Theoretical insights into the mechanism of photocatalytic reduction of CO2 over semiconductor catalysts

Photocatalytic reduction of CO₂ is one important approach to alleviate greenhouse gas emission and energy crisis, which has gained huge attention in the past decades. However, the lack of understanding complex reaction mechanism impedes new catalysts design. It is also very difficult to understand the mechanism by using only experimental approaches. For this concern, theoretical calculations can effectively supplement the experimental deficiency and thus play an important role. Recently theoretical calculations have been performed on adsorption, migration and reduction of CO₂ molecule on the photocatalyst surface, leading to useful information that have contributed greatly to this field. This review summarizes recent advances in first-principles calculations about CO₂ photoreduction over various semiconductor photocatalysts like metal oxides, sulfides and g-C₃N₄. The methods, models, adsorption and reaction pathways have been discussed in detail. The perspective about future investigation on the photocatalytic reduction of CO₂ using first principles calculations is also presented.     

Sunlight-induced effective heterogeneous photocatalytic decomposition of aqueous organic pollutants to CO2 assisted by a CO2 sorbent, amine-containing mesoporous silica

Photocatalytic mineralization of aqueous formic acid and phenol on pure TiO(2) under sunlight irradiation was substantially accelerated to give a reliable photocatalytic efficiency by conducting the reactions in the presence of a CO(2) sorbent, amine-containing SBA-15, placed in the gas phase of the reactor.     

Biocidal effects of photocatalytic semiconductor TiO2

Photocatalytic action of the commercial TiO₂ was the subject of study on the destruction of the microbes within the biofilms. The TiO₂ powder was characterized by X-ray diffraction (XRD) for identifying its type and the particle size was determined. The biofilm was allowed to form over TiO₂ coatings over glass slides irradiated with polychromatic light for different time durations and distances. It indicates that a five-fold decrease in bacterial count due to the formation of H₂O₂ at TiO₂/biofilm interface. The formation of H₂O₂ at the TiO₂/biofilm interface is estimated and it does not destroy the entire bacterial population within the biofilm. Bacterial killing effect is supported by FT-IR analysis.     

CO2-fixation into organic carbonyl compounds in visible-light-induced photocatalysis of linear aromatic compounds

CO₂ is fixed into benzophenone using poly(p-phenylene) as a photocatalyst and triethylamine as an electron donor under visible light irradiation. The reaction was enhanced by the addition of onium cation with soft character. The enhanced formation of the radical anion and the dianion species in poly(p-phenylene) by sodium reduction was confirmed by the cation addition. The role of the anion species formed in the photocatalytic system is discussed as the electron pool for the photofixation.     

几种典型有毒有机污染物的光催化降解研究

本文研究了氯代苯酚和苯胺类化合物的光催化降解,探讨了这些有机污染物分子的氯取代和化合物毒性与光催化降解之间的关系.研究表明,氯代苯酚和苯胺类化合物的光催化降解均符合表观一级反应动力学模型.有机物分子中氯取代基的引入加快了化合物的光催化降解,且反应速率随氯取代基个数的增多而增大.光催化降解的表观速率常数与正辛醇-水分配系数之间存在有较好的线性关系:logk=alogKow+b,该模型可以用于有机污染物的光催化降解性预测.     

Defective TiO2 for photocatalytic CO2 conversion to fuels and chemicals

Photocatalytic conversion of CO₂ into fuels and valuable chemicals using solar energy is a promising technology to combat climate change and meet the growing energy demand. Extensive effort is going on for the development of a photocatalyst with desirable optical, surface and electronic properties. This review article discusses recent development in the field of photocatalytic CO₂ conversion using defective TiO₂. It specifically focuses on the different synthesis methodologies adapted to generate the defects and their impact on the chemical, optical and surface properties of TiO₂ and, thus, photocatalytic CO₂ conversion. It also encompasses theoretical investigations performed to understand the role of defects in adsorption and activation of CO₂ and identify the mechanistic pathway which governs the formation and selectivity of different products. It is divided into three parts: (i) general mechanism and thermodynamic criteria for defective TiO₂ catalyzed CO₂ conversion, (ii) theoretical investigation on the role of defects in the CO₂ adsorption–activation and mechanism responsible for the formation and selectivity of different products, and (iii) the effect of variation of physicochemical properties of defective TiO₂ synthesized using different methods on the photocatalytic conversion of CO₂. The review also discusses the limitations and the challenges of defective TiO₂ photocatalysts that need to be overcome for the production of sustainable fuel utilizing solar energy.

This review discusses photocatalytic CO₂ conversion using defective TiO₂, with emphasis on the mechanism, the role of defects on CO₂ adsorption–activation and product selectivity, as well as challenges of defective TiO₂ to produce solar fuels.     

Solubility of room-temperature ionic liquid in supercritical CO2 with and without organic compounds

Ionic liquid can dramatically dissolve in supercritical (sc) CO2 with polar organic compounds (ethanol, acetone) especially as the concentration of the compounds in scCO2 exceeds 10 mol%, while the effect of a nonpolar organic compound (n-hexane) in scCO2 on the solubility is very limited even when its concentration is as high as 30 mol%.     

Abatement of volatile organic compounds using an annular photocatalytic reactor: Study of gaseous acetone

Photocatalytic oxidation of organic compounds in gas phase appears to be a promising process for remediation of polluted air. In the present work, the photocatalytic degradation of acetone, which is a typical pollutant of indoor air, was investigated by using an annular photoreactor. After a modelling by a cascade of elementary continuously stirred tank reactor, the annular photoreactor was assimilated to a plug flow reactor (PFR). No transfer limitation (external and internal) has been demonstrated for this reactor with the fibreglass photocatalytic support. The influence of several kinetic parameters has been studied such as pollutant concentration, incident light irradiance, contact time and humidity content. The Langmuir–Hinshelwood model has been verified for acetone. It can be noticed that no by-products have been detected by FID suggesting almost total mineralization. The possible minor gaseous by-products have been accumulated into a mixture of ethanol–liquid nitrogen at −50 °C then a sample of it has been injected into a GC/MS for analysis. A mechanistic pathway is then proposed for the photocatalytic degradation of acetone.     

A TiO2-nanotube-array-based photocatalytic fuel cell using refractory organic compounds as substrates for electricity generation

A TiO(2)-nanotube-array-based photocatalytic fuel cell system was established for generation of electricity from various refractory organic compounds and simultaneous wastewater treatment. The present system can respond to visible light and produce obviously enhanced cell performance when a narrow band-gap semiconductor (i.e. Cu(2)O and CdS) was combined with TiO(2) nanotubes.     

Comparison of photocatalytic degradation kinetic characteristics of different organic compounds at anatase TiO2 nanoporous film electrodes

Photoelectrochemical method was adopted to investigate the photocatalytic oxidation of different organic compounds. It was found that at the anatase TiO₂ nanoporous electrode, the potential bias changes the rate-determining step from electron migration in the film at low potentials to photohole capture at relatively high potentials. When the applied potential bias is sufficient, the steady state photocurrent obtained reflects exclusively the rate of photohole capture at TiO₂ surface. Under such conditions, the photocatalytic degradation of various organic compounds with different chemical structures was studied.At very low concentrations, the linear increase of steady state photocurrent with the concentration was observed for all compounds investigated, due to the mass transfer limitation, although the number of electron needed for complete mineralization of these compounds differs markedly. It was demonstrated that the substrate molecules that reaches the electrode surface have been exhaustively mineralized under mass transfer-controlled conditions regardless of their chemical nature.At high concentration, substrate molecules (or intermediates) are accumulated on the surface (or in the reaction zone). As a result, the steady state photocurrent deviated from the linear relationship. Under such conditions, the interaction of substrate molecules and/or partially degraded intermediates with TiO₂ determines the overall photohole capture rate. The differences in photohole capture rate among different organic compounds were observed. The kinetic characteristics of different organic compounds at high concentrations were also explained based on the structural differences.     

Hybrid composites of monodisperse pi-conjugated rodlike organic compounds and semiconductor quantum particles

Composite materials of quantum particles (Q-particles) arranged in layers within crystalline powders of pi-conjugated, rodlike dicarboxylic acids are reported. The synthesis of the composites, either as three-dimensional crystals or as thin films at the air-water interface, comprises a two-step process: 1) The preparation of the Cd salts 6 (Cd), 8 (Cd) or Pb salts 6 (Pb), 8 (Pb) of the oligo(p-phenyleneethynylene)dicarboxylic acids 6 (H), 8 (H), in which the metal ions are arranged in ribbons and are separated by the long axis of the organic molecules, as demonstrated by X-ray powder diffraction analysis of the solids and grazing incidence X-ray diffraction analysis of the films on water. 2) Topotactic solid/gas reaction of these salts with H(2)S to convert the metal ions into Q-particles of CdS or PbS embedded in the organic matrix that consists of the acids 6 (H) and 8 (H). These hybrid materials have been characterized by X-ray photoelectron spectroscopy and transmission electron microscopy.     

Highly CO2-selective organic molecular cages: what determines the CO2 selectivity

A series of novel organic cage compounds 1-4 were successfully synthesized from readily available starting materials in one-pot in decent to excellent yields (46-90%) through a dynamic covalent chemistry approach (imine condensation reaction). Covalently cross-linked cage framework 14 was obtained through the cage-to-framework strategy via the Sonogashira coupling of cage 4 with the 1,4-diethynylbenzene linker molecule. Cage compounds 1-4 and framework 14 exhibited exceptional high ideal selectivity (36/1-138/1) in adsorption of CO(2) over N(2) under the standard temperature and pressure (STP, 20 °C, 1 bar). Gas adsorption studies indicate that the high selectivity is provided not only by the amino group density (mol/g), but also by the intrinsic pore size of the cage structure (distance between the top and bottom panels), which can be tuned by judiciously choosing building blocks of different size. The systematic studies on the structure-property relationship of this novel class of organic cages are reported herein for the first time; they provide critical knowledge on the rational design principle of these cage-based porous materials that have shown great potential in gas separation and carbon capture applications.     

Synergic effect of simultaneous fluorination and platinization of TiO2 surface on anoxic photocatalytic degradation of organic compounds

Simultaneously surface fluorinated and platinized TiO2 (F-TiO2/Pt) exhibits a novel photocatalytic activity for the anoxic degradation of organic compounds, which is attributed to the unique synergic effect of surface fluorination and platinization on the photo-induced charge transfer process.     

A nature inspired molecular Ni-catalyst for efficient photocatalytic CO2 reduction to CO under visible light

Science China Chemistry - Development of efficient molecular catalysts for photocatalytic CO2 reduction is desirable but challenging. At present, the majority of reported molecular catalysts...     

Amide-bridged conjugated organic polymers: efficient metal-free catalysts for visible-light-driven CO2 reduction with H2O to CO

The visible-light-driven photoreduction of CO₂ to value-added chemicals over metal-free photocatalysts without sacrificial reagents is very interesting, but challenging. Herein, we present amide-bridged conjugated organic polymers (amide-COPs) prepared via self-condensation of amino nitriles in combination with hydrolysis, for the photoreduction of CO₂ with H₂O without any photosensitizers or sacrificial reagents under visible light irradiation. These catalysts can afford CO as the sole carbonaceous product without H₂ generation. Especially, amide-DAMN derived from diaminomaleonitrile exhibited the highest activity for the photoreduction of CO₂ to CO with a generation rate of 20.6 μmol g⁻¹ h⁻¹. Experiments and DFT calculations confirmed cyano/amide groups as active sites for CO₂ reduction and second amine groups for H₂O oxidation, and suggested that superior selectivity towards CO may be attributed to the adjacent redox sites. This work presents a new insight into designing photocatalysts for artificial photosynthesis.

Amino nitrile-derived conjugated organic polymers can realize the photoreduction of CO₂ with water to CO without H₂ generation efficiently.     

Preface: Special topic on photocatalytic or electrocatalytic reduction of CO2

Science China Chemistry -     

Development of a panchromatic photosensitizer and its application to photocatalytic CO2 reduction

We designed and synthesized a heteroleptic osmium(ii) complex with two different tridentate ligands, Os. Os can absorb the full wavelength range of visible light owing to S–T transitions, and this was supported by TD-DFT calculations. Excitation of Os using visible light of any wavelength generates the same lowest triplet metal-to-ligand charge-transfer excited state, the lifetime of which is relatively long (τ_em = 40 ns). Since excited Os could be reductively quenched by 1,3-dimethyl-2-(o-hydroxyphenyl)-2,3-dihydro-1H-benzo[d]imidazole, Os displays high potential as a panchromatic photosensitizer. Using a combination of Os and a ruthenium(ii) catalyst, CO₂ was photocatalytically reduced to HCOOH via irradiation with 725 nm light, and the turnover number reached 81; irradiation with light at λ_ex > 770 nm also photocatalytically induced HCOOH formation. These results clearly indicate that Os can function as a panchromatic redox photosensitizer.

The osmium(ii) complex functioned as a panchromatic photosensitizer and drove CO₂ reduction.