Toward High-Value Hydrocarbon Generation by Photocatalytic

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Toward High-Value Hydrocarbon Generation by Photocatalytic Reduction of CO2 in Water Vapor Naixu Li, Bingbing Wang, Yitao Si, Fei Xue, Jiancheng Zhou, Youjun Lu, and Maochang Liu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00223 • Publication Date (Web): 09 May 2019 Downloaded from http://pubs.acs.org on May 10, 2019

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ACS Catalysis

Revised MS No. cs-2019-00223p.R2, 2019. 05

Toward

High-Value

Hydrocarbon

Generation

by

Photocatalytic Reduction of CO2 in Water Vapor

Naixu Li†,§, Bingbing Wang†, Yitao Si‡, Fei Xue‡, Jiancheng Zhou†,§,*, Youjun Lu‡, Maochang Liu‡,#,*

†School

of Chemistry and Chemical Engineering, Southeast University, Nanjing

211189, P.R. China ‡International

Research Center for Renewable Energy, State Key Laboratory of

Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, P. R. China §Jiangsu

Key Laboratory for Biomass Energy and Material, Nanjing 210042, P. R.

China #Suzhou

Academy of Xi’an Jiaotong University, Suzhou, Jiangsu 215123, P. R. China

*To whom correspondence should be addressed. Email: [email protected] (M.L.) and [email protected] (J. Z.)

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ACS Paragon Plus Environment

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Abstract Semiconductor crystals with well-defined morphology, porous nanostructure, and spatially separated active sites are attractive for use as photocatalysis. This paper describes a controlled synthesis of cake-like porous TiO2 photocatalyst with surfacelocalized doping of copper and cobalt by using a well-defined MIL-125(Ti) metal organic framework as template precursor. The series of the modified TiO2 photocatalysts present the improved activity for photocatalytic CO2 reduction with water vapor. It is found that 1%Cu-doped TiO2 shows an enhanced behavior for breaking C=O bonds. In this case, the outcomes are primary as CO and CH4, yielding up to 135.94 and 127.05 μmol, respectively, under the irradiation of simulated sun light for 3 hours. The performance can be further improved by incorporating trace cobalt. Besides the improved property for CO and CH4 production, the selectivity also shifts to high-value hydrocarbons (C2+). The yields for C2H6 and C3H8 can be up to 267.60 and 10.07 μmol, respectively, by using 0.02%Co-1%Cu/TiO2. Our in-situ Fourier transform infrared spectra together with theoretical calculations indicate that efficient charge separation on copper and cobalt ions is achieved. This altered charge behavior leads to the generation and enrichment of methyl radicals on the surface of cobalt ions, giving rise to the production of C2+ hydrocarbons. This work demonstrates a vibrant catalyst platform for solar fuel generation by photocatalytic CO2 conversion in water.

Keywords: Photocatalysis, MIL-125(Ti), CO2 reduction, solar fuel, high-value hydrocarbons

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1. Introduction Carbon dioxide (CO2) generated from excessive combustion of fossil fuels has led to both global warming and energy crisis, thus triggered worldwide concerns.1-5 Many approaches including hydrogenation, photocatalysis, and electrocatalysis, to this end, have been attempted for the reduction of CO2.6 While ongoing study has been devoted to develop more feasible procedures, photocatalytical conversion of CO2 and H2O into CO, CH4, high-value hydrocarbons (C2+), and so on, using solar irradiation has been considered as one of the most preferential methods.7-10 Although incredible development has been achieved on this reaction, the efficiency is still quite low. The reaction rate is generally in the order of micromoles per hour, which is too far from any practical application. More importantly, the major products in most reported literatures are CH4 and CO. More preferred high-value C2+ hydrocarbons are rarely reported. It is limited by the chemical nature of a given catalyst that has both required redox behavior and suitable binding affinity to CO2 for selective generation of C2+ hydrocarbons.11 As a special type of functionalized inorganic-organic hybrids, metal-organic frameworks (MOFs) represent ideal platforms for this application. They are constructed form molecular building units possessing high and tunable pore metrics and thus surface areas.12-15 In this regard, one can integrate well-defined and highly selective catalyst species to the backbone of the architectures to gain desired product and purity thereof. More importantly, while the big surface area and regularly pore structure benefit rapid diffusion of reactants to the active sites, the activity can be further improved by engineering their band structures. Consequently, modification of MOFs, for example, by employing them as template precursors to design and synthesize functional metal oxide photocatalysts, is of particular interest.16 These obtained metal oxide photocatalysts thus can maintain the hierarchical inter-connected pores as well as the big surface area. Studies have shown that porous metal oxides (MOs) such as Fe2O3, Co3O4 and ZnO constructed from MOF precursors exhibit excellent properties.17-22 MIL-125, formulated as Ti8O8(OH)4(BDC)6, is the first reported Ti-based MOF material.14,15,23-27 Here, BDC represents benzene-1,4-dicarboxylate. It is a three3


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dimensional microporous framework constructed by eight TiO6 octahedron units linked by BDC molecules. Herein, using MIL-125(Ti) as both precursor and template, an porous cake-like TiO2 photocatalyst with mixed anatase-rutile phases was in-situ prepared. Generally, TiO2 is active for CO2 reduction in UV light that only occupies an energy fraction of 4% from the sun. We then introduced Cu into the surface layer of TiO2 matrix during the synthesis to gain visible light response. The photocatalytic activity toward CO and CH4 generation can be remarkably improved under simulated sun light irradiation, yet without notable selectivity over high-value C2+ hydrocarbons. By further co-incorporating cobalt ions into the surface of TiO2 photocatalyst, the obtained Co-Cu/TiO2 photocatalyst showed both good selectivity and high activity for C2H6 and C3H8 generation. The roles as well as the synergy of doped Cu and Co that responsible for the enhancement were then investigated both theoretically and experimentally.

2. Experimental 2.1 Materials Titanium tetraisopropanolate (C12H28O4Ti), 1,4-benzenedicarboxylic acid (C8H6O4 or H2BDC, 99%), N,N-dimethylformamide ((CH3)2NCHO or DMF, 99.9%), methanol (CH3OH, 99,9%), copper nitrate trihydrate (Cu(NO3)2·3H2O, 98.0%), and cobalt nitrate hexahydrate (Co(NO3)2·6H2O, 98.0%) were obtained from Shanghai Sinopharm Chemical Reagent Co., Ltd.. No additional treatment was applied before using these chemicals for a synthesis. Carbon dioxide in Argon (1.0% CO2, 99.0% Ar) and nitrogen gas (N2, 99.99%) were purchased from Nanjing Shangyuan Industrial Gas Plant. 2.2 Synthesis 2.2.1 Preparation of MIL-125(Ti) MOF The MIL-125 MOF was prepared based upon a previous report by Horiuchi et al. with some modification28. Briefly, 2.1 mL C12H28O4Ti and 3.53 g of H2BDC were introduced into a solution containing 54 mL DMF and 6 mL CH3OH. The mixture was pretreated by magnetic stirring at 25 oC for 30 min, which resulted a transparent homogeneous solution. The solution was subsequently moved to a 100 mL Teflon-lined 4


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stainless-steel autoclave which was placed in a 150 oC furnace. The reaction was allowed for 16 hours. Once the temperature was decreased to a normal level (~ 25 oC), the reaction was completely stopped. The obtained product was separated by repeating a process of centrifugation and washing with 80 mL methanol for three times. Finally, a white solid, i.e., the MIL-125(Ti) powder, was obtained by drying the product at 80 oC

in an oven for 16 hours.

2.2.2 Synthesis of Pure Porous TiO2, Cu/TiO2, and Co-Cu/TiO2 Photocatalysts Typically, a 60 mL methanol suspension containing 1g of evenly suspended MIL125(Ti) was prepared firstly. A methanol solution containing certain amount of Cu(NO3)2·3H2O was introduced into the MOF suspension. The resulted mixture was stirred for an additional 1 hour to fully adsorb copper ions. The obtained suspension was then filtered, washed with methanol, and dried at 80 °C for 16 hours. The generated powder was finally heated in a 450 oC muffle furnace (heating rate of 5 oC min-1) for 6 hours to get Cu doped TiO2 (Cu/TiO2). For the synthesis of Co-Cu co-doped TiO2 (0.02%Co-1%Cu/TiO2), similar procedure was employed except that an additional amount of Co(NO3)2·6H2O and 9.25 mg Cu(NO3)2·3H2O were introduced into the suspension. The synthesis process is shown in the Figure S1. The content of Cu and Co was controlled by the introduction of different amount of Cu(NO3)2·3H2O and Co(NO3)2·6H2O. Specifically, the addition of 4.63, 9.25, 18.50, and 27.75 mg Cu(NO3)2·3H2O gave the 0.5 wt%, 1 wt%, 2 wt%, and 3 wt% Cu doped TiO2, designating as x%Cu/TiO2 (x = 0.5, 1, 2, and 3). For the preparation of 0.01 wt%, 0.02 wt%, and 0.05 wt% Co co-doped 1%Cu/TiO2, 0.12, 0.24, and 0.60 mg of Co(NO3)2·6H2O were used respectively. The resulted composites were designated as y%Co-1%Cu/TiO2 (y = 0.01, 0.02, and 0.05). 2.3 Photocatalytic Tests Photocatalysis of CO2 reduction was conducted in a visualized micro-autoclave with a polytetrafluoroethylene liner (see Figure S2 for the test system). The reactor had a capacity of 250 cm3 and a quartz window having a thickness of 10 nm was integrated on its top. The thickness of the window was 10 mm for the introduction of light from an Xe lamp (300 W). Prior to irradiation, the inwall of the polytetrafluoroethylene liner 5


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was covered by a piece of silver paper to reflect light. The catalyst (0.1 g) was uniformly placed in a 0.5 g quartz wool that was set inside the lining. A certain space was left between the quartz wool and the bottom of the lining, allowing necessary magnetic stirring to enhance gas flow exchange. The reactor was swept by nitrogen gas for 30 minutes. Finally, the reactor chamber was filled by 1.0 mL H2O and CO2 gas. Light was turned on once the pressure of CO2 reached 1.0 MPa. The reaction products were determined by an online gas chromatograph (GC-9860-5C) every 30 min. A thermal conductivity detector was integrated for O2 quantification while a flame ionized detector was used for hydrocarbons measurement. To gain reliable data, all the tests were repeated for three times or more. The external standard curves were used as the base lines to quantify the products. Controlled isotope-labeled photocatalytic tests were conducted by replacing

12CO

2

with

13CO

2

while keeping other reaction condition

unchanged. The obtained products were analyzed on a gas chromatography−mass spectrometry (GC-MS, Agilent 7890A and 5975C). 2.4 Instruments Ambient powder X-ray diffraction patterns (XRD) were obtained on a Bruker D8Discover X-ray diffractometer with a Cu-Kα irradiation (λ = 0.1542 nm). Scanning electron microscope (SEM, JSM-6700F) and transmission electron microscope (TEM, Hitachi H-600) were used to investigate the morphology, microstructure, and particle size and distribution. The optical property of the samples was revealed by UV-vis spectra (Shimadzu UV 3600, wavelength: 200 - 800 nm) and infrared radiation (IR) spectra containing a liquid nitrogen-cooled HgCdTe (MCT) detector (Thermo fisher, Nicolet iZ10 spectrometer). Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements were conducted in-situ. In brief, a Harrick Scientific HVCDRP reaction chamber coupled with a Praying Mantis DRIFTS accessory were employed. The base of reaction cell is surrounded by a coil for cooling water circulation. The sample cup was set in the center of the cell which had three windows on its dome. Two of them (KBr type) for the transmission of infrared radiation and the left one (quartz type) for light introduction from a 300 W simulated sun light. BET specific surface area were determined by a Mieromerities ASAP 2010 BET 6


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instrument which applied the BET method of the N2 absorption/desorption isotherm using liquid nitrogen. The photoluminescence (PL) spectra were carried out on a Fluoromax-4 fluorescence spectrophotometer. X-ray photoelectron spectroscopy (XPS) was measured over a 2000 XPS facility by using Al-Kα irradiation as the light source and applying C 1s signal (284.6 eV) for spectrum calibration. CO2 temperatureprogrammed desorption (CO2-TPD) test was implemented at a temperature ramping rate of 10 °C/min and in a gas stream (flow rate: 30 mL/min) containing 4% Ar. An electrochemical workstation (CHI 660E, China) was used for the measurement of electrochemical impedance and transient photocurrent. The workstation equipped with a standard three-electrode system containing a counter electrode (Pt foil), a reference electrode (Ag/AgCl in saturated KCl solution), and a working electrode (photocatalyst coated ITO glass). Typically, for the preparation of a working electrode, a slurry was firstly obtained by grinding a mixture containing the as-prepared photocatalyst (0.2 mg), Triton X-100 (10 μL), and deionized water (20 μL). The slurry then coated by a doctorblade approach on an ITO substrate (1×1 cm2) with its conductive surface as the connecting layer, and subsequently was transferred an oven with the temperature set to 100 oC for 5 h. Note that scotch tape as a spacer was employed in this process to make the coating more uniform. The measurement of transient photocurrent was proceeded in a Na2SO4 aqueous solution (0.5 M) in the presence of radiation from a 300 W Xe lamp. The electrochemical impedance spectra (EIS) were obtained by conducting the tests in an aqueous solution containing KCl (0.1 M), K3[Fe(CN)6] (25 mM), and K4[Fe(CN)6] (25 mM), with the frequency ranging from 0.1 Hz to100 KHz. Simultaneously, the carrier density as well as flat-band potential was analyzed based upon Mott-Schottky plots that were generated at the frequency of 1 KHz. 2.5 Computational Details Vienna ab initio simulation package (VASP, version: 5.3.5) was employed for spin polarized calculations and the projection operators are evaluated in real space with GGA-PBE adopted as the exchange-correlation functional. Grimme’s semi-empirical D3 scheme was employed for dispersion correction. The plane wave basis having a kinetic energy cut-off of 400 eV was set to describe valence electrons. For Brillouin 7


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zone integration, the k points were generated according to Г-centred Monkhorst-Pack scheme. Г-point only was used during the relaxation, where it changed to a finer (3 × 3 × 1) mesh for the electron structure calculations. Geometry relaxation converged with corresponding residual forces decreased below 0.02 eV/Å on each atom. The dipole correction was also switched on to counterbalance the finite size effect induced by the surface polarity of the (101) TiO2 anatase surface. The constructed slab model of a 2 × 3 × 2 (101) TiO2 anatase surface including 108 atoms and 20-Å vacuum width was relaxed as the pristine structure. During the relaxation, the uppermost buckle of three Ti layers and four O layers were fully relaxed, while other atoms were fixed to simulate the bulk structure. The doped structures with Cu, Co substituting to the surface Ti5c atom of the as-mentioned model were relaxed on the same conditions (see Figure S3). Besides, for the electron structure calculations, the DFT+U method was used to treat the localized d electrons with U(Ti) = 4 eV, U(Cu) = 5.2 eV, and U(Co) = 4 eV, respectively.

3. Results and Discussion XRD patterns were firstly used to study the crystal structures of the prepared MIL125(Ti), porous TiO2, and metal ion doped TiO2 samples (see Figure 1). MIL-125(Ti) shows its typical diffraction peaks that are well matched with the reported values.15 For TiO2, it presents mixed crystal phases with the diffraction peaks being well-assigned to the anatase and rutile TiO2 (see JCPDS card No. 21-1272 and 21-1276 for reference). No notable changes were observed once Cu was introduced. A slight low-angle shift was observed if the content of Cu was larger than 1% (see an amplified peak in Figure 1a). This notion indicates that the copper ions were successfully incorporated into the TiO2 matrix while most of them should be localized at the surface. Embedding of cobalt ions, similarly, has shown little impact to the crystal structure of TiO2 (see amplification in Figure 1b). This result also indicates that both Cu and Co ions were highly dispersed. Moreover, decreasing of the XRD peak intensity should be resulted from the growth limiting effect of Cu and Co ions that suppresses the crystallization of TiO2. 29, 30 The microstructure and morphology of the samples were then investigated by SEM 8


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and TEM. Figure 2a shows that MIL-125(Ti) adopt a perfect cake-like morphology with diameters of 800 nm. Calcination of the structure removes the terephthalic acid organic ligands, leading to a shrink of the crystal. As a result, as shown in Figure 2b, TiO2 tablets with a diameter of about 700 nm were generated. Significantly, once copper and cobalt were introduced, both the size and cake structure were well maintained (Figure 2, c and d). The phenomenon can be also deciphered by the growth limiting effect taken by Cu and Co ions. In this case, TiO2 grains can hardly connected in a well-defined manner during ligands remove. However, this hindering effect has also led to a much rougher surface of the obtained doped TiO2 photocatalysts. This proposed growth behavior was also validated by TEM characterization. As shown in Figure 2e, the cake-like architecture was made up of many small nanoparticles that have a mean size of several nanometers. HRTEM image of 0.02%Co-1%Cu/TiO2 shows clear grain boundaries (Figure 2f) involved in the crystal. The lattice spacing of 3.5 Å corresponds to anatase TiO2 (101) plane, while the value of 3.2 Å matches the (110) plane of rutile TiO2. The results are consistent with the observation from XRD. Moreover, we also noticed the lattice spacings of 2.4 Å and 0.27 Å, which could be indexed to the Cu2O (111) and CuO (110) lattice planes, respectively. We further determined the elemental distribution of the co-doped TiO2 by STEM and energy dispersive X-ray spectroscopy (i.e., EDX analysis). Taking 0.02%Co-1%Cu/TiO2 as a model photocatalyst, as shown in Figure 3, all elements were homogeneous distributed in the composite. We then tried to determine that whether the pore structures were well maintained. Figure 4a shows the N2 adsorption-desorption isotherms of MIL-125(Ti), TiO2, 1%Cu/TiO2, and 0.02%Co-1%Cu/TiO2. Obviously, these samples exhibit type IV isotherm and have H3 hysteresis loop. It indicates the existence of the mesopore structure involved in the catalysts. This notion was also supported by further pore-size measurements as shown in Figure 4b. Generally, pores of MIL-125(Ti) originate from its atomic configuration. It leads to a narrow size distribution of the pores, herein, ~3.8 nm for the MOF. While this small pore structure was well inherited during preparation of TiO2, 1%Cu/TiO2, and 0.02%Co-1%Cu/TiO2, new broad pores ranging from 5-12 9


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nm, resulted from removal of organic ligands and contraction of structure were also generated. The BET specific surface areas, as well as the characteristics of the pores of the various catalysts were also summarized in Table 1. Particularly, the surface area was remarkably influenced by the introduction of Cu and Co. The value of MIL-125(Ti) was 1088.429 m2/g, then sharply declined to 42.016 m2/g after annealing into porous TiO2, and further decreased to 38.748 and 37.967 m2/g for 1%Cu/TiO2 and 0.02%Co1%Cu/TiO2, respectively. The total pore volume gained at P/P0= 0.985, showed the same decreasing trends. We further characterized the prepared TiO2 and doped photocatalysts by using UVvis absorption spectra (Figure S4). The porous TiO2 showed its intrinsic band edge at about 400 nm. Band-gap energy (Eg) was estimated according to the formula shown below: 𝛼ℎ𝜈 = A(ℎ𝜈 ― Eg)𝑛/2

(1)

Here, α is the absorption coefficient of a given material. h and ν represent the Planck constant and frequency of the incident light, respectively. A is designated as a proportionality constant. For direct transition semiconductor, such as TiO2, n equals to 1.31 It was thus determined that Eg of pure TiO2 was 3.06 eV. Introduction of a foreign ion would narrow the band gap. For example, Eg of 0.02%Co-1%Cu/TiO2 was reduced to 2.90 eV. This reduction made it successful in visible light response of the catalyst. In addition to UV-vis spectra, PL spectra can offer in-depth insight into photocatalysis by analyzing charge separation/recombination properties. As shown Figure S5, a and b, almost identical photoluminescence peak positions were observed on TiO2, Cu/TiO2, Co-Cu/TiO2 photocatalysts that were excited at 325 nm. Introduction of Cu could reduce the PL intensity. It indicates that these copper ions would serve as a trapping sites for either photogenerated holes or electrons, preventing them from extensive recombination. However, too heavy doping would induce additional recombination centers. Therefore, the optimized concentration should be 1% for copper to get the lowest PL intensity (Figure S5a). The PL peaks were quenched dramatically by surface co-doping of Cu and Co (Figure S5b). The behavior of the charge carriers was further 10


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investigated by transient photocurrents and EIS. As depicted in Figure S5c, the photocurrents changed in an order of 0.02%Co-1%Cu/TiO2 > 1%Cu/TiO2 > TiO2, indicating better electron transfer and charge separation property achieved over the doped catalysts. There are three possible reason leading to this improvement: (i) Cu and Co doping has visible light response realized on TiO2, (ii) CuOx can promote the migration of excited electrons from TiO2 to ITO base, (iii) CoOx can selectively capture the excited holes. To further reveal the charge carrier dynamics involved in the working electrode, Nyquist plots from EIS tests over the samples were also provided.32,33 As shown in Figure S5d, the much smaller semicircle diameters of both 0.02%Co1%Cu/TiO2 and 1%Cu/TiO2, together with the electron transfer resistance (Ret) values of 7.18 and 7.66 ohm, respectively, indicate the more favorable charge transfer dynamics of them than that of pristine TiO2 (Ret = 20.12 ohm). These results suggest that surface doping of the Cu and Co would be advantageous for photocatalytic CO2 transformation. To understand the electronic structure change by Cu and Co doping, first-principle density functional theory calculation was employed. The modeling is based upon the optimized structural models shown in Figure S3. Figure 5 describes the electronic density of states (DOS) of TiO2 and Cu/Co-doped photocatalysts. Obviously, pure TiO2 possessed separated conduction and valence bands (CB and VB) that mainly consisted of Ti 3d and O 2p electronic states, respectively (Figure 5a). Introduction of Cu led to a decrease of VB, thus reducing the band gap of TiO2 (Figure 5b). Significantly, it is found that Cu 3d electronic states also partially contributed to CB minimum formation. Basically, this small contribution implies possible electron transportation from Ti to Cu, and this possibility becomes much greater when an oxidative species such as CO2 was adsorbed on the surface of Cu.34-37 The results obtained by Co doping, on the contrary, are a little different from that obtained by Cu doping (see Figure 5c). While the CB position of TiO2 have been changed, only isolated impurity band levels consisted of Co 3d and O 2p orbitals were generated without any dedication to the CB formation. In principle, the isolated impurity band states nearby the VB maximum can significantly improve the transportation of photogenerated holes to cobalt. This notion 11


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also explains why such doping shall be favorable for enhanced photocatalytic performance.38,39 However, it is worth pointing out that two separated deep energy levels also formed in the case of Co doping. Given the fact such deep levels within the band gap would readily become carrier recombination center, the doping content of Co needs to be balanced. The band structures of TiO2, 1%Cu/TiO2 and 0.02%Co1%Cu/TiO2 were studied by coupling Mott-Schottky (M-S) plots with XPS VB spectra. The M-S plots (Figure S6, a-c) indicate the intrinsic n-type feature of these semiconductors.40 The flat band potentials (Efb) were determined to be -0.28, -0.59, and -0.61 V vs Ag/AgCl (i.e., -0.08, -0.39, and -0.41 V vs NHE) for TiO2, 1%Cu/TiO2, and 0.02%Co-1%Cu/TiO2, respectively. The exact VB positions of these catalysts were then analyzed by the XPS VB spectra (Figure S6d), which were determined to be 2.98, 2.60, and 2.49 V for TiO2, 1%Cu/TiO2, and 0.02%Co-1%Cu/TiO2, respectively. Taken together, we could calculate the band gaps to be 3.06, 2.99, and 2.90 eV for the corresponding catalysts, which well agreed with the results obtained from UV-vis spectra (Figure S4). We then investigated the chemical states of each element in the 0.02%Co1%Cu/TiO2 photocatalyst by using XPS (Figure 6). Figure 6a presents the survey spectrum of the sample, clearly demonstrating the existence of C, O, Ti, and Cu, and consistent with the EDX results described in Figure 3f. All the spectra were calibrated according to the binding energy of C 1s located at 284.6 eV (Figure 6b). The extremely low content of cobalt is below the detecting limit of the instrument. In this case, no signals were discovered for Co. While O and Ti were showed their typical chemical states in TiO2 (Figure 6, c and d),41 Cu was found with two divided states, as demonstrated by its LMM spectrum shown in Figure 6e. The kinetic energies located at 913.38 and 914.23 eV should be assigned to Cu+ and Cu2+, respectively.42,43 This notion is also validated by Figure 6f, the spin-orbit splitting spectrum of Cu 2p. The main signals at 932.51 and 952.74 eV should be attributed to Cu 2p1/2 and 2p3/2 in Cu2O, while the peaks around peaks around 934.55 eV and 954.39 eV can be a result of CuO.44-50 The as-prepared samples were evaluated as photocatalysts for CO2 reduction in water 12


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vaper assisted by simulated solar irradiation. Blank experiments without employing either the catalyst or light irradiation showed no notable changes on the composition of the mixed gas, indicating the reaction is photocatalytically proceeded. Figure 7, a-c present the initial time-course photocatalytic gas evolution over TiO2 and the Cu/TiO2 photocatalysts. Clearly, the main products were CO and CH4. The activity of TiO2 could be significantly improved by integrating Cu ions. While the Cu/TiO2 photocatalysts showed similar selectivity, the activity was sensitive to the doping amount of Cu. A volcano relationship was obtained for the photocatalytic activity relative to the content of Cu. 1%Cu/TiO2 showed the highest rates for the production of both CO and CH4, achieving 135.94 μmol and 127.05 μmol for 3-h tests. The selectivity toward high-value hydrocarbon (C2+) products could be, however, negligible. Interestingly, incorporating trace amount Co ions into the Cu/TiO2 matrix could provide notable selectivity toward C2+ paraffin production (Figure 7, d-f). Moreover, C2H6 turned to the main product that was even larger than CO and CH4. Although the yield of C3H8 was not high enough, we still expect further improvement by modifying the Co-Cu/TiO2 photocatalyst. The highest activity was achieved over 0.02%Co-1%Cu/TiO2 photocatalyst. The yields of CO and CH4 reached 150.58 and 169.79 μmol for 3-h tests, while it was remarkably increased to 267.60 μmol for C2H6 production. These results clearly demonstrated crucial role of cobalt ions for the transition of reaction pathway. The origination of carbon involved in the products is of crucial significance to the reaction. As shown in Figure S7, the STEM mapping examination indicates that a small amount of carbon residue was also reserved during the synthesis. To exclude the possibility of the produced fuels generated from the catalyst or other synthesis residues, 13C-labeled

isotopic tests by substituting

12CO

2

with

13CO

2

were implemented. The

results of GC-MS tests for the products were presented in Figure 8. The peaks at 1.82, 1.98, and 5.22 min in the GC spectrum (Figure 8a) could be assigned to 13CO, 13CH4, and 13C2H6, respectively, with the associated m/z values of 29, 17, and 32 (see Figure 8, b-d, the MS spectra). It is worth pointing out that the detectors used here are much more sensitive to CO than alkanes. As a result, the amount of generated alkanes could be underestimated. However, it is evident that the generated products were completely 13


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stemmed from CO2. By using 0.02%Co-1%Cu/TiO2 as a model photocatalyst, we further investigated the photostability for the reaction. As shown in Figure S8, the photocatalyst is generally stable during five-reaction cycles despite slight decrement. We believe very trace amount of carbon could be generated and deposit on the catalyst surface during the reaction. The covering of active sites led to the slightly decrease of CO, CH4, C2H6, and C3H8. However, this reduction will eventually stop as carbon deposition will reach a balanced state during the reaction. The durability of the catalyst was also analyzed by XPS spectra of key elements of the catalyst before and after the reaction. As shown in Figure S9 and S10, the valence states of Cu and Co did not change significantly before and after the photocatalytic reaction. Both elements showed mixed valence states with Co composed of Co2+ and Co3+, and Cu consisting of Cu+ and Cu2+. The result provides an additional proof of the stable photocatalytic behavior of the catalyst. We next sought to understand the photocatalytic mechanism by using in-situ DRIFTS characterization. This method can be used to explore the key reaction intermediates during photoreduction of CO2 in water. Figure 9, a-c compared the DRIFTS spectra of samples which had absorbed CO2 and H2O in the dark for 30 min and subsequently were irradiated under simulated solar light for 30, 60, 120, and 180 min. The absorption band (Figure 9a) in the wavenumber from 3500 to 3800 cm-1 can be ascribed to a joint stretching vibration coupling the H-bound OH groups with the physically adsorbed H2O.51 Significant reduction of the original 3726 and 3550-3750 cm-1 bands was observed during the reaction. This decrement is probably due to the desorption of H2O molecules that were weakly bonded on the surface of the catalyst. The new emerged wavenumber at 3743 cm-1 should be induced by the Ti3+-OH vibration, which is due to the transformation of Ti4+ to Ti3+ during light irradiation, along with the generation of surface vacancy simultaneously.52 We also try to determine the valence state of CO adsorbed Cu (see Figure 9b). A signal at 2076 cm-1 that could be assigned to Cu+-CO species was noticed. However, no obvious bands represented Cu2+-CO species was observed. Generally, Cu2+-CO complex involves a relatively weak σ-component that leads to the instable behavior of the complex.53-55 In addition, 14


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it might also be a result of the internal conversion of Cu2+/Cu+ upon interaction with CO.53 This photocatalytic process also contained the formation of bidentate carbonates (b-CO32- at 1557 cm-1 and m-CO32- at 1518 cm-1), bicarbonate (HCO3- at 1222 cm-1), •CH3 (at 1382, 1450 cm-1), and •CO2- (at 1676 cm-1) as depicted in Figure 9c. Significantly, the observation of •CO2- implies that the activation of CO2, in the way of CO2 + e- → •CO2-, can be achieved by shoving excess charge to the adsorbed CO2 at the surface defect sites. HCO3- species, generated from CO2 and OH groups, serves as a possible intermediate for the preparation of CO and C1 fuels associate with the emergence of dissociative H atoms. Both bidentate carbonate and bicarbonate are the important intermediates that transform into products CO.56-60 Characteristic peaks of methyl radicals (•CH3) well demonstrated the production of CH4 and the possibility of further evolving C2+ alkanes. CO2-TPD is a versatile method to reveal the active sites that responsible for CO2 adsorption. Although there are many modes for CO2 adsorption, CO2 and carbonate molecules are usually the primary and preferred adsorbed forms adopted by metal oxide catalyst. Generally, it is the directly reaction of CO2 with oxygen atom or in a way with both oxygen and metal atoms from the catalyst that leads the formation of m-CO32- and b-CO32-.61 On the other hand, the combination of CO2 and surface hydroxyls gives rise to the formation HCO3-.62-64 Several characteristic TPD peaks were observed in Figure 9d, which can be assigned to HCO3- (300-420 °C), b-CO32- (500-700 °C), and m-CO32(680-850 °C). The uptake capacity of CO2 on the photocatalysts followed the order of 0.02%Co-1%Cu/TiO2 > 1%Cu/TiO2 > TiO2 in correlation with their specific surface areas. Introduction of copper and cobalt increased the adsorption amount of CO2. Particularly, the peaks of 1%Cu/TiO2 and 0.02%Co-1%Cu/TiO2 shifted evidently to the low temperature side, which could be explained by the higher coordination number for metals (Ti, Cu, Co) as well as oxygen on their surface.63,65 By simply comparing the adsorption peak areas, it can be concluded that 0.02%Co-1%Cu/TiO2 is the best for CO2 adsorption. The surface of 0.02%Co-1%Cu/TiO2 is thus preferred to be covered by adsorbing more CO2-derived species, contributing to the better photoactivity for CO2 reduction. 15


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Based upon the above analysis, a possible photocatalytic reaction pathway (Figure 10a) and the corresponding mechanism involved in the process (Figure 10b) responsible for CO2 reduction with water vapor were proposed. Copper and cobalt played significant roles in the effective charge separation. Photoelectrons were generated on the conduction band of TiO2 upon exposure to simulated sunlight. They were rapidly migrated to the nearby CuxO species to conduct the reduction reaction.66 The photogenerated holes were, on the contrary, transferred to CoxOy, facilitating the generation of H+. More specifically, after photoexcitation, while electrons are transferred to have CO2 reduced into •CO2- radicals, the holes are captured by H2O to initiate the photooxidation, yielding protons (H+) and hydroxyl radicals (•OH). These •OH radicals will have H2O further oxidized into O2 and generate more protons.67 These protons can also be reduced into •H radicals, which subsequently react with surface carbon radicals. This combination will in-situ generate many intermediate radicals such as •CH, CH2, and •CH3, eventually leading to the formation of CH4 or high-value hydrocarbons. The major possible steps involved in this photocatalytic reaction are listed as equations shown in Eqs. S1-S23 in the Supporting Information.1,3,6,53,68-72 Notably, the impurity levels formed by Co and Cu co-doping will also have the light absorption range of TiO2 largely extended. Moreover, the mixed valence states of both Cu and Co and the internal transformation therein are considered important for efficient charge transportation and the subsequent surface redox reaction. To further reveal the role of Co, we investigated the yield changes during photocatalysis on CO2 and water vapor for 3 hours with TiO2, 0.02%Co/TiO2, 1%Cu/TiO2, and 0.02%Co-1%Cu/TiO2. It can be proved from Figure S11, the yields of CO and CH4 were 12.92 and 6.91 μmol for 3-h tests on pure TiO2, while it reached to 135.94 and 127.05 μmol for 3-h tests on 1%Cu/TiO2. What’s more, the new product of C2H6 appeared with a yield of 16.66 μmol for 3-h tests. When introducing Co species, the yields of CO and CH4 were stabilized at 150.58 and 169.79 μmol for 3-h tests on 0.02%Co-1%Cu/TiO2. Amazingly, the yield of C2H6 was remarkably increased to 267.60 μmol and C3H8 was generated with 10.07 μmol. In contrast, for 0.02%Co/TiO2 photocatalyst, the products were only trace amount of CO and CH4 (yields: 37.58 and 16


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10.75 μmol). These results clearly demonstrated crucial role of cobalt ions for altering the reaction pathway. More specifically, cobalt species captures a large number of holes to enhance oxidation capability, leading to the formation of excessive protons. These protons on cobalt site will accelerate the continuous reduction of CO2 by further combining photogenerated electrons, giving rise to the formation of more methyl radicals (•CH3). More interestingly, the excessive protons will also promote the capture of methane from the hole center to generate methyl radicals (•CH3) simultaneously. (see Eq. S20) Accumulation of these methyl radicals is the key to produce C2+ hydrocarbons (see Eq. S21).

4. Conclusion Photocatalytic transformation of mixed CO2 and H2O gas into solar fuels with superior activity and selectivity was achieved over Cu and Co co-doped TiO2 photocatalyst. The templated synthesis allowed us to control the doping more localized at the surface layer of the composite, giving rise to the visible light response of TiO2. While the photocatalytic activity of TiO2 was significantly improved by Cu doping, high selectivity toward C2+ products could be gained only by co-doping of Cu and Co. The best catalytic property was gained over the 0.02%Co and 1%Cu co-modified TiO2 photocatalyst, with CO, CH4, C2H6, and C3H8 yielding up to 150.58, 169.79, 267.60, 10.07 μmol, respectively, under simulated solar light irradiation of 3 hours. This work not only offers a robust route for embedding active species into the surface matrix of a given photocatalysts, but also marks an important step for photocatalytic production of high-value solar fuels.

Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org Proposed reaction mechanism, schematic illustration of catalyst synthesis, the schematic diagram and the photograph of the reaction device, the structures of pristine TiO2, Cu/ TiO2 and Co/TiO2 slab model, UV−vis, PL, photocurrent, EIS, MottSchottky plots, VB-XPS spectra of TiO2, Cu/TiO2, and Co-Cu/TiO2 photocatalysts, STEM image of 0.02%Co-1%Cu/TiO2, photocatalytic stability, XPS spectra of the 17


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catalysts before and after photoreaction, comparison of catalytic behaviors of TiO2, 0.02%Co/TiO2, 1%Cu/TiO2, and 0.02%Co-1%Cu/TiO2 photocatalysts.

Acknowledgement We acknowledge the financially support from the Natural Science Foundation of China (No. 51602052, No. 51876173, and No. 21576050), Natural Science Foundation of Jiangsu Province (No. BK20150604), and China Fundamental Research Funds for the Central Universities (No. 3207045403, 3207045409, 3207046414). This research was also supported by Jiangsu Key Laboratory for Biomass Energy and Material (Foundation No. JSBEM201805), Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions, and “Zhongying Young Scholars” Program of Southeast University. We also greatly appreciate Dr. Chao Gao from University of Science and Technology of China for the help of 13C tests.

References (1) Abdullah, H.; Khan, M. M. R.; Ong, H. R.; Yaakob, Z. Modified TiO2 Photocatalyst for CO2 Photocatalytic Reduction: An Overview. J. CO2 Util. 2017, 22, 15-32. (2) Izumi, Y. Recent Advances in The Photocatalytic Conversion of Carbon Dioxide to Fuels with Water and/or Hydrogen Using Solar Energy and Beyond. Coordin. Chem. Rev. 2013, 257, 171-186. (3) Ola, O.; Maroto-Valer, M. M. Review of Material Design and Reactor Engineering on TiO2 Photocatalysis for CO2 Reduction. J. Photoch. Photobio. C 2015, 24, 1642. (4) Seneviratne, S. I.; Donat, M. G.; Pitman, A. J.; Knutti, R.; Wilby, R. L. Allowable CO2 Emissions Based on Regional and Impact-Related Climate Targets. Nature 2016, 529, 477-483. (5) Forkel, M.; Carvalhais, N.; Rödenbeck, C.; Keeling, R.; Heimann, M.; Thonicke, K.; Zaehle, S.; Reichstein, M. Enhanced Seasonal CO2 Exchange Caused by Amplified Plant Productivity in Northern Ecosystems. Science 2016, 351, 696-699. (6) Gattrell, M.; Gupta, N.; Co, A. A Review of the Aqueous Electrochemical Reduction of CO2 to Hydrocarbons at Copper. J. Electroanal. Chem. 2006, 594, 118


Page 18 of 37

Page 19 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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19. (7) Tahir, B.; Tahir, M.; Amin, N. A. S. Photo-Induced CO2 Reduction by CH4/H2O to Fuels over Cu-Modified g-C3N4 Nanorods under Simulated Solar Energy. Appl. Surf. Sci. 2017, 419, 875-885. (8) Reñones, P.; Moya, A.; Fresno, F.; Collado, L.; Vilatela, J. J.; de la Peña O’Shea, V. A. Hierarchical TiO2 Nanofibres as Photocatalyst for CO2 Reduction: Influence of Morphology and Phase Composition on Catalytic Activity. J. CO2 Util. 2016, 15, 24-31. (9) Song, H.; Meng, X. G.; Wang, Z. J.; Wang, Z. A.; Chen, H. L.; Weng, Y. X.; Ichihara, F.; Oshikiri, M.; Kako, T.; Ye, J. H. Visible-Light-Mediated Methane Activation for Steam Methane Reforming under Mild Conditions: A Case Study of Rh/TiO2 Catalysts. ACS Catal. 2018, 8, 7556-7565. (10) Wang, C.; Xie, Z.; deKrafft, K. E.; Lin, W. Doping Metal-Organic Frameworks for Water Oxidation, Carbon Dioxide Reduction, and Organic Photocatalysis. J. Am. Chem. Soc. 2011, 133, 13445-13454. (11) Diercks, C. S.; Liu, Y.; Cordova, K. E.; Yaghi, O. M. The Role of Reticular Chemistry in the Design of CO2 Reduction Catalysts. Nat. mater. 2018, 17, 301307. (12) Xia, W.; Qu, C.; Liang, Z.; Zhao, B.; Dai, S.; Qiu, B.; Jiao, Y.; Zhang, Q.; Huang, X.; Guo, W.; Dang, D.; Zou, R.; Xia, D.; Xu, Q.; Liu, M. High-Performance Energy Storage and Conversion Materials Derived from a Single Metal-Organic Framework/Graphene Aerogel Composite. Nano lett. 2017, 17, 2788-2795. (13) Jiao, L.; Wang, Y.; Jiang, H. L.; Xu, Q. Metal-Organic Frameworks as Platforms for Catalytic Applications. Adv. Mater. 2018, 30, 1703663. (14) Dan-Hardi, M.; Serre, C.; Frot, T.; Rozes, L.; Maurin, G.; Sanchez, C.; Férey, G. A New Photoactive Crystalline Highly Porous Titanium (IV) Dicarboxylate. J. Am. Chem. Soc. 2009, 131, 10857-10859. (15) Fu, Y.; Sun, D.; Chen, Y.; Huang, R.; Ding, Z.; Fu, X.; Li, Z. An AmineFunctionalized Titanium Metal-Organic Framework Photocatalyst with VisibleLight-Induced Activity for CO2 Reduction. Angew. Chem. Int. Edit. 2012, 51, 336419


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3367. (16) Dhakshinamoorthy, A.; Asiri, A. M.; Garcia, H. Metal-Organic Framework (MOF) Compounds: Photocatalysts for Redox Reactions and Solar Fuel Production. Angew. Chem. Int. Edit. 2016, 55, 5414-5445. (17) Wang, L.; Xu, H. W.; Chen, P. C.; Zhang, D. W.; Ding, C. X.; Chen, C. H. Electrostatic Spray Deposition of Porous Fe2O3 Thin Films as Anode Material with Improved Electrochemical Performance for Lithium-Ion Batteries. J. Power Sources 2009, 193, 846-850. (18) Tian, D.; Zhou, X. L.; Zhang, Y. H.; Zhou, Z.; Bu, X. H. MOF-Derived Porous Co3O4 Hollow Tetrahedra with Excellent Performance as Anode Materials for Lithium-Ion Batteries. Inorg. Chem. 2015, 54, 8159-8161. (19) Zou, F.; Hu, X.; Li, Z.; Qie, L.; Hu, C.; Zeng, R.; Jiang, Y.; Huang, Y. MOFDerived Porous ZnO/ZnFe2O4/C Octahedra with Hollow Interiors for High-Rate Lithium-Ion Batteries. Adv. Mater. 2014, 26, 6622-6628. (20) Shen, L.; Xu, C.; Qi, X.; Cao, Y.; Tang, J.; Zheng, Y.; Jiang, L., Highly Efficient CuxO/TiO2 Catalysts: Controllable Dispersion and Isolation of Metal Active Species. Dalton T. 2016, 45, 4491-4495. (21) Wu, R.; Qian, X.; Yu, F.; Liu, H.; Zhou, K.; Wei, J.; Huang, Y. MOF-Templated Formation of Porous CuO Hollow Octahedra for Lithium-Ion Battery Anode Materials. J. Mater. Chem. A 2013, 1, 11126-11129. (22) Chen, G.; Gao, R.; Zhao, Y.; Li, Z.; Waterhouse, G. I. N.; Shi, R.; Zhao, J.; Zhang, M.; Shang, L.; Sheng, G.; Zhang, X.; Wen, X.; Wu, L. Z.; Tung, C. H.; Zhang, T. Alumina-Supported CoFe Alloy Catalysts Derived from Layered-DoubleHydroxide Nanosheets for Efficient Photothermal CO2 Hydrogenation to Hydrocarbons. Adv. Mater. 2018, 30, 1704663. (23) McNamara, N. D.; Neumann, G. T.; Masko, E. T.; Urban, J. A.; Hicks, J. C. Catalytic Performance and Stability of (V) MIL-47 and (Ti) MIL-125 in the Oxidative Desulfurization of Heterocyclic Aromatic Sulfur Compounds. J. Catal. 2013, 305, 217-226. (24) Kim, S.-N.; Kim, J.; Kim, H.-Y.; Cho, H.-Y.; Ahn, W.-S. Adsorption/Catalytic 20


Page 20 of 37

Page 21 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Properties of MIL-125 and NH2-MIL-125. Catal. Today 2013, 204, 85-93. (25) Wang, H.; Yuan, X.; Wu, Y.; Zeng, G.; Chen, X.; Leng, L.; Wu, Z.; Jiang, L.; Li, H. Facile Synthesis of Amino-Functionalized Titanium Metal-Organic Frameworks and Their Superior Visible-Light Photocatalytic Activity for Cr(VI) Reduction. J. Hazard. Mater. 2015, 286, 187-194. (26) Yu, J.; Low, J.; Xiao, W.; Zhou, P.; Jaroniec, M. Enhanced Photocatalytic CO2Reduction Activity of Anatase TiO2 by Coexposed {001} and {101} Facets. J. Am. Chem. Soc. 2014, 136, 8839-8842. (27) Zhang, T.; Lin, W. Metal-Organic Frameworks for Artificial Photosynthesis and Photocatalysis. Chem. Soc. Rev. 2014, 43, 5982-5993. (28) Horiuchi, Y.; Toyao, T.; Saito, M.; Mochizuki, K.; Iwata, M.; Higashimura, H.; Anpo, M.; Matsuoka, M. Visible-Light-Promoted Photocatalytic Hydrogen Production by Using an Amino-Functionalized Ti(IV) Metal-Organic Framework. J. Phys. Chem. C 2012, 116, 20848-20853. (29) Geng, H.; Liu, X.; Shi, G.; Bai, G.; Ma, J.; Chen, J.; Wu, Z.; Song, Y.; Fang, H.; Wang, J. Graphene Oxide Restricts Growth and Recrystallization of Ice Crystals. Angew. Chem. Int. Edit. 2017, 56, 997-1001. (30) Zheng, X.; Zhang, M.; Shi, X.; Wang, G.; Zheng, L.; Yu, Y.; Huang, A.; Chu, P. K.; Gao, H.; Ren, W.; Di, Z.; Wang, X. Fluorinated Graphene in Interface Engineering of Ge-Based Nanoelectronics. Adv. Funct. Mater. 2015, 25, 1805-1813. (31) Zhao, Y.; Li, C.; Liu, X.; Gu, F.; Jiang, H.; Shao, W.; Zhang, L.; He, Y. Synthesis and Optical Properties of TiO2 Nanoparticles. Mater. Lett. 2007, 61, 79-83. (32) Cachet, H.; Sutter, E. Kinetics of Water Oxidation at TiO2 Nanotube Arrays at Different pH Domains Investigated by Electrochemical and Light-Modulated Impedance Spectroscopy. J. Phys. Chem. C 2015, 119, 25548-25558. (33) Bao, S.; Wu, Q.; Chang, S.; Tian, B.; Zhang, J. Z-scheme CdS-Au-BiVO4 with Enhanced Photocatalytic Activity for Organic Contaminant Decomposition. Catal. Sci. Technol. 2017, 7, 124-132. (34) Zhang, H.; Yu, X.; McLeod, J. A.; Sun, X. First-Principles Study of Cu-Doping and Oxygen Vacancy Effects on TiO2 for Water Splitting. Chem. Phys. Lett. 2014, 21


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

612, 106-110. (35) Guo, M.; Du, J. First-Principles Study of Electronic Structures and Optical Properties of Cu, Ag, and Au-Doped Anatase TiO2. Physica B. 2012, 407, 10031007. (36) Seriani N., Pinilla C., Crespo Y. Presence of Gap States at Cu/TiO2 Anatase Surfaces: Consequences for the Photocatalytic Activity, J. Phys. Chem. C. 2015, 119, 6696-6702. (37) Wang, S.; Pan, L.; Song, J. J.; Mi, W.; Zou, J. J.; Wang, L.; Zhang, X. TitaniumDefected Undoped Anatase TiO2 with p-Type Conductivity, Room-Temperature Ferromagnetism, and Remarkable Photocatalytic Performance. J. Am. Chem. Soc. 2015, 137, 2975-2983. (38) Xu, F.; Chen, H.; Xu, C.; Wu, D.; Gao, Z.; Zhang, Q.; Jiang, K. Ultra-Thin Bi2WO6 Porous Nanosheets with High Lattice Coherence for Enhanced Performance for Photocatalytic Reduction of Cr (VI). J. Colloid. Interf. Sci. 2018, 525, 97-106. (39)Wang, T.; Meng, X.; Liu, G.; Chang, K.; Li, P.; Kang, Q.; Liu, L.; Li, M.; Ouyang, S.; Ye, J. In Situ Synthesis of Ordered Mesoporous Co-Doped TiO2 and Its Enhanced Photocatalytic Activity and Selectivity for the Reduction of CO2. J. Mater. Chem. A. 2015, 3, 9491-9501. (40) Kim, J. K.; Chai, S. U.; Ji, Y.; Levy-Wendt, B.; Kim, S. H.; Yi, Y.; Heinz, T. F.; Nørskov, J. K.; Park, J. H.; Zheng, X. Resolving Hysteresis in Perovskite Solar Cells with Rapid Flame-Processed Cobalt-Doped TiO2. Adv. Energy Mater. 2018, 8, 1801717. (41) Mayer, J. T.; Diebold, U.; Madey, T. E.; Garfunkel, E. Titanium and Reduced Titania Overlayers on Titanium Dioxide(110). J. Electron Spectrosc. 2015, 73, 111. (42) Lee, W. J.; Lee, Y. S.; Rha, S. K.; Lee, Y. J.; Lim, K. Y.; Chung, Y. D.; Whang, C. N. Adhesion and Interface Chemical Reactions of Cu/Polyimide and Cu/TiN by XPS. Appl. Surf. Sci. 2003, 205, 128-136. (43) Abbas, M.; Chen, Z.; Zhang, J.; Chen, J. Highly Dispersed, Ultra-Small and Noble Metal-Free Cu Nanodots Supported on Porous SiO2 and Their Excellent Catalytic 22


Page 22 of 37

Page 23 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Hydrogenation of Dimethyl Oxalate to Methyl Glycolate. New J. Chem. 2018, 42, 10290-10299. (44) Colón, G.; Maicu, M.; Hidalgo, M. C.; Navío, J. A. Cu-Doped TiO2 Systems with Improved Photocatalytic Activity. Appl. Catal. B: Environ. 2006, 67, 41-51. (45) Singhal, N.; Ali, A.; Vorontsov, A.; Pendem, C.; Kumar, U. Efficient Approach for Simultaneous CO and H2 Production via Photoreduction of CO2 with Water Over Copper Nanoparticles Loaded TiO2. Appl. Catal. A: Gen. 2016, 523, 107-117. (46) Zhu, L.; Hong, M.; Ho, G. W. Fabrication of Wheat Grain Textured TiO2/CuO Composite Nnofibers for Enhanced Solar H2 Generation and Degradation Performance. Nano Energy 2015, 11, 28-37. (47) Adekoya, D. O.; Tahir, M.; Amin, N. A. S. g-C3N4/(Cu/TiO2) Nanocomposite for Enhanced Photoreduction of CO2 to CH3OH and HCOOH under UV/Visible Light. J. CO2 Util. 2017, 18, 261-274. (48) Xu, S.; Du, A. J.; Liu, J.; Ng, J.; Sun, D. D. Highly Efficient CuO Incorporated TiO2 Nanotube Photocatalyst for Hydrogen Production from Water. Int. J. Hydrogen Energy 2011, 36, 6560-6568. (49) Li, P.; Liu, J.; Nag, N.; Crozier, P. A. In Situ Preparation of Ni-Cu/TiO2 Bimetallic Catalysts. J. Catal. 2009, 262, 73-82. (50) Lee, S. S.; Bai, H.; Liu, Z.; Sun, D. D. Novel-Structured Electrospun TiO2/CuO Composite Nanofibers for High Efficient Photocatalytic Cogeneration of Clean Water and Energy from Dye Wastewater. Water Res. 2013, 47, 4059-4073. (51) Walter, K.; Buyevskaya, O. V.; Wolf, D.; Baerns, M. Rhodium-Catalyzed Partial Oxidation of Methane to CO and H2. In Situ DRIFTS Studies on Surface Intermediates. Catal. Lett. 1994, 29, 261-270. (52) Wu, S.; Zhang, L.; Wang, X.; Zou, W.; Cao, Y.; Sun, J.; Tang, C.; Gao, F.; Deng, Y.; Dong, L. Synthesis, Characterization and Catalytic Performance of FeMnTiOx Mixed Oxides Catalyst Prepared by a CTAB-Assisted Process for Mid-Low Temperature NH3-SCR. Appl. Catal. A: Gen. 2015, 505, 235-242. (53) Liu, L.; Zhao, C.; Li, Y. Spontaneous Dissociation of CO2 to CO on Defective Surface of Cu(I)/TiO2-x Nanoparticles at Room Temperature. J. Phys. Chem. C 2012, 23


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116, 7904-7912. (54) Guo, J.; Hou, Z.; Gao, J.; Zheng, X. DRIFTS Study on Adsorption and Activation of CH4 and CO2 over Ni/SiO2 Catalyst with Various Ni Particle Sizes. Chinese J. Catal. 2007, 28, 22-26. (55) Gamarra, D.; Fernándezgarcía, M.; Belver, C.; Martínezarias, A. Operando DRIFTS and XANES Study of Deactivating Effect of CO2 on a Ce0.8Cu0.2O2 COPROX Catalyst. J. Phys. Chem. C 2010, 114, 18576-18582. (56) Yin, G.; Huang, X.; Chen, T.; Zhao, W.; Bi, Q.; Xu, J.; Han, Y.; Huang, F. Hydrogenated Blue Titania for Efficient Solar to Chemical Conversions: Preparation, Characterization, and Reaction Mechanism of CO2 Reduction. ACS Catal. 2018, 8, 1009-1017. (57) Li, K.; Peng, T.; Ying, Z.; Song, S.; Zhang, J. Ag-Loading on Brookite TiO2 Quasi Nanocubes with Exposed {210} and {001} Facets: Activity and Selectivity of CO2 Photoreduction to CO/CH4. Appl. Catal. B: Environ. 2016, 180, 130-138. (58) Liu, L.; Zhao, H.; Andino, J. M.; Li, Y. Photocatalytic CO2 Reduction with H2O on TiO2 Nanocrystals: Comparison of Anatase, Rutile, and Brookite Polymorphs and Exploration of Surface Chemistry. ACS Catal. 2012, 2, 1817-1828. (59) Munera, J. F.; Irusta, S.; Cornaglia, L. M.; Lombardo, E. A.; Cesar, D. V.; Schmal, M. Kinetics and Reaction Pathway of the CO2 Reforming of Methane on Rh Supported on Lanthanum-Based Solid. J. Catal. 2007, 245, 25-34. (60) Wang, X.; Shi, H.; Kwak, J. H.; Szanyi, J. Mechanism of CO2 Hydrogenation on Pd/Al2O3 Catalysts: Kinetics and Transient DRIFTS-MS Studies. ACS Catal. 2015, 5, 6337-6349. (61) Mao, J.; Ye, L.; Li, K.; Zhang, X.; Liu, J.; Peng, T.; Zan, L. Pt-Loading Reverses the Photocatalytic Activity Order of Anatase TiO2 {001} and {010} Facets for Photoreduction of CO2 to CH4. Appl. Catal. B-Environ. 2014, 144, 855-862. (62) Fadzil, N. A. M.; Rahim, M. H. A.; Maniam, G. P. Room Temperature Synthesis of Ceria by the Assisted of Cationic Surfactant and Aging Time. Malays. J. Anal. Sci. 2018, 22, 404-415. (63)Zhao, J.; Wang, Y.; Li, Y.; Yue, X.; Wang, C. Phase-Dependent Enhancement for 24


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CO2 Photocatalytic Reduction over CeO2/TiO2 Catalysts. Catal. Sci. Technol. 2016, 6, 7967-7975. (64) Im, Y.; Lee, J. H.; Kang, M. Effective Photoconversion of CO2 into CH4 over Ti30Si70MCM-41 Nanoporous Catalyst Photosensitized by A Ruthenium Dye. Korean J. Chem. Eng. 2017, 34, 1669-1677. (65) Quesada, J.; Arreola-Sánchez, R.; Faba, L.; Díaz, E.; Rentería-Tapia, V. M.; Ordóñez, S. Effect of Au Nanoparticles on the Activity of TiO2 for Ethanol Upgrading Reactions. Appl. Catal. A-Gen. 2018, 551, 23-33. (66) Shen, H.; Ni, D.; Niu, P.; Zhou, Y.; Zhai, T.; Ma, Y. Enhancing Photocatalytic H2 Evolution from Water on CuO-Co3O4/TiO2: The Key Roles of Co3O4 Loading Amounts. Int. J. Hydrogen Energy 2017, 42, 30559-30568. (67) Tan, S. S.; Zou, L.; Hu, E. Photocatalytic Reduction of Carbon Dioxide into Gaseous Hydrocarbon Using TiO2 Pellets. Catal. Today 2006, 115, 269-273. (68) Tahir, M.; Amin, N. S. Indium-Doped TiO2 Nanoparticles for Photocatalytic CO2 Reduction with H2O Vapors to CH4. Appl. Catal. B: Environ. 2015, 162, 98-109. (69) Tan, S. S.; Zou, L.; Hu, E. Kinetic Modelling for Photosynthesis of Hydrogen and Methane Through Catalytic Reduction of Carbon Dioxide with Water Vapour. Catal. Today 2008, 131, 125-129. (70) Tahir, M.; Amin, N. S. Photocatalytic CO2 Reduction and Kinetic Study over In/TiO2 Nanoparticles Supported Microchannel Monolith Photoreactor. Appl. Catal. A: Gen. 2013, 467, 483-496. (71) Tahir, M.; Tahir, B.; Saidina Amin, N. A.; Alias, H. Selective Photocatalytic Reduction of CO2 by H2O/H2 to CH4 and CH3OH over Cu-Promoted In2O3/TiO2 Nanocatalyst. Appl. Surf. Sci. 2016, 389, 46-55. (72) Ye, S.; Wang, R.; Wu, M.-Z.; Yuan, Y.-P. A Review on g-C3N4 for Photocatalytic Water Splitting and CO2 Reduction. Appl. Surf. Sci. 2015, 358, 15-27.

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Table 1. Specific surface area, pore size and pore volume of the different photocatalysts Catalyst

SBET (m2/g)

Dpore (Å)

MIL-125(Ti)

1088.429

3.816

0.6080

TiO2

42.016

6.549

0.0851

1%Cu/TiO2

38.748

6.521

0.0668

0.02%Co-1%Cu/TiO2

37.967

6.056

0.0652

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Vpore (cm3/g)

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Figure 1. XRD patterns of (a) MIL-125, porous TiO2, 0.5%Cu/TiO2, 1%Cu/TiO2, 2%Cu/TiO2 and 3%Cu/TiO2 and (b) 0.01%Co-1%Cu/TiO2, 0.02%Co-1%Cu/TiO2, and 0.05%Co-1%Cu/TiO2.

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Figure 2. SEM images of (a) MIL-125(Ti), (b) TiO2, (c) 1%Cu/TiO2, and (d) 0.02%Co1%Cu/TiO2. (e) TEM image of 0.02%Co-1%Cu/TiO2 and (f) HRTEM of 0.02%Co1%Cu/TiO2 catalyst.

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Figure 3. (a) STEM image of 0.02%Co-1%Cu/TiO2 and (b-e) corresponding EDX mappings of 0.02%Co-1%Cu/TiO2, indicating the particular distributions of (b) Ti, (c) O, (d) Cu, and (e) Co. (f) EDX pattern of 0.02%Co-1%Cu/TiO2 catalyst.

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Figure 4. (a) N2 adsorption–desorption isotherms and (b) pore size distribution curves of the as-prepared catalysts.

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Figure 5. Density of states (DOS) for (a) TiO2, (b) Cu/TiO2, and (c) Co/TiO2.

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Figure 6. XPS spectra of 0.02%Co-1%Cu/TiO2 catalyst. (a) Survey spectrum, (b) C 1s, (c) O 1s, (d) Ti 2p, (e) CuLMM, and (f) Cu 2p.

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Figure 7. Effect of irradiation time on products of CO2 and water vaper: (a) CO with a series of Cu/TiO2, (b) CH4 with a series of Cu/TiO2, (c) 1%Cu/TiO2, (d) 0.01%Co1%Cu/TiO2, (e) 0.02%Co-1%Cu/TiO2, and (f) 0.05%Co-1%Cu/TiO2.

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Figure 8. Results of GC-MS analysis of the products generated from the 13CO2 isotope experiments. (a) GC spectrum, (b)-(d) mass spectra showing 13CO (m/z=29), 13CH4 (m/z=17), and 13C2H6 (m/z=32) produced over 0.02%Co-1%Cu/TiO2.

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Figure 9. In situ DRIFTS spectra of (a) OH groups on catalysts (3500-3800 cm-1), (b) CO adsorption (2000-2200 cm-1), (c) CO2 interaction with 0.02%Co-1%Cu/TiO2 under the photoirradiation (1100-1800 cm-1), and (d) CO2-TPD profiles for TiO2, 1%Cu/TiO2, and 0.02%Co-1%Cu/TiO2.

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Figure 10. (a) Possible reaction pathways for photoreduction of CO2 with H2O and (b) the mechanism involved on the surface of the composited photocatalyst.

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