Gas-phase oxidation of carbon

Heckman and Harling57 examined the gas-phase oxidation of carbon black micro-structures and showed that oxidative attack of carbon crystallites was concentrated on the small crystallites, at the edges of layer planes and at lattice defects. Partial graphitization of a carbon black, so that only the outermost surface layers are well-ordered, causes oxidative corrosion within the core of the carbon particle, leaving an outer shell . Consequently, similar behavior can be expected for ungraphitized and partially graphitized carbons in electrochemical environments. 

It should be stressed that a eatalytic sequenee representing the reaction pathway may not contain a RDS. As an example, consider the catalytie gas-phase oxidation of carbon monoxide. On some metals the surface reaction between adsorbed CO moleeules and O atoms is the RDS, then, if is an active site, the sequence can be represented as ... 

The gas-phase oxidation of carbon blacks by oxygen and/or water is strongly catalyzed by the presence of catalytically active metals, such as platinum (Rewick et al. 1974, Stevens and Dahn 2005), whereby several weight percent of platinum on carbon can increase the gas-phase oxidation rate by orders of magnitude. This, however, is not the case for the electrochemical oxidation of carbon blacks, where at potentials of 0.8 V and higher (vs. RHE) the carbon corrosion rate is within a factor of 2 between that for noncatalyzed and platinum-catalyzed carbon blacks (Roen et al. 2004, Passalacqua et al. 1992, Kinoshita 1988). Therefore, gas-phase oxidation tests to screen potential carbon-black supports is not a reliable method for predicting their stability in the electrochemical environment, so it is essential to measure the carbon corrosion rates directly in an electrochemical cell. 

Gas-phase oxidation of propylene using oxygen in the presence of a molten nitrate salt such as sodium nitrate, potassium nitrate, or lithium nitrate and a co-catalyst such as sodium hydroxide results in propylene oxide selectivities greater than 50%. The principal by-products are acetaldehyde, carbon monoxide, carbon dioxide, and acrolein (206—207). This same catalyst system oxidizes propane to propylene oxide and a host of other by-products (208). 

Propylene oxide is also produced in Hquid-phase homogeneous oxidation reactions using various molybdenum-containing catalysts (209,210), cuprous oxide (211), rhenium compounds (212), or an organomonovalent gold(I) complex (213). Whereas gas-phase oxidation of propylene on silver catalysts results primarily in propylene oxide, water, and carbon dioxide as products, the Hquid-phase oxidation of propylene results in an array of oxidation products, such as propylene oxide, acrolein, propylene glycol, acetone, acetaldehyde, and others. 

The gas phase oxidation of naphthalene to phthalic anhydride over V2Os-based catalysts is one of the oldest successful partial oxidation processes and is still of industrial importance today. Common commercial catalysts are modified silica-supported V—K—S—O catalysts and catalysts similar to those used for benzene or o-xylene oxidation. Maximum phthalic anhydride yields of 80—85 mol. % (92—98 wt. %) at 350—400°C are reported. By-products are naphthoquinone (2—5%), maleic anhydride (2— 5%) and carbon oxides. 

Again, the homogeneous reaction of carbon monoxide and oxygen is implicitly included in the parameter, y of Equation (Rl). Further, we neglect the gas phase oxidation of hydrogen... 

Mikhalovsky, S.V., Zaitsev, and Yu. P. (1997). Catalytic properties of activated carbons I. Gas-phase oxidation of hydrogen sulphide. Carbon, 35, 1367—74. 

S. J. Gentry and A. Jones, "Poisoning and Inhibition of Catalytic Oxidations. I. The Effect of Silicone Vapour on the Gas-Phase Oxidations of Methane, Propane, Carbon Monoxide and Hydrogen over Platinum and Palladium Catalysts", J. Appl. Chem. Biotech.. 1978,727... 

Carbon balancing and branching ratios for the gas phase oxidation of toluene by OH radicals in the absence of NOx... 

Reduction of Reaction Mechanism for Gas-Phase Oxidation of Formaldehyde in the Presence of Carbon Oxide (H)... 

Reaction mechanism for gas-phase oxidation of formaldehyde in the presence of carbon oxide (II), suggested by LA. Vardanyan, G.A. Sachyan, A.B. Nalbandyan [71] and presented in Table 3.3, is rather complicated. However, it is chosen dehberately as a useful one for the clarification of the methods to reduce kinetic models, based on the sensitivity analysis method... 

Based on selected data array on the sensitivity of species concentrations for the rate constants of steps at the reaction time 5-10 s, E.P. Dougherty, J.T. Hwang, H.G. Rabitz [69] determined both the dominant and imimportant steps of the reaction mechanism, as presented in Table 3.3. Further, using a more systematic method to analyze data on the sensitivity of concentrations [56] and rate-of-production of the reaction species [28], with respect to variations of the step rate constants (the method of principal components analysis) S. Vajda, P. Valko, T. Turanyi [56] ranked the role of steps for the reaction mechanism of gas-phase oxidation of formaldehyde in the presence of carbon oxide (II). They succeeded also in separating the minimal reaction mechanism involving 13 steps, which gives the same results, accurate to within 2% (for the reaction time of 5 10 s), as the initial kinetic model consisiting of 25 steps (see Table 3.5). 

The work at Montecatini-Edison on the u.v.-initiated reaction of oxygen with tetrafluoroethylene, hexafiuoropropene, and hexafluorobuta-1,3-diene has been summarized. A typical distribution of products from the gas-phase oxidation of hexafiuoropropene comprises (% based upon carbon... 

Nickel sulfate also is made by the reaction of black nickel oxide and hot dilute sulfuric acid, or of dilute sulfuric acid and nickel carbonate. The reaction of nickel oxide and sulfuric acid has been studied and a reaction induction temperature of 49°C deterrnined (39). High purity nickel sulfate is made from the reaction of nickel carbonyl, sulfur dioxide, and oxygen in the gas phase at 100°C (40). Another method for the continuous manufacture of nickel sulfate is the gas-phase reaction of nickel carbonyl and nitric acid, recovering the soHd product in sulfuric acid, and continuously removing the soHd nickel sulfate from the acid mixture (41). In this last method, nickel carbonyl and sulfuric acid are fed into a closed-loop reactor. Nickel sulfate and carbon monoxide are produced the CO is thus recycled to form nickel carbonyl. 

Mitsubishi Gas Chemical Co. in Japan produces pyromellitic dianhydtide in the same unit used for trimellitic anhydtide production (105). This process starts with pseudocumene, which is first carbonylated with carbon monoxide in the presence of boron trifluotide and hydrogen fluotide to form 2,4,5-trimethylbenzaldehyde. The Hquid-phase oxidation of the trimethylbenzaldehyde to pyromellitic acid and subsequent processing steps ate much the same as described for the Mitsubishi Gas Chemical process in the trimellitic acid section. The production of pyromellitic anhydtide is in conjunction with a joint venture agreement with Du Pont. 

Very recently, considerable effort has been devoted to the simulation of the oscillatory behavior which has been observed experimentally in various surface reactions. So far, the most studied reaction is the catalytic oxidation of carbon monoxide, where it is well known that oscillations are coupled to reversible reconstructions of the surface via structure-sensitive sticking coefficients of the reactants. A careful evaluation of the simulation results is necessary in order to ensure that oscillations remain in the thermodynamic limit. The roles of surface diffusion of the reactants versus direct adsorption from the gas phase, at the onset of selforganization and synchronized behavior, is a topic which merits further investigation. 

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