Changes In Surface Coverage Research Articles

CO Oxidation and the CO/NO Reaction on Pd(110) Studied Using “Fast” XPS and a Molecular Beam Reactor

We have combined the use of a molecular beam reactor and in situ spectroscopy (XPS) in order to correlate changes in the rate of CO oxidation and the CO–NO reaction with the coverages of the adsorbates and intermediates on the surface. In the reactor, both reactions exhibit an isothermal “light-off” phenomenon in which the rate autocatalytically increases with time. In the case of the CO oxidation reaction this is due to the desorption of CO which releases extra sites for O2 dissociation which, in turn, removes more CO, and hence the acceleration. In effect the reaction can be written as 2COa + O2g + 2S → 2CO2g + 4S, the acceleration coming from the release of extra adsorption sites S, which are involved in the reaction itself. “Fast XPS”, carried out in situ during the course of the reaction, shows domination of the surface by COa below 390 K and domination by Oa above that temperature, with a rapid change in surface coverage over a very narrow temperature window. On high surface area samples this acceleration is further reinforced due to a rapid temperature increase because of the highly exothermic nature of the overall reaction. The situation for the CO–NO reaction is broadly similar, except that the surface is dominated by NO at low temperature, not CO which tends to be displaced from the surface by NO. “Light-off” is dictated by the onset of the dissociation of NOa, which occurs at ∼400 K. Once Na and Oa are formed, N2O production is immediate and accelerates due to the creation of vacant sites for both NO and CO adsorption, the latter removing Oa as CO2g. Again, the reaction self-accelerates and there is a rapid change of surface coverage from NOa to Oa at ∼450 K. The overall self acceleration is due to the following overall reaction, 2NOa + COg + S → N2Og + CO2g + 3S, again producing more adsorption sites (S) in carrying out the reaction step. The rate is reduced at high temperature due to domination of the surface by Oa and to the reduced coverages of the molecular species.

Probe Molecule Kinetic Studies of Adsorption on MCM-41

Gases and vapors of varying dimensions were used as probes to investigate the adsorption processes on MCM-41 and precipitated silica, prepared by the same method as for MCM-41, but without the template. The adsorption kinetics of nitrogen, carbon dioxide, n-nonane, and α-pinene were studied for different amounts of preadsorbed gas/vapor as a function of relative pressure. The adsorption kinetics follow a linear driving force model for changes in surface coverage up to the adsorption isotherm plateaus. The variation of rate constant with relative pressure for MCM-41 and precipitated silica showed marked differences associated with adsorption in mesoporous structure, which are attributed to alignment of molecules in the mesopores leading to a pore blocking effect. The results are discussed in terms of differences in the adsorption mechanisms of gas/vapor diffusion in porous materials.

Effects of carbonate species on the kinetics of dechlorination of 1,1,1-trichloroethane by zero-valent iron.

The effect of precipitates on the reactivity of iron metal (Fe0) with 1,1,1-trichloroethane (TCA) was studied in batch systems designed to model groundwaters that contain dissolved carbonate species (i.e., C(IV)). At representative concentrations for high-C(IV) groundwaters (approximately 10(-2) M), the pH in batch reactors containing Fe0 was effectively buffered until most of the aqueous C(IV) precipitated. The precipitate was mainly FeCO3 (siderite) but may also have included some carbonate green rust. Exposure of the Fe0 to dissolved C(IV) accelerated reduction of TCA, and the products formed under these conditions consisted mainly of ethane and ethene, with minor amounts of several butenes. The kinetics of TCA reduction were first-order when C(IV)-enhanced corrosion predominated but showed mixed-order kinetics (zero- and first-order) in experiments performed with passivated Fe0 (i.e., before the onset of pitting corrosion and after repassivation by precipitation of FeCO3). All these data were described by fitting a Michaelis-Menten-type kinetic model and approximating the first-order rate constant as the ratio of the maximum reaction rate (Vm) and the concentration of TCA at half of the maximum rate (K(1/2)). The decrease in Vm/K(1/2) with increasing C(IV) exposure time was fit to a heuristic model assuming proportionality between changes in TCA reduction rate and changes in surface coverage with FeCO3.

Emissions of O− Radical Anions and Electrons from Silver on a Solid Electrolyte

The emissions of O− radical anions and electrons from a silver electrode on a solid electrolyte, yttria-stabilized zirconia (YSZ) or a quartz glass, have been observed using a quadrupole mass spectrometer. A simple experimental apparatus was designed to incorporate a gap between a silver electrode on a YSZ anode and a platinum space electrode. By applying a DC voltage across the gap, O− radical anions and electrons were observed. The results of Arrhenius plots with respect to O− show that the value of activation energy Ed for O− dissociation from a silver electrode on YSZ is approximately 91.5–142.3 kJ/mol with respect to different electric fields. These values are considerably lower than the bonding energy between silver and oxygen atoms (4.2 eV). It is suggested that the change in the activation energy of O− desorption with an electric field is due to the change of surface coverage of O− on silver and O− is correlated with backspillover oxygen.

Transient plasma-induced emission analysis of laser-desorbed species during Cl2 plasma etching of Si

The surface during the etching of Si in a Cl2 inductively-coupled plasma (ICP) was analyzed by laser desorption (LD), followed by detection of the desorbed species by monitoring the transient changes by plasma-induced emission (PIE). Optical emission from Si, SiCl, SiCl2, and possibly other species was detected in situ using this LD-PIE method as a function of rf power, substrate bias, and pressure. The surface coverage of chlorine was determined by normalizing the LD-PIE signal by either of two ways: by the electron density, as measured by microwave interferometry, or by using the background PIE signal. Little change in surface coverage was observed as the ion density was changed by varying the rf power supplied to the ICP, confirming the observation made using laser-induced fluorescence (LIF) detection (LD-LIF). The LD-PIE signal is related to the density of species desorbed from the surface and subsequently excited by electrons. LD-PIE analysis of the surface is more versatile than LD-LIF—sometimes it can detect the desorbed species while LD-LIF cannot. Still, it requires calibration to account for the electron collision-induced excitation of the laser-desorbed species.

A self assembled shell of 11-mercaptoundecanoic aminophenylboronic acids on gold nanoclusters

Sensors employing gold clusters coated with biomimetic compounds offer considerable advantages like long-term stability and simple sensor handling in comparison to sensors based on bioaffinity interactions. We took advantage of the unique properties of boronate molecules in binding hexose as well as their ability to form self assembled monolayers (SAMs) on 14 nm Gold nanoclusters. 11-Mercaptoundecanoic aminophenylboronic acid was synthesised in a one pot reaction step. The hydroxyl group of 11-mercaptoundecanoic acid was coupled with aminophenylboronic acid via activation by carbonyldiimidazole (CDI). Gold nanoclusters were covered with a monolayer of these boronate molecules by a self assembling process. At a pH of 9.5 thiols were found to bind irreversibly to the surface of gold nano-clusters, producing a particle exposing sugar binding sites to the solution; these clusters were stable against centrifugation, high salt and low pH. Such clusters were bound to the sensor surface at defined distance from a mirror layer. Thereby cluster-absorption was strongly enhanced. This phenomenon enables us to transduce affinity interactions highly amplifying changes in surface coverage of clusters. The optical properties of the sensors were found to be dependent on the size and number of the metal-clusters as well as their distance to the metal layer sputtered on the support surface.

On the Adsorption of Dimethyl Methylphosphonate on Self-Assembled Alkanethiolate Monolayers: Influence of Humidity

Humidity influences the adsorption of dimethyl methylphosphonate (DMMP) on three different organic model interfaces to quite different extents. The model interfaces, characterized by the following tail groups (-OH CH 3 , -COOH), were prepared by solution self-assembly of w-substituted alkanethiols on gold, and the coadsorption of water and DMMP was followed by a surface acoustic wave sensor and infrared reflection absorption spectroscopy. The surfaces show both quantitative and qualitative differences concerning the interaction with DMMP. Humidity is found to influence the hydrogen bonds formed between DMMP and the tail groups, increasing the strength ofinteraction. However, the changes in surface coverage of DMMP, as measured by infrared absorption, are not fully registered by the surface acoustic wave sensor, because of a concomitant desorption of water. Indeed there is even a loss in sensor signal due to the presence of humidity in the case of the -COOH-terminated surface, where the strongest interaction with DMMP is found. After compensation for the loss of water, the coverage of DMMP increases with the relative humidity on the OH surface and decreases with the relative humidity on the COOH surface. The consequences of the results are discussed in terms of their impact on the design of sensing interfaces based on hydrogen bonding.

Elementary Processes in Molecular Beam Epitaxy Studied by in situ Scanning Electron Microscopy

The recent advancement in the in situ scanning electron microscopy of molecular beam epitaxy of GaAs is reviewed. The topics include two-dimensional (2D) nucleation and growth, three-dimensional (3D) roughening processes, 2D-to-step-propagation growth mode change and annealing processes after growth. In the nucleation-and-growth mode, delayed island nucleation and nonlinear change of surface coverage are found. The mechanism of delayed nucleation is discussed in terms of site-dependent successive atomic layer growth processes. It is shown that under a critical growth condition between the 2D mode and the step propagation mode, the local growth mode changes from the 2D mode to the step propagation mode. It is found that there are three types of surface roughening, or onset of 3D growth. A 3D/2D/1D growth mode phase diagram is obtained. Some of the results are compared with the growth of Si on (111) surface. The behavior of monolayer-deep holes during postgrowth annealing is observed. It is found that some holes are left for a long time and form big holes with each other. They extend in the [110] direction but, at the same time, shrink in the [1-10] direction. Finally, they shrink in both directions and disappear .It takes about 10 min for all the holes to disappear. Related work by other authors is briefly described.

A surface coverage model for pulse-plating of binary alloys exhibiting a displacement reaction

Models for the prediction of the composition of pulse-plated copper-nickel alloys in the presence and absence of a displacement reaction during the pulse off-time are reviewed and critically evaluated. For very long and very short pulse off-times the composition of copper-nickel alloys deposited from electrolytes containing an excess of nickel ion is entirely controlled by mass transport of copper ion. At intermediate pulse off-times, however, the kinetics of nickel dissolution play a role. A modified model is proposed which takes into account the role of surface coverage by copper for the rate of nickel dissolution. Assuming a step function for the variation of surface coverage with off-time the model predictions can be fitted to experimental data using the critical off-time which corresponds to a change in surface coverage from zero to one as adjustable parameter. The model is used to estimate for different experimental conditions the amount of copper leading to a suppression of the displacement reaction. More copper is deposited by the displacement reaction in sulfamate than in citrate electrolytes.

Specific binding of low molecular weight ligands with direct optical detection

The characterization of low molecular weight ligand interaction with receptor molecules is of importance for the investigation of biological processes and for drug research. We report on the investigation of the binding of low molecular weight ligands to immobilized receptors by label-free detection. Reflectometric interference spectroscopy, an optical transducer which allows the monitoring of a few picograms per square millimetre changes in surface coverage, was used to study two model systems. In both cases detection of the binding event was clearly detectable at receptor surface concentrations as low as streptavidin was clearly detectable at receptor surface concentrations as low as 1–2 × 10 10 binding sites/mm 2. Linear correlation between the receptor surface concentration and the response to biotin binding was observed. Using immobilized DNA, we investigated the binding of common intercalators with respect to kinetics and thermodynamics by evaluation of the association and the dissociation part of the binding curve. Bi-exponential increase and decrease of intercalator loading was observed, indicating complex interaction kinetics. The four structurally different intercalators showed significant distinction in binding kinetics and equilibrium signals. Improvement of experimental parameters is required to obtain more reliable kinetic data.

Statistical multiple diffuse scattering from rough surfaces in RHEED — beyond the distorted-wave Born approximation

In reflection high-energy electron diffraction (RHEED) of growing surfaces in molecular beam epitaxy (MBE), diffuse scattering is generated by atom vibrations, point vacancies and growth islands (or surface roughness). Most of the existing RHEED theories have been developed under the first-order diffuse scattering approximation, and thus they are restricted for surfaces whose roughness is relatively low. In fact, crystal surfaces grown by MBE are usually rough; the change of surface coverage from 0 to 1 monolayer accounts for the observed RHEED oscillation. In this paper, a formal dynamical theory of RHEED has been developed to calculate the diffuse scattering produced by both atom vibrations and point vacancies at surfaces. The theory is aimed at recovering the multiple diffuse scattering that has been dropped by the distorted-wave Born approximation (DWBA). With the inclusion of a complex potential in the dynamical calculation, a rigorous proof is given to show that the high-order diffuse scattering terms are recovered in the calculation using the equation originally derived under the DWBA. This conclusion establishes the basis for expanding the RHEED theories developed under the first-order diffuse scattering to cases where the degree of surface roughness is high, allowing dynamical calculation of RHEED rocking curves for any growing surface. The statistical time and structure averages over the distorted crystal potential are evaluated analytically before numerical calculation. The dynamic form factor is calculated with consideration of anisotropic surface atom vibration and point vacancies at a growing surface.

Studies on the suppression of Ge segregation during Si overgrowth on [formula omitted] substrates by gas-source MBE

Surfactant effects of hydrogen atoms on the Ge segregation during Si growth on Ge Si(100) substrates by Si 2H 6-source molecular beam epitaxy have been studied. The layer-by-layer growth rates were examined in-situ by reflection high energy electron diffraction (RHEED) intensity oscillation periods. The growth rate of Si overlayers decreases exponentially with increasing Si thickness and eventually approaches to that of Si Si(100) homoepitaxial growth at substrate temperatures of 440–550°C, which corresponds to the change in surface coverage of Ge atoms. The decay length of the surface concentration of Ge atoms is suppressed by an increase in Si 2H 6 gas pressure. These facts can be explained as an effect of H atoms existing on the growing surface. Furthermore, the atomic mixing phenomenon in the initial stage of growth is observed, which is related with the strain of Ge layers. The addition of excess hydrogen atoms during Si growth leads to a further reduction of the decay length under the appropriate growth condition and to the island growth at higher temperatures and for lower Si 2H 6 pressures.

Spatially resolved infrared spectroscopy: a novel technique for in situ study of spatial surface coverage during CO oxidation on supported catalysts

A spatially resolved infrared (IR) imaging technique to monitor the linear adsorbed CO coverage on supported catalyst surface combining an IR bandpass filter and an IR thermography camera has been developed. Images acquired during the CO adsorption/desorption and ignition indicate that the technique provides an excellent method to image the change of surface coverage with a spatial resolution. It is expected that the combination of infrared thermography with spatially resolved imaging of surface coverage will provide a deeper insight in the dynamics of spatio-temporal patterns on heterogeneous catalysts.

A Thermal Desorption Model for the Outgassing Rate versus Time Characteristics in the Pump-Down of an Unbaked Stainless Steel Chamber.

In order to explain theoretically the outgassing rate versus time characteristics in the pump-down of an unbaked vacuum system, a thermal desorption model has been proposed. In this model, four parameters are introduced, the surface coverage θ, the mean residence time τ, the sticking probability s and the pumping parameter a/A (a is the effective area of the pumping orifice and A the adsorbing area), and it is assumed for the pump-down of the vacuum chamber that the initial surface coverage changes by less than one monolayer and adsorption is fully reversible. In addition the activation energy of desorption is represented with the variation of the heat of adsorption in the Temkin isotherm. Two conservation of mass equations are derived for gas molecules at the surface and in the pump-down of the vacuum chamber, by combining these two mass balance equations a differential equation of the outgassing rate is derived, in which it is expressed with a variable separation form of F (θ) (-dθ/dt) = constant (F (θ) is a function of coverage). The discussion shows that it is possible to solve the differential equation analytically, since F (θ) can be approximated with a simple function exp (-bθ) in the range of 0.05<θ<1. As a result, it is suggested that the differential equation gives an outgassing rate which depends not only on pumping time but also on pumping speed.

Determination of Kinetic Constants for the Interaction Between the Platelet Glycoprotein IIb-IIIa and Fibrinogen by Means of Surface Plasmon Resonance

The binding reaction between purified human platelet glycoprotein IIb-IIIa and fibrinogen was investigated by real-time measurements using the surface-plasmon-resonance sensor technology. In these experiments, either glycoprotein IIb-IIIa or fibrinogen was immobilized on a sensor surface. The time-dependent change in surface coverage that occurred immediately upon contact with a solution of the complementary protein was then detected. The ability to record this dynamic event from its initiation allowed the collection of kinetic and thermodynamic data over an extended time period. These data indicated that initially, in fast reaction, a reversible low-affinity complex with an equilibrium dissociation constant, Kd, of 155-180 nM was formed. In a subsequent slower reaction this complex was transformed into a more stable high-affinity complex with a Kd of 20-70 nM. Efficient dissociation of the high-affinity complex could only be induced in the presence of a competitive inhibitor such as RGDV. These data demonstrate that the binding between glycoprotein IIb-IIIa and fibrinogen is not a single monophasic reaction, but is composed of at least two consecutive processes both with their own kinetics.

Monte Carlo simulation of temperature programmed reaction and surface explosion during CO oxidation on a Pt catalyst

A Monte Carlo model of CO oxidation on a Pt(111) surface that includes finite rates of adsorption-desorption and reaction and the effect of the catalyst temperature is presented. The results show that, as expected from the reaction-adsorption probabilities, the surface coverage changes from being almost completely covered by CO at low temperature (60°C), to being completely covered by oxygen at high temperature (160°C). Furthermore, it was found that an unstable state occurs when cooling down the oxygen covered surface from 160°C to 60°C. It is shown that if a site for CO adsorption is created under this metastable state, a surface explosion that propagates spatially occurs. Thus the MC simulations provide a method to describe a catalytic reaction on surfaces with strongly non-linear spatio-temporal dynamics.

Oscillations in the carbon monoxide oxidation on platinum surfaces observed by field electron microscopy

Platinum field emitter tips ((111)-, (001)-, and (012)-oriented) are used to investigate local surface properties during the oscillatory regime of the CO-oxidation at T = 430–480 K and partial pressures p o 2 ≈5×10 -4 mbar and p CO≈5×10 -5 mbar. Changes in surface coverage - and catalytic activity - are indicated by variations of the local work function and field emission current. Self-sustaining isothermal oscillations could be observed on different crystal planes. (i) A fast “switching on” of emission currents is initially noticed when the oxygen adsorption layer is transferred into a CO-layer. Starting - after a certain induction period - from the (011)-region the oxygen side is quickly switched into the CO-side, which slowly returns back to the catalytically active oxygen side. (ii) The formation of reactive wave fronts starts in the (011)-region and propogates in the direction of (111)- and (100)-planes. The CO-wavefront and the O-wavefront move alternately in opposite directions and display different propagation velocities, which could be measured. The frequencies and amplitudes of oscillations critically depends on temperatures and partial pressures. Compared to single crystal planes a broadening and shift of oscillatory regimes in parameter space is noticed. In certain cases period doubling could be measured.

Surface species on a palladium/zirconia CO oxidation catalyst prepared from a glassy metal precursor. Characterization by diffuse reflectance FTIR spectroscopy

CO adsorption and oxidation on the surface of a palladium/zirconia catalyst prepared from an amorphous Pd25Zr75 precursor have been studied by diffuse reflectance FTIR (DRIFT) spectroscopy. In pure CO atmosphere, the most abundant surface species observed is bridge-bound CO. When the catalyst is exposed to a reactant feed consisting of CO and N2–O2(purified air), the coverage of two-fold coordinated CO molecules decreases, and singly and triply bound CO species appear on the surface. Apparently the bridge-bound CO reacts with subsurface and/or gas-phase oxygen, and is subsequently desorbed from the catalyst surface as CO2. When the oxygen partial pressure is increased, the triply bound CO species also participate in the surface reaction. A simple model is proposed to account, in a qualitative way, for the changes in surface coverage observed under oxidative conditions

Dopant effects on the response of gas-sensitive resistors utilising semiconducting oxides

Conductivity changes induced in semiconducting oxides as a consequence of the presence of small concentrations (ppm level) of reactive gases in air are presumed to be caused by non-equilibrium changes in surface coverage of ionised oxygen species (surface acceptor states). Responses of porous pellets of different oxide materials to small concentrations of different reducing gases in the air can usually be classified as either p-type (resistance increases) or n-type (resistance decreases). However, some materials show n-type responses to some reducing gases and p-type to others, and some materials can show a change in sign of the response with increasing concentration of the reducing gas; such effects can be brought about by systematic substitution into the lattice of complex oxides. This paper illustrates these effects and develops a simple model, the principal features of which are that the bulk donor density is low enough, the average grain size small enough and the surface acceptor state density high enough that the grains are fully depleted of conduction electrons. It is then shown that the conductivity goes through a minimum with increasing surface density of acceptor states. Since it is the surface density of acceptor states which is varied as a consequence of interaction with the reducing gas, the observations are thereby accounted for.

Molecular beam epitaxy/atomic layer epitaxy growth processes of wide-band-gap II–VI compounds: Characterization of surface stoichiometry by reflection high-energy electron diffraction

A good agreement is found between the observed temporal dependence of the reflection high-energy electron diffraction (RHEED) intensity variation of a specular spot and the calculated temporal dependence of surface coverage during sublimation and adsorption processes in atomic layer epitaxy. Here the surface coverage is calculated on the basis of a phenomenological model in which precursor states are taken into account. The RHEED oscillation behavior during molecular beam epitaxy (MBE) growth of Zn chalcogenides is investigated for various growth conditions. The RHEED intensity saturates at a certain level and the saturation level is closely correlated with the surface stoichiometry or the surface coverage. It is concluded that the RHEED intensity variation corresponds to the change in surface coverage during ALE and MBE growth. It is possible to characterize the surface stoichiometry during atomic layer epitaxy (ALE) and MBE growth by RHEED.