Methane oxidation, photocatalytic

The methane oxidation to methanal is thus realized in the catalytic cycle in which atmospheric 02 is the oxidant and the OH radicals are the catalyst, and which is coupled to photoassisted dissociation of nitrogen dioxide (Figure 9.7). The latter process yields two ozone molecules per photocatalytic cycle. 

On the other hand, Ito et al. (99) found that the oxidative dimerization of methane to yield ethylene and ethane can be achieved with a high yield and good selectivity on Li-doped MgO catalysts. Since this pioneering work, many oxidic systems have been studied. Anpo et al. (100) found that surface sites of low coordination produced by the incorporation of Li into MgO play a vital role in the methane oxidative coupling reaction. Thus, although it was known that MgO acts as an acid-base catalyst, both the catalytic and photocatalytic activities of the MgO catalysts seem to be associated with the existence of surface ions in low coordination located on MgO microcrystals. 

Gas-phase oxidation of methane could be enhanced by the addition of a small amount of NO or N02 in the feed gas.1077 Addition of methanol to the CH4-02-N02 mixture results in a further increase in methane reactivity.1078 Photocatalytic conversion of methane to methanol is accomplished in the presence of water and a semiconductor photocatalyst (doped W03) at 94°C and atmospheric pressure.1079 The yield of methanol significantly increased by the addition of H202 consistent with the postulated mechanism that invokes hydroxyl radical as an intermediate in the reaction. 

Photochemical reduction of C02 was also achieved in the presence of the p-type semiconductor (copper oxide) or silicon carbide electrodes [97]. Irradiation of this system generates methanol and methane as the main products in the case of CuO electrode whereas hydrogen (with efficiency about 80%), methanol (16%), methane, and carbon monoxide in the case of SiC electrode. Also Ti02/CuO systems appeared relatively efficient (up to 19.2% quantum yield) in photocatalytic C02 to CH3OH reduction [98]. 

Few redox studies with cubic mesoporous materials have been reported [52]. The large, complex, three-dimensional pore system offers a unique environment. Ti- and Cr-substituted MCM-48 have been studied for the selective oxidation of methyl methacrylate and styrene to methyl pyruvate and benzaldehyde, respectively, using peroxides as oxidants and were found to outperform TS-1. Ti-MCM-48 has also been found to be better than Ti-MCM-41, TS-1 and Ti02 for the photocatalytic reduction of CO2 and H2O to methane and methanol. Ti-grafted MCM-48 has also been reported as the first functional biomimic of vanadium bromoperoxidase, active at neutral pH and used in the peroxidative halogenation of bulky organic dyes. 

Preparation of isolated highly dispersed titanium oxides on silica by sol-gel method for photocatalytic non-oxidative direct methane coupling... 

Other subjects recently discussed include the photocoabsorption of carbon monoxide and oxygen on ZnO,664 the relative photoabsorption of oxygen and methane over ZnO,665 the photocatalytic properties of ZnO and MgO,66 the effect of irradiation on the dielectric properties of Ti02,667 the photoluminescence of an oxide layer on aluminium,668 the singlet-triplet splitting of the free A-exciton in ZnO,659 the properties of the photosensitive film of copper in iodine solution,580 and the photosensitivity of layers of CuCla and [Fe(ox)8]3-.681... 

The reaction between alkanes (especially methane) and water is endothermic and produces syngas (see books, reviews [51], recent papers [52] and patents [53]). The photocatalytic oxidation of methane with water to produce methanol and molecular hydrogen has been patented [54]. 

P-12 - Nb- and Ti-containing silica-based mesoporous molecular sieves as catalysts for photocatalytic oxidation of methane... 

Nb- and Ti-containing silica-based mesoporous molecular sieves were used as catalysts for photocatalytic oxidation of methane. It is found that methanol was formed at 323 K and water pre-adsorbed samples exhibited higher catalytic activity than their pure metal oxides. By comparison with Nb-MCM-41, Ti-MCM-41 gave better methane conversion and methanol yield although traces of formaldehyde were observed over Nb-MCM-41 catalyst. Interestingly, OH radical was detected by the in-situ ESR spectroscopy using DMPO as trapper. 

Selective Photocatalytic Oxidation of Methane into Methanol... 

Selective Photocatalytic Oxidation of Methane into Methanol on Mesoporous Silica Containing the V Oxide Species (V-MCM-41)... 

NO was used as an oxidant. Methane conversion and methanol selectivity reached 6% and 88%, respectively, after UV irradiation for 3 h. As shown in Figure 18.9, the yield of methanol increases with an increase in the V content up to 0.6 wt%, and then decreases with a further increase in the V content. The observed good correspondence between the yield of methanol and the intensity of the photoluminescence indicates that the isolated tetrahedral oxide species act as active sites for the photocatalytic oxidation of methane with NO into methanol. 

Photocatalytic decomposition of alcohol Electro-oxidation of hydrogen Electroreduction of oxygen Ammonia synthesis Carbon monoxide methanation Carbon monoxide methanation Carbon monoxide oxidation Propene hydrogenation Benzene hydrogenation Oxidation of ethylene Coal liquefaction Electroreduction of oxygen Dehydrogenation of butadiene... 

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