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

π-π stack driven competitive /complementary adsorption of aromatic compounds on MIL-53(Al)

In this study, the selective adsorption of aromatic compounds on mesoporous MIL-53(Al) was investigated, and followed the order: Biphenyl (Biph) > Triclosan (TCS) > Bisphenol A (BPA) > Pyrogallol (Pyro) > Catechol (Cate) > Phenol (Phen), and exhibited high selectivity toward TCS in binary compounds. In addition to hydrophobicity and hydrogen bonding, π-π interaction/stacking predominated, and more evidently with double benzene rings. TCS-containing halogens could increase π interaction on the benzene rings via forming Cl-π stacking with MIL-53(Al). Moreover, site energy distribution confirmed that complementary adsorption mainly occurred in the Phen/TCS system, as evidenced by ΔQ^pri (the decreased solid-phase TCS concentration of the primary adsorbate) < Q^sec (the solid-phase concentrations of the competitor (Phen)). In contrast, competitive sorption occurred in the BPA/TCS and Biph/TCS systems within 30 min due to ΔQ^pri = Q^sec, followed by substitution adsorption in the BPA/TCS system, but not for the Biph/TCS system, likely attributed to the magnitude of energy gaps (E_g) and bond energy of TCS (1.80 eV, 362 kJ/mol) fallen between BPA (1.74 eV, 332 kJ/mol) and Biph (1.99 eV, 518 kJ/mol) according to the density-functional theory of Gaussian models. Biph with a more stable electronic homeostasis than TCS lead to the occurrence of substitution adsorption in the TCS/BPA system, but not in the TCS/Biph system. This study provides insight into the mechanisms of different aromatic compounds on MIL-53(Al).

Hierarchical 3D Flower-like Metal Oxides Micro/Nanostructures: Fabrication, Surface Modification, Their Crucial Role in Environmental Decontamination, Mechanistic Insights, and Future Perspectives

Hierarchical micro/nanostructures are constructed by micro-scaled objects with nanoarchitectures belonging to an interesting class of crystalline materials that has significant applications in diverse fields. Featured with a large surface-to-volume ratio, facile mass transportation, high stability against aggregation, structurally enhanced adsorption, and catalytical performances, three dimenisional (3D) hierarchical metal oxides have been considered as versatile functional materials for waste-water treatment. Due to the ineffectiveness of traditional water purification protocols for reclamation of water, lately, the use of hierarchical metal oxides has emerged as an appealing platform for the remediation of water pollution owing to their fascinating and tailorable physiochemical properties. The present review highlights various approaches to the tunable synthesis of hierarchical structures along with their surface modification strategies to enhance their efficiencies for the removal of different noxious substances. Besides, their applications for the eradication of organic and inorganic contaminants have been discussed comprehensively with their plausible mechanistic pathways. Finally, overlooked aspects in this field as well as the major roadblocks to the implementation of these metal oxide architectures for large-scale treatment of wastewater are provided here. Moreover, the potential ways to tackle these issues are also presented which may be useful for the transformation of current water treatment technologies.

Metal nanoparticle-decorated germanane for selective photocatalytic aerobic oxidation of benzyl alcohol

Two dimensional materials such as germanane have attracted substantial research interest due to their unique chemical, optical, and electronic properties. A variety of methods for introducing diverse functionalities to their surfaces have been reported and these materials have been exploited as photocatalysts. Herein, we report the preparation of metal nanoparticle (Au, Ag, Cu, Pd, Pt) decorated germanane (M@GeNSs) via facile surface-mediated reduction and investigate their structure, composition, as well morphology using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). These functional materials were subsequently explored as photocatalysts for selective visible light-induced oxidation of benzyl alcohol to benzaldehyde as freestanding nanosystems and thin films and a reaction mechanism of the photocatalytic oxidation of benzyl alcohol is proposed.

Photo-Induced Holes Initiating Peroxymonosulfate Oxidation for Carbamazepine Degradation via Singlet Oxygen

Peroxymonosulfate (PMS) has been intensively used to enhance the photocatalytic activity of catalysts, which is adopted as an electron acceptor to inhibit the recombination of electrons and holes. However, the effect of holes generated by visible light (VL) on PMS activation is always overlooked. Herein, the VL/Bi2WO6/PMS process was constructed for the efficient removal of organics, in which the degradation rate of carbamazepine (CBZ) increased by over 33.0 times by the introduction of PMS into Bi2WO6 under visible light. The radical quenching and determination experiments confirmed that the photogenerated holes could firstly oxidize PMS to form SO5•− and react with HSO5− to produce 1O2, then inducing the formation of other reactive species to greatly enhance the performance of pollutant removal by the VL/Bi2WO6/PMS process. Density functional theory (DFT) predicted that sites with high Fukui index (f0) on CBZ were more susceptible to being attacked, resulting in hydroxylation, ring closure, and C=C bond cleavage of CBZ. Toxicity estimation indicated that photocatalysis degradation products from CBZ were less toxic compared to the parent compound. This study provides a potential avenue for improving photocatalytic efficiency and widening the application of photocatalytic technology in wastewater purification.

Droplet Flow Assisted Electrocatalytic Oxidation of Selected Alcohols under Ambient Condition

This study reports using a droplet flow assisted mechanism to enhance the electrocatalytic oxidation of benzyl alcohol, 2-phenoxyethanol, and hydroxymethylfurfural at room temperature. Cobalt phosphide (CoP) was employed as an active electrocatalyst to promote the oxidation of each of the individual substrates. Surface analysis of the CoP electrocatalyst using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), as well as electrochemical characterization, revealed that it had excellent catalytic activity for each of the substrates studied. The combined droplet flow with the continuous flow electrochemical oxidation approach significantly enhanced the conversion and selectivity of the transformation reactions. The results of this investigation show that at an electrolysis potential of 1.3 V and ambient conditions, both the selectivity and yield of aldehyde from substrate conversion can reach 97.0%.

Efficient photocatalytic degradation of volatile organic compounds over carbon quantum dots decorated Bi2WO6 under visible light

A series of CQDs/Bi₂WO₆ (CBW) hybrid materials with different loading amounts of carbon quantum dots (CQDs) were prepared via a facile in situ hydrothermal method. As determined by multiple techniques including XRD, TEM, UV-vis, PL, TPR, and XPS, the CBW possessed expanded visible light response interval, decreased recombination rate of the photogenerated electron hole, and enhanced oxidation ability as compared with the pristine Bi₂WO₆. In addition, with different loading amounts of CQDs, the optical and electronic properties of the corresponding CBW were different. These CBW materials performed superior activities to the pristine Bi₂WO₆ in the photodegradation of VOCs under visible light, among which the CBW-2 demonstrated the best activity of almost complete degradation within only 120 min. Moreover, the CBW-2 exhibited high stability and reusability after five cycles.

Plasma Treatment as a Promising Environmentally Benign Approach for Synthesis of Valuable Multi-gas Doped Nano-TiO2 -P25: An Efficient Way to Boost the Photocatalytic Performance under Visible Light Illumination

An ingenious prospect has been established to synthesize a wide range of non-metal-doped TiO₂ -P25 by plasma technique. Different atmospheres (Air, O₂ , N₂ , Ar and CO₂ ) have been embedded on the surface of TiO₂ -P25 by plasma treating as an effective alternative to wet chemical pretreatment processes. This approach is clean beyond recognition by employing pure gases as well as no need to poison precursors or organic solvents without producing waste stream, which surprisingly can meet green chemistry purposes. More specifically, plasma has been a contributing factor in the narrowing band gap energies of doped photocatalysts in comparison with pure TiO₂ -P25. Synthesized photocatalysts gained enormous benefit from the plasma treatment in the selective oxidation of benzyl alcohols to associating aldehydes under blue LED illumination with excellent yields, which dramatically decreased the time reaction to many folds. Additionally, benzaldehyde formation under influence of various wavelengths of visible light, including blue photons (λ_max = 460 nm), green photons (λ_max = 510 nm) and red photons (λ_max = 630 nm) was compared to assess the effect of plasma treating on photoactivity of nano-TiO₂ -P25. Furthermore, as-prepared photocatalysts were investigated by diverse characterization techniques.

Ag nanoparticle-decorated 2D/2D S-scheme g-C3N4/Bi2WO6 heterostructures for an efficient photocatalytic degradation of tetracycline

The 2D/2D S-scheme Ag–g-C₃N₄–Bi₂WO₆ composite with efficient charge transfer, displayed a remarkable degradation activity of tetracycline under visible light irradiation.

Amine-functionalized metal–organic framework integrated bismuth tungstate (Bi2WO6/NH2-UiO-66) composite for the enhanced solar-driven photocatalytic degradation of ciprofloxacin molecules

The Bi2WO6/NH2-UiO-66 composite with Z-Scheme heterojunction formation offered efficient charge separation and strong redox property towards ciprofloxacin degradation under solar light irradiation.

New insight into reactive oxidation species (ROS) for bismuth-based photocatalysis in phenol removal

Bismuth-based semiconductors (BBS) are a group of promising candidates applied to visible light-induced photocatalysis. With deep positions of valence bands (2.34-4.04 eV), BBS exhibited excellent activity in oxidation processes. Fundamental studies on the reactive oxidation species primarily focused on TiO₂ under ultraviolet, and it was recognized that OH radicals were effective reactive oxidative species in photocatalytic oxidation processes. This verdict may not be applicable for all other photocatalytic systems. In this study, the reactive oxidation species for BBS in the photocatalytic decomposition of phenol were explored. BBS were prepared with Hierarchical structures and high crystallinity. It was found that OH radicals and superoxide radicals were negligibly produced in most BBS photocatalytic systems. Instead, separated holes on the valence band may directly react with adsorbed species including organics, and acted as the primary ROS. One of the possible explanations of this phenomenon may be due to the shorter lifetime of photogenerated charge carriers on most BBS (212.3-415.7 ms) compared to that of TiO₂ (1193.8 ms). Photocatalytic reaction pathways of degradation of phenol were also different between BBS and TiO₂, which were proposed. This work shed light on the significance of addressing and clarifying the reactive oxidation species in BBS photocatalysis.

Microwave Solvothermal Synthesis of Three-Dimensional Bi2MoO6 Microspheres with Enhanced Photocatalytic Activity

In this study, three-dimensional (3D) Bi₂MoO₆ microspheres were successfully fabricated by a facile, rapid, and mild microwave solvothermal strategy for the first time. The resultant 3D Bi₂MoO₆ microspheres exhibited superior adsorption capacity and photocatalytic efficiency in the degradation of the representative antibiotic ciprofloxacin under visible light, for which the reaction kinetic rate constant is 7.5 times as high as that of the as-synthesized zero-dimensional Bi₂MoO₆ nanoparticles. The 3D hierarchical porous structure and the high Brunauer-Emmett-Teller surface area providing abundant reactive sites mainly contributed to the enhanced photocatalytic activity. The results highlight the feasibility of 3D Bi₂MoO₆ microspheres as an efficient visible-light-responsive photocatalyst for antibiotic removal in an aqueous system.

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.

In situ assembly of CQDs/Bi2WO6 for highly efficient photocatalytic degradation of VOCs under visible light

A facile strategy of the assembly of CQD/Bi₂WO₆ hybrid materials, which exhibit highly efficient photocatalytic degradation of pollutants under visible light.

Enhanced d–d transitions in HKUST/Bi2WO6 nanocomposite mediated visible-light driven selective conversion of benzyl alcohol to benzaldehyde

Graphical representation of the involvement of the d–d transition in the photocatalytic conversion of benzyl alcohol to benzaldehyde.

Synthesis of Bi-deficient monolayered Bi2WO6 nanosheets with enhanced photocatalytic activity under visible light irradiation

The strong protonated hydroxyl groups around Bi vacancies could efficiently promote the separation of photoexcited electron–hole pairs.

Photocatalytic oxidation of arylalcohols to aromatic aldehydes promoted by hydroxyl radicals over a CoP/CdS photocatalyst in water with hydrogen evolution

Hydroxyl free radicals generated by visible-light-catalyzed water splitting over CoP/CdS drive the oxidation of arylalcohols with hydrogen evolution.

Two-Dimensional Bi2WO6 Nanosheets as a Robust Catalyst toward Photocyclization

The present work describes the improved photocatalytic activity of cetyl trimethylammonium bromide (CTAB)-assisted Bi₂WO₆ (CBTH) toward the synthesis of bioactive benzazoles. X-ray diffraction analysis of CBTH suggests that crystal growth has occurred along the (200) plane, whereas field-emission scanning electron microscopy images confirm two-dimensional rose bud morphology and high-resolution transmission electron microscopy analysis suggests the formation of thin nanosheets possessing an orthorhombic structure. Temperature-programmed desorption of ammonia and Py-IR measurements indicate substantial acidity with the generation of Brønsted acid sites on the surface of CBTH. Raman spectra of CBTH also corroborate these observations with the formation of defects within [Bi₂O₂]²⁺ layers, resulting in decreased thickness and shapes of nanoplates. These beneficial properties are explored toward the photochemical synthesis of benzazoles using a 35 W tungsten lamp and a CBTH photocatalyst, resulting in better yields at lesser exposure time. It is observed that the catalytic activity is retained up to five consecutive cycles with marginal decrease in % yield. Such a feature can be ascribed to the photostability of the photocatalyst even after continuous exposure to light, implying that the surface active sites remained unaltered as evident from the X-ray photoelectron spectroscopy analysis of pre- and post-characterization of CBTH. Moreover, decrease in the surface hydroxyl groups after five catalytic cycles also accounts for the generation of enhanced Brønsted sites owing to the presence of Bi-O on the surface of CBTH. It exhibits better catalytic activity as compared to other photocatalysts employed for the synthesis of benzazoles. Thus, CBTH serves as a robust photocatalyst for the facile synthesis of these heterocycles in a sustainable manner.

Photocatalytic degradation of 2-(4-methylphenoxy)ethanol over TiO2 spheres

The photocatalytic TiO₂-assisted decomposition of 2-(4-methylphenoxy)ethanol (MPET) in aqueous solution has been studied for the first time. The intermediate compounds of MPET photodegradation have been also determined. A toxic p-cresol is formed in significant quantities during the photocatalytic reaction. A solvent-exchange approach for a template-free preparation of spherical TiO₂ particles has been described, which is based solely on precipitation of hydrous titania from aqueous titanium peroxo complex by using organic solvents. The proposed method favours the formation of spherical titania particles with a mean size varying from 50 to 260nm depending on the choice of solvent. The procedure for converting nonporous titania spheres into mesoporous material maintaining the same spherical morphology has been developed. The synthesized TiO₂ spheres demonstrate a degree of MPET photo-degradation close to that of the commercial titania Aeroxide P25, besides being successfully recovered and reused for four reaction cycles without loss of photocatalytic activity. The effectiveness of the commercial Aeroxide P25 in MPET photodegradation, on the other hand, suffers 10-time drop during the third reaction cycle, which is attributed to its poor recoverability because the photocatalyst is composed of small particles of 20nm size.

Ultrathin TiO2(B) Nanosheets as the Inductive Agent for Transfrering H2O2 into Superoxide Radicals

A reflux method to synthesize ultrathin polycrystalline TiO₂(B) nanosheets (NSs) which are assembled by single crystals, and further stacked into nanoflower structures, is described. On the basis of the theoretical calculations and experiments, H₂O₂ can easily substitute the ethylene glycol adsorbed on the surface of TiO₂(B) NSs, forming H₂O₂-NS due to the lower adsorption energy and the unique structural features of ultrathin TiO₂(B) nanosheets. TiO₂(B) NSs and the H₂O₂ system can be accelerated to generate superoxide radicals under heat or light and thus exhibit a great degradation property on dye molecules; the total organic carbon (TOC) removal rate was 6 times higher than that for H₂O₂ alone. Meanwhile, TiO₂(B) NSs and the H₂O₂ system have a good application on the selective oxidation due to the reactive species of superoxide radicals avoiding overoxidization of benzyl alcohol. The conversion of benzyl alcohol oxidized to benzaldehyde in water solution under low temperature and atmospheric pressure was 51.13%, while the selectivity was close to 100%. We believe that the present findings will provide valuable methods for highly efficient generation of superoxide radicals and broaden their applications in catalysis.

Synthesis of Ultrasmall Platinum Nanoparticles on Polymer Nanoshells for Size-Dependent Catalytic Oxidation Reactions

It is highly desirable for the synthesis and stabilization of noble metal nanoparticles of uniform, precisely tunable sizes, especially in the range of angstroms to a few nanometers, for many catalytic applications in pursuit of optimal activity and selectivity. Herein, we report a novel strategy for the synthesis of uniform platinum (Pt) nanoparticles of ultrasmall sizes (average size: 0.9-2.3 nm), which are stabilized on hollow polymer nanoshells formed by polymerization of sodium dodecyl benzenesulfonate (SDBS) at the interface of an ethanol/water emulsion. The resulting composite represents a highly active catalyst for effective oxidation of alcohols under ambient conditions. Strong size-dependent catalytic activity of Pt nanoparticles has been revealed in aerobic oxidation of 1-phenylethanol to yield acetophenone, demonstrating a volcano-shape profile, with Pt nanoparticles of ∼1.7 nm showing the highest activity. The size effect has been attributed to the size-dependent d-band electron structure of the Pt nanoparticles. This work reveals the size effect of Pt nanoparticles in general organic oxidation reactions, and thus provides a general methodology and a lot of opportunities in the design of metal-nanoparticle-based catalysts for fine-chemical production.

Aerosol-Sprayed Gold/Ceria Photocatalyst with Superior Plasmonic Hot Electron-Enabled Visible-Light Activity

Integration of nanoscale plasmonic metals with semiconductors is a promising strategy for utilizing visible and near-infrared light to enhance chemical reactions. Here we report on the preparation of Au/CeO₂ microsphere photocatalysts through aerosol spray and the study of their photocatalytic activity toward the aerobic oxidation of 1-phenylethanol under visible light. The microsphere catalysts exhibit a remarkable photocatalytic performance with their turnover frequency values reaching 108 h^-1, which is more than 23 times that of (Au core)@(CeO₂ shell) nanostructures and much larger than those obtained previously for the visible-light photocatalytic oxidation of 1-phenylethanol. In addition, the Au/CeO₂ catalyst shows the best performance among eight types of oxide semiconductor supports. Moreover, the photocatalytic mechanism of the Au/CeO₂ catalyst is systematically investigated. This study offers insights for plasmonic hot electron-enabled photocatalysis, which will be valuable for the design of various efficient (plasmonic metal)/semiconductor photocatalysts.

Designed C3N4/CdS–CdWO4 core–shell heterostructure with excellent photocatalytic activity

Charge separation efficiency is improved by the enhanced built-in photogenerated electric field.

Facile synthesis of porous Bi2WO6 nanosheets with high photocatalytic performance

Compared with the well-known three-dimensional Bi2WO6 nanosheet-assembled nanostructures, the free-standing two-dimensional porous Bi2WO6 nanosheets have seldom been reported. The possible reason is that Bi2WO6 nanosheets with a high surface-to-volume ratio usually tend to self-assemble or aggregate to form microspheres to reduce their surface energy. To prevent their aggregation, in this study, a new and facile self-assembled route, which includes the in situ ion-exchange reaction of Na2WO4 solution with the Bi(NO3)3 solid powder and the following high-temperature calcination, has been successfully developed to prepare the free-standing porous Bi2WO6 nanosheets. The ion-exchange reaction between the Bi(NO3)3 solid and Na2WO4 solution can in situ produce amorphous Bi2WO6 nanosheets, while the high-temperature calcination (500 °C) causes the formation of homogeneously porous structures in individual nanosheets during their phase transformation from amorphous to crystalline. The resultant porous nanosheets are composed of one-layer Bi2WO6 nanoparticles with a size of 30-50 nm, and there is a strong coupling interface among these nanoparticles. Photocatalytic experimental results suggest that the resultant porous Pt/Bi2WO6 nanosheets show a high photocatalytic performance for the decomposition of phenol solution. Considering their facile preparation, the present synthetic route may provide new insights for the design and fabrication of other nanostructured materials with various potential applications.

A graphene-coupled Bi2WO6 nanocomposite with enhanced photocatalytic performance: a first-principles study

Under the built-in electric field and the band edge potential well in graphene/Bi₂WO₆, the photogenerated electrons in Bi₂WO₆ can flow to graphene. Graphene can efficiently capture and transport photogenerated electrons. Photocatalytic oxidation and reduction reaction can occur on the surface of Bi₂WO₆ and graphene, respectively.

Critical design of heterogeneous catalysts for biomass valorization: current thrust and emerging prospects

Catalysis in the heterogeneous phase plays a crucial role in the valorization of biorenewable substrates with controlled reactivity, efficient mechanical process separation, greater recyclability and minimization of environmental effects.

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.

A green and efficient photocatalytic route for the highly-selective oxidation of saturated alpha-carbon C–H bonds in aromatic alkanes over flower-like Bi2WO6

A green and efficient photocatalytic route has been developed for the highly-selective oxidation of aromatic alkanes over flower-like Bi₂WO₆ under solvent-free conditions.