Showing papers on "Chemisorption published in 1980"

A molecular beam study of the catalytic oxidation of CO on a Pt(111) surface

Abstract: The oxidation of carbon monoxide catalyzed by Pt(111) was studied in ultrahigh vacuum using reactive molecular beam–surface scattering. Under all conditions studied, the reaction follows a Langmuir–Hinshelwood mechanism: the combination of a chemisorbed CO molecule and an oxygen adatom. When both reactants are at low coverage, the reaction proceeds with an activation energy E*LH =24.1 kcal/mole and a pre‐exponential υ4 =0.11 cm2 particles−1 sec−1. At very high oxygen coverage, E*LH decreases to about 11.7 kcal/mole and υ4 to about 2×10−6 cm2 particles−1 sec−1. This is largely attributed to the corresponding increase in the energy of the adsorbed reactants. When a CO molecule incident from the gas phase strikes the surface presaturated with oxygen, it enters a weakly held precursor state to chemisorption. Desorption from this state causes a decrease in chemisorption probability with temperature. Once chemisorbed, the CO molecule then has almost unit probability of reacting to produce CO2 below 540 K. The CO2 product angular distribution varies from cosγ to cos4γ depending sensitively upon the adsorbed reactant concentrations.

Oxygen chemisorption on tin oxide: Correlation between electrical conductivity and EPR measurements

Abstract: Chemisorption of oxygen on tin oxide is studied. Correlations between electric conductivity and electron paramagnetic resonance (EPR) measurements are reported.

Effective-medium theory of chemical binding: Application to chemisorption

Abstract: An approximate theory of the total energy change connected with the embedding of an atom in an inhomogeneous host is developed. The primary effect of the inhomogeneous environment is included by replacing it with a homogeneous electron gas of a density equal to that of the host at the atom site. The lowest-order corrections to this simple picture are derived. The scheme, which is computationally very simple, is tested against first-principles calculations for several chemisorption systems. It is found that including a simple first-order correction gives excellent agreement for H and O adsorbates. For less electronegative atoms like Si and Li, it seems that a second-order term involving the polarizability of the atom in a homogeneous electron gas must be included.

Surface effects and the formation of metal hydrides

Abstract: Hydrogen is stored atomically in metal hydrides. Dissociative chemisorption and associative desorption are therefore important steps in the hydrogen absorption and desorption processes. We summarize the theoretical and experimental knowledge of hydrogen chemisorption on clean and precovered metal surfaces and correlate it with the techniques for preparing metal hydrides. At the surface of hydride-forming intermetallics, precipitates of d metals and a metallic subsurface are produced by surface segregation and decomposition. The subsurface and the precipitates are able to dissociate H2. Our recent work on the surface analysis of LaNi5, FeTi, Mg2Ni and ErFe2 is reviewed.

Surface poisoning of LaNi5, FeTi and (Fe,Mn)Ti by O2, Co and H2O☆

Abstract: This paper is a brief summary of an extensive experimental program designed to understand the surface poisoning of metal hydrides by impurities in the hydrogen used. The alloys investigated were LaNi5, FeTi and Fe0.85Mn0.15Ti. The gaseous impurities studied were O2, H2O and CO. The nature of the surface structures formed and consequently the degree of poisoning and ease of reactivation vary markedly from alloy to alloy and from impurity to impurity. Examples of complex compound film formation, chemisorption and possibly physisorption can be seen. The results show the hope of developing surface structures with significant resistance to impurity gases and especially the possibility of designing practical regeneration cycles.

Electron spectroscopy study of silicon surfaces exposed to XeF2 and the chemisorption of SiF4 on silicon

Abstract: The surface chemistry of silicon exposed to reactive XeF2 gas and the chemisorption of SiF4 on Si at −150 and 25 °C have been studied using XPS and AES. While SiF4 can be condensed at −150 °C, XeF2 is dissociatively chemisorbed and Xe does not stick on the surface. For both Si/SiF4 and Si/XeF2 at 25 °C, a layer of SiF2‐like surface species is identified from the characteristic core level chemical shifts. The formation of this fluorinated surface layer hinders the adsorption of SiF4, but XeF2 reacts with this layer to form volatile SiF4. The behavior of fluorine chemisorption on silicon is illustrated for the first time and the role of surface fluorine in the silicon etching process is discussed in light of the new results.

Systematics of the binding energy of oxygen and hydrogen on transition-metal surfaces. I.

Abstract: The chemisorption energy of simple gases on transition metals follows some remarkable systematic trends. We construct a simple theory which leads to an understanding of the observed trends and relates the chemisorption energy to the essential parameters characterizing the transition metals, viz., the mean energy of their density of states, the bandwidth, and the number of $d$ electrons. The theory is applied to the adsorption of hydrogen and oxygen on $3d$ and $4d$ transition metals. The positions of the atomic levels of hydrogen and oxygen (and nitrogen) with respect to the $d$ bands of the transition metals is such that the primary binding comes from transfer of $d$ electrons from surface metal atoms to the adatoms and to low-energy bonding resonances induced in the metal atoms. However, there is large enough hybridization of the adatom orbitals with the surface metal-atom orbitals that the local density of states of the metal atoms is significantly altered throughout the band. Correspondingly the metal parameters that determine the binding energy are the average position of the $d$ band with respect to the adatom orbital energy and the overall width of the $d$ band.

The chemisorption of acetylene and ethylene on Rh(111): A low energy electron diffraction (LEED), high resolution electron energy loss (ELS), and thermal desorption mass spectrometry (TDS) study

Abstract: We report the results of a detailed investigation of the chemisorption and reactivity of acetylene (C2H2) and ethylene (C2H4) on the Rh(111) single crystal surface. Below 270 K ELS measurements indicate that acetylene chemisorbs on Rh(111) with its C–C bond oriented parallel to the surface forming an approximately sp2 hybridized species. LEED investigations show that both C2H2 and C2H4 form metastable (2×2) surface structures on Rh(111) below 270 K. An irreversible order–order transformation occurs between 270 and 300 K to a stable c(4×2) hydrocarbon overlayer. The stable species formed from both molecules are identical. Hydrogen addition to chemisorbed acetylene is necessary to complete this conversion. The geometry of the adsorbed ethylene species does not change during this transformation although the overlayer structure does. This stable hydrocarbon species is identical to the hydrocarbon species formed from the chemisorption of either C2H4 or C2H4 and hydrogen on Pt(111) above 300 K. The addition of ...

Photoelectron study of the interaction of carbon monoxide with zinc oxide

Abstract: Ultraviolet photoelectron spectroscopy (UPS) has been used to study the chemisorption of CO on four low-index surfaces of ZnO, an active methanol catalyst. These surfaces have significant differences with respect to their coordination unsaturation; thus, a correlation between the adsorption behavior and the surface structure enables the geometric requirements for chemisorption to be determined. The He II photoelectron peak associated with the 4 sigma molecular orbital of the CO molecule serves as a probe of the amount of adsorbed CO. The adsorption is found to be reversible, and measurements of the equilibrium coverage as a function of temperature and pressure demonstrate that the bonding interactions are very similar on all four surfaces, with an initial heat of adsorption of 12.0 +- 0.4 kcal/mol, which decreases approximately linearly with coverage. Chemisorbed CO is readily displaced by NH/sub 3/, indicating the involvement of zinc ions in the bonding of CO to the surface. This is strongly supported by the relative CO coverage of the four surfaces under equivalent conditions, which generally correlates with the availability of unsaturated zinc sites. Angle-resolved measurements of the 4 sigma intensity are used to confirm this binding mode. The bonding of CO to ZnO is cominated bymore » the sigma donor interaction of the carbon end of the molecule with the zinc ion as determined from the observed decrease in the splitting of the energies of the 4 sigma and 5 sigma molecular orbitals. The lack of significant ..pi.. back-bonding is supported by the increase of the dipole moment of the coordinated CO molecule to approximately 0.6 D as determined from He I photoelectron work function measurements. This mode of binding of CO is consistent with the observed increase in the CO stretching frequency relative to the gas-phase value and suggests possible relevance to catalysis.« less

Chemisorption theory for metallic surfaces: Electron localization and the description of surface interactions

Abstract: A theory for describing molecule-solid surface interactions using correlated configuration-interaction (CI) wave functions for the surface region is described. Starting with a delocalized self-consistent-field wave function for the lattice, approximated as a cluster of atoms, a local surface region is defined by a unitary, localization transformation of the single-particle orbitals of the lattice wave function based on electron exchange maximization with the surface sites of interest. CI calculations on the resulting $N$-electron subspace plus adsorbate permit an accurate description of bonding at the surface. Ab initio computational techniques for treating the many-electron problem and large clusters of metal atoms are described.

Oxidation of polycrystalline indium studied by x‐ray photoelectron spectroscopy and static secondary ion mass spectroscopy

Abstract: The chemisorption of oxygen on clean polycrystalline indium surfaces is investigated by the combined techniques of x‐ray photoelectron spectroscopy (XPS) and the static mode of secondary ion mass spectroscopy (SIMS). Oxygen uptake is characterized by a moderately rapid formation of In2O3, with no preoxidation adsorbed O2 phase discernable in the XPS spectra. A high binding energy O (1s) peak becomes prominent after one monolayer of In2O3 has formed, which we ascribe to adsorbed oxygen on the oxide. Formation of In2O3 continues into the bulk either via migration of the adsorbed oxygen through the oxide layer to the metal‐In2O3 interface or by the outward migration of indium cations to the surface. Both the formation of oxide and the onset of adsorption are reflected in the secondary ion yield ratios InO−2/InO− and O−2/O−. The behavior of SIMS molecular cluster ion yields with oxygen exposure are consistent with recent theoretical calculations of the cluster formation process, in which considerable local atomic order of the surface is present in the cluster.

Reduction of GaAs surface recombination velocity by chemical treatment

Abstract: Chemisorbed ruthenium ions on the surface of n‐GaAs decrease the surface recombination velocity of electrons and holes from 5×105 to 3.5×104 cm/sec. It is shown that the ions, in a one‐third monolayer thickness, are confined to the surface and do not form a new junction by diffusing into the GaAs. This use of Ru appears to be the first observation of the reduction of the surface recombination velocity for GaAs by the simple chemisorption of ions.

Electron donor–acceptor interactions and surface semiconductivity in molecular crystals as a function of ambient gas

Abstract: Quantitative studies of the magnitude, rate and reversibility of changes in surface semiconductivity of single crystals of phthalocyanines, perylene, tetracyanoquinodimethane (TCNQ) and molecular complexes as a function of ambient gas are reported. NO2+ N2O4 increases the surface conductivity of phthalocyanines, by factors of up to 108. At low pressures, the magnitude of the increase follows the Freundlich adsorption isotherm, while the rate obeys the Elovich equation. The saturation conductivity at high pressures is similar for all the phthalocyanines and corresponds to complete surface coverage, each adsorbed molecule producing one ionised state. Reversibility on heating in vacuo depends on the metal : (metal free, Ni, Cu, Zn) > (Co, Mn) > Pb. In all cases, treatment with low pressures of NH3 gives rapid reversal of the effects. BF3 gives smaller, irreversible enhancements. The surface conductivity of perylene is enhanced by a factor up to 108 in BF3 and the effect is reversed on treatment with NH3. NO2+ N2O4 produces smaller effects (104), easily reversible. Perylene—TCNQ shows small conductivity increases (102) with both NO2+ N2O4 and NH3, while TCNQ shows similar effects only with NH3. Semiconduction activation energies in the presence of gases enhancing conductivity are reduced to values comparable with those obtained from temperature dependence of photoconduction (0.1–0.2 eV). Conductivity changes are interpreted in terms of production of ionised states following weak chemisorption involving donor–acceptor interactions. The magnitude and reversibility of the changes depends on the nature of the orbitals involved in these interactions, and provides scope for selective gas detection.