Kinetic model of catalytic ethylene oxidation

The kinetics of ethylene oxidation on a pumice supported silver catalyst promoted with chlorine has been studied. A simplified kinetic equation with two parameters is given to describe the process over a wide range of ethylene concentrations. The parameters of the equation have been determined as a function of the temperature.

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Stochastic bi-resonance in non-isothermal carbon monoxide catalytic oxidation system

The Pt(110)/CO O2 system subject to reaction heat, heat conduction and radiative heat transfer is non-isothermal and its temperature varies in time and space. In this paper, taking support temperature (ST) as the control parameter, the effect of the ST fluctuations in the oscillatory dynamics of the non-isothermal Pt(110)/CO O2 system is numerically studied. It is found that the ST fluctuations may induce stochastic oscillations and the oscillations exhibit stochastic bi-resonance (SBR) with the change of the strength or correlation time of the fluctuations. This result shows that the temperature fluctuations may enhance the chemical reaction oscillations. Moreover, the system can selectively and repeatedly employ the temperature fluctuations to enhance its reaction oscillations. It is also shown when the distance of the ST temperature to the oscillatory region increases a little, the effect of the temperature fluctuations would obviously weaken.     

Mechanism of carbon monoxide oxidation on nickel oxide and its solid solutions

On the basis of studying the isotope exchange of oxygen and the position of the Fermi level in the system MiO–Li_xNi_1–xO, an explanation is proposed for the compensation effect in the reaction of CO oxidation on nickel oxide and its solid solutions.

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NiO–Li_xNi_1–xO CO .

     

Quantum chemical analysis of the Eley-Rideal and Langmuir-Hinshelwood mechanisms of the catalytic oxidation of carbon monoxide on the nickel surface

The studies concerned with the oxidation of carbon monoxide on the nickel surface are reviewed. The Eley-Rideal (ER) collision and Langmuir-Hinshelwood (LH) adsorption mechanisms of the oxidation are analyzed. Calculations of the activation barriers of the oxidation of carbon monoxide on the Ni (111), (100), and (110) faces were performed for the first time and involved optimization of the reaction paths by the collision and adsorption mechanisms. It is shown that on the Ni (111) and (110) faces the ER collision mechanism of the reaction is preferable with the activation barriers ΔE _dis ^O ₂=62 kJ/mole and ΔE _trans ^O A21F5001₂x=25 kJ/mole for Ni (111) and ΔE _dis ^O ₂=72 kJ/mole and ΔE _trans ^O ₂=20 kJ/mole for Ni (110); on the Ni (100) face, the LH adsorption mechanism with the activation barriers ΔE _dis ^O ₂=75 kJ/mole and ΔE _trans ^O ₂=42 kJ/mole is favored. Analysis of the potential barriers for the catalytic oxidation of carbon monoxide on the Ni surfaces suggests the LH mechanism to be preferential, although insignificant differences in the activation barries can lead to the oscillatory reaction mechanism, which is confirmed experimentally. The calculations were performed by the LCAO MO SCF method in the MINDO/3 approximation. Kiev Polytechnical Institute. Translated fromZhurnal Struktumoi Khimii, Vol. 37, No. 4, pp. 628–645, July–August, 1996. Translated by I. Izvekova     

A model for explosions during carbon monoxide oxidation

During the oxidation of carbon monoxide containing a trace of water, ten well-known atomic and molecular species can be identified as of potential significance. All conceivable reactions of these species in their ground electronic states were considered, and rate constants for all those that are of potential importance are either known or can be estimated with considerable confidence. For compositions and temperatures of experimental interest an isothermal system goes to a single steady state that is stable to perturbation and will neither explode nor oscillate. These steady-state computations also predict that as the temperature is raised above about 1000 K most of the water is converted to H, OH, and HO₂ radicals. Under such conditions, exothermic reactions would be so rapid that strong thermal gradients would develop in any real system of plausible dimensions. A simple model based on these calculations predicts explosion limits consistent with those observed experimentally. Simultaneous behavior in time and in space must be calculated in detail before it is clear whether or not this model based on ground electronic states can model the oscillations that are sometimes observed in this system.     

Experimental activation entropy for heterogeneous catalytic oxidation of hydrogen and carbon monoxide

The entropy of the activated complex S, calculated from experimental kinetic data, is approximately constant in the series of oxide catalysts used for oxidation of hydrogen, which shows that these contacts are of the same type. The low S values indicate low mobility in the activated complex. A similar result is obtained for the oxidation of carbon monoxide on the oxides. The high S values observed in the oxidation of hydrogen on platinum and palladium indicate surface mobility of the transitional state and also a difference in the nature of metallic and of oxide catalysts for the oxidation of hydrogen.     

Stochastic resonance in surface catalytic oxidation of carbon monoxide induced by colored noise

Noise is generally considered as a disadvanta-geous factor, which would smear weak signals, there-fore, people always try to reduce its influence. How-ever, recent studies show that, for a nonlinear system, under certain circumstances, noise can enhance sys-tem抯 response to a signal. At specific noise intensity, the response of a system to a weak signal may reach its maximum, which is called 搒tochastic resonance?(SR). The concept of SR was originally put forward by Benzi and his collaborat…     

Kinetics of carbon monoxide methanation on nickel catalysts

The kinetics of CO methanation in excess H₂ on CaO- and CeO₂-doped nickel catalysts supported on Al₂O₃ and TiO₂ was studied at atmospheric pressure in a temperature range of 180–240°C. It was found that the same rational fractional rate equation corresponding to the reaction taking place at high surface coverages, is valid for all of the catalysts. The activity of nickel catalysts in the methanation reaction and their adsorption capacity with respect to reaction mixture components depend on the nature of the support and dopants.     

The mercury-sensitized oxidation of carbon monoxide

The mercury-photosensitized oxidation of CO was studied at 275°C over a wide range of [O₂]/[CO] ratios in the absence and presence of the oxygen atom scavenger 2-trifluoromethylpropene (TMP) and at 25°C at low [O₂]/[CO] ratios in the presence of TMP. By following the quantum yield of CO₂ production, Φ {CO₂}, as a function of the [O₂]/[CO] ratio, the reactions of vibrationally excited CO (v υ 9) and electronically excited O₂, probably in the c¹Σ^?_u state, were studied. At low [O₂]/[CO] ratios the predominant reactions are of vibrationally excited CO (v υ 9). Relative rate constants for chemical reaction versus deactivation of CO (v υ 9) were obtained. At higher [O₂]/[CO] ratios, the principal reactions are of electronically excited O₂. Relative rate constants for chemical reactions and deactivation of this electronically excited O₂ with CO, O₂, and TMP were obtained. From the effect of total pressure on Φ {CO₂}, it is proposed that an intermediate CO₃ is formed in the reaction of electronically excited O₂ with CO.     

The oxidation of carbon monoxide on iron oxide

The oxidation of CO on α-Fe₂O₃ was studied in a flow reactor. The conversion was complete at 650–660 K. The catalytic activity of iron oxide was higher than that of the ferrite-containing xMgOyFe₂O₃ catalyst. The adsorption of CO on iron oxide and the kinetics of interaction of carbon monoxide with oxygen atomically adsorbed on the surface of α-Fe₂O₃ were studied. The kinetic parameters of the oxidation of CO are evidence of the participation of adsorbed oxygen atoms, whose binding energy on the surface of α-Fe₂O₃ is lower than that on the surface of the magnesium ferrite-containing catalyst.     

Kinetic studies on the oxidation of carbon monoxide over pure,reduced, and doped indium sesquioxides

Kinetic studies on the oxidation of carbon monoxide have been carried out between 413 and 473 K at different partial pressures of carbon monoxide and oxygen by means of the static method using vacuum-activated, hydrogen-reduced, and NiO-doped In₂O₃ semiconductors as catalysts. A strong carbon dioxide inhibiting effect is observed. The experimental data satisfactorily fit an equation derived by assuming the controlling step to be the adsorption of gaseous oxygen on the surface of catalyst. CO and CO₂ adsorb on the lattice oxygens (O), while O₂ adsorbs on the oxygen vacancies (V) formed by vacuum-activation, H₂-reduction, and NiO-doping of In₂O₃. When CO₂ formed during the reaction is removed by means of liquid nitrogen trap, the oxidation is found to be first-order with respect to CO and to be half-order with respect to O₂. The concentration of oxygen vacancy in the solid catalyst is shown to be the controlling factor for the oxidation of carbon monoxide. A possible reaction mechanism can be explained by the n-type character of In₂O₃ and proposed from the experimentally obtained kinetic data and conductivities.     

Intrinsic catalytic role of mesoporous silica in preferential oxidation of carbon monoxide in excess hydrogen

We have studied the intrinsic catalytic role of MCM-41 mesoporous silica in preferential oxidation of CO in excess H(2) (PROX). Two types of MCM-41 supports (MCM-41A and MCM-41B) were obtained from the same pristine as-synthesized materials by using different procedures for surfactant removal: one-step calcination or two-step extraction-calcination. Although two kinds of Pt catalysts prepared from the MCM-41 supports exhibit high similarity in apparent physicochemical parameters such as Pt morphology, particle-size distribution, electronic states, support architecture, and pore-size distribution, they show a dramatic difference in catalytic activity (ca. 100?% versus 10?% CO conversions at 298?K). This feature motivated us to investigate the catalytic role of MCM-41 in the PROX reaction. By means of infrared experiments with the isotope tracer technique, it was revealed that the reactive microenvironment at the interface between Pt and the MCM-41A support is the origin of the high activity. On the highly active Pt/MCM-41A catalyst, interfacial silanols play a decisive role in the ignition of CO oxidation, and gaseous O(2) and H(2) are dissociated on CO-free Pt sites created by the interfacial reaction. The dissociated oxygen and hydrogen are proposed to sustain the catalytic cycle in the form of regenerated silanols on the support, which is catalyzed by the Pt surface in the presence of H(2).     

Influence of the nonmetallic component in binary chromium compounds on their catalytic activity in carbon monoxide oxidation

Oxidation kinetics of carbon monoxide on chromium carbide, nitride and boride has been studied. The activity of chromium compounds was compared with the electronic state of the chromium atom according to XPS data. A reaction mechanism is proposed and an assumption is made on the predominant effect of the nonmetallic component on the character of change in the catalytic activity of chromium compounds.

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Polyethylene-bound complexes in carbon monoxide catalytic reactions

Catalytic reactions of carbon monoxide are investigated in presence of polyethylene-bound palladium and rhodium complexes and it was shown that such catalysts with temperature-dependent solubility are applicable for carbonylation of organic halides and hydroformylation of alkenes due to high activity and recoverability.     

Chemisorption and oxidation of carbon monoxide on palladium alloys

Kinetic and adsorption data (programmed thermal desorption in situ) for the oxidation of carbon monoxide on palladium alloys with silver and gold have been discussed. It has been shown that the local reaction plays a determining role and that active cluster sites are best for catalysis which are monoatomic with respect to palladium, weakly binding the chemisorbed CO molecule. The role of silver is to activate the second component: oxygen. The cluster model makes it possible to predict the optimum composition of Pd-Ag catalysts for the oxidation of CO and to explain the dependence of Pd-Au activity on the composition.Kiev University. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 5, pg. 574–578 September–October, 1991. Original article submitted July 12, 1991.     

Preferential oxidation of carbon monoxide on supported gold catalysts

Catalyst precursors containing 1% Au were synthesized by the impregnation of Al₂O₃ and CeO₂-Al₂O₃ supports with an aqueous solution of HAuCl₄ followed by drying, treatment with aqueous ammonia for the removal of chlorine residues, and final drying at 90°C. The oxidation of CO in gas containing ～1 vol % O₂, ～1 vol % CO, 60 vol % H₂, and the balance N₂ on the activated catalysts was studied. In a number of experiments, to 18 vol % water was added to the gas mixture. The activation of precursors by the initial gas was studied. It was found that the prolonged storage of a precursor in air made its activation difficult to perform. The 1%Au/Al₂O₃ catalyst activated by the gas mixture stably operated in the preferential oxidation of CO at room temperature with the occurrence of the reaction in the mode of catalyst surface ignition (a hot spot at the bed inlet) under a change in the feed gas flow rate by a factor of 3. The effect of the presence of additional CO₂ (to 39 vol %) on the oxidation of H₂ was studied: the catalyst activity decreased. Because the reaction of CO₂ reduction to CO did not occur, the effect can be due to the adsorption of CO₂ on gold. The effect of the addition of water vapor to the feed gas was studied with the use of 1%Au/CeO₂-Al₂O₃ as an example. The exo/endo effects related to the adsorption/desorption of water on the catalyst surface were detected upon steam supply and shutoff at a bed temperature of 100–150°C. It was noted that the addition of water vapor to a certain level favorably affected the selectivity (decreased the residual concentration of CO). The boundary water concentration, at which the effect became negative, depended on catalyst bed temperature. The higher was the bed temperature, the greater amount of water could be added until the manifestation of a negative effect.