On the mechanism of photocatalytic degradation of chlorinated phenols to CO2 and HCl by polyoxometalates

A lacunary tungstomolybdophosphate as an electronic pendulum: The "blue" electron under examination

The photoreduction of a Keggin type lacunary tungstomolybdophosphate, α-(Bu₄N)₄[H₃PW₉Mo₂O₃₉], in acetonitrile, led to the formation of a monoreduced lacunary heteropoly anion, or a one electron reduced "heteropoly blue" species, whereby the added "blue" electron was captured by the molybdenum atoms. The magnetic properties and behavior of the "blue" electron were studied by a modified Evans nuclear magnetic resonance method (small downshift of the ³¹P signal) and variable-temperature electron paramagnetic resonance (g = 1.936 for Mo^V). The intermolecular exchange of the "blue" electron was limited by a geometrical factor, which requires the contact between Mo caps to exchange it between the heteropoly couple. The intramolecular exchange of the "blue" electron between Mo atoms was rather fast (5.3 × 10⁹ s^-1), with a rate of more than six orders of magnitude larger than the intermolecular exchange rate. Density functional theory was used to determine the most prevalent protonation sites in the mixed lacunary isomers with the aim of studying the intramolecular electron transfer pathway in the isolated [H₄PW₉Mo₂O₃₉]^4- species. The singly occupied molecular orbital (SOMO) is essentially localized in one of the two nonequivalent molybdenum sites. The kinetics of the intramolecular electron exchange equilibrium Mo^V + Mo^VI → Mo^VI + Mo^V between the two molybdenum atoms bridged by an oxygen atom was found to be fast in agreement with the experimental result. The transition state is of mixed-valence type, with the SOMO delocalized over the Mo-O-Mo group. Spectroscopic parameters were found to be in fair agreement with experimental results.

Self-Assembly Hydrothermal Synthesis of Silverton-Type Polyoxometalate-Based Photocatalysts for Enhanced Degradation

The synthesis of some complex polyoxometalates (POMs) is critical to develop potential photocatalysts with high catalytic activity and selectivity. Here, we address this challenge by a hydrothermal self-assembly route to obtain a novel POM-based Co₄W₆O₂₁(OH)₂·4H₂O with a hierarchical microsphere structure. The Co₄W₆O₂₁(OH)₂·4H₂O crystallizes in the cubic space group Im3̅ with cell parameters: a = b = c = 12.878 Å, α = β = γ = 90°, and Z = 4. The structure is further characterized by X-ray diffraction, X-ray photoelectron spectroscopy, UV-vis spectroscopy, thermogravimetric analysis, and Fourier transform infrared spectra. After depositing Ag₂O nanoparticles on the 3D Co₄W₆O₂₁(OH)₂·4H₂O microsphere by photochemical synthesis, the Co₄W₆O₂₁(OH)₂·4H₂O/Ag₂O heterojunction presents enhanced photocatalytic activity for RhB compared with P25 and pristine Ag₂O. Moreover, we confirm the key role of holes for the Co₄W₆O₂₁(OH)₂·4H₂O/Ag₂O and put forward a possible mechanism for the photocatalytic degradation reaction.

Polyoxometalates in photocatalysis

Recent developments in polyoxometalate photochemistry are discussed with a focus on visible light driven productive chemical reactions. Special attention is given to the fundamental photochemistry of polyoxometalates and the effects on the resulting photoprocesses.

Byproducts of aqueous chlorination of equol and their estrogenic potencies

While the phytoestrogen metabolite equol has been reported to exist in surface water, its behavior in drinking water treatment plants remains unrevealed. In this study, eight products including four chlorinated equols (monochloro-equol, dichloro-equol, trichloro-equol, and tetrachloro-equol) were identified in an aqueous chlorinated equol solution by UHPLC-quadrupole-orbitrap-HRMS. Two main pathways of chlorination reaction are proposed: (1) chlorine-substitution reactions on the aromatic ring and subsequent dehydration to form the chlorine-substituted equols, and (2) break-up of the heterocyclic ring with oxygen followed by oxidation of aldehyde to carboxyl. The human estrogen receptor (hER) activating activity for monochloro-equol (EC₅₀ = 3456 nM) and dichloro-equol (EC₅₀ = 2456 nM) were slightly stronger than that of equol (EC₅₀ = 3889 nM). This is the first report on the behavior of equol in drinking water chlorination, which provided an important information on the risk assessment of equol in drinking water.

Abatement of 2,4-D by H2O2 solar photolysis and solar photo-Fenton-like process with minute Fe(III) concentrations

The Photo-Fenton-like (PF-like) process with minute Fe(III) concentrations and the Hydrogen Peroxide Photolysis (HPP), using Xe-lamp or solar light as sources of irradiation, were efficiently applied to eliminate the herbicide 2,4-D from water. PF-like experiments concerning ferric and H₂O₂ concentrations of 0.6 mg L^-1 and 20 mg L^-1 respectively, using Xenon lamps (Xe-lamps) as a source of irradiation and 2,4-D concentrations of 10 mg L^-1 at pH 3.6, exhibited complete 2,4-D degradation and 77% dissolved organic carbon (DOC) removal after 30 min and 6 h of irradiation respectively whereas HPP (in absence of ferric ions) experiments showed a 2,4-D reduction and DOC removal of 90% and 7% respectively after 6 h of irradiation. At pH 7.0, HPP process achieved a 2,4-D abatement of approximately 75% and a DOC removal of 4% after 6 h. PF-like exhibited slightly improved 2,4-D and DOC removals (80% and 12% respectively) after the same irradiation time probably due to the low pH reduction (from 7.0 to 5.6). Several chlorinated-aromatic intermediates were identified by HPLC-MS. These by-products were efficiently removed by PF at pH 3.6, whereas at neutral PF-like and acid or neutral HPP, they were not efficiently degraded. With natural solar light irradiation, 10 and 1 mg L^-1 of 2,4-D were abated using minor H₂O₂ concentrations (3, 6, 10 and 20 mg L^-1) and iron at 0.6 mg L^-1 in Milli-Q water. Similar results to Xe-lamp experiments were obtained, where solar UV-B + A light H₂O₂ photolysis (HPSP) and solar photo-Fenton-like (SPF-like) played an important role and even at low H₂O₂ and ferric concentrations of 3 and 0.6 mg L^-1 respectively, 2,4-D was efficiently removed at pH 3.6. Simulated surface water at pH 3.6 containing 1 mg L^-1 2,4-D, 20 mg L^-1 H₂O₂ and 0.6 mg L^-1 Fe(III) under natural sunlight irradiation efficiently removed the herbicide and its main metabolite 2,4-DCP after 30 min of treatment while at neutral pH, 40% of herbicide degradation was achieved. In the case of very low iron concentrations (0.05 mg L^-1) at acid pH, 150 min of solar treatment was required to remove 2,4-D.

Capacity and recycling of polyoxometalate applied in As(III) oxidation by Fe(II)-Amended zero-valent aluminum

Arsenic remediation is often initiated by oxidizing As(III) to As(V) to alleviate its toxicity and mobility. Due to the easy availability, zero-valent Al (ZVAl) like Al can was considered as potential alternatives to facilitate As(III) oxidation. This study determined the capability and recycling of polyoxometalate (POM) to catalyze As(III) oxidation in Fe(II)-amended ZVAl systems. POM acquired electrons from ZVAl more effectively at pH 1 than at pH 2. While 76% of the reduced POM [POM(e^-)] reacted with O_2(g) to generate H₂O₂ at pH 1, only 60% of POM(e^-) was used to produce H₂O₂ at pH 2. The remaining POM(e^-) was oxidized by the generated H₂O₂. Such additional consumption of POM(e^-) and H₂O₂ led to the incomplete As(III) oxidation in the system without residual ZVAl and emphasized the need for a continuous electron supply from ZVAl to compensate the depletion of POM(e^-). After the hydrolyzation at pH 6.0, the XANES data evidenced that not only As(V) but WO₄ released from the POM retained on surfaces of Al/Fe hydroxides. The competition for sorption sites on Al/Fe hydroxides between As(V) and WO₄ led to the incomplete As removal. Despite the loss of WO₄, the POM re-polymerized at pH 1 still showed the comparable capability to catalyze As(III) oxidation with original POM. This study revealed electron transfer pathways from ZVAl to As(III) as catalyzed by POM and evidenced the effective POM recycling after As removal, which lowers the cost of POM application and turns the ZVAl/Fe(II)/POM/O₂ system into a practical strategy for As remediation.

Titania hollow spheres modified with tungstophosphoric acid with enhanced visible light absorption for the photodegradation of 4-chlorophenol

Titania hollow spheres were synthesized using silica nanospheres as the template. The core was removed using NaOH solution. They were subsequently impregnated with tungstophosphoric acid (TPA) solutions and annealed at two different temperatures (100 and 500 °C). These materials were characterized by several physicochemical techniques (XRD, BET, SEM, DRS, FT-IR, FT-Raman and ³¹P MAS-NMR). The ³¹P MAS-NMR and FT-IR characterization showed that the main species present in the samples was the [PW₁₂O₄₀]^3- anion, which was partially transformed into the [P₂W₂₁O₇₁]^6- anion during the synthesis and drying step. ³¹P MAS-NMR, and FT-Raman characterization revealed the evidence of a strong interaction between the Keggin anion of TPA and TiO₂ surfaces, possibly due to the formation of surface heteropolyacid-TiO₂ complexes. The DRS results showed that the absorption threshold onset continuously shifted to the visible region with increased TPA concentration and calcination at 500 °C. The enhanced visible light absorption could be related to the formation of a surface complex TPA Keggin anion-TiO₂. The catalytic activity of the materials in the photodegradation of 4-chlorophenol under UV and visible light irradiation increased when the TPA content and the calcination temperature of the samples were raised.

Phosphomolybdic acid immobilized on graphite as an environmental photoelectrocatalyst

A new phosphomolybdic acid (PMA)/Graphite surface was prepared based on electrostatic interactions between phosphomolybdic acid and graphite surface. The PMA/Graphite was characterized by cyclic voltammetry (CV) analysis and scanning electron microscope (SEM). SEM images showed that the phosphomolybdic acid particles were well stabilized on the graphite surface and they were evidenced the size of particles (approximately 10 nm). The CV results not only showed that the modified surface has good electrochemical activity toward the removal of the dyestuff, but also exhibits long term stability. The PMA/Graphite was used as a photoanode for decolorization of Reactive Yellow 39 by photoelectrocatalytic system under UV irradiation. The effects of parameters such as the amount of phosphomolybdic acid used in preparation of PMA/Graphite surface, applied potential on anode electrode and solution pH were studied by response surface methodology. The optimum conditions were obtained as follows: dye solution pH 3, 1.5 g of immobilized PMA on graphite surface and applied potential on anode electrode 1 V. Under optimum conditions after 90 min of reaction time, the decolorization efficiency was 95%.

Two cationic [(CuxIy)x−y]n motif based coordination polymers and their photocatalytic properties

Two cationic [(Cu_xI_y)^x−y]_n based coordination polymers, featuring a 3,6-connected net or a coordination chain, and interesting supramolecular structures, exhibit photochemistry properties towards degradation of methyl blue.

A series of Anderson-type polyoxometalate-based metal–organic complexes: their pH-dependent electrochemical behaviour, and as electrocatalysts and photocatalysts

Seven novel polyoxometalate-based metal–organic complexes are synthesized and structurally characterized, exhibiting high electrochemical sensitivity to pH and photocatalytic activities for the degradation of organic dyes under UV irradiation.

Polyoxometalate-based layered nano-tubular arrays: facile fabrication and superior performance for catalysis

Keggin-type polyoxoanion PW₁₂O₄₀³⁻-containing one-dimensional nano-tubular arrays fabricated within porous templates show a superior performance just through simple filtrating processes.

Construction of POMOFs with different degrees of interpenetration and the same topology

Three POMOFs with different degrees of interpenetration are reported and the influence of the organic molecules on the degree of interpenetration of the compounds is discussed.

Harvesting visible light for aerobic oxidation of alcohols by a novel and efficient hybrid polyoxometalate

The synthesis, characterization and photocatalytic application of a novel hybrid polyoxometalate, ((n-C₄H₉)₄N)₄[SiW₁₁O₃₉(Si(CH₂)₃NCHC₁₄H₉)₂O], which can absorb light in the visible region, in the oxidation of alcohols under an O₂ atmosphere are reported.

Photocatalytic degradation of C. I. Reactive Red 24 solution with K₆SiW₁₁O₃₉Sn(II.)

Environmental friendly materials, K6SiW11O39Sn (SiWSn), was synthesized. SiWSn photocatalytic decomposition of C. I. Reactive Red 24 (RR24) with the UV-lamp (253.7 nm, 20 W), Xenon lamp filtered less than 390 nm light (500 W) and sun light was investigated. The results showed that RR24 solution could be effectively decolorized with the SiWSn photocatalyst. The photocatalytic degradation efficiency of RR24 with SiWSn was affected by the initial concentration of RR2 solution, the amount of SiWSn and the photolysis time. It is demonstrated that the process of photodegradation of RR24 with SiWSn is a pesudo first-order reaction, which can be described by Langmuir-Hinshelwood equation. Hydroxyl radicals and holes are both the main oxidants in the photocatalytic reaction of RR24 with SiWSn.

Comparative Study of Homogeneous and Heterogeneous Photocatalytic Redox Reactions: PW(12)O(40)(3-) vs TiO(2)

Polyoxometalates (POMs) as a homogeneous photocatalyst and semiconductor oxide as a heterogeneous photocatalyst share many aspects of similarity in their operating mechanisms. This study systematically compares various photocatalytic oxidation and reduction reactions of PW12O403- (a POM) and TiO2 in water to demonstrate that the two photocatalysts are very different in many ways. Both POM and TiO2 can photooxidize various organic compounds with comparable rates, but the POM-mediated mineralization is markedly slower than the mineralization with TiO2 under the experimental conditions employed in this study. Kinetic studies using tert-butyl alcohol as an OH radical scavenger suggest that OH radicals are the sole dominant photooxidant in POM-mediated degradations regardless of the kind of substrates tested, whereas both OH radicals and direct hole transfers take part in TiO2 photocatalysis. POM immobilization on silica support and surface fluorination of TiO2 significantly modified the kinetics and intermediate distribution. POM-mediated photoreductive dechlorination of CCl4 and trichloroacetate was negligible, whereas the dechlorination with TiO2 was markedly faster. The rate of electron transfer from POM- to reducible substrates seems to be significantly slower than the rate of conduction band electron transfer on TiO2 mainly due to the strong electron affinity of POM. The effects of H2O2 addition on photocatalytic reactivity are also very different between POM and TiO2. Detailed kinetic and mechanistic comparisons between PW12O403- and TiO2 photocatalysts are presented and discussed to understand the similarities and differences.

Efficient degradation of methabenzthiazuron photoinduced by decatungstate anion in water: kinetics and mechanistic studies

This study concerns the elimination of methabenzthiazuron (MBTU) photocatalysed by sodium decatungstate salts W10O32(4-)·(DTA) in aqueous solution under irradiation at 365 nm. Ninety percentage of MBTU (10(-4) M) is mineralised in the presence of the photocatalyst (2×10(-4) M) after 7 d under exposure and the formation of nitrate, sulphate and ammonium confirmed this phenomenon. In aerated conditions, the photodegradation rate of MBTU clearly increased in the presence of DTA by a factor of 40 when compared to direct photolysis with ΦMBTU=2.5×10(-2) and t1/2 (MBTU)=1.4 h. Oxygen appeared essential since 2 times inhibition of MBTU disappearance and the photocatalytic cycle interrupt were observed in the absence of oxygen. The degradation mechanism has been elucidated through the photoproducts identification by LC-ESI-MS analysis. Two processes were implied in the degradation: electron transfer and H atom abstraction reactions both involving W10O32(4-∗) excited state species. In the primary steps of the degradation, the aromatic ring hydroxylation was observed by electron transfer leading to OH-MBTU isomers and H atom abstraction reaction gave benzthiazuron and a supposed demethylated product. Secondary oxidations permitted the hydroxylation of both products.

Photocatalytic mineralization of hydroperfluorocarboxylic acids with heteropolyacid H₄SiW₁₂O₄₀ in water

The decomposition of hydroperfluorocarboxylic acids [H-PFCAs; HC(n)F(2)(n)COOH (n=4 and 6)] induced by heteropolyacid photocatalyst H(4)SiW(12)O(40) in water was investigated, and the results are compared with the results for the corresponding perfluorocarboxylic acids (PFCAs; C(n)F(2)(n)(+1)COOH). This is the first report on the photochemical decomposition of H-PFCAs, which are being developed as alternative surfactants to environmentally persistent and bioaccumulative PFCAs. H-PFCAs were not decomposed by irradiation with UV-Visible light (>290 nm) in the absence of H(4)SiW(12)O(40). In contrast, UV-Visible light irradiation of H-PFCAs in the presence of H(4)SiW(12)O(40) efficiently decomposed H-PFCAs to F(-) and CO(2). The decomposition reactions showed pseudo-first-order kinetics, and the decomposition rate constants were 1.8-2.5 times higher than those for the corresponding PFCAs. The reaction mechanism can be explained by elimination of H(+) from the ω-H atom of the H-PFCAs by the excited catalyst, followed by formation of perfluorodicarboxylic acids.

Chromate reduction by zero-valent Al metal as catalyzed by polyoxometalate

In spite of a high reduction potential of zero-valent Al (ZVAl), its ability to reduce Cr(VI), a widespread pollutant, to less toxic Cr(III) remains to be uncovered. In the present study, Cr(VI) reduction by ZVAl was conducted to evaluate the potential application of Al as a reductant for Cr(VI). Polyoxometalate (POM, HNa(2)PW(12)O(40)), a catalyst, was used to accelerate Cr(VI) reduction by Al. The reaction of 0.192mM Cr(VI) on ZVAl was investigated in the presence of N(2) or O(2) at pH 1. A slight decrease in Cr(VI) concentration was observed on as-received (uncleaned) ZVAl due to the presence of oxide layer with a low surface area (ca. 3.4x10(-3)m(2)/g) of ZVAl. On addition of 0.1mM POM, Cr(VI) reduction on uncleaned ZVAl increased significantly. This is attributed to the unique properties of POM, which has a Brphinsted acidity higher than usual inorganic acids such as H(2)SO(4) and HCl. Thus, POM could remove rapidly the oxidize layer on ZVAl, followed by acting as a shuttle for electron transfer from ZVAl to Cr(VI). Under a N(2) atmosphere, one- or two-electron reduction of POM by ZVAl was responsible for Cr(VI) reduction in the early stage of the reaction. However, during reaction with ZVAl over 120min, three-electron reduction of POM predominated over Cr(VI) reduction. On interaction of O(2) with reduced POM, the formation of H(2)O(2) was responsible for subsequent Cr(VI) reduction. The results suggest that POM is an efficient catalyst for Cr(VI) reduction by Al due to the extremely rapid consumption of reduced POM or H(2)O(2) by Cr(VI).