View Article Online View Journal Journal of Materials Chemistry A Accepted Manuscript This article can be cited before page numbers have been issued, to this please use: M N Ha, G Lu, Z Liu, L wang and Z Zhao, J Mater Chem A, 2016, DOI: 10.1039/C6TA05402A This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available You can find more information about Accepted Manuscripts in the Information for Authors Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content The journal’s standard Terms & Conditions and the Ethical guidelines still apply In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains www.rsc.org/materialsA Page of 11 Pleaseofdo not adjust margins A Journal Materials Chemistry View Article Online Journal Name ARTICLE 3DOM-LaSrCoFeO6-δ as a highly active catalyst for thermal and photothermal reduction of CO2 with H2O to CH4 Published on 21 July 2016 Downloaded by LA TROBE UNIVERSITY on 29/07/2016 02:21:59 Minh Ngoc Haa,b,c, Guanzhong Lu*a,b, Zhifu Liub, Lichao Wangb and Zhe Zhao*b Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/ The double perovskite LaSrCoFeO6-δ (LSCF) and LaSrCoFeO6-δ with three-dimensionally ordered macroporous structure (3DOM-LSCF) were successfully synthesized by a facile combustion process The crystal structure, morphology, BET surface area, band gap and catalytic properties were characterized in details Phase pure of the double perovskite LSCF and 3DOMLSCF can be obtained by calcination at 550-950 oC for h The ordered and interconnected pore structure generated by PMMA template can be remained successfully in the 3DOM-LSCF catalyst Both catalysts had good catalytic performance in either CH4 selectivity and total yield Production of CH4 from CO2 and H2O can reach 351.32 µmol g-1 for LSCF and 557.88 µmol g-1 for 3DOM-LSCF under photothermal (350 oC + Vis-light) in h The high solar-to-methane (STM) energy conversion efficiency was 1.217% of LSCF and 1.933% of 3DOM-LSCF under photothermal mode The results also show that the yield of CH4 in photothermal mode is times of that in thermal reduction The double perovskite LSCF and 3DOM-LSCF are promising photothermal catalytic materials for CO2 reduction to hydrocarbon fuels Introduction The rapid development of the industry has been accompanied by increasing concentrations of atmospheric pollutants Global warming caused by emissions of greenhouse gases such as carbon dioxide (CO2), chlorofluorocarbons (CFCs), and nitrous oxide (N2O) to the atmosphere, is widely regarded as one of the most severe environmental issues of recent years The atmospheric concentration of CO2 has gradually increased mainly owing to human activities.1 Beside, thermal pollution is also the most current pollution and it is a result of large-scale industrialization The extremely large amounts of these waste heat will be useful if they can be harvested and used for sustainable energy generation In addition, as we know solar energy can be used not only for thermal power generation, but also for chemical manufacture.2 The discovery of new costeffective and highly active catalysts for directly energy conversion using solar energy, transforming CO2 and heat emission into hydrocarbon fuels and storage is of prime importance to address climate change challenges and develop storage options for renewable energies a Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, China Email: gzhlu@ecust.edu.cn b School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, China E-mail: zhezhao@kth.se c Faculty of Chemistry, Hanoi University of Science, Vietnam National University, Hanoi 10000, Vietnam Electronic Supplementary Information (ESI) available: [Experimental details and catalytic measurements] See DOI: 10.1039/x0xx00000x Photocatalysis, as an efficient, green, and promising solution to the current energy crisis and environmental deterioration, has attracted considerable interest In general, the photocatalytic reduction of CO2 is a possible avenue to convert CO2 into hydrocarbon fuels, because reducing the amount of CO2 will not only meet the purpose of environmental protection but also provide raw materials for chemical industry Since Halmann discovered the photoelectrochemical reduction of CO2 into organic compounds in 19783 and Hiroshi and co-worker reported that the photocatalytic reduction of CO into organic compounds over suspending semiconductor particles in water, a growing interest in the development of semiconductor photocatalyst has evolved The present invention combines photo, thermal, electric and chemical processes to develop a new method, maximizing the efficiency and the conversion rate of thermal radiation to chemical potential, in the form of CO2 reduction to CO, C and O2 and H2O reduction to H2 and O2 in the same system The dissociation of CO2 and H2O may occur in the same system simultaneously or either one of them can be performed alone Photothermal combines photo and thermal reaction conditions in one way to reduction of CO2 with H2O vapor to CH4 had more attention.2,20 In our previous study have shown that the photothermal process has improved catalytic performance better than reduction of CO2 with H2O vapor to CH4 under thermal only.20 The photothermal process was good to be combined advantages of photochemical and thermochemical catalytic, while it promoted and supported together in the one reaction system to provide high efficiency and reaction rate To date, many kinds of photocatalyst have been investigated to catalyze the CO2 reduction.3,5-9 For heterogeneous photocatalyst, many efforts still focus on TiO 2-based6,8,10,11 This journal is © The Royal Society of Chemistry 20xx J Name., 2013, 00, 1-3 | Please not adjust margins Journal of Materials Chemistry A Accepted Manuscript DOI: 10.1039/C6TA05402A Pleaseofdo not adjust margins A Journal Materials Chemistry Journal Name materials while other catalysts such as SrTiO 3,12 Zn2GeO4,13 ZnGa2O4,14 CaFe2O4,15 ALa4Ti4O15 (A = Ca, Sr, and Ba),16 NiO/InTaO4,17 and BiVO4,18 ZnO@Cu-Zn-Al,19 WO3,20 NaNbO321 and so forth have also been reported Among them, perovskite oxides with general formula ABO3 possess unique properties, such as metal-insulator transition, spin blockade, colossal magnetoresistance, ferroelectricity, and superconductivity, which make them attractive in technological applications such as electrocatalysis, catalysis, sensor devices, magnetoresistance devices, and spintronics.22-24 Perovskite-type La1-xSrxCo1-yFeyO3δ oxides with mixed electronic and ionic conductivities are known mainly as good candidates for cathode materials used in solid oxide fuel cells25 and for membrane materials with high oxygen permeability as well as phase/chemical stability 26 Excellent catalytic properties of La1-xSrxCo1-yFeyO3-δ, as powders intended for membrane reactors, were found for partial oxidation of natural gas.25 It is also highly efficient catalyst towards methane and propane combustion, 27 toluene combustion28 and methanol decomposition to CO and H 2,29 VOC combustion,30 catalysts in automobile exhaust systems, and as gas sensors.31 La1-xSrxCo1-yFeyO3-δ perovskite possess oxygen vacancies,32-35 which may act as Lewis acid sites necessary for the reaction of phenol catalytic alkylation.36 In addition, double perovskite oxides with a general formula AA’BB’O6 or A2BB’O6 (where A and A’ are alkaline-earth and/or rare-earth metals and B and B’ are transition metals) have been widely investigated for their catalytic, magnetic, dielectric properties and colossal magnetoresistance (CMR).37,38 After the discovery of room temperature CMR and tunnelling magnetoresistance (TMR) in the double perovskite Sr2FeMoO6 and Sr2FeReO6, respectively,39,40 there have been growing interests worldwide in researching for effective methods to make double perovskite materials.41-43 Unfortunately, the traditional methods involve in high-temperature solid-state reactions, leading to the destruction of pore structures and hence to low surface areas, unfavorable for enhancement in the catalytic performance of the obtained perovskite materials Therefore, it is highly desirable to develop an effective strategy for the controlled preparation of porous perovskite materials that are high in surface area Recently, this problem has been solved using the colloidal crystal templating method, by which one can create a three-dimensionally ordered macroporous (3DOM) structure Perovskite-type oxides with 3DOM structure possess relatively large surface areas, high thermal stability, and good catalytic performance.44,45 The unique ordered macroporous structure can provide easy mass transfer to the reactant molecules, facile accessibility to the active sites, and convenient loading of active components.46 Therefore, 3DOM-structured ABO3 is considered to be one of the most promising catalytic materials 47-49 Therefore, we report the preparation, characterization, and comparing the catalytic properties of the double perovskite LSCF and 3DOM-LSCF for thermal and photothermal reduction of CO2 with H2O vapor to CH4 The aim of this work was to investigate the effect of temperature on morphology, crystal structure, band gap, catalytic performance and the thermal, photothermal reaction mechanism of the double perovskite LSCF and 3DOM-LSCF for the CO2 reduction In a typical experiment, the double perovskite and View LSCF Article Online DOI: 10.1039/C6TA05402A 3DOM-LSCF were prepared by a convenient and efficient modified combustion process The samples were calcined in air for h at different temperatures between 550 and 950 oC The 3DOM-LSCF catalyst with well-defined 3DOM structure could be prepared using the PMMA template The catalytic experiments were carried out in a gas-closed circulation system The volume of the reaction system was about 150 mL The evaluation of catalytic activity was performed at 150, 250, 350 oC without light (thermal) and 350 oC with visible light (photothermal), the light source was used a 300 W Xe lamp with a UV-light filter (λ>420 nm) Taking samples per hour and quantitative analysis was performed on a GS-Tek (Echromtek A90) equipment with a capillary column The quantification of CH4 yield product was based on the external standard and the use of calibration curve (ESI S1) Results and discussion The prepared double perovskite LSCF and 3DOM-LSCF powders were calcined in air for h at different temperatures between 550 and 950 °C to investigate the evolution of crystalline phases X-ray diffraction patterns (XRD) for the heat-treated double perovskite LSCF and 3DOM-LSCF powders are shown in Fig 1a and Fig 1b, respectively The diffraction peaks of two samples are in good agreement with the standard file, which corresponds to pure perovskite phase with a cubic system (space group Pm-3m, Ref Code 01-089-5720) All the characteristic diffraction peaks, which belong to the double perovskite LSCF and 3DOM-LSCF are observed in the patterns of all the samples indicating that the obtained catalysts possessed AA’BB’O6 double perovskite-type structure with disordered cubic structure Fig 1a shows the XRD pattern of LSCF calcined in air at 750 oC for h with diffraction peaks at 2θ = 22.97°, 32.77°, 40.41°, 47.04°, 52.96°, 58.52°, 68.73°, 73.57°, and 78.25°, which could be perfectly indexed to the (1 0), (1 0), (1 1), (2 0), (2 0), (2 1), (2 0), (3 0), and (3 0) crystal faces of double perovskite, respectively Fig 1b shows the XRD pattern of 3DOM-LSCF calcined at the same condition with diffraction peaks at 2θ = 23.10°, 32.81°, 40.47°, 47.10°, 53.10°, 58.59°, 68.84°, 73.60°, and 78.38°, which could be perfectly indexed to the (1 0), (1 0), (1 1), (2 0), (2 0), (2 1), (2 0), (3 0), and (3 0) crystal faces of double perovskite, respectively The XRD pattern of 3DOM-LSCF with main peak indexed to the (1 0) crystal face shifted to higher angle than peak of LSCF, it means lattice parameter of 3DOM-LSCF decrease and diffraction peaks move to the high angle side Furthermore, the diffraction peaks shifted to higher angle, higher intensity and sharper when increasing temperature, indicating that perovskite crystal structure affected by temperature The crystallite size of the double perovskite LSCF and 3DOM-LSCF also increase when increasing temperature It was affected the surface electronic structure, electrical transport properties of the catalysts The results of Rietveld structure refinement for the double perovskite LSCF and 3DOMLSCF are summarized in Table The stability of complex perovskite structures can be well explained with the use of | J Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please not adjust margins Journal of Materials Chemistry A Accepted Manuscript Published on 21 July 2016 Downloaded by LA TROBE UNIVERSITY on 29/07/2016 02:21:59 ARTICLE Page of 11 Journal Materials Chemistry Pleaseofdo not adjust margins A Page of 11 ARTICLE tolerance factors (t) For the materials studied here, the tolerance factors can be determined by equation (eqn) (1): Published on 21 July 2016 Downloaded by LA TROBE UNIVERSITY on 29/07/2016 02:21:59 t rLa  rSr  rO r r 2( Co Fe  rO ) (1) where rLa, rSr, rCo, rFe and rO are the ionic radii of La, Sr, Co, Fe and O ions, respectively.51,52 Shannon’s ionic radii52 are frequently employed to determine the tolerance factors Hines et al suggested (solely by analysis of the tolerance factor) that the perovskite will be cubic if 0.9 < t < 1.0, and orthorhombic if 0.75 < t 350 °C > 250 °C > 150 °C The detail values of catalytic performance under photothermal after h was arranged by 557.88, 120.86, 39.02, and 2.81 µmol g-1 for 3DOMLSCF and 351.32, 65.88, 24.94 and 1.89 µmol g-1 for LSCF Figure 7b shown the yield of methane over 3DOM-LSCF catalyst under photothermal after 8h (557.88 µmol g-1) is about 1.6 times of LSCF catalyst (351.32 µmol g-1) The results also shown that the best catalytic performance is under photothermal and it is higher times than catalytic performance under thermal only The results may consider on the comparative surface area, pore volume, and crystallite size.63-65 The band gap energy is also correlated to the photocatalytic activity The double perovskite LSCF and 3DOM-LSCF have similar band gap but the 3DOM-LSCF catalyst has a positions of the CB and VB more suitable for CO2 photoreduction than LSCF catalyst It has a more negative CB and less positive VB than LSCF catalyst In addition, the BET specific surface area of 3DOM-LSCF catalyst is 21.86 m2 g-1, which is larger than LSCF catalyst with BET specific surface area of 8.46 m2 g-1 In addition, the high catalytic performance of the double perovskite LSCF and 3DOM-LSCF may consider on the photo-thermal coupling effect, self-formed oxygen vacancies, | J Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please not adjust margins Journal of Materials Chemistry A Accepted Manuscript Published on 21 July 2016 Downloaded by LA TROBE UNIVERSITY on 29/07/2016 02:21:59 ARTICLE Page of 11 Journal Materials Chemistry Pleaseofdo not adjust margins A Page of 11 Journal Name ARTICLE Table Thermal and photothermal catalytic activity and physical properties of the double perovskite LSCF and 3DOMLSCF BET µmol g-1 h-1 Samples 150 o C LSCF 3DOM- Published on 21 July 2016 Downloaded by LA TROBE UNIVERSITY on 29/07/2016 02:21:59 LSCF 250 o C 350 o C Band 350 oC gap + Vis- (eV) Crystallite size (nm) light Total surface pore area volume (m2 g- (cm3 g- ) ) 1.35 18.09 53.35 300.69 2.84 20.06 8.46 0.048 2.11 28.03 93.91 467.63 2.83 25.79 21.68 0.095 small crystallite size and high porous material Furthermore, the 3DOM-LSCF may process self-formed heterostructures with 3DOM structure all play positive role in the separation process of photogenerated electrons and holes In such a way, the presence of heterostructures interface the recombination of photogenerated electrons and holes were suppressed effectively, and the photocatalytic activity is greatly enhanced The reusability of the catalyst is important for its practical application In order to evaluate the activity stability of the catalyst, the reuse experiment was carried out From Fig 7c, it can be seen that the catalytic activity of the double perovskite LSCF and 3DOM-LSCF remain high catalytic activity after reuse of times and the catalytic activity of 3DOM-LSCF catalyst is better than LSCF catalyst The double perovskite 3DOM-LSCF shown considerable stability in the catalytic process Addition, it is difficult to directly compare the methane production rate of the double perovskite LSCF and 3DOM-LSCF with rates reported for other photocatalysts because of the variance in experimental conditions (such as light intensity, illumination area, and photocatalyst dosage), morphological features, surface areas, and co-catalysts However, the catalytic performance of the double perovskite LSCF and 3DOM-LSCF are comparable to, and perhaps better than other reported photocatalysts that convert CO2 into methane using solar irradiation and without using noble metal co-catalysts, including in Table (ESI S3) The reaction mechanism for the thermal and photothermal reduction of CO2 with H2O vapor to CH4 over double perovskite LSCF and 3DOM-LSCF catalysts were proposed base on last study20,66,67 and illustrated in Fig The rate of photocatalytic reaction can be controlled by several steps: photoexcitation of the double perovskite LaSrCoFeO6-δ surface, creating electronhole pairs, followed by their transfer to CO and H2O The surface defects and hydration are often considered to be important for heterogeneous catalysis as well, since these particular factors also play important roles in the reactantsurface binding and the formation of bonds between the surface atoms and H2O, CO2 molecules To correlate surface structures with photocatalytic activity, interaction between H 2O, CO2 molecules and the surface of the photocatalyst was examined.68-73 An understanding of the interaction between catalyst surface and the CO2 and H2O molecules is vital for developing its role in the photocatalytic reduction of CO The CO2 and H2O molecules could be adsorbed on the double perovskite LaSrCoFeO6-δ surface A variety of possible binding Fig Thermal and photothermal catalytic activity of a) LSCF, b) 3DOM-LSCF and d) Reuse of the catalyst configurations of H2O and CO2 on the perfect and defective catalysts surfaces in terms of geometries, energies, and net charges were explored Five models were constructed to determine the adsorption energy of the system (Fig 8) Li Liu and co-workers70,71 shown that the adsorbed CO2 molecules are partially negatively charged, indicating that CO2 accepted electrons from the surface and formed a partially and negatively charged CO2δ- species This negatively charged CO2δintermediate has also been described in experimental 74,75 and theoretical work.76,77 For defective surfaces, surface oxygen This journal is © The Royal Society of Chemistry 20xx J Name., 2013, 00, 1-3 | Please not adjust margins Journal of Materials Chemistry A Accepted Manuscript Rate of CH4 evolution View Article Online DOI: 10.1039/C6TA05402A Pleaseofdo not adjust margins A Journal Materials Chemistry Journal Name defects were found to play an important role and can significantly influence the interaction of CO with the surface: the oxygen vacancies are the active sites on the defective surfaces; the nearby oxygen vacancies can significantly enhance the adsorption energy of CO2 molecule compared to the perfect surfaces; CO2 can not only be activated but can also be further dissociated into CO and O on the surface oxygen defect site and the oxygen vacancy defect can be healed by the oxygen atom released during the dissociation process Through analysis of the dissociative adsorption mechanism of CO on defective surfaces, the results shown that the dissociative adsorption of CO2 favours the stepwise dissociation mechanism and the dissociation process can be described in eqn (2): CO2 + Vo  CO2δ- /Vo  COadsorbed + Osurface (2) Furthermore, H2O adsorbed on perfect surfaces could spontaneously dissociate into an H atom and an OH group The presence of oxygen defects was found to strongly promote H2O dissociation on the (0 0) surface The results revealed that the interaction of CO2 and H2O with catalyst surfaces was dependent on the structure, crystal plane and active site on surface.70,71 Fig Possible configurations of adsorbed CO2 (a, b, c, d, e) and H2O (f, g, h, i, k) molecule on the double perovskite LaSrCoFeO 6δ surface In order to understand the reaction process, a possible catalytic mechanism of the double perovskite LSCF and 3DOMLSCF for the reduction of CO2 with H2O vapor to CH4 is shown in Fig and equations Photocatalytic reduction of CO2 with H2O vapor on semiconductor oxide catalyst surfaces using solar energy to yield fuels/chemicals (CH4, CH3OH, etc.) involves two major steps, splitting of H2O to yield H2, which in turn helps in the reduction of CO2 to different hydrocarbon products in the second step The complex sequence of process steps that follow, involving two, four, six or eight electrons for reduction, lead to the formation of formic acid/CO, formaldehyde, methanol and methane respectively7 depending on the type ofView catalyst and Article Online DOI: 10.1039/C6TA05402A reaction conditions employed The first step involving photocatalytic splitting of water follows the well-accepted elementary steps as shown in eqn (3)(8): LaSrCoFeO6-δ + hυ  e− + h+ (3) H2Oads + h+ OH− + H+ (4) H+ + e−  •H (5) OH− + h+  •OH (6) 2•OH  H2O2 + h+  O2− + 2H+ (7) O2− + h+  O2 (8) The second step for activation and reduction of CO2 to CH4 could then follow20,67,78,79 it shows in eqn (9)-(14): CO2ads + e−  •CO2− (9) •CO2− + •H  CO +OH− (10) CO + e−  •CO− (11) •CO− + •H  •C +OH− (12) •C +H+ +e−  •CH  •CH2  •CH3 (13) •CH3 + H+ + e−  CH4 (14) Possible thermocatalysis mechanism activation and reduction of CO2 to CH4 shows in eqn (15)-(19) and the total reaction under photothermal coupling effected shows in eqn (20) LaSrCoFeO6-δ + H2O  LaSrCoFeO6 + H2 (15) LaSrCoFeO6-δ + δ/2CO2  LaSrCoFeO6 + δ/2C(s) (high Vo) (16) LaSrCoFeO6-δ + CO2  LaSrCoFeO6 + CO (low Vo) (17) Vo (oxygen vacancies) CO + H2  C + H2O (18) C + 2H2  CH4 (19) The total reaction under photo-thermal coupling CO2 + 2H2O  CH4 + 2O2 (20) Tabata and co-worker88 reported that CO2 could be decomposed completely to carbon with oxygen-deficient ferrites, Zn(II), Mn(II) and Ni(II) bearing ferrites 81-84 at low temperature near 300 °C In this study, water used as a hydrogen source, under optimized reaction conditions the double perovskite LaSrCoFeO6-δ with self-formed oxygen vacancy could split H2O into element H under 350 °C The combination of the two splitting reactions improved conversion of CO2 to CH4 of high selectivity and high yield High selectivity was due to the splitting of CO2 more tend to form C (eqn (16)) as intermediate product of CH4 under low temperature (