Showing papers on "Chemisorption published in 1993"

Physicochemical properties of small metal particles in solution: "microelectrode" reactions, chemisorption, composite metal particles, and the atom-to-metal transition

Abstract: The study of nanometer and subnanometer colloidal metal particles in aqueous solution complements the investigation of small particles in molecular beams, frozen solutions, and inorganic clusters. The electronic properties of the metal particles are changed by surface modification, for example by chemisorption of a nucleophilic molecule or by deposition of a second metal. The resulting changes in the chemical, photochemical, and optical properties are discussed. Methods are described which enable one to store excess electrons or positive holes on the particles in a controlled manner and to investigate the accompanying changes in the optical properties. Metal particles carrying excess electrons initiate electrochemical reactions such as the reduction of water or the deposition of metals

Structure and binding of alkanethiolates on gold and silver surfaces: implications for self-assembled monolayers

Abstract: We performed ab initio geometry optimization of HS and CH 3 S on cluster models of Au(111), Au(100), Ag(111), and Ag(100) surfaces, at the RECP Hartree-Fock+electron correlation (MBPT2) level. From these we determined classical force field parameters, thus opening the possibility for realistic molecular dynamics (MD) simulations of self-assembled alkanethiolate monolayers on gold and silver surfaces. We find that there are two chemisorption modes, very close in energy, for thiolates on Au(111) surfaces. In the first, the surface-S-C bond angle is ∼180 o (sp hybridization), while in the second it is ∼104 o (spα 3 hybridization). This suggests a possible mechanism for the annealing of alkanethiolate monolayers

Interaction of oxygen with Al(111) studied by scanning tunneling microscopy

Abstract: The interaction of oxygen with Al(111) was studied by scanning tunneling microscopy (STM). Chemisorbed oxygen and surface oxides can be distinguished in STM images, where for moderate tunnel currents and independent of the bias voltage the former are imaged as depressions, while the latter appear as protrusions. An absolute coverage scale was established by counting O adatoms. The initial sticking coefficient is determined to so=0.005. Upon chemisorption at 300 K the O adlayer is characterized by randomly distributed, immobile, individual O adatoms and, for higher coverages, by small (1×1) O islands which consist of few adatoms only. From the random distribution of the thermalized O adatoms at low coverages a mobile atomic precursor species is concluded to exist, which results from an internal energy transfer during dissociative adsorption. These ‘‘hot adatoms’’ ‘‘fly apart’’ by at least 80 A, before their excess energy is dissipated. A model is derived which explains the unusual island nucleation scheme by trapping of the hot adatoms at already thermalized oxygen atoms. Oxidation starts long before saturation of the (1×1) O adlayer, at coverages around ΘO≂0.2. For a wide coverage range bare and Oad covered surfaces coexist with the surface oxide phase. Upon further oxygen uptake both chemisorbed and oxide phase grow in coverage. Oxide nucleation takes place at the interface of Oad islands and bare surface, with a slight preference for nucleation at upper terrace step edges.Further oxide formation progresses by nucleation of additional oxide grains rather than by growth of existing ones, until the surface is filled up with a layer of small oxide particles of about 20 A in diameter. At very large exposures up to 5×105 L they cover the entire surface as a relatively smooth, amorphous layer of aluminum oxide. The difference in Al atom density between Al metal and surface oxide is accommodated by short range processes, with no indication for any long range Al mass transport. Based on our data we discuss a simpler two step model for the interaction of oxygen with Al(111), without making use of an additional subsurface oxygen species. The complex spectroscopic data for the O/Al(111) system are rationalized by the wide coexistence range of bare and Oad covered surface with surface oxide and by differences in the electronic and vibronic properties of the surface atoms depending on the number of neighboring O adatoms in the small Oad islands.

Effect of rotation on the translational and vibrational energy dependence of the dissociative adsorption of D2 on Cu(111)

Abstract: We have investigated the dependence on the rotational and vibrational states of the translational energy of D2(v,J) formed in recombinative desorption from Cu(111). These results provide information about the effect of rotational energy relative to that of vibrational and translational energy on the dissociative chemisorption of D2 on Cu(111). The range of rovibrational states measured includes rotational states J=0–14 for vibrational state v=0, J=0–12 for v=1, and J=0–8 for v=2. D2 molecules were detected in a quantum‐state‐specific manner using three‐photon resonance‐enhanced multiphoton ionization (2+1 REMPI). Kinetic energies of desorbed molecules were obtained by measuring the flight time of D2+ ions in a field‐free region. The mean kinetic energies determined from these measurements depend strongly on the rotational and vibrational states. Analyzing these results using the principle of detailed balance confirms previous observations that vibrational energy is effective, though not as effective as tr...

Self‐Assembled Layers of Alkanethiols on Copper for Protection Against Corrosion

Abstract: Self-assembled monolayers of alkanethiols C[sub n]H[sub 2n+1]SH(n = 6 [approximately] 18) adsorbed on the surface of a polycrystalline bulk cu were constructed and characterized by x-ray photoelectron (XPS) and surface-enhanced Raman scattering (SERS) spectroscopy and contact angle and impedance measurements. The protection ability of the alkanethiol monolayers against Cu corrosion in an aerated 0.5 M Na[sub 2]SO[sub 4] solution was examined by impedance and polarization techniques. Results of XPS, SERS, and contact angle measurements showed that alkanethiols were chemisorbed on the Cu surface by the formation of strong bonds between Cu and S atoms following cleavage of a S-H bond and formed densely packed, water-repellent monolayers on the surface. The advancing contact angle of these monolayer films was comparable to that of alkanethiolate monolayers adsorbed at a vapor-deposited Cu on a Si wafer. However, sufficiently high protection abilities of the films against Cu corrosion were not obtained in 0.5 M Na[sub 2]SO[sub 4]. A preliminary experiment demonstrated the formation of a promising protective film which was prepared on the Cu surface by modification of self-assembled 11-mercapto-1undecanol monolayer with octyltrichlorosilane to form cross-linkages between the thiol molecules with siloxane bonds.

Growth model for metal films on oxide surfaces: Cu on ZnO(0001)-O.

Abstract: The structural and electronic properties of Cu films vapor deposited on the oxygen terminated ZnO(0001)-O surface at 130 K have been characterized using x-ray photoemission spectroscopy (XPS), ${\mathrm{He}}^{+}$-ion-scattering spectroscopy, low-energy electron diffraction, work-function, and band-bending measurements, angular-resolved XPS, and CO and ${\mathrm{H}}_{2}$O chemisorption. These results show that Cu is cationic at tiny coverages, but becomes nearly neutral at coverages beyond a few percent. The Cu clusters into two-dimensional (2D) metallic islands at these coverages. Further deposition of Cu leads to spreading of these 2D islands without forming thicker layers, until about 50% of the surface is covered. Thereafter, these Cu islands grow thicker without filling the gaps between the islands except at a rate much slower than the rate at which Cu is deposited into these clean spaces. The annealing behavior of these films has also been studied between 130 and 850 K. These results show that the Cu has a tendency to cluster into thick 3D islands which only cover a small fraction of the surface. We present a model here based on the energetics of the system which readily explains the apparent contradiction between this tendency for 3D clustering, and the dynamical effect which leads to efficient wetting for coverages up to 1/2 monolayer at low temperatures. This model shows that a large fraction of the surface can first be covered by a 2D film even when the metal's self-adsorption energy significantly exceeds its adsorption energy on the oxide, provided the difference in these energies does not exceed the energy of 2D evaporation from kinks onto terraces. This model helps to explain a variety of confusing results in the growth of metal films on oxide surfaces.

Origin of non-faradaic electrochemical modification of catalytic activity

Abstract: X-ray photoelectron spectroscopy (XPS) was used to investigate the effect of electrochemical oxygen pumping on Pt catalyst films interfaced with an O 2 -conducting yttria-stabilized zirconia solid electrolyte. It was found that electrochemical oxygen pumping to the catalyst causes spillover of significant amounts of anionic oxygen from the solid electrolyte onto the platinum film surface. The spillover oxygen species has an XPS binding energy 528.8 eV compared to 530.4 eV for chemisorbed oxygen, which is also observed on the surface, and is less reactive than chemisorbed oxygen with the reducing ultrahigh vacuum background. The detection of the anionic oxygen species upon electrochemical pumping confirms the previously proposed explanation of the non-Faradaic electrochemical modification of catalytic activity (NEMCA) or electrochemical in catalysis

Hydrogen chemisorption on ceria: influence of the oxide surface area and degree of reduction

Abstract: The chemisorption of hydrogen on two ceria samples (CeO2-BS, 4 m2 g–1; CeO2-SM, 56 m2 g–1) reduced at temperatures ranging from 623 to 1173 K has been studied by Fourier-transform infrared (FTIR) spectroscopy and temperature-programmed desorption followed by thermal conductivity (TPD-TC) and mass spectrometry (TPD-MS). The concentration of the oxygen vacancies created by the reduction treatments was determined by using a combination of O2 pulses and temperature-programmed oxidation. According to our TPD-MS study, hydrogen can be desorbed from ceria as both H2(reversible adsorption) and H2O (irreversible adsorption), the relative contribution of these two forms depending on the reduction temperature. For samples reduced at 773 K or higher temperatures, H2 was the only desorption product. From this observation, some earlier TPD-TC and TPR-TC results could be better understood. Upon reduction at 773 K, the amount of H2 chemisorbed per mole of CeO2 was ten times larger for CeO2-SM than for CeO2-BS. Likewise, the molar chemisorptive capability of CeO2-SM strongly decreased (45 times) with the reduction temperature. No simple relationship could be observed between the amount of chemisorbed hydrogen and the total concentration of oxygen vacancies in the oxide. In contrast to earlier results on the contribution of a massive bronze-like phase when chemisorbing H2 at 195–500 K, the results reported here show that the hydrogen chemisorption on reduced ceria is a surface-related process. Furthermore, the highest value for the hydrogen chemisorption we have obtained, 7.1 H atom nm–2(BET), suggests a pure surface process.

Electron theory of thin-film gas sensors

Abstract: The simple charge-transfer model usually applied to thin-film gas sensors is critically discussed and the fundamental difficulties are elucidated from a quantum-chemical point of view. In order to overcome the inconsistency of the charge-transfer model, a microscopic model of a thin-film gas sensor is developed in the framework of the electron theory of chemisorption. On the basis of Volkenstein's theory, two forms of chemisorption are introduced: the neutral weak-chemisorbed form and the charged strong-chemisorbed form. For the case of acceptor-like chemisorption and n-type metal-oxide semiconductors, the pressure dependence of the thin-film conductivity is derived, solving self-consistently the one-dimensional Poisson equation. Depending on the geometric and electronic structure of the thin-film gas sensor, relationships are derived in a consistent manner between the catalytic surface properties and the electronic bulk properties. It turns out that different types of adsorption isotherms can arise, depending on the bulk doping level. An interesting feature of the catalytic behaviour of the proposed microscopic model is the saturation effect of charged strong-chemisorbed species, which already arises at very low partial pressure (often known as the Weisz limitation in the sensor literature). In contrast to the charged strong-chemisorbed species, the neutral weak-chemisorbed species show a typical Langmuir behaviour. The main result is the power-law behaviour of the thin-film conductivity over a wide pressure region, σ(p) ∼ p−m. Whereas in the sensor literature the different power laws are explained by applying mass-action laws, it is shown that the power, m, is determined by different, fundamental parameters: ξ=D/LD (LD = Debye length), film thickness D, temperature and the surface-state energy. Hence it cannot be described by the stoichiometric coefficient alone. Furthermore, the theoretical values of the power vary between 0 and 1, in agreement with various experimental results. A modified Helmholtz equation is proposed, which can connect the change of the work function to both the change of the surface potential due to the charged strong-chemisorbed species and the change of the electron affinity due to the dipole moment of the neutral, weak-chemisorbed species. The effect of an external field on chemisorption is investigated for the case of a simplified one-dimensional ‘suspended’-gate MESFET (metal-semiconductor FET).

Formation of surface-bonded methoxy groups in the sorption of methanol and methyl iodide on zeolites studied by carbon-13 MAS NMR spectroscopy

Abstract: Methanol and methyl iodide as methylating agents react chemically with the surface atoms of zeolites and, under favorable conditions, yield surface-bonded methoxy groups. Zeolites with well-defined structures of faujasites, mordenites and ZSM5-containing various cations (Na + , Rb + , Mg 2+ , Ca 2+ ) and /or acid protons were investigated in chemisorption experiments using CH 3 I. The observed signals in the 53-59 ppm TMS region were assigned to the methoxy groups, i.e., methyl groups surface-bonded to the lattice oxygens. The chemical shift of these signals depends linearly on the equalized electronegativity of the oxygen of the given zeolite structure and becomes a measure of the lattice oxygen basicity (nucleophility)

Chemisorption of nitric oxide on char. 1. Reversible nitric oxide sorption

Abstract: The process of chemisorption of nitric oxide (NO) on a carbon derived from phenol-formaldehyde resin has been studied at temperatures between 323 and 473 K and at NO partial pressures between 2.02 and 10.1 kPa. The process of chemisorption can be split into two separate pathways-one a reversible chemisorptive pathway forming nitric oxide surface complexes C(NO) and the other an irreversible chemisorptive pathway yielding N[sub 2] and carbon surface oxides C(O) and C(O[sub 2]). The enthalpy of formation of C(NO) is between 42 and 46 kJ/mol exothermic. The reversible pathway is governed by isotherms that are linear in NO partial pressure, within the range of pressures studied here. This same process, however, appears to be kinetically second order with respect to NO partial pressure, suggesting a possible role of NO dimer as an intermediate in the process. 35 refs., 11 figs., 1 tab.

Surface organometallic chemistry of tin: reactivity of tetraalkyltin complexes and tributyltin hydride toward silica

Abstract: Tetraalkyltin complexes, SnR, (R = Me, Et, i-Pr, Bu), and tributylhydridotin were reacted with the surface of partially dehydroxylated silica. Interaction between the complexes and the silica surface was followed by IR and I3C and "%n MAS NMR spectroscopies and analysis of the evolved gases. At room temperature, all the studied complexes are physisorbed: a hydrogen-type bonding between the terminal methyl group of the alkyl ligands and/or the hydride ligand and the surface silanol groups is evidenced. Above 100 OC, with silica dehydroxylated at 500 OC (SiO,(SOO)), a reaction occurs, leading to the formation at 200 OC of essentially one surface complex, hSiO-SnR3 (1) with evolution of 1 mol of alkane per mol of surface tin. 1 is already formed at room temperature by reaction of silica with Bu3SnH; the reaction is slow and liberates 1 mol of H2/mol of surface Sn. At low surface coverage, and for R # Me, the alkyl ligands of 1 are folded toward the surface, interacting with it via hydrogen-type bonding. The same interaction and reaction are observed when the surface of silica is less dehydroxylated, SiO2(2O0): nevertheless 1 is less stable, probably due to the availability of surface silanol groups in the neighborhood of 1.

Mechanisms of island formation of alkali-metal adsorbates on Al(111)

Abstract: Using density functional theory total-energy calculations we investigate the chemisorption of Na and K alkali metals on the (111) surface of aluminum from very low coverages (\ensuremath{\Theta}=1/16) up to a full monolayer. The calculations predict site changes of the adsorbate atoms, as well as a condensation into islands with increasing coverage. Some processes are activated, which implies a possibly irreversible temperature dependence. This is shown to have important consequences for the coverage dependency of the work function change.

Chlorine bonding sites and bonding configurations on Si(100)–(2×1)

Abstract: A combination of experimental methods has been employed for the study of Cl2 adsorption and reaction on Si(100)–(2×1). At 100 K, Cl2 adsorption occurs rapidly to a coverage of ∼0.7 Cl/Si. This is followed by slower adsorption kinetics with further Cl2 exposure. Two Cl adsorption states are observed experimentally. One of the adsorption states is terminally bonded Cl on the inclined dangling bond of the symmetric Si2 dimer sites, with a vibrational frequency, ν(SiCl) of 550∼600 cm−1. These bonded Cl atoms give four off‐normal Cl+ ESDIAD emission beams from the orthogonal domains of silicon dimer sites. The Si–Cl bond angle for this adsorption configuration is estimated to be inclined 25°±4° off‐normal. The second Cl adsorption state, a minority species, is bridge bonded Cl with ν(Si2Cl) of ∼295 cm−1 which produces Cl+ ion emission along the surface normal direction. Both adsorption states are present at low temperatures. Irreversible conversion from bridge bonded Cl to terminally bonded Cl begins to occur ...

Adsorption of CO2 over univalent cation-exchanged ZSM-5 zeolites

Abstract: Isotherms and IR spectra were measured for CO2-MZSM-5 (M = Li+, Na+, K+, Rb+, Cs+) adsorption systems. The observed adsorption characteristics were well approximated by a physical adsorption where the van der Waals force and an electrostatic interaction force are operating: the degree of irreversible adsorption (chemisorption) was less than 10% of the total adsorption. Although the adsorbed CO2 molecule interacts with both the cation and the pore wall, only the CO2-cation interaction can convert the IR inactive ν1 vibration into an IR active state. The adsorption model proposed enables us to calculate the initial heat of adsorption as well as the molecular orientation angle of CO2 against the cation site.

Dissociative adsorption of H2 on Ni(111)

Abstract: Ab initio configuration interaction calculations are performed to study the dissociative adsorption of H2 on a Ni(111) surface. The lattice is modeled as an embedded three‐layer 41‐atom cluster. Ni 3d orbitals are explicitly included on seven Ni atoms on the surface. H is preferentially chemisorbed at a threefold site on Ni(111) and the calculated binding energy of 62 kcal/mol, H–Ni distance of 1.86 A, and H vibrational frequency of 1176 cm−1 are in excellent agreement with experimental data. H adsorbed at bridge and on‐top Ni sites is 2.5 and 8.1 kcal/mol less stable, respectively. The heat of reaction H2 (gas)→2 H (ads) is calculated to be 22.0 kcal/mol exothermic. When two H atoms are adsorbed as nearest neighbors to the same Ni atom, threefold sites are preferred with H atoms adsorbed at fcc–fcc, hcp–hcp, or across atom fcc–hcp sites. These structures are consistent with the observed (2×2)−2H low energy electron diffraction pattern. The average adsorption energy per H is calculated to be 62 kcal/mol f...

Molecular dynamics with combined quantum and empirical potentials: C2H2 adsorption on Si(100)

Abstract: Classical trajectory calculations were employed to study the reaction of acetylene with dimer sites on the Si(100) surface at 105 K. Two types of potential energy functions were combined to describe interactions for different regions of the model surface. A quantum mechanical potential based on the semiempirical AM1 Hamiltonian was used to describe interactions between C2H2 and a portion of the silicon surface, while an empirically parametrized potential was developed to extend the size of the surface and simulate the dynamics of the surrounding silicon atoms. Reactions of acetylene approaching different sites were investigated, directly above a surface dimer, and between atoms from separate dimers. In all cases, the outcome of C2H2 surface collisions was controlled by the amount of translational energy possessed by the incoming molecule. Acetylene molecules with high translational energy reacted with silicon dimers to form surface species with either one or two Si–C bonds. Those molecules with low transl...

Structure of monolayers of short chain n-alkanoic acids (CH3(CH2)nCOOH, n = 0-9) spontaneously adsorbed from the gas phase at silver as probed by infrared reflection spectroscopy

Abstract: This paper presents the results of an infrared spectroscopic and wettability examination of monolayers formed by the spontaneous adsorption of short chain n-alkanoic acids (CH[sub 3](CH[sub 2])[sub n]COOH, n = 0-9) from the gas phase onto Ag. The infrared spectral data, collected in an external reflection mode, indicate that the monolayers chemisorb as the corresponding salt and that the chains contain all-trans conformational sequences for n = 1-9. The chemisorption process is further exemplified by the formation of a carboxylate group that is symmetrically bound at Ag as either a bidentate or bridging ligand. The conformational insights are developed on the basis of an unusually prominent series of the methylene wagging progression. An in-depth analysis resulted in the complete assignment of the fundamental modes in the progression as well as a reassignment of the CC stretch in the region of this progression. Structural insights developed from the infrared spectral data and contact angle measurements are compared to analogous monolayers formed from dilute solution. This comparison points to the subtle, but important, role of solvent in influencing the structure of the monolayer. 48 refs., 8 figs., 1 tab.

Dissociative chemisorption of nitrogen on Ru(0001)

Abstract: The dissociative chemisorption of nitrogen on clean and cesiated Ru(0001) surfaces has been studied using high‐resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). N2 (at 300 K) chemisorbs dissociatively with a sticking coefficient of 2×10−6, independent of substrate temperature which was varied between 420 and 700 K. The saturation coverage is found at 0.5 monolayer. The energy of the N–Ru stretching vibration is 71 meV at the bare surface and 69 meV at the cesiated Ru(0001) surface. The activation energy for desorption is about 190 kJ/mol for small coverages. The kinetic data suggest the existence of an activation barrier in the entrance channel of adsorption. Preadsorption of 0.08 monolayer of Cs increases the sticking coefficient only by a factor of 1.3, and the maximum amount of adsorbed N is reduced due to blocking of adsorption sites through Cs.

Adsorption of oxygen on Cu(100). I. Local structure and dynamics for two atomic chemisorption states.

Abstract: By means of temperature-dependent surface-extended x-ray-absorption fine structure (SEXAFS) measurements we characterize two different chemisorption state of atomic O/Cu(100), one corresponding to a reconstructed surface and the other to an unreconstructed surface, respectively. The unreconstructed state is characterized by an O-Cu nearest-neighbor (NN) bond length of 1.88(3) A, the O atoms being located 0.8(3) A above the first Cu layer