Rate Of Oxygen Adsorption Research Articles

Microscopic control of semiconductor surface oxidation

We examined the effect of ultrathin (0.1–10 Å) chromium overlayers on the reactivity with oxygen of Si(111) and GaAs(110) cleavage surfaces. Synchrotron radiation photoemission shows that for Cr coverages below a critical threshold coverage the overlayer does not affect substantially the oxygen adsorption rate. For chromium coverage above threshold the overlayer sharply enhances the oxygen adsorption kinetics so that most semiconductor atoms in the surface and near surface region appear oxidized at activated oxygen exposures as low as 100 L. The critical threshold coverage corresponds to the onset of reactive interdiffusion at the Si(111)–Cr and GaAs(110)–Cr interfaces. Therefore we suggest that ultrathin Si–Cr and As–Cr reacted phases created at the surface act as activation layers for semiconductor oxidation.

Optically enhanced low temperature oxygen chemisorption on GaAs(110)

Optically enhanced oxidation of GaAs is promising both for insight into fundamental mechanisms of oxygen chemisorption and for possible application in GaAs direct write. For atomically clean GaAs(110), we have found that sample temperature is a key parameter in determining the increase of oxygen uptake under low-intensity argon ion laser illumination (1 W/cm2 at 2.41 eV) relative to the ‘‘dark’’ uptake. Although cooling to 185 K suppresses the dark oxygen adsorption rate, the oxygen uptake with optical irradiation is the same at both 185 and 400 K. The ratio of laser-assisted to dark uptake for 108 L exposure then approaches the enhancement necessary for spatially selective oxidation, showing an increase from 2.5 at room temperature to well over 10 at 185 K. After ruling out sample heating and direct oxygen gas excitation, we suggest that the light-induced increase in uptake is due to electronic excitations in the GaAs. To explain the enhancement, one also needs to assume a weakly adsorbed precursor state that converts to the chemisorption state when thermally or optically activated; the implications for previously suggested oxidation mechanisms will be discussed.

The oxidation of CO on pt(100): Mechanism and structure

Using dynamic LEED measurements of spot intensities and profiles, together with thermal desorption data, we have investigated the oxidation of CO on Pt(100)−(1 × 1). At T = 355 K, either CO or O was preadsorbed and reacted off with the other species. Results from both titration sequences point to the following conclusions: Titration of preadsorbed oxygen with CO g leads to rapid reaction, with a reaction probability of unity for each chemisorbed CO. Adsorbed CO does not accumulate on the surface until θ o ⩽ 0.05, i.e. an intermediate, rather clean (1 × 1) Pt surface is obtained. Further evidence for this clean intermediate is provided by the fact that characteristics of the diffraction spots of the c(2 × 2) of CO develop identically during this reaction sequence and during adsorption of CO on a clean (1 × 1) Pt surface. In the reverse case, titration of preadsorbed CO with O 2,g, the reaction rate is slower than the oxygen adsorption rate, leading to a pressure-dependent development of coexisting O ad and CO ad domains, which we observe directly with LEED. The stable phases coexisting are the c(2 × 2) of CO and the oxygen-related (3 × 1). Thermal desorption peak shapes, together with LEED observations, indicate that the CO in this case is held in c(2 × 2) islands by a matrix of surrounding oxygen atoms. In no case do mixed structures form, nor is an existing structure compressed by subsequent adsorption of the second species. Starting from a Langmuir-Hinshelwood mechanism, the differences between the two reaction sequences are discussed in terms of different activation barriers for reaction and different sticking coefficients of the adsorbing species. Special attention is given to the mobilities of the adsorbed reactants.

Fire spread mechanisms along steel cylinders in high pressure oxygen

Fire spread along steel cylinders in high pressure oxygen has been studied. Upward and downward fire spread experiments were conducted in a cylindrical, high pressure oxygen chamber of about 2.5 × 10 7 mm 3 capacity. The fire spread phenomena were recorded photographically and by television and analyzed. Based on the experimental results, the mechanisms of fire spread are discussed. Fire spread was found to depend on the test piece diameter, oxygen pressure, and spread direction. Upward fire spread was rather steady, although periodic detachment of molten iron oxide mass and cyclic variation in spread rate occurred. Two different modes were observed of downward fire spread. One was observed at a lower oxygen pressure. The molten iron oxide mass detached from the burning test piece end to become a droplet, so that the mode of fire spread in this case was much the same as that of upward fire spread. The other was observed at a higher oxygen pressure. The molten mass flowed down along the test piece. The fire spread in this case was accelerative and its rate was much larger than that in the aforementioned cases. An analysis for steady fire spead has been performed by considering heat transfer from the burning region to the unburned solid iron. It was shown that the ratio of the molten mass surface areas to the test piece cross section area is an important parameter for discussing the fire spread mechanisms. Based on the dependence of this ratio on the oxygen pressure or test piece diameter, it was inferred that the spread rate depends on the rate of oxygen adsorption at the molten mass surface and convective heat transfer at the molten-solid boundary.

Concentration oscillations of carbon monoxide, oxygen and 1-butene over a platinum supported catalyst

The effect of a wide range of experimental parameters on the sinusoidal and multiple spike oscillations observed during the oxidation of mixture of carbon monoxide and 1-butene over platinum supported on γ-alumina has been investigated in a CSTR. An elementary step model, where the rate constants are assumed independent of surface coverages, is found to be able to account largely for the observed oscillatory phenomena. The carbon monoxide and oxygen adsorption and desorption rate constant values in the model have been evaluated by fitting experimental transients, while 1-butene parameter values were obtained from stability conditions. Rate constant values and operating conditions have a profound effect on the observed and predicted oscillations. A parameter of great importance is the ratio of the number of adsorption sites to reactor volume.

Oxygen adsorption on a Ge(100) surface: II. Alkali metal-covered surfaces

Oxygen adsorption on an alkali(Na, K, Cs)-covered Ge(100) surface was studied by Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS), thermal desorption (TD) and work function (WF) measurements. It was found that the preadsorbed alkali metal (AM) results in enhancement of the oxygen adsorption rate. The initial values of the oxygen sticking coefficient are exponential functions of AM induced WF changes. The EELS data show that low AM concentrations (θ AM < 0.1) change the oxidation state of the bare Ge(100) surface, i.e. an increase of the oxygen coordination number in the GeO x unit is observed. At high θ AM values, the EELS and TDS data imply the formation of a Ge-O-AM surface “compound” rather than of simple GeO x (1 < x < 2).

Oxygen adsorption on Ge(111) surface: II. Alkali metal-covered surfaces

Oxygen adsorption on an alkali metal (a.m.)-covered Ge(111) surface has been studied by means of Auger electron spectroscopy (AES), electron energy loss spectroscopy (ELS), thermal desorption (TD), and work function measurements (WF). It was found that the presence of a.m. results in enhancement of the oxygen adsorption rate. The initial values of the sticking coefficient, S 0, are exponential functions of the work function changes caused by the a.m. adsorption. It was shown that no germanium oxide phases are formed on an alkali-covered Ge surface at 300 K. The oxidation rate at high temperatures is limited by the rearrangement processes taking place in the surface GeO layer. The results obtained show that the alkali metal perturbs the GeO bond to a certain extent but no alkali oxide formation was observed at a.m. covertages under investigation.

The role of CH bond breaking in the epoxidation of ethylene

Ethylene and deuterated ethylene epoxidation on silver was studied on chlorine-modified catalyst in order to elucidate the mechanism by which the epoxidation of deuterated ethylene proceeds at a higher rate than that of ethylene, as was reported by Cant and Hall. The isotope effect strongly decreased with increasing selectivity as a result of the chlorinate surface and it increased with increasing temperature. The rate of epoxide formation increased and that of carbon dioxide formation decreased when ethylene was replaced by deuterated ethylene, in agreement with the results by Cant and Hall. Kinetic studies suggested that the rate of oxygen adsorption was rate-limiting. The results indicated that the surface oxygen species responsible for epoxide formation can also react with ethylene to give an intermediate ethylene species which undergoes total combustion.

Electron stimulated adsorption: Surface activation and preferential binding of oxygen to sulfur on CdS

Highly non-reactive clean surfaces of CdS, prepared by ion-bombardment and subsequent annealing, exhibit a significant rate of oxygen adsorption only upon irradiation with electrons in the presence of water vapor at pressures as low as 10 -10 Torr. AES and XPS studies show that this electron stimulated adsorption (ESA) on CdS occurs due to a local activation of the surface rather than the dissociation of previously adsorbed species assumed in earlier studies. It is shown that during the initial stages of ESA oxygen bonds to sulfur atoms; further electron irradiation results in the oxidation of CdS with an increasing amount of oxygen bonded to cadmium atoms.

Electron stimulated adsorption: Surface activation and preferential binding of oxygen to sulfur on CdS

Highly non-reactive clean surfaces of CdS, prepared by ion-bombardment and subsequent annealing, exhibit a significant rate of oxygen adsorption only upon irradiation with electrons in the presence of water vapor at pressures as low as 10 −10 Torr. AES and XPS studies show that this electron stimulated adsorption (ESA) on CdS occurs due to a local activation of the surface rather than the dissociation of previously adsorbed species assumed in earlier studies. It is shown that during the initial stages of ESA oxygen bonds to sulfur atoms; further electron irradiation results in the oxidation of CdS with an increasing amount of oxygen bonded to cadmium atoms.

Study of oxygen adsorption on a silver catalyst modified with chloride ions

The rate of oxygen adsorption was found to decrease strongly with the increase of the chloride-covered fraction of the surface. Adsorption is not the rate-controlling step in ethylene oxidation on both non-modified and chloride-modified silver catalysts.

Adsorption of hydrogen and oxygen and oxidation of methanol on ruthenium electrodes

Sorption of hydrogen and oxygen on ruthenium in 1N H 2SO 4 have been investigated by potentiodynamic and potentiostatic methods. It has been shown that adsorption and absorption of hydrogen and oxygen take place on a smooth ruthenium electrode. The solubility of hydrogen grows linearly with potential in the range 0·41–0·04 V, whereas the solubility of oxygen increases linearly with potential in the range 0·37–1·3 V. Adsorption of hydrogen on a smooth ruthenium electrode takes place in the potential range 0·4–0·0 V; on ruthenized ruthenium electrode the main portion of hydrogen is adsorbed at potentials 0·2–0·0 V. No maxima corresponding to specific types of chemisorbed oxygen exist on the potentiodynamic curves of oxygen adsorption. The oxygen adsorption rate on ruthenium is of the same order as on platinum. The kinetics of methanol oxidation on smooth and ruthenized ruthenium electrodes were investigated. The chemisorption of methanol was found to be the limiting step in both cases in the potential range 0·4–1·0 V; the chemisorption rate on smooth ruthenium exceeds the chemisorption rate on ruthenized electrode by two orders of magnitude. This results is consistent with the difference in the surface bond energies of adsorbed hydrogen for smooth and ruthenized ruthenium.

Carbon molecular sieves for the concentration of oxygen from air

The micropore structure of a number of American coals varying in rank from LVB to lignite has been modified by two methods: 1. (1) heat treatment in nitrogen to 800–900 °C, and 2. (2) air oxidation near the ignition temperature followed by heat treatment to 900–1000 °C in nitrogen. The second method of treatment is a necessity for coking coals. The chars from HVC coal using method (1) and from LVB coal using method (2) showed selective oxygen adsorption from air. Preferential oxygen uptake is attributed to an initial rate of oxygen adsorption that is higher than that for nitrogen owing to molecular sieve structure. Carbons obtained from coconut shells and some highly crosslinked synthetic polymers (for example, polydivinyl benzene) also showed selective oxygen adsorption. Adsorption of both gases on these carbons is primarily physical, that is, adsorption and desorption cycles can be performed by pressure swing with no significant loss of capacity. The slope of the oxygen isotherm is higher than that of the nitrogen isotherm, indicating that oxygen adsorption is enhanced over that of nitrogen at higher pressures. Operating at pressures around 6.3 MPa and 25 °C with air, oxygen concentration in the desorption product stream can be increased to about 36%. Using three beds of carbon in series, the final product gas can be concentrated to about 80% oxygen. The fraction, by volume, of the inlet gas which is realized as product gas depends upon a number of variables.

The initial oxidation of molybdenum: V. A model for the nucleation of epitaxial oxide and its application to molybdenum

A new model for the phenomenon of epitaxial oxidation nucleation is presented growth characteristics of the oxide suggest that the prenucleation growth is governed by the oxygen adsorption rate of the oxide/gas boundary. As the oxide grows the calculated rate of dissolution of cations at the oxide/metal interface falls until this becomes rate determining. At this point low work function facet sided pits are etched in the substrate with corresponding bulges in the outer oxide film. These bulges are identified as epitaxial nuclei. Based on this model several parameters relating to oxide growth may be calculated. The agreement between theory and experiment is very good. In particular a lower and upper temperature limit for nucleation are found and the pressure dependence is shown to be quite complicated. The basis of the model can be extended to the chemisorption regime. It is shown that above a surface potential of about 1 V cations migrate rapidly through the oxygen layer. The onset of this rapid migration is identified with molecular oxide production and the start of faceting.

Surface interactions between chemisorbed species on platinum: Carbon monoxide, hydrogen, oxygen, and methanol

By means of ultrahigh vacuum techniques, including thermal flash desorption and residual gas analysis, the surface interactions of single-component and multi-component adsystems were examined. The gases studied included CO, H 2, O 2, and CH 3OH in contact with a polycrystalline platinum ribbon. Three binding states were observed for CO, each of which exhibited high sticking probability, relative to that of oxygen or hydrogen. Carbon contamination of the Pt surface greatly reduced the adsorption rate of oxygen. Competitive sorption studies revealed displacement of chemisorbed hydrogen by carbon monoxide. Preadsorbed oxygen interacted readily with gaseous carbon monoxide, but very slow reaction was noted in the case of preadsorbed CO interacting with gaseous oxygen. During the methanol decomposition the surface interactions exhibited similar characteristics to those observed for the hydrogen-carbon monoxide. The results are interpretable in terms of the formation of a bound three-atomic intermediate species, such as [HCO], produced by hydrogen abstraction from the methyl group during methanol decomposition, and subsequent reaction with hydrogen adatoms by way of rupture of the HCO bond to yield H 2 and CO. These surface studies on the molecular level elucidate some of the elementary steps involved in the catalytic reactions involving methanol.

Oxidation of Palladium during the Olefin Oxidation over Palladium Salts-Active Charcoal Catalysts

Oxidation of reduced palladium on active charcoal during the ethylene oxidation over palladium salts-active charcoal catalysts was studied. Effect of oxygen pressure on the reaction rate was determined from the rate of oxygen adsorption using the apparatus with constant volume and constant pressure equipment. Effect of oxygen pressure became evident only when its value was very little.If we assume that the over-all reaction rate is determined in terms of the dynamic equilibrium between the oxidation of ethylene by palladium salts and the re-oxidation of reduced palladium compounds by oxygen, the conclusion, that the rate of re-oxidation is represented by the equation (01), can be derived from the analysis of experimental results, where K is the dissociative adsorption constant of oxygen. This suggests that the reduced palladium is oxidized by oxygen which is dissociatively adsorbed on active charcoal.In the equation (01), κ2√K which is the measure of the re-oxidation activity, was found to vary with temperature, ethylene pressure and anions. The order of re-oxidation activity of palladium salts was Pd(NO3)2>PdCl2>PdSO4 under the same reaction condition.

Oxygen Interchange on Vanadium Oxide Catalysts

It is often difficult to compare results for heterogeneous reactions as the data reported in the literature are presented in. various ways. A common example of this difficulty is the reporting of oxygen adsorption rate constants in units such as 1 (g-catalyst)−1 s−1, 1 m−2 h−1, g m−2 h−1, g-mol m−2 h−1, etc. In this paper a number of studies involving oxygen interchange with vanadium pentoxide or vanadium pentoxide – potassium sulfate promoted catalysts are re-examined. Where possible, a first-order oxygen adsorption rate constant is evaluated from published data on rates of oxidation and rates of isotopic exchange, and expressed in units of 1 h−1 m−2 of catalyst surface. The rate constants so evaluated and compared as to catalyst composition, temperature, and activation energy show reasonable agreement, even though a variety of catalysts, processes, and techniques are involved.

Oxygen chemisorption on well cleaned carbon surfaces

Graphon, a highly graphitized carbon black, was first oxidized to 14.4% weight loss in O 2 at 625°C to introduce significant active surface area. Following the cleaning of the activated Graphon surface by heating at 975°C in a vacuum of 10 −8 torr, chemisorption of oxygen between 25–400°C was studied. The saturation amounts of oxygen adsorbed sharply increase at temperatures above 250°C, suggesting the presence of at least two types of active sites. The maximum amount of oxygen adsorbed is estimated to occupy 2.8 m 2/g or 2.6% of the total surface. The rate of oxygen adsorption on the more active sites is given by k o Cn∞(1− θ) 2, where k 0 = 7.14 × 10 9 exp (−7,400/RT) cc/sec mole O 2. Following a large number of adsorption-desorption cycles, the Graphon surface is additionally activated to yield (in part) a fraction of very active sites that are not produced at comparable burn-offs by higher temperature activation between 500–625°C.

Effect of Adsorbed Olefines on the Oxidation of Aluminium Films

THE presence of adsorbed species on metal surfaces has been observed to affect subsequent interactions of the metal with reactive gases1–3. To determine the effect adsorbed olefines have on the oxidation of aluminium, clean aluminium films were exposed to olefine vapour and the subsequent rate of oxygen adsorption was measured. The olefines used included ethylene, butene-1, hexene-1, octene-1, and cetene-1.

Researches on plant respiration. II.―Variations in the respiratory quotient during germination of seeds with different food reserves

Determinations of respiratory activity in plants have nearly always been made by measuring the rate of carbon dioxide evolution by the tissues. Much less frequently the rate of oxygen adsorption has been used as a measure of the process. In comparatively few researches have determinations been made of both carbon dioxide evolution and oxygen adsorption. Yet it is clear that a true measure of respiratory activity can only be given by one or other of these quantities alone, it they bear a fixed numerical relation to one another; where this is not so, it is obvious that at least one of these quantities cannot be taken as a measure of respiration, and it may not always easy in such cases to decide which of them is to be regarded as giving the truer value of this process. Indeed, a determination of both carbon dioxide output and oxygen adsorption by respiring tissues seems eminently desirable as providing data allied should assist in the elucidation of the mechanism of the respiratory process. In the work described in this paper, which deals with the course of respiratory activity of plants of a number of species during germination of the seeds and the early development of the seedling, we have accordingly determined not only the rate of carbon dioxide evolution, but the rate of oxygen adsorption. Such determinations appear to be the more desirable as the existing information on the respiratory quotient of germinating seeds is scanty, and such data as have been recorded are highly contradictory.