Heterogeneous Photocatalytic Oxidation Research Articles

Hydroxyl Radical Mediated Heterogeneous Photocatalytic Baeyer-Villiger Oxidation over Covalent Triazine/Heptazine-Based Frameworks.

The Baeyer-Villiger oxidation of ketones to the corresponding lactones/esters is a classic and essential reaction in the chemical industry. However, this oxidation process has not yet been achieved in ambient conditions with the aid of oxygen and heterogeneous photocatalysts. In this study, we developed an organic photocatalytic system using covalent triazine/heptazine-based frameworks (CTF-TB/CHF-TB) to enable the B-V oxidation reaction under mild conditions through a cascade reaction pathway. Experimental data and theoretical calculations showed that heptazine/triazine units can "chelate" and decompose the in situ generated H2O2 into hydroxyl radicals (•OH). Compared to conventional methods that primarily involve metal-activated benzaldehyde at elevated temperatures (e.g., 60 oC), the •OH generated in our study can readily cleave the C-H bond of benzaldehyde, forming an active intermediate that drives subsequent sequential processes: O2→H2O2→•OH→Ph-CO•→Ph-COOO•. By employing this photocatalytic process, a yield of 91% and a selectivity of over 99 % were obtained in the oxidation of cyclohexanone to caprolactone at room temperature. This performance is comparable to the state-of-the-art catalysts, and our CHF-TB catalyst demonstrates impressive reusability, maintaining a high yield after 5 consecutive runs. This work presents a straightforward approach for C-H cleavage by organocatalysts to produce ε-caprolactone in a mild manner by Baeyer-Villiger oxidation.

Heterogeneous photocatalytic methanol oxidation coupled with oxygen reduction toward pyrimidines synthesis

Heterogeneous photocatalytic methanol oxidation coupled with oxygen reduction toward pyrimidines synthesis

Heterogeneous Photocatalytic Oxidation and Detoxification of Simulated Agricultural Wastewater Contaminated with Boscalid Fungicide Using g-C3N4 Catalyst

In the present study, the photocatalytic oxidation and detoxification of aqueous matrices contaminated with boscalid using g-C3N4 catalyst and UV-A light was investigated. The UV-A/g-C3N4 process was found to achieve higher than 83% removal of boscalid in both matrices, with h+ and O2•− being the main species. UHPLC-HRMS analysis allowed the identification of five TPs, while the main degradation pathways involved hydroxylation, cyclization, and dechlorination. Scenedesmus rubescens microalgae species was exposed to boscalid solutions and lake water spiked with the fungicide before the photocatalytic treatment and inhibition in the growth rate was observed. An increase in the toxicity was also observed during the first stages of the treatment. The results from the in silico study correlate with the observed evolution of ecotoxicity during the application of the process, as some of the identified TPs were found to be toxic or very toxic for aquatic organisms. However, prolonged application of the process can lead to detoxification. It was also observed that the g-C3N4 catalyst can retain its photochemical stability and activity after at least three cycles. However, a slight decrease in the activity was observed when repeated another two times. This study demonstrated that the suggested photocatalytic process can both decrease the harmful effects of boscalid as well as effectively lower its concentration in water.

Reaction Steps in Heterogeneous Photocatalytic Oxidation of Toluene in Gas Phase-A Review.

A review of the current literature shows there is no clear consensus regarding the reaction mechanisms of air-borne aromatic compounds such as toluene by photocatalytic oxidation. Potential oxidation reactions over TiO2 or TiO2-based catalysts under ultraviolet and visible (UV/VIS) illumination are most commonly considered for removal of these pollutants. Along the pathways from a model pollutant, toluene, to final mineralization products (CO2 and H2O), the formation of several intermediates via specific reactions include parallel oxidation reactions and formation of less-reactive intermediates on the TiO2 surface. The latter may occupy active adsorption sites and causes drastic catalyst deactivation in some cases. Major hazardous gas-phase intermediates are benzene and formaldehyde, classified by the International Agency for Research on Cancer (IARC) as Group 1 carcinogenic compounds. Adsorbed intermediates leading to catalyst deactivation are benzaldehyde, benzoic acid, and cresols. The three most typical pathways of toluene photocatalytic oxidation are reviewed: methyl group oxidation, aromatic ring oxidation, and aromatic ring opening.

Pyridinium-derived polycationic covalent organic polymers for aromatic C–H bond photocatalytic oxidation

Using oxygen in the air as the sole oxidant to oxidize hydrocarbons into high value-added compounds is a highly promising synthesis strategy with economic advantages. However, the oxidation of hydrocarbons with molecular oxygen under mild conditions is challenging due to the large C–H bond energy in hydrocarbons. Herein, a metal-free two-dimensional covalent organic polymers (COP) functionalized by photoactive pyridinium units has been developed for heterogeneous photocatalytic oxidation of hydrocarbons. This is the first kind of COPs material that can achieve photocatalytic oxidation of hydrocarbons without any additives or stoichiometric oxidants except for the oxygen in the air.

Novel Indium Vanadium Oxide Nanosheet-Supported Nickel Iron Oxide Nanoplate Heterostructure for Synergistically Enhanced Photocatalytic Degradation of Tetracycline

Semiconductor-based heterogeneous photocatalytic oxidation processes have received considerable attention for the remediation of toxic pollutants. Herein, InVO4/NiFe2O4 nanocomposites were synthesized using a facile hydrothermal technique. Furthermore, various characterization results revealed the successful loading of NiFe2O4 nanoplates over InVO4 nanosheets, thereby signifying the formation of a heterostructure. The performance of the synthesized photocatalyst was tested for tetracycline (TC) antibiotic removal. The optimized InVO4/NiFe2O4 nanocomposite exhibits maximum photodegradation of TC molecules (96.68%) in 96 min; this is approximately 6.47 and 4.93 times higher than that observed when using NiFe2O4 and InVO4, respectively. The strong interaction between the InVO4 nanosheets and NiFe2O4 nanoplates can improve the visible-light absorption and hinder the recombination of charge carriers, further enhancing the photocatalytic performance. Moreover, hydroxyl radicals play a crucial role in the photodegradation of TC antibiotics.

CFD modeling of ethylene degradation in gas-phase photocatalytic reactors.

Photocatalytic oxidation is a promising technology to degrade volatile organic compounds. The performance of photocatalytic reactors is affected by the hydrodynamics, radiation transfer, mass transfer and reaction kinetics. Baffles may improve the hydrodynamics. The effect of baffles on heterogeneous photocatalytic oxidation of gas-phase ethylene in three annular reactors is simulated using computational fluid dynamics. ANSYS Fluent is used to solve all governing equations. Baffles can improve the uniformity of flow and prolong the residence time. The residence time of the C-type reactor and B-type reactor is 0.5% greater than the unbaffled reactor. Baffles have little effect on the radiation distribution. The concentric arrangement of lamp and the reactor leads to a radial dominance of radiation. The effect of baffles on the diffusion of ethylene is complex. The effective diffusion coefficient at the catalyst surface in the C-type reactor decreases 9.5% and that in the B-type reactor increases 3% with respect to the unbaffled reactor. The outlet ethylene concentration is 4.19 ppmv for the U-type reactor, 3.93 ppmv for the C-type reactor and 3.62 ppmv for the B-type reactor. The optimal performance in the B-type reactor is due to the large diffusion coefficient of ethylene. The arrangement of baffles should enlarge the effective diffusion coefficient at the catalyst surface as far as possible.

Phenol degradation of waste and stormwater on a flat plate photocatalytic reactor with TiO2 on glass slide: An experimental and modelling investigation

Heterogeneous photocatalytic oxidation process using TiO2 (titanium dioxide) and UV light is a promising method to degrade a wide variety of recalcitrant organic substances such as phenol, pesticides, herbicides, etc. from waste and stormwater. Due to lack of suitable computational fluid dynamics (CFD) model, the industrial-scale application of this technique poses a few challenges indicating the need to clearly understand the hydrodynamic behaviour of the photocatalytic system and investigate the ways of improving their performance. Inadequate mixing and turbulence in a fixed photocatalytic reactor limit the performance of the reactor in terms of pollutant mass transport to the catalyst surface and pollutant degradation. In this study, a 3D flat plate photocatalytic reactor was designed and developed, and numerically analysed using CFD code FLUENT to investigate the performance of the reactor. The CFD modelling results were validated against the experimental data at various flow conditions. The CFD results including velocity and phenol concentration was found in strong agreement with the experimental results with R2 value of 0.998. The experimental results shows that the phenol degradation was 85.5% when the irradiation was 70.6 Wm−2 and initial concentration of phenol was 20 PPM The experimental results also show that the phenol degradation increases with the increase of irradiation but decreases with the increase of initial concentration. The results show that phenol degradation of the reactor decreases with the increase in flow rate as expected. The CFD model can accurately predict the velocity and the concentration of phenol at different flow rates.

Response mechanisms of different antibiotic-resistant bacteria with different resistance action targets to the stress from photocatalytic oxidation

The stress response of antibiotic-resistant bacteria (ARB) and the spread of antibiotic resistance genes (ARGs) pose a serious threat to the aquatic environment and human beings. This study mainly explored the effect of the heterogeneous photocatalytic oxidation (UVA-TiO2 system) on the stress response mechanism of ARB with different antibiotic resistance action targets, including the cell wall, proteins, DNA, RNA, folate and the cell membrane. Results indicate that the stress response mechanism of tetracycline- and sulfamethoxazole-resistant E. coli DH5α, which targets the synthesis of protein and folate, could rapidly induce global regulators by the overexpression of relative antibiotic resistance action target genes. Different stress response systems were mediated via cross-protection mechanism, causing stronger tolerance to an adverse environment than other ARB. Moreover, the photocatalytic inactivation mechanism of bacterial cells and a graded response of cellular stress mechanism caused differences in the intensity of the stress mechanism of antibiotic resistance action targets. E. coli DH5α resistant to cefotaxime and polymyxin, targeting synthesis of the cell wall and cell membrane, respectively, could confer greater advantages to bacterial survival and higher conjugative transfer frequency than E. coli DH5α resistant to nalidixic acid and rifampicin, which target the synthesis of DNA and RNA, respectively. This new perspective provides detailed information on the practical application of photocatalytic oxidation for inactivating ARB and hampering the spreading of ARGs in the aquatic environment.

Quantitative evaluation of structure-activity relationships in heterogeneous photocatalytic oxidation towards organic contaminants

Photocatalysis based advanced oxidation processes (AOPs) have drawn increasing attention for the removal of organic contaminants, while the relationships of the target organic contaminants with the intrinsic properties of the photocatalysts are usually neglected. Herein, the photocatalytic behaviors among ten kinds of organic contaminants and two heterojunction photocatalysts were comparatively investigated. Photocatalysts BiOCl0.75I0.25/g-C3N4 (BCI-CN) with abundant oxygen vacancies exhibited stronger absorption capacity towards light and quicker transfer efficiency of photoinduced carriers (factor η1×2) than BiOCl/g-C3N4 (BC-CN). Nevertheless, the redox ability (factor η3) of BCI-CN was weaker than that of BC-CN. Factor η1×2 and η3 synergistically and competitively affect the photocatalytic activities. Based on the differences and connections about intrinsic properties of BC(I)-CN photocatalysts, quantitative evaluation of structure-activity relationships among different organic contaminants and two photocatalysts were constructed, which provide valuable guidance for objectively evaluating the photocatalytic performance and precisely recognizing the compatibility among pollutants and photocatalysts.

Strategies and Challenges on Selectivity of Photocatalytic Oxidation of Organic Substances

AbstractMost organic compounds are conventionally synthesized under high temperature and elevated pressure, resulting in significant energy consumption and wastage. Heterogeneous photocatalytic oxidation has the potential to provide a green route for the synthesis of the majority of industrially important chemicals. The major limiting factor affecting the efficiency of photocatalytic organic synthesis is the lack of high selectivity; therefore, substantial effort has been devoted to solving this problem. Responding to this fast‐moving progress, this review gives an overview of the fundamental understanding of the reactive oxygen species involved in photocatalytic organic oxidations and furthermore, the general mechanisms of a few crucial oxidation reactions. The focus of this critical review is on the discussion of diverse strategies to improve the selectivity of high value chemicals, including band engineering, metal loading, hybrid materials, and defect engineering, with the primary aim of highlighting the catalyst design protocols based on the reaction mechanism. Finally, the difficulties and challenges of developing highly selective oxidation photocatalysts are comprehensively discussed in order to help direct the focus of future research.

Recent Advances on Photocatalytic and Electrochemical Oxidation for Ammonia Treatment from Water/Wastewater

Heterogeneous photocatalytic (PC) and electrochemical (EC) oxidation of ammonia/ammonium pollutants in water/wastewater have been thoroughly investigated for ammonia abatement from aqueous streams,...

A heterogeneous photocatalytic sulfate radical-based oxidation process for efficient degradation of 4-chlorophenol using TiO2 anchored on Fe oxides@carbon

A photocatalytic sulfate radical-based advanced oxidation process was studied to degrade 4-chlorophenol (4-CP) using persulfate (PS) catalyzing UV light and heterogeneous catalyst. In this regards, ferroferric oxide nanoparticles anchored on activated carbon (FOC) was applied as a supporter of TiO2 nanoparticles for fabricating of a recoverable photocatalyst (FOCT). Some analysis techniques including PL, UV–vis DRS, XRD, FESEM-EDS, BET, TEM and VSM were utilized to determine the optical, structural, textural and physicochemical characteristics of FOCT. The reaction mechanisms of PS activation and the generation of free radicals were described in details. Moreover, degradation intermediate products were identified and reaction pathway were proposed. It was found that photocatalytic activity and oxidation performance of pure TiO2 were improved after decoration on FOC. An excellent elimination efficiency was achieved in coupling of FOCT and UV irradiation with PS. Over 99 % of 4-CP (60 mg/L) and 49 % of TOC were removed by FOCT/UV/PS at optimum operational conditions. Furthermore, high PCP decontamination rate (> 86 %) was obtained, even at high concentrations (100 mg/L). During consecutive five runs of catalyst use, the degradation rate was decreased slightly to 88 % with a negligible decline in Fe and Ti leaching, demonstrating the excellent reusability and durability of FOCT in oxidation process. SO4– and HO radicals were detected as main reactive species that involved in the degradation of 4-CP over FOCT/UV/PS system. Thanks to good performance and easy recovery of catalyst, FOCT/UV/PS hybrid system has a great potential for environmental remediation perspectives.

Modelling heterogeneous photocatalytic oxidation using suspended TiO2 in a photoreactor working in continuous mode: Application to dynamic irradiation conditions simulating typical days in July and February

AbstractCompared to more conventional techniques, advanced oxidation processes (AOP) hold significant promise in terms of elimination of organic (especially persistent) compounds and microorganisms (disinfection) in wastewater. If the objective is to power these processes using solar energy, we need to be able to manage the intermittency in the solar resource. This is an essential step for design and to ensure efficient operation of the treatment processes. As solar radiation is inherently variable due to day/night cycles, seasonal cycles, and weather meteorological conditions, solar AOP performances are difficult to establish using conventional measures. To address this gap, we carry out experimental campaigns under controlled conditions and develop modelling tools capable of describing dynamic‐mode photocatalytic degradation. Here we develop a way to capture the responses of a photoreactor subjected to various stresses, including irradiation conditions, via an LED panel. Using a model that considers the influence of UV flux density and pollutant concentration made, it was possible to represent photoreactor responses under different irradiation conditions and feeds (concentration or flow at the input). The ultimate objective is to study the photocatalytic capacity of the photoreactor under irradiation conditions simulating a real day of sunshine.

Effects of catalyst preparation method and reaction parameters on the ultrasound assisted Photocatalytic oxidation of reactive yellow 84 dye.

In this study, the Heterogeneous Sono-photocatalytic Process was used to degrade Reactive Yellow 84 (RY 84) dye dissolved in water over iron containing TiO2 and TiO2-Ce catalysts. The catalysts were prepared by sol-gel and incipient wetness impregnation methods and characterized using XRD, SEM, Nitrogen adsorption, UV-Vis DRS and ICP-AES measurements. The TiO2 catalyst containing 1% (in weight) iron, prepared by incipient wetness impregnation technique and calcined at 300°C (1%Fe/TiO2-300°C (IW)) was found to be the most effective catalyst. Parametric study was carried out over this catalyst and COD removal of 55% and TOC removal of 38% were achieved while the decolorization efficiency reached 100% after 45min of reaction at the optimum conditions of, (25mg/L of RY 84 solution, 0.5g/L of catalyst, 5mM of H2O2, a temperature of 25°C, pH = 6 and US at a power of 40W). Decolorization of RY 84 obeyed the first order kinetics with an activation energy of 20.7kJ/mol. Sonication increased the decolorization efficiency of the heterogeneous Fenton process (UV + Catalyst+H2O2) from 92.7% to 97.5% after 30min of reaction, with the COD and TOC reductions increasing from 87% to 90% and 48% to 57% after 120min of reaction, respectively. US also decreased the toxicity of the RY 84 dye. The results obtained from this study show that, iron containing TiO2 and TiO2-Ce catalysts could be efficiently used in the hybrid process of ultrasound assisted heterogeneous photocatalytic oxidation in a wide range of experimental conditions.

UV Light-Irradiated Photocatalytic Degradation of Coffee Processing Wastewater Using TiO2 as a Catalyst

The coffee industry generates a significant amount of wastewater that is rich in organic loads and is highly acidic. The present study investigates the potential of the heterogeneous photocatalytic oxidation process to reduce the pollutant load in coffee processing wastewater. The experimental runs were conducted to evaluate the effect of operative parameters such as pH, catalyst dosage, intensity of UV light irradiation, and addition of oxidant on Chemical Oxygen Demand (COD) and colour reduction. Significant results for COD and colour removal, 67%, and 70% respectively, were achieved at a pH of 4 with titanium dioxide (TiO2), and a catalyst dosage of 500 mg/L, using four ultraviolet-C (UV-C) lamps of 16 W each. With the addition of hydrogen peroxide (H2O2) as an oxidant, the removal efficiency increased to 84% and 75% for COD and colour, respectively. Finally, the best results obtained by photocatalytic degradation using UV light were compared to those using solar light. Based on the investigation, it was inferred that the pollutant removal efficiency in coffee pulping wastewater was also considerably high under sunlight. These findings may have relevance in terms of application in countries where coffee processing is carried out and where sunlight irradiance is usually strong: the technique could be exploited to decrease the pollutant content of this wastewater sustainably.

Photocatalytic oxidation of psychoactive drug Duloxetine: Degradation kinetics, inorganic ions and phytotoxicity evaluation

Pharmaceutically active compounds, emerging extensively in ecosystems as pollutants, have become an important environmental and public health issue, since they can contaminate drinking water and pose threat to wildlife and human health. Therefore, efforts should be made in order to establish proper methods for their inactivation or elimination in the environment. The photocatalytic oxidation of psychoactive drug Duloxetine (DLX) has been investigated. In the case of heterogeneous photocatalytic oxidation, the effect of TiO2 P25 concentration (0.1–1 g L-1), initial concentration of H2O2 (0.25–0.2 g L-1) and Fe3+ (0.00175–0.014 g L-1) and pH of the solution (3–10) on initial reaction rates were evaluated, while for homogeneous photocatalytic oxidation the effect of the amount of H2O2 (0.25–0.2 g L-1) and Fe3+ (0.00175–0.014 g L-1) were investigated. Additionally, the conversion of the heteroatoms in the molecule of DLX to inorganic ions (NO3-, NH4+, SO42-) during photocatalytic process has been observed, and phytotoxicity testing, using three plant species, was carried out in order to examine the effect of photocatalytic oxidation on the toxicity of DLX. According to the results presented in this study, both heterogeneous and homogeneous photocatalytic oxidation is an efficient methodology for DLX degradation.

Modeling of gas-phase heterogeneous photocatalytic oxidation reactor in the presence of mass transfer limitation and axial dispersion

Mass transfer plays a critical role in the efficiency of photocatalytic oxidation (PCO) technology for air purification applications. There has been limited work on the exploration of mass transfer in the PCO reactor with the non-ideal flow. In this work, the performance of a continuous heterogeneous ultraviolet photocatalytic oxidation (UV-PCO) reactor was investigated and addressed under mass transfer limitation and axial dispersion. First, CFD modeling was used to determine the flow distribution in the reactor at various airflow rates. The residence time distribution (RTD) analysis with a tracer gas (CO2) was carried out for the experimental validation of the simulation model and good agreement between experimental and simulation data was achieved. Further, a quick and straightforward methodology employing an axial dispersion plug flow model was used to study the RTD. The proposed model could predict the residence time distribution of CO2 with high accuracy. Results of the RTD for CO2 in the presence and absence of a PCO filter (silica fiber felts (SFF) modified with TiO2) were almost identical due to the high porosity of the filter. The model was then used for the evaluation of the axial dispersion of methyl ethyl ketone (MEK) at different flow rates. Owing to a low value of the Peclet number (Pe < 100), the flow in the reactor deviated from an ideal plug flow and dispersion could not be ignored. The photocatalysis reaction of MEK by the SFF filter under the mass-transfer-controlled regime was further addressed. A steady-state mass balance equation was implemented for the assessment of the performance of the UV-PCO reactor in the presence of axial dispersion and mass transfer limitation. The mass transfer coefficient was calculated with the use of the developed model and a correlation for the Sherwood number was proposed to relate the mass transfer coefficient to the flow rate and fiber structure. The proposed correlation was assessed and compared with the other empirical formulas available in the literature.

Heterogeneous photocatalytic anaerobic oxidation of alcohols to ketones by Pt-mediated hole oxidation.

We report a platinum nanocluster/graphitic carbon nitride (Pt/g-C3N4) composite solid catalyst with a photocatalytic anaerobic oxidation function for highly active and selective transformation of alcohols to ketones. The desirable products were successfully obtained in good to excellent yields from various functionalized alcohols at room temperature, including unactivated alcohols. Mechanistic studies indicated that the reaction could proceed through a Pt-mediated hole oxidation initiating an α-alcohol radical intermediate followed by a two-electron oxidation pathway. The merit of this strategy offers a general approach towards green and sustainable organic synthetic chemistry.

Understanding the role of catalytic active sites for heterogeneous photocatalytic oxidation of methanol and thermal reduction of NOx

Identification and understanding the role of catalytic active sites are crucial to realize the performance of a catalyst. The conventional heterogeneous metal catalysts are still far away from the understanding of the nature of active sites. In this work, the solid solutions of Ti1-xPtxO2-δ (x = 0.01, 0.03, and 0.05) were synthesized by a microwave assisted hydrothermal method, where bivalent Pt was doped into the lattice of TiO2 in order to create the oxide ion vacancies. The as-prepared materials were reduced to leach out the substituted Pt in metallic state. Both the sets of materials were thoroughly characterized structurally and morphologically. The catalytic role of Pt was probed in these materials for two different and quite important reactions; room temperature photo-oxidation of methanol to hydrogen, and thermal reduction of NO to N2. The active sites of the catalysts were identified from the experimental observations. The elementary reaction mechanism over the active sites were proposed, and supported with modelling. The results revealed that the oxidized and the reduced catalyst behave in a completely different manner in the oxidation and reduction reactions chosen in this work.