Photocatalytic reduction of CO2 with H2O on TiO2 and Cu/TiO2 catalysts

Photoinduced reduction of CO₂ by H₂O to produce CH₄ and CH₃OH has been investigated on wellcharacterized standard TiO₂ catalysts and on a Cu²⁺ loaded TiO₂ catalyst. The efficiency of this photoreaction depends strongly on the kind of catalyst and the ratio of H₂O to CO₂. Anatase TiO₂, which has a large band gap and numerous surface OH groups, shows high efficiency for photocatalytic CH₄ formation. Photogenerated Ti³⁺ ions, H and CH₃ radicals are observed as reactive intermediates, by ESR at 77 K. Cu-loading of the small, powdered TiO₂ catalyst (Cu/TiO₂) brings about additional formation of CH₃OH. XPS studies suggest that Cu⁺ plays a significant role in CH₃OH formation.     

Back Cover: Illustrating the Fate of Methyl Radical in Photocatalytic Methane Oxidation over Ag−ZnO by in situ Synchrotron Radiation Photoionization Mass Spectrometry (Angew. Chem. Int. Ed. 32/2023)

Photocatalysis has emerged as an ideal method for the direct activation and conversion of methane under mild conditions. In this reaction, methyl radical (⋅CH₃) was deemed a key intermediate that affected the yields and selectivity of the products. However, direct observation of ⋅CH₃ and other intermediates is still challenging. Here, a rectangular photocatalytic reactor coupled with in situ synchrotron radiation photoionization mass spectrometry (SR-PIMS) was developed to detect reactive intermediates within several hundred microseconds during photocatalytic methane oxidation over Ag−ZnO. Gas phase ⋅CH₃ generated by photogenerated holes (O⁻) was directly observed, and its formation was demonstrated to be significantly enhanced by coadsorbed oxygen molecules. Methoxy radical (CH₃O⋅) and formaldehyde (HCHO) were confirmed to be key C1 intermediates in photocatalytic methane overoxidation to CO₂. The gas-phase self-coupling reaction of ⋅CH₃ contributes to the formation of ethane, which indicates the key role of ⋅CH₃ desorption in the highly selective synthesis of ethane. Based on the observed intermediates, the reaction network initiated from ⋅CH₃ of photocatalytic methane oxidation could be clearly illustrated, which is helpful for studying the photocatalytic methane conversion processes.     

锐钛矿(001)与(101)晶面在光催化反应中的作用

采用水热法制备了(001)和(101)晶面暴露的单晶锐钛矿TiO₂颗粒. 利用光还原沉积贵金属(Au, Ag, Pt)和光氧化沉积金属氧化物(PbO₂, MnO_x)的方法研究了暴露的锐钛矿(001)和(101)晶面在光催化中的作用. 通过透射电子显微镜(TEM)、扫描电子显微镜(STM)、能量色散X射线光谱仪(EDX)和X射线光电子能谱(XPS)的表征, 发现发生光还原反应生成的贵金属粒子主要沉积在暴露的锐钛矿(101)晶面上, 而发生光氧化反应产生的金属氧化物颗粒主要沉积在暴露的锐钛矿(001)晶面上. 此结果表明光激发产生的电子与空穴主要并分别分布在单晶锐钛矿TiO₂的(101)与(001)晶面上, 并在其上参与光催化还原反应和氧化反应. 同时也表明暴露的不同晶面对光生电荷具有分离效应. 基于本研究可以认为同时暴露分别进行氧化和还原反应的晶面可以有效促进光催化反应.     

Simultaneous co-Photocatalytic CO2 Reduction and Ethanol Oxidation towards Synergistic Acetaldehyde Synthesis

Photocatalytic conversion of CO₂ is of great interest but it often suffers sluggish oxidation half reaction and undesired by-products. Here, we report for the first the simultaneous co-photocatalytic CO₂ reduction and ethanol oxidation towards one identical value-added CH₃CHO product on a rubidium and potassium co-modified carbon nitride (CN-KRb). The CN-KRb offers a record photocatalytic activity of 1212.3 μmol h⁻¹g⁻¹ with a high selectivity of 93.3 % for CH₃CHO production, outperforming all the state-of-art CO₂ photocatalysts. It is disclosed that the introduced Rb boosts the *OHCCHO fromation and facilitates the CH₃CHO desorption, while K promotes ethanol adsorption and activation. Moreover, the H⁺ stemming from ethanol oxidation is confirmed to participate in the CO₂ reduction process, endowing near ideal overall atomic economy. This work provides a new strategy for effective use of the photoexcited electron and hole for high selective and sustainable conversion of CO₂ paired with oxidation reaction into identical product.     

An easy one-step photocatalytic synthesis of 1-aryl-2-alkylbenzimidazoles by platinum loaded TiO2 nanoparticles under UV and solar light

One-pot synthesis of disubstituted benzimidazoles from N-substituted 2-nitroanilines or 1,2-diamines with 3-12 nm-sized platinum particles loaded on the TiO₂ using solar and UV-A light is described. 1-Aryl-2-alkylbenzimidazoles from 2-nitrodiphenylamines are formed by combined redox photocatalytic reaction, condensation and catalytic dehydrogenation on Pt-TiO₂. In case of diamines, this reaction is proceeded by Pt-TiO₂ assisted photocatalytic oxidation of an alcohol and a catalytic dehydrogenation of the intermediate on the surface of platinum nanoparticles. In both cases product formation was achieved by tandem photocatalytic and catalytic reactions on Pt-TiO₂.     

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.     

Cover Feature: The Role of Common Alcoholic Sacrificial Agents in Photocatalysis: Is It Always Trivial? (Chem. Eur. J. 64/2021)

Photocatalytic hydrogen production is proposed as a sustainable energy source. Simultaneous reduction and oxidation of water is a complex multistep reaction with high overpotential. Photocatalytic processes involving semiconductors transfer electrons from the valence band to the conduction band. Sacrificial substrates that react with the photochemically formed holes in the valence band are often used to study the mechanism of H₂ production, as they scavenge the holes and hinder charge carrier recombination (electron-hole pairs). Here, we show that the desired sacrificial agent is one forming a radical that is a fairly strong reducing agent, and whose oxidized form is not a good electron acceptor that might suppress the hydrogen evolution reaction (HER). In an acidic medium, methanol was found to fulfill both these requirements better than ethanol and propan-2-ol in the TiO₂-(M⁰-NPs) (M=Au or Pt) system, whereas in an alkaline medium, the alcohols exhibit a reverse order of activity. Moreover, we report that CH₂(OH)₂ is by far the most efficient sacrificial agent in a nontrivial mechanism in acidic media. Our study provides general guidelines for choosing an appropriate sacrificial substrate and helps to explain the variance in the performance of alcohol scavenger-based photocatalytic systems.     

Improving photoreduction of CO_2 with water to CH_4 in a novel concentrated solar reactor

CO_2 photoreduction is an attractive process which allows the storage of solar energy and synthesis of solar fuels. Many different photocatalytic systems have been developed, while the alternative photo-reactors are still insufficiently investigated. In this work, photoreduction of CO_2 with H_2O into CH_4 was investigated in a modified concentrating solar reactor, using TiO_2 and Pt/TiO_2 as the catalysts. The TiO_2 and Pt/TiO_2 samples were extensively characterized by different techniques including powder X-ray diffraction(XRD), N_2 adsorption/desorption and UV–vis absorption. The catalytic performance of the TiO_2 and Pt/TiO_2 samples in the gas phase was evaluated under unconcentrated and concentrated Xe-lamp light and nature solar light with different concentrating ratios. Various parameters of the reaction system and the catalysts were investigated and optimized to maximize the catalytic performance of CO_2 reduction system. Compared with the normal light irradiation, the TiO_2 and Pt/TiO_2 samples show higher photocatalytic activity(about 6–7 times) for reducing CO_2 into CH_4 under concentrated Xe-lamp light and nature solar light. In the range of experimental light intensity, it is found that the concentration of the light makes it suitable for the catalytic reaction, and increases the utilization efficiency of the TiO_2 and Pt/TiO_2 samples while does not decrease the quantum efficiency.     

Catalytic Role of TiO2 Terminal Oxygen Atoms in Liquid‐Phase Photocatalytic Reactions: Oxidation of Aromatic Compounds in Anhydrous Acetonitrile

On the basis of experiments carried out with controlled amounts of residual oxygen and water, or by using oxygen‐isotope‐labeled Ti¹⁸O₂ as the photocatalyst, we demonstrate that ¹⁸O_s atoms behave as real catalytic species in the photo‐oxidation of acetonitrile‐dissolved aromatic compounds such as benzene, phenol, and benzaldehyde with TiO₂. The experimental evidence allows a terminal‐oxygen indirect electron‐transfer (TOIET) mechanism to be proposed, which is a new pathway that involves the trapping of free photogenerated valence‐band holes at O_s species and their incorporation into the reaction products, with simultaneous generation of oxygen vacancies at the TiO₂ surface and their subsequent healing with oxygen atoms from either O₂ or H₂O molecules that are dissolved in the liquid phase. According to the TOIET mechanism, the TiO₂ surface is not considered to remain stable, but is continuously changing in the course of the photocatalytic reaction, challenging earlier interpretations of TiO₂ photocatalytic phenomena.     

Metal-Free Photocatalytic CO2 Reduction to CH4 and H2O2 under Non-sacrificial Ambient Conditions

Photocatalytic CO₂ reduction to CH₄ requires photosensitizers and sacrificial agents to provide sufficient electrons and protons through metal-based photocatalysts, and the separation of CH₄ from by-product O₂ has poor applications. Herein, we successfully synthesize a metal-free photocatalyst of a novel electron-acceptor 4,5,9,10-pyrenetetrone (PT), to our best knowledge, this is the first time that metal-free catalyst achieves non-sacrificial photocatalytic CO₂ to CH₄ and easily separable H₂O₂. This photocatalyst offers CH₄ product of 10.6 μmol ⋅ g⁻¹ ⋅ h⁻¹ under non-sacrificial ambient conditions (room temperature, and only water), which is two orders of magnitude higher than that of the reported metal-free photocatalysts. Comprehensive in situ characterizations and calculations reveal a multi-step reaction mechanism, in which the long-lived oxygen-centered radical in the excited PT provides as a site for CO₂ activation, resulting in a stabilized cyclic carbonate intermediate with a lower formation energy. This key intermediate is thermodynamically crucial for the subsequent reduction to CH₄ product with the electronic selectivity of up to 90 %. The work provides fresh insights on the economic viability of photocatalytic CO₂ reduction to easily separable CH₄ in non-sacrificial and metal-free conditions.     

Redox system for perfluoroalkylation of arenes and α-methylstyrene derivatives using titanium oxide as photocatalyst

Photoirradiation of titanium oxide (TiO₂) excites the electrons from the valence band to the conduction band, leaving holes in the valence band. Using these holes and electrons, it is possible to perform one-electron oxidations and reductions. We developed a method for the photocatalytic perfluoroalkylation of aromatic rings such as benzene and its derivatives, naphthalene and benzofuran with perfluoroalkyl iodide by the combination of reduction and oxidation reactions with TiO₂. Perfluoroalkyl iodide was reduced to a perfluoroalkyl radical by the excited electrons in the conduction band of TiO₂, and the resulting radical reacted with an aromatic ring to form an arenium radical that was successively oxidized to a cation by the holes in the valence band of TiO₂. Similarly, the photocatalytic reaction of α-methylstyrene with perfluoroalkyl iodide afforded perfluoroalkylated α-methylstyrene, in which the perfluoroalkyl group is on a methyl carbon.     

The effects of local environment towards yield and selectivity on photocatalytic CO2 reduction catalyzed by metal-organic framework

The CO₂ photoconversion is sensitive to the local reaction environment, of which activity and selectivity can be regulated by the change of reaction systems. This paper focuses on investigating the photocatalytic CO₂ reduction behaviors of MOFs with the involvement of water under different reaction modes, including gas-solid and liquid-solid systems. The CO₂ photoreduction in a liquid-solid system shows high performance in generating HCOOH with the selectivity of 100%. In contrast, the gas-solid system referring to the synergistic interaction of MOFs and H₂O vapor benefits to the formation of gas-phase products, such as CO and CH₄. The possible mechanisms of photocatalytic CO₂ reaction in two modes were investigated by in-situ Fourier-transform infrared spectroscopy, which indicates that the distinction in reaction consequence may result from the difference in CO₂ chemisorbed modes and the proton provision. The choice of reaction system plays an important role in the achievement of high efficiency and selectivity for photocatalytic CO₂ reduction, which is of great practical value in real-world applications.     

TiO2/Cu2O Core/Ultrathin Shell Nanorods as Efficient and Stable Photocatalysts for Water Reduction

P‐type Cu₂O has been long considered as an attractive photocatalyst for photocatalytic water reduction, but few successful examples has been reported. Here, we report the synthesis of TiO₂ (core)/Cu₂O (ultrathin film shell) nanorods by a redox reaction between Cu²⁺ and in‐situ generated Ti³⁺ when Cu²⁺‐exchanged H‐titanate nanotubes are calcined in air. Owing to the strong TiO₂‐Cu₂O interfacial interaction, TiO₂ (core)/Cu₂O (ultrathin film shell) nanorods are highly active and stable in photocatalytic water reduction. The TiO₂ core and Cu₂O ultrathin film shell respectively act as the photosensitizer and cocatalyst, and both the photoexcited electrons in the conduction band and the holes in the valence band of TiO₂ respectively transfer to the conduction band and valence band of the Cu₂O ultrathin film shell. Our results unambiguously show that Cu₂O itself can act as the highly active and stable cocatalyst for photocatalytic water reduction.     

Unravelling the Promotional Effect of La2O3 in Pt/La-TiO2 Catalysts for CO2 Hydrogenation

This work reports the preparation of a La₂O₃-modified Pt/TiO₂ (Pt/La-TiO₂) hybrid through an excess-solution impregnation method and its application for CO₂ hydrogenation catalysis. The Pt/La-TiO₂ catalyst is characterized by XRD, H₂ temperature-programmed reduction (TPR), TEM, X-ray photoelectron spectroscopy (XPS), Raman, EPR, and N₂ sorption measurements. The Pt/La-TiO₂ composite starts to catalyze the CO₂ conversion reaction at 220 °C, which is 30 °C lower than the Pt/TiO₂ catalyst. The generation of CH₄ and CO of Pt/La-TiO₂ is 1.6 and 1.4 times greater than that of Pt/TiO₂. The CO₂ temperature-programmed desorption (TPD) analysis confirms the strengthened CO₂ adsorption on Pt/La-TiO₂. Moreover, the in situ FTIR experiments demonstrate that the enhanced CO₂ adsorption of Pt/La-TiO₂ facilitates the formation of the active Pt–CO intermediate and subsequently boosts the evolution of CH₄ and CO. The cycling tests reveal that Pt/La-TiO₂ shows reinforced stability for the CO₂ hydrogenation reaction because the La species can prevent Pt nanoparticles (NPs) from sintering. This work may provide some guidance on the development new rare-metal-modified hybrid catalysts for CO₂ fixation.     

Study of the Efficiency of UV and Visible‐Light Photocatalytic Oxidation of Methanol on Mesoporous RuO2–TiO2 Nanocomposites

Mesoporous RuO₂–TiO₂ nanocomposites at different RuO₂ concentrations (0–10 wt %) are prepared through a simple one‐step sol–gel reaction of tetrabutyl orthotitanate with ruthenium(III) acetylacetonate in the presence of an F127 triblock copolymer as structure‐directing agent. The thus‐formed RuO₂–TiO₂ network gels are calcined at 450 °C for 4 h leading to mesoporous RuO₂–TiO₂ nanocomposites. The photocatalytic CH₃OH oxidation to HCHO is chosen as the test reaction to examine the photocatalytic activity of the mesoporous RuO₂–TiO₂ nanocomposites under UV and visible light. The photooxidation of CH₃OH is substantially affected by the loading amount and the degree of dispersion of RuO₂ particles onto the TiO₂, which indicates the exclusive effect of the RuO₂ nanoparticles on this photocatalytic reaction under visible light. The measured photonic efficiency ξ=0.53 % of 0.5 wt % RuO₂–TiO₂ nanocomposite for CH₃OH oxidation is maximal and the further increase of RuO₂ loading up to 10 wt % gradually decreases this value. The cause of the visible‐light photocatalytic behavior is the incorporation of small amounts of Ru⁴⁺ into the anatase lattice. On the other hand, under UV light, undoped TiO₂ shows a very good photonic efficiency, which is more than three times that for commercial photocatalyst, P‐25 (Evonik–Degussa); however, addition of RuO₂ suppresses the photonic efficiency of TiO₂. The proposed reaction mechanism based on the observed behavior of RuO₂–TiO₂ photocatalysts under UV and visible light is explored.     

CO2在纳米SiO2/TiO2悬浮体系中的光催化还原

用水热法合成了氧化硅改性的具有高比表面积、高催化活性的锐钛型二氧化钛, 并在其悬浮体系中将CO₂光催化还原合成甲醇. 采用XRD, TEM, 物理吸附, UV-Vis吸收光谱和FTIR等表征手段对催化剂结构特征进行了研究. 结果表明: 添加氧化硅后, 氧化硅和二氧化钛之间形成Si—O—Ti键, 抑制了TiO₂晶粒生长, 提高了锐钛型TiO₂的比表面积, 且随着含硅量的增加, SiO₂/TiO₂的UV吸收逐步蓝移, 禁带宽度增加. 还原反应结果表明: SiO₂/TiO₂具有光催化还原活性, 且随着含硅量的增加先增加后减小, 当SiO₂质量分数为3.5%时, SiO₂/TiO₂复合催化剂反应活性最强, 5 h内甲醇产量可达到21.0 mg/L, 并有少量甲醛生成.     

Construction of the reaction networks for heterogeneous catalytic reactions: Fischer—Tropsch synthesis and related reactions

The key approaches to the generation of reaction networks for the synthesis of products from CO and H₂ are considered. The selection rules for the elementary steps on the surface of heterogeneous catalysts are formulated. Data on the surface compounds and steps related to reactions of CO and H₂ are analyzed and a set of transforms (models of elementary steps) for generation of the reaction network are selected. Eight variants of generation of reaction networks for the formation of C1 products with different sets of transforms (12 to 31) were tested in computer experiments, and eight reaction networks comprising 34 substances and 132 to 1647 elementary steps were obtained. The pathways to CO₂, CH₄, and CH₃OH and pairs of compounds CH₄, CO₂ and CH₄, HCOOH obtained from the reaction network (220 elementary steps) are compared with the published schemes.     

Template‐free Fabrication of ZnS/TiO2 Photocatalyst with Macrochannels

Constructing a porous structure in photocatalysts is an effective strategy for improving the photocatalytic activity because of its enhanced molecule transfer capability and light capturing efficiency. In this work, a hierarchical macro‐/mesoporous ZnS/TiO₂ composite with macrochannels was successfully synthesized without using templates by the simple dropwise addition of an ethanol solution of tetrabutyl titanate and zinc acetate into a sodium sulfide aqueous solution, which was then calcined at 450°C. Compared with pure TiO₂, the ordered porous ZnS/TiO₂ composite exhibited an enhanced photocatalytic activity on methylene blue removal under UV‐light irradiation. The results indicate that the macro‐/mesoporous structure, the large specific surface area, and the heterostructure combination between ZnS and TiO₂ play a synergistic effect on the enhanced photocatalytic activity via improving the light absorption and the diffusion of organic molecules, providing more reactive sites for the photocatalytic reaction and improving the separation of photogenerated electron–hole pairs, respectively. Radical trapping experiments demonstrated that holes (h⁺) and superoxide anion radicals (O₂⁻) play an important role in the photocatalytic oxidation process.     

Fabrication of S-scheme hollow TiO2@Bi2MoO6 composite for efficiently photocatalytic CO2 reduction

Herein, an S-scheme hollow TiO₂@Bi₂MoO₆ heterojunction was synthesized for photocatalytic reduction of CO₂ under simulated sunlight. Among all prepared composites, the TiO₂@Bi₂MoO₆ with 20% of TiO₂ exhibited the highest CO yield (183.97 μmol/g within 6 h), which was 4.0 and 2.4 times higher than pristine TiO₂ and Bi₂MoO₆, respectively. The improved photocatalytic activity may be due to the formation of S-scheme heterojunction to promote the separation and transfer of photogenerated charge carriers. Additionally, this hollow structure provided abundant sites in terms of CO₂ adsorption and activation. Meanwhile, the photogenerated charge transfer mechanism of the S-scheme was verified by work function calculations, Electron paramagnetic resonance (EPR) measurements as well as X-ray photoelectron spectroscopy (XPS). This research presents a novel approach to improve photocatalytic reduction of CO₂ via morphology modulation and the fabrication of S-scheme heterojunction.     

Porous TiO2 Nanotubes with Spatially Separated Platinum and CoOx Cocatalysts Produced by Atomic Layer Deposition for Photocatalytic Hydrogen Production

Efficient separation of photogenerated electrons and holes, and associated surface reactions, is a crucial aspect of efficient semiconductor photocatalytic systems employed for photocatalytic hydrogen production. A new CoO_x/TiO₂/Pt photocatalyst produced by template‐assisted atomic layer deposition is reported for photocatalytic hydrogen production on Pt and CoO_x dual cocatalysts. Pt nanoclusters acting as electron collectors and active sites for the reduction reaction are deposited on the inner surface of porous TiO₂ nanotubes, while CoO_x nanoclusters acting as hole collectors and active sites for oxidation reaction are deposited on the outer surface of porous TiO₂ nanotubes. A CoO_x/TiO₂/Pt photocatalyst, comprising ultra‐low concentrations of noble Pt (0.046 wt %) and CoO_x (0.019 wt %) deposited simultaneously with one atomic layer deposition cycle, achieves remarkably high photocatalytic efficiency (275.9 μmol h⁻¹), which is nearly five times as high as that of pristine TiO₂ nanotubes (56.5 μmol h⁻¹). The highly dispersed Pt and CoO_x nanoclusters, porous structure of TiO₂ nanotubes with large specific surface area, and the synergetic effect of the spatially separated Pt and CoO_x dual cocatalysts contribute to the excellent photocatalytic activity.