Photocatalytic reduction of nitrobenzene over titanium dioxide: by-product identification and possible pathways

Photocatalytic performance of SiO2@TiO2 spheres in selective conversion of oxidation of benzyl alcohol to benzaldehyde and reduction of nitrobenzene to aniline

Selective photocatalytic oxidation of benzyl alcohol to benzaldehyde and reduction of nitrobenzene to aniline reactions are investigated by using SiO2@TiO2 spheres produced in a simple route using chitosan as a template. The spheres are predominantly macroporous and, the XRD points out an amorphous crystallographic profile suggesting the uniform distribution of TiO2. Under low-power lighting for 4 hours, the conversions achieved was of the order of 49% and 99% for benzyl alcohol and nitrobenzene, respectively, with selectivity to benzaldehyde and aniline of 99% in both reactions. The study also follows the effects of the solvent and the presence of O2.

Selective Photocatalytic Reduction of Nitrobenzene to Aniline Using TiO2 Embedded in sPS Aerogel

In recent years, aromatic substances have become the focus of environmental pollution-related concern due to their high stability and mutagenicity. In this regard, researchers have focused their attention on the development of photocatalytic processes to convert nitroaromatic compounds into aniline. In this work, the photocatalytic conversion of nitrobenzene (NB) to aniline (AN) was studied. The photocatalytic reaction was performed using commercial TiO₂ (P25) and a photocatalytic aerogel, based on P25 embedded in syndiotactic polystyrene (sPS) aerogel (sPS/P25 aerogel) as photocatalysts. Different alcohols were used as hydrogen sources during the photocatalytic experiments. At the optimized operating conditions (photocatalysts dosage: 0.5 mg/L and 50% (v/v) EtOH%), an AN yield of over 99% was achieved. According to the results, this work could open avenues toward effective production of AN from NB using mild reaction conditions with sPS/P25 aerogel-in view of a possible scale-up of the photocatalytic process.

Solar-light-driven ternary MgO/TiO2/g-C3N4 heterojunction photocatalyst with surface defects for dinitrobenzene pollutant reduction

Defect engineering and heterojunction are promising strategies to improve the photocatalytic performance of particular catalyst through effective charge carrier separation and transport. Herein, we developed Z-scheme MgO/TiO₂/g-C₃N₄ ternary heterojunction photocatalyst with surface defects and effective charge separation for reduction of recalcitrant dinitrobenzene isomers under simulated solar light irradiation. Mott-Schottky (MS) plot analysis and electron spin resonance (ESR) radical trapping experiment suggested the formation of Z-scheme heterojunction at the interface of TiO₂/g-C₃N₄, which played a crucial role in the electron-hole separation. Incorporating MgO into the structure further enhances charge separation via Ti³⁺ and oxygen vacancy (OV) defects formation at the TiO₂/MgO interface as confirmed by electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) analyses. Besides, the surface basicity of MgO enhanced conversion of dinitrobenzene (DNB) isomers through formation of nitrophenylhydroxylamine intermediate which can easily be reduced to phenylenediamines (PDAs). As confirmed by high performance liquid chromatography (HPLC) analysis, excellent selectivity for PDAs (95-98%) was achieved in 90 min with ternary MgO/TiO₂/g-C₃N₄ composite compared to the binary MgO/TiO₂ and TiO₂/g-C₃N₄. A possible reaction pathway and photocatalytic reduction mechanism were proposed and elucidated. This work demonstrated an effective strategy to reduce recalcitrant dinitrobenzene isomers using efficient, low-cost, and environmental benign photocatalyst with a facile identification of reaction intermediates.

Tandem Photocatalysis Protocol for Hydrogen Generation/Olefin Hydrogenation Using Pd-g-C3N4-Imine/TiO2 Nanoparticles

An unprecedented visible-light-driven photocatalytic system consisting of Pd nanoparticles stabilized on g-C₃N₄-imine-functionalized TiO₂ nanoparticles was discovered for photoassisted hydrogen generation followed by olefin hydrogenation under mild conditions. The structural integrity of the as-synthesized photocatalyst was corroborated by Fourier transform infrared spectroscopy, X-ray powder diffraction, energy-dispersive X-ray spectroscopy, inductively coupled plasma atomic emission spectroscopy, X-ray photoelectron spectroscopy, ultraviolet-diffuse reflectance spectroscopy, Brunauer-Emmett-Teller measurements, and thermogravimetric analysis (TGA). Transmission electron microscopy and high-resolution scanning electron microscopy revealed the nanoscopic nature of the catalyst. The photocatalyst promoted several different transformations in a one-pot reaction sequence: hydrogen evolution through photocatalytic acceptorless formation of benzimidazoles as important therapeutic agents followed by visible-light-driven photocatalytic reduction of olefins with a high hydrogen utilization efficiency of up to 92% under mild conditions. A significant volume of H₂ was produced under blue light-emitting diode (LED) irradiation during the selective formation of benzimidazole, while the selectivity reduced significantly under a Xe lamp or in the dark. The in situ-generated H₂ could be activated by the as-prepared Pd-C₃N₄-imine/TiO₂ photocatalyst to effectively hydrogenate olefins under mild conditions at appropriate time exposed to blue LED irradiation. The light-dependent photocatalytic performance of the title catalyst was assessed using action spectra by calculating the apparent quantum efficiency (AQE), which exhibited the maximum AQEs at 410 and 550 nm, at which the highest performance for styrene hydrogenation was obtained. The improved photoredox activity of the title nanohybrid could be caused by the synergistic effects of the heterojunction of carbon nitride-Pd on TiO₂ nanoparticles evidenced by photoluminescence spectra and catalytic reactions. The catalyst proved to be air-stable, robust, recyclable, and very active in the absence of any undesirable additives and reducing agents. Thus, this work presents a new protocol for improving the photocatalytic properties of semiconducting materials for various photocatalytic applications under environmentally friendly conditions.

In-situ fabrication of 3D hierarchical flower-like β-Bi2O3@CoO Z-scheme heterojunction for visible-driven simultaneous degradation of multi-pollutants

Development of an efficient heterojunction catalyst with a superior visible-light driven activity is regarded as a promising strategy to decontaminate organic wastewater. Herein, a novel direct Z-scheme β-Bi₂O₃@CoO heterojunction was well designed and successfully fabricated by in situ incorporating the two energy band-matched semiconductors. The obtained β-Bi₂O₃@CoO hybrid presented a unique 3D hierarchical structure with a mass of open channels and mesoporous, which afforded not only rapid mass transfer of targets but also good light-harvesting in view of the multiple reflections. Compared to the pristine β-Bi₂O₃ and CoO, the β-Bi₂O₃@CoO hybrid exhibited remarkably improved photocatalytic activity towards the simultaneous degradation of chlorotetracycline (CTC), tetracycline hydrochloride (TCH), oxytetracycline (OTC) and nitrobenzene (NB) under visible-light irradiation. The possible intermediates and degradation pathways were also tracked by mass spectra (MS) analysis. Moreover, a direct Z-scheme charge transfer mechanism in the intimate contact interface between β-Bi₂O₃ and CoO was verified for the improved catalytic activity, endowing the effective separation/transportation of the photo-excited charge carriers and maintenance of the strong redox ability in β-Bi₂O₃@CoO heterojunction. The present work affords a simple approach to design and construct 3D hierarchical direct Z-scheme photocatalysts with promising applications in water environment remediation.

Experimental study of nitrobenzene degradation in water by strong ionization dielectric barrier discharge

Nitrobenzene (NB) is toxic and carcinogenic aromatic compound widely used in several industries which is ultimately found in their effluents. In this work, dielectric barrier discharge (DBD) reactor was employed for the degradation of nitrobenzene in aqueous solution. Active species like O₃ and ^•OH produced by DBD reactor were mixed with water which degraded the NB. The results indicated that the lower NB concentrations slightly acidic conditions and high voltage ranges showed the optimum efficiencies. Moreover, the impacts of active species inhibitors isopropyl alcohol (IPA), tert-butanol (TBA), inorganic ions for instance sulfates ( S O 4 2 - ), bicarbonates ( H C O 3 - ), nitrates ( N O 3 - ), carbonates ( C O 3 2 - ) and chlorides (Cl^-) on the degradation of NB were examined. This analysis showed that the hydroxyl radical was captured by the addition of these inhibitors and resulted in the decrease in efficiencies. Byproducts produced during the degradation of nitrobenzene were assessed by analytical techniques of high-performance liquid chromatography (HPLC), liquid chromatography-mass spectrometry (LC-MS), UV-visible spectroscopy and total organic carbon (TOC) analysis. Main intermediate products were nitrophenols and low molecular weight organic acids including oxalic acid and acetic acid that were eventually mineralized to CO₂ and H₂O. The dielectric barrier discharge technology was found productive for the degradation of nitroaromatic compounds.

An Efficient and Robust Method for Selective Conversion of Aniline to Azobenzene Using nano-TiO2 -P25-SO3 H, under Visible Light Irradiation

Nano-TiO₂ -P25-SO₃ H as our previous report has successfully been utilized to synthesize azobenzene through the selective conversion of aniline under visible light irradiation. According to PL emission spectra, the immobilizing a solid Brønsted acid of -SO₃ H groups on the pure-TiO₂ -P25 surface with a close interface is an approach to amplify the nano-TiO₂ -P25 response to visible light, which can productively hinder the recombination rate of photogenerated electrons and holes as carriers. Therefore, the photocatalytic activity of the semiconductor is highly likely to increase. Photooxidation of aniline to azobenzene was achieved by applying nano-TiO₂ -P25-SO₃ H (E_g  = 2.6 eV) that activated by blue photons (λ_max  = 460 nm), green photons (λ_max  = 510 nm) and red photons (λ_max  = 630 nm) which is introducing as a sustainable procedure. Central composite design (CCD) was employed for evaluating the effects of photocatalyst amount, oxidant concentration and irradiation time on the synthesis of azobenzene by this approach. Easily synthesizing, recyclability of the photocatalyst, mild reaction condition and short reaction time could be considered as plus points of this process.

Synthesis and Characterization of Zinc Peroxide Nanoparticles for the Photodegradation of Nitrobenzene Assisted by UV-Light

The contamination of both soil and water by nitrobenzene (NB) is a problem that has been studied, where several reactive agents have been developed for the degradation of this compound as well as different methods. Nanoparticles with semiconductive properties have been studied for organic compounds photodegradation due to their assistance in optimizing the degradation processes. Two of the most promising photocatalysts are ZnO and TiO2 because of their optimal results. In the present work the performance of the zinc peroxide (ZnO2) nanoparticles was evaluated. ZnO2 nanoparticles were synthesized from zinc acetate and hydrogen peroxide using the Sol-Gel method under ultrasound assistance. The characterization was carried out by UV–Vis spectroscopy, infrared Fourier transform total reflectance (ATR-FT-IR) spectroscopy, Raman spectroscopy, X-ray diffraction (XRD), Zeta potential, dynamic light scattering (DLS), field emission scanning electron microscopy (FE-SEM), and Energy Dispersive X-ray spectroscopy (EDX). The experiments for the degradation of NB were carried out in a photoreactor with UV lamps of 254 nm at 25 °C, using a solution of nitrobenzene with the nanoparticles. The best conditions for NB photodegradation were 30 ppm (ZnO2) and 15 ppm (NB) at pH 2, reaching up to 90% degradation in 2 h. The intermediates formed during the photodegradation of NB were identified by gas chromatography mass spectrometry.

Chemoselective photocatalytic oxidation of alcohols to aldehydes and ketones by nitromethane on titanium dioxide under violet 400 nm LED light irradiation

In this study, for the first time, nitroalkanes, especially nitromethane, have been used as electron acceptors for the highly chemoselective oxidation of alcohols in the presence of a TiO2 photocatalyst under 400 nm LED irradiation. The reactions showed excellent selectivity for the production of aldehydes. Interestingly, aldehydes such as benzaldehyde and p-methoxybenzaldehyde are stable under the reaction conditions. In the case of the use of 2-nitropropane and 2-methyl-2-nitropropane, the product imine, which is the result of the reaction of the aldehyde with aliphatic amine, is also obtained.

First-principles study on the mechanism of photocatalytic reduction of nitrobenzene on the rutile TiO2(110) surface

Oxygen vacancies and excess electrons play vital roles in the photocatalytic reduction of nitrobenzene on the TiO₂ surface.

Kinetics and Optimization of the Photocatalytic Reduction of Nitrobenzene

The photocatalytic reduction of nitrobenzene to aniline in alcoholic solutions appears as an interesting alternative to the classical hydration. However, little is known about the influence of reaction parameters on the kinetics of the reaction which were therefore studied herein. The effects of light intensity, catalyst concentration, initial concentration, and temperature were systematically investigated under more than 50 different conditions and accurately described with an appropriate kinetic model. The results show that the efficiency of the reaction is extremely high and apparent quantum yields of up to 142 % were observed under optimized conditions. Particularly interesting is the fact high efficiencies were also obtained at high reaction rates of up to 74.3 mM h-1. Overall these results demonstrate that heterogeneous photocatalytic reactions can be very efficient and productive at the same time and may therefore present a powerful tool in synthetic organic chemistry.

Ultra-low content of Pt modified CdS nanorods: Preparation, characterization, and application for photocatalytic selective oxidation of aromatic alcohols and reduction of nitroarenes in one reaction system

A series of Pt nanoparticles (with size of 3-4 nm) decorated CdS nanorods were prepared via a simple solvothermal method. The samples were then used for photocatalytic selective oxidation (SO) of aromatic alcohols and reduction (SR) of nitroarenes in one reaction system. The platinized samples showed enhanced activity for the conversions than pristine CdS as Pt can serve as e^- trapping and reaction sites, by which the recombination of photoinduced charge carriers can be suppressed and the adsorption of reactants and the SR of nitroarenes can be promoted. The sample loaded with only of 0.03% Pt showed the highest performance and, after irradiation for 4 h, the conversions of p-methoxybenzyl alcohol and nitrobenzene are as high as 92.7% and 94.8%, while the yields of p-methoxybenzaldehyde and aniline are 80.5% and 36.0%. The activities are about 2.0 times higher than that of CdS. The coupling reaction mechanism for the SO of aromatic alcohols to aldehydes and SR of nitroarenes to anilines in the reaction system was finally proposed.

Aromatic organoarsenic compounds (AOCs) occurrence and remediation methods

Many researchers at home and abroad have made a body of researches and have gained great achievements on the environmental occurrence, fate, and toxicity of inorganic arsenic. But there is less research on the use of aromatic organoarsenic compounds (AOCs), which are common feed additives for livestock in the poultry industry. In this review, we outline the current state of knowledge acquired on the occurrence and remediation of AOCs, respectively. We also identify knowledge gaps and research needs, including the elucidation of the environmental fate of AOCs, metabolic pathway, the impact of metabolic modification on toxicity, and advanced analytical or repaired methods that allows for monitoring, identification or removal of the degradation products.

Titanium(iv) oxide having a copper co-catalyst: a new type of semihydrogenation photocatalyst working efficiently at an elevated temperature under hydrogen-free and poison-free conditions

A copper-loaded titanium(iv) oxide photocatalyst exhibited perfect selectivity in hydrogenation of alkynes to alkenes in an alcohol solution at 298 K under hydrogen-free and poison-free conditions. A slight elevation in the reaction temperature to 323 K greatly increased the reaction rate with the selectivity being preserved and the formation of an H2 by-product being suppressed. The apparent activation energy of 4-octyne semihydrogenation was determined to be 54 kJ mol-1, indicating that the rate determining step of this photocatalytic reaction was not an electron production process but a thermocatalytic hydrogenation process under light irradiation.

Ring hydrogenation of aromatic compounds in aqueous suspensions of an Rh-loaded TiO2 photocatalyst without use of H2 gas

Aromatics with a carboxyl group were almost quantitatively hydrogenated to corresponding cyclohexanes over an Rh–TiO₂ photocatalyst under H₂-free conditions.

Photocatalytic hydrogenation of furan to tetrahydrofuran in alcoholic suspensions of metal-loaded titanium(iv) oxide without addition of hydrogen gas

Furan was almost quantitatively hydrogenated to tetrahydrofuran in methanolic suspension of a Pd-TiO₂ photocatalyst with an apparent quantum efficiency of 37%.

Organically modified titania having a metal catalyst: a new type of liquid-phase hydrogen-transfer photocatalyst working under visible light irradiation and H2-free conditions

Organically modified titania having a metal catalyst (OMTC), 2,3-dihydroxynaphthalene-modified titania having palladium metal, successfully worked as a hydrogen-transfer (C[double bond, length as m-dash]C hydrogenation) photocatalyst in the presence of triethanolamine as the hydrogen source under visible light irradiation and hydrogen-free conditions.

Development of photocatalysts for selective and efficient organic transformations

One of the main goals of organic chemists is to find easy, environmentally friendly, and cost effective methods for the synthesis of industrially important compounds. Photocatalysts have brought revolution in this regard as they make use of unlimited source of energy (the solar light) to carry out the synthesis of organic compounds having otherwise complex synthetic procedures. However, selectivity of the products has been a major issue since the beginning of photocatalysis. The present article encompasses state of the art accomplishments in harvesting light energy for selective organic transformations using photocatalysts. Several approaches for the development of photocatalysts for selective organic conversions have been critically discussed with the objective of developing efficient, selective, environmental friendly and high yield photocatalytic methodologies.

Theoretical study on catalyzed selective photoreduction mechanism for 4-bromobenzaldehyde in two different solvents

Photoreduction of 4-bromobenzaldehyde is selective in different solvents. Both photoelectrons and alcohol radicals play important roles in this reaction.

Controlled growth of hexagonal Zn2GeO4 nanorods on carbon fibers for photocatalytic oxidation of p-toluidine

Hexagonal Zn₂GeO₄ nanorods grown on the surface of carbon fiber substrates have been prepared for photocatalytic oxidation of p-toluidine.

Probing the role of surface energetics of electrons and their accumulation in photoreduction processes on TiO₂

We address the role of the energetics of photogenerated electrons in the reduction of 4-nitrobenzaldehyde on TiO2. This model molecule bears two functional groups featuring different reducibilities. Electrochemistry shows that reduction to 4-aminobenzyl alcohol occurs in entirely distinct potential ranges. Partial reduction of the -NO2 group, affording 4-aminobenzaldehyde, takes place through surface states at potentials positive of the flatband potential (E(fb)). Dark currents caused by reduction of the aldehyde group are observed only at potentials more negative than E(fb), and the process requires an electron accumulation regime. Photocatalysis with TiO2 suspensions agrees with the electrochemical data. In particular, reduction of the nitro group is a relatively fast process (k=0.059 s(-1)), whereas that of the aldehyde group is slower (k=0.001 s(-1)) and requires electron photoaccumulation. Control of the photogenerated charge is a prospective means for achieving chemoselective reductions.

Photocatalytic hydrogenation of alkenes to alkanes in alcoholic suspensions of palladium-loaded titanium(iv) oxide without the use of hydrogen gas

Styrene was successfully hydrogenated to ethylbenzene in alcoholic suspensions of a Pd–TiO₂ photocatalyst.

Novel method of screening the oxidation and reduction abilities of photocatalytic materials

A methodology for understanding the photocatalytic abilities of materials is presented. The conversion of simple organic molecules was monitored in situ in photocatalytic microreactors.

Photocatalytic degradation of herbicide bentazone in aqueous suspension of TiO2: mineralization, identification of intermediates and reaction pathways

Semiconductor-mediated hydrogen peroxide-assisted photocatalytic degradation of a selected herbicide, Bentazone (1) has been investigated in aqueous suspensions of TiO2 under a variety of conditions. The degradation was studied by monitoring the depletion in total organic carbon content as a function of irradiation time. The degradation kinetics was investigated under different conditions such as type of TiO2 (Anatase/Anatase-Rutile mixture), reaction pH, catalyst dosage and hydrogen peroxide (H202) concentration. The degradation rates were found to be strongly influenced by all the above parameters. Titanium dioxide Degussa P25 was found to be more efficient as compared with other two commercially available TiO2 powders like Hombikat UV100 and PC500 from Millennium Inorganic Chemicals. Gas Chromatography-Mass Spectrometry (GC-MS) analysis of the irradiated mixture of Bentazone (1) indicates the formation of several intermediate products which have been characterized on the basis of molecular ion/mass fragmentation pattern and also on comparison with the National Institute of Standards and Technology (NIST) library. Plausible mechanism for the formation of different products during photocatalytic treatment of Bentazone in the presence of TiO2 has been proposed. The use of H202 substantially increased the efficiency of TiO2 photocatalytic degradation.

Synthesis of graphene-ZnO-Au nanocomposites for efficient photocatalytic reduction of nitrobenzene

A simple hydrothermal method of preparing highly photocatalytic graphene-ZnO-Au nanocomposites (G-ZnO-Au NCs) has been developed. Zinc acetate and graphene oxide are reduced by catechin to form graphene-zinc oxide nanospheres (G-ZnO NSs; average diameter of (45.3 ± 3.7) nm) in the presence of ethylenediamine (EDA) as a stabilizing agent and gold nanorods (Au NRs) at 300 °C for 2 h. Then Au NRs are deposited onto as-formed G-ZnO NSs to form G-ZnO-Au NCs. Upon ultraviolet light activation, G-ZnO-Au NCs (4 mg mL(-1)) in methanol generates electron-hole pairs. Methanol (hydroxyl group) assists in trapping holes, enabling photogenerated electrons to catalyze reduction of nitrobenzene (NB) to aniline with a yield of 97.8% during a reaction course of 140 min. The efficiency of G-ZnO-Au NCs is 3.5- and 4.5-fold higher than those provided by commercial TiO2 and ZnO NSs, respectively. Surface assisted laser desorption/ionization mass spectrometry has been for the first time applied to detect the intermediates (nitrosobenzene and phenylhydroxylamine) and major product (aniline) of NB through photoelectrocatalytic or photocatalytic reactions. The result reveals that the reduction of NB to aniline is through nitrosobenzene to phenylhydroxylamine in the photoelectrocatalytic reaction, while via nitrosobenzene directly in the photocatalytic reaction. G-ZnO-Au NC photocatalyst holds great potential in removal of organic pollutants like NB and in the production of aniline.

Unveiling chemical reactivity and structural transformation of two-dimensional layered nanocrystals

Two-dimensional (2D) layered nanostructures are emerging fast due to their exceptional materials properties. While the importance of physical approaches (e.g., guest intercalation and exfoliation) of 2D layered nanomaterials has been recognized, an understanding of basic chemical reactions of these materials, especially in nanoscale regime, is obscure. Here, we show how chemical stimuli can influence the fate of reaction pathways of 2D layered nanocrystals. Depending on the chemical characteristics (Lewis acid ((1)O2) or base (H2O)) of external stimuli, TiS2 nanocrystal is respectively transformed to either a TiO2 nanodisc through a "compositional metathesis" or a TiO2 toroid through multistage "edge-selective structural transformation" processes. These chemical reactions can serve as the new design concept for functional 2D layered nanostructures. For example, TiS2(disc)-TiO2(shell) nanocrystal constitutes a high performance type II heterojunction which not only a wide range solar energy coverage (~80%) with near-infrared absorption edge, but also possesses enhanced electron transfer property.