Wanhong Ma

Semiconductor-mediated photodegradation of pollutants under visible-light irradiation

The semiconductor and the surface-adsorbed antenna molecule (dyes or other color species) can constitute efficient photochemical systems for environmental remediation. The major advantage of these systems is that they are able to achieve the degradation of organic pollutants by using visible light from the sun as energy and O(2) in the air as the oxidant under ambient conditions. In this tutorial review, the unique mechanistic characteristics, the constitution of photodegradation systems and their performance are described. The involved radical reactions during the degradation are also discussed.

Change of Adsorption Modes of Dyes on Fluorinated TiO2 and Its Effect on Photocatalytic Degradation of Dyes under Visible Irradiation

Surface-fluorinated TiO2 (F-TiO2) particles were prepared via the HF etching method. The surface characteristics of fluorinated TiO2, the adsorption modes of dyes, and the reaction pathways for the photocatalytic degradation of dye pollutants under visible light irradiation were investigated. It was found that, in the treatment of TiO2 by HF etching, F(-) not only displaces surface HO(-) but also substitutes some surface lattice oxygen. Using zwitterionic Rhodamine B (RhB) dye as a model, the change of the adsorption mode of RhB on F-TiO2 relative to that on pure TiO2 was validated by adsorption isotherms, X-ray photoelectron spectroscopy (XPS), and IR techniques for the first time. RhB preferentially anchors on pure TiO2 through the carboxylic (-COOH) group, while its adsorption group is switched to the cationic moiety (-NEt 2 group) on F-TiO2. Both the photocatalytic degradation kinetics and mechanisms were drastically changed after surface fluorination. Dyes with positively charged nitrogen-alkyl groups such as methylene blue (MB), malachite green (MG), Rhodamine 6G (Rh6G), and RhB all underwent a rapid N-dealkylation process on F-TiO2, while on pure TiO2 direct cleavage of dye chromophore ring structures predominated. The relationship between surface fluorination and the degradation rate/pathway of dyes under visible irradiation was also discussed in terms of the effect of fluorination on the surface adsorption of dyes and on the energy band structure of TiO2.

Selective Formation of Imines by Aerobic Photocatalytic Oxidation of Amines on TiO2

Imine derivatives are important building blocks for the synthesis of fine chemicals and pharmaceuticals. Generally, their synthesis involves condensation of an amine and a carbonyl compound. To circumvent the problem caused by the excessively active nature of ketones or aldehydes, an alternative strategy for the direct oxidation of amines has attracted much interest. Dioxygen, or more preferably dioxygen in the air, embodies the quintessential oxidant for chemical synthesis. Unfortunately, the reactivity of dioxygen is difficult to control; it typically reacts under harsh conditions with poor selectivity. Recently, much attention has been paid to organic synthesis photocatalyzed by TiO2. [3] In H2O, the selectivity of TiO2 photocatalysis is especially poor, so that, it is efficient in the degradation of organic pollutants. Even when the reactions are performed in inert organic solvent to prevent generation of OH radicals, the selectivity is usually very low owing to the unselective autooxidation of the photogenerated radicals. However, it has been shown that high selectivity can be realized in the TiO2 photocatalytic system when both generation of the OH radical and unselective autooxidation are avoided. By using O-labeling experiments, our research group has discovered that an oxygen-atom transfer dominates the photocatalytic transformation of alcohols into the corresponding carbonyl compounds on TiO2 in benzotrifluoride. This transformation has a different mechanism than that of noble-metal/transition-metal complex catalysis. All the experimental results support a mechanism as shown in Equation (1). Such a mechanism avoids the unselective autooxidation processes, which commonly occur in TiO2 photocatalysis in the presence of dioxygen, and hence should lead to high selectivity in the photocatalytic oxidation of alcohols. This mechanistic insight can direct us to envision new reactions. Herein, we report our findings on a series of benzylic amines that were selectively transformed into the corresponding imines using 1 atm of air as the oxidant by TiO2 photocatalysis in acetonitrile. The formation of imines involves a two-step process: a selective oxygenation step to generate the aldehydes and a subsequent condensation step to afford the imines. The high selectivity for the formation of imines is attributed to both the highly selective formation of aldehydes from amines [Eq. (2)] via a similar mechanism to

Oxygen atom transfer in the photocatalytic oxidation of alcohols by TiO2: oxygen isotope studies.

The selective oxidation of alcohols into carbonyl compounds using dioxygen in lieu of toxic or corrosive stoichiometric oxidants such as ClO , Cr, and Cl2, is one of the most challenging functional group transformations. The oxidation of alcohols using dioxygen as the oxidant has been successfully realized by using noble-metal and transitionmetal complexes for catalysis. TiO2 photocatalysis has also attracted much attention as a potential and promising strategy for this aim, because of its high oxidation ability, environmentally friendly properties, and the benefit of using O2 as an oxidant and light as the driving force. A few successful cases involving TiO2 photocatalysis in acetonitrile, water, or solvent-free systems have recently been reported. Molecular oxygen plays a vital role in the aerobic oxidation of alcohols in all these systems. Therefore, it is significant and necessary to reveal how the dioxygen participates in the reaction process. In the noble-metal catalysis system, the role of dioxygen has been proven to oxidize the reduced noble-metal center (for example, M or M hydride species) without an O-atom transfer from dioxygen to the products. In the aerobic oxidation of alcohols in the cytochrome P450 system, a gem-diol intermediate is formed, in which one hydroxyl group comes from the alcohol substrate (ca. 100 % O abundance) and the other from O2 (98% O labeled). Such a gem-diol intermediate leads to approximately 50% of the carbonyl product having incoorporated O atoms. Unlike these thermal catalytic systems, the essential role of dioxygen in the oxidation of alcohols by TiO2 photocatalysis has not been completely clarified yet. Herein we disclose an unexpected phenomenon: when the photocatalytically oxidative transformation of isotopelabeled alcohols was performed over pure anatase TiO2 in organic solvents, such as benzotrifluoride (BTF), the oxygen atom in the substrate alcohol is completely replaced by one of the oxygen atoms of dioxygen, that is, the photocatalytic process involves a selective cleavage of the C O bond of the alcohol with concomitant formation of a new C=O bond in the product aldehyde in which the O atom comes from dioxygen. This finding adds fundamental insight to the oxidation process of alcohols on the TiO2 surface, which is of importance for both the TiO2 photocatalysis and the selective oxidation of alcohols. O-enriched benzyl alcohol and cyclohexanol were used for the TiO2 photocatalytic oxidation (Table 1). The original abundance of O in the O-enriched benzyl alcohol was 65% (Table 1, entries 1–2) and 90% (Table 1, entries 4–7), respec-

Selective Aerobic Oxidation Mediated by TiO2 Photocatalysis

TiO2 is one of the most studied metal oxide photocatalysts and has unparal-leled efficiency and stability. This cheap, abundant, and non-toxic material has the potential to address future environmental and energy concerns. Understanding about the photoinduced interfacial redox events on TiO2 could have profound effect on the degradation of organic pollutants, splitting of H2O into H2 and O2, and selective redox organic transformations. Scientists traditionally accept that for a semiconductor photocatalyst such as TiO2 under the illumination of light with energy larger than its band gap, two photocarriers will be created to carry out their independent reduction and oxidation processes. However, our recent discoveries indicate that it is the concerted rather than independent effect of both photocarriers of valence band hole (hvb(+)) and conduction band electron (ecb(-)) that dictate the product formation during interfacial oxidation event mediated by TiO2 photocatalysis. In this Account, we describe our recent findings on the selective oxidation of organic substrates with O2 mediated by TiO2 photocatalysis. The transfer of O-atoms from O2 to the corresponding products dominates the selective oxidation of alcohols, amines, and alkanes mediated by TiO2 photocatalysis. We ascribe this to the concerted effect of both hvb(+) and ecb(-) of TiO2 in contribution to the oxidation products. These findings imply that O2 plays a unique role in its transfer into the products rather than independent role of ecb(-) scavenger. More importantly, ecb(-) plays a crucial role to ensure the high selectivity for the oxygenation of organic substrates. We can also use the half reactions such as those of the conduction band electron of TiO2 for efficient oxidation reactions with O2. To this end, efficient selective oxidation of organic substrates such as alcohols, amines, and aromatic alkanes with O2 mediated by TiO2 photocatalysis under visible light irradiation has been achieved. In summary, the concerted effect of hvb(+) and ecb(-) to implement one oxidation event could pave the way for selective oxofunctionalization of organic substrates with O2 by metal oxide photocatalysis. Furthermore, it could also deepen our understanding on the role of O2 and the elusive nature of oxygen species at the interface of TiO2, which, in turn, could shed new light on avant-garde photocatalytic selective redox processes in addressing the energy and environmental challenges of the future.

Photooxidation of azo dye in aqueous dispersions of H2O2/α-FeOOH

Abstract The photodegradation of an azo dye, Mordant Yellow 10 (MY10), in aqueous dispersions of goethite (α-FeOOH)/H2O2 at neutral pHs under UV irradiation was studied in comparison with the dark reaction. It was found that UV irradiation could significantly accelerate the degradation of MY10. This provides an alternative approach to the treatment of non-biodegradable azo dye pollutants. The degradation process were investigated by infrared (IR), 1 H NMR, total organic carbon (TOC) and gas chromatography–mass spectroscopy (GC–MS) analysis. Evidence for enhancement of OH radical generation by UV irradiation was obtained using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) spin-trapping EPR spectroscopy. A possible degradation mechanism of MY10 by α-FeOOH/H2O2 under UV irradiation was proposed, which involves the photolysis of the surface complex of H2O2 with the oxide surface metal centers to produce OH radicals, leading to the decomposition of organic compounds.

Visible‐Light‐Induced Selective Photocatalytic Aerobic Oxidation of Amines into Imines on TiO2

Imines are important intermediates for the synthesis of fine chemicals, pharmaceuticals, and agricultural chemicals. Selective oxidation of amines into their corresponding imines with dioxygen is one of the most-fundamental chemical transformations. Herein, we report the oxidation of a series of benzylic amines into their corresponding imines with atmospheric dioxygen as the oxidant on a surface of anatase TiO(2) under visible-light irradiation (λ>420 nm). The visible-light response of this system was caused by the formation of a surface complex through the adsorption of a benzylic amine onto the surface of TiO(2). From the analysis of products of specially designed benzylic amines, we demonstrated that a highly selective oxygenation reaction proceeds via an oxygen-transfer mechanism to afford the corresponding carbonyl compound, whose further condensation with an amine would generate the final imine product. We found that when primary benzylic amines (13 examples), were chosen as the substrates, moderate to excellent selectivities for the imine products were achieved (ca. 38-94%) in moderate to excellent conversion rates (ca. 44-95%). When secondary benzylic amines (15 examples) were chosen as the substrates, both the corresponding imines and aldehydes were detected as the main products with moderate to high conversion rates (ca. 18-100%) and lower selectivities for the imine products (ca. 14-69%). When tribenzylamine was chosen as the substrate, imine (27%), dibenzylamine (24%), and benzaldehyde products (39%) were obtained in a conversion of 50%. This report can be viewed as a prototypical system for the activation of C-H bonds adjacent to heteroatoms such as N, O, and S atoms, and oxofuctionalization with air or dioxygen as the terminal oxidant under visible-light irradiation using TiO(2) as the photocatalyst.

Template-Free Synthesis of Cube-like Ag/AgCl Nanostructures via a Direct-Precipitation Protocol: Highly Efficient Sunlight-Driven Plasmonic Photocatalysts

In this paper, we report that cube-like Ag/AgCl nanostructures could be facilely fabricated in a one-pot manner through a direct-precipitation protocol under ambient conditions, wherein no additional issues such as external energy (e.g., high temperature or high pressure), surfactants, or reducing agents are required. In terms of using sodium chloride (NaCl) as chlorine source and silver acetate (CH₃COOAg) as silver source, it is disclosed that simply by adding an aqueous solution of NaCl into an aqueous solution of CH₃COOAg, Ag/AgCl nanostructures with a cube-like geometry, could be successfully formulated. We show that thus-formulated cube-like Ag/AgCl nanospecies could be used as high-performance yet durable visible-light-driven or sunlight-driven plasmonic photocatalysts for the photodegradation of methyl orange (MO) and 4-chlorophenol (4-CP) pollutants. Compared with the commercially available P25-TiO₂, and the Ag/AgCl nanospheres previously fabricated via a surfactant-assisted method, our current cube-like Ag/AgCl nanostructures could exhibit much higher photocatalytic performance. Our template free protocol might open up new and varied opportunities for an easy synthesis of cube-like Ag/AgCl-based high-performance sunlight-driven plasmonic photocatalysts for organic pollutant elimination.

Nonmetal P-doped hematite photoanode with enhanced electron mobility and high water oxidation activity

Nonmetal P-doped hematite photoanodes synthesized by a simple impregnation method during the hydrothermal preparation showed remarkable photocurrent densities of 2.3 mA cm−2 and 2.7 mA cm−2 at 1.23 VRHE, for electrodes annealed at 650 °C and 750 °C, respectively. After “Co–Pi” cocatalyst modification, a photocurrent density of over 3 mA cm−2 at 1.23 VRHE could be achieved. The high activity can be attributed to enhanced electron mobility, as verified by DFT + U calculations, electrochemical impedance spectroscopy and structural characterizations.

The Surface‐Structure Sensitivity of Dioxygen Activation in the Anatase‐Photocatalyzed Oxidation Reaction

The reaction pathways by which oxygen is incorporated into the substrate in the photocatalytic oxidation of terephthalic acid (TPTA) are vastly different on {001} and {101} facets of an anatase single crystal. This was established by controlling the percentage of {101} and {001} facets, isotopically tracing the origins of the hydroxy group, and studying dioxygen consumption and variance in the concentration of hydroxylation intermediate.