Reactions on Oxides

Catalytic reaction mechanisms on oxide catalysts often involve oxygen atoms from the lattice ending up in product molecules. Multicomponent oxides are used as selective oxidation catalysts. 

A consequence of the coordinative unsaturation at oxide surfaces is that the oxygen anions in the surface have weaker bonds with the underlying lattice than do the fully coordinated oxygen ions of the interior. As a result, lattice oxygens at the surface may participate in oxidation reactions, where they become incorporated in product molecules. This type of mechanism, named after Mars and Van Krevelen, 

Where total oxidation is relevant for cleaning industrial exhaust gases, partial or selective oxidation is of great importance for the production of chemicals. The oxides used as catalysts are often multicomponent systems. The (Bi203)/Mo03) system is of practical use in the oxidation of propylene to acroleine 

Acrolein and acrylonitrile are important intermediates for polymer production. 

Multicomponent (P04)jf(V205), catalysts are applied in processes to produce maleic anhydride and phthalic anhydride intermediates for polymers. In the classical industrial process, maleic anhydride is the product from benzene and phthalic anhydride the product from naphthalene. Vanadium oxide-based catalysts have a stronger interaction with their substrates than molybdenum oxide-based catalysts. Whereas on M0O3 catalysts the hydrocarbon skeleton remains intact, oxidation on vanadium oxides proceeds with rupture of carbon-carbon bonds and the total number of carbon atoms in the molecule is not maintained in the reaction products. A new development is the use of butane as a reactant in these processes. 

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Schmickler,W. Electron Transfer Reactions on Oxide-Covered Metal Electrodes 17... 

Thirdly, if it is not possible to apply the SRS technique, it can be established whether a primary, secondary or tertiary alcohol is present by oxidizing the alcohol on the chromatographic zone and then subjecting the oxidation product to a detection reaction. On oxidation primary alcohols form aldehydes, secondary alcohols ketones and tertiary alcohols are not oxidized. 

Recently, Stair and coworkers [10, 11] developed a method to produce gas-phase methyl radicals, and used this to study reactions of methyl groups on Pt surfaces [12] and on molybdenum oxide thin films [13]. In this approach, methyl radicals are produced by pyrolysis of azomethane in a tubular reactor locat inside an ulttahigh vacuum chamber. This method avoids the complications of co-adsorbcd halide atoms, it allows higher covraages to be reached, and it allows tiie study of reactions on oxide and other surfaces that do not dissociate methyl halides effectively. 

DESIGN OF REACTION INTERMEDIATE AND TRANSITION-STATE ANALOGUE FOR A TARGET REACTION ON OXIDE SURFACES... 

Indeed the distinction between the mechanism for the reaction of hydrocarbons with hydrogen on metals compared to oxides may be that the metals have open the above avenue for a,)3 exchange while the reaction on oxides is restricted to a single atomic center 37). 

Scott MJ, Morgan JJ. 1995. Reactions on oxide surfaces. 1. Oxidation of As(III) by synthetic bimessite. Environmental Science and Technology 29 1898-1905. 

In the investigation of hydrocarbon partial oxidation reactions the study of the factors that determine selectivity has been of paramount importance. In the past thirty years considerable work relevant to this topic has been carried out. However, there is yet no unified hypothesis to address this problem. In this paper we suggest that the primary reaction pathway in redox type reactions on oxides is determined by the structure of the adsorbed intermediate. When the hydrocarbon intermediate (R) is bonded through a metal oxygen bond (M-O-R) partial oxidation products are likely, but when the intermediate is bonded through a direct metal-carbon bond (M-R) total oxidation products are favored. Results on two redox systems are presented ethane oxidation on vanadium oxide and propylene oxidation on molybdenum oxide. 

Photo-induced oxidation reactions on oxide surfaces have been discussed in reviews by Bickley (206, 405) and by Formenti and Teichner (406). It is characteristic of these reactions that the energy of the irradiating light needs to be not less than that of the absorption band of the oxide. Under these conditions both electrons and holes are produced, which can then react with molecules adsorbed on the surface. Since in some cases the surface lattice ions can absorb light at an energy less than the bulk oxide (7), photo-oxidation may be observed at lower wavelengths than expected from the bulk absorption band. 

Kosiba and Dienes 145) also investigated the effect of exposure of the graphite to gamma-irradiation during reaction on oxidation rates. On the... 

A similar oxidation-reduction mechanism in the carbon monoxide oxidation reaction on oxide catalysts has been proposed by Benton (71), Bray (72), Frazer (73), and Schwab (74). In this reaction also, Mooi and Selwood (57) found that a decrease in the percentage of iron oxide, manganese oxide or copper oxide on the alumina support first increased the rate, and then at lower percentages decreased the rate, of carbon monoxide oxidation, indicating that valence stabilization is again operative in these cases. 

C. Ethene and Hydrogenation Reactions on Oxide-Supported Pt Catalysts... 

Note that careful evaluation and minimization of uncertainties and errors in CTMs is requested to enable the application of these CTMs to the study of observed changes in 03 as small as < 1.5 %/yr. However, actually 03 concentrations are simulated by the models within 20-50%. Chemical reaction rates are also uncertain, for instance in the 90 s determinations of the rates of CH4 and CH3CC13 reactions with OH suggested that these reactions are about 20% slower than believed. Similarly OH reaction with N02 which is an important sink for NOx in the troposphere is measured to be 10-30% lower than earlier estimates [23]. Thus, the past years a number of studies (mainly based on Monte Carlo simulations) focused on the identification and evaluation of the importance of various chemical reactions on oxidant levels to highlight topics crucial for error minimization. Temperature dependence of reaction rates can also introduce a 20-40% uncertainty in 03 and H20 computations in the upper troposphere. It has been also shown that 03 simulations are particularly sensitive to the photolysis rates of N02 and 03 and to PAN chemistry. 

The features of behavior described above for the reactions on oxides bear a close resemblance to the kinetic characteristics of reactions on metals these include, for example, ranges of obedience to Eq. (2), the magnitudes of the calculated values of B, e, and a, and other features. Again compensation trends were found within groups of rate processes that involved either a common chemical transformation catalyzed by several oxides or related reactions on a single oxide. Representative instances of such observed obedience to Eq. (2) are included in Table V. The quantity of kinetic information available for reactions on particular oxides was, however, often insufficient to enable values of B and e to be estimated meaningfully. Also in the... 

Equation (16.1) is characteristic of oxidation reactions on oxide catalysts, with most oxides displaying a reaction order with respect to... 

Many of the early studies on kinetics of soil chemical processes were obviously concerned with diffusion-controlled exchange phenomena that had half-lives (r1/2) of 1 s or greater. However, we know that time scales for soil chemical processes range from days to years for some weathering processes, to milliseconds for degradation, sorption, and desorption of certain pesticides and organic pollutants, and to microseconds for surface-catalyzed like reactions. Examples of the latter include metal sorption-desorption reactions on oxides. 

To study rapid reactions, traditional batch and flow techniques are inadequate. However, the development of stopped flow, electric field pulse, and particularly pressure-jump relaxation techniques have made the study of rapid reactions possible (Chapter 4). German and Japanese workers have very successfully studied exchange and sorption-desorption reactions on oxides and zeolites using these techniques. In addition to being able to study rapid reaction rates, one can obtain chemical kinetics parameters. The use of these methods by soil and environmental scientists would provide much needed mechanistic information about sorption processes. 

Sudoh M, Kodera T, Hino H, Shimamura H. Effect of anodic and cathodic reactions on oxidative degradation of phenol in an undivided bipolar electrolyzer. J Chem Eng Japan 1988 21(2) 198-203. 

In this section the reactivity of the adsorbed O ion is discussed for some elementary reactions on oxide surfaces for which there is direct spectroscopic evidence of the participation of the ion. Evidence from the gas phase shows that free O" ion reacts with H2, the lower hydrocarbons, and also with oxygen both associative and hydrogen abstraction reactions are known to occur (99, 100). 

Casey W. H. and Cheney M. A. (1993) Bronsted reactions on oxide mineral surfaces and the temperature-dependence of their dissolution rates. Aquat. Sci. 55, 304-313. 

Idriss H, Seebauer EG (2000) Ethanol reactions on oxide surfaces. J Mol Catal A Chem 152 201... 

Hakkarainen, R., Salmi, T., and Keiski, R.L. Comparison of the dynamics of the high-temperature water-gas shift reaction on oxide catalysts. Catalysis Today, 1994, 20, 395. 

The literature concerned with hydrocarbon oxidation has been critically reviewed by Germain [1], who was specifically concerned with patterns of activity and selectivity of reactions on oxide catalysts. Although the relative activites of a... 

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