Selective Photocatalytic Oxidation of Alcohols to Aldehydes in Water by TiO2 Partially Coated with WO3

Direct Photocatalytic Methane Oxidation to Formaldehyde by N Doping Co-Decorated Mixed Crystal TiO2

In this paper, N-doped TiO2 mixed crystals are prepared via direct calcination of TiN for highly selective oxidation of CH4 to HCHO at room temperature. The structures of the prepared TiO2 samples are characterized to be N-doped TiO2 of anatase and rutile mixed crystals. The crystal structures of TiO2 samples are determined by XRD spectra and Raman spectra, while N doping is demonstrated by TEM mapping, ONH inorganic element analysis, and high-resolution XPS results. Significantly, the production rate of HCHO is as high as 23.5 mmol·g-1·h-1 with a selectivity over 90%. Mechanism studies reveal that H2O is the main oxygen source and acts through the formation of ·OH. DFT calculations indicate that the construction of a mixed crystal structure and N-doping modification mainly act by increasing the adsorption capacity of H2O. An efficient photocatalyst was prepared by us to convert CH4 to HCHO with high yield and selectivity, greatly promoting the development of the photocatalytic CH4 conversion study.

Acetal photocatalytic formation from ethanol in the presence of TiO2 rutile and anatase

The decomposition of ethanol, one of the most important biomass platform molecules, was investigated under green conditions, ambient temperature, atmospheric pressure and air for the synthesis of acetal in the presence of TiO₂ activated under UV-A radiation. The impact of ethanol concentration, of the nature of TiO₂ (rutile, anatase or mixture), of the photo-deposition of Pt under air or argon were all factors under investigation. Whatever the conditions and the nature of catalyst used, acetaldehyde was initially formed before reacting with ethanol to form acetal, a promising fuel additive. However, the subsequent generation of acetal differs depending on the conditions and the nature of catalyst. In the absence of a noble metal, rutile TiO₂ leads to an increase in acetal formation at equivalent acetaldehyde formation. This behavior is discussed considering the acidic and basic properties of rutile and anatase phases together with H⁺ generated under UV. In the presence of Pt, under air or Ar, the acetal formation begins at a lower concentration of acetaldehyde due to the in-situ photo-deposition of Pt. However, whereas acetal formation is similar for Pt/anatase and Pt/rutile phase under air, under Ar, less acetal is generated on Pt/rutile in agreement with the production of more H₂.

An overview on recent development in visible light-mediated organic synthesis over heterogeneous photo-nanocatalysts

The implementation of heterogeneous photo-nanocatalysts in organic syntheses has been investigated greatly in the last decade as a result of the increasing demand to achieve the organic reactions via the use of green approaches and through the availability of visible light source. Herein, the presented results describe the basic concepts and state-of-the-art of fundamental insight into key features that influence the catalytic performance in organic reactions to investigate and optimize a broad range of catalyzed organic transformations, that benefit the researchers in academia and chemical industry fields.

Preparation and Photocatalytic Performances of WO3/TiO2 Composite Nanofibers

The use of sunlight for photocatalytic oxidation is an ideal strategy, but it is limited by factors such as insufficient light absorption intensity of the photocatalyst and easy recombination of photogenerated electron holes. TiO2 is favored by researchers as an environment-friendly catalyst. In this paper, TiO2 is combined with WO3 to obtain a nanofiber with excellent catalytic performance under sunlight. The WO3/TiO2 composite nanofibers were synthesized by using the electrospinning method. The X-ray diffraction (XRD) analysis indicated that WO3 was successfully integrated onto the surface of TiO2. The photodegradation performance and photocurrent analysis of the prepared nanofibers showed that the addition of WO3 really improved the photocatalytic performance of TiO2 nanofibers, methylene blue (MB) degradation rate increased from 72% to 96%, and 5% was the optimal composite mole percentage of W to Ti. The scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), UV-Vis diffuse reflectance spectra (UV-Vis DRS), and Brunauer-Emmett-Teller (BET) analysis further characterized the properties of 5% WO3/TiO2 nanofibers. The H2 generation rate of 5% WO3/TiO2 nanofibers was 107.15 μmol·g−1·h−1, in comparison with that of TiO2 nanofibers (73.21 μmol·g−1·h−1) under the same condition. The 5% WO3/TiO2 produced ·OH under illumination, which played an important role in the MB degradation. Also, the enhanced photocatalytic mechanism was also proposed based on the detailed analysis of the band gap and the active species trapping experiment. The results indicated that the effective separation of Z-scheme photogenerated electron-hole pairs and transfer system constructed between TiO2 and WO3 endowed the excellent photocatalytic activity of 5% WO3/TiO2 nanofibers.

Aqueous solution photocatalytic synthesis of p-anisaldehyde by using graphite-like carbon nitride photocatalysts obtained via the hard-templating route

Graphite-like carbon nitride (GCN)-based materials were developed via the hard-templating route, using dicyandiamide as the GCN precursor and silica templates. That resulted in urchin-like GCN (GCN-UL), 3D ordered macroporous GCN (GCN-OM) and mesoporous GCN (GCN-MP). The introduction of silica templates during GCN synthesis produced physical defects on its surface, as confirmed by SEM analysis, increasing their specific surface area. A high amount of nitrogen vacancies is present in modified catalysts (revealed by XPS measurements), which can be related to an increase in the reactive sites available to catalyse redox reactions. The textural and morphological modifications induced in GCN an enhanced light absorption capacity and reduced electron/hole recombination rate, contributing to its improved photocatalytic performance. In the photocatalytic conversion of p-anisyl alcohol to p-anisaldehyde in deoxygenated aqueous solutions under UV-LED irradiation, the GCN-UL was the best photocatalyst reaching 60% yield at 64% conversion for p-anisaldehyde production after 240 min of reaction. Under oxygenated conditions (air), the process efficiency was increased to 79% yield at 92% conversion only after 90 min reaction. The GCN-based photocatalyst kept its performance when using visible-LED radiation under air atmosphere. Trapping of photogenerated holes and radicals by selective scavengers showed that under deoxygenated conditions, holes played the primary role in the p-anisaldehyde synthesis. Under oxygenated conditions, the process is governed by the effect of reactive oxygen species, namely superoxide radicals, with a significant contribution from holes.

Alkaline treatment as a means to boost the activity of TiO2 in selective photocatalytic processes

In this work, the activity enhancement of TiO₂ photocatalysts by alkaline treatment has been investigated.

Utilising anatase nano-seeds coupled with a visible-light antennae system (Cu–Pd–N) for effective photo-organic transformations

Bandgap tuning TiO₂ nano-seeds with a three-component strategy (Cu, Pd, and N) has facilitated the selective photo-oxidation of cyclic alcohols.

Anaerobic Alcohol Conversion to Carbonyl Compounds over Nanoscaled Rh-Doped SrTiO3 under Visible Light

Photocatalytic oxidation of organic compounds on semiconductors provides a mild approach for organic synthesis and solar energy utilization. Herein, we identify the key points for the photocatalytic oxidation over Pt-loaded Rh-doped strontium titanate allowing the conversion of alcohols efficiently and selectively to aldehydes and ketones under anaerobic conditions and visible light with an apparent quantum efficiency of pure benzyl alcohol oxidation at 420 nm of ≤49.5%. Mechanistic investigations suggest that thermodynamically the controlled valence band edge position via Rh doping provides a suitable oxidation ability of photogenerated holes, avoiding the powerful hydroxyl radical intermediates prone to overoxidation resulting in high selectivity. Kinetically, oxygen vacancies induced by Rh³⁺ substitution in the SrTiO₃ lattice not only favor the dissociative adsorption of alcohols yielding alkoxy species but also induce the weakening of the α-C-H bond facilitating its cleavage by the photogenerated holes. Pt nanoparticles deposited as a cocatalyst contribute to the final hydrogen evolution.

Photocatalytic properties of a new Z-scheme system BaTiO3/In2S3 with a core–shell structure

It's highly desired to design an effective Z-scheme photocatalyst with excellent charge transfer and separation, a more negative conduction band edge (E_CB) than O₂/·O₂⁻ (−0.33 eV) and a more positive valence band edge (E_VB) than ·OH/OH⁻ (+2.27 eV).

Spatial separation of dual-cocatalysts on bismuth vanadate for selective aerobic oxidation of benzylalcohols to benzaldehydes under visible light irradiation

Efficient converting of alcohols to aldehydes under visible light can be achieved on BiVO₄ crystals with spatial separation of dual-cocatalysts.

Modification of heterogeneous photocatalysts for selective organic synthesis

This review elaborates on recent strategies of modifying heterogeneous photocatalysts for high-efficiency selective organic synthesis.

Photochemical synthesis of benzo[f]chromene

The one-pot photocatalytic synthesis of benzo[f]chromene was performed at room temperature by the addition of benzyl alcohol, malononitrile and β-naphthol in contact with an illuminated nano-SnO₂/TiO₂ composite. This study illustrated the potential of photochemical synthesis of benzo[f]chromene. A simple procedure was proposed for the synthesis of a nano-SnO₂/TiO₂ composite using the sol-gel method by hydrolysis of tin(iv) chloride and titanium(iv) chloride at room temperature in the presence of polyethylene glycol 200, diethanolamine, and distilled water. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and FT-IR were employed to characterize the proposed photocatalyst.

Photocatalytic materials and technologies for air purification

Since there is increasing concern for the impact of air quality on human health, the present work surveys the materials and technologies for air purification using photocatalytic materials. The coverage includes (1) current photocatalytic materials for the decomposition of chemical contaminants and disinfection of pathogens present in air and (2) photocatalytic air purification systems that are used currently and under development. The present work focuses on five main themes. First, the mechanisms of photodegradation and photodisinfection are explained. Second, system designs for photocatalytic air purification are surveyed. Third, the photocatalytic materials used for air purification and their characteristics are considered, including both conventional and more recently developed photocatalysts. Fourth, the methods used to fabricate these materials are discussed. Fifth, the most significant coverage is devoted to materials design strategies aimed at improving the performance of photocatalysts for air purification. The review concludes with a brief consideration of promising future directions for materials research in photocatalysis.

Selective photocatalytic oxidation of aromatic alcohols into aldehydes by tungsten blue oxide (TBO) anchored with Pt nanoparticles

Hypostoichiometric Pt/WO₃ shows efficient, selective and stable conversion of aromatic alcohols into corresponding aldehydes under simulated sunlight in aqueous solution.

Surface modification of TiO₂ nanocrystals by WO(x) coating or wrapping: solvothermal synthesis and enhanced surface chemistry

TiO2 anatase nanocrystals were prepared by solvothermal processing of Ti chloroalkoxide in oleic acid, in the presence of W chloroalkoxide, with W/Ti nominal atomic concentration (R(w)) ranging from 0.16 to 0.64. The as-prepared materials were heat-treated up to 500 °C for thermal stabilization and sensing device processing. For R(0.16), the as-prepared materials were constituted by an anatase core surface-modified by WO(x) monolayers. This structure persisted up to 500 °C, without any WO3 phase segregation. For R(w) up to R(0.64), the anatase core was initially wrapped by an amorphous WO(x) gel. Upon heat treatment, the WO(x) phase underwent structural reorganization, remaining amorphous up to 400 °C and forming tiny WO3 nanocrystals dispersed into the TiO2 host after heating at 500 °C, when part of tungsten also migrated into the TiO2 structure, resulting in structural and electrical modification of the anatase host. The ethanol sensing properties of the various materials were tested and compared with pure TiO2 and WO3 analogously prepared. They showed that even the simple surface modification of the TiO2 host resulted in a 3 orders of magnitude response improvement with respect to pure TiO2.

Visible light induced green transformation of primary amines to imines using a silicate supported anatase photocatalyst

Catalytic oxidation of amine to imine is of intense present interest since imines are important intermediates for the synthesis of fine chemicals, pharmaceuticals, and agricultural chemicals. However, considerable efforts have been made to develop efficient methods for the oxidation of secondary amines to imines, while little attention has until recently been given to the oxidation of primary amines, presumably owing to the high reactivity of generated imines of primary amines that are easily dehydrogenated to nitriles. Herein, we report the oxidative coupling of a series of primary benzylic amines into corresponding imines with dioxygen as the benign oxidant over composite catalysts of TiO2 (anatase)-silicate under visible light irradiation of λ > 460 nm. Visible light response of this system is believed to be as a result of high population of defects and contacts between silicate and anatase crystals in the composite and the strong interaction between benzylic amine and the catalyst. It is found that tuning the intensity and wavelength of the light irradiation and the reaction temperature can remarkably enhance the reaction activity. Water can also act as a green medium for the reaction with an excellent selectivity. This report contributes to the use of readily synthesized, environmentally benign, TiO2 based composite photocatalyst and solar energy to realize the transformation of primary amines to imine compounds.

Highly efficient and selective oxidation of aromatic alcohols photocatalyzed by nanoporous hierarchical Pt/Bi2WO6 in organic solvent-free environment

Selective conversion of aromatic alcohols into corresponding aldehydes is important from energy and environmental stance. Here, we describe highly selective (>99%) and efficient conversion (>99%) of aromatic alcohols (e.g., 4-methoxybenzyl alcohol and 4-nitrobenzyl alcohol) into their corresponding aldehydes in the presence of Pt-modified nanoporous hierarchical Bi2WO6 spheres in water under simulated sunlight at ambient conditions. Overoxidation of p-anisaldehyde, formed during photooxidation process, was not observed until comprehensive alcohol oxidation was attained. Furthermore, the catalyst showed substantial oxidation under dark and course of conversion was different than that of under light. Dependency of alcohol oxidation on substrate concentration, photocatalyst amount, and Pt loading was studied. The effect of various radical scavengers was investigated, and the rate-determining step was elucidated. It has been envisaged that the reduction site of semiconductor photocatalysts plays more decisive role in determining the selectivity as alcohol preferably get oxidized over that of water. Furthermore, the chemical stability and recyclability of the photocatalyst were investigated.

Enhanced photocatalytic activities of visible-light driven green synthesis in water and environmental remediation on Au/Bi2WO6 hybrid nanostructures

Deposition of Au NPs enhances photocatalytic activity toward selective oxidation of alcohol in water and Cr(vi) reduction over Bi₂WO₆ nanosheets.

Enhanced photocatalytic activity of Pt/WO3photocatalyst combined with TiO2nanoparticles by polyelectrolyte-mediated electrostatic adsorption

An electrostatic adsorption approach was used to realize a composite structure in which larger WO₃crystalline particles were surrounded by TiO₂nanoparticles. The Pt/WO₃–TiO₂composite photocatalyst showed a high activity under visible light irradiation (λ> 420 nm).

Selective photoredox using graphene-based composite photocatalysts

Graphene (GR) has proven to be a promising candidate to construct effective GR-based composite photocatalysts with enhanced catalytic activities for solar energy conversion. During the past few years, various GR-based composite photocatalysts have been developed and applied in a myriad of fields. In this perspective review, compared with the traditional applications of GR-based nanocomposites for the "non-selective" degradation of pollutants, photo-deactivation of bacteria and water splitting to H2 and O2, we mainly focus on the recent progress in the applications of GR-based composite photocatalysts for "selective" organic transformations, including reduction of CO2 to renewable fuels, reduction of nitroaromatic compounds to amino compounds, oxidation of alcohols to aldehydes and acids, epoxidation of alkenes, hydroxylation of phenol, and oxidation of tertiary amines. The different roles of GR in these GR-based nanocomposite photocatalysts such as providing a photoelectron reservoir and performing as an organic dye-like macromolecular photosensitizer have been summarized. In addition, graphene oxide (GO) as a co-catalyst in GO-organic species photocatalysts and GO itself as a photocatalyst for selective reduction of CO2 have also been demonstrated. Finally, perspectives on the future research direction of GR-based composite photocatalysts toward selective organic redox transformations are discussed and it is clear that there is a wide scope of opportunities awaiting us in this promising research field.

Bi2WO6: A highly chemoselective visible light photocatalyst toward aerobic oxidation of benzylic alcohols in water

The visible-light-driven flower-like Bi₂WO₆ photocatalyst toward “green” chemistry oriented selective organic transformations in water is an essential pathway to sustainable development.