Partial photocatalytic oxidations

In this section some experimental results obtained in our laboratories on PM Rs are reported. In particular two different photocatalytic membrane reactors, used in total and partial photocatalytic oxidations are described. 

AugugUaro V, El Nazerb H A, Loddo V, Mele A, Palmisauo G, Pahnisauo L and Yurdakal S (2010), Partial photocatalytic oxidation of glycerol in TiOj water suspensions , Catal Today, 151,21-28. 

Li C-J, Xu G-R, Zhang B, Gong JR (2012) High selectivity in visible-light-driven partial photocatalytic oxidation of benzyl alcohol into benzaldehyde over single-crystalline rutile Ti02 nanorods. Appl Catal B 115 201-208... 

Lewandowski and Ollis have proposed a simple kinetic model describing the transient photocatalytic oxidation of aromatic contaminants [50]. The model considered three chemical species an aromatic contaminant, preadsorbed onto the catalyst in the dark and refreshed continuously from the gas phase a strongly bound, recalcitrant reaction intermediate and final reaction products (CO or CO2), assumed for simplicity to be strictly gas-phase species. The model also assumed that two types of catalyst site were present on the photocatalyst surface, with the first suitable for the adsorption of aromatic contaminants, as well as reaction intermediates, and the second type considered to be more polar in nature, suitable only for adsorption of partially oxidized reaction intermediates. 

It has recently been recognized that crystal structure and particle size can also influence photoelectrochemical activity. For example, titanium dioxide crystals exist in the anatase phase in samples which have been calcined at temperatures below 500 °C, as rutile at calcination temperatures above 600 °C, and as a mixture of the two phases at intermediate temperature ranges. When a range of such samples were examined for photocatalytic oxidation of 2-propanol and reduction of silver sulfate, anatase samples were found to be active for both systems, with increased efficiency observed with crystal growth. The activity for alcohol oxidation, but not silver ion reduction, was observed when the catalyst was partially covered with platinum black. On rutile, comparable activity was observed for Ag, but the activity towards alcohol oxidation was negligibly small . Photoinduced activity could also be correlated with particle size. 

Under similar conditions, the photocatalytic oxidation of 2-or 3-methyl-l-butene and of 2-methyl-2-butene over Ti(>2 yielded carbonyl compounds as partial oxidation products (18). However, the selectivity to a particular aldehyde or ketone was reduced by cleavages not only of the double bond but also of the Cg-Cy bond. 

Palmisano et al. [41] in a study on the selectivity of hydroxyl radical in the partial oxidation of different benzene derivatives have investigated how the substituent group affect the distribution of the hydroxylated compounds. The reported results show that the primary photocatalytic oxidation of compounds containing an electron donor group (phenol, phenylamine, etc.) leads to a selective substitution in ortho and para positions of aromatic molecules while in the presence of an electron-withdrawing group (nitrobenzene, benzoic acid, cyanobenzene, etc.) the attack of the OH radicals is nonselective, and a mixture of all the three possible isomers is obtained. 

Since Bahnemann and co-workers have observed that a comparatively high amount of trapped holes are formed when partially platinized titanium dioxide particles are subjected to ultra band gap irradiation (cf. Fig. 7.6), they have chosen this system to study the dynamics of the photocatalytic oxidation of the model compounds dichloroacetate, DCA", and SCN- [7]. To explain their experimental observations these authors have used a model assuming two energetically different types of hole traps (see our detailed discussion above). 

Intermediates in colloidal titania-catalysed photoreactions have been detected, and a study of the heterogeneous and homogeneous photooxidation of various organic compounds in both gas and liquid phases has associated the photocatalytic centre of the catalyst with the Ti-O bond. During light absorption the Ti ion changes its oxidation state and coordination number. The photocatalytic oxidation of ethane has been studied at room temperature, au d in particular the effect on the rate of ethame oxidation of the partial pressure of react ults, the UV light... 

Potentially there are a number of methods for mustard gas detoxification. Among them are nucleophilic substitution of chlorine moieties with hydroxide group, partial oxidation to the corresponding sulfoxide, or deep oxidation of HD. Photocatalytic oxidation over Ti02 bears some resemblance to burning but takes place at ambient temperature. Due to the presence of carbon, hydrogen, chlorine, and sulfur, the final products can be expected to be carbon dioxide, water, hydrogen chloride, and sulfuric acid. 

Pichat, R, Partial or complete heterogeneous photocatalytic oxidation of... 

Picant, P. (1994) Partial or complete heterogeneous photocatalytic oxidation of organic compounds in liquid organic or aqueous phases. Catal Today, 19 (2), 313-333. 

Partial or complete heterogeneous photocatalytic oxidation of neat toluene and 4-picoline in liquid organic oxygenated dispersions containing pure or iron-doped titania photocatalysts. /. Mol Catal A. Chem., 104 (3), 329-339. 

Tsukamoto D, Ikeda M, Shiraishi Y, Kara T, Ichikuni N, Tanaka S, Hirai T (2011) Selective photocatalytic oxidation of alcohols to aldehydes in water by Ti02 partially coated with WO3. ChemEurJ 17 9816-9824... 

Recently, it is reported that Xi02 particles with metal deposition on the surface is more active than pure Ti02 for photocatalytic reactions in aqueous solution because the deposited metal provides reduction sites which in turn increase the efficiency of the transport of photogenerated electrons (e ) in the conduction band to the external sjistem, and decrease the recombination with positive hole (h ) in the balance band of Xi02, i.e., less defects acting as the recombination center[l,2,3]. Xhe catalytic converter contains precious metals, mainly platinum less than 1 wt%, partially, Pd, Re, Rh, etc. on cordierite supporter. Xhus, in this study, solutions leached out from wasted catalytic converter of automobile were used for precious metallization source of the catalyst. Xhe XiOa were prepared with two different methods i.e., hydrothermal method and a sol-gel method. Xhe prepared titanium oxide and commercial P-25 catalyst (Deagussa) were metallized with leached solution from wasted catalytic converter or pure H2PtCl6 solution for modification of photocatalysts. Xhey were characterized by UV-DRS, BEX surface area analyzer, and XRD[4]. 

In this chapter, photoelectrochemical control of size and color of silver nanoparticles, i.e., multicolor photo-chromism [1], is described. Silver nanoparticles are deposited on UV-irradiated Ti02 by photocatal5dic means [2]. Size of the nanoparticles can be roughly controlled in the photocatalytic deposition process. However, it is rather important that this method provides nanoparticles with broadly distributed sizes. The deposited silver nanoparticles are able to be dissolved partially and reduced in size by plasmon-induced photoelectrochemical oxidation in the presence of an appropriate electron acceptor such as oxygen. If a monochromatic visible light is used, only the particles that are resonant with the light are dissolved. That is, size-selective dissolution is possible [3]. This is the principle of the multicolor photochromism. 

When referring to Ti02-based photocatalytic systems it is important to note that, in most cases, the semiconducting oxide is associated there with a noble metal or/and a noble metal oxide catalyst. While the role played by these catalysts in (partial) cathodic reactions seems relatively well understood it remains less clear with regard to the photoanodic reactions. In particular, the exact function of the extensively used ruthenium dioxide catalyst has been questioned The role of Ru02 as a hole-transfer catalyst has, for example, been established through laser-photolysis kinetic studies in the case of photo-oxidation of halide (Br and CP) ions in colloidal titanium dioxide dispersions. In fact, the yields of Brf and ClJ radical anions, photogenerated in the course of these reactions. 

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