Dynamic Evolution of Atomically Dispersed Cu Species for CO2

State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University,. Fuzhou .... In addition, the mesoporous ch...
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Dynamic Evolution of Atomically Dispersed Cu Species for CO2 Photoreduction to Solar Fuels Lan Yuan, Sung-Fu Hung, Zi-Rong Tang, Hao-Ming Chen, Yujie Xiong, and Yi-Jun Xu ACS Catal., Just Accepted Manuscript • Publication Date (Web): 17 Apr 2019 Downloaded from http://pubs.acs.org on April 17, 2019

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Dynamic Evolution of Atomically Dispersed Cu Species for CO2 Photoreduction to Solar Fuels Lan Yuan†,‡, Sung-Fu Hung§, Zi-Rong Tang‡, Hao Ming Chen§, Yujie Xiongǁ, Yi-Jun Xu†,‡,* State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, 350116, P. R. China ‡ College of Chemistry, New Campus, Fuzhou University, Fuzhou, 350116, P. R. China § Department of Chemistry, National Taiwan University, Taipei 106, Taiwan †

ǁHefei

National Laboratory for Physical Sciences at the Microscale, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), School of Chemistry and Materials Science, and National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China ABSTRACT: Probing the dynamic evolution of catalyst structure and chemical state under operating conditions is highly important for investigating the reaction mechanism of catalysis more in depth, which in turn advances rational design of redox catalysis in using renewable energy to produce fuels. Herein, the evolution of atomically dispersed Cu species supported by mesoporous TiO2 (mTiO2) during the in situ photocatalytic reduction of CO2 with H2O to valuable solar fuels has been reported. The results unveil that the initial atomically dispersed Cu(II) undergoes reduction to Cu(I) and ultimately to Cu(0); and the mixture of Cu(I)/Cu(0) is proposed to be more effective for CH4 formation. In addition, the enhanced CO2 adsorption ability benefited from the structure advantage of mTiO2 and the elevated charge carrier transfer synergistically contribute to the CO2 photoreduction. It is anticipated that this work would guide rational design of Cu-based light harvesting catalyst for artificial CO2 reduction to value-added feedstocks, and inspire further interest in using in situ techniques to study the structure-activity interplay of photocatalysts in operating reaction conditions. KEYWORDS: dynamic evolution, atomically dispersed, Cu species, photocatalysis, CO2 reduction

problem is that other structure differences would inevitably be introduced to the various catalysts simultaneously, thus making the Cu valence state correlated activity comparison far from determined. Therefore, it is highly necessary to in situ monitor the dynamic structure and composition of the same Cu-based catalysts under operating conditions, thereby revealing how the catalysts evolve in the course of reaction and establishing the realistic structure-function correlation of the catalysts.23-24 In addition, recent works have emphasized that metal atoms with unsaturated coordination are probably to be the catalytic reactive sites, and it therefore is highly desired to reduce the catalyst size and improve the exposed fraction of metal atoms that has unsaturated coordination simultaneously to optimize the catalysts.25-27 On the basis of this aspect, atomically dispersed Cu species anchored on specific support are expected to have more catalytic centers for enhanced activity and stability, which could provide a promising platform for in situ investigating the structure-catalytic performance for CO2 photoreduction; however, these studies are still lacking until now. Herein, we report the facile synthesis of atomically dispersed Cu species onto mesoporous TiO2 spheres

INTRODUCTION Artificial photosynthesis that utilizes inexhaustible solar energy to convert CO2 and H2O into renewable feedstocks is considered to be one of the promising ways to meet our long-term global sustainability goal.1-4 In this context, it is critical to design an integrated catalyst system, consisting of a light capturing component and a catalytic reaction part, which can efficiently capture solar energy, generate electron-hole charge carriers, and drive target redox catalysis reactions.5-10 Owing to its natural abundance and potential of favoring hydrocarbons formation, Cu is one of the most attractive elements for creating cost-effective Cubased hybrid catalysts for CO2 reduction.11-15 However, studies on Cu species are confronted with one challenge from transient changes in the structural composition, a key factor in determining the CO2 photoreduction activity.2,6,1619 To date, it still remains controversial what are the most active Cu species for CO2 photoreduction.20-22 Despite efforts devoted to evaluation of Cu-based catalysts with different Cu valence state prepared separately, such as by calcinating the same precursor catalysts under different protective atmosphere20,22, one

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Figure 1. (a) Illustration for the synthesis of mTiO2 and Cu-mT; (b) TEM image of 1Cu-mT and (c) corresponding elemental mapping results; (d) magnified HAADF-STEM image of 1Cu-mT and (e, f) the corresponding intensity profiles for the atomically dispersed Cu species. TIP: titanium isopropoxide; HDA: hexadecylamine; mTiO2: mesoporous TiO2 spheres; CumT: Cu species grafted mTiO2. (mTiO2) for efficient CO2 photoreduction with H2O in the gas phase, and in particular use the in situ techniques to probe the dynamic evolution of the catalysts during the course of catalytic reaction. The results reveal that upon light irradiation, the initial atomically dispersed Cu(II) species on mTiO2 gradually undergo reduction to Cu(I) and ultimately to Cu(0), thereby photocatalyzing the reduction of CO2 into solar fuels in a dynamic manner; and the mixture of Cu(I)/Cu(0) is proposed to be the most efficient for CH4 formation. In addition, the mesoporous channel of mTiO2 support and the enhanced charge carrier transfer together promotes the adsorption, activation and reduction of CO2. It is hoped that our work would promote ongoing interest in the in-depth understanding of fundamental structure and composition of catalysts during in situ operating conditions, which is indispensible for rationally designing efficient and selective catalysts for CO2 reduction.

(Cu-mT with x% molar ratio of Cu are denoted as xCu-mT). Because of the high dispersion or low content of the Cu species, no characteristic diffraction peaks for Cu species (metal Cu or CuxO) can be observed in the X-Ray diffraction (XRD) patterns of xCu-mT (Figure S4a).18,30 UV-vis diffuse reflectance spectra (DRS) of the samples are displayed in Figure S4b-c. Besides the onset absorption of mTiO2 at 400 nm, the absorption shoulders centered at ∼450 nm can be assigned to interfacial charge transfer (IFCT) from the valence band (VB) of TiO2 to Cu species, indicating that the photoexcited electrons from TiO2 are directly transferred to the Cu species upon light irradiation.17-18,31 Another absorption band starting from 580 nm can be ascribed to the Cu(II) d-d orbital transition.32-33 The electron paramagnetic resonance (EPR) spectra of xCu-mT also show large signals of Cu2+, which are located in the region of 26003600 G (Figure S4d).34 TEM was conducted to detect the microscopic structures of the samples (taking 1Cu-mT as an example, since it shows the best activity for CO2 photoreduction according to the later activity test, and without further specification, following characterizations will also mainly focus on 1CumT). The TEM images of 1Cu-mT (Figure 1b) suggest that the mesoporous structure of mTiO2 was well maintained after the successful deposition of Cu species, which can be confirmed by the energy-dispersive X-ray (EDX) (Figure S5) and corresponding elemental mapping results (Figure 1c). The aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADFSTEM) image of 1Cu-mT certifies that the Cu species are

RESULTS AND DISCUSSION The fabrication of Cu species grafted mTiO2 (denoted as Cu-mT) is schematically illustrated in Figure 1a. Initially, mTiO2 precursor beads possessing smooth surfaces and amorphous structure were synthesized, based on which mTiO2 with rough surfaces and well-resolved crystal form of anatase TiO2 were produced after solvothermal and calcination (Figure S1).28-29 Transmission electron microscopy (TEM) images (Figure S2) and N2 adsorptiondesorption isotherms (Figure S3) disclose the disordered mesoporous structure of mTiO2. The deposition of Cu species onto mTiO2 was achieved by a precipitation method

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Figure 2. Typical high-resolution XPS spectra for the core levels of (a) Ti 2p, (b) O 1s and (c) Cu 2p for mTiO2 and 1Cu-mT; (d) the Cu K-edge XANES spectra and (e) their first derivatives for reference materials (including metal Cu, Cu2O, CuO, and Cu(OH)2) and 1Cu-mT; (f) fourier transformed EXAFS spectra for the reference materials and 1Cu-mT. atomically dispersed on the mTiO2 support (Figure 1d).25 Homogeneously dispersed single Cu sites can clearly be observed from the corresponding intensity profiles (Figure 1e, f).25 X-ray photoelectron spectroscopy (XPS) has been performed to further investigate elemental compositions and chemical states of the samples, with the survey XPS spectra shown in Figure S6. Figure 2a shows the peaks in the region of Ti 2p. As for bare mTiO2, two peaks located at 458.0 and 463.6 eV correspond to the binding energy (BE) of Ti 2p3/2 and Ti 2p1/2, respectively, which indicates that the Ti element presents as the chemical state of Ti4+.35 As for 1Cu-mT, both of the peak locations shift toward higher BE by 0.4 eV, manifesting strong electronic interaction between Cu species and TiO2.36 O 1s core-level spectra are illustrated in Figure 2b. The peak appearing at ca. 529.2 eV for mTiO2 is assigned to Ti-O, which also shifts to 529.6 eV by 0.4 eV for 1Cu-mT, resulting from Cu species interacting with Ti-O.36 Another peak located at ca. 531.0 eV stands for the surface hydroxides, which was largely enhanced for 1Cu-mT as compared to bare mTiO2 due to the employment of NaOH aqueous for Cu species deposition. As illustrated in the Cu 2p core-level spectra (Figure 2c), no Cu 2p peak is observed for mTiO2; while 1Cu-mT shows the BE of 932.8 and 952.6 eV for Cu 2p3/2 and Cu 2p1/2, respectively, which are ascribed to that derived from Cu(I) species.30,37

Taking into account that Cu(II) species can be reduced to Cu(I) by the bombardment effect of X-ray irradiation during XPS analysis in high vacuum, especially when Cu is very low in content and presents in a highly dispersed state,18-19 the catalysts were further subjected to X-ray absorption spectroscopy (XAS) measurements for more accurate investigations. The Cu K-edge X-ray absorption near-edge structure (XANES) spectra for reference materials including metal Cu, Cu2O, CuO, Cu(OH)2 and as-prepared 1Cu-mT are shown in Figure 2d. It seems that the spectrum of 1Cu-mT resembles that of Cu(OH)2. To make the features more evident, the XANES spectra were subjected to first derivation and the results are exhibited in Figure 2e. It can be seen that the derivative spectra of Cu0 and Cu+ show the edge-energy features at ca. 8979 and 8980 eV, respectively, which represent the 1s→4p transition.38 That of Cu2+ in CuO and Cu(OH)2 show a presence of a weak feature at 8977 eV for the 1s→3d transition.38 A main peak for CuO located at 8983 eV stands for the 1s → 4p transition of Cu2+ in the tetragonal symmetry. The Cu2+ species in the octahedral symmetry as for Cu(OH)2 shows an edge-energy at 8985.5 eV, which is ca. 2.5 eV larger than that of Cu2+ in CuO.32,38 Both the 1s→3d and 1s→4p features of Cu2+ can be found in the spectrum of the 1Cu-mT catalysts, which implys that the Cu species exist in +2 state.18-19 As for the feature of the 1s → 4p transition, the Cu2+ species in 1Cu-mT show a

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EPR has been conducted to explore the catalysts properties after different reaction times. It can be seen that the EPR signals for Cu2+ largely decreased after the first 2-h reaction (Figure 3d), which might be because that paramagnetic Cu2+ were reduced to diamagnetic Cu+/Cu0 by the photoinduced electrons.24,34 No further decrease of Cu2+ for the reaction time from 3 h to 4 h can be observed. Yet, we cannot exclude the rapid re-oxidation of the as-formed Cu+/Cu0 back to Cu2+ that caused by the exposure of them to air during the samples preparation process for EPR measurement. Additionally, a small signal appears at the gfactor of 2.003 for 1Cu-mT during the reaction, which is attributable to trapped holes.34 Yet, it cannot be detected for mTiO2, indicating the better prevented recombination of holes with electrons in 1Cu-mT than that in mTiO2. To better understand the connection between structure and activity of the catalysts, in situ XAS analyses were employed to track the real-time evolution of Cu species on 1Cu-mT during the reaction process of photocatalytic CO2 reduction. Figure 3e shows the in situ XANES spectra at Cu K-edge for 1Cu-mT under the operating conditions of CO2 photoreduction, and their first derivatives were conducted and displayed in Figure 3f. We can see that before light irradiation, the fresh 1Cu-mT sample displays a distinct peak at 8986 eV for Cu(II).47 Upon light irradiation, the peaks for Cu(0) and Cu(I) appeared at 0.5 h, while that for Cu(II) significantly decreased, indicating that Cu(II) species was fastly reduced upon irradiation.19 This is reasonable because electrons in the CB of TiO2 can transfer to Cu(II) and reduce it.19,30 A detailed inspection on the derivative spectra reveals that further light irradiation resulted in the everincreasing Cu(0), whereas the Cu(II) disappeared at 1.5 h. The Cu(I) species increased at first and reached the maximun at 2.0 h but then decreased and disappered at 3.0 h. When further prolonging the light irradiation time from 3 h to 4 h, the XANES spectra almost keep unchanged, suggesting that a steady state of Cu(0) species was reached. The in situ extended X-ray absorption fine structure (EXAFS) in Figure 3g further confirms the gradual transformation of Cu(II) to Cu(0). Combined with the recycling experiments for CH4 production in Figure 3c, it can be inferred that the mixture state of Cu(I)/Cu(0) in the second cycle (2h-4h) is more favorable for CH4 formation, as compared to mixed Cu(II)/Cu(I)/Cu(0) in the first cycle (0h-2h) and pure Cu(0) in the third (4h-6h) to fifth cycles (8h-10h). This could be attributed to the synergistic effect of the Cu(I) and Cu(0) species, since Cu(0) with low Fermi level is more efficient for attacking electrons48, and Cu(I) is an active site to promote the formation of CH437. In addition, as for the mixed state of Cu(I)/Cu(0), photoexcited electrons trapped by Cu(I) could reduce it to Cu(0), while Cu(0) would trap the photoexcited holes, thus separating electrons and holes more efficiently.20 The HAADF-STEM images of the used 1Cu-mT (Figure S9) confirm that the Cu species still remained as isolated, which is consistent with the EXAFS analysis (Figure S10). These results suggest that no obvious aggregation was happened

positive energy shift of ca. 0.5 eV as compared to that of Cu2+ in the Cu(OH)2, indicating that a part of the Cu2+ species are in a site distorted.32 The fourier transformed (FT) extended X-ray absorption fine structure (EXAFS) spectra of the samples are displayed in Figure 2f. We can see that the chemical environment of Cu2+ species in 1Cu-mT resembles that of Cu2+ in CuO. Yet, the Cu species in pure CuO is surrounded by Cu and O ions as the outer shell, forming large grain size of CuO,38 with bond distance near 3.1 Å (phase uncorrected). The lack of the peak at 3.1 Å in the EXAFS of 1Cu-mT discloses the high dispersion of Cu2+ on the surface of mTiO2.19 The above results together reveal that the Cu species are atomically attached onto the mTiO2 surface with a distorted fivecoordinate square pyramidal structure, in which the apical oxygen approaches the Cu(II) sites with –O or –OH.18-19,32 We have then evaluated the activity of the samples for gas-phase photocatalytic CO2 reduction with H2O under UVvis light irradiation. In agreement with previous literature reports39-45, H2, CO and CH4 were found to be the major products. A set of experiments controlling the reaction conditions (Figure S7) have been performed to confirm that the reaction is driven by photocatalysis: the removal of light irradiation or catalysts results in the absence of products detection, indicating that the reaction is a photocatalytic process; once CO2 was replaced with Ar, only a trace amount of CO was observed, suggesting that the carbon-containing products from decomposition of possible carbon residues on photocatalysts are negligible.46 Comparing with bare mTiO2, the products evolution rates are obviously improved on xCu-mT (Figure S8), among which 1Cu-mT achieves the best activity, where both the produced CH4 and CO are almost 5 folds as those over mTiO2, with a apparent quantum yield of 0.36% and 0.09% for CH4 and CO, respectively. Further increment in the Cu content leads to a decrease in the reaction rate, which can be connected with the light-screening effect of excessive Cu species or their roles of acting as recombination centers of electrons and holes.32-33 Based on the optimal 1Cu-mT sample, the long-term activity tests have been conducted, and the time-dependent product yields are shown in Figure 3a. To further trace the carbon source of CO and CH4, isotopic 13CO2 was used as the reactant to perform the photocatalytic reaction under identical conditions, and the results (Figure 3b) confirmed that the carbon of CO and CH4 indeed originates from photocatalytic CO2 reduction. The recycling experiments have been carried out for further study. As shown in Figure 3c, the general trend of slightly declined products evolution should be ascribed to the saturation of the adsorption sites over the catalysts surface with intermediate products after continuous reaction, which is in agreement with previous reports on CO2 photoreduction in the gas phase.20,47 Extraordinarily, significantly enhanced CH4 formation was observed in the second cycle, which could indicate other changes of catalyst properties.

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Figure 3. (a) Time-dependent products yield over mTiO2 and 1Cu-mT; (b) mass spectra of 13CH4 (m/z = 17) and 13CO (m/z = 29) produced over 1Cu−mT in photocatalytic reduction of 13CO2; (c) recycling experiments over 1Cu-mT during every 2 h reaction time. (d) EPR spectra of mTiO2 and 1Cu-mT catalysts after different reaction times for CO2 reduction; Experimental parameters: microwave frequency 9.4731 GHz, microwave power 6.35 mW; modulation frequency 100 kHz, modulation amplitude 1 G, modulation phase 0, time constant 10.24 s, sweep time 41.98 s. (e) in situ Cu K-edge XANES spectra, (f) the first derivatives and (g) EXAFS spectra for 1Cu-mT under photocatalytic CO2 reduction reaction conditions. to the Cu species after the CO2 photoreduction reaction. In addition, TiO2 nanoparticles and Cu species modified TiO2 nanoparticles (1Cu-T) were prepared (Figure S11) for comparative activity study. Progressively enhanced production for CH4, CO and H2 are observed for the samples in the order of TiO2 < mTiO2 < 1Cu-T < 1Cu-mT (Figure S12). Figure 4a depicts the total converted amount of CO2 over the samples, from which we can see that as compared to TiO2 nanoparticles, mTiO2 improves the activity by 2 times, while the loading of Cu species further boosts the activity by 4 times. As a result, the activity for CO2 photoreduction over 1Cu-mT is 8-folds as that over bare TiO2. Significantly, the CH4/CO molar ratio for the products increases from 1.5 for TiO2 to 5.2 for 1Cu-mT, implying the promoted multi-electron reduction of CO2 over 1Cu-mT catalysts. The enhancement in 1Cu-mT in comparison with bare TiO2 is on one hand due to the enhanced mass transfer of the reactants on catalyst surface, which is widely accepted as the preceding step influencing the reaction dynamics of the photocatalytic process for CO2 reduction.7,49-51 The specific surface areas were measured to compare the

available active sites of the samples (Figure S13). As clarified in Table S1, the surface area of TiO2 (47m2/g) is much lower than that of mTiO2 (100 m2/g). 1Cu-mT hybrids (99 m2/g) inherit the high surface area of mTiO2, as well as the large availability of active sites. Corresponding CO2 uptake capability of the sample were investigated by the CO2 adsorption isotherms. As shown in Figure 4b, both mTiO2 and 1Cu-mT exhibit a rapid rise in CO2 adsorption as compared to TiO2, benefiting from the largely enhanced surface areas and pore volumes (Table S1). The slightly decreased CO2 adsorption of 1Cu-mT as compared to bare mTiO2 might be caused by the deposition of Cu species into the porous structure of mTiO2, in consistence with the a little lower BET surface area of 1Cu-mT than mTiO2 (Table S1). More information about the adsorption active sites for CO2 molecules on the catalysts surface were acquired by the CO2 temperature-programmed desorption (CO2-TPD) analysis. As shown in Figure 4c, the desorption peaks of α, β, and γ can be denoted as the weak, moderate, and strong CO2 adsorption active sites on the surface of the catalysts,

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Figure 4. (a) The converted amount of CO2 as well as produced CH4/CO molar ratios over the samples of TiO2, mTiO2 and 1CumT; (b) CO2 adsorption isotherm curves at 273 K; (c) MS signals of CO2 desorption for CO2-TPD profiles, and (d) electron lifetime calculated from the decay of OCP in dark over the samples of TiO2, mTiO2 and 1Cu-mT; (e) probable reaction mechanisms for photocatalytic reduction of CO2 with H2O over the Cu-mT catalysts. respectively.52-53 Only one CO2 desorption peak (α) located at 80 °C is observed for TiO2 and mTiO2, and the peak intensity for mTiO2 is stronger than that of TiO2, indicating that mTiO2 with higher surface area promotes the weak CO2 adsorption.54 As for 1Cu-mT, besides the further enhanced α peak, two new CO2 desorption peaks (β and γ) appear, suggesting the formation of stronger CO2 adsorption active sites.55-56 As illustrated in Figure S14a, the H2O desorption peaks for mTiO2 and 1Cu-mT show a slight shift from 200 °C to a higher temperature of 220 °C along with a largely increased peak area, as compared to TiO2, showing the enhanced H2O adsorption ability of the samples in the order of 1Cu-mT > mTiO2 > TiO2.56 The enhanced CO2 and H2O adsorption over 1Cu-mT ultimately offers more opportunities for the electrons transfer from catalysts surface to CO2 and subsequent reaction with protons deriving from H2O oxidation, thereby leading to higher activity of CO2 photoreduction. On the other hand, the separation efficiency of electronhole pairs within catalytic materials is critical for an efficient photocatalytic process. A measurement of opencircuit photovoltage decay (OCPD) of the catalysts was performed (Figure S14b), based on which the photoelectron lifetime as a function of VOC was calculated.5759 As shown in Figure 4d, it is obvious to see that mTiO 2 shows longer electron-lifetime than TiO2, which can be ascribed to the enhanced interparticle charge transfer60; while 1Cu-mT demonstrates distinctly prolonged electron lifetime in comparison with mTiO2 and TiO2, because the photoelectrons can migrate from TiO2 to Cu species and the holes tend to be trapped by more abundant surface hydroxy groups, thereby preventing their recombination efficiently.17,30,47 The steady-state photoluminescence (PL) spectra (Figure S15) further show the obvious PL quenching of 1Cu-mT, suggesting the establishment of an electron transfer in a nonradiative quenching pathway.30,61 Corresponding photo- and electrochemical results

including polarization curves, cyclic voltammograms (CV), electrochemical impedance spectroscopy (EIS), and photocurrent responses additionally verify that the increment of the charge carriers transfer efficiency for the materials follows the sequence of 1Cu-mT >> mTiO2 > TiO2 (Figure S16). Thus far, the possible reaction mechanisms for the photoreduction of CO2 with H2O over the Cu-mT catalyst are schematically illustrated in Figure 4e. Firstly, the mixture gas of CO2 and H2O vapor diffused into the porous structure of mTiO2 and adsorbed on the catalyst surface. Upon light illumination, mTiO2 was band-gap excited to generate electron-hole pairs, and the grafted Cu species and surface hydroxy groups respectively played the role in trapping electrons and holes, thus boosting the spatial separation of charge carriers. Notably, the initial Cu(II) species can be gradually reduced to Cu(I) by the electrons and further to Cu(0), while Cu(0) can trap the holes to be re-oxidized to Cu(I) and further to Cu(II)22. Since Cu species are more efficient for attacking electrons, the reduction process was faster than the oxidation process, thus ultimately transferred the Cu species to Cu(0)48, which dynamically catalyzed the photoreduction of CO2. Adsorbed H2O and surface hydroxy groups on the surface of Cu-mT were subsequently activated by photoinduced holes to form O2 and protons. The production of O2 was confirmed by the gradually increased O2/N2 ratios along with the light irradiation over the catalyst (Figure S17).62-63 Yet, its experimental amount detected is much less than the theoretical O2 evolution amount, which might be because that the O2 and/or O species were readsorbed or remained on the catalysts surface, or the produced O2 was reconsumed.62-63 Finally, the photoelectrons were transferred to react with CO2/CO or/and protons to form the products. In particular, the supported atomically dispersed Cu species can efficiently accumulate the

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photoinduced electrons to accelerate the multielectron reduction of CO2 to CH4.

CONCLUSION In summary, Cu species have been atomically dispersed onto mTiO2 for gas-phase photocatalytic CO2 reduction with H2O, during which the dynamic evolution coupled synergy catalysis was revealed. The enhanced CO2 adsorption/activation and charge carriers separation collectively catalyze the efficient and stable CO2 reduction in a synergistic manner; and it is demonstrated that the atomically supported Cu(II) species in situ undergoes reduction to Cu(I) and ultimately to Cu(0), and the mixture of Cu(I)/Cu(0) is proposed to be more efficient for CH4 formation. It is expected that this work would inspire further interest in developing efficient artificial photocatalysts composed of cost-effective and nontoxic Cu element for CO2 transformation to value-added fuels, and using various in situ techniques to study on the structureactivity interplay of the catalyst in operating reaction conditions.

EXPERIMENTAL SECTION Materials. Titanium (IV) isopropoxide (TIP, 97%) and hexadecylamine (HDA, 90%) were supplied by SigmaAldrich. Potassium chloride (KCl), sodium hydroxide (NaOH), cupric nitrate (Cu(NO3)2), ethanol (C2H5OH) and N, N-dimethylformamide (C3H7NO, DMF) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Deionized (DI) water was prepared in local laboratory. Preparation of mesoporous TiO2 spheres (denoted as mTiO2). The amorphous precursors of mTiO2 were prepared via a sol-gel strategy, during which HDA and KCl were used to direct the structure and control the monodispersity of the precursors, respectively.28-29 Typically, HDA (1.75 g) was firstly added and dissolved into ethanol (200 mL), followed by the injection of KCl solution (0.07 M, 1.3 mL). Then, TIP (4.32 mL) was quickly injected into the solution along with vigorous stirring under ambient temperature. The as-obtained milky white suspension was kept static for 18 h, and then washed by ethanol and dried at room temperature. Mesoporous TiO2 spheres (mTiO2) were prepared through a solvothermal treatment of the precursors and calcination. Typically, the amorphous precursors (1.6 g) were dispersed in the mixture of ethanol (20 mL) and DI water (10 mL), and then sealed within a Teflon-lined autoclave (50 mL) and heated under 433 K for 16 h. After washing and drying, the powders were calcined at 773 K for 2 h in air to remove organic components and form the mTiO2 products. For comparison, TiO2 nanoparticles (denoted as TiO2) were synthesized by the same process without the addition of HDA. Fabrication of Cu species modified mesoporous TiO2 spheres (denoted as Cu-mT). Cu species modified mTiO2 photocatalysts were fabricated through a precipitation method.30 Before use, mTiO2 powder was further heated at 623 K under air for 12 h and washed several times with DI water to remove the possible residual organic contaminants. Then, mTiO2 (0.04 g) was dispersed in NaOH aqueous solution (0.25 M, 40 mL), and then a certain

amount of Cu(NO3)2 aqueous solution (0.0077 M) was added dropwise into the solution under vigorous stirring at room temperature. After further stirring of 6 h, the precipitate was washed with DI water several times until the PH=7. Finally, the washed precipitates were dried at 353 K for 12 h to obtain Cu species decorated mTiO2 (denoted as xCu-mT, where x represents for different molar ratios of Cu species in the Cu-mT composites, including 0.25%, 0.5%, 1%, 2% and 3%). For comparison, the deposition of 1% molar addition ratio of Cu species onto TiO2 nanoparticles (denoted as 1Cu-T) was also prepared under the same experimental conditions. Characterizations. Transmission electron microscopy (TEM) images were acquired on a JEOL model JEM 2010 EX instrument at an accelerating voltage of 200 kV. Aberrationcorrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and corresponding EELS were obtained on a JEOL JEMARM200F TEM/STEM system. The X-ray diffraction (XRD) patterns of the samples were collected on a Bruker D8 Advance X-ray diffractometer with Ni-filtered Cu Kα radiation at 40 kV and 40 mA. The UV-vis diffuse reflectance spectroscopy (DRS) of the samples were collected using a UV-vis spectrophotometer (Cary 500, Varian Co.), where BaSO4 was employed as the internal reflectance standard. N2 adsorption-desorption isotherms and the BrunauerEmmett-Teller (BET) surface areas were measured by Micromeritics ASAP2020 equipment. The sample was degassed at 393 K for 5 h and then analyzed at 77 K. CO2 adsorption isotherms was determined at 273 K using Micromeritics ASAP2020 equipment. The sample was degassed at 393 K for 8 h and then analyzed at 273 K. CO2 Temperature-programmed desorption (CO2-TPD) measurements were conducted on the Micromeritics Autochem II 2910 instrument. The measurements were started from 353 K to 773 K and the desorbed CO2 and H2O were detected by thermal conductivity detector (TCD). Meanwhile, the mass spectrometry (MS) was employed to detect the contents changes of CO2 and H2O (m/z values of 44 and 18, respectively). X-ray photoelectron spectroscopy (XPS) of the samples were collected on a Thermo Scientific ESCA Lab 250 spectrometer with monochromatic Al Ka as the X-ray source. All of the binding energies were calibrated by the C1s peak at 284.6 eV. Electron paramagnetic resonance (EPR) signals of the samples were measured using a Bruker ESP 300 E electron paramagnetic resonance spectrometer under vacuum at 77 K. X-ray absorption spectra (XAS) of Cu K-edge, including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), were recorded at room temperature in fluorescence mode using ion chambers at BL01C1 at National Synchrotron Radiation Research Center (NSRRC, Taiwan). In situ XAS spectra were collected under a home-made reactor (Figure S18) that the gas mixture of CO2 and H2O was filled while irradiating with UV-vis light from a Xe lamp (0.8 W cm−2). The spectra were normalized by the incident beam intensity. The photon energy was calibrated with the first inflection point of the Cu K-edge in Cu metal foil. The as-obtained XAS data were processed with software IFEFFIT. Photoelectrochemical measurements. The photoluminescence spectra (PL) of the samples were analyzed on an Edinburgh Analytical Instrument F900

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spectrophotometer, and the excitation wavelength was 340 nm. To make the PL spectra comparable, the experimental parameters, including the excitation wavelength, slit width, and the samples amount, were keep identical. The electrochemical and photoelectrochemical analysis was measured in the normal three-electrode cell, where the Pt plate and Ag/AgCl electrodes are used as the counter and reference electrode, respectively. The working electrode was prepared on fluorine doped tin oxide (FTO) glass that was sufficiently cleaned and dried at 353 K for 2 h. The boundary of FTO glass was protected by scotch tape to make a exposed area of 0.25 cm2. Then, 5 mg sample was fully dispersed in 0.5 mL DMF by sonication to get slurry, which was spread onto the exposed area of FTO glass. After drying at 393 K for 2 h, the tape was unstuck, and the uncoated area of the electrode was isolated by epoxy resin. The photocurrents were measured on an electro-chemical workstation (Autolab, PGSTAT204) without bias under UVvis light irradiation and the employed electrolyte was Na2SO4 aqueous solution (0.2 M, pH = 6.8). The cathodic polarization curves were obtained using the linear sweep voltammetry technique with a scan rate of 0.5 mV s−1. The electrochemical impedance spectroscopy (EIS) was measured in the KCl solution (0.5 M, containing 5.0 mM K3[Fe(CN)6]/K4[Fe(CN)6]) under open circuit potential (OCP) conditions by applying an AC voltage with 5 mV amplitude in a frequency range from 1 Hz to 100 kHz. The cyclic voltammograms were conducted in the same solution as that of the EIS measurement. Photoactivity testing. The photoactivity of the samples for CO2 reduction in the gas-solid mode was performed in a quartz reactor (volume, ~25 mL). Typically, 20 mg catalyst was firstly loaded into the reactor, which was evacuated by a mechanical pump and then filled with pure CO2 gas, and the evacuation-filling process was repeated five times. After that, a certain amount of evacuated liquid water (0.5 mL) was introduced into the sample cell (5 mL) hanging in the reactor with a syringe via the septum. The system was then kept in dark for 2 h to reach the saturated water vapour pressure and establish the adsorption-desorption equilibrium before the irradiation. A 300W Xe arc lamp (PLS-SXE 300C, Beijing Perfect light Co., Ltd.) was employed as the irradiation source. The energy output was measured to be 0.8 W cm−2 by a Thorlabs PM100 optical power and energy meter. The temperature of the reactor was kept at 298 K by an electronic fan. After certain reaction time, 1 mL sample gas was taken from the reactor with a syringe for timely analysis. The products were quantified by gas chromatography (GC 2014C, Shimadzu), which is equipped with a high-sensitivity thermal conductivity detector (TCD) and a flame ionization detector (FID). After the products effluents containing CO2, H2, O2, CO and CH4 were separated by the PN pre-column and 5A molecular sieve column, H2 and O2 will be quantified by the TCD. CO will be further transformed to CH4 by a methanation reactor. CH4 will be quantified by the FID. The detection limit of TCD is 0.004 µmol while that of FID is 0.002 µmol. The control experiments were performed by replacing CO2 with Ar under other reaction conditions identical. Isotope experiments were performed by gas chromatography-mass spectrometry (GC-MS, 7890B and 5977A, Agilent). As for the recycling experiment, after every reaction of two hours, the reactor was purged and then filled with pure reaction

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gas. The reactor was kept in dark for 2 h to establish the adsorption-desorption equilibrium before the irradiation for the following cycle. The products (O2, H2, CO and CH4) were measured directly by the evolved rate of the product within a certain time period under light irradiation by per gram of catalyst (Eqn. A), provided that no other products were detected by the analysis of gas chromatography. The unit of R is µmol g−1 h−1. The conversion of CO2 is calculated by the sum of the evolved rate of CO and CH4 (Eqn. B). The apparent quantum yield (AQY) of the photocatalyst was estimated using the Eqn. C-D. 𝑛(𝐻2, 𝑂2, 𝐶𝑂, 𝐶𝐻4) 𝑅 (𝐻2, 𝑂2, 𝐶𝑂, 𝐶𝐻4) = (𝐴) 𝑇𝑖𝑚𝑒 ∙ 𝑚 (𝐶𝑎𝑡𝑎𝑙𝑦𝑠𝑡𝑠) 𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 𝑜𝑓 𝐶𝑂2 = 𝑅 (𝐶𝑂) + 𝑅 (𝐶𝐻4) (𝐵) 𝑁(𝐻2, 𝐶𝑂, 𝐶𝐻4) = 6.022 × 1023 × 𝑛(𝐻2, 𝐶𝑂, 𝐶𝐻4) (𝐶) 2 𝑁 (𝐶𝑂)/8 𝑁(𝐶𝐻4) 𝐴𝑄𝑌 𝑓𝑜𝑟 (𝐶𝑂, 𝐶𝐻4) = × 100% (𝐷) 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 Details about the calculation of the “Number of incident photons” are given in the Appendix of Supporting Information.

AUTHOR INFORMATION Corresponding Author * [email protected]

Notes The authors declare no competing financial interest.

ASSOCIATED CONTENT Supporting Information. Additional experimental details, characterization and photoactivity results. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT The support from the National Natural Science Foundation of China (NSFC) (21872029, U1463204, 21173045), the Award Program for Minjiang Scholar Professorship, the Natural Science Foundation (NSF) of Fujian Province for Distinguished Young Investigator Rolling Grant (2017J07002), the Independent Research Project of State Key Laboratory of Photocatalysis on Energy and Environment (NO. 2014A05), the 1st Program of Fujian Province for Top Creative Young Talents, and the Program for Returned High-Level Overseas Chinese Scholars of Fujian province is gratefully acknowledged.

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ACS Catalysis

Table of Contents (TOC) Atomically dispersed Cu species attached mesoporous TiO2 spheres (mTiO2) towards gas-phase CO2 photoreduction with H2O to solar fuels, during which the dynamic evolution of Cu species has been monitored by in situ X-ray absorption spectra (XAS).

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