[Show abstract][Hide abstract] ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate that two large categories of compounds, namely,
polyoxometallates (produced by acid condensation of mainly tungstates
and/or molybdates) and aggregates of various metal oxides (formed by
stirring, sonication etc) have similar properties. Emphasis is
given to the photocatalytic properties of the two systems. The various
steps in the photocatalytic cycles are described and several examples, from
both systems, have been used to demonstrate that their photocatalytic
performance is similar in terms of the overall mechanism of
photodecomposition of organic compounds, the intermediate species involved
and the final photodegradation products (i.e., CO2,
H2O and inorganic anions).
No preview · Article · Jan 2001 · Chemical Society Reviews
[Show abstract][Hide abstract] ABSTRACT: Polyoxometalates (POM), at least PW12O403−, SiW12O404− and W10O324−, are effective photocatalysts for the mineralization of diversified organic pollutants, such as, lindane, cresol, phenol,
chlorophenols and polychlorinated phenols, and chloroacetic acid. Key reactions are the formation of OH radicals, the high
affinity of organic pollutants for POM, and the regeneration of catalyst by dioxygen. The mineralization (i.e. formation of
CO2, H2O and inorganic anions) proceeds via several intermediates resulting from H-atom abstraction, hydroxylation and to a lesser
extent dehalogenation. The breaking of the aromatic ring is followed by the formation of several saturated and unsaturated
organic acids. In all cases so far, ethanoic acid has been detected. The formation of -CH2 and -CH3 groups from aromatic carbons (i.e. -CH groups) suggests that a reductive pathway accompanies the oxidation process. The overall
photobehavior of POM resembles the highly publicized photodecomposition of organic pollutants by TiO2.
[Show abstract][Hide abstract] ABSTRACT: Homogeneous aqueous solutions of organochlorine pesticides and chlorophenols, namely, lindane, hexachlorobenzene and 2,4-dichlorophenol
(2,4DCP), undergo effective photodegradation upon photolysis with UV and near visible light in the presence of a characteristic
polyoxometallate catalyst PW12O40
3−. These substrates remained, practically, intact (lindane, HCB) or underwent minor degradation under similar conditions in
absence of catalyst. The main oxidant appears to be OH radicals formed by the reaction of the excited polyoxometallate with
H2O. The system compares with the widely published TiO2. Chlorohydroquinone (ClHQ), hydroquinone (HQ), chlorobenzoquinone (ClBQ), benzoquinone (BQ), 3,5-dichlorocatechol (3,5DCC)
and 4-chlorocatechol (4CC), among others, were identified as the main aromatic intermediates in the photodegradation of 2,4DCP.
Acetic acid was detected as ring cleavage product. In all cases the final photodegradation leads to complete mineralization
of substrates to CO2 and HCl.
No preview · Article · Dec 1999 · Research on Chemical Intermediates
[Show abstract][Hide abstract] ABSTRACT: The excited state of polyoxometalates, (POM) arising from absorption of light at the O→M charge transfer (CT) band (near-VIS and UV light), is a powerful oxidizing reagent. The oxidizing ability is manifested, mainly, through formation of OH radicals arising from the reaction of the excited POM with adsorbed water. The currently accepted mechanism of H-abstraction as the initial reaction of excited POM with organic substrates (mainly alcohols) is modified by addition of one more step that involves the formation of OH radicals which, as is well known, react with organic substrates, mainly alcohols, by H-abstraction. In view of the formation of OH radicals, and the high oxidizing ability of the excited POM, the photochemical selective oxidation of organic substrates, reported so far, in aqueous solutions, should be considered with reservation. Propan-2-ol is, indeed, selectively oxidized to propanone in the presence of PW12O403-. The selectivity is, however, a kinetic phenomenon rather than a thermodynamic one. Both photoreactions, as is the case with numerous other organic compounds, give, as final products, CO2 and H2O.
No preview · Article · Feb 1999 · Physical Chemistry Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: Photolysis of lindane in aqueous solution, using near-visible and UV light (λ >320 nm) and in the presence of the polyoxometallate PW12O403-, results in its conversion to CO2 and HCl. Initial photodecomposition takes place within a few minutes both in the presence and absence of dioxygen. The effective mineralization in the absence of dioxygen suggests that OH radicals act as the primary oxidant in this case.
No preview · Article · Jun 1997 · Pesticide Science
[Show abstract][Hide abstract] ABSTRACT: Polyoxometalates appear to be effective photocatalysts for a variety of organic pollutants, leading to their decomposition to CO2 and H2O and Cl− in case of chlorinated hydrocarbons. Various aromatic derivatives and chlorinated acetic acids, undergo mineralization upon photolysis with near visible and UV light, in presence of W10O4−32, PW12O3−40, and SiW12O4−40. These catalysts are, at least, as effective as TiO2. The main oxidant appears to be OH radicals formed by the reaction of the excited polyoxometalates with H2O. At the initial stages of photoreaction, dioxygen has a profound effect (up to eighteen times faster) on the photodegradation of chloroacetic acid, whereas, it has minor effect on chlorophenols. On the contrary, at the final stages of photodegradation of chlorophenols, the rates were an order of magnitude faster in presence than in absence of dioxygen. Substitution of WO moiety of SiW12O4−40 for transition metals, namely, [H2OMnIISiW11O39]6− and [H2OCuIISiW11O39]6− reduced, by about an order of magnitude, the effectiveness of the catalyst.
No preview · Article · Dec 1996 · Journal of Molecular Catalysis A Chemical
[Show abstract][Hide abstract] ABSTRACT: Photolysis of homogeneous solutions of polyoxometalates W10O324−, PW12O403−, and SiW12O404− with near visible and UV light in presence of chlorophenols, leads to complete decomposition of substrates to CO2 and HCl. At the initial stages of photooxidation, decomposition rates, in presence and absence of dioxygen, practically complete with each other. On the contrary, at the final stages of decomposition, the rate in presence of dioxygen is an order of magnitude faster than in absence of dioxygen. The main function of dioxygen seems to be the reoxidation (regeneration) of the catalyst. Several hydroxyl intermediates justify the formation of OH radicals. The hydroxyl aromatic derivatives detected are in accordance with the electrophilicity of OH radicals. Hydroxy butanedioic acid and acetic acid were detected in the three monochlorophenols, prior to mineralization.
Preview · Article · Feb 1996 · Journal of Photochemistry and Photobiology A Chemistry
[Show abstract][Hide abstract] ABSTRACT: Homogeneous oxygenated aqueous solutions of chlorophenols, undergo photodegradation (λ > 320 nm) to CO2 and Cl− by polyoxometalates W10O4−32, PW12O3−40, and SiW12O4−40. Mineralization takes place, also, in the absence of dioxygen but the process is more than an order of magnitude slower. In presence of dioxygen the effectiveness of these catalysts compares with TiO2 suspension.
No preview · Article · Sep 1994 · Journal of Molecular Catalysis