TiO2@Pt@CeO2 nanocomposite as a bifunctional catalyst for enhancing photo-reduction of Cr (VI) and photo-oxidation of benzyl alcohol

Insight into the photocatalytic reduction of hexavalent chromium using photodeposited metal nanoparticle-TiO2 photocatalysis

Hexavalent chromium (Cr(VI)) is carcinogenic to organisms. It is widely used in several industries. In this work, we investigated the Cr(VI) photocatalytic reaction with a scavenger on Pt and Cu-TiO2 photocatalysts. Metal-deposited TiO2 was successfully synthesized by a photodeposition method. TEM-EDX, XRD, and UV-DR were analyzed to study the changes in morphology, crystallinity, and the electronic properties of photocatalysts. The rate of charge recombination during reduction and photoluminescence (PL) spectroscopy was used to examine the catalysts in depth. Cu-TiO2 demonstrates the highest photocatalytic activity for 63.74% of Cr(VI) removal. To understand the photoreduction of Cr(VI), the fate transformation of Cr species during the adsorption and reaction was investigated using in situ XANES. The results demonstrated that the Cr(III) was noticeably main component adsorbed over the catalyst, particularly in Cu-TiO2. The presence of humic acid can boost the Cr(VI) removal efficiency and enhanced the Cr(VI) reduction to Cr(III). We believe that the extensive research on Cr(VI) photoreduction on metal-TiO2 heterojunction will provide a comprehensive understanding of catalytic behaviors, paving the way for rationally designed novel Cr reduction catalysts.

An intriguing Z-scheme titania loaded on fibrous silica ceria for accelerated visible-light-driven photocatalytic degradation of ciprofloxacin

A novel Z-scheme titania loaded on fibrous silica ceria (Ti-FSC) was triumphantly fabricated via hydrothermal followed by electrolysis method and evaluated for the visible-light degradation of ciprofloxacin (CIP). Noticeably, Ti-FSC exhibits as an efficient photocatalyst for CIP photodegradation with 95% as followed by titania loaded on fibrous silica (Ti-FS) (68%), Ti-CeO₂ (35%), FSC (47%), FS (22%), and CeO₂ (17%). The combination of the inherent merits of Ti loaded on FSC is able to realize the crucial role of Ce in harnessing the high dispersion of Ti, which could beneficial for improving the performance proven by XRD, FESEM, TEM and FTIR. Consequently, high dispersion of Ti on FSC has worthwhile towards the interaction of the Si-O-Ti, Ce-O-Ti, and Si-O-Ti, which could enhance the CIP photodegradation by providing more surface defects, narrowing the band gap, improving electron-hole separation and suppressing electron-hole recombination that revealed by XPS, UV-vis/DRS, Nyquist plots and PL studies, respectively. The scavenger study revealed that the controlling species in the system was hydroxyl radical and holes. A potential Z-scheme heterojunction mechanism for Ti-FSC was deduced from the band structure analysis. The possible photodegradation pathway was proposed based on GCMS analysis. Besides, the acceptable reusability, which exceeded 90% of degradation indicated the great application potential of Z-scheme Ti-FSC in wastewater treatment and others application.

A review on synergistic coexisting pollutants for efficient photocatalytic reaction in wastewater remediation

With the tremendous development of the economy and industry, the pollution of water is becoming more serious due to the excessive chemical wastes that need to remove thru reduction or oxidation reactions. Simultaneous removal of dual pollutants via photocatalytic redox reaction has been tremendously explored in the last five years due to effective decontamination of pollutants compared to a single pollutants system. In a photocatalysis mechanism, the holes in the valence band can remarkably promote the oxidation of a pollutant. At the same time, photoexcited electrons are also consumed for the reduction reaction. The synergistic between the reduction and oxidation inhibits the recombination of electron-hole pairs extending their lifetime. In this review, the binary pollutants that selectively removed via photocatalysis reduction or oxidation are classified according to heavy metal-organic pollutant (HM/OP), heavy metal-heavy metal (HM/HM) and organic-organic pollutants (OP/OP). The intrinsic between the pollutants was explained in three different mechanisms including inhibition of electron-hole recombination, ligand to metal charge transfer and electrostatic attraction. Several strategies for the enhancement of this treatment method which are designation of catalysts, pH of mixed pollutants and addition of additive were discussed. This review offers a recent perspective on the development of photocatalysis system for industrial applications.

Au/CeO2 Photocatalyst for the Selective Oxidation of Aromatic Alcohols in Water under UV, Visible and Solar Irradiation

Au nanoparticles supported on CeO2 have been prepared and investigated as photocatalysts for the photocatalytic selective oxidation of benzyl alcohol and 4-methoxybenzyl alcohol to the correspondent benzaldehydes, in aqueous suspensions and room conditions under UV, visible and natural solar light irradiation. Au nanoparticles have been supported by impregnation (1 and 3 wt.%) on two types of CeO2 (i.e., a commercial one and a home prepared oxide obtained in the presence of NaOH as precipitation agent). The Au impregnated samples showed strong visible radiation absorption at 565–570 nm associated to localized surface plasmon resonance (LSPR). The bare CeO2 samples are activated by UV light and resulted virtually inactive under visible irradiation, whereas the presence of Au improved both the conversion of the alcohols and the selectivity of the reaction towards the aldehyde, giving rise to good results, particularly under visible and natural solar light irradiation. The activity of the materials increased by increasing the Au content.

Ex Situ Method for Photoreduction of the Cadmium Ion from Terbium-Loaded Bismuth Vanadium Oxide

The photoreduction of Cd (II) to Cd (0) was performed using Bi4V2O11, which was tremendously enhanced by Tb3+-doped Bi4V2O11. The relationship between charge carrier isolation and light harvesting was studied in depth in this research, and a promising technique for fabricating effective photocatalysts for heavy metals was discovered. Lattice disorder effects due to size variance between V5+ and Tb3+ cations in Bi4V2O11 nanomaterials substituted with an invariable Tb3+ cation at different concentrations (x = 15, 20, and 25%). Bi4V2O11 and 15% Tb/Bi4V2O11 evidenced a coexistence of monoclinic (α-phase) with a CS/m symmetry, while 25% Tb/Bi4V2O11 was tetragonal (γ-phase) with an I4/mmm symmetry. Raman scattering experiments elucidated the changes in Bi4V2O11 lattice corresponding to oxygen motion, suggesting significant destabilization of the VO4 tetrahedra after addition of Tb3+. The SEM micrograph depicted a disparity in the microstructure with reduced grain size in 25% Tb/Bi4V2O11 samples. However, the TEM micrographs of 25% Tb/Bi4V2O11 nanomaterials revealed that crystallite sizes of 25-35 nm were obtained, presenting a single tetragonal phase, highly homogeneous in nature. Impedance spectroscopy was used to study the conductivity of these compounds in the temperature range of 300 °C. At 300 °C, the compounds with x = 25% showed a conductivity of 15.92 S cm-1. The conductivity values were found to be comparable with the highest values reported in the literature for similar compounds.

Ce-based catalysts used in advanced oxidation processes for organic wastewater treatment: A review

Refractory organic pollutants in water threaten human health and environmental safety, and advanced oxidation processes (AOPs) are effective for the degradation of these pollutants. Catalysts play vital role in AOPs, and Ce-based catalysts have exhibited excellent performance. Recently, the development and application of Ce-based catalysts in various AOPs have been reported. Our study conducts the first review in this rapid growing field. This paper clarifies the variety and properties of Ce-based catalysts. Their applications in different AOP systems (catalytic ozonation, photodegradation, Fenton-like reactions, sulfate radical-based AOPs, and catalytic sonochemistry) are discussed. Different Ce-based catalysts suit different reaction systems and produce different active radicals. Finally, future research directions of Ce-based catalysts in AOP systems are suggested.

Complementary behavior of doping and loading in Ag/C-ZnTa2O6 for efficient visible-light photocatalytic redox towards broad wastewater remediation

This work reports on the simple fabrication of a silver loaded and carbon doped zinc tantalate (Ag/C-ZnTa₂O₆) photocatalyst with visible light photocatalytic activity toward broad wastewater remediation, including high photo-reduction of Cr(vi) (98.4% in 210 min), excellent photo-oxidation of tetracycline hydrochloride (94.7% in 210 min), and superior photo-degradation of multiple dyes (>99.0% within 210 min). The optimal photocatalytic performance of Ag/C-ZnTa₂O₆ is mainly due to the excellent visible light absorption capacity and superior electron-hole separation efficiency, which is ascribed to the complementary behavior between carbon doping and silver loading. Particularly, the generation of defects due to C-doping is greatly inhibited by Ag-loading, and the SPR effect of Ag nanoparticles is enhanced due to the obstruction of Ag⁺ by C doping.

Synthesis of a Rationally Designed Multi-Component Photocatalyst Pt:SiO2:TiO2(P25) with Improved Activity for Dye Degradation by Atomic Layer Deposition

Photocatalysts for water purification typically lack efficiency for practical applications. Here we present a multi-component (Pt:SiO₂:TiO₂(P25)) material that was designed using knowledge of reaction mechanisms of mono-modified catalysts (SiO₂:TiO₂, and Pt:TiO₂) combined with the potential of atomic layer deposition (ALD). The deposition of ultrathin SiO₂ layers on TiO₂ nanoparticles, applying ALD in a fluidized bed reactor, demonstrated in earlier studies their beneficial effects for the photocatalytic degradation of organic pollutants due to more acidic surface Si–OH groups which benefit the generation of hydroxyl radicals. Furthermore, our investigation on the role of Pt on TiO₂(P25), as an improved photocatalyst, demonstrated that suppression of charge recombination by oxygen adsorbed on the Pt particles, reacting with the separated electrons to superoxide radicals, acts as an important factor for the catalytic improvement. Combining both materials into the resulting Pt:SiO₂:TiO₂(P25) nanopowder exceeded the dye degradation performance of both the individual SiO₂:TiO₂(P25) (1.5 fold) and Pt:TiO₂(P25) (4-fold) catalysts by 6-fold as compared to TiO₂(P25). This approach thus shows that by understanding the individual materials’ behavior and using ALD as an appropriate deposition technique enabling control on the nano-scale, new materials can be designed and developed, further improving the photocatalytic activity. Our research demonstrates that ALD is an attractive technology to synthesize multicomponent catalysts in a precise and scalable way.

Photoreduction and Removal of Cadmium Ions over Bentonite Clay-Supported Zinc Oxide Microcubes in an Aqueous Solution

Cadmium ion is toxic to organisms and shows persistence because of its nondegradability. Photoreduction of the cadmium ion (Cd(II)) was studied using a bentonite-supported Zn oxide (ZnO/BT) photocatalyst in an aqueous medium under ultraviolet light. The prepared ZnO/BT photocatalyst was characterized by diffuse reflectance spectroscopy, field-emission scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, photoluminescence spectroscopy, transmission electron microscopy, energy-dispersive spectroscopy, and Brunauer-Emmett-Teller/Barrett-Joyner-Halenda analysis. The effects of main parameters including pH, contact time, initial concentration of cadmium(II) ion, light intensity, temperature, and the photocatalyst dosage were investigated for obtaining appreciate reduction/removal efficiency. The maximum reduction/removal efficiency of 74.8% was obtained at optimized values which were found to be at pH 5, 6 h contact time, 6 ppm Cd(II) ion, 200 W UV light, 45 °C temperature, and 4 g/L of ZnO/BT. Reduction/removal of Cd(II) was significantly affected by light intensity so that the increment in UV intensity from 0 to 200 increased the reduction/removal efficiency from 61.2 to 76.8%. This study reports an inexpensive and environmentally friendly photocatalyst for Cd²⁺ reduction in real samples and prospective photoelectric materials.

The Effect of the ZrO2 Loading in SiO2@ZrO2-CaO Catalysts for Transesterification Reaction

The effect of the ZrO₂ loading was studied on spherical SiO₂@ZrO₂-CaO structures synthetized by a simple route that combines the Stöber and sol-gel methods. The texture of these materials was determined using S_BET by N₂ adsorption, where the increment in SiO₂ spheres' surface areas was reached with the incorporation of ZrO₂. Combined the characterization techniques of using different alcoholic dissolutions of zirconium (VI) butoxide 0.04 M, 0.06 M, and 0.08 M, we obtained SiO₂@ZrO₂ materials with 5.7, 20.2, and 25.2 wt % of Zr. Transmission electron microscopy (TEM) analysis also uncovered the shape and reproducibility of the SiO₂ spheres. The presence of Zr and Ca in the core-shell was also determined by TEM. X-ray diffraction (XRD) profiles showed that the c-ZrO₂ phase changed in to m-ZrO₂ by incorporating calcium, which was confirmed by Raman spectroscopy. The purity of the SiO₂ spheres, as well as the presence of Zr and Ca in the core-shell, was assessed by the Fourier transform infrared (FTIR) method. CO₂ temperature programmed desorption (TPD-CO₂) measurements confirmed the increment in the amount of the basic sites and strength of these basic sites due to calcium incorporation. The catalyst reuse in FAME production from canola oil transesterification allowed confirmation that these calcium core@shell catalysts turn out to be actives and stables for this reaction.

Surface modification of graphene oxide via noncovalent functionalization with porphyrins for selective photocatalytic oxidation of alcohols

GO–porphyrin composites were simply prepared by a self-assembly process. The noncovalent interactions between the porphyrins and oxygen-containing functional groups on GO play a crucial role in controlling their photocatalytic activities.

Ultrasound-assisted method to improve the structure of CeO2@polyprrole core-shell nanosphere and its photocatalytic reduction of hazardous Cr6

In this work, the CeO₂@polypyrrole (CeO₂@PPy) core-shell nanosphere has been synthesized via an ultra-sonication method using bath type (WUC-D22H, Daihan Scientific, Korea) and they are utilized for the photo-reduction of hazardous Cr⁶⁺ to benign Cr³⁺. The ultrasonic frequency and power were 20 kHz and 100 W, respectively. The PPy shielded CeO₂ in aqueous solution could prevent the dissolution of CeO₂ and to improve the photocatalytic ability of CeO₂. X-ray diffraction was used to confirm the crystalline structure of as prepared CeO₂@PPy core-shell and FT-IR was used to identify the functional groups. The uniform sized core of PPy and shell of CeO₂ were observed by transition electron microscopy. The ultrasonic assisted synthesized CeO₂@PPy core-shell exhibits a narrow bandgap (UV-DRS) and good reduction efficiency with higher reusability and stability compared to pure CeO₂, PPy and mechanical mixing of CeO₂@PPy. Moreover, the synergistic effect of CeO₂ and PPy core-shell structure facilitate a higher electron transfer rate and prolong lifetime of photogenerated electron-hole pairs which can achieve good reduction rate of 98.6% within 30 min. In particular, the pH, catalyst, and Cr⁶⁺ concentration effects were optimized in photocatalytic reduction reactions. Meanwhile, this photocatalysis with fast and effective electron transfer mechanism for the Cr⁶⁺ reduction was elucidated. This method opens a new window for simple fabrication of conducting polymers-based metal oxide nanocomposite towards wastewater remediation and beyond.

Platinum-enhanced amorphous TiO2-filled mesoporous TiO2 crystals for the photocatalytic mineralization of tetracycline hydrochloride

The adsorption ability and photoactivity of a photocatalyst largely determine the mineralization efficiency of antibiotics. Herein, aiming to enhance the adsorption and mineralization of antibiotics, we constructed a hierarchical porous core-shell structure by filling amorphous TiO₂ in the pores of Pt-doped mesoporous TiO₂ crystals (MCs). The physical-chemical properties of the prepared samples were investigated by surface photovoltage spectroscopy, X-ray photoelectron spectroscope, etc. Adsorption and photocatalysis experiments were conducted with tetracycline hydrochloride as the model antibiotic. Pt nanoparticles doped at the interface of the rutile-amorphous homojunction remarkably enhanced the built-in electric field. The enhanced electric field increased the hole transfer to the catalyst surface, and the Pt doping treatment promoted the growth of amorphous TiO₂ into the mesopores of the MCs. The optimization increased the surface area of the catalyst without increasing the thickness of the amorphous TiO₂ shell, thereby reducing the charge migration distance from the core-shell interface to the catalyst surface. The adsorption amount and mineralization efficiency of tetracycline hydrochloride for the porous core-shell composite were 6.7 and 3.8 times of those for MCs, respectively.

Understanding the Photocatalytic Properties of Pt/CeOx /TiO2 : Structural Effects on Electronic and Optical Properties

Ceria-titania interfaces play a crucial role in different chemical processes but are especially promising for the photocatalytic splitting of water using light in the visible wavelength region when Pt is added to the system. However, the complexity of this hierarchical structure hampers the study of the origin of its outstanding properties. In this article, the structural, electronic and optoelectronic properties of CeO₂ /TiO₂ systems containing 1D, 2D, and 3D particles of ceria are analyzed by means of density functional calculations. Adsorption sites and vacancy effects have been studied to model Pt adsorption. Density of states calculations and absorption spectra simulations explain the behavior of these systems. Finally, these models are used for the screening of other metals that can be combined with this heterostructure to potentially find more efficient water splitting photocatalysts.

Enhanced visible light photoreduction of aqueous Cr(VI) by Ag/Bi4O7/g-C3N4 nanosheets ternary metal/non-metal Z-scheme heterojunction

In this paper, we successfully constructed a ternary metal/non-metal nanomaterial which can synergize Z-scheme heterojunction and plasmonic metal/semiconductor for enhancing visible light photoreduction of aqueous Cr(VI). The as-formed ternary metal/non-metal nanomaterial was composed of g-C₃N₄ nanosheets mutual modified by Ag nanoparticles and Bi₄O₇. In the ternary nanomaterial, g-C₃N₄ nanosheets and Bi₄O₇ fabricated Z-scheme heterojunction, g-C₃N₄ nanosheets and Ag nanoparticles formed plasmonic metal/semiconductor system, synchronously. The photodeposition experiments were designed to indirect demonstrate the Z-scheme heterojunction via photocatalytic oxidation and reduction deposition reactions, AgNO₃ was used as the electron acceptor and Pb(NO₃)₂ as the electron donor, photocatalytic reduction product and oxidation product were selectively deposited on g-C₃N₄ nanosheets and Bi₄O₇, respectively, revealing the Z-scheme heterojunction was definite formed. In addition, X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), UV-vis absorption, and Fourier-transform infrared spectroscopy (FTIR) were studied to indicate that the plasmonic metal/semiconductor system was existent in the ternary nanomaterial. The visible light photoreduction of ternary Ag/Bi₄O₇/g-C₃N₄ nanosheets was evaluated by photoreducing aqueous Cr(VI). The photoreduction results show that the ternary Ag/Bi₄O₇/g-C₃N₄ nanosheets is much higher than the pristine bulk g-C₃N₄ and g-C₃N₄ nanosheets.

Magnetic Fe₂O₃⁻SiO₂⁻MeO₂⁻Pt (Me = Ti, Sn, Ce) as Catalysts for the Selective Hydrogenation of Cinnamaldehyde. Effect of the Nature of the Metal Oxide

The type of metal oxide affects the activity and selectivity of Fe₂O₃–SiO₂–MeO₂–Pt (Me = Ti, Sn, Ce) catalysts on the hydrogenation of cinnamaldehyde. The double shell structure design is thought to protect the magnetic Fe₂O₃ cores, and also act as a platform for depositing a second shell of TiO₂, SnO₂ or CeO₂ metal oxide. To obtain a homogeneous metallic dispersion, the incorporation of 5 wt % of Pt was carried out over Fe₂O₃–SiO₂–MeO₂ (Me = Ti, Sn, Ce) structures modified with (3-aminopropyl)triethoxysilane by successive impregnation-reduction cycles. The full characterization by HR-TEM, STEM-EDX, XRD, N₂ adsorption isotherm at −196 °C, TPR-H₂ and VSM of the catalysts indicates that homogeneous core-shell structures with controlled nano-sized magnetic cores, multi-shells and metallic Pt were obtained. The nature of the metal oxide affects the Pt nanoparticle sizes where the mean Pt diameter is in the order: –TiO₂–Pt > –SnO₂–Pt > –CeO₂–Pt. Among the catalysts studied, –CeO₂–Pt had the best catalytic performance, reaching the maximum of conversion at 240 min. of reaction without producing hydrocinnamaldehyde (HCAL). It also showed a plot volcano type for the production of cinnamic alcohol (COL), with 3-phenyl-1-propanol (HCOL) as a main product. The –SnO₂–Pt catalyst showed a poor catalytic performance attributable to the Pt clusters’ occlusion in the irregular surface of the –SnO₂. Finally, the –TiO₂–Pt catalyst showed a continuous production of COL with a 100% conversion and 65% selectivity at 600 min of reaction.

Cr(VI) remediation from aqueous environment through modified-TiO2-mediated photocatalytic reduction

Cr(VI) exhibits cytotoxic, mutagenic and carcinogenic properties; hence, effluents containing Cr(VI) from various industrial processes pose threat to aquatic life and downstream users. Various treatment techniques, such as chemical reduction, ion exchange, bacterial degradation, adsorption and photocatalysis, have been exploited for remediation of Cr(VI) from wastewater. Among these, photocatalysis has recently gained considerable attention. The applications of photocatalysis, such as water splitting, CO2 reduction, pollutant degradation, organic transformation reactions, N2 fixation, etc., towards solving the energy crisis and environmental issues are briefly discussed in the Introduction of this review. The advantages of TiO2 as a photocatalyst and the importance of its modification for photocatalytic reduction of Cr(VI) has also been addressed. In this review, the photocatalytic activity of TiO2 after modification with carbon-based advanced materials, metal oxides, metal sulfides and noble metals towards reduction of Cr(VI) was evaluated and compared with that of bare TiO2. The photoactivity of dye-sensitized TiO2 for reduction of Cr(VI) was also discussed. The mechanism for enhanced photocatalytic activity was highlighted and attributed to the resultant properties, namely, effective separation of photoinduced charge carriers, extension of the light absorption range and intensity, increase of the surface active sites, and higher photostability. Advantages and limitations for photoreduction of Cr(VI) over modified TiO2 are depicted in the Conclusion. The various challenges that restrict the technology from practical applications in remediation of Cr(VI) from wastewater were addressed in the Conclusion section as well. The future perspectives of the field presented in this review are focused on the development of whole-solar-spectrum responsive, TiO2-coupled photocatalysts which provide efficient photocatalytic reduction of Cr(VI) along with their good recoverability and recyclability.

BiVO4 quantum dot-decorated BiPO4 nanorods 0D/1D heterojunction for enhanced visible-light-driven photocatalysis

Herein, we report a BiVO₄ quantum dot (QDs)-decorated BiPO₄ nanorods 0D/1D heterojunction via an in situ growth method.

A promazine derivative-oriented chemical sensor for ultra-sensitive chromium determination

The study reports a simple and highly selective spectrofluorometric method for trace determination and speciation of chromium(iii & vi) species in water using 10-(3-dimethylaminopropyl) phenothiazine hydrochloride, namely, promazine hydrochloride (PMH) as a fluorescence tagging reagent.

Controllable synthesis and photoreduction performance towards Cr(vi) of BiOCl microrods with exposed (110) crystal facets

BiOCl microrod exposed (110) facets was synthesized via a simple hydrothermal method using sodium citrate as capping agent. It exhibits outstanding photoreduction performance towards Cr(vi) at neutral and acid condition.