Photocatalytic conversion of benzene to phenol using modified TiO2 and polyoxometalates

Advanced oxidation processes for removal of monocyclic aromatic hydrocarbon from water: Effects of O3/H2O2 and UV/H2O2 treatment on product formation and biological post-treatment

Several oxidative treatment technologies, such as ozonation or Fenton reaction, have been studied and applied to remove monocyclic hydroaromatic carbon from water. Despite decades of application, little seems to be known about formation of transformation products while employing different ozone- or ^∙OH-based treatment methods and their fate in biodegradation. In this study, we demonstrate that O₃/H₂O₂ treatment of benzene, toluene, ethylbenzene (BTE), and benzoic acid (BA) leads to less hydroxylated aromatic transformation products compared to UV/H₂O₂ as reference system - this at a similar ^∙OH exposure and parent compound removal efficiency. Aerobic biodegradation tests after oxidation of 0.15 mM BA (12.6 mg C L^-1 theoretical DOC) revealed that a less biodegradable DOC fraction > 4 mg C L^-1 was formed in both oxidative treatments compared to the BA control. No advantage of ozonation over UV/H₂O₂ treatment was observed in terms of mineralization capabilities, however, we detected less transformation products after oxidation and biodegradation using high-resolution mass spectrometry. Biodegradation of BA that was not oxidized was more complete with minimal organic residual. Overall, the study provides new insights into the oxidation of monocyclic aromatics and raises questions regarding the biodegradability of oxidation products, which is relevant for several treatment applications.

UVA-irradiated dual photoanodes and dual cathodes photocatalytic fuel cell: mechanisms and Reactive Red 120 degradation pathways

To enhance dye removal and energy recovery efficiencies in single-pair electrode photocatalytic fuel cell (PFC-AC), dual cathodes PFC (PFC-ACC) and dual photoanodes PFC (PFC-AAC) were established. Results revealed that PFC-AAC yielded the highest decolorization rate (1.44 h-1) due to the promotion of active species such as superoxide radical (•O2-) and hydroxyl radical (•OH) when the number of photoanode was doubled. The results from scavenging test and UV-Vis spectrophotometry disclosed that •OH was the primary active species in dye degradation of PFC. Additionally, PFC-AAC also exhibited the highest power output (17.99 μW) but the experimental power output was much lower than the theoretical power output (28.24 μW) due to the strong competition of electron donors of doubled photoanodes to electron acceptors at the single cathode and its high internal resistance. Besides, it was found that the increments of dye volume and initial dye concentration decreased the decolorization rate but increased the power output due to the higher amount of sacrificial agents presented in PFC. Based on the abovementioned findings and the respective dye intermediate products identified from gas chromatography-mass spectrometry (GC-MS), the possible degradation pathway of RR120 was scrutinized and proposed.

Recent Advances in the Heterogeneous Photocatalytic Hydroxylation of Benzene to Phenol

Phenol is an important chemical material that is widely used in industry. Currently, phenol is dominantly produced by the well−known three−step cumene process, which suffers from severe drawbacks. Therefore, developing a green, sustainable, and economical strategy for the production of phenol directly from benzene is urgently needed. In recent years, the photocatalytic hydroxylation of benzene to phenol, which is economically feasible and could be performed under mild conditions, has attracted more attention, and development of highly efficient photocatalyst would be a key issue in this field. In this review, we systematically introduce the recent achievements of photocatalytic hydroxylation of benzene to phenol from 2015 to mid−2022, and various heterogeneous photocatalysts are comprehensively reviewed, including semiconductors, polyoxometalates (POMs), graphitic carbon nitride (g−C3N4), metal–organic frameworks (MOFs), carbon materials, and some other types of photocatalysts. Much effort is focused on the physical and chemical approaches for modification of these photocatalysts. The challenges and future promising directions for further enhancing the catalytic performances in photocatalytic hydroxylation of benzene are discussed in the end.

Evaluation of Au/γ-Al2O3 nanocatalyst for plasma-catalytic decomposition of toluene

The emission of toluene into the atmosphere can seriously affect the environmental quality and endanger human health. A dielectric barrier discharge reactor filled with a small amount of Au nanocatalysts was used to decompose toluene in He and O₂ gases mixtures at room temperature and atmospheric pressure. Normally, the oxidation of toluene using Au nanocatalysts suffers from low reaction activity and facile catalyst deactivation. Herein, the effects of Au loading, calcination time and calcination temperature were systematically investigated. It was found that 0.1 wt%Au/γ-Al₂O₃ calcined at 300 °C for 5 h can keep an average size around 6 nm with good dispersion on γ-Al₂O₃ surface and display the best catalytic performance. Moreover, the influences of energy density, gas flow rate, toluene concentration and O₂ concentration on toluene degradation using 0.1 wt%Au/γ-Al₂O₃ were evaluated. It showed the best catalytic performance of near 100% conversion for toluene degradation under the reaction conditions of the energy density was 20 J/L, the gas flow rate was 300 mL/min, the concentration of toluene was 376 mg/m³ and the oxygen content was 10%. Combining experimental results and theoretical calculations, the values of reaction constant k were 8.6 × 10^-5, 3.53 × 10^-5 and 3.09 × 10^-5 m⁶/(mol*J), when O₂ concentration, power or flow rate changed, respectively. Therefore, O₂ concentration has the greatest effect on toluene decomposition compared to other factors in the presence of Au/γ-Al₂O₃.

Enhanced Photocatalytic Hydrogen Production of the Polyoxoniobate Modified with RGO and PPy

The development of high-efficiency, recyclable, and inexpensive photocatalysts for water splitting for hydrogen production is of great significance to the application of solar energy. Herein, a series of graphene-decorated polyoxoniobate photocatalysts Nb₆/PPy-RGO (Nb₆ = K₇HNb₆O₁₉, RGO = reduced graphene oxide, PPy = polypyrrole), with the bridging effect of polypyrrole were prepared through a simple one-step solvothermal method, which is the first example of polyoxoniobate-graphene-based nanocomposites. The as-fabricated photocatalyst showed a photocatalytic H₂ evolution activity without any co-catalyst. The rate of 1038 µmol g⁻¹ in 5 h under optimal condition is almost 43 times higher than that of pure K₇HNb₆O₁₉·13H₂O. The influencing factors for photocatalysts in photocatalytic hydrogen production under simulated sunlight were studied in detail and the feasible mechanism is presented in this paper. These results demonstrate that Nb₆O₁₉ acts as the main catalyst and electron donor, RGO provides active sites, and PPy acted as an electronic bridge to extend the lifetime of photo-generated carriers, which are crucial factors for photocatalytic H₂ production.

One-Step Catalytic or Photocatalytic Oxidation of Benzene to Phenol: Possible Alternative Routes for Phenol Synthesis?

Phenol is an important chemical compound since it is a precursor of the industrial production of many materials and useful compounds. Nowadays, phenol is industrially produced from benzene by the multi-step “cumene process”, which is energy consuming due to high temperature and high pressure. Moreover, in the “cumene process”, the highly explosive cumene hydroperoxide is produced as an intermediate. To overcome these disadvantages, it would be useful to develop green alternatives for the synthesis of phenol that are more efficient and environmentally benign. In this regard, great interest is devoted to processes in which the one-step oxidation of benzene to phenol is achieved, thanks to the use of suitable catalysts and oxidant species. This review article discusses the direct oxidation of benzene to phenol in the liquid phase using different catalyst formulations, including homogeneous and heterogeneous catalysts and photocatalysts, and focuses on the reaction mechanisms involved in the selective conversion of benzene to phenol in the liquid phase.

Treatment of wastewater from petroleum industry: current practices and perspectives

Petroleum industry is one of the fastest growing industries, and it significantly contributes to economic growth in developing countries like India. The wastewater from a petroleum industry consist a wide variety of pollutants like petroleum hydrocarbons, mercaptans, oil and grease, phenol, ammonia, sulfide, and other organic compounds. All these compounds are present as very complex form in discharged water of petroleum industry, which are harmful for environment directly or indirectly. Some of the techniques used to treat oily waste/wastewater are membrane technology, photocatalytic degradation, advanced oxidation process, electrochemical catalysis, etc. In this review paper, we aim to discuss past and present scenario of using various treatment technologies for treatment of petroleum industry waste/wastewater. The treatment of petroleum industry wastewater involves physical, chemical, and biological processes. This review also provides scientific literature on knowledge gaps and future research directions to evaluate the effect(s) of various treatment technologies available.

First Insights into Photocatalytic Degradation of HDPE and LDPE Microplastics by a Mesoporous N–TiO2 Coating: Effect of Size and Shape of Microplastics

Microplastics (MPs), which are small plastic debris of ≤5 mm size, are polluting the oceans with negative consequences for their biota. In this work, visible-light photocatalysis of high-density polyethylene (HDPE) and low-density polyethylene (LDPE) MPs in aqueous medium using a mesoporous N–TiO2 coating is proposed as an alternative for fighting MP pollution. Spherical primary HDPE MPs were extracted from commercially available facial scrubs, while film-shaped secondary LDPE MPs were obtained from a plastic bag. For each plastic, two different sizes were tested. Degradation was measured by mass-loss and carbonyl-index (CI) calculation. The results obtained reveal that the photocatalytic degradation of HDPE and LDPE MPs using an N–TiO2 coating was affected by the size and shape of the MPs. Smaller MPs led to higher degradation, while film-shaped MPs led to lower degradation that was related to a poorly illuminated and oxygenated reaction medium. These results set the basis for further investigation on the on the design of more effective photocatalytic-reaction systems for decreasing MP inputs to the environment.

In situ treatment of metalworking wastewater by chemical addition-dissolved air flotation coupled with UV, H2O2 & ZnO

The hybrid treatment techniques have been proven that could be a proper solution to the metalworking fluid (MWF) wastewaters disposal challenge. Hence, this investigation was conducted aim to the assessment of chemical addition-dissolved air floatation (CA-DAF) unit followed with a heterogeneous photocatalytic (PC) process as UV/H₂O₂/ZnO to treat MWF wastewater produced in one of the central industrial estates in the Middle East. The CA-DAF unit was implemented as trial and errors and had an appropriate efficiency. However, the environmental discharge standards were not achieved only by this unit, so that, a PC process considered for this purpose in a pilot-scale reactor. And also, Chemical oxygen demand (COD), total petroleum hydrocarbons (TPHs) were considered as physicochemical parameters to analyze the applied photochemical reaction throughout the concentrations of ZnO and H₂O₂, and pH value as variables of the study. The ideal and optimized conditions were observed at pH 10, 600 mg l^-1 of ZnO, and 13.11 g l^-1 of H₂O₂ via 99.87% and 97.9% reduction rates in total COD and TPH, respectively. These removal rates were obtained for this integrated strategy under the optimized reaction. By evaluating the synergistic effect, it was found that UV/ZnO could be a predominant reaction in this process. The organic and intermediates analysis appeared 78.46% reduction for all detected organic matters. Besides, PC generation of bis phthalate, mono phthalate, benzene, and benzoic acid to 2,6-bis (1,1-dimethylethyl)-4-mthyl phenol was the reason of the residual phenolic compound concentration in the reaction solution with low removal rate. The kinetic study showed that this reaction could be well fitted with the pseudo-first-order kinetic model by R ² equal with 0.973 and 0.988 in turn for COD and TPH. The expenditures to treat 1 m³ of the CA-DAF was estimated at 5.335 us$ via cost analysis. Finally, the collected findings indicate that CA-DAF integrated with UV/H₂O₂/ZnO can be an efficient approach in the MWF wastes disposal or treatment for reuse.

Metal/Semiconductor Nanocomposites for Photocatalysis: Fundamentals, Structures, Applications and Properties

Due to the capability of utilizing light energy to drive chemical reactions, photocatalysis has been widely accepted as a green technology to help us address the increasingly severe environment and energy issues facing human society. To date, a large amount of research has been devoted to enhancing the properties of photocatalysts. As reported, coupling semiconductors with metals is one of the most effective methods to achieve high-performance photocatalysts. The excellent properties of metal/semiconductor (M/S) nanocomposite photocatalysts originate in two aspects: (i) improved charge separation at the metal-semiconductor interface; and (ii) increased absorption of visible light due to the surface plasmon resonance of metals. So far, many M/S nanocomposite photocatalysts with different structures have been developed for the application in environmental remediation, selective organic transformation, hydrogen evolution, and disinfection. Herein, we will give a review on the M/S nanocomposite photocatalysts, regarding their fundamentals, structures (as well as their typical synthetic approaches), applications and properties. Finally, we will also present our perspective on the future development of M/S nanocomposite photocatalysts.

Interpreting Interactions between Ozone and Residual Petroleum Hydrocarbons in Soil

We evaluated how gas-phase O₃ interacts with residual petroleum hydrocarbons in soil. Total petroleum hydrocarbons (TPH) were 18 ± 0.6 g/kg soil, and TPH carbon constituted ∼40% of the dichloromethane-extractable carbon (DeOC) in the soil. At the benchmark dose of 3.4 kg O₃/kg initial TPH, TPH carbon was reduced by nearly 6 gC/kg soil (40%), which was accompanied by an increase of about 4 gC/kg soil in dissolved organic carbon (DOC) and a 4-fold increase in 5-day biochemical oxygen demand (BOD₅). Disrupting gas channeling in the soil improved mass transport of O₃ to TPH bound to soil and increased TPH removal. Ozonation resulted in two measurable alterations of the composition of the organic carbon. First, part of DeOC was converted to DOC (∼4.1 gC/kg soil), 75% of which was not extractable by dichloromethane. Second, the DeOC containing saturates, aromatics, resins, and asphaltenes (SARA), was partially oxidized, resulting in a decline in saturates and aromatics, but increases in resins and asphaltenes. Ozone attack on resins, asphaltenes, and soil organic matter led to the production of NO₃^-, SO₄^2-, and PO₄^3-. The results illuminate the mechanisms by which ozone gas interacted with the weathered petroleum residuals in soil to generate soluble and biodegradable products.

Hydroxylation of Benzene via C-H Activation Using Bimetallic CuAg@g-C3N4

Bimetallic CuAg@g-C3N4 catalyst system has been designed and synthesized by impregnating copper and silver nanoparticles over the graphitic carbon nitride surface. Its application has been demonstrated in the hydroxylation of benzene under visible light.

Efficient photocatalytic oxidation of benzene to phenol by metal complex-clay/TiO2 hybrid photocatalyst

Synthetic saponite containing a photosensitizing metal complex was complexed with colloidal anatase and used for the visible light photocatalytic reaction of aqueous benzene to phenol.

A new composite membrane based on Keggin polyoxotungstate/poly(vinylidene fluoride) and its application in photocatalysis

A new composite membrane based on Keggin POMs and PVDF not only exhibits good photocatalytic activity and repeatability, but also can be easily handled and recycled.

Visible light-driven decomposition of gaseous benzene on robust Sn2+-doped anatase TiO2 nanoparticles

The Sn²⁺-doped TiO₂ nanoparticles are excellent and promising visible-light photocatalysts for the decomposition of benzene with robust photostability.

Effects of electrode geometry on the performance of dielectric barrier/packed-bed discharge plasmas in benzene degradation

In this study, the effects of electrode geometry on benzene degradation in a dielectric barrier/packed-bed discharge plasma reactor with different electrodes were systematically investigated. Three electrodes were employed in the experiments, these were coil, bolt, and rod geometries. The reactor using the coil electrode showed better performance in reducing the dielectric loss in the barrier compared to that using the bolt or rod electrodes. In the case of the coil electrode, both the benzene degradation efficiency and energy yield were higher than those for the other electrodes, which can be attributed to the increased role of surface mediated reactions. Irrespective of the electrode geometry, the packed-bed discharge plasma was superior to the dielectric barrier discharge plasma in benzene degradation at any specific applied voltage. The main gaseous products of benzene degradation were CO, CO2, H2O, and formic acid. Discharge products such as O3, N2O, N2O5, and HNO3 were also detected in the outlet gas. Moreover, the presence of benzene inhibited the formation of ozone because of the competing reaction of oxygen atoms with benzene. This study is expected to offer an optimized approach combining dielectric barrier discharge and packed-bed discharge to improve the degradation of gaseous pollutants.

Methane from benzene in argon dielectric barrier discharge

A first-time account of direct, on-line, instantaneous and efficient chemical conversion of gas phase benzene to methane in argon Dielectric Barrier Discharge (DBD) is presented. In the absence of another overt hydrogen-donating source, potency of analogous parents toward methane generation is found to follow the order: benzene>toluene>p-xylene. Simultaneous production of trace amounts of phenolic surface deposits suggest (a) prompt decomposition of the parent molecules, including a large fraction yielding atomic transients (H-atom), (b) continuous and appropriate recombination of such parts, and (c) trace moisture in parent contributing OH radicals and additional H-atoms, which suitably react with the unreacted fraction of the parent, and also other intermediates. Results highlight Ar DBD to be a simple and exploitable technology for transforming undesirable hazardous aromatics to usable/useful low molecular weight open-chain products following the principles of green chemistry and engineering.

Nanofiber-Membrane-Supported TiO2as a Catalyst for Oxidation of Benzene to Phenol

We applied a simple, low-cost design of glass capillary microreactor for the catalytic oxidation of benzene to phenol at ambient conditions. Polyvinylchloride-nanofiber-membrane-supported titania nanoparticle (TiO2-PVC) as catalyst and in situ production of hydroxyl radicals as oxidant. The reaction was monitored by gas chromatography-mass spectrometry (GC-MS). The reaction conditions were optimized and the performance of the microreactor was then compared with the conventional laboratory scale reaction which used hydrogen peroxide as oxidant. The microreactor gave a better yield of 14% for phenol compared to 0.14% in the conventional laboratory scale reaction. Reaction conditions such as reaction time, reaction pH, and applied potential were optimized. With optimized reaction conditions selectivity of >37% and >88% conversion of benzene were obtained.

Photocatalytic Degradation of Organic Pollutants: A Review

Water pollution is increasing at an ever increasing pace and the whole world is in the cancerous grip of this pollution. Various industries are discharging their untreated effluents into the nearby water resources; thus, adding to the existing water pollution to a great extent. Hence, there is a pressing demand to develop an alternate technology for wastewater treatment and in this context; photocatalysis has emerged as an Advanced Oxidation Process with green chemical approach for such a treatment. This chapter deals with photocatalytic degradation of different kinds of organic pollutants; mainly surfactants, pesticides, dyes, phenols, chloro compounds, nitrogen containing compounds etc. Mechanisms of their degradation have also been discussed with hydroxyl and allied radicals as the main active oxidizing species.

A review of photocatalysis using self-organized TiO2 nanotubes and other ordered oxide nanostructures

Photocatalytic approaches, that is the reaction of light-produced charge carriers at a semiconductor surface with their environment, currently attract an extremely wide scientific interest. This is to a large extent due to the high expectations: i) to convert sunlight directly into an energy carrier (H(2)), ii) to stimulate chemical synthetic reactions, or iii) to degrade unwanted environmental pollutants. Since the early reports in 1972, TiO(2) has been the most investigated photocatalytic material by far; this originates from its outstanding electronic properties that allow for a wide range of applications. Not only the material, but also its structure and morphology, can have a considerable influence on the photocatalytic performance of TiO(2). In recent years, particularly 1D (or pseudo 1D) structures such as nanowires and nanotubes have received great attention. The present Review focuses on TiO(2) nanotube arrays (and similar structures) that grow by self-organizing electrochemistry (highly aligned) from a Ti metal substrate. Herein, the growth, properties, and applications of these tubes are discussed, as well as ways and means to modify critical tube properties. Common strategies are addressed to improve the performance of photocatalysts such as doping or band-gap engineering, co-catalyst decoration, junction formation, or applying external bias. Finally, some unique applications of the ordered tube structures in various photocatalytic approaches are outlined.

Progress in Heterogeneous Photocatalysis: From Classical Radical Chemistry to Engineering Nanomaterials and Solar Reactors

The field of heterogeneous photocatalysis has expanded rapidly in the last four decades, having undergone various evolutionary phases related to energy and the environment. The two most significant applications of photocatalysis are geared toward solar water splitting and the purification of air and water. Notably, the interdisciplinary nature of the field has increased significantly, incorporating semiconductor physics, surface sciences, photo and physical chemistry, materials science, and chemical engineering. Whereas this forms the basis on which the field continues to grow, adequate bridging of multidisciplinary knowledge remains essential. By recalling some of the classical fundamentals of photocatalysis, this Perspective provides contemporary views on heterogeneous photochemical conversion, encompassing charge transport characteristics, radical chemistry and organic degradation mechanisms, photocatalyst design, and photoreactor engineering.

One-pot synthesis of CdS and Ni-doped CdS hollow spheres with enhanced photocatalytic activity and durability

CdS and Ni-doped CdS hollow spheres were synthesized via a simple template-free one-pot method. The products were characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, energy-dispersive spectroscopy analysis, X-ray photoelectron spectroscopy, and UV-vis absorption spectroscopy. The formation mechanism for the Ni-doped CdS hollow spheres was discussed. The prepared CdS and Ni-doped CdS hollow spheres showed the superior photocatalytic activity for the degradation of RhB under visible light (λ > 420 nm) irradiation, and 1.2 mol % Ni-doped CdS hollow spheres were found to be highly efficient for organic pollutants RhB and phenol removal. Moreover, this catalyst showed improved stability, and the activity did not decrease significantly after four recycles. The unique hollow spheres structure may favor the harvesting of exciting light due to multiple scattering within the interior space, and the doping of Ni(2+) may facilitate the generation of electrons and holes pairs and inhibit their recombination rate by act as a temporary trapping sites of photoinduced electrons.

Effect of surface fluorination on the electrochemical and photoelectrocatalytic properties of nanoporous titanium dioxide electrodes

Titanium dioxide is a widely used photocatalyst whose properties can be modified by fluoride adsorption. This work is focused on the effect of surface fluorination on the electrochemical and photoelectrocatalytic properties of TiO(2) nanoporous thin films. Surface fluorination was achieved by simple addition of HF to the working solution (pH 3.5). Open circuit potential as well as ex situ XPS measurements verify that surface modification takes place. Fluorination triggers a significant capacitance increase in the accumulation potential region, as revealed by dark voltammetric measurements for all the TiO(2) samples studied. The photoelectrocatalytic properties (measured as photocurrents under white light illumination) depend on the substrate being oxidized and, in some cases, on the nature of the TiO(2) sample. In particular, the results obtained for electrodes prepared with a mixed phase (rutile + anatase) commercial nanopowder (PI-KEM) indicate that the processes mediated by surface trapped holes, such as the photooxidation of water or methanol, are accelerated while those occurring by direct hole capture from the adsorbed state (formic acid) are retarded. The photooxidation of catechol and phenol is also enhanced upon fluorination. In such a case, the effect can be rationalized on the basis of a diminished recombination and a surface displacement of both the oxidizable organic substrates and the poisoning species formed as a result of the organics oxidation. Photoelectrochemical and in situ infrared spectroscopic measurements support these ideas. In a more general vein, the results pave the way toward a better understanding of the photocatalysis phenomena, unravelling the importance of the reactant adsorption processes.