Carbon oxidation reactions

The SR process involves two reactions, the conversion of hydrocarbon with steam to form hydrogen and carbon oxides [reaction (9.1)] and the WGS reaction for the conversion of carbon monoxide into carbon dioxide [reaction (9.2)] ... 

The JANAF thermochemical tables deal with the two forward solid carbon oxidation reactions producing CO and CO2. The CO oxidation is a matter of subtraction for the equilibrium constant. 

Despite its low equilibrium potential, the rate of the carbon oxidation reaction (COR) (reaction 17) is negligible at potentials less than 1.8 V because of its very small exchange current density (j = 6x 10 A/cm ). ... 

Tlie reaction rate of catalyzed carbon oxidation reactions depends on a number of intrinsic parameters ... 

Carbon oxidation reactions nsnally result in the formation of alcohol products, but in some instances they produce desaturated metabolites. Early examples are provided by the P450-cata-lyzed oxidative A -desaturation of valproic acid [88,89], A -desaturation of testosterone [90], and A -desaturation of sterols [91, 92] (Fig. 4.11). Additional examples are provided by the desaturation of lovastatin [93], ezlopitant [94], and capsaicin [95] (Fig. 4.12). In all these examples, hydroxylation to give the normally expected alcohol product also is observed, which suggests that in these substrates desaturation diverges at some point from the normal substrate hydroxylation reaction. 

Hu, J. et al. 2008. Modeling and simulations on mitigation techniques for carbon oxidation reaction caused by local fuel starvation in a PEMFC. ECS Transactions 16 1313-1322. 

All the carbon-oxidation reactions involving molecular oxygen, carbon dioxide, steam and oxides of nitrogen are catalyzed by inorgaiuc material contained within the carbon. Not only are the chemical kinetics of the reactions changed by the catalytic process, but the topographical kinetics (reaction anisotropy) are also changed. 

Li et al. (2010) tested four different Australian raw coals as fuels in direct carbon MCFCs. They found that the cell performances are highly dependent on a coal s intrinsic properties, in particular its chemical composition and concentration of oxygen-containing surface groups. Impurities such as AI2O3 and Si02 lead to an inhibitive effect, whereas CaO, MgO, and Fc203 exhibit a catalytic effect on the electrochemical carbon oxidation reaction. 

The carbon oxidation reaction can be affected by temperature, interfacial electrode potential, and water vapor pressure electrode potential is the most aggressive of these factors [94]. The presence of Pt can also catalyze carbon oxidation at lower potentials [95]. Corrosion of the catalyst s carbon support can occur at both the cathode and the anode. If the cathode is held at relatively high potentials, its catalyst carbon support will be oxidized, whereas if the anode is fuel starved, oxidation of the anode catalyst s carbon support may occur. For example, when the fuel level is insufficient to provide the expected current for PEM fuel cells, the potential value of the anode can increase to >0.21 V, or even to > 1.23 V, at which point water electrolysis and carbon oxidation at the anode will occur to provide the required protons and electrons for the ORR at the cathode. 

Fig. 11 A proton exchange membrane fuel cell experiencing Hj/air front start/stop, showing the major electrochemical reactions considered during start/stop operations. ORR Oxygen reduction reaction, COR Carbon oxidation reaction, OER Oxygen evolution reaction, and HOR Hydrogen oxidation reaction...

Fig. 4 Principle of the reverse current region. For details see the text. COR carbon oxidation reaction, OER oxygen evolution reaction, ORR oxygen reduction reaction, HOR hydrogen oxidation reaction, GDL gas-diffusion layer, CL catalyst layer...

A simple explanation for the apparent similarity of properties with respect to start/stop cycling can be found in the differences of the relative humidity. The low-tanperature fuel cell is operated with humidified gases (66% relative humidity), which drives the carbon oxidation reaction to the product (CO ) side (1). Interestingly, we recently found very similar results by a comparison of low- and high-temperature MEAs operated continuously at arotmd 0.8 V (Schmidt 2(X)6a, b). 

Ketene can be obtained by reaction of carbon oxides with ethylene (53). Because ketene combines readily with acetic acid, forming anhydride, this route may have practical appHcations. Litde is known about the engineering possibiHties of these reactions. 

Oxidation. The oxidation reactions of organoboranes have been reviewed (5,7,215). Hydroboration—oxidation is an anti-Markovnikov cis-hydration of carbon—carbon multiple bonds. The standard oxidation procedure employs 30% hydrogen peroxide and 3 M sodium hydroxide. The reaction proceeds with retention of configuration (216). 

Reaction 21 is the decarbonylation of the intermediate acyl radical and is especially important at higher temperatures it is the source of much of the carbon monoxide produced in hydrocarbon oxidations. Reaction 22 is a bimolecular radical reaction analogous to reaction 13. In this case, acyloxy radicals are generated they are unstable and decarboxylate readily, providing much of the carbon dioxide produced in hydrocarbon oxidations. An in-depth article on aldehyde oxidation has been pubHshed (43). 

The carboxyl group of acids appears to deactivate the hydrogens on the alpha carbon atom toward attack by the free-radical flux in oxidation reactions. Acetic acid, therefore, is particularly inert toward further oxidation (hydrogens are both primary and deactivated) (48). For this reason, it is feasible to produce acetic acid by the oxidation of butane (in the Hquid phase), even under rather severe oxidation conditions under which most other products are further oxidized to a significant extent (22). 

At still higher temperatures, when sufficient oxygen is present, combustion and "hot" flames are observed the principal products are carbon oxides and water. Key variables that determine the reaction characteristics are fuel-to-oxidant ratio, pressure, reactor configuration and residence time, and the nature of the surface exposed to the reaction 2one. The chemistry of hot flames, which occur in the high temperature region, has been extensively discussed (60-62) (see Col ustion science and technology). 

The radicals are then involved in oxidations such as formation of ketones (qv) from alcohols. Similar reactions are finding value in treatment of waste streams to reduce total oxidizable carbon and thus its chemical oxygen demand. These reactions normally are conducted in aqueous acid medium at pH 1—4 to minimize the catalytic decomposition of the hydrogen peroxide. More information on metal and metal oxide-catalyzed oxidation reactions (Milas oxidations) is available (4-7) (see also Photochemical technology, photocatalysis). 

Quantitative Analysis of All llithium Initiator Solutions. Solutions of alkyUithium compounds frequentiy show turbidity associated with the formation of lithium alkoxides by oxidation reactions or lithium hydroxide by reaction with moisture. Although these species contribute to the total basicity of the solution as determined by simple acid titration, they do not react with allyhc and henzylic chlorides or ethylene dibromide rapidly in ether solvents. This difference is the basis for the double titration method of determining the amount of active carbon-bound lithium reagent in a given sample (55,56). Thus the amount of carbon-bound lithium is calculated from the difference between the total amount of base determined by acid titration and the amount of base remaining after the solution reacts with either benzyl chloride, allyl chloride, or ethylene dibromide. 

The manufacture of the highly pure ketene required for ketenization and acetylation reactions is based on the pyrolysis of diketene, a method which has been employed in industrial manufacture. Conversion of diketene to monomeric ketene is accompHshed on an industrial scale by passing diketene vapor through a tube heated to 350—600°C. Thus, a convenient and technically feasible process for producing ketene uncontaminated by methane, other hydrocarbons, and carbon oxides, is available. Based on the feasibiHty of this process, diketene can be considered a more stable form of the unstable ketene. 

Weak to moderate chemiluminescence has been reported from a large number of other Hquid-phase oxidation reactions (1,128,136). The Hst includes reactions of carbenes with oxygen (137), phenanthrene quinone with oxygen in alkaline ethanol (138), coumarin derivatives with hydrogen peroxide in acetic acid (139), nitriles with alkaline hydrogen peroxide (140), and reactions that produce electron-accepting radicals such as HO in the presence of carbonate ions (141). In the latter, exemplified by the reaction of h on(II) with H2O2 and KHCO, the carbonate radical anion is probably a key intermediate and may account for many observations of weak chemiluminescence in oxidation reactions. 

Butane-Based Fixed-Bed Process Technology. Maleic anhydride is produced by reaction of butane with oxygen using the vanadium phosphoms oxide heterogeneous catalyst discussed earlier. The butane oxidation reaction to produce maleic anhydride is very exothermic. The main reaction by-products are carbon monoxide and carbon dioxide. Stoichiometries and heats of reaction for the three principal reactions are as follows ... 

---