A review on heterogeneous photocatalysis for environmental remediation:From semiconductors to modification strategies

Heterogeneous photocatalysis,an advanced oxidation process,has garnered extensive attention in the field of environmental remediation because it involves the direct utilization of solar energy for the removal of numerous pollutants.However,the application of heterogeneous photocatalysis in environmental remediation has not achieved the expected consequences due to enormous challenges such as low photocatalytic efficiencies and high costs of heterogeneous photocatalysts in large-scale practical applications.Furthermore,pollutants in the natural environment,including water,air,and solid phases,are diverse and complex.Therefore,extensive efforts should be made to better understand and apply heterogeneous photocatalysis for environmental remediation.Herein,the fundamentals of heterogeneous photocatalysis for environmental remediation are introduced.Then,potential semiconductors and their modification strategies for environmental photocatalysis are systematically presented.Finally,conclusions and prospects are briefly summarized,and the direction for the future development of environmental photocatalysis is explored.This review may provide reference directions toward understanding,researching,and designing photocatalytic remediation systems for various environmental pollutants.

Control of selectivity in organic synthesis via heterogeneous photocatalysis under visible light

Organic synthesis driven by heterogeneous catalysis is a central research theme to both fundamental research and industrial production of fine chemicals.However,the employment of stoichiometric strong oxidizing or reducing reagents(e.g.,K_(2)Cr_(2)O_(7) and LiAlH_(4))and harsh reaction conditions(e.g.,high temperature and pressure)always leads to the products of overreaction and other by-product residues(e.g.,salt and acid waste).Thus the poor control of product selectivity and tremendous energy consumption result in the urgent demand to develop novel technologies for heterogeneous catalysis.Given the current global theme of development in CO_(2) reduction and sustainable energy utilization,one promising protocol is heterogeneous photocatalysis.It enables sustainable solar-to-chemical energy conversion under mild conditions(e.g.,room temperature,ambient pressure,and air as the oxidant)and offers unique reaction pathways for improved selectivity control.To accurately tailor the selectivity of desired products,the electronic structure(e.g.,positions of valence-band maximum and conduction-band minimum),geometric structure(e.g.,nanorod,nanosheet,and porous morphology),and surface chemical micro-environment(e.g.,vacancy sites and co-catalysts)of heterogeneous photocatalysts require rational design and construction.In this review,we will briefly analyze some effective photocatalytic systems with the excellent regulation ability of product selectivity in organic transformations(mainly oxidation and reduction types)under visible light irradiation,and put forward opinions on the optimal fabrication of nanostructured photocatalysts to realize selective organic synthesis.

Solar Photocatalysis of A Pesticide in A Tubular Reactor on Titaniferous Sand as A New Semi-Conductor

This present study comes in addition to overcome the problems of separation of fine particles of TiO2 in heterogeneous photocatalysis after treatment. It aims to show the potential for using titaniferous sand as a new semiconductor under solar irradiation. The photocatalytic efficiency of this titaniferous sand was tested on a pesticide (Azadirachtin). A tubular photocatalytic reactor with recirculation of the polluting solution was designed for the elimination of the pesticide in an aqueous solution. Before its use as a photocatalyst, the titaniferous sand has undergone a specific treatment that consists of calcination at 600℃ followed by extraction of the calcined natural organic materials, which can interfere with the measurement of analytical parameters such as COD. The titaniferous sand was also characterized by X-ray fluorescence spectroscopy (XRF). XRF analyses have shown that TiO2 is predominant in the titaniferous sand with a percentage that has been estimated at 46.34%. The influence of various experimental parameters such as the flow rate of the polluting solution, the concentration of titaniferous sand, the presence of oxygen and the intensity of the overall rate of sunshine, was studied to optimize the photocatalytic degradation of the pesticide. The results showed that the highest removal rate (70%) was observed under the following conditions: a pH of 6, a titaniferous sand concentration of 150 g/L, a flow rate of 0.3 mL/min, and a sunshine rate of 354 W/m2 and in the presence of atmospheric oxygen. Under these experimental conditions, the rate of photodegradation of the pesticide follows the pseudo first order kinetic model of Langmuir Hinshelwood with a coefficient of determination R2 of 0.9869 and an apparent rate constant of 0.0029 min-1. The results clearly demonstrated the potential of titaniferous sand as a photocatalyst sensitive to sunlight for the effective removal of pesticides in the aquatic environment.

Selective CO_(2) reduction into formate using Ln-Ta oxynitrides combined with a binuclear Ru(Ⅱ)complex under visible light

Hybrid materials constructed from a visible-light-absorbing semiconductor and a functional metal complex have attracted attention as efficient photocatalysts for CO_(2) reduction with high selectivity to a desired product.In this work,defect fluorite-type Ln-Ta oxynitrides LnTaO_(x)N_(y)(Ln=Nd,Sm,Gd,Tb,Dy and Ho)were examined as the semiconductor component in a hybrid photocatalyst system combined with known Ag nanoparticle promoter and binuclear ruthenium(Ⅱ)complex(RuRu’).Among the LnTaO_(x)N_(y) examined,TbTaO_(x)N_(y) gave the highest performance for CO_(2) reduction under visible light(k>400 nm),with a Ru Ru0-based turnover number of 18 and high selectivity to formate(>99%).Physicochemical analyses indicated that high crystallinity and more negative conduction band potential of Ln Ta O_(x)N_(y)with the absence of Ln-4 f states in the band gap structure contributed to higher activity of the hybrid photocatalyst.

Heterogeneous photocatalytic borylation of aryl iodides mediated by isoreticular 2D covalent organic frameworks

Metal-free heterogeneous photocatalysts provide an environmental-friendly and cost-efficient avenue for green organic synthesis.Covalent organic frameworks(COFs)as heterogeneous photocatalysts showcase promising potential in the field of photocatalytic organic reactions due to their high porosity,insolubility and tailor-made functions.However,thus far,COF-based catalysts only mediated a few types of reactions.Herein,we developed a series of isoreticular nitrogen-rich covalent organic frameworks(N-COFs)with comparable porous structures as photocatalysts which effectively mediated the borylation of aryl iodides with broad substrate scope.Remarkably,6N-COF exhibits excellent photocatalytic efficiency and superior recyclability.It suggests a new pathway to construct efficient heterogeneous photocatalysts for the borylation of aryl halides.

Development of a Construction Material for Indoor and Outdoor, Metakaolinite-Based Geopolymer, with Environmental Properties

Environmental problems that came from the human activity have many facets,since pollution of the atmosphere arises from vehicles,industrial emissions while pollution of water could be from organic compounds,pesticides etc.These can cause serious health effects,such as respiratory diseases,including asthma and lung cancer.Hence,in the present work,a kinetic study on the effective adsorption and photo degradation of methylene blue(MB)dye,under ultraviolet A(UVA)irradiation of an alkali activated inorganic polymer(geopolymer)with homogeneously dispersed titanium dioxide(TiO2)micro-particles is presented.In addition,antimicrobial testing of the alkali activated TiO2 material was performed showing a bacteriostatic effect.

Removal of the Methylene Blue Dye (MB) with Catalysts of Au-TiO₂: Kinetic and Degradation Pathway

Au-TiO2 catalysts were used in the photocatalytic degradation of the methylene blue dye (MB). The synthesis of titanium oxide (TiO2) was carried out by sol-gel method. Subsequently, particles of Au were deposited on the surface of the semiconductor by photo-deposition, thus modifying the surface of the semiconductor. For the characterization of the catalyst obtained, the techniques of X-ray Diffraction (DRX), Scanning Electron Microscopy (SEM), Spectroscopy with Diffuse Reflectance (DR) and Surface Area by the BET (Brunauer, Emmett y Teller) were used. The solid obtained was tested experimentally as a catalyst in the photocatalytic degradation of a solution of MB. The data obtained were analyzed by UV-vis Spectroscopy and Total Organic Carbon (TOC) and the results indicated conversions were greater than 80%. The intermediate products were evaluated by mass coupled gas chromatography (GC-MS) and the MB decomposition route was by hydroxylation, obtaining aromatic intermediates, esters and products of the chemical degradation of the molecule.

Evaluation of TiO_(2),ZnO,CuO and Ga_(2)O_(3)on the Photocatalytic Degradation of Phenol Using an Annular-Flow Photocatalytic Reactor

Even with rigorous environmental regulations, phenol still is a major contaminant. One possible solution is the use of heterogeneous photocatalysis due to low chemical addition, feasibility and reliability to be implanted on cost-effective industrial process. TiO2 is the most employed photocatalyst because of its favorable (photo) chemical properties and ZnO is considered one of the best alternative for that. Other oxides were tested in lesser proportions, like CuO and Ga2O3. When the photocatalyst is dispersed as slurry, higher degradation rates are achieved due to high solid to liquid contact area when compared with supported form. The aim of this work was to develop a batch recirculating photocatalytic reactor and evaluate its efficiency when assisted by the photocatalysts TiO2 P25, ZnO, CuO and β-Ga2O3. TiO2 achieved 95% mineralization after 200 min reaction in an average degradation rate of 0.68 mg·L﹣1·min﹣1 and ZnO was less efficient (0.41 mg·L﹣1·min﹣1). Ga2O3 and CuO presented poor performance, mainly due to low surface area for the CuO syntesized and the absorption of the UV radiation by the reactor walls, decreasing Ga2O3 activity. Degradation intermediates were detected in diverse concentrations and at different operational times for each oxide tested, which indicate different degradation mechanisms.

Solar-assisted photocatalytic water splitting using defective UiO-66 solids from modulated synthesis

Metal-organic frameworks(MOFs)are attracting increasing interests as photocatalysts for solar-driven hydrogen production from water.This paper reports on a comparative study of using either acetic acid(AA)or trifluoroacetic acid(TFA)as the representative UiO-66 organic modulators for synthesizing visible light responsive UiO-66(Zr)-X(X:NH_(2) or NO_(2))photocatalysts for water splitting.The results show that photocatalytic hydrogen generation from a water/methanol mixture can be improved by varying the nature and amount of the modulator employed to prepare the different UiO-66(Zr)-X(X:NH_(2) or NO_(2))solid derivatives.UiO-66(Zr)-NH_(2) was the most active photocatalyst,followed by UiO-66(Zr)-NO_(2),both prepared with 12 equivalents of AA with respect to the organic ligand.This UiO-66(Zr)-NH_(2) solid was more active than the parent MOF in photocatalytic overall water splitting(OWS)(H_(2) and O_(2) production of 450 and 160μmol·g^(-1),respectively,in 5 h;apparent quantum yield(AQY)at 400 nm of 0.06%)in the absence of methanol and compares favourably with analogous reports.Information on the photocatalytic activity of the most active solids of both series was obtained by means of a series of techniques,including ultraviolet-visible(UV-vis)diffuse reflectance,X-ray photoelectron spectroscopy(XPS),laser flash photolysis(LFP),electron spin resonance(ESR),photoluminescence and photoelectrochemical measurements together with density functional theory(DFT)calculations.The results showed that organic acid modulators can be used to enhance the photocatalytic activity of missing linker UiO-66 defective materials in solar-powered water splitting.

Semi-heterogeneous photo-Cu-dual-catalytic cross-coupling reactions using polymeric carbon nitrides

A merger of copper catalysis and semiconductor photocatalysis using polymeric carbon nitride(PCN)for multi-type cross-coupling reactions was developed.This dual-catalytic system enables mild C-H arylation,chalcogenation,and C-N cross-coupling reactions under visible light irradiation with a broad substrate scope.Good-to-excellent yields were obtained with appreciable site selectivity and functional group tolerance.Metal-free and low-cost PCN photocatalyst can easily be recovered and reused several times.

Donor-acceptor 2D covalent organic frameworks for efficient heterogeneous photocatalyticα-oxyamination

Covalent organic frameworks(COFs)have received widespread interest due to their high porosity,excellent crystallinity,tailorable structures,and broad application prospects.It has been demonstrated that proper combination and arrangement of electron donor and acceptor units in 2D conjugated COF lattice could promote efficient charge separation and electron transfer,and thus is beneficial for photocatalysis.In this article,three donor-acceptor(D-A)2D COFs were prepared by Schiff base reaction of electron acceptor 4,4′,4′′,4′′′-(benzo[1,2-d:4,5-d′]bis(oxazole)-2,4,6,8-tetrayl)tetraaniline(BBO)with different electron donors:thieno[3,2-b]thiophene-2,5-dicarbaldehyde(TT),benzo[1,2-b:4,5-b’]dithiophene-2,6-dicarboxaldehyde(BDT)and terephthalaldehyde(Ph),respectively.These D-A 2D COFs exhibited prominent photocatalytic activity towardsα-oxyamination of 1,3-dicarbonyl with 2,2,6,6-tetramethyl-1-piperdinyloxy(TEMPO)upon visible light irradiation.Among these D-ABBO-COFs,DTT-ABBO-COF exhibited the highest photocatalytic rates,which can be ascribed to the more negative highest occupied molecular orbital(HOMO)and narrower bandgap.The excellent stability,high activity and superior recyclability render DTT-ABBO-COF as a potential and environmentally friendly heterogeneous catalyst forα-oxyamination.

Photo-induced surface frustrated Lewis pairs for promoted photocatalytic decomposition of perfluorooctanoic acid

Heterogeneous photocatalysis has gained substantial research interest in treating per-and polyfluoroalkyl substances(PFAS)-contaminated water.However,sluggish degradation kinetics and low defluorination efficiency compromise their practical applications.Here,we report a superior photocatalyst,defected Bi_(3)O(OH)(PO_(4))_(2),which could effectively degrade typical PFAS,perfluorooctanoic acid(PFOA),with high defluorination efficiency.The UV light irradiation could in situ generate oxygen vacancies on Bi_(3)O(OH)(PO_(4))_(2) through oxidation of the lattice hydroxyls,which further promotes the formation of Lewis acidic coordinately unsaturated bismuth sites.Then,the Lewis acidic sites couple with the proximal surface hydroxyls to constitute the surface frustrated Lewis pairs(SFLPs).With the in-depth spectroscopic analysis,we revealed that the photo-induced SFLPs act as collection centers to effectively adsorb PFOA and endow accessible pathways to transfer photogenerated holes to PFOA rapidly.Consequently,activation of the terminal carboxyl,a ratelimiting step for PFOA decomposition,could be easily achieved over the defected Bi_(3)O(OH)(PO_(4))_(2) photocatalyst.These results suggest that SFLPs exhibit great potential in developing highly efficient photocatalysts to degrade persistent organic pollutants.

Fabrication of Schiff base coordinated ZnS nanoparticles for enhanced photocatalytic degradation of chlorpyrifos pesticide and detection of heavy metal ions

We report the simple synthesis of zinc sulfide nanoparticles(ZnS NPs)by a co-precipitation method using Schiff base,(2-[(4-methoxy-phenylimino)-methyl]-4-nitrophenol)as a capping agent.The formation of ZnS NPs and their optical,structural,thermal properties and morphologies were studied by means of UVevis DRS,PL,FTIR,XRD,SEM,TEM,and TGA.The optical properties and quantum confinement effect of the products were confirmed by means of spectroscopic measurements.We have accessed the photocatalytic ability of the prepared ZnS and Schiff base capped ZnS NPs in the degradation of chlorpyrifos under UV light irradiation for the prevention of environmental pollution.The prepared ZnS NPs exhibited a selective probe detection of Fe^(3+),Cr^(2+) and Cd^(2+) ions by fluorometrically and the emission band which disappears in the presence of increasing concentrations of Fe^(3+),Cr^(2+) and Cd^(2+) ions.Our work suggested that the synthesized Schiff base capped ZnS NPs could be a potential selective photocatalyst for the degradation of toxic pollutants and a selective probe sensor for the detection of heavy metal ions.

Iron sites on defective BiOBr nanosheets:Tailoring the molecular oxygen activation for enhanced photocatalytic organic synthesis

Sunlight-driven activation of molecular oxygen(O_(2))for organic oxidation reactions offers an appealing strategy to cut down the reliance on fossil fuels in chemical industry,yet it remains a great challenge to simultaneously tailor the charge kinetics and promote reactant chemisorption on semiconductor catalysts for enhanced photocatalytic performance.Herein,we report iron sites immobilized on defective BiOBr nanosheets as an efficient and selective photocatalyst for activation of O_(2) to singlet oxygen(^(1)O_(2)).These Fe^(3+) species anchored by oxygen vacancies can not only facilitate the separation and migration of photogenerated charge carrier,but also serve as active sites for effective adsorption of 02.Moreover,low-temperature phosphorescence spectra combined with X-ray photoelectron spectroscopy(XPS)and electronic paramagnetic resonance(EPR)spectra under illumination reveal that the Fe species can boost the quantum yield of excited triplet state and accelerate the energy transfer from excited triplet state to adsorbed O2 via a chemical loop of Fe^(3+)/Fe^(2+),thereby achieving highly efficient and selective generation of ^(1)O_(2).As a result,the versatile iron sites on defective BiOBr nanosheets contributes to near-unity conversion rate and selectivity in both aerobic oxidative coupling of amines to imines and sulfoxidation of organic sulfides.This work highlights the significant role of metal sites anchored on semiconductors in regulating the charge/energy transfer during the heterogeneous photocatalytic process,and provides a new angle for designing high-performance photocatalysts.

Pillar[5]arene based conjugated macrocycle polymers with unique photocatalytic selectivity

The development of heterogeneous catalysts with substrate shape,size or electronic constitution selectivity is a huge challenge in photocatalysis.Reported herein is a host-guest interaction strategy to endow photocatalysts with special selectivity.By adjusting the precursors,conjugated macrocycle polymers(CMPs)with pillar[5]arene struts(CMP-1 and CMP-2)and a corresponding non-pillar[5]are necontained conjugated organic polymer(COP-1)were prepared and the photocatalytic activities toward sulfide derivatives were investigated.The sulfides showed similar conve rsions when COP-1 was used as a photocatalyst,but exhibited significant differences when it turned to the CMPs.Remarkably,the conversion yield of S-1 achieved near 18 folds over the one of S-2 when CMP-2 was used as a catalyst.Mechanism studies confirmed that the"host-guest"effect of pillar[5]arene struts in CMPs was the main cause of the difference.The present work establishes CMPs as novel heterogeneous photocatalysts with substrate selectivity,and such a method will inspire the researchers concerning preparation of heterogeneous catalysts with excellent selectivity.

Effect of transition metaldoping under reducing calcination atmosphere on photocatalytic property of TiO_2 immobilized on SiO_2 beads

TiO2 immobilized on SiO2 （TiO2 /SiO2 ） have been prepared by sol-gel method and various ions of transition metals （Cr3＋ , Co2＋ , Ni2＋ , Cu2＋ , and Zn2＋ ）weredoped on the photocatalyst usingwet impregnation method under reducing calcination atmosphere. The photocatalytic activity of metaldoped TiO2 /SiO2 towards phenoldegradation under black light irradiationwere investigated and comparedwith undoped TiO2 /SiO2 . The results showed that the photoresponse of Cu2＋ and Zn2＋doped TiO2 /SiO2were larger than undoped TiO2 /SiO2 , indicating that the photogenerated carrierswere separated more efficiently in Cu2＋ and Zn2＋doped TiO2 /SiO2 . The reactivitywas in the order of Cu2＋ 〉 Zn2＋ 〉 Ni2＋ 〉 Cr3＋ 〉 Co2＋ . Thedifferent photoreactivitywas ascribed to combine effect of thedifferent ionic radii and photocorrison tendency of thedopants. The samplewas also characterized by surface analytical methods such as X-raydiffraction, scanning electron micrograph/electrondispersive X-ray analyzer and UV-Vis absorption spectrum.

Organic photocatalysts:From molecular to aggregate level

Organic semiconductors(OSCs)have the advantages of tunable molecular structures,suitable band gaps,and exceptional optoelectronic properties.The π-π stacking ability of OSCs also leads to appealing molecular stacking structure,function,and stability.So far,organic photocatalysts have engaged in homogeneous or heterogeneous photocatalysis in the form of free molecules,supported molecules,or nanostructures.Meanwhile,researches on organic photocatalysts have expanded from small organic molecules to the organic macromolecules,as well as their various nanostructures and nanocomposites including isolated zero-dimensional(0D),one-dimensional(1D),two-dimensional(2D),three-dimensional(3D)nanostructures,and their combinations.Therefore,many versatile strategies have been explored to improve photocatalytic ability and practicality either from molecular synthetic modification,crystal,or interface engineering.In this review,we first discuss the photophysical and photochemical processes of organic photocatalysts that govern the ultimate photocatalytic efficiency;we then summarize different forms of organic photocatalysts,their rational design strategies,and mechanistic pathways,as well as their applications in H_(2) evolution,CO_(2) reduction,and environmental purification,aiming to highlight the structure/property relationships;we lastly propose ongoing directions and challenges for future development of organic photocatalysts in real use.

Progress in organic photocatalysts

Organic materials have advantages of diversity,ease of functionality, self-assembly, etc. The varied mechanistic pathways also make it conceivable to design an appropriate photocatalyst for an identical reaction. From this perspective, organic photocatalysts find wide applications in homogeneous, heterogeneous photocatalysis and photoelectrochemical（PEC） solar cells. In this review, the form of the employed organic photocatalysts ranging from molecules, supported molecules, to nanostructures or thinfilm aggregates will be firstly discussed. Rational design strategies relating to each form are also provided, aiming to enhance the photoenergy conversion efficiency. Finally,the ongoing directions for future improvement of organic materials in high-quality optoelectronic devices are also proposed.

Metal-free and water-compatible nanocatalysts for green photocatalytic dehydrogenation in aqueous medium

Two types of poly-porphyrin-based heterogeneous photocatalysts including insoluble polymer and water-soluble nanoparticles exhibited excellent photocatalytic activity in visible-light-induced dehydrogenation in water.The extremely low catalyst loading(0.01 mol%)of water-soluble nanoparticles provided so far the highest turnover frequency(TOF)value(333 h^(-1))for such transformation among reported records.