 consisting of oxygen vacancies. Discussion of the properties of vacancies as deduced from measurements of electrical conductivity, EPR and IR spectra and the stability of different oxygen vacant sites as obtained from quantum-chemical calculations is given. Mechanism of the reduction of V 2 O 5 through crystallographic shear, resulting in the formation of V 6 O 13 or V 4 O 9 , is illustrated. It is shown that because of a pronounced anisotropy of V 2 O 5 crystal structure sensitivity of catalytic reactions appears. Two types of crystal planes are exposed. The (0 1 0) basal plane has all chemical bonds almost fully saturated. The non-bonding d-orbitals of V ions have the LUMO character and act as Lewis acid sites, whereas the lone electron pairs of bridging oxygen atoms have the HOMO character and behave as Lewis basic sites. On the (1 0 0) and (0 0 1) planes cleavage leaves coordinatively unsaturated vanadium and oxygen ions, which develop Bronsted acid-base interactions with reacting molecules, causing the heterolytic chemisorption. Oxygen vacancies in the lattice are replenished through oxidation by gas phase oxygen, which sometimes is considered as oxygen chemisorption.

Kinetic study of the reverse water-gas shift reaction over CuO/ZnO/Al2O3 catalysts

Abstract: The kinetics of the reverse water-gas shift (RWGS) reaction over CuO/ZnO/Al2O3 catalysts was studied by use of CO2H2 cycles, hydrogen chemisorption and catalytic tests performed in both differential and integral plug flow reactors. The effect of the reactant composition on the reaction rate was specifically studied by changing the PH20/PCO20 ratio between 9.0 and 0.3. It was found that different reagents become rate limiting depending upon pressure. While in a H2-rich region the rate increases strongly with CO2 partial pressure and is zero order in hydrogen, under low PH20/PCO20 ratios the reaction is less active and is strongly positive order in hydrogen and low order in carbon dioxide. The experimental data were modeled by considering that the reaction proceeds through a surface redox mechanism, copper being the active metal. A good agreement between experimental and calculated data was obtained by assuming that in the redox mechanism either the dissociative CO2 adsorption (H2-rich region) or both the CO2 dissociation and the water formation (H2-lean region) determine the rate of the overall reaction. Based on previous studies performed on copper crystal surfaces, such a change in kinetics may be explained by assuming that under H2-rich atmosphere a surface structural or phase transition occurs involving a change in reactivity with respect to CO2 dissociation.

Adsorption of water on Si(100)-(2×1): A study with density functional theory

Abstract: Adsorption of water on the Si(100)-(2×1) surface has been investigated using density functional theory and cluster models of the surface. The reaction pathway and geometries of the product, the transition state and a molecular precursor state are described. There is no energy barrier to dissociative chemisorption. Adsorbed H and OH fragments are most stable when bonded to the same surface dimer with the hydroxyl oriented away from the surface dimer bond. The orbital and electrostatic interactions that determine the adsorbate and transition state geometries are analyzed. Surface distortion (dimer buckling) is a recurring theme in this analysis. Interactions of adsorbed molecular fragments with each other and with dangling bonds have significant effects, modifying the adsorbate geometry and leading to adsorbate islanding. Calculated vibrational frequencies of adsorbed H2O on Si(100)-(2×1) are discussed. The theoretical results are consistent with most available experimental results, and provide a microscopi...

Complex pathways in dissociative adsorption of oxygen on platinum

Abstract: Gas adsorption on solid surfaces is the basis of heterogeneous catalysis. Gas–surface interactions may be complex and in many cases the fundamental mechanisms of the chemisorption process are hard to discern. The macroscopic kinetics of a heterogeneous catalytic reaction are usually modelled within the Langmuir model1, which assumes that free adsorption sites are occupied at random. The adsorption of oxygen on a platinum (111) surface has been studied extensively as a model system for surface chemical processes generally2,3,4,5,6,7,8,9,10,11,12,13,14,15, owing to its significance in platinum catalysed oxidation reactions such as that of CO and NO. Here we show that even for this well studied system the chemisorption process may be much more complicated than the Langmuir model implies. Our observations with the scanning tunnelling microscope show that the dissociation probability for an oxygen molecule becomes affected by chemisorbed species in the vicinity that have dissociated already. This introduces a dynamic heterogeneity in the adsorption mechanism which leads to kinetically limited ordering of the adsorbate. This effect is likely to be quite general and to affect the bulk kinetics of catalytic reactions conducted at the high temperatures and pressures of most industrial heterogeneous catalysis.

Atomistic studies of O2 dissociation on Pt(111) induced by photons, electrons, and by heating

Abstract: The adsorption and subsequent dissociation of O2 on Pt(111) was studied by variable temperature scanning tunneling microscopy in the temperature range of 40 to 215 K. Tight clustering of bridge site molecules is observed on terraces between 40 and 70 K, indicating a highly mobile precursor to chemisorption. Coexistence of bridge and fcc hollow site molecules in fractal-shaped islands is observed after dosing between 70 and 95 K. Dissociation of these species was induced by uv radiation, inelastic tunneling electrons, and heating. In all three cases, two O atoms are found within two lattice constants of the original molecule and one to three lattice constants apart.

Gas phase hydrogenation of crotonaldehyde over Pt/Activated carbon catalysts. Influence of the oxygen surface groups on the support

Abstract: Three activated carbons were prepared with different content of oxygen surface complexes and impregnated with aqueous solutions of [Pt(NH34]Cl2. The catalysts were characterized by H2 and CO chemisorption at room temperature, temperature-programmed decomposition (TPD) and X-ray photoelectron spectroscopy, and their catalytic behavior in the vapor phase hydrogenations of benzene and crotonaldehyde (trans-2-butenal) was determined. Metal dispersion is highly dependent on the degree of support oxidation, being lower for the catalyst support containing the highest amount of surface acidic complexes. This is attributed to the decomposition of the surface complexes, which act as anchoring centers for the platinum precursor, upon the reduction treatments at which the catalysts are subjected. The specific catalytic activity in the gas phase hydrogenation of crotonaldehyde is higher as the starting support is more oxidized, and the activity per gram of platinum increases with reduction temperature. The selectivity for the hydrogenation of the carbonilic CO bond instead of the olefinic CC bond is also improved when an oxidized support is used, and the production of the unsaturated alcohol, crotyl alcohol, is enhanced when catalysts are reduced at higher temperature, especially those prepared with oxidized supports.

Evaluation of the acid-base surface properties of several oxides and supported metal catalysts by means of model reactions

Abstract: Acid-base properties of oxides (Al2O3, SiO2, ZrO2, CeO2, MgO, SiO2Al2O3 and CeO2Al2O3) were investigated by means of model reactions: 3,3-dimethylbut-1-ene isomerization (33DMB1), methylene cyclohexane isomerization (MECH), cyclohexanol conversion (CHOL) and CO2 chemisorption at room temperature. The effect of acid (Cl−, SO2−4) and basic (K+) promoters on certain oxides (Al2O3, ZrO2) was also studied. Surface acidity was evaluated by means of 33DMB1 or MECH isomerization and of CHOL dehydration into cyclohexene. CO2 chemisorption as well as the cyclohexanone to cyclohexene ratio in CHOL conversion were used to measure the surface basicity of the solids. Except for very few cases, all the tests gave coherent results which led to well-defined scales of acidity and of basicity. Contrary to what could be observed with the conventional isopropanol test, the CHOL test proves to be little sensitive to the redox sites of the oxides. The presence of metals can create significant perturbations in these acid-base tests, except in the case of 33DMB1 isomerization. The latter reaction is not catalyzed by Pt and Rh surfaces, which allows to measure (in the presence of these metals) the acid properties of the support and to investigate the changes in surface acidity, resulting from the metal impregnations (metal → support electronic effect, presence of anions,…).

Spatially Resolved Raman Spectroscopy of Carbon Electrode Surfaces: Observations of Structural and Chemical Heterogeneity

Abstract: Raman spectroscopy and Raman imaging were used to examine several types of carbon electrode materials, including glassy carbon (GC) and highly ordered pyrolytic graphite (HOPG). Variations in the intensity ratio of the D and E2g Raman bands across the carbon surface indicated varying carbon microstructure. The D/E2g ratio for polished GC and pyrolytic graphite edge (PG) was relatively constant, while that of basal HOPG and PG varied significantly due to defects. The spatial heterogeneity of Rhodamine 6G Raman intensity following physisorption to carbon surfaces indicated that adsorption occurs at disordered regions, particularly defects on HOPG. This observation provides visual confirmation of previously reported correlations of defect area and physisorption. Chemisorption of dinitrophenylhydrazine was observed only at edge plane regions, confirming the localization of surface carbonyl groups on graphitic edge plane. Finally, chemisorption of nitroazobenzene radical formed from a diazonium precursor occur...

Oxygen chemisorption and oxide film growth on Ni{100}, {110}, and {111}: Sticking probabilities and microcalorimetric adsorption heats

Abstract: Using single-crystal adsorption calorimetry, heat data have been measured for the adsorption of oxygen on the three low-index planes of Ni at 300 K along with corresponding sticking probabilities. New data are presented with coadsorbed potassium on each plane, and temperature-dependent data for O2/Ni{100}. The initial heats of adsorption of oxygen on Ni{100}, {110}, and {111} are 550, 475, and 440 kJ (mol O2)−1, respectively, at 300 K, and the heat is found to drop rapidly with coverage in the chemisorption regime, indicating strong interadsorbate interactions. However, this rapid decline is not seen with coadsorbed potassium, a difference discussed both in terms of electron availability and coadsorbate attractions. The integral heats of adsorption for oxide film formation are 220, 290, and 320 kJ mol−1, respectively. Corresponding sticking probability measurements show initial values, all less than unity, of 0.63, 0.78, and just 0.23, again for the {100}, {110}, and {111} surfaces in that order. The coverage dependence of the sticking probability is consistent in each case with a passivating oxide film four layers thick. Comparable data for Ni{100} obtained using a pyroelectric detector gave good agreement with the conventional results at 300 K. At 410 K, however, the heat-coverage curve was flat up to 0.25 monolayers. Data were also obtained at 90 K. Analysis and Monte Carlo simulation of the temperature-dependent adsorption heat curves indicates that the large drop in adsorption heat with coverage seen at room temperature is consistent with a local second-nearest neighbor adatom–adatom repulsion rather than a long-range electronic effect.

Surface-enhanced raman spectroscopy of phosphate anions : adsorption on silver, gold, and copper electrodes

Abstract: The adsorption of phosphate anions on Ag, Au, and Cu electrodes from H2O and D2O solutions has been studied by means of surface-enhanced Raman spectroscopy (SERS). The interpretation of the spectra based on the solution Raman data and frequency shifts upon solution H2O/D2O exchange are presented. The prominent band at 1070−1100 cm-1, observed from adsorbed PO43- and HPO42- ions, exhibits downshifts of about 10 cm-1 in D2O solutions and has been assigned to the asymmetric P−O stretching mode. The corresponding asymmetric deformation mode has been assigned to the band located at ∼570 cm-1 which shows an upshift of 9−15 cm-1 in D2O solutions. Monodentate surface coordination of the PO43- and HPO42- ions is proposed. The dependence of the relative intensity of the internal modes on electrode potential was interpreted in terms of the migration of P−O groups from the surface as potential became more negative. Spectroscopic evidence was found for chemisorption of H2PO4- ion on the Cu electrodes, but no such evid...

X-ray photoemission spectroscopy study of UV/ozone oxidation of Au under ultrahigh vacuum conditions

Abstract: (111) textured Au films were oxidized under UHV conditions by exposing them to UV radiation and ozone. The Au films were characterized by x-ray photoelectron spectroscopy. The gold surface becomes heterogeneous upon oxidation: approximately 0.8 monolayer (ML) of the adsorbed oxygen is present as chemisorbed species. The rest of the adsorbed oxygen forms, on the average, a few ML thick gold oxide with a stoichiometry close to Au2O3. Both the oxide and the chemisorbed oxygen disappear slowly when heated to 150 °C. During isothermal decomposition at 200 °C, the oxide is transformed to chemisorbed oxygen prior to desorption. Both the chemisorbed oxygen and the gold oxide can be removed by rinsing with water. The surface is hydrophilic after the rinse. When contaminated gold films are exposed to UV/ozone, impurity oxides appear at higher binding energies than the binding energies of oxygen on clean gold samples. The impurity oxides are thermally much more stable than Au2O3.

Hydrolysis and Condensation of Self-Assembled Monolayers of (3-Mercaptopropyl)trimethoxysilane on Ag and Au Surfaces

Abstract: Self-assembled monolayer films of (3-mercaptopropyl)trimethoxysilane (3MPT) and their hydrolysis products on Ag and Au surfaces are characterized using Raman spectroscopy, FTIR spectroscopy, ellipsometry, X-ray photoelectron spectroscopy (XPS), and electrochemistry. 3MPT monolayers are formed through metal−thiolate bonding through the S atom on both metals, similar to other alkanethiol chemisorption chemistries. The orientation of 3MPT molecules in these monolayers is similar on both metals. Prior to hydrolysis, the molecules form an organized monolayer with the methoxy headgroups oriented largely parallel to the surface and the propyl chain in a largely trans conformation. When the methoxy groups are hydrolyzed, the 3MPT molecules cross-link through the formation of siloxane bonds. Although the surface vibrational spectroscopy indicates the presence of a small number of unreacted silanol groups on the siloxane surface, cyclic voltammetry of underpotential deposition of Pb suggests that the Si−O−Si networ...

Raman spectroscopic determination of the structure and orientation of organic monolayers chemisorbed on carbon electrode surfaces.

Abstract: Azobenzene (AB) and 4-nitroazobenzene (NAB) were covalently bonded to carbon surfaces by electrochemical reduction of their diazonium derivatives. The N(1s) features of XPS spectra of modified surfaces had intensities expected for monolayer coverage. However, the Raman spectra were significantly more intense than expected, implying an increase in scattering cross section upon chemisorption. A likely explanation is resonance enhancement of the carbon/adsorbate chromophore analogous to that reported earlier for dinitrophenylhydrazine (DNPH) chemisorption. Vibrational assignments indicate that the C-C vibration between azobenzene and the carbon surface is in the 1240-1280 cm(-1) region, and this conclusion is supported by spectra obtained from [(13)C]graphite. Observation of depolarization ratios for 4-nitroazobenzene and DNPH on graphite edge plane indicate that NAB is able to rotate about the NAB/carbon C-C bond, while chemisorbed DNPH is not. The partial multiple bond character of the DNPH linkage to graphite is consistent with the observation that the DNPH π system remains parallel to the graphitic planes.

Characterization of mono- and bi-metallic platinum catalysts using CO FTIR spectroscopy. Size effects and topological segregation

Abstract: CO chemisorption on Pt catalysts supported on non-acidic alumina prepared from platinum acetylacetonate has been studied by FTIR spectroscopy. The stretching frequency for isolated CO species is observed at 2032 cm−1 for highly dispersed Pt samples (dispersion 0.99) and at 2067 cm−1 for poorly dispersed samples (dispersion 0.09). This frequency shift is in agreement with single-crystal and on supported Pt catalyst data, and may be ascribed to the change of coordination at the Pt chemisorption site. Large shifts, reaching 180 cm−1 can be observed upon coadsorption of ammonia suggesting the possibility of long-range interactions involving the collective electronic properties. CO adsorption has also been studied on well dispersed bimetallic samples obtained by modifying the well dispersed Pt/Al2O3 catalyst with Sn or Ge. Quite different effects of the two additives were observed: whereas Ge decreased the dipole–dipole coupling in the adsorbed layer, Sn did not. This suggests that Sn segregates at sites of low coordination at the surface of the Pt particles, while Ge is more homogeneously distributed. The coadsorption of ammonia and CO on the bimetallic particles also shows that the electronic properties of Pt were dramatically modified by Ge and unaffected by Sn.

Dissociative chemisorption of methane on Ir(111): Evidence for direct and trapping-mediated mechanisms

Abstract: Molecular beam and bulb gas techniques were employed to study dissociative chemisorption of methane on Ir(111). The initial dissociative chemisorption probability (S0) was measured as a function of incident kinetic energy (Ei), surface temperature, and angle of incidence (θi). As the incident kinetic energy increases, the value of S0 first decreases and then increases with Ei indicating that a trapping-mediated chemisorption mechanism dominates methane dissociation at low kinetic energy, and a direct mechanism dominates at higher kinetic energies. The values of the reaction probability determined from molecular beam experiments of methane on Ir(111) are modeled as a function of Ei, θi, and surface temperature. These fits are then integrated over a Maxwell–Boltzmann energy distribution to calculate the initial chemisorption probability of thermalized methane as a function of gas and surface temperature. The calculations are in excellent agreement with results obtained from bulb experiments conducted with r...

XPS characterisation of surface modified Ni-Ti shape memory alloy

Abstract: Ni-Ti shape memory alloy has been surface amorphised by N+ ion implantation and by controlled shot peening in order to improve surface mechanical properties. X-ray photoelectron spectroscopy (XPS) and surface wetting measurements have been used to characterise the surface modified Ni-Ti to provide a fuller understanding of the biomaterial potential of such a material. The results of this study show that both the modified and unmodified Ni-Ti surfaces were predominantly covered with TiO2 and the underlying substrate crystallography determined both the affinity for surface OH−/chemisorbed water and ultimately the wetting behaviour of distilled water. Additionally, N+ ion implanted Ni-Ti contained a TiN phase within the surface which reduced wetting, demonstrating a reduced interfacial energy. The surface concentrations of Ni were unaffected by the surface modifications, with all samples containing less than 3 at.% Ni. This study has shown that the surface TiO2 oxide layer was maintained despite the surface amorphisation treatments. It can be assumed that the TiO2 layer is almost identical in unimplanted and implanted surfaces and they will display the same biocompatibility.

Hydrogen Chemisorption on Silica-Supported Pt Clusters: In Situ X-ray Absorption Spectroscopy

Abstract: Hydrogen chemisorption on small silica-supported Pt clusters was investigated using in situ extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray absorption near-edge structure (XANES) spectroscopy. The clusters were found to exhibit a bulklike Pt first nearest neighbor (NN) distance (2.76 A) and low disorder while covered by chemisorbed hydrogen. In contrast, bare Pt clusters produced by heating in vacuo at 300 °C are characterized by a contracted Pt NN distance (2.66 A) and greater disorder. These effects are reversed by re-exposure of the bare Pt clusters to H2 at 25 °C. The metal−support interface is characterized by a short Pt−O distance, irrespective of the presence of chemisorbed hydrogen. An apparent L3 edge shift of 0.8 eV relative to bulk Pt is observed for the hydrogen-covered clusters. This shift is attributed to a decrease in the Pt L3 edge resonance (white line) intensity, as no corresponding shift is observed at the L2 edge. A hydrogen-related L2,3 XANES feature at 9 eV ap...