Hydrocarbons photocatalytic reactions

Hashimoto, K., Kawai, T., and Sakata, T., Photocatalytic reactions of hydrocarbons and fossil fuels with water. Hydrogen production and oxidation, ]. Phys. Chem., 88, 4083,1984. 

That products of intermediate oxidation level can be detected in the photocatalytic reactions of hydrocarbons and fossil fuels is also consistent with a surface bound radical intermediate . Photocatalytic isotope exchange between cyclopentane and deuterium on bifunctional platinum/titanium dioxide catalysts indicates the importance of weakly adsorbed pentane at oxide sites. The platinum serves to attract free electrons, decreasing the efficiency of electron-hole recombination, and to regenerate the surface oxide after exchange. Much better control of the exchange is afforded with photoelectrochemical than thermal catalysis > ) As before, hydrocarbon oxidations can also be conducted at the gas-solid interface... 

Selective photocatalytic reactions are also used for the conversion of hydrocarbons (cyclohexane, toluene, methylcydohexane, ethylbenzene, and cumene) to their... 

UV irradiation of Ti02 containing a sufficient amount of H2O in the presence of unsaturated hydrocarbons such as CsHg and C4H6 leads to the formation of hydrogenolysis reaction products and oxygen-containing products (214, 242, 243). As shown in Fig. 52, the rates of these photocatalytic reactions increase when the ionization potential of the reactants decreases (224). 

In the photocatalytic reaction over each sample, CO, H2 and hydrocarbons such as C2H6 and C2H4 were obtained. The representative results are shown in Table 1. The CO and H2 should be mainly produced through the photocatalytic reduction of CO2 with methane (the PRCM, eq. 1), and C2H6 and other hydrocarbons should be produced through the photocatalytic coupling of methane (PCM, eq. 2) and consecutive delydro-genation, as previously reported [1]. 

Photocatalysis can be associated not only with a biological treatment, but also with other processes whose properties differ in some aspects from photocatalytic treatment. For instance, an initial ultrasonic treatment can allow one to destroy the CF3 group (124,125), which withstands most oxidation reactions. Ultrasound can also be more appropriate to start the destruction of hydrophobic compounds with long hydrocarbon chains, which interact poorly with Ti02 in water (125). 

The C—H bond can be activated by a metal complex, particularly when the complex plays the role of catalyst or photocatalyst. The reactions of hydrocarbons with metal complexes occur at low temperatures and can be selective. There are different pathways for C—H bond activation (i) by low-valence metal complexes, (ii) by high-valent metal-oxo compounds, (iii) by molecular oxygen and oxygen atom donors, (iv) by biological oxidation, or (v) by photocatalytic enhancement (21). 

The ubiquitous and reversible formation of radical cations in photoelectrochemical transformations allows pericyclic reactions to take place upon photocatalytic activation since the barriers for pericyclic reactions are often lower in the singly oxidized product than in the neutral precursor. For example, ring openings on irradiated CdS suspensions are known in strained saturated hydrocarbons [176], and formal [2 -I- 2] cycloadditions have been described for phenyl vinyl ether [ 177] and A-vinyl carbazole [178]. The cyclization of nonconjugated dienes, such as norbomadiene, have also been reported [179]. A recent example involves a 1,3-sigmatropic shift [180]. 

Major limitation associated with carbon dioxide reduction is the accuracy of the analytical measurements employed. The photocatalytic process is a multielectron transfer process, hence the reaction leads to the formation of a variety of products like carbon monoxide, methane, higher hydrocarbons, alcohol, aldehydes, carboxylic acid etc., with some intermediates. The identification and quantification of the products are needed for the best selection of photocatalyst, comparison and elucidation of reaction mechanisms. Currently there is no standard analysis method that has been developed for product analysis of carbon dioxide reduction. Hence the results of these measurements also include the products derived from the carbon contamination invariably present in the reaction sys-... 

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