Comparison of CO2 Reduction Performance with NH3 and H2O between Cu/TiO2 and Pd/TiO2

The aim of this study is to clarify the effect of doped metal type on CO₂ reduction characteristics of TiO₂ with NH₃ and H₂O. Cu and Pd have been selected as dopants for TiO₂. In addition, the impact of molar ratio of CO₂ to reductants NH₃ and H₂O has been investigated. A TiO₂ photocatalyst was prepared by a sol-gel and dip-coating process, and then doped with Cu or Pd fine particles by using the pulse arc plasma gun method. The prepared Cu/TiO₂ film and Pd/TiO₂ film were characterized by SEM, EPMA, TEM, STEM, EDX, EDS and EELS. This study also has investigated the performance of CO₂ reduction under the illumination condition of Xe lamp with or without ultraviolet (UV) light. As a result, it is revealed that the CO₂ reduction performance with Cu/TiO₂ under the illumination condition of Xe lamp with UV light is the highest when the molar ratio of CO₂/NH₃/H₂O = 1:1:1 while that without UV light is the highest when the molar ratio of CO₂/NH₃/H₂O = 1:0.5:0.5. It is revealed that the CO₂ reduction performance of Pd/TiO₂ is the highest for the molar ratio of CO₂/NH₃/H₂O = 1:1:1 no matter the used Xe lamp was with or without UV light. The molar quantity of CO per unit weight of photocatalyst for Cu/TiO₂ produced under the illumination condition of Xe lamp with UV light was 10.2 μmol/g, while that for Pd/TiO₂ was 5.5 μmol/g. Meanwhile, the molar quantity of CO per unit weight of photocatalyst for Cu/TiO₂ produced under the illumination condition of Xe lamp without UV light was 2.5 μmol/g, while that for Pd/TiO₂ was 3.5 μmol/g. This study has concluded that Cu/TiO₂ is superior to Pd/TiO₂ from the viewpoint of the molar quantity of CO per unit weight of photocatalyst as well as the quantum efficiency.     

Photocatalytic Reduction of CO2 on TiO2 and Other Semiconductors

Rising atmospheric levels of carbon dioxide and the depletion of fossil fuel reserves raise serious concerns about the ensuing effects on the global climate and future energy supply. Utilizing the abundant solar energy to convert CO₂ into fuels such as methane or methanol could address both problems simultaneously as well as provide a convenient means of energy storage. In this Review, current approaches for the heterogeneous photocatalytic reduction of CO₂ on TiO₂ and other metal oxide, oxynitride, sulfide, and phosphide semiconductors are presented. Research in this field is focused primarily on the development of novel nanostructured photocatalytic materials and on the investigation of the mechanism of the process, from light absorption through charge separation and transport to CO₂ reduction pathways. The measures used to quantify the efficiency of the process are also discussed in detail.     

Pivotal Role of Holes in Photocatalytic CO2 Reduction on TiO2

Evidence is provided that in a gas-solid photocatalytic reaction the removal of photogenerated holes from a titania (TiO₂) photocatalyst is always detrimental for photocatalytic CO₂ reduction. The coupling of the reaction to a sacrificial oxidation reaction hinders or entirely prohibits the formation of CH₄ as a reduction product. This agrees with earlier work in which the detrimental effect of oxygen-evolving cocatalysts was demonstrated. Photocatalytic alcohol oxidation or even overall water splitting proceeds in these reaction systems, but carbon-containing products from CO₂ reduction are no longer observed. H₂ addition is also detrimental, either because it scavenges holes or because it is not an efficient proton donor on TiO₂. The results are discussed in light of previously suggested reaction mechanisms for photocatalytic CO₂ reduction. The formation of CH₄ from CO₂ is likely not a linear sequence of reduction steps but includes oxidative elementary steps. Furthermore, new hypotheses on the origin of the required protons are suggested.     

Constructing Ordered Three‐Dimensional TiO2 Channels for Enhanced Visible‐Light Photocatalytic Performance in CO2 Conversion Induced by Au Nanoparticles

As a typical photocatalyst for CO₂ reduction, practical applications of TiO₂ still suffer from low photocatalytic efficiency and limited visible‐light absorption. Herein, a novel Au‐nanoparticle (NP)‐decorated ordered mesoporous TiO₂ (OMT) composite (OMT‐Au) was successfully fabricated, in which Au NPs were uniformly dispersed on the OMT. Due to the surface plasmon resonance (SPR) effect derived from the excited Au NPs, the TiO₂ shows high photocatalytic performance for CO₂ reduction under visible light. The ordered mesoporous TiO₂ exhibits superior material and structure, with a high surface area that offers more catalytically active sites. More importantly, the three‐dimensional transport channels ensure the smooth flow of gas molecules, highly efficient CO₂ adsorption, and the fast and steady transmission of hot electrons excited from the Au NPs, which lead to a further improvement in the photocatalytic performance. These results highlight the possibility of improving the photocatalysis for CO₂ reduction under visible light by constructing OMT‐based Au‐SPR‐induced photocatalysts.     

Catalysis of Carbon Dioxide Photoreduction on Nanosheets: Fundamentals and Challenges

The transformation of CO₂ into fuels and chemicals by photocatalysis is a promising strategy to provide a long‐term solution to mitigating global warming and energy‐supply problems. Achievements in photocatalysis during the last decade have sparked increased interest in using sunlight to reduce CO₂. Traditional semiconductors used in photocatalysis (e.g. TiO₂) are not suitable for use in natural sunlight and their performance is not sufficient even under UV irradiation. Some two‐dimensional (2D) materials have recently been designed for the catalytic reduction of CO₂. These materials still require significant modification, which is a challenge when designing a photocatalytic process. An overarching aim of this Review is to summarize the literature on the photocatalytic conversion of CO₂ by various 2D materials in the liquid phase, with special attention given to the development of novel 2D photocatalyst materials to provide a basis for improved materials.     

光催化CO2转化为碳氢燃料体系的综述

传统化石能源燃烧产生CO₂引起的地球变暖和能源短缺已经成为一个严重的全球性问题. 利用太阳光和光催化材料将CO₂还原为碳氢燃料, 不仅可以减少空气中CO₂浓度, 降低温室效应的影响, 还可以提供碳氢燃料, 缓解能源短缺问题, 因此日益受到各国科学家的高度关注. 本文综述了光催化还原CO₂为碳氢燃料的研究进展, 介绍了光催化还原CO₂的反应机理, 并对现阶段报道的光催化还原CO₂材料体系进行了整理和分类, 包括TiO₂光催化材料, ABO₃型钙钛矿光催化材料, 尖晶石型光催化材料, 掺杂型光催化材料, 复合光催化材料, V、W、Ge、Ga基光催化材料及石墨烯基光催化材料. 评述了各种材料体系的特点及光催化性能的一些影响因素. 最后对光催化还原CO₂的研究前景进行了展望.     

Facilely anchoring Cu2O nanoparticles on mesoporous TiO2 nanorods for enhanced photocatalytic CO2 reduction through efficient charge transfer

Semiconductor-employed photocatalytic CO₂ reduction has been regarded as a promising approach for environmental-friendly conversion of CO₂ into solar fuels. Herein, TiO₂/Cu₂O composite nanorods have been successfully fabricated by a facile chemical reduction method and applied for photocatalytic CO₂ reduction. The composition and structure characterization indicates that the Cu₂O nanoparticles are coupled with TiO₂ nanorods with an intimate contact. Under light illumination, all the TiO₂/Cu₂O composite nanorods enhance the photocatalytic CO₂ reduction. In particular, the TiO₂/Cu₂O-15% sample exhibits the highest CH₄ yield (1.35 µmol g^-1 h^-1) within 4 h irradiation, and it is 3.07 and 15 times higher than that of pristine TiO₂ nanorods and Cu₂O nanoparticles, respectively. The enhanced photoreduction capability of the TiO₂/Cu₂O-15% is attributed to the intimate construction of Cu₂O nanoparticles on TiO₂ nanorods with formed p-n junction to accelerate the separation of photogenerated electron-hole pairs. This work provides a reference for rational design of a p-n heterojunction photocatalyst for CO₂ photoreduction.     

Z-scheme heterojunction of SnS2-decorated 3DOM-SrTiO3 for selectively photocatalytic CO2 reduction into CH4

The rapid recombination of photoinduced electron-hole pairs as well as the deficiency of high-energy carriers restricted the redox ability and products selectivity. Herein, the heterojunction of SnS₂-decorated three-dimensional ordered macropores (3DOM)-SrTiO₃ catalysts were in-situ constructed to provide transmit channel for high-energy electron transmission. The suitable band edges of SnS₂ and SrTiO₃ contribute to the Z-scheme transfer of photogenerated carrier. The 3DOM structure of SrTiO₃-based catalyst possesses the slow light effect for enhancing light adsorption efficiency, and the surface alkalis strontium is benefit to the boosting adsorption for CO₂. The in-situ introduced SnS₂ decorated on the macroporous wall surface of 3DOM-SrTiO₃ altered the primary product from CO to CH₄. The Z-scheme electron transfer from SnS₂ combining with the holes in SrTiO₃ occurred under full spectrum photoexcitation, which improved the excitation and utilization of photogenerated electrons for CO₂ multi-electrons reduction. As a result, (SnS₂)₃/3DOM-SrTiO₃ catalyst exhibits higher activity for photocatalytic CO₂ reduction to CH₄ compared with single SnS₂ or 3DOM-SrTiO₃, i.e., its yield and selectivity of CH₄ are 12.5 μmol g^-1 h^-1 and 74.9%, respectively. The present work proposed the theoretical foundation of Z-scheme heterojunction construction for enhancing photocatalytic activity and selectivity for CO₂ conversion.     

Engineering a Copper Single-Atom Electron Bridge to Achieve Efficient Photocatalytic CO2 Conversion

Developing highly efficient and stable photocatalysts for the CO₂ reduction reaction (CO₂RR) remains a great challenge. We designed a Z-Scheme photocatalyst with N−Cu₁−S single-atom electron bridge (denoted as Cu-SAEB), which was used to mediate the CO₂RR. The production of CO and O₂ over Cu-SAEB is as high as 236.0 and 120.1 μmol g⁻¹ h⁻¹ in the absence of sacrificial agents, respectively, outperforming most previously reported photocatalysts. Notably, the as-designed Cu-SAEB is highly stable throughout 30 reaction cycles, totaling 300 h, owing to the strengthened contact interface of Cu-SAEB, and mediated by the N−Cu₁−S atomic structure. Experimental and theoretical calculations indicated that the SAEB greatly promoted the Z-scheme interfacial charge-transport process, thus leading to great enhancement of the photocatalytic CO₂RR of Cu-SAEB. This work represents a promising platform for the development of highly efficient and stable photocatalysts that have potential in CO₂ conversion applications.     

Pure and modified TiO2 photocatalysts and their environmental applications

Semiconductor photocatalysis is a process that harnesses light energy in chemical conversions. In particular, its applications to environmental remediation have been intensively investigated. The characteristics of TiO₂, the most popular photocatalyst, is briefly described and selected studies on the degradation/conversion of various recalcitrant pollutants using pure and modified TiO₂ photocatalysts, which were carried out in this group, are reviewed. Photocatalytic reactions are multi-phasic and take place at interfaces of not only water/TiO₂ and air/TiO₂ but also solid/TiO₂. Examples of photocatalytic reactions of various organic and inorganic substrates that are converted through the photocatalytic oxidation or reduction are introduced. TiO₂ has been modified in various ways to improve its photocatalytic activity. Surface modifications of TiO₂ that include surface platinization, surface fluorination, and surface charge alteration are discussed and their applications to pollutants degradation are also described in detail.     

Biochar modified Co–Al LDH for enhancing photocatalytic reduction CO2 performance and mechanism insight

This study prepared a biochar-based photocatalyst (Co–Al LDH–C) via facile ultrasonic-assisted solvent treatment. The Co–Al LDH–C photocatalyst shows better photocatalytic activity in CO₂ reduciton than the pure Co–Al LDH without biochar modification. The Co–Al LDH–C affords a CO generation rate of 29.2 µmol g^?1. The enhanced CO₂ reduction activity is attributed to the biochar in Co–Al LDH enhanced the light absorption property and separation efficiency of the charge carriers. Additionally, a mechanism insight of Co–Al LDH reduction CO₂ is also investigated by a series of characterizations and experiments results. This work offers a new insight for CO₂ reduction by waste utilization of biomass and improved the performance of Co–Al LDH, and extends the broad potential application of biochar-based photocatalyst in the photocatalytic conversion from solar to carbon resource.

     

Rational Design of Carbon-Based 2D Nanostructures for Enhanced Photocatalytic CO2 Reduction: A Dimensionality Perspective

Photocatalytic CO₂ reduction is a revolutionary approach to solve imminent energy and environmental issues by replicating the ingenuity of nature. The past decade has witnessed an impetus in the rise of two-dimensional (2D) structure materials as advanced nanomaterials to boost photocatalytic activities. In particular, the use of 2D carbon-based materials is deemed as highly favorable, not only as a green material choice, but also due to their exceptional physicochemical and electrical properties. This Review article presents a diverse range of alterations and compositions derived from 2D carbon-based nanomaterials, mainly graphene and graphitic carbon nitride (g-C₃N₄), which have remarkably ameliorated the photocatalytic CO₂ performance. Herein, the rational design of the photocatalyst systems with consideration of the aspect of dimensionality and the resultant heterostructures at the interface are systematically analyzed to elucidate an insightful perspective on this pacey subject. Finally, a conclusion and outlook on the limitations and prospects of the cutting-edge research field are highlighted.     

Development of TiO<Subscript>2</Subscript> photocatalysts suitable for practical use and their applications in environmental cleanup

Recently, environmental disruption is proceeding on a global scale through the consumption of huge amounts of fossil fuels and the emission of various chemical substances. However, these substances resist bio-treatment. TiO₂ generates electrons and holes by irradiation with light. Most organic micro-pollutants, including dioxins, are decomposed into carbon dioxide and water by the effect of the holes with high oxidative potential. By using such a photocatalytic reaction, various applications are feasible for environmental cleanup. In general, TiO₂ powder has been utilized as photocatalyst, although TiO₂ powder photocatalyst has several disadvantages: (1) it is difficult to handle, (2) photocatalytic reaction is slow and it takes a lot of time for treatment and (3) it is difficult to apply to plastics and textiles, because the photocatalyst decomposes them. We have developed a photocatalyst suitable for practical use and have developed high-activity photocatalysts such as TiO₂ photocatalytic transparent film, photocatalytic silica-gel, apatite-coated TiO₂ photocatalyst usable for plastics and textiles, photocatalytic paper, photocatalytic blue charcoal and photocatalytic oxygen scavenger. The application of these high-activity photocatalysts has been studied in deodorization, anti-bacterial, self-cleaning, anti-stain, water treatment, air purification such as photocatalytic decomposition of dioxins and VOC, and NO _x removal. Now various photocatalytic articles using these new photocatalyst materials are on the market in Japan. Photocatalytic technology can create many valuable products for environmental use all over the world.     

2D Catalysts for CO2 Photoreduction: Discussing Structure Efficiency Strategies and Prospects for Scaled Production Based on Current Progress

Currently, the excessive consumption of fossil fuels is accompanied by massive emissions of CO₂, leading to severe energy shortages and intensified global warming. It is of great significance to develop and use renewable clean energy while reducing the concentration of CO₂ in the atmosphere. Photocatalytic technology is a promising strategy for carbon dioxide conversion. Clearly, the achievement of the above goals largely depends on the design and construction of catalysts. This review is mainly focused on the application of 2D materials for photocatalytic CO₂ reduction. The contribution of synthetic strategies to their structure and performance is emphasized. Finally, the current challenges, and prospects of 2D materials for photoreduction of CO₂ with high efficiency, even for practical applications are discussed. It is hoped that this review can provide some guidance for the rational design, controllable synthesis of 2D materials, and their application for efficient photocatalytic CO₂ reduction.     

Hydrothermal preparation and visible-light photocatalytic activity of Bi2WO6 powders

Bi₂WO₆ powder photocatalyst was prepared using Bi(NO₃)₃ and Na₂WO₄ as raw materials by a simple hydrothermal method at 150 °C for 24 h, and then calcined at 300, 400, 500, 600 and 700 °C for 2 h, respectively. The as-prepared samples were characterized with UV-visible diffuse reflectance spectra, fourier transform infrared spectra (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and N₂ adsorption-desorption measurement. The photocatalytic activity of the samples was evaluated using the photocatalytic oxidation of formaldehyde at room temperature under visible light irradiation. It was found that post-treatment temperature obviously influenced the visible-light photocatalytic activity and physical properties of Bi₂WO₆ powders. At 500 °C, Bi₂WO₆ powder photocatalyst showed the highest visible-light photocatalytic activity due to the samples with good crystallization and high BET surface area.     

Efficient Decomposition of Organic Pollutants Over In2O3/TiO2 Nanocomposite Photocatalyst Under Visible Light Irradiation

The heterojunction structures of In₂O₃/TiO₂, exhibiting visible light photocatalytic efficiency, has been synthesized by utilizing maleic acid as an organic linker to combine In₂O₃ and Degussa P25 (TiO₂) nanoparticles. The prepared nanocomposite has been characterized by FESEM, TEM, XRD and UV?CVisible reflectance spectra. The photocatalytic efficiency of the composite photocatalyst has been investigated based on the decomposition of 2-propanol (IP) in gas phase and 1,4-dichlorobenzene (DCB) in aqueous phase under visible light (??????420?nm) irradiation. The results reveal that the In₂O₃/TiO₂ composite photocatalyst with 7?wt% In₂O₃ demonstrated 6.3 times of efficiency in evolving CO₂ from gaseous IP and 8.7 times of efficiency in removing aqueous DCB in compare with Degussa P25. In this In₂O₃/TiO₂ composite system, TiO₂ seems to be the principal photocatalyst whereas the function of In₂O₃ is to sensitize TiO₂ by absorbing visible light (??????420?nm). The extraordinary high photocatalytic efficiency of this composite In₂O₃/TiO₂ under visible light has been explained on the basis of relative energy band positions of the component semiconductors.     

A Large-Area Patterned TiO2/SnO2 Bilayer Type Photocatalyst Prepared by Gravure Printing

Anatase TiO₂ films (thickness = 50 nm) were formed in shape of stripes (width = 1.6 mm, interval = 0.4 mm) by gravure printing on commercially available SnO₂ coated soda-lime glass substrates (dimension = 300 × 300 mm). Its photocatalytic activity was examined for the gas-phase oxidation of CH₃CHO in comparison with a simple TiO₂ photocatalyst formed on a silica glass. The patterned TiO₂/SnO₂ bilayer type photocatalyst showed a high photocatalytic activity in an H₂O bearing atmosphere. On the other hand, neither the patterning nor stacking effect was observed for the same reaction under dry conditions. These results could be explained in terms of the reducing potential of the electrons in the conduction band of the SnO₂ layer.     

Rare-Earth Single Erbium Atoms for Enhanced Photocatalytic CO2 Reduction

The solar-driven photocatalytic reduction of CO₂ (CO₂RR) into chemical fuels is a promising route to enrich energy supplies and mitigate CO₂ emissions. However, low catalytic efficiency and poor selectivity, especially in a pure-water system, hinder the development of photocatalytic CO₂RR owing to the lack of effective catalysts. Herein, we report a novel atom-confinement and coordination (ACC) strategy to achieve the synthesis of rare-earth single erbium (Er) atoms supported on carbon nitride nanotubes (Er₁/CN-NT) with a tunable dispersion density of single atoms. Er₁/CN-NT is a highly efficient and robust photocatalyst that exhibits outstanding CO₂RR performance in a pure-water system. Experimental results and density functional theory calculations reveal the crucial role of single Er atoms in promoting photocatalytic CO₂RR.     

Recent progress in TiO2-based photocatalysts for hydrogen evolution reaction: A review

TiO₂ has gained tremendous attention as a cutting-edge material for application in photocatalysis. The performance of TiO₂ as a photocatalyst depends on various parameters including morphology, surface area, and crystallinity. Although TiO₂ has shown good catalytic activity in various catalysis systems, the performance of TiO₂ as a photocatalyst is generally limited due to its low conductivity and a wide optical bandgap. Numerous different studies have been devoted to overcome these problems, showing significant improvement in photocatalytic performance. In this study, we summarize the recent progress in the utilization of TiO₂ for the photocatalytic hydrogen evolution reaction (HER). Strategies for modulating the properties toward the high photocatalytic activity of TiO₂ for HER including structural engineering, compositional engineering, and doping are highlighted and discussed. The advantages and limitations of each modification approach are reviewed. Finally, the remaining obstacles and perspective for the development of TiO₂ as photocatalysts toward high efficient HER in the near future are also provided.     

Preparation of SnO2–TiO2/Fly Ash Cenospheres and its Application in Phenol Degradation

SnO₂–TiO₂/fly ash cenospheres (FAC) were prepared via hydrothermal method and used as an active photocatalyst in a photocatalytic system. Scanning electron microscopy, X‐ray diffraction analysis, UV–Vis diffuse reflectance spectroscopy, Fourier transform infrared spectroscopy and N₂ adsorption–desorption measurements were used to determine the structure and optical property of SnO₂–TiO₂/FAC. Phenol was selected as the model substance for photocatalytic reactions to evaluate catalytic ability. Results showed that the degradation efficiency of phenol by SnO₂–TiO₂/FAC was 90.7% higher than that decomposed by TiO₂/FAC. Increased efficiency could be due to the enhanced synergistic effect of semiconductors and FAC could provide more adsorption sites for the pollutant in the photocatalytic reaction. Furthermore, SnO₂–TiO₂/FAC composites exhibited excellent photocatalytic stability in four reuse cycles. Radical‐trapping experiments further revealed the dominating functions of holes in the photocatalytic reaction.