Review of material design and reactor engineering on TiO2 photocatalysis for CO2 reduction

Recent Progress on Tailoring the Biomass-Derived Cellulose Hybrid Composite Photocatalysts

Biomass-derived cellulose hybrid composite materials are promising for application in the field of photocatalysis due to their excellent properties. The excellent properties between biomass-derived cellulose and photocatalyst materials was induced by biocompatibility and high hydrophilicity of the cellulose components. Biomass-derived cellulose exhibited huge amount of electron-rich hydroxyl group which could promote superior interaction with the photocatalyst. Hence, the original sources and types of cellulose, synthesizing methods, and fabrication cellulose composites together with applications are reviewed in this paper. Different types of biomasses such as biochar, activated carbon (AC), cellulose, chitosan, and chitin were discussed. Cellulose is categorized as plant cellulose, bacterial cellulose, algae cellulose, and tunicate cellulose. The extraction and purification steps of cellulose were explained in detail. Next, the common photocatalyst nanomaterials including titanium dioxide (TiO2), zinc oxide (ZnO), graphitic carbon nitride (g-C3N4), and graphene, were introduced based on their distinct structures, advantages, and limitations in water treatment applications. The synthesizing method of TiO2-based photocatalyst includes hydrothermal synthesis, sol-gel synthesis, and chemical vapor deposition synthesis. Different synthesizing methods contribute toward different TiO2 forms in terms of structural phases and surface morphology. The fabrication and performance of cellulose composite catalysts give readers a better understanding of the incorporation of cellulose in the development of sustainable and robust photocatalysts. The modifications including metal doping, non-metal doping, and metal-organic frameworks (MOFs) showed improvements on the degradation performance of cellulose composite catalysts. The information and evidence on the fabrication techniques of biomass-derived cellulose hybrid photocatalyst and its recent application in the field of water treatment were reviewed thoroughly in this review paper.

Gold nanoparticle decoration potentiate the antibacterial enhancement of TiO2 nanotubes via sonodynamic therapy against peri-implant infections

Inflammatory damage from bacterial biofilms usually causes the failure of tooth implantation. A promising solution for this challenge is to use an implant surface with a long-term, in-depth and efficient antibacterial feature. In this study, we developed an ultrasound-enhanced antibacterial implant surface based on Au nanoparticle modified TiO2 nanotubes (AuNPs-TNTs). As an artificial tooth surface, films based on AuNPs-TNTs showed excellent biocompatibility. Importantly, compared to bare titania surface, a larger amount of reactive oxygen radicals was generated on AuNPs-TNTs under an ultrasound treatment. For a proof-of-concept application, Porphyromonas gingivalis (P. gingivalis) was used as the model bacteria; the as-proposed AuNPs-TNTs exhibited significantly enhanced antibacterial activity under a simple ultrasound treatment. This antibacterial film offers a new way to design the surface of an artificial implant coating for resolving the bacterial infection induced failure of dental implants.

Photocatalytic hydrogen generation using TiO2: a state-of-the-art review

This review is focusing on photocatalytic hydrogen (H2) production as a viable fuel. The limitations of different production methods for H2 generation and the importance of photocatalytic process are discussed, which renders this process as highly promising to meet the future energy crises. TiO2 is one of most effective material to generate the H2 via photocatalytic processes. Therefore, advantages of the catalyst over other semiconductors have been thoroughly analyzed. Starting from synthesis of TiO2 and factors affecting the whole process of photocatalytic H2 production have been discussed. Modifications for improvement in TiO2 and the photocatalytic reaction are critically reviewed as well as the mechanism of TiO2 modification has been described. Metal doping, non-metal doping, impurity addition and defect introduction processes have been analyzed and the comparison of experimental results is developed based on H2 production efficiency. A critical review of the literature from 2004 to 2021 concludes that H2 production as fuel using TiO2 photocatalytic method is efficient and environment friendly, which have potential for practical applications for H2 generation.

Photocatalytic Hydrogen Production using Porous 3D Graphene-Based Aerogels Supporting Pt/TiO2 Nanoparticles

Composites involving reduced graphene oxide (rGO) aerogels supporting Pt/TiO2 nanoparticles were fabricated using a one-pot supercritical CO2 gelling and drying method, followed by mild reduction under a N2 atmosphere. Electron microscopy images and N2 adsorption/desorption isotherms indicate the formation of 3D monolithic aerogels with a meso/macroporous morphology. A comprehensive evaluation of the synthesized photocatalyst was carried out with a focus on the target application: the photocatalytic production of H2 from methanol in aqueous media. The reaction conditions (water/methanol ratio, catalyst concentration), together with the aerogel composition (Pt/TiO2/rGO ratio) and architecture (size of the aerogel pieces), were the factors that varied in optimizing the process. These experimental parameters influenced the diffusion of the reactants/products inside the aerogel, the permeability of the porous structure, and the light-harvesting properties, all determined in this study towards maximizing H2 production. Using methanol as the sacrificial agent, the measured H2 production rate for the optimized system (18,800 µmolH2h-1gNPs-1) was remarkably higher than the values found in the literature for similar Pt/TiO2/rGO catalysts and reaction media (2000-10,000 µmolH2h-1gNPs-1).

Recent review of BixMOy (M=V, Mo, W) for photocatalytic CO2 reduction into solar fuels

The utilization of solar energy for CO₂ conversion not only enables a green and low-carbon recycling of CO₂ with renewable energy, but also solves ecological problems. Bi_xMO_y (M = V, Mo, W) materials have typical layered structures and unique electronic properties that provide suitable band gaps and potential to meet the basic conditions for CO₂ reduction. However, pristine Bi_xMO_y faces with problems such as small specific surface area, insufficient active sites, low charge carriers' separation and utilization efficiency. This review comprehensively described the basic principles and reaction pathways of photocatalytic CO₂ reduction, and further presented the research progress of Bi_xMO_y catalysts in CO₂ conversion reactions. In this perspective, we further focus on the design concepts and modification strategies to improve the photocatalytic CO₂ reduction activity of Bi_xMO_y, such as morphology control, constructing surface vacancies and heterojunction fabrication. Finally, based on representative researches, the present review will be expected to provide updated information and insights for developing advanced Bi_xMO_y materials to further improve CO₂ reduction activity and selectivity.

One-Step Hydrothermal Synthesis of Anatase TiO2 Nanotubes for Efficient Photocatalytic CO2 Reduction

The hydrothermal dissolution-recrystallization process is a key step in the crystal structure of titania-based nanotubes and their composition. This work systematically studies the hydrothermal conditions for directly synthesizing anatase TiO₂ nanotubes (ATNTs), which have not been deeply discussed elsewhere. It has been well-known that ATNTs can be synthesized by the calcination of titanate nanotubes. Herein, we found the ATNTs can be directly synthesized by optimizing the reaction temperature and time rather than calcination of titanate nanotubes, where at each temperature, there is a range of reaction times in which ATNTs can be prepared. The effect of NaOH/TiO₂ ratio and starting materials was explored, and it was found that ATNTs can be prepared only if the precursor is anatase TiO₂, using rutile TiO₂ leads to forming titanate nanotubes. As a result, ATNTs produced directly without calcination have excellent photocatalytic CO₂ reduction than titanate nanotubes and ATNTs prepared by titanate calcination.

WO3/Ag/ZnO S-scheme heterostructure thin film spinning disc photoreactor for intensified photodegradation of cephalexin antibiotic

The presence of antibiotics in wastes and drinking water has led to serious environmental and health concerns, further necessitating the development of an advanced sustainable strategy to eliminate antibiotics from aquatic media. In this context, the present research reports the successful fabrication of a spinning disc photoreactor (SDPR) supported ZnO/Ag/WO₃ S-scheme visible-light-driven thin-film photocatalyst to study the degradation of cephalexin (CPX) as a target pollutant under blue light irradiation. The optical, electrochemical and physicochemical characterization of the as-prepared thin-film samples were carried out by XRD, top-view FE-SEM, EDS-mapping, UV-Vis-DRS, contact angle, EIS, transient photocurrent, mott Schottky and AFM techniques. The rod shape morphology of the samples with moderate surface roughness, desirable hydrophobicity, low bandgap and remarkable band structure alignment confirmed the applicability of as-prepared thin-film with an average photon flux of 1.94 × 10^-4-8.61 × 10^-5 E's m^-2 s^-1. The use of a rotating catalytic disc impressively declined the photon propagation distance, decremented the probability of light absorption by the solution, and intensified the mass transfer rate. The maximum throughputs of 98.8% efficiencies for CPX degradation were achieved at a rotational speed of 180 rpm, the solution flow rate of 1.0 L min^-1, the light intensity of 11 mW cm^-2, and initial CPX concentration of 40 mg L^-1, illumination time of 80 min, and pH of 6. Damkohler number (Da) value was found to be 1.23 × 10^-2 at the optimum conditions, indicating the negligibility of the external mass transfer resistance in the SDPR. The photocatalytic mechanism was elucidated for finding the most operative radical species, suggesting the crucial role of ·O₂^- in photodegradation of CPX and a drastic improvement of the charge separation by S-scheme heterostructure and facilitation by Ag mediator. Findings indicated that the developed reusable and robust SDPR benefited from an s-scheme photocatalyst can be a promising technology for degradation of the organic compounds.

An Artificial Photosystem of Metal-Insulator-CTF Nanoarchitectures for Highly Efficient and Selective CO2 Conversion to CO

It is of pivotal significance to explore robust photocatalysts to promote the photoreduction of CO₂ into solar fuels. Herein, an intelligent metal-insulator-semiconductor (MIS) nano-architectural photosystem was constructed by electrostatic self-assembly between cetyltrimethylammonium bromide (CTAB) insulator-capped metal Ni nanoparticles (NPs) and covalent triazine-based frameworks (CTF-1). The metal-insulator-CTF composites unveiled a substantially higher CO evolution rate (1254.15 μmol g^-1  h^-1 ) compared with primitive CTF-1 (1.08 μmol g^-1  h^-1 ) and reached considerable selectivity (98.9 %) under visible-light irradiation. The superior photocatalytic CO₂ conversion activity over Ni-CTAB-CTF nanoarchitecture could be attributed to the larger surface area, reinforced visible-light response, and CO₂ capture capacity. More importantly, the Ni-CTAB-CTF nanoarchitecture endowed the photoexcited electrons on CTF-1 with the ability to tunnel across the thin CTAB insulating layer, directionally migrating to Ni NPs and thereby leading to the efficient separation of photogenerated electrons and holes in the photosystem. In addition, isotope-labeled (¹³ CO₂ ) tracer results verified that the reduction products come from CO₂ rather than the decomposition of the photocatalysts. This study opens a new avenue for establishing a highly efficient and selective artificial photosystem for CO₂ conversion.

Preparation and Real World Applications of Titania Composite Materials for Photocatalytic Surface, Air, and Water Purification: State of the Art

The semiconducting transition metal oxide TiO2 is a rather cheap and non-toxic material with superior photocatalytic properties. TiO2 thin films and nanoparticles are known to have antibacterial, antiviral, antifungal, antialgal, self, water, and air-cleaning properties under UV or sun light irradiation. Based on these excellent qualities, titania holds great promises in various fields of applications. The vast majority of published field and pilot scale studies are dealing with the modification of building materials or generally focus on air purification. Based on the reviewed papers, for the coating of glass, walls, ceilings, streets, tunnels, and other large surfaces, titania is usually applied by spray-coating due to the scalibility and cost-efficiency of this method compared to alternative coating procedures. In contrast, commercialized applications of titania in medical fields or in water purification are rarely found. Moreover, in many realistic test scenarios it becomes evident that the photocatalytic activity is often significantly lower than in laboratory settings. In this review, we will give an overview on the most relevant real world applications and commonly applied preparation methods for these purposes. We will also look at the relevant bottlenecks such as visible light photocatalytic activity and long-term stability and will make suggestions to overcome these hurdles for a widespread usage of titania as photocalyst.

Photocatalytic CO2 Conversion Using Anodic TiO2 Nanotube-CuxO Composites

Nanosized titanium dioxide (TiO2) is currently being actively studied by the global scientific community, since it has a number of properties that are important from a practical point of view. One of these properties is a large specific surface, which makes this material promising for use in photocatalysts, sensors, solar cells, etc. In this work, we prepared photocatalysts based on TiO2 nanotubes for converting carbon dioxide (CO2) into energy-intensive hydrocarbon compounds. Efficient gas-phase CO2 conversion in the prepared single-walled TiO2 nanotube-CuxO composites was investigated. Parameters of defects (radicals) in composites were studied. Methanol and methane were detected during the CO2 photoreduction process. In single-walled TiO2 nanotubes, only Ti3+/oxygen vacancy defects were detected. The Cu2+ centers and O2− radicals were found in TiO2 nanotube-CuxO composites using the EPR technique. It has been established that copper oxide nanoparticles are present in the TiO2 nanotube-CuxO composites in the form of the CuO phase. A phase transformation of CuO to Cu2O takes place during illumination, as has been shown by EPR spectroscopy. It is shown that defects accumulate photoinduced charge carriers. The mechanism of methane and methanol formation is discussed. The results obtained are completely original and show high promise for the use of TiO2-CuxO nanotube composites as photocatalysts for CO2 conversion into hydrocarbon fuel precursors.

Sustainable approaches towards green synthesis of TiO2 nanomaterials and their applications in photo-catalysis mediated sensingtomonitor environmental pollutions

Nanomaterials are gaining enormous interests owing to their novel applications that have been explored nearly in every field of our contemporary society. In this scenario, preparations of nanomaterials following green routes have attracted widespread attention in terms of sustainable, reliable and environmentally friendly practice to produce diverse nanostructures. In this review, we summarized the fundamental processes and mechanisms of green synthesis approaches of TiO₂ NPs. We explore the role of plants and microbes as natural bioresources to prepare TiO₂ NPs. Particularly, focused have been made to explore the potential of TiO₂ based nanomaterials to design variety of sensing platforms by exploiting the photo-catalysis efficiency under the influence of light source. Such types of sensing can of massive importance to monitor the environmental pollutions and thereby to invent advanced strategies to remediate hazardous pollutants to offer clean environment.

Enhanced solar light photocatalytic and antimicrobial activity of green noble metal/TiO2 nanorods

Au/TiO2 and Pt/TiO2 nanocomposites have been processed using a green method. Au and Pt colloidal nanoparticles have been primarily synthesized by mixing their corresponding metal ions with an aqueous solution of corn husk extract, followed by anchoring on the as-synthesized TiO2 nanorods. The structural and morphological properties of green-prepared nanomaterials were systematically investigated by various techniques. The UV–VIS absorption measurements confirmed the formation of colloidal Au and Pt with λmax at 550 and 345 nm, respectively. TEM results show anchoring of spherical Au particles (40 nm) on TiO2 nanorods while smaller Pt particles have been observed on Pt/TiO2 composite. It has been shown that the highest visible light harvesting capability was earned for Au/TiO2 composite due to the surface plasmon resonance (SPR) of Au nanoparticles. The photocatalytic and antimicrobial activity of the nanomaterials was investigated for disinfection of Escherichia coli under solar light irradiation. The green synthesized nanocomposites showed enhanced solar light photocatalytic and antimicrobial activity. The best photocatalytic and antimicrobial activity was obtained for Au/TiO2. This evidences the enhanced SPR of Au nanoparticles and hence an enhancement in the solar light accessible by TiO2 so that a higher amount of reactive oxygen species can be generated, which enhances photocatalytic and antibacterial activity.

Spin-Polarized Photocatalytic CO2 Reduction of Mn-Doped Perovskite Nanoplates

"Spin" has been recently reported as an important degree of electronic freedom to improve the performance of electrocatalysts and photocatalysts. This work demonstrates the manipulations of spin-polarized electrons in CsPbBr₃ halide perovskite nanoplates (NPLs) to boost the photocatalytic CO₂ reduction reaction (CO₂RR) efficiencies by doping manganese cations (Mn²⁺) and applying an external magnetic field. Mn-doped CsPbBr₃ (Mn-CsPbBr₃) NPLs exhibit an outstanding photocatalytic CO₂RR compared to pristine CsPbBr₃ NPLs due to creating spin-polarized electrons after Mn doping. Notably, the photocatalytic CO₂RR of Mn-CsPbBr₃ NPLs is significantly enhanced by applying an external magnetic field. Mn-CsPbBr₃ NPLs exhibit 5.7 times improved performance of photocatalytic CO₂RR under a magnetic field of 300 mT with a permanent magnet compared to pristine CsPbBr₃ NPLs. The corresponding mechanism is systematically investigated by magnetic circular dichroism spectroscopy, ultrafast transient absorption spectroscopy, and density functional theory simulation. The origin of enhanced photocatalytic CO₂RR efficiencies of Mn-CsPbBr₃ NPLs is due to the increased number of spin-polarized photoexcited carriers by synergistic doping of the magnetic elements and applying a magnetic field, resulting in prolonged carrier lifetime and suppressed charge recombination. Our result shows that manipulating spin-polarized electrons in photocatalytic semiconductors provides an effective strategy to boost photocatalytic CO₂RR efficiencies.

Hybrid Metal Oxide/Biochar Materials for Wastewater Treatment Technology: A Review

This paper discusses the properties of metal oxide/biochar systems for use in wastewater treatment. Titanium, zinc, and iron compounds are most often combined with biochar; therefore, combinations of their oxides with biochar are the focus of this review. The first part of this paper presents the most important information about biochar, including its advantages, disadvantages, and possible modification, emphasizing the incorporation of inorganic oxides into its structure. In the next four sections, systems of biochar combined with TiO₂, ZnO, Fe₃O₄, and other metal oxides are discussed in detail. In the next to last section probable degradation mechanisms are discussed. Literature studies revealed that the dispersion of a metal oxide in a carbonaceous matrix causes the creation or enhancement of surface properties and catalytic or, in some cases, magnetic activity. Addition of metallic species into biochars increases their weight, facilitating their separation by enabling the sedimentation process and thus facilitating the recovery of the materials from the water medium after the purification process. Therefore, materials based on the combination of inorganic oxide and biochar reveal a wide range of possibilities for environmental applications in aquatic media purification.

Exploring CO2 Bio-Mitigation via a Biophotocatalytic/Biomagnetic System for Wastewater Treatment and Biogas Production

Carbon dioxide (CO2) emissions from fossil fuels have led industries to seek cheaper carbon abatement technologies to mitigate environmental pollution. Herein, the effect of a magnetic photocatalyst (Fe-TiO2) on biogas production in anaerobic digestion (AD) of wastewater was investigated with three bioreactors coupled with UV-light (18 W). Three experimental setups defined as the control (AD system with no Fe-TiO2), biophotoreactor (BP), and biophotomagnetic (BPM) systems were operated at a mesophilic temperature (35 ± 5 °C) for a hydraulic retention time (HRT) of 30 days. The control system (ADs) had no Fe-TiO2 additives. The BPMs with 2 g Fe-TiO2 were exposed to a magnetic field, whereas the BPs were not. The removal rate of the chemical oxygen demand (COD), volatile solids (VS), and total solids (TS), together with biogas production and composition were monitored for each reactor. The degree of degradation of 75% COD was observed for the BPMs at a pH of 6.5 followed by the BPs (65% COD) and the ADs (45% COD). The results showed that the rate of degradation of COD had a direct correlation with the cumulative biogas production of the BPMs (1330 mL/d) > BPs (1125 mL/d) > AD (625 mL/d). This finding supports the use of biophotomagnetic systems (BPMs) in wastewater treatment for resource recovery and CO2 reduction (0.64 kg CO2/L) as an eco-friendly technology.

Solar heterogeneous photocatalytic degradation of phenol on TiO2/quartz and TiO2/calcite: a statistical and kinetic approach on comparative efficiencies towards a TiO2/glass system

Phenol is a widely used synthetic organic compound, which according to global estimations, is discharged into the environment at a rate of 10 tons/year through industrial waste. Phenol is a recalcitrant pollutant, and human exposure to water containing phenolic substances can lead to health issues. It has been found both in drinking water and wastewater. Solar heterogeneous photocatalytic phenol degradation, measured through chemical oxygen demand, was performed on a thin film tilted plate reactor with TiO₂ immobilized onto different support materials. A full factorial experimental design (4 × 3 × 3) was carried out to statistically evaluate if the independent variables' effects were significant. Four advanced oxidation processes (photolysis, photolysis + H₂O₂, heterogeneous photocatalysis, and heterogeneous photocatalysis + H₂O₂), three support materials (quartz, calcite, and glass), and three pH levels (3, 5.4, and 9) were evaluated. Reaction kinetics were fitted to the pseudo-first-order reaction rate and data was analyzed with an ANCOVA and means test, considering solar light intensity as a covariate. Photolysis/calcite at pH 5.4 and heterogeneous photocatalysis + H₂O₂/glass plate at pH 3 gave the best results, with a reaction rate constant k_ph = 3.047 × 10^-3 min^-1 and k_phC = 4.498 × 10^-3 min^-1, respectively. The three independent variables and their interactions had a significant effect in the phenol degradation (p < 0.05).

Comparative Efficiencies for Phenol Degradation on Solar Heterogeneous Photocatalytic Reactors: Flat Plate and Compound Parabolic Collector

Phenol is a recalcitrant anthropogenic compound whose presence has been reported in both wastewater and drinking water; human exposure to phenolic substances can lead to health problems. The degradation of phenol (measured as COD decrease) through solar heterogeneous photocatalysis with immobilized TiO2 was performed in two different reactors: a flat-plate reactor (FPR) and a compound parabolic collector (CPC). A 23 full factorial experimental design was followed. The variables were the presence of TiO2, H2O2 addition, and the type of reactor. Data were fitted to the pseudo-first-order reaction-rate-kinetics model. The rate constant for photocatalytic phenol degradation with 1 mM of H2O2 was 6.6 × 10−3 min−1 for the FPR and 5.9 × 10−3 min−1 in the CPC. The calculated figures of merit were analyzed with a MANCOVA, with UV fluence as a covariate. An ANCOVA showed that the type of reactor, H2O2 addition, or fluence had no statistically significant effect on the results, but there was for the presence of TiO2. According to the MANCOVA, fluence and TiO2 presence were significant (p < 0.05). The CPC was on average 17.4% more efficient than the FPR when it came to collector area per order (ACO) by heterogeneous photocatalysis and 1 mM H2O2 addition.

Review on optofluidic microreactors for photocatalysis

Four interrelated issues have been arising with the development of modern industry, namely environmental pollution, the energy crisis, the greenhouse effect and the global food crisis. Photocatalysis is one of the most promising methods to solve them in the future. To promote high photocatalytic reaction efficiency and utilize solar energy to its fullest, a well-designed photoreactor is vital. Photocatalytic optofluidic microreactors, a promising technology that brings the merits of microfluidics to photocatalysis, offer the advantages of a large surface-to-volume ratio, a short molecular diffusion length and high reaction efficiency, providing a potential method for mitigating the aforementioned crises in the future. Although various photocatalytic optofluidic microreactors have been reported, a comprehensive review of microreactors applied to these four fields is still lacking. In this paper, we review the typical design and development of photocatalytic microreactors in the fields of water purification, water splitting, CO2 fixation and coenzyme regeneration in the past few years. As the most promising tool for solar energy utilization, we believe that the increasing innovation of photocatalytic optofluidic microreactors will drive rapid development of related fields in the future.

The Importance of the Macroscopic Geometry in Gas-Phase Photocatalysis

Photocatalysis has the potential to make a major technological contribution to solving pressing environmental and energy problems. There are many strategies for improving photocatalysts, such as tuning the composition to optimize visible light absorption, charge separation, and surface chemistry, ensuring high crystallinity, and controlling particle size and shape to increase overall surface area and exploit the reactivity of individual crystal facets. These processes mainly affect the nanoscale and are therefore summarized as nanostructuring. In comparison, microstructuring is performed on a larger size scale and is mainly concerned with particle assembly and thin film preparation. Interestingly, most structuring efforts stop at this point, and there are very few examples of geometry optimization on a millimeter or even centimeter scale. However, the recent work on nanoparticle-based aerogel monoliths has shown that this size range also offers great potential for improving the photocatalytic performance of materials, especially when the macroscopic geometry of the monolith is matched to the design of the photoreactor. This review article is dedicated to this aspect and addresses some issues and open questions that arise when working with macroscopically large photocatalysts. Guidelines are provided that could help develop novel and efficient photocatalysts with a truly 3D architecture.

A review on TiO2/SnO2 heterostructures as a photocatalyst for the degradation of dyes and organic pollutants

Industrialization, civilization and human activities have all grown steadily in recent years. As a result, small and large industries discharge many organic pollutants into the environment and contribute to environmental pollution. These compounds are quite stable and challenging to break down over time, posing a long-term risk. The heterogeneous advanced oxidation processes technology has gained tremendous attention. It depends on the light-induced formation of e^-/h⁺ pairs, which combine with water and aqueous oxygen to generate highly reactive hydroxyl radicals that degrade the organic pollutants in a solution and convert them ultimately into non-toxic products. In this paper, the synergetic impact of TiO₂-SnO₂ coupling with other semiconductor materials and their photodegradation performance on toxic contaminants in an aqueous medium has been reviewed. In addition, multiple approaches for the synthesis of TiO₂-SnO₂ photocatalysts have been discussed. Among them, hydrothermal, sol-gel, electrospinning, precipitation and even their combination are extensively used to synthesize various forms of nanostructures. These techniques demonstrate better tunability for visible absorption, suppression of e^-/h⁺ pair recombination and enhanced e^-/h⁺ separation to improve photocatalytic performance. This paper also summarises the role of different operating factors such as catalyst loading, pH, pollutants variation concentration, various light sources and oxidizing agents on the photodegradation of organic pollutants.

Effect of Metal Cocatalysts and Operating Conditions on the Product Distribution and the Productivity of the CO2 Photoreduction

The CO2 photoreduction is a promising way to convert one of the most abundant greenhouse gases to valuable chemicals. The photoreduction in the liquid phase is limited by the low solubility of CO2 in water, but this point is overcome here by using an innovative photoreactor, which allows one to work up to pressures of 20 bar, improving the overall productivity. The photoreduction was performed in the presence of Na2SO3 and using in primis commercial titanium dioxide (P25) and a set of titania catalysts functionalized by surface deposition of either monometallic or bimetallic cocatalysts. The gaseous products were hydrogen and traces of CO, while, in the liquid phase, formic acid/formate, formaldehyde and methanol were quantitatively detected. The pH was observed to shift the products distribution. A neutral environment led mainly to hydrogen and methanol, while, at pH 14, formate was the most abundant compound. The trend for monometallic cocatalysts showed enhanced productivity when using noble metals (i.e., gold and platinum). In order to limit the cost of the catalytic material, bimetallic cocatalysts were explored, adding titania with Au+Ag or Au+Pt. This may open to the possibility of performing the reaction with a smaller amount of the most expensive metals. In the end, we have expressed some conclusions on the cost of the photocatalysts here employed, to support the overall feasibility assessment of the process.

Investigation of CO2 Photoreduction in an Annular Fluidized Bed Photoreactor by MP-PIC Simulation

Carbon dioxide (CO₂) photoreduction is a promising process for both mitigating CO₂ emissions and providing chemicals and fuels. A gas–solid two-phase annular fluidized bed photoreactor (FBPR) would be preferred for this process due to its high mass-transfer rate and easy operation. However, CO₂ photoreduction using the FBPR has not been widely researched to date. The Lagrangian multiphase particle-in-cell (MP-PIC) simulation with computational fluid dynamic models is a new and robust approach to explore the multiphase reaction system in the gas–solid fluidized bed. Therefore, the purpose of this paper is to investigate CO₂ photoreduction in the FBPR by MP-PIC modeling to understand the intrinsic mechanism of solid flow, species mass transfer, and CO₂ photoreaction. The MP-PIC models for solid flow in the FBPR were validated by the bed expansion height and bubble size. The results showed the particle stress of the Lun model, the drag of the Ergun-WenYu (Gidaspow) model, and the coefficient of restitution e = 0.95 with the wall parameters e_w = 0.9 and μ_w = 0.6 are the best fit to the experimental empirical correlations. The MP-PIC models developed in this work proved to be better than the Eulerian two-fluid modeling in the prediction of the bed expansion height and bubble size. It was also found from the simulation results that the maximum radiation intensity is in the half reactor height area, and the photocatalytic reaction mainly occurred around the inner wall. It showed that the gas velocity and catalyst loading were two crucial operating parameters to control the process. The results reported here can provide guidance for the operation and reactor design of the CO₂ photoreduction process.

Process enhancing strategies for the reduction of Cr(VI) to Cr(III) via photocatalytic pathway

This discourse aimed at providing insight into the strategies that can be adopted to boost the process of photoreduction of Cr(VI) to Cr(III). Cr(VI) is amongst the highly detestable pollutants; thus, its removal or reduction to an innocuous and more tolerable Cr(III) has been the focus. The high promise of photocatalysis hinged on the sustainability, low cost, simplicity, and zero sludge generation. Consequently, the present dissertation provided a comprehensive review of the process enhancement procedures that have been reported for the photoreduction of Cr(VI) to Cr(III). Premised on the findings from experimental studies on Cr(VI) reductions, the factors that enhanced the process were identified, dilated, and interrogated. While the salient reaction conditions for the process optimization include the degree of ionization of reacting medium, available photogenerated electrons, reactor ambience, type of semiconductors, surface area of semiconductor, hole scavengers, quantum efficiency, and competing reactions, the relevant process variables are photocatalyst dosage, initial Cr(VI) concentration, interfering ion, and organic load. In addition, the practicability of photoreduction of Cr(VI) to Cr(III) was explored according to the potential for photocatalyst recovery, reactivation, and reuse reaction conditions and the process variables.

Improved Methane Production by Photocatalytic CO2 Conversion over Ag/In2O3/TiO2 Heterojunctions

In this work, the role of In₂O₃ in a heterojunction with TiO₂ is studied as a way of increasing the photocatalytic activity for gas-phase CO₂ reduction using water as the electron donor and UV irradiation. Depending on the nature of the employed In₂O₃, different behaviors appear. Thus, with the high crystallite sizes of commercial In₂O₃, the activity is improved with respect to TiO₂, with modest improvements in the selectivity to methane. On the other hand, when In₂O₃ obtained in the laboratory, with low crystallite size, is employed, there is a further change in selectivity toward CH₄, even if the total conversion is lower than that obtained with TiO₂. The selectivity improvement in the heterojunctions is attributed to an enhancement in the charge transfer and separation with the presence of In₂O₃, more pronounced when smaller particles are used as in the case of laboratory-made In₂O₃, as confirmed by time-resolved fluorescence measurements. Ternary systems formed by these heterojunctions with silver nanoparticles reflect a drastic change in selectivity toward methane, confirming the role of silver as an electron collector that favors the charge transfer to the reaction medium.

Construction of the Sn-doped defect pyrochlore oxide KNbMoO6·H2O/g-C3N4 composite and its photocatalytic reduction of CO2

The Sn-doped defect pyrochlore oxide KNbMoO6·H2O/g-C3N4 composite can be used as a photocatalyst for conversion of CO2 in order to deal with energy and environmental issues.

Application of Porous Materials for CO2 Reutilization: A Review

CO2 reutilization processes contribute to the mitigation of CO2 as a potent greenhouse gas (GHG) through reusing and converting it into economically valuable chemical products including methanol, dimethyl ether, and methane. Solar thermochemical conversion and photochemical and electrochemical CO2 reduction processes are emerging technologies in which solar energy is utilized to provide the energy required for the endothermic dissociation of CO2. Owing to the surface-dependent nature of these technologies, their performance is significantly reliant on the solid reactant/catalyst accessible surface area. Solid porous structures either entirely made from the catalyst or used as a support for coating the catalyst/solid reactants can increase the number of active reaction sites and, thus, the kinetics of CO2 reutilization reactions. This paper reviews the principles and application of porous materials for CO2 reutilization pathways in solar thermochemical, photochemical, and electrochemical reduction technologies. Then, the state of the development of each technology is critically reviewed and evaluated with the focus on the use of porous materials. Finally, the research needs and challenges are presented to further advance the implementation of porous materials in the CO2 reutilization processes and the commercialization of the aforementioned technologies.

Semiconductor photocatalysts and mechanisms of carbon dioxide reduction and nitrogen fixation under UV and visible light

The review summarizes the current knowledge about heterogeneous semiconductor photocatalysts that are active towards photocatalytic reduction of carbon dioxide and molecular nitrogen under visible and near-UV light. The main classes of these photocatalysts and characteristic features of their application in the target processes are considered. Primary attention is given to photocatalysts based on titanium dioxide, which have high activity and stability in the carbon dioxide reduction. For the first time, the photofixation of nitrogen under irradiation in the presence of various semiconductor materials is considered in detail.

The bibliography includes 264 references.

Novel Two-Dimensional MA2N4 Materials for Photovoltaic and Spintronic Applications

We have systematically investigated a family of newly proposed two-dimensional MA₂N₄ materials (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W; A = Si, Ge) using first-principles calculation. We categorize the potential of these materials into three different applications based on accurate simulation of band gap (using Hybrid HSE06 functional) and the associated descriptors. Three candidate materials (MoGe₂N₄, HfSi₂N₄, and NbSi₂N₄) turn out to be extremely promising for three different applications. MoGe₂N₄ and HfSi₂N₄ monolayers show strong optical absorption in the visible range, including high transition probability from the valence to conduction band. The GW+BSE calculations confirm a strong excitonic effect in both the systems. With a band gap of 1.42 eV, multilayer MoGe₂N₄ shows reasonably large simulated efficiency (∼15.40%) and hence can be explored for possible photovoltaic applications. High optical absorption, suitable band gap/edge positions, and the CO₂ activation make HfSi₂N₄ monolayer a promising candidate for photocatalytic CO₂ reduction. NbSi₂N₄, on the other hand, belongs to a new class of spintronic material called a bipolar magnetic semiconductor, recommended for spin-transport-based applications.

Current and future perspectives on catalytic-based integrated carbon capture and utilization

There exist several well-known methods with varying maturity for capturing carbon dioxide from emission sources of different concentrations, including absorption, adsorption, cryogenics and membrane separation, among others. The capture and separation steps can produce almost pure CO₂, but at substantial cost for being conditioned for transport and final utilization, with high economical risks to be considered. A possible way for the elimination of this conditioning and cost is direct CO₂ utilization, whether on-site in a further process but within the same plant, or in-situ, coupling both capture and conversion in the same unit. This approach is usually called integrated carbon capture and utilization (ICCU) or integrated carbon capture and conversion (ICCC), and has lately started receiving considerable attention in many circles. As CO₂ is already industrially employed in other sectors, such as food preservation, water treatment and conversion to high added-value chemicals and fuels such as methanol, methane, etc., among others, it is of great interest to explore the global ICCC approach. Catalytic-based processes play a key role in CO₂ conversion, and different technologies are gaining great attention from both academia and industry. However, the 'big picture of ICCU' and in which technology the efforts should focus on at large scale is still unclear. This review analyzes some promising concepts of ICCU specifically on CO₂ catalytic conversion, highlighting their current commercial relevance as well as challenges that have to be faced today and in the next future.