Hydrogen oxidation processes

As a result of the high rate of the hydrogen oxidation process at platinum in contact with this ionomeric medium, the interfacial potential drop at a well-humidified H2 anode in a PEFC operating at 80 °C at 1 A/cm has been usually considered negligible. It should be remembered, however, that this would not be the case when ... 

The PEFC Anode with Pure Hydrogen Feed Probably as a result of the high performance of (pure) hydrogen Pt anodes in PEFCs, relatively little work has been done on the rate and mechanism of the hydrogen oxidation process at an impurity-free Pt/ionomer interface. Impedance spectroscopy work performed by Raistrick [65] on a model system of a flat Pt electrode pressed against a Nafion membrane, revealed details of the mechanism of hydrogen oxidation at the Pt/ionomer interface at room temperature. 

The rate of the hydrogen oxidation process at the Pt-based catalyst at 80 is very high, as long as the catalyst surface is not contaminated by adsorbed impurities. 

Enthalpies, Entropies and Gibbs Free Energy for Hydrogen Oxidation Processes... 

Some values for and (3 for electrochemical reactions of importance are given in table A2.4.6, and it can be seen that the exchange currents can be extremely dependent on the electrode material, particularly for more complex processes such as hydrogen oxidation. Many modem electrochemical studies are concerned with understanding the origin of tiiese differences in electrode perfomiance. 

Methyl ester hydrogenation process subtotal 474 Oxidation processes ... 

The electrons undergo the equivalent of a partial oxidation process ia a dark reaction to a positive potential of +0.4 V, and Photosystem I then raises the potential of the electrons to as high as —0.7 V. Under normal photosynthesis conditions, these electrons reduce tryphosphopyridine-nucleotide (TPN) to TPNH, which reduces carbon dioxide to organic plant material. In the biophotolysis of water, these electrons are diverted from carbon dioxide to a microbial hydrogenase for reduction of protons to hydrogen ... 

Reactions and Uses. The common reactions that a-hydroxy acids undergo such as self- or bimolecular esterification to oligomers or cycHc esters, hydrogenation, oxidation, etc, have been discussed in connection with lactic and hydroxyacetic acid. A reaction that is of value for the synthesis of higher aldehydes is decarbonylation under boiling sulfuric acid with loss of water. Since one carbon atom is lost in the process, the series of reactions may be used for stepwise degradation of a carbon chain. 

The anhydride can be made by the Hquid-phase oxidation of acenaphthene [83-32-9] with chromic acid in aqueous sulfuric acid or acetic acid (93). A postoxidation of the cmde oxidation product with hydrogen peroxide or an alkaU hypochlorite is advantageous (94). An alternative Hquid-phase oxidation process involves the reaction of acenaphthene, molten or in alkanoic acid solvent, with oxygen or acid at ca 70—200°C in the presence of Mn resinate or stearate or Co or Mn salts and a bromide. Addition of an aHphatic anhydride accelerates the oxidation (95). 

Fig. 2. Steps in advanced oxidation process (AOPs) involving o2one, hydrogen peroxide, and uv light of 254 nm. ( D) represents the doublet state ( ) represents quantum yield, and the other numbers associated with the reaction arrows are rate constants in units of (Af-s). Dashed arrows indicate...

Of the raw material hydrogen sources—natural gas, coal, and petroleum fractions—natural gas is the most often employed in ammonia plants in the 1990s and steam reforming is by far the most often used process. Partial oxidation processes are utilized where steam-reformable feeds are not available or in special situations where local conditions exist to provide favorable economics. Table 5 fists the contribution of the various feedstocks to world ammonia... 

The / f/-butanol (TBA) coproduct is purified for further use as a gasoline additive. Upon reaction with methanol, methyl tert-huty ether (MTBE) is produced. Alternatively the TBA is dehydrated to isobutylene which is further hydrogenated to isobutane for recycle ia the propylene oxide process. 

The sulfur removed via these fixed-bed metal oxide processes is generally not recovered. Rather the sulfur and sorbent material both undergo disposal. Because the sorbent bed has a limited capacity and the sulfur is not recovered, the appHcation of these processes is limited to gas streams of limited volumetric rate having low concentrations of hydrogen sulfide. 

Other wet oxidation processes under development as of the mid-1990s include Marathon Oil s Hysulf process which uses an organic solvent to remove the hydrogen sulfide. One significant distinction of the Hysulf process is that in addition to sulfur, hydrogen is produced. 

Another butadiene oxidation process to produce butanediol is based on the 1,4-addition of /-butyl hydroperoxide to butadiene (108). Cobalt on siHca catalyzes the first step. This is followed by hydrogenation of the resulting olefinic diperoxide to produce butanediol and /-butyl alcohol. 

Ethers. In the presence of anhydrous agents such as ferric chloride (88), hydrogen bromide, and acid chlorides, ethers react to form esters (see Ethers). Esters can also be prepared from ethers by an oxidative process (89). With mixed sulfonic—carboxyhc anhydrides, ethers are converted to a mixture of the corresponding carboxylate and sulfonate esters (90) ... 

---