Activation Energy For Chemisorption Research Articles

Precursor adsorption of CO on Ni(111) near 5 K and the activation energy for chemisorption

Measurements of low energy electron transmission spectrum and work function φ of a Ni(111) surface held at T = 5.5 K during exposure to CO provide direct evidence for a precursor adsorption preceding chemisorption. Subsequent warming of the CO covered surface leads to the chemisorption state. An analysis of the kinetics of the increase in φ that accompanies this transition gives an activation energy of ≈ 4 meV for the reaction.

Precursor Adsorption of Oxygen on Ni(111) and the Activation Energy for Chemisorption

Measurements of electron transmission spectra and work function $\ensuremath{\varphi}$ of a Ni(111) surface held at $T=5.5$ K during exposure to ${\mathrm{O}}_{2}$ provide direct evidence for a precursor adsorption preceding dissociative chemisorption. The form of the electron transmission spectra indicates that the precursor adsorption involves oxygen molecules. Subsequent warming of the oxygen-covered surface leads to the chemisorption state. An analysis of the kinetics of the increase in $\ensuremath{\varphi}$ that accompanies this transition gives an activation energy of \ensuremath{\sim} 13 meV for the reaction.

Kinetics of hydrogen absorption-desorption by niobium

The kinetics of hydrogen absorption and desorption by clean, recrystallized Nb foils, (110) orientation, were measured as a function of temperature. The results are discussed on the basis of a previously published model for the kinetics. It was found that, contrary to the original assumption, the initial sticking coefficient of hydrogen on the clean Nb(110) surface is temperature dependent and can be described by the expression ${S}_{i}={S}_{0}\mathrm{exp}[\ensuremath{-}(\frac{2{E}_{I}}{\mathrm{RT}})]$, where ${S}_{0}$ was found to be close to unity and ${E}_{I}=(1.27\ifmmode\pm\else\textpm\fi{}0.01)$ kcal/mol H. This led to a modification of the theoretical model by the inclusion of an activation energy for chemisorption. Furthermore, an analysis of the temperature-dependent data yielded values for the chemisorption energy of hydrogen ${E}_{D}=(14.8\ifmmode\pm\else\textpm\fi{}0.2)$ kcal/mol H, the vibrational entropy at the surface, the surface coverage, as well as other parameters related to the energy barrier at the surface.

3930. Precursor adsorption of O 2 on tin and the activation energy for chemisorption. (USA)

3930. Precursor adsorption of O 2 on tin and the activation energy for chemisorption. (USA)

Precursor adsorption of O2 on tin and the activation energy for chemisorption

Condensation of a thick layer of O2 on Sn at 5 K causes an increase in resistance corresponding to a loss of metallic conductivity by less than 0.1 Å of Sn. The small magnitude must mean that oxygen is initially adsorbed in a molecular physisorbed state since tying up even one Sn conduction electron per surface atom in chemical bonds would cause a resistance increase more than 10 times that observed. When warmed above 13 K the resistance of the oxygen-covered Sn films increases irreversibly with time, a change we attribute to chemisorption. The rate increases with increasing temperature and decreases as a total change corresponding to removal of 11 Å of Sn is approached. Analysis of the data for oxidation thicknesses between 1.5 and 8 Å gives consistent values for an activation energy of 0.035 eV.

Hydrocarbon adsorption on an aluminum—Chromium—Calcium catalyst

1. Temperature intervals have been marked out over which paraffin hydrocarbons and benzene will undergo simultaneous chemical and physical adsorption on an aluminum—chromium—calcium catalyst. 2. The fact that the rate of chemisorption was directly proportional to the pressure indicated that the adsorption complex contained a single hydrocarbon molecule. 3. The chemisorption activation energy increased with rising paraffin molecular weight; it also increased on replacing the potassium of the aluminum—chromium—potassium catalyst with calcium.

Relations between electronic properties of clean surfaces and activated adsorption

Surface reconstruction and steps alter the electron affinity and ionization potential of surfaces by a few tenth of an eV. In an simple potential model these variations are related to the changes in the activation energy for chemisorption. Without introducing fitted parameters the model describes semiquantitatively the influence of surface re-construction, step density and doping on the oxygen sticking coefficient on silicon and gallium arsenide. Possible applications of the model to stepped metal surfaces and to the influence of steps on catalysis in general are discussed as well.

Chemisorption of paraffin hydrocarbons on an aluminum-chromium-potassium catalyst

1. Processes of physical and chemical adsorption of C2-C6 paraffin hydrocarbons occur on an aluminum-chromium-potassium catalyst. Chemisorption, in contrast to physical adsorption, is a slow process, requiring an activation energy, nonequilibrium in the catalytic temperature region. 2. The activation energy of chemisorption increases with increasing molecular weight of the hydrocarbon. 3. Chemisorption and subsequent dehydrogenation of paraffin hydrocarbons occur on active centers containing Cr2+ ions. 4. The slowest step in the process of catalytic dehydrogenation is the decomposition of the chemisorption complex.

Kinetics of chemisorption of oxygen on diamond

The rate of chemisorption of oxygen on diamond powder of 20m 2g −1 total surface area has been studied. Adsorption conditions ranged from 28 to 141°C at 100 mTorr O 2 pressure. Also at 28°C rates were measured over the O 2 pressure range 11–100 mTorr. Prior to adsorption measurements, the sample was outgassed at 1000°C for 10–12 hr at a residual gas pressure of 10 −8Torr. Adsorption was followed up to 24 hr. Plots of amount of oxygen adsorbed vs. log of time were linear and showed no breaks. The activation energy for chemisorption increased linearly from 1·5 to 14·1 kcal mole −1 with increasing oxygen coverage up to 53 × 10 18 atoms g −1. It is concluded that under the conditions used, chemisorption was occurring predominantly on the (111) surface of diamond. Chemisorption at least on the (100) surface is expected to occur primarily at higher adsorption temperatures.

Adsorption kinetics and electron desorption of O 2 on polycrystalline tungsten

Electron desorption parameters and sticking coefficients of O 2 adsorbed on polycristalline tungsten have been studied. Two clearly-distinct adsorption states, β 1 and β 2, are identified with desorption energies of 110 kcal mol and 96 kcal mol ; the desorption temperatures are 2150 and 1900°K respectively. Desorption from β 1 yields atomic oxygen and from β 2 a complex WO n . The populations are n β 1 = 8.7 × 10 14 molecules cm 2 and n β 2 = 2 × 10 14 molecules cm 2 at 77 °K. The sticking coefficients, as a function of coverage for both states, have been determined ( s 0β 1⋍ 1 , s 0 β 2 = 0.8 at 77 °K and s 0 β 1 = 0.98, s 0 β 2 = 0.76 at 300 °K), being the experimental points fitted by a theoretical expression. From the values of so, ( E A − E P) for both states has been calculated, E A being the activation energy of chemisorption and E P the depth of the potential well in the preadsorbed state.

The chemisorption of several diatomic gases onto a W(210) plane and a polycrystalline tungsten surface

Plots of the sticking probabilities of CO and H 2 on the (210) face of tungsten against the fraction of the surface covered were found to be sigmoid. This shape has already been observed for at least one other system (15). It is explained by suggesting that adsorption on neighboring positions linearly increased the chemisorption activation energy experienced by a molecule in the mobile precursor state. The desorption spectra of CO on the W (210) plane contained an α and a β peak and an analysis of the spectra produced by CO on polycrystalline tungsten demonstrated that the β 3 peak desorbed by a second order reaction. The change in the work function when CO adsorbed on the W(210) face showed that the surface filled in two stages. The surface moment was constant during each stage. Hydrogen initially increased the work function of the W(210) plane. When the fraction of the surface covered reached 0.75, the work function was at a maximum and then decreased somewhat.

Studies on ultra-clean carbon surfaces—III. Kinetics of chemisorption of hydrogen on graphon

The chemisorption of hydrogen on activated Graphon was studied over the range 80–600°C at a hydrogen pressure of 0.10 Torr and at 500°C over the pressure range 0.020–0.47 Torr. The Graphon was activated by preoxidation in oxygen to 18.6 per cent weight loss at 625°C. The oxidized sample had a BET surface area of 100 m 2/g. Prior to adsorption measurements, the sample was outgassed at 1000°C at residual pressures < 10 −8 Torr. Adsorption was followed for times up to 24 hr. Plots of amount of hydrogen adsorbed vs. log of time showed linear regions. In all, four linear regions were identified. The activation energy for chemisorption changed little within a linear region. However, it changed abruptly between linear regions, increasing over-all from 5.7 kcal/mole for the first linear region to 30.4 kcal/mole for the last. It is concluded that each linear region represents chemisorption on different discrete configurations of sites on the carbon surface.

Anisotropic Reaction Kinetics of Oxygen With Pyrolytic Graphite.

Because of the possible use of pyrolytic graphite in aerospace vehicles its reaction with oxygen-containing gases is of interest, particularly with respect to chemical anisotropy. Although several authors have found different degrees of reactivity of graphite in different directions they disagree regarding the existence of a temperature coefficient. This disagreement, however, is probably due to investigators studying single crystals in some cases and pyrolytic graphite in others and in some cases different oxidizers. New data for pyrolytic graphite are presented which were obtained by oxidizing in each run several pieces of differing geometry. These together with earlier data show that the rate ratio for the two major directions is temperature dependent with an activation energy difference of about 19 kilojoules (4500 thermochemical calories) per gram atom of carbon reacted. It is shown that this difference in rates probably arises because the relative number of sites available in these directions is sufficiently different to cause a shift in which reaction step is rate-controlling. On the "faces" chemisorption is the rate controlling step followed by rapid decomposition of surface oxides. On the "edges" decomposition is the rate controlling step. This interpretation applied to the data yield 131 kJ/mol for the activation energy of chemisorption and 150 kJ/mol for decomposition (31 and 36 kcal/mol, respectively).

Studies on ultraclean carbon surfaces: II. Kinetics of chemisorption of oxygen on graphon

The chemisorption of oxygen on activated Graphon was studied over the temperature range −78° to 160°C at an oxygen pressure of 100 millitory. The Graphon was activated by preoxidation to 16.6% weight loss at 625°C. The oxidized sample had a BET surface area of 100 m 2/gm. Prior to adsorption measurements, the sample was outgassed at 1000°C at residual pressures <10 −8 torr. Adsorption was followed for times up to 24 hr. Plots of amount of oxygen adsorbed versus log of time showed linear regions. In all, five linear regions were identified. The activation energy for chemisorption changed little within a linear region. However, it changed abruptly in going from one linear region to the next, increasing overall from 3.1 kcal/mole for the first linear region to 12.4 kcal/mole for the last. It is concluded that each linear region represents adsorption on different discrete types of sites on the carbon surface.

The reactivity of Brominated Activated Carbon (BAC)

The manner of preparation of BAC determines its capacity for reaction with ethylene, in the following order: slow isothermal bromination > rapid bromination > rapid bromination followed by heating at 550°C. The latter method produces unreactive BAC. It is conluded that heat supplies the activation energy for chemisorption of the bromine, and that the chemisorbed bromine is unreactive. The ability of BAC to retain its bromine was found to be related to the pore structure of the carbon. At saturation pressure, capillary condensation of the bromine occurs. Carbons with larger pore diameters desorb their bromine more rapidly and to a greater extent than do carbons with smaller pore diameters. Recommendations for applications to air purification are suggested.

The decomposition of nitrous oxide catalysed by palladium-gold alloy wires

The rate of decomposition of nitrous oxide has been examined by pressure measurements, at temperatures between 500 and 900 °C and pressures between 10-2and 1 torr. The reaction is first order, but shows retardation by oxygen, but not nitrogen. Over the range of alloys, from Pd to nearly 40 at. % Pd, the velocity at 650 °C falls by a factor of 104, the apparent activation energy falls from 30 to 13 kcal/mole, and the retarding effect of oxygen falls to zero. Over this range of alloys the Fermi level which lies in thedband hardly changes but the concentration of thedband vacancies falls to zero. Over the range of alloys from 40 at. % Pd to Au the velocity at 650 °C remains constant but the apparent activation energy and frequency factor, which show an abrupt increase at 40 at. % Pd, show a continuous fall. The retarding effect of oxygen remains zero. In this range the Fermi level has entered thesband and increases to Au. A steady state treatment of an irreversible dissociative chemisorption of nitrous oxide, together with an oxygen chemisorption equilibrium, yields an equation for the velocity in quantitative agreement with the results found. It is also possible to account for the increase in apparent activation energy with oxygen coverage of the surface. The heat of adsorption of oxygen is derived as 32-2±2 kcal/mole, and the activation energy for chemisorption of nitrous oxide as 12-7 ±0-5 kcal/mole.

Elovich adsorption kinetics and the heterogeneous surface

Earlier treatments of the rate of adsorption on a heterogeneous surface have given results which do not agree with the Elovich equation at very short times. It is shown that the familiar, simple model of a nonuniform surface does, in fact, lead to a rate equation in good agreement with the Elovich equation at very short as well as at very long times. The model gives a physical significance to the constant t 0 and, when t 0 is appreciable, leads to Elovich plots having two linear sections. Similar experimental plots can be found. The common assumption that the activation energy for chemisorption increases with the energy of adsorption is incompatible with the model when the energy of chemisorption decreases with coverage and is thus difficult to justify: The apparent justification to be found in earlier treatments of the simple model is shown to be unsound.