Abiotic-Biological Hybrid Systems for CO2 Conversion to Value-Added Chemicals and Fuels

Abiotic-biological hybrid systems that combine the advantages of abiotic catalysis and biotransformation for the conversion of carbon dioxide(CO2)to value-added chemicals and fuels have emerged as an appealing way to address the global energy and environmental crisis caused by increased CO2 emission.We illustrate the recent progress in this field.Here,we first review the natural CO2 fixation pathways for an in-depth understanding of the biological CO2 transformation strategy and why a sustainable feed of reducing power is important.Second,we review the recent progress in the construction of abiotic-biological hybrid systems for CO2 transformation from two aspects:(i)microbial electrosynthesis systems that utilize electricity to support whole-cell biological CO2 conversion to products of interest and(ii)photosynthetic semiconductor biohybrid systems that integrate semiconductor nanomaterials with CO2-fixing microorganisms to harness solar energy for biological CO2 transformation.Lastly,we discuss potential approaches for further improvement of abiotic-biological hybrid systems.

Molybdenum carbide clusters for thermal conversion of CO2 to CO via reverse water-gas shift reaction

Molybdenum carbides are highly active for CO2 conversion to CO via the reverse water-gas shift(RWGS)reaction, however the large grain size up to micrometers renders its relatively lower active sites utilization efficiency while generating CH4 as a by-product. In this work, a homogeneously dispersed molybdenum carbide hybrid catalyst with sub-nanosized cluster(the average size as small as 0.5 nm) is prepared via a facile carbothermal treatment for highly selective CO2-CO reduction. The partially disordered Mo2C clusters are characterized by synchrotron high-resolution XRD and atomic resolution HAADF-STEM analysis, for which the source cause of the disorder is pinpointed by XAFS analysis to be the nitrogen intercalants from the carbonaceous precursor. The partially disordered Mo2C clusters show a RWGS rate as high as 184.4 μmol gMo2C-1s-1 at 400 ℃ with a superior selectivity toward CO(> 99.5%). This work 2 highlights a facile strategy for fabricating highly dispersed and partially disordered Mo2C clusters at a sub-nano size with beneficial N-doping for delivering high catalytic activity and operational stability.

Co-MOF as an electron donor for promoting visible-light photoactivities of g-C3N4 nanosheets for CO2 reduction

A possible mechanism for boosting the visible-light photoactivities of graphitic carbon nitride(g-C3N4)nanosheets for CO2 reduction via coupling with the electron donor Co-metal-organic framework(MOF)is proposed in this study.Specifically,Co-MOF as an electron donor is capable of transferring the photogenerated electrons in the lowest unoccupied molecular orbital(LUMO)to the conduction band of g-C3N4 to facilitate charge separation.As expected,the prepared Co-MOF/g-C3N4 nanocomposites display excellent visible-light-driven photocatalytic CO2 reduction activities.The CO production rate of 6.75μmol g–1 h–1 and CH4 evolution rate of 5.47μmol g–1 h–1 are obtained,which are approximately 2 times those obtained with the original g-C3N4 under the same conditions.Based on a series of analyses,it is shown that the introduction of Co-MOF not only broadens the range of visible-light absorption but also enhances the charge separation,which improves the photocatalytic activity of g-C3N4 to a higher level.In particular,the hydroxyl radical(·OH)experiment was operated under 590 nm(single-wavelength)irradiation,which further proved that the photogenerated electrons in the LUMO of Co-MOF can successfully migrate to g-C3N4.This work may provide an important strategy for the design of highly efficient g-C3N4-based photocatalysts for CO2 reduction.

Revalorization of CO2 for methanol production via ZnO promoted carbon nanofibers based Cu-ZrO2 catalytic hydrogenation

A series of novel carbon nanofibers(CNFs)based Cu-ZrO2 catalysts were synthesized by deposition precipitation method.To investigate the influence of promoter,catalysts were loaded with 1,2,3 and 4 wt%ZnO and characterized by ICP-OES,HRTEM,BET,N2O chemisorption,TPR,XPS and CO2-TPD techniques.The results revealed that physicochemical properties of the catalysts were strongly influenced by incorporation of ZnO to the parent catalyst.Copper surface area(SCu)and dispersion(DCu)were slightly decreased by incorporation of ZnO promoter.Nevertheless,SCuand DCuwere remarkably decreased when ZnO content was exceeded beyond 3 wt%.The catalytic performance was evaluated by using autoclave slurry reactor at a pressure and temperature of 30 bar and 180℃,respectively.The promotion of CuZrO2/CNFs catalyst with 3 wt%of ZnO enhanced methanol synthesis rate from 32 to 45 g kg^-1 h^-1.Notably,with the ZnO promotion the selectivity to methanol was enhanced to 92%compared to 78%of the un-promoted Cu-ZrO2/CNFs catalyst at the expense of a lowered CO2 conversion.In addition,the catalytic activity of this novel catalyst system for CO2 hydrogenation to methanol was compared with the recent literature data.

Size control synthesis of sulfur doped titanium dioxide (anatase) nanoparticles,its optical property and its photo catalytic reactivity for CO_2 + H_2O conversion and phenol degradation

Sulfur doped anatase TiO2 nanoparticles （3 nm- 12 nm） were synthesized by the reaction of titanium tetrachloride, water and sulfuric acid with addition of 3 M NaOH at room temperature. The electro-optical and photocatalytic properties of the synthesized sulfur doped TiO2 nanoparticles were studied along with Degussa commercial TiO2 particles （24 nm）. The results show that band gap of TiO2 particles decreases from 3.31 to 3.25 eV and for that of commercial TiO2 to 3.2 eV when the particle sizes increased from 3 nm to 12 nm with increase in sulfur doping. The results of the photocatalytic activity under UV and sun radiation show maximum phenol conversion at the particle size of 4 nm at 4.80% S-doping. Similar results are obtained using UV energy for both phenol conversion and conversion of CO2＋H2O in which formation of methanol, ethanol and proponal is observed. Production of methanol is also achieved on samples with a particle size of 8 and 12 nm and sulfur doping of 4.80% and 5.26%. For TiO2 particle of 4 nm without S doping, the production of methanol, ethanol and proponal was lower as compared to the S-doped particles. This is attributed to the combined electronic effect and band gap change, S dopant, specific surface area and the light source used.

Protic vs aprotic ionic liquid for CO2 fixation:A simulation study

The cycloaddition of CO2 with epoxides catalyzed by ionic liquids(ILs)has been a widely ongoing studied hot topic over the years.Recent experimental research has shown that the protic ionic liquids(PILs)behave stronger hydrogen proton donating ability than aprotic ionic liquids(APILs),and can effectively catalyze the cycloaddition of CO2.Unfortunately,the mechanistic explanation remains primarily unraveled.Herein,a detailed simulation study on the cycloaddition reaction catalyzed by PIL([HDBU][Mim])in comparison with APIL([MeDBU][Mim])re-action catalysts was conducted,including the three-step route(ring-opening of PO(propylene oxide),insertion of CO2 and ring-closure of propylene carbonate(PC))and two-step route(simultaneously ring-opening of PO and addition of CO2,and then ring-closure of PC).Based on the activation energy barrier of the rate-determining step,PIL preferentially activates PO as the optimal route for the reaction with the energy barrier of 23.2 kcal mol-1,while that of APIL is 31.2 kcal mol-1.The role of[HDBU]+in the reaction was also explored and found that the direct formation of intermolecular hydrogen bond(H-bond)between[HDBU]+and the reactants(PO+CO2)was unfavorable for the reaction,while the cooperation with the anion[Mim]-to assist indirectly was more conducive.To fully consider the reaction microenvironment of ILs,ONIOM calculation was used to study the solvent effect.At last,the above conclusions were further verified by the analysis of intermediates with charge,non-covalent interaction(NCI),and atoms in molecules(AIM)methods.The computational findings show that ILs studied in this work have dual functions of catalyst and solvent,enabling a microscopic understanding of the ILs catalyst for CO2 utilization as well as providing guidance for the rational design of more efficient ILs-based catalysts.

Electrocatalytic conversion of CO_2 to liquid fuels using nanocarbon-based electrodes

Recent advances on the use of nanocarbon-based electrodes for the electrocatalytic conversion of gaseous streams of CO2 to liquid fuels are discussed in this perspective paper. A novel gas-phase electrocatalytic cell, different from the typical electrochemical systems working in liquid phase, was developed. There are several advantages to work in gas phase, e.g. no need to recover the products from a liquid phase and no problems of CO2 solubility, etc. Operating under these conditions and using electrodes based on metal nanoparticles supported over carbon nanotube （CNT） type materials, long C-chain products （in particular isopropanol under optimized conditions, but also hydrocarbons up to C8-C9） were obtained from the reduction of CO2. Pt-CNT are more stable and give in some cases a higher productivity, but Fe-CNT, particular using N-doped carbon nanotubes, give excellent properties and are preferable to noble-metal-based electrocatalysts for the lower cost. The control of the localization of metal particles at the inner or outer surface of CNT is an importact factor for the product distribution. The nature of the nanocarbon substrate also plays a relevant role in enhancing the productivity and tuning the selectivity towards long C-chain products. The electrodes for the electrocatalytic conversion of CO2 are part of a photoelectrocatalytic （PEC） solar cell concept, aimed to develop knowledge for the new generation artificial leaf-type solar cells which can use sunlight and water to convert CO2 to fuels and chemicals. The CO2 reduction to liquid fuels by solar energy is a good attempt to introduce renewables into the existing energy and chemical infrastructures, having a higher energy density and easier transport/storage than other competing solutions （i.e. H2）.

Multidimensional In_(2)O_(3)/In_(2)S_(3) heterojunction with lattice distortion for CO_(2) photoconversion

Photocatalytic CO_(2)reduction to sustainably product of fuels is a potential route to achieve clean energy conversion.Unfortunately,the sluggish charge transport dynamics and poor CO_(2)activation performance result in a low CO_(2)conversion efficiency.Herein,we develop a multidimensional In_(2)O_(3)/In_(2)S_(3)(IO/IS)heterojunction with abundant lattice distortion structure and high concentration of oxygen defects.The close contact interfaces between the junction of the two phases ensure undisturbed transmission of electrons with high‐speed.The increased free electron concentration promotes the adsorption and activation of CO2 on the catalyst surface,leaving the key intermediate*COOH at a lower energy barrier.The perfect combination of the band matching oxide and sulfide effectively reduces the internal energy barrier of the CO2 reduction reaction.Furthermore,the lattice distortion structure not only provides additional active sites,but also optimizes the kinetics of the reaction through microstructural regulation.Remarkably,the optimal IO/IS heterojunction exhibits superior CO_(2)reduction performance with CO evolution rate of 12.22μmol g^(−1)h^(−1),achieving about 4 times compared to that of In_(2)O_(3)and In2S3,respectively.This work emphasizes the importance of tight interfaces of heterojunction in improving the performance of CO_(2)photoreduction,and provides an effective strategy for construction of heterojunction photocatalysts.

Review on porous nanomaterials for adsorption and photocatalytic conversion of CO_2

Photocatalytic conversion of“greenhouse gas”CO2is considered to be one of the most effective ways to alleviate current energy and environmental problems without additional energy consumption and pollutant emission.The performance of many traditional semiconductor photocatalysts is not efficient enough to satisfy the requirements of practical applications because of their limited specific surface area and low CO2adsorption capacity.Therefore,the exploration of photocatalysts with high CO2uptake is significant in the field of CO2conversion.Recently the porous materials appeared to be a kind of superior candidate for enriching the CO2molecules on the surface of photocatalysts for catalytic conversion.This paper first summarizes the advances in the development of nanoporous adsorbents for CO2capture.Three main classes of porous materials are considered:inorganic porous materials,metal organic frameworks,and microporous organic polymers.Based on systematic research on CO2uptake,we then highlight the recent progress in these porous‐material‐based photocatalysts for CO2conversion.Benefiting from the improved CO2uptake capacity,the porous‐material‐based photocatalysts exhibited remarkably enhanced efficiency in the reduction of CO2to chemical fuels,such as CO,CH4,and CH3OH.Based on reported recent achievements,we predict a trend of development in multifunctional materials with both high adsorption capability and photocatalytic performance for CO2utilization.

Investigation of operating parameters on CO2 splitting by dielectric barrier discharge plasma

Experiments of CO_2 splitting by dielectric barrier discharge（DBD） plasma were carried out, and the influence of CO_2 flow rate, plasma power, discharge voltage, discharge frequency on CO_2 conversion and process energy efficiency were investigated. It was shown that the absolute quantity of CO_2 decomposed was only proportional to the amount of conductive electrons across the discharge gap,and the electron amount was proportional to the discharge power; the energy efficiency of CO_2 conversion was almost a constant at a lower level, which was limited by CO_2 inherent discharge character that determined a constant gap electric field strength. This was the main reason why CO_2 conversion rate decreased as the CO_2 flow rate increase and process energy efficiency was decreased a little as applied frequency increased. Therefore, one can improve the CO_2 conversion by less feed flow rate or larger discharge power in DBD plasma, but the energy efficiency is difficult to improve.

Microporous polymer based on hexaazatriphenylene-fused triptycene for CO2 capture and conversion

Chemical conversion of carbon dioxide(CO2)to value-added useful chemicals like cyclic carbonates represents one potential solution to climate warming.Here,a kind of porous organic polymer(HAT-TP)with large surface area and excellent carbon dioxide uptake capacity is prepared via a condensation reaction to introduce hexaazatriphenylene(HAT)units into triptycene(TP)-based microporous polymer.HAT-TP can coordinate with zinc ions,and the resulting polymer(Zn/HAT-TP)can be utilized as an efficient recyclable catalyst for chemical conversion of CO2 into cyclic carbonates with epoxides.

Theoretical Study of Hydrogen-Bond Interactions of CO_(2) in Organic Absorbent 1,3-Diphenylguanidine

Carbon capture and storage technology have been rapidly developed to reduce the carbon dioxide(CO2)emission into the environment.It has been found that the amine-based organic molecules could absorb CO_(2) efficiently and form the bicarbonate salts through hydrogen-bond(H-bond)interactions.Recently,the aqueous 1,3-diphenylguanidine(DPG)solution was developed to trap and convert CO_(2) to valuable chemicals under ambient conditions.However,how the DPG molecules interact with CO_(2) in an aqueous solution remains unclear.In this work,we perform molecular dynamics simulations to explore the atomistic details of CO_(2) in the aqueous DPG.The simulated results reveal that the protonated DPGH+and the bicarbonate anions prefer to form complexes through different H-bond patterns.These double H-bonds are quite stable in thermodynamics,as indicated from the accurate density functional theory calculations.This study is helpful to understand the catalytic mechanism of CO_(2) conversion in the aqueous DPG.

Modification of CaO-based sorbents prepared from calcium acetate for CO_2 capture at high temperature

CaO-based sorbent is considered to be a promising candidate for capturing CO_2 at high temperature. However,the adsorption capacity of CaO decreases sharply with the increase of the carbonation/calcination cycles. In this study, CaO was derived from calcium acetate(CaAc_2), which was doped with different elements(Mg, Al,Ce, Zr and La) to improve the cyclic stability. The carbonation conversion and cyclic stability of sorbents were tested by thermogravimetric analyzer(TGA). The sorbents were characterized by N_2 isothermal adsorption measurements, scanning electron microscopy(SEM) and X-ray diffraction(XRD). The results showed that the cyclic stabilities of all modified sorbents were improved by doping elements, while the carbonation conversions of sorbents in the 1st cycle were not increased by doping different elements. After 22 cycles, the cyclic stabilities of CaO–Al, CaO–Ce and CaO–La were above 96.2%. After 110 cycles, the cyclic stability of CaO–Al was still as high as 87.1%. Furthermore, the carbonation conversion was closely related to the critical time and specific surface area.

Polyoxometalate-based ionic liquids-promoted CO_2 conversion

Polyoxometalates(POMs) are a class of molecular metal oxides, showing numerous applications in various chemical processes due to their unique acid/base and redox features. By adjusting the tunable molecular structures of the anions and counter cations, plenty of POM-based ionic liquids(POM-based ILs) have been fabricated to be used in various fields, such as catalysis, structural chemistry and material science. As a class of excellent catalysts, POM-based ILs have shown advantages in the emerging field of CO_2 utilization such as CO_2 capture, cycloaddition of CO_2 to epoxides, and reduction of CO_2, owing to the efficient activation of CO_2 by POM anions. This review summarizes recent advances in the catalysis of POM-based ILs, and particularly highlights the areas that are related to CO_2 conversion.

Recent advances in the catalytic conversion of CO_2 to value added compunds

CO2 is a major greenhouse gas,and it can also be used as a chemical feedstock for synthesis of chemicals and fuels by passing the petrochemical source.Herein,we present the recent progress of our research work in the catalytic conversion of CO2 to chemicals,with particular attention paid to catalytic reactivity and reaction mechanism.We also give the recommendations regarding the challenges and potential directions of the future research in this field.

Kinetics and rate-limiting mechanisms of dolomite dissolution at various CO_2 partial pressures

Techniques of rotating-disk and catalyst were used in investigating the kinetics of dolomite dissolution in flowing CO2-H2O system. Experiments run in the solutions equilibrated with various CO2 partial pressures (PCO2) from 30 to 100000 Pa. It shows that dissolution rates ofdolomite are related with rotating speeds at conditions far from equilibrium. This was explained by modified diffusion boundary layer (DBL) model. In addition, the dissolution rates increase after addition of carbonic anhydrase (CA) to solutions, where the CA catalyzes CO2 conversion. However, great differences occur among various CO2 partial pressures. The experimental observations give a conclusion that the modified DBL model enables one to predict dissolution rates and their behaviour at various PCO2 with satisfactory precision at least far from equilibrium.

Synthesis of Cyclic Carbonates from Alkenyl and Alkynyl Substrates

As an extensively used chemical product,cyclic carbonate was generally synthesized by transesterification,or the cycloaddition of epoxides,diols with CO2.To reduce the production costs and expand the raw materials,alkenyl and alkynyl substrates have caused much attention in the synthesis of cyclic carbonates,such as olefins,allyl alcohols and propargylic alcohols.Based on the alkenyl substrate,the synthetic process involves a continuous reaction of oxidative carboxylation,with epoxide or halohydrin as an intermediate usually.Therefore,peroxides and nucleophiles （halogens or organic bases） are often necessary in the conversion.Using propargylic alcohols to produce a-alkylidene cyclic carbonates,noble metal catalysts play crucial roles in alkynyl activation,and organic bases are considered to assist the intramolecular and intermolecular proton transfer and combine CO2 molecular.As the carboxyl sources in products,inorganic carbonates and organic carboxylic acids also have some applications instead of CO2.In this review,we summarized the synthetic routes of cyclic carbonates from alkenyl and alkynyl substrates in the aspect of catalyst,mechanism and the development tendency.

Prolonged lifetime and enhanced separation of photo- generated charges of nanosized α-Fe2O3 by coupling SnO2 for efficient visible-light photocatalysis to convert C02 and degrade acetaldehyde

To develop efficient visible-light photocatalysis on α-Fe2O3, it is highly desirable to promote visible-light-excited high-energy-level electron transfer to a proper energy platform thermodynamically. Herein, based on the transient-state surface photovoltage responses and the atmosphere-controlled steady-state surface photovoltage spectra, it is demonstrated that the lifetime and separation of photogenerated charges of nanosized α-Fe2O3 are increased after coupling a proper amount of nanocrystalline SnO2. This naturally leads to greatly improved photocatalytic activities for CO2 reduction and acetaldehyde degradation. It is suggested that the enhanced charge separation results from the electron transfer from α-Fe2O3 to SnO2, which acts as a proper energy platform. Based on the photocurrent action spectra, it is confirmed that the coupled SnO2 exhibits longer visible-light threshold wavelength （-590 nm） compared with the coupled TiO2 （-550 nm）, indicating that the energy platform introduced by SnO2 would accept more photogenerated electrons from α-Fe2O3. Moreover, electrochemical reduction experiments proved that the coupled SnO2 possesses better catalytic ability for reducing CO2 and O2. These are well responsible for the much efficient photocatalysis on SnO2-coupled α-Fe2O3.

Facile synthesis and enhanced visible-light photocatalytic activity of Ti3 ＋-doped Ti02 sheets with tunable phase composition

Ti3＋-doped TiO2 nanosheets with tunable phase composition （doped TiO2 （A/R）） were synthesized via a hydrothermal method with high surface area anatase TiO2 nanosheets TiO2 （A） as a substrate, structure directing agent, and inhibitor; the activity was evaluated using a probe reaction-photocatalytic CO2 conversion to methane under visible light irradiation with H2 as an electron donor and hydrogen source. High-resolution transmission electron microscope （HRTEM）, field emission scanning electron microscope, UV-Vis diffuse reflectance spectra, and X-ray diffraction （XRD） etc., were used to characterize the photocatalysts. XRD and HRTEM measurements confirmed the existence of anatase-rutile phase junction, while Ti3-and single-electron-trapped oxygen vacancy in the doped TiO2 （A/R） photocatalyst were revealed byelectron paramagnetic resonance （EPR） measurements. Effects of hydrothermal synthesis temperature and the amount of added anatase TiO2 on the photocatalytic activity were elucidated. Significantly enhanced photo- catalytic activity of doped TiO2 （A/R） was observed; under the optimized synthesis conditions, CH4 generation rate of doped TiO2 （A/R） was 2.3 times that of Ti3＋-doped rutile TiO2.

A review on metal-organic frameworks for photoelectrocatalytic applications

Semiconductor-based photoelectrocatalytic processes have attracted considerable research interest for solar energy collection and storage.Photoelectrocatalysis is a heterogeneous photocatalytic process in which a bias potential is applied to a photoelectrode,and thus the photoelectrocatalytic performance is closely related to the photoelectrode prepared by semiconductors.Among various semiconductors,metal-organic frameworks(MOFs)have attracted more and more attention because of their unique properties such as optical properties and adjustable structure.Herein,a comprehensive review on different MOFs(Ti-based,Zn-based,Co-based,Fe-based,Cu-based,and mixed metal-based MOFs)for heterogeneous photoelectrocatalysis is carried out and,in particular,the application of this technique for CO2 conversion and water splitting is discussed.In addition,the challenges and development prospects of MOFs in photoelectrocatalysis are also presented.