Photocatalytic Transformations Research Articles

Improving the Visible Light Photoactivity of In2S3–Graphene Nanocomposite via a Simple Surface Charge Modification Approach

We report an efficient and easily accessible self-assembly route to synthesize In2S3-GR nanocomposites via electrostatic interaction of positively charged In2S3 nanoparticles with negatively charged graphene oxide (GO) followed by a hydrothermal process for reduction of GO to graphene (GR). The as-synthesized In2S3-GR nanocomposites exhibit much higher visible light photocatalytic activity toward selective reduction of nitroaromatic compounds in water than bare In2S3 nanoparticles and In2S3-GR-H that is obtained from the simple "hard" integration of GR nanosheets with solid In2S3 nanoparticles without modification of surface charge. On the basis of the joint characterizations and structure-photoactivity correlation it is disclosed that the enhanced photocatalytic performance of In2S3-GR is mainly ascribed to the more efficient interfacial contact between In2S3 and the GR nanosheets than In2S3-GR-H, which would amplify the use of electron conductivity and mobility of GR to improve the lifetime and transfer of photogenerated charge carriers more efficiently and thus boost the photoactivity more effectively. This work highlights the significant effect of preparation methods on the photoactivity of GR-semiconductor nanocomposites. It is expected that such a simple electrostatic self-assembly strategy could aid to rationally fabricate more efficient GR-semiconductor nanocomposites with improved interfacial contact and photocatalytic performance toward various photocatalytic selective transformations.

Catalytic and Photocatalytic Transformations on Metal Nanoparticles with Targeted Geometric and Plasmonic Properties

Heterogeneous catalysis by metals was among the first enabling technologies that extensively relied on nanoscience. The early intersections of catalysis and nanoscience focused on the synthesis of catalytic materials with high surface to volume ratio. These synthesis strategies mainly involved the impregnation of metal salts on high surface area supports. This would usually yield quasi-spherical nanoparticles capped by low-energy surface facets, typically with closely packed metal atoms. These high density areas often function as the catalytically active surface sites. Unfortunately, strategies to control the functioning surface facet (i.e., the geometry of active sites that performs catalytic turnover) are rare and represent a significant challenge in our ability to fine-tune and optimize the reactive surfaces. Through recent developments in colloidal chemistry, chemists have been able to synthesize metallic nanoparticles of both targeted size and desired shape. This has opened new possibilities for the design of heterogeneous catalytic materials, since metal nanoparticles of different shapes are terminated with different surface facets. By controlling the surface facet exposed to reactants, we can start affecting the chemical transformations taking place on the metal particles and changing the outcome of catalytic processes. Controlling the size and shape of metal nanoparticles also allows us to control the optical properties of these materials. For example, noble metals nanoparticles (Au, Ag, Cu) interact with UV-vis light through an excitation of localized surface plasmon resonance (LSPR), which is highly sensitive to the size and shape of the nanostructures. This excitation is accompanied by the creation of short-lived energetic electrons on the surface of the nanostructure. We showed recently that these energetic electrons could drive photocatalytic transformations on these nanostructures. The photocatalytic, electron-driven processes on metal nanoparticles represent a new family of chemical transformations exhibiting fundamentally different behavior compared with phonon-driven thermal processes, potentially allowing selective bond activation. In this Account, we discuss both the impact of the shape of metal nanoparticles on the outcome of heterogeneous catalytic reactions and the direct, electron-driven photocatalysis on plasmonic metal nanostructures of noble metals. These two phenomena are important examples of taking advantage of physical properties of metal materials that are controlled at nanoscales to affect chemical transformations.

Efficient visible-light-induced hydrogenation over composites of CdS and ruthenium carbonyl complexes

Hybrid systems built by coupling CdS with a series of Ru carbonyl complexes ([Ru(bpy)2(CO)2](PF6)2 (RC-1), [Ru(phen)2(CO)2](PF6)2 (RC-2), [Ru(bpy)(CO)2Cl2] (RC-3), [Ru(Me2bpy)(CO)2Cl2] (RC-4), and [Ru(phen)(CO)2Cl2] (RC-5)) showed activity for the hydrogenation of carbonyl compounds under visible-light irradiations in the presence of appropriate sacrificial agent. The influences of the irradiations mode, sacrificial agent, substrates, and the structure of the Ru complexes on the hydrogenation performance of these composites were investigated. Results indicated that CdS act as the photosensitizer, while the Ru carbonyl complexes as catalytic active component. This study provides some guidance in the development of feasible hybrid photocatalytic systems encompassing visible-light-responsive semiconductors and redox-active transition metal complexes, which are expected to find great potentials in the photocatalytic organic transformations. It is believed that such photocatalytic systems can be built in a more controllable way based on our understanding on the structure–activity relationship of the molecular electro-catalyst.

Visible‐Light‐Induced Photocatalytic Reductive Transformations of Organohalides

Bei Licht betrachtet: Ein durch sichtbares Licht angeregter Iridiumkatalysator überträgt Elektronen von einem Amin auf ein Organohalogenid. Der Elektronentransfer löst daraufhin die reduktive Spaltung der Kohlenstoff-Halogen-Bindung aus und erzeugt das entsprechende Alkyl-, Alkenyl- oder Arylradikal, das Cyclisierungs- und Hydrodehalogenierungsreaktionen eingehen kann. DIPEA=N,N-Diisopropylethylamin.

Visible‐Light‐Induced Photocatalytic Reductive Transformations of Organohalides

A photo opportunity: A visible-light-excited iridium catalyst delivers electrons from an amine to an organohalide. The electron transfer then induces reductive scission of the carbon–halogen bond, generating the corresponding alkyl, alkenyl, and aryl radical that can undergo cyclization and hydrodehalogenation reactions. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

ChemInform Abstract: Radical Carbon—Carbon Bond Formations Enabled by Visible Light Active Photocatalysts

AbstractReview: 30 refs.

Radical Carbon–Carbon Bond Formations Enabled by Visible Light Active Photocatalysts

This mini-review highlights the Stephenson group's contribution to the field of photoredox catalysis with emphasis on carbon-carbon bond formation. The realization of photoredox mediated reductive dehalogenation initiated investigations toward both intra- and intermolecular coupling reactions. These reactions commenced via visible light-mediated reduction of activated halogens to give carbon-centered radicals that were subsequently involved in carbon-carbon bond forming transformations. The developed protocols using Ru and Ir based polypyridyl complexes as photoredox catalysts were further tuned to efficiently catalyze overall redox neutral atom transfer radical addition reactions. Most recently, a simplistic flow reactor technique has been utilized to affect a broad scope of photocatalytic transformations with significant enhancement in reaction efficiency.

Fabrication of coenocytic Pd@CdS nanocomposite as a visible light photocatalyst for selective transformation under mild conditions

Coenocytic Pd@CdS nanocomposite has been successfully prepared via a facile wet chemistry approach. Its structure and properties have been characterized by a series of techniques, including field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), ultra-violet/visible diffuse reflectance spectroscopy (DRS), nitrogen adsorption–desorption and electron spin resonance spectroscopy (ESR). The results demonstrate that the Pd nanoparticles as multiple cores are evenly distributed inside the photoactive CdS shell, forming a coenocytic nanostructure. It is found that the concentration of precursors and the intrinsic nature of noble metal colloids play essential roles in the growth process for such noble metal@semiconductor nanocomposite. Accordingly, a possible formation mechanism for the coenocytic Pd@CdS nanocomposite is proposed. The visible light photocatalytic activity of Pd@CdS has been evaluated by selective oxidation of a range of alcohols using molecular oxygen as oxidant under mild conditions. The results show that the coenocytic Pd@CdS exhibits enhanced photocatalytic activity as compared to blank-CdS obtained by the same procedure in the absence of Pd colloid nanoparticles as seeds. The enhanced photocatalytic performance of the coenocytic Pd@CdS can be ascribed to the coupling interaction of enhanced light absorption intensity, the longer lifetime of photogenerated charge carriers and its favorable adsorptivity. It is expected that our work could provide useful information for fabricating other core-shell nanocomposites and open an avenue to utilizing them in the field of photocatalytic selective organic transformations.

Autoinhibition of photocatalytic reduction of safranin and dichromate ions at anatase

The photocatalytic transformations of safranin and dichromate ion at titanium dioxide involve the formation of reduction products at the first stage. With increase in the concentration of catalyst the rate constant increases at [E] 1 g/L and subsequently reaches a plateau in accordance with the Michaelis-Menten equation. With increase in the concentration of the substrate, however, the rate of the process increases, passes through a maximum, and then decreases. The retardation of the process is due to inhibition by the substrate or by the product of the photocatalytic transformation. The proposed equation predicts a hyperbolic dependence of the rate and the rate constant on [E] and an extremal dependence of these kinetic parameters on the concentration of the substrate.

Doping Metal–Organic Frameworks for Water Oxidation, Carbon Dioxide Reduction, and Organic Photocatalysis

Catalytically competent Ir, Re, and Ru complexes H(2)L(1)-H(2)L(6) with dicarboxylic acid functionalities were incorporated into a highly stable and porous Zr(6)O(4)(OH)(4)(bpdc)(6) (UiO-67, bpdc = para-biphenyldicarboxylic acid) framework using a mix-and-match synthetic strategy. The matching ligand lengths between bpdc and L(1)-L(6) ligands allowed the construction of highly crystalline UiO-67 frameworks (metal-organic frameworks (MOFs) 1-6) that were doped with L(1)-L(6) ligands. MOFs 1-6 were isostructural to the parent UiO-67 framework as shown by powder X-ray diffraction (PXRD) and exhibited high surface areas ranging from 1092 to 1497 m(2)/g. MOFs 1-6 were stable in air up to 400 °C and active catalysts in a range of reactions that are relevant to solar energy utilization. MOFs 1-3 containing [Cp*Ir(III)(dcppy)Cl] (H(2)L(1)), [Cp*Ir(III)(dcbpy)Cl]Cl (H(2)L(2)), and [Ir(III)(dcppy)(2)(H(2)O)(2)]OTf (H(2)L(3)) (where Cp* is pentamethylcyclopentadienyl, dcppy is 2-phenylpyridine-5,4'-dicarboxylic acid, and dcbpy is 2,2'-bipyridine-5,5'-dicarboxylic acid) were effective water oxidation catalysts (WOCs), with turnover frequencies (TOFs) of up to 4.8 h(-1). The [Re(I)(CO)(3)(dcbpy)Cl] (H(2)L(4)) derivatized MOF 4 served as an active catalyst for photocatalytic CO(2) reduction with a total turnover number (TON) of 10.9, three times higher than that of the homogeneous complex H(2)L(4). MOFs 5 and 6 contained phosphorescent [Ir(III)(ppy)(2)(dcbpy)]Cl (H(2)L(5)) and [Ru(II)(bpy)(2)(dcbpy)]Cl(2) (H(2)L(6)) (where ppy is 2-phenylpyridine and bpy is 2,2'-bipyridine) and were used in three photocatalytic organic transformations (aza-Henry reaction, aerobic amine coupling, and aerobic oxidation of thioanisole) with very high activities. The inactivity of the parent UiO-67 framework and the reaction supernatants in catalytic water oxidation, CO(2) reduction, and organic transformations indicate both the molecular origin and heterogeneous nature of these catalytic processes. The stability of the doped UiO-67 catalysts under catalytic conditions was also demonstrated by comparing PXRD patterns before and after catalysis. This work illustrates the potential of combining molecular catalysts and MOF structures in developing highly active heterogeneous catalysts for solar energy utilization.

Water Splitting on Composite Plasmonic-Metal/Semiconductor Photoelectrodes: Evidence for Selective Plasmon-Induced Formation of Charge Carriers near the Semiconductor Surface

A critical factor limiting the rates of photocatalytic reactions, including water splitting, on oxide semiconductors is the high rate of charge-carrier recombination. In this contribution, we demonstrate that this issue can be alleviated significantly by combining a semiconductor photocatalyst with tailored plasmonic-metal nanostructures. Plasmonic nanostructures support the formation of resonant surface plasmons in response to a photon flux, localizing electromagnetic energy close to their surfaces. We present evidence that the interaction of localized electric fields with the neighboring semiconductor allows for the selective formation of electron/hole (e(-)/h(+)) pairs in the near-surface region of the semiconductor. The advantage of the formation of e(-)/h(+) pairs near the semiconductor surface is that these charge carriers are readily separated from each other and easily migrate to the surface, where they can perform photocatalytic transformations.

ChemInform Abstract: Photocatalytic Transformations of Organic Sulfur Compounds and H2S

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 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.

Selective Photocatalytic Transformations on Microporous Titanosilicate ETS-10 Driven by Size and Polarity of Molecules

Photocatalytic activity of microporous titanosilicate ETS-10 has been studied in water. The photoactivated ETS-10 shows catalytic activity driven by size and polarity of substrates. ETS-10 efficiently catalyzes a conversion of substrates with a size larger than the pore diameter of ETS-10. In contrast, the reactivity of small substrates depends strongly on substrate polarity; less polar substrates show higher reactivity on ETS-10. Electron spin resonance analysis reveals that large substrates or less polar substrates scarcely diffuse inside the highly polarized micropores of ETS-10 and, hence, react efficiently with hydroxyl radicals (*OH) formed on titanol (Ti-OH) groups exposed on the external surface of ETS-10. In contrast, small polar substrates diffuse easily inside the micropores of ETS-10 and scarcely react with *OH, resulting in low reactivity. The photocatalytic activity of ETS-10 is successfully applicable to selective transformations of large reactants or less polar reactants to small polar products, enabling highly selective dehalogenation and hydroxylation of aromatics.

LC-MS for identifying photodegradation products of pharmaceuticals in the environment

Abiotic processes, such as direct and indirect photolysis, are of great importance in determining the aquatic fate of pharmaceutical compounds. Photolytic reactions are often complex, involving various competing or parallel pathways and leading to multiple reaction products. The identification of transformation products and the elucidation of photolysis-reaction pathways are of crucial importance in understanding their fate in the aquatic environment, but are complicated and cumbersome. This article focuses on advanced liquid chromatography-mass spectrometry (LC-MS) methods for identifying transformation products of abiotic processes that lead to the transformation of pharmaceutical compounds in environmental systems (direct and indirect photolysis) and photocatalytic transformations in water- and wastewater-treatment systems, using titanium dioxide or the photo-Fenton process. We discuss the capabilities and the limitations of single-MS, tandem-MS and hybrid-MS techniques, and present practical examples (triple-quadrupole-MS analysis of phototransformation products of fluoxetine, ion-trap-MS analysis of photoproducts of carbamazepine, and time-of-flight-MS elucidation of the reaction pathways of photolytic and photocatalytic degradation of diclofenac).

Photocatalytic Destruction of a Thiosulfonate

This paper presents experimental and quantum chemical consideration of photocatalytic and photochemical transformations in acetonitrile of CH3C6H4S(O2)SCH2CH2N(iPr)2 (diisopropylaminoethyl 4-methylbenzenethiosulfonate, DAMT), an imitant of chemical agent VX with close structural properties and ionization energy. Diisopropylaminoethyl disulfide and 4-methylbenzenesulfonic acid were detected as major products of photocatalytic and photochemical degradation. Photocatalytic degradation stopped at about 60% DAMT conversion due to acidification of the system. Photochemical degradation proceeded slower but to a higher conversion since DAMT protonation did not stop photolysis completely. DAMT· + forms at the first step of photocatalytic degradation, then it splits into CH3C6H4SO 2 · and SCH2CH2N+(iPr)2. The former fragment transforms into sulfonic acid and the latter decomposes into SCH2 and CH2N+(iPr)2 in the absence of TiO2, the route observed in mass spectrometry. However it is stable in adsorbed state on the TiO2 surface and after reduction with photogenerated electron recombines to produce the disulfide product. The results suggest that photocatalytic and photolytic detoxification of VX is possible provided conditions are adjusted for its complete conversion.

Photocatalytic transformations of aminopyrimidines on TiO2 in aqueous solution

The photocatalytic degradation in aqueous solution of aminopyrimidines (2-aminopyrimidine, 4-aminopyrimidine, 4-methyl-2-aminopyrimidine, 2,6-dimethoxy-4-aminopyrimidine) on TiO2 has been investigated. Their transformation pathways seem to be closely related to the position of the amino group in the heterocyclic ring; especially the fate of the organic nitrogen is deeply influenced.Both the ratio [NH4+]/[NO3−] and the extent of their formation are connected to the nature of the substituents and the amino group position in the heteroaromatic ring. When the amino group is located in C4, the transformation pathways proceed mainly through the cleavage of the CN bonds, with the formation of oxygenated structures as intermediates and the release of nitrogen mainly as ammonium ions. In this case, the stoichiometric conversion of nitrogen into inorganic ions and the complete mineralization is achieved within few hours of irradiation.On the contrary, when the amino group is held in C2, a lack in both nitrogen and carbon mineralization is observed. It is attributable to the stoichiometric conversion of the NC(NH2)N moiety into guanidine, a very stable compound for which the mineralization is obtained only at long irradiation times (>70h). Interestingly, in this latter case the nitrogen is mainly released as nitrate ions.

Immobilization of TiO2 on pumice stone for the photocatalytic degradation of dyes and dye industry pollutants

An easy method is proposed to immobilize TiO2 for photocatalytic transformations of organic pollutants in aqueous solution. It consists of impregnation of pumice stone pellets with commercially available TiO2. Pumice stone is a soft material, but this disadvantage can be eliminated by fixing pellets on a hard surface (cement or polycarbonate) and using a thin-film fixed bed reactor. Examples of application are given with 3-nitrobenzenesulfonic acid (3-NBSA), Acid Orange-7 (AO-7, a dye) and real wastewaters collected after biological treatment.

Experimental upper bound on phosphate radical production in TiO2 photocatalytic transformations in the presence of phosphate ions

The spin trapping technique using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) was used to investigate the involvement of phosphate radicals in TiO2-photocatalyzed systems. A persistent (HO)2PO2–DMPO˙ adduct (aN = 14.6 G, aHβ = 12.4 G, aHγ = 1.1 G and aP = 0.5 G) was first synthesized by UV photolysis of aqueous peroxodiphosphate solutions, at pH 4.0, in the presence of DMPO. The trapping efficiency of the phosphate radicals, η, determined at [DMPO] = 0.3 mM was η = 35%. In the heterogeneous system, the rate of HO–DMPO˙ aminoxyl radical generation under continuous UV irradiation of TiO2 sols, at [DMPO] = 0.3 mM and pH = 4.0, becomes gradually inhibited increasing [(HO)2PO2−] above 0.1 mM. However, no evidence of (HO)2PO2–DMPO˙ could be obtained in the whole range of conditions explored in this work (0.1 ≤ [DMPO]/mM ≤ 1 mM, 0.1 ≤ [(HO)2PO2−]/mM ≤ 100 mM). Based on the above experiments, we estimated a conservative upper limit for the quantum yield of (HO)2PO2˙ production, Φ = 2.0 × 10−4, in phosphate loaded TiO2 particles. The analysis of this result and its implications for the mechanism of the TiO2 photocatalytic oxidations of organic compounds in the presence of phosphate are discussed.

Photocatalytic transformations of hydrocarbons at the sea water/air interface under solar radiation

The kinetics and mechanisms of photocatalytic oxidation of dodecane and toluene have been considered in pure water and in synthetic sea water on different titanium dioxide powders under simulated solar radiation. Diluted hydrocarbons disappear in pure water in the presence of TiO2 and simulated solar radiation with a half live of ca. 2 h for dodecane (2.9 × 10−4 M) and 5 min for toluene (5.4 × 10−4M), respectively, and are mineralized for more than 95%. Possible mechanisms are presented based on detected intermediates and related systems. In synthetic sea water little changes in degradation rates are observed, probably attributable to modifications of the surface properties of the catalyst. No chlorinated compounds have been detected. Thin films of dodecane and dodecane/toluene mixtures have also been considered in the presence of TiO2 as micrometer powder, dispersed powder and silanized particles with hydrophobic properties. Whereas TiO2 P25 either naked or dispersed in 1-octanol, after initial activity at the water/oil interface sinks, leading to a decrease of efficiency, Aerosil T805 exhibits a sustained activity.

Photocatalytic transformation of aromatic compounds in aqueous zinc oxide suspensions: effect of substrate concentration on the distribution of products

The photocatalytic transformations of 4-methoxybenzyl alcohol and 4-hydroxybenzyl alcohol were studied as a function of the substrate concentration in the presence or absence of a hydroxyl radical scavenger (isopropanol). In the absence of isopropanol, hydroxylated derivatives and aldehydic products are formed. The formation of aldehydic products is clearly favoured at high substrate concentrations. In the presence of isopropanol, the formation of hydroxylated derivatives is suppressed, whereas the formation of aldehydic products is only partially reduced. These results show that hydroxyl radicals and positive holes are both involved in the transformation of these compounds. Direct oxidation by positive holes, yielding the aldehydic products, is the main process at high substrate concentrations. The photocatalytic transformation of 4-hydroxyphenethyl alcohol yields many products. In the presence of isopropanol, only one product is observed. Its formation rate is clearly enhanced in the presence of the hydroxyl radical scavenger.