Photocatalytic Reduction Process Research Articles

Photocatalytic applications of heterostructure Ag2S/TiO2 nanotube arrays for U(VI) reduction and phenol degradation

The photocatalytic reduction of U(VI) is an efficient strategy for dealing with uranium-containing wastewater. Herein, a heterostructure material of Ag2S deposited in titanium dioxide nanotube arrays (Ag2S/TNTAs) was prepared by the successive ionic layer adsorption method (SILAR) and applied as an effective photocatalyst for U(VI) reduction. Compared with pure TNTAs, the Ag2S/TNTAs-7 (the number of SILAR cycles) achieved the optimal photoreduction rate of 94% for U(VI) with the rate constant of 0.0105 min−1, which was a 21-fold boost over TNTAs. Impressively, the photocatalytic reduction process of U(VI) by Ag2S/TNTs-7 could be achieved without any sacrifice agents. Furthermore, phenol was introduced to replace the action of methanol, which would not only enhance U(VI) removal efficiency, but also realize degradation of phenol. Mechanism studies have shown that photogenerated electrons and ·O2− act as the reactive species to reduce U(VI) to immobile (UO2) O2⋅2H2O. This work paves a feasible pathway for treatment of uranium-containing wastewater and also demonstrates the potential application prospect of Ag2S/TNTAs-7 in the treatment of organic pollutants.

Light-induced halogen defects as dynamic active sites for CO2 photoreduction to CO with 100% selectivity

Dynamic defects on halide perovskite materials, caused by ion dissociation and migration under light illumination, typically result in undesirable energy dissipation and limited energy conversion efficiency. However, in this work, we demonstrated that dynamic halogen defects generated by the same process in bismuth oxyhalide (Bi5O7Cl) materials can act as active sites to promote charge separation and photocatalytic efficiency. Mechanistic studies and density functional theory calculations revealed that dynamic Cl defects affected the electronic structure of Bi5O7Cl and photocatalytic CO2 reduction process. As active sites, these defects promoted charge transfer, leading to the activation of adsorbed CO2 molecules and reduction of the energy barrier of the rate-determining step. Thus, CO2 was spontaneously converted into COOH− intermediate and finally reduced to CO with a high efficiency of 108.60 μmol g−1 and selectivity of 100% after 4-h of CO2 photoreduction. This work is highly instructive and valuable to the exploration of dynamic defects on halide-containing materials applied in solar energy conversion.

Microenvironments Enabled by Covalent Organic Framework Linkages for Modulating Active Metal Species in Photocatalytic CO2 Reduction

AbstractCovalent organic frameworks (COFs) are promising platforms for understanding photocatalytic CO2 reduction processes owing to their predesignable structures and tailor‐made functions. Herein, a nickel‐modified COF composed of N‐acylhydrazone‐linked electron‐donor and electron‐acceptor dyads (H‐COF‐Ni) is reported. H‐COF‐Ni generates 5694 µmol g−1 of CO with 96% selectivity over H2 evolution in 2 h under visible light irradiation, which greatly outperforms that of typical imine‐linked counterpart. Experimental and theoretical results have demonstrated that metal active sites in host frameworks are deprived by 2,2′‐bipyridine additive to form new catalytic active species, the separation and transfer process of the photogenerated charge carriers are not main reason for their activity difference. The linkage‐dependent activation of CO2 molecules on Ni centers is responsible for high photocatalytic efficiency. This study provides new protocols to improve CO2 photoreduction performance through the modification of linkage microenvironments.

Selective reduction of nitrate into N2 by novel Z-scheme NH2-MIL-101(Fe)/BiVO4 heterojunction with enhanced photocatalytic activity

Nitrate and its metabolites as common pollutants in water had attracted widespread attentions. Converting nitrate to nontoxic and harmless nitrogen via photocatalysis was a promising approach. In this study, a novel Z-scheme NH2-MIL-101(Fe)/BiVO4 heterojunction was successfully prepared. As-prepared Z-scheme heterojunction along with built-in electric field facilitated the charge separation and enhanced the photocatalytic activity in nitrate reduction. The results showed that 0.10-MBiVO photocatalyst exhibited the highest nitrate removal rate of 94.8% (initial concentration 100 mgN/L) and final selectivity to N2 of 93.4% in 50 min under ultraviolet irradiation. Moreover, formic acid was proved as better hole scavenger compared with methanol and oxalic acid. And the concentration of formic acid had significant influence on the process of nitrate photocatalytic reduction. 0.10-MBiVO photocatalyst exhibited excellent reusability in the recycling tests, indicating its great potential in practical application of nitrate photocatalytic removal. The mechanism of the enhancement as well as reaction pathways for nitrate photocatalytic reduction on NH2-MIL-101(Fe)/BiVO4 were comprehensively explored and described at the end.

TiO2@MOF Photocatalyst for the Synergetic Oxidation of Microcystin-LR and Reduction of Cr(VI) in Aqueous Media

The coexistence of pollutants presents a great challenge to the implementation of photocatalysts. In this work, a novel MIL-101(Fe)/TiO2 composite prepared by in situ growth of MIL-101(Fe) on TiO2 was developed for the synergetic oxidation of MC-LR and Cr(VI) reduction. The heterojunction material shows elevated photocatalytic behavior under ultraviolet compared with the unary pollutant system. Furthermore, quenching experiments and electron spin resonance confirm that the enhanced photodegradation behavior is related to the synergistic effect between the photocatalytic reduction and oxidation process, in which MC-LR consumes the holes and Cr(VI) captures electrons, followed by efficient charge separation through the conventional double-transfer mechanism between MIL-101(Fe) and TiO2. This investigation provides a deeper understanding of the construction of MOFs/semiconductor heterojunctions for the pollutants removal in multi-component contaminants system.

Unique Dual-Sites Boosting Overall CO2 Photoconversion by Hierarchical Electron Harvesters.

Low selectivity and poor activity of photocatalytic CO2 reduction process are usually limiting factors for its applicability. Herein, a hierarchical electron harvesting system is designed on CoNiP hollow nano-millefeuille (CoNiP NH), which enables the charge enrichment on CoNi dual active sites and selective conversion of CO2 to CH4 . The CoNiP serves as an electron harvester and photonic "black hole" accelerating the kinetics for CO2 -catalyzed reactions. Moreover, the dual sites form from highly stable CoONiC intermediates, which thermodynamically not only lower the reaction energy barrier but also transform the reaction pathways, thus enabling the highly selective generation of CH4 from CO2 . As an outcome, the CoNiP NH/black phosphorus with dual sites leads to a tremendously improved photocatalytic CH4 generation with a selectivity of 86.6% and an impressive activity of 38.7µmolg-1 h-1 .

One-step oxidation of benzene to phenol with H3PW12O40 under photoirradiation: Effect of operating conditions

The synthesis of phenol by one-pot oxidation of benzene with dioxygen (O2) is a significant conceptual and practical challenge because benzene has low reactivity to O2 at room temperature and ambient pressure. Furthermore, phenol is more reactive than benzene and is easily overoxidized, so complete oxidation is more likely to proceed in the benzene oxidation reaction, which generally results in lower phenol yield and selectivity. Herein, we report that a heteropolyacid H3PW12O40 functions as a photocatalyst for benzene oxidation reaction in an aqueous acetonitrile solution using O2 in high phenol yield (23–41%) and selectivity (80–90%). The addition of acetonitrile to the reaction solution significantly inhibited the complexation between phenol and H3PW12O40, preventing phenol overoxidation. Furthermore, no decomposition of H3PW12O40 during the reaction was evidenced by UV–VIS spectra. This photocatalyst can be used repeatedly with only a slight decrease in the rate of phenol formation. For the mechanistic study, the initial rates, which depended on the reaction conditions, were investigated in detail. The benzene oxidation under anaerobic conditions was also conducted as a control experiment. A color change of reaction solution from colorless to blue has confirmed the reduction of H3PW12O40 during the catalytic process and the formation of reduced H3PW12O40 detected in the UV–VIS spectra. A kinetic isotope effect (KIE), kH/kD = 1.15, is found for the oxidation of benzene/benzene‑d6, indicating electron transfer is the principal C−H bond activation process in the mechanism. We identified hydrogen peroxide as an intermediate and found that phenol was generated in both the photocatalytic reduction and reoxidation process in our system.

A Critical Review on Black Phosphorus-Based Photocatalytic CO2 Reduction Application.

Energy shortages and greenhouse effects are two unavoidable problems that need to be solved. Photocatalytically converting CO2 into a series of valuable chemicals is considered to be an effective means of solving the above dilemmas. Among these photocatalysts, the utilization of black phosphorus for CO2 photocatalytic reduction deserves a lightspot not only for its excellent catalytic activity through different reaction routes, but also on account of the great preponderance of this relatively cheap catalyst. Herein, this review offers a summary of the recent advances in synthesis, structure, properties, and application for CO2 photocatalytic reduction. In detail, the review starts from the basic principle of CO2 photocatalytic reduction. In the following section, the synthesis, structure, and properties, as well as CO2 photocatalytic reduction process of black phosphorus-based photocatalyst are discussed. In addition, some possible influencing factors and reaction mechanism are also summarized. Finally, a summary and the possible future perspectives of black phosphorus-based photocatalyst for CO2 reduction are established.

The fabrication of two-dimensional g-C3N4/NaBiO3·2H2O heterojunction for improved photocatalytic CO2 reduction: DFT study and mechanism unveiling

Photocatalytic CO2 reduction is typically limited by the separation efficiency of photogenerated carriers for a single semiconductor. Thus, fabricating a two-dimensional/two-dimensional (2D/2D) heterojunction photocatalyst with high separation efficiency of photogenerated carriers has become a research priority. Here, a 2D/2D g-C3N4/NaBiO3·2H2O (CN/NBO) heterojunction photocatalyst was successfully synthesized for CO2 photoreduction. With the assistance of the nature of CN, the 10CN/NBO composite showed the best performance with the production yield rates of 110.2 and 43.8 µmol g−1 for CO and CH4, respectively. Our experiments showed that the introduction of CN in CN/NBO composites, which is under the step-scheme (S-step) transfer direction of photogenerated carriers, could greatly inhibit the recombination of photogenerated e−-h+ pairs to prolong the carriers' lifetime, which was further confirmed by analysis of photoluminescence and photochemical characterization. As we expected, the CN/NBO composites show improved photocatalytic CO2 reduction activity. The in situ infrared spectroscopy was also performed to study the intermediate products of the photocatalytic CO2 reduction process. This study provides a way to design CN-based heterojunction photocatalysts for CO2 photoreduction.

Constructing the Z-scheme TiO2/Au/BiOI nanocomposite for enhanced photocatalytic nitrogen fixation

Aiming at comparing the charge separation efficiencies of various heterojunctions, such as p-n junction, metal–semiconductor junction, Z-scheme junction, TiO2-Au, TiO2-BiOI, TiO2-BiOI-Au and TiO2-Au-BiOI were synthesized by using n-type TiO2, p-type BiOI and plasmon metal Au as components for photocatalytic nitrogen fixation. The as-prepared “Z-scheme” TiO2-Au-BiOI photocatalyst showed the highest photocatalytic performance for N2 photofixation with 543.53 μmol L−1h−1 g−1 in the pure water system, which is 1.68 times higher than that of TiO2-BiOI-Au nanocomposite, 1.85 times higher than that of TiO2-BiOI and 6.69 times higher than that of TiO2-Au, revealing superiority of the heterojunctions of TiO2-Au-BiOI in kinetics among these different heterostructured photocatalysts during the photocatalytic N2 reduction process. Moreover, the as-prepared TiO2-Au-BiOI exhibited considerable performance under real natural environment, showing a promising potential in future application.

Vinylene-linked covalent organic frameworks with enhanced uranium adsorption through three synergistic mechanisms

So far, it remains a challenge to synthesize uranium adsorbents with robust stability, high adsorption capacity, excellent photocatalytic activity and easy regeneration. Herein, we report the first example of vinylene-linked covalent organic framework (Tp-TMT) with enhanced uranium adsorption through combined selective ligand binding, chemical reduction and photocatalytic reduction. The unique structure and excellent photocatalytic activity of Tp-TMT make it very suitable for photo-enhanced uranium adsorption through three synergistic mechanisms, thus exhibiting an outstanding uranium adsorption capacity (2362.4 mg g−1). In the dark, a large number of hydroxyl groups in the Tp-TMT framework serve as selective binding sites for uranium, and reduce part of U(VI) to U(IV), thereby greatly improving the adsorption capacity. Meanwhile, the synergistic effect of the triazine units and hydroxyl groups in the highly conjugated framework greatly decreases the optical band gap of Tp-TMT, and an additional U(VI) photocatalytic reduction process can occur under light irradiation, further increasing the adsorption kinetics and capacity. This work explored the structural and functional design of covalent organic frameworks for the adsorption and reduction of uranium in nuclear industry wastewater.

Multichannel Electron Transmission and Fluorescence Resonance Energy Transfer in In2S3/Au/rGO Composite for CO2 Photoreduction.

Efficient electron transmission is an important step in the process of CO2 photoreduction. In this paper, a multi-interface-contacted In2S3/Au/reduced graphene oxide (rGO) photocatalyst with the fluorescence resonance energy transfer (FRET) mechanism has been successfully prepared by the solvothermal, self-assembly, and hydrothermal reduction processes. Photocatalytic CO2 reduction experiments showed that the In2S3/Au/rGO (IAr-3) composite exhibited excellent photoreduction performance and photocatalytic stability. The yields of CO and CH4 obtained after the photoreduction process with IAr-3 as the catalyst were around 4 and 6 times higher than those of pure In2S3, respectively. Photoelectrochemical analysis showed that the multi-interface contact and FRET mechanism greatly improved the generation, transmission, and separation efficiency of carriers photogenerated within the photocatalyst. In situ FTIR test was applied to analyze the photocatalytic CO2 reduction process. 13C isotope tracer test confirmed that the carbon source of CO and CH4 was the CO2 molecules in the photoreduction process rather than the decomposition of catalyst or TEOA. A potential enhanced photocatalytic mechanism has been discussed in total.

Alkynyl-Based sp 2 Carbon-Conjugated Covalent Organic Frameworks with Enhanced Uranium Extraction from Seawater by Photoinduced Multiple Effects

Biofouling is a major obstacle to the efficient extraction of uranium from seawater due to the numerous marine microorganisms in the ocean. Herein, we report a novel amidoxime (AO) crystalline cova...

Photocatalytic Reduction of Methyl Orange, Antibacterial and Antibreast Cancer Activities of Biogenic Silver Nanoparticle Synthesized from Beta vulgaris Extract

Studies based on a biogenic synthesis of noble metal nanomaterials has become a promising one in today’s biomedical approach in therapy and diagnosis due to multidimensional applications. Hence, the present study is to explore the antibacterial, antibreast cancer and photocatalytic efficacy of silver nanoparticles (AgNPs) synthesized from an extract of Beta vulgaris (BV). The band at 398 nm in the UV-visible spectra confirmed the formation of AgNPs. The characteristic shift in OH and C=O peak after the formation of silver nanoparticles shows the participation of Beta vulgaris extracts in the reduction process which is further supported by from SEM morphology. The average size of the particle (17 nm) was determined from XRD analysis using Scherrer’s equation. Antibacterial results of Beta vulgaris mediated BV-AgNPs show the maximum zone of inhibition against Candida albicans. On anticancer activity, BV-AgNPs reveals the toxicity effect on the MCF-7 cell line with an IC50 value of 40.65 μg/mL. Similarly, it reduces 81.3% of methyl orange at 180 min on the photocatalytic reduction process. This study has suggested an effective replacement for the hazardous chemical methods and leads to a cost-effective, environmentally-friendly method that can also be used as antibacterial and anticancer agents.

CsPbBr3-CdS heterostructure: stabilizing perovskite nanocrystals for photocatalysis.

The instability of cesium lead bromide (CsPbBr3) nanocrystals (NCs) in polar solvents has hampered their use in photocatalysis. We have now succeeded in synthesizing CsPbBr3–CdS heterostructures with improved stability and photocatalytic performance. While the CdS deposition provides solvent stability, the parent CsPbBr3 in the heterostructure harvests photons to generate charge carriers. This heterostructure exhibits longer emission lifetime (τave = 47 ns) than pristine CsPbBr3 (τave = 7 ns), indicating passivation of surface defects. We employed ethyl viologen (EV2+) as a probe molecule to elucidate excited state interactions and interfacial electron transfer of CsPbBr3–CdS NCs in toluene/ethanol mixed solvent. The electron transfer rate constant as obtained from transient absorption spectroscopy was 9.5 × 1010 s−1 and the quantum efficiency of ethyl viologen reduction (ΦEV+˙) was found to be 8.4% under visible light excitation. The Fermi level equilibration between CsPbBr3–CdS and EV2+/EV+˙ redox couple has allowed us to estimate the apparent conduction band energy of the heterostructure as −0.365 V vs. NHE. The insights into effective utilization of perovskite nanocrystals built around a quasi-type II heterostructures pave the way towards effective utilization in photocatalytic reduction and oxidation processes.

In-situ preparation of MOFs/SiC/PVA-Co-PE nanofiber membranes for efficient photocatalytic reduction of CO2

In this paper, we prepared a polyvinyl alcohol-polyethylene (PVA-Co-PE) composite nanofiber membrane catalyst decorated by Fe-MOFs/SiC and completed the photocatalytic reduction of CO2 performance. The results show that the CO2 conversion rate of composite film materials under visible light irradiation is increased by 31 times compared with powdered Fe-MOFs/SiC materials. Through SEM, XRD, BET, FTIR, DRS and other characterization methods, the influencing factors of the photocatalytic CO2 reduction process of the composite nanofiber membrane were investigated. The synergistic effect of Fe-MOFs/SiC and nanofiber membrane photocatalysis is beneficial to electron-hole pairs. The effective separation and strong absorption in the visible light region make it exhibit excellent photocatalytic activity in the photocatalytic CO2 reduction reaction.

Furfural mediated synthesis of silver nanoparticles for photocatalytic reduction of hexavalent chromium

Industrial effluents contain various toxic substances and the removal or conversion of these substances to non-toxic products is highly crucial. The work emphasized on the green synthesis of silver nanoparticles with furfural, a promising derivative of lignocellulosic biomass (FAgNPs) for exploring its catalytic efficiency in the photocatalytic reduction of hexavalent chromium [Cr(VI)]. A peak at 410 nm was obtained with a molar ratio of 1:1 (silver nitrate to furfural) at pH 9. The effect of initial concentration of Cr(VI), pH, catalyst dosage, time on reduction of Cr(VI) has been investigated. The photocatalytic efficiency of 1 g/L FAgNPs was studied and 9% to 88% Cr(VI) reduction was observed with decreasing Cr(VI) concentration at pH 2. Under UV illumination, electron–hole pairs generated in FAgNPs resulted in the migration of photo generated electrons for the reduction process which is highly pH dependent. The photocatalytic reduction process can be explained by Langmuir–Hinshelwood model and fits into pseudo-first order kinetics. HaCaT cells exposed to Cr(VI) after being treated with FAgNPs for various time period exhibited an increase in cell viability with time which further substantiated its efficiency. Therefore, the study was an attempt to prepare a nanomaterial by a simple, facile method for the photocatalytic reduction of hexavalent chromium with a focus on environmental remediation.

Research Progress on Photocatalytic Reduction of Cr(VI) in Polluted Water

Abstract More and more wastewater containing hexavalent chromium (Cr(VI)), which causes increasingly threatening environmental events including death of plants or organisms, soil inactivation and canceration of human organs, has been caused by rapid industrial growth. Various methods, such as photocatalytic reduction, physical adsorption, electrochemical and photoelectrochemical approaches have been proposed to detoxify/remove Cr(VI) contained in wastewater. Quite significantly, photocatalytic Cr(VI) reduction grabs increasing attention with many advantages, including environmental friendliness, no sludge, low secondary pollution risk, high utilization of solar energy and low dosage of chemical reagents. For the purpose of improving the Cr(VI) removal efficiency during the photocatalytic reduction process, various kinds of catalysts were developed. In this mini-review, the photocatalytic reduction of Cr(VI) by ion doping photocatalysts, faceted photocatalysts, and heterostructure photocatalysts are briefly introduced. Furthermore, some suggestions for modifying photocatalysts to enhance their photocatalytic performance on Cr(VI) reduction are put forward.

Ultrasonic preparation of a new composite poly(GMA)@Ru/TiO2@Fe3O4: Application in the catalytic reduction of organic pollutants

This work concerns the preparation of a new composite material based on poly(Glycidyl methacrylate), Ru/TiO2 and Fe3O4 using ultrasound treatment. Several composites were prepared by varying the percentage of Ru/TiO2 and Fe3O4. Then, they were applied as catalysts for the reduction of several organic pollutants. 4-Nitrophenol (4-NPh), Methylene blue (MB) and Orange G (OG) were used as a model to study the photocatalytic reduction process in the presence of NaBH4 at room temperature. The obtained photocatalysts were characterized by different methods such as XRD, FTIR, XPS, SEM, EDX, TEM, STEM, TGA, ultraviolet–visible (UV–vis) spectroscopy and Zeta potential measurements. The results showed that the use of ultrasound gave a good dispersion of all species into the polymeric matrix. These solids have shown good activity via the photocatalytic reduction of organic pollutants. The best performance was obtained for the photocatalyst having the higher content of metal NPs “CP(30)”. The rate constant Kapp of composite CP(30) towards catalytic reduction of MB, OG and 4-NPh reached a high level up to 0.55 min−1, 0.241 min−1 and 0.3 min−1, respectively. Finally, the recyclability of the photocatalyst CP(30) was also evaluated. The results showed that the performance of this photocatalyst was satisfactory during its reuse.

Visible light induced selective photocatalytic reduction of CO2 to CH4 on In2O3-rGO nanocomposites

Maintaining the atmospheric CO2 by its utilization in the production of valuable solar fuel like methane is one of the major challenges for humankind. Photocatalytic reduction of CO2 is a vital approach for its consumption by conversion into a valuable chemical product. Here, we prepared In2O3-rGO nanocomposites with varying reduced graphene oxide (rGO) content for the investigation of photocatalytic reduction of CO2. The product yield obtained by photocatalytic reduction of CO2 by visible light source was quantified by exploring the gas chromatography measurements. The catalytic activity of In2O3-rGO nanocomposites towards the photocatalytic reduction process of CO2 to CH4 gets enhanced as compared to In2O3 nanostructures due to the enhancement of lifetime of separation of photogenerated electron-hole pairs, which assist in improvement of charge transfer from In2O3 to rGO under visible light source. The In2O32wt%rGO nanocomposite exhibits a maximum methane yield of 953.72 μmol.g−1. The highest catalytic performance of In2O3-2wt%rGO nanocomposite may be attributed to improvement in its optical properties such as optical band gap and oxygen vacancies (Vo) defects by the addition of rGO in it. A mechanism based on the oxygen vacancies mediated charge transfer process is invoked to explain the observed enhancement in the photocatalytic reduction of CO2.