Oxygen vacancy generation and stabilization in CeO2-x by Cu

catalysts have been studied, such as TiO2. 2, ZnO3, g-C3N4. 4,. CeO2. 4-5, BiOCl6 et al. However, two main issues, the relatively low photo-catalytic ...
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Oxygen vacancy generation and stabilization in CeO2-x by Cuintroduction with improved CO2 photocatalytic reduction activity Min Wang, Meng Shen, Xixiong Jin, Jianjian Tian, Mengli Li, Yajun Zhou, Lingxia Zhang, Yongsheng Li, and Jianlin Shi ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03975 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 12, 2019

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Oxygen vacancy generation and stabilization in CeO2-x by Cuintroduction with improved CO2 photocatalytic reduction activity Min Wang,1,2,3 Meng Shen,1,2 Xixiong Jin,1,2 Jianjian Tian,1,2 Mengli Li,4 Yajun Zhou,3 Lingxia Zhang,1,2* Yongsheng Li,3 and Jianlin Shi1,2*. 1State

Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding-xi Road, Shanghai, 200050, P. R. China. E-mail: *[email protected], *[email protected] Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, P. R. China. 2

3School

of Materials Science and Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai, 200237, P. R. China. 4School

of Biology and Chemical Engineering, Jiaxing University, No. 56 South Yuexiu Road, Jiaxing, Zhejiang, 314001, P. R. China. ABSTRACT: Introducing O vacancies into the lattice of a semiconductor photocatalyst can alter its intrinsic electronic properties and band-gap, thus enhancing the visible light absorption, promoting the separation/transfer of photogenerated charge carriers, and resultantly elevating the photocatalytic activity of oxide semiconductors. Moreover, O vacancies can help adsorb and activate CO2 on photocatalyst surface, which, however, are prone to being filled by O atoms during the photo-reduction reaction. In this work, Cu was introduced to increase the O vacancy concentration in CeO2-x and promote the photocatalytic activity of CeO2-x. The sample Cu/CeO2-x-0.1 showed the highest photocatalytic activity with a CO yield of 8.25 μmol g-1 under 5 h irradiation, which is about 26 times that on CeO2-x. According to the analysis of Raman and XPS spectra, it has been evidenced that Cu introduction benefits the chemical stabilization of O vacancies in CeO2-x during photocatalytic CO2 reduction, which is responsible for the improved and sustained photocatalytic activity.

KEYWORDS. Photocatalysis, CeO2, CO2 reduction, Cu introduction, Oxygen vacancy.

INTRODUCTION The continuous consumption of conventional fossil fuels has induced serious energy crisis and environmental problems (i.e. global warming). The photo-catalytic reduction of CO2 with water into useful fuels was considered to be a clean and promising solution in reducing the pressure of energy crisis and global warming1. Nowadays, a variety of semiconductor photocatalysts have been studied, such as TiO22, ZnO3, g-C3N44, CeO24-5, BiOCl6 et al. However, two main issues, the relatively low photo-catalytic activity and stability, have extremely hindered the development and application of photo-catalysts. In order to enhance photocatalytic activity of semiconductors, several strategies have been developed, such as heteroatom doping, metal deposition, semiconductor composition, and so on. Recently, the surface defect engineering, especially oxygen vacancy

introduction, has attracted strong interest among researchers. Xie et al.6 synthesized gram-scale single-unitcell o-BiVO4 with abundant V vacancies on the surface, which exhibited excellent CO2 photo-reduction performance (methanol yield as high as 398.3 μmol g-1 h-1) and highly efficient and stable catalytic activity (in more than 96 h). Density functional theory (DFT) calculations uncovered that a new defect level was generated after V vacancies introduction and the concentration of holes also increased near Fermi level, which can increase the photo-absorption and electronic conductivity of BiVO4. Lee et al.2 found that CO2 was preferentially adsorbed at the O vacancy defects on TiO2 (1 1 0) surface by scanning tunneling microscopy (STM) investigation. And during the CO2 reduction process, one oxygen atom of CO2 filled the O vacancy and the concentration of surface O vacancies thus decreased. O vacancies on the surface can provide active sites and increase the CO2 adsorption energy, which makes CO2 molecules more easily adsorbed

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and activated on photocatalyst surface. Moreover, O vacancies are capable of generating a defect energy level, which may change the carriers’ transfer path, suppress the recombination of electron-hole and prolong the lifetime of carriers7. Zhu et al. introduced surface O vacancies on ZnO and BiPO4 by H2 reduction3, 8, which showed that O vacancies were able to broaden the valence band width and narrow the band gap of the photo-catalysts, thus efficiently improving the photo-catalytic performance and photocurrent intensity. As a typical n-type semiconductor, CeO2 has the advantages such as non-toxicity, stability against photoirradiation, and so on9. However, the major drawbacks like low electronic conductivity and wide band gap extremely impede its application in photocatalytic reaction10. The positions of its conduction and valence bands are located at -1.02 V and 1.53 V (versus NHE), respectively, which are suitable to photocatalyze CO2 reduction. However, the performance of photocatalytic CO2 reduction on pure CeO2 is extremely poor limited by its wide band gap and low light absorption. Heterogeneous atom doping is an effective strategy to narrow its band gap and improve its photo-reductive activity11. For instance, Cr or Fe doped mesoporous CeO2 containing enhanced concentrations of Ce3+ and O vacancies, exhibited improved visible light absorbance and elevated photocatalytic CO2 reduction to CO and CH412. Guo et al. investigated the effects of doping Pd, Ru, and Cu on CO2 adsorption/activation, CO2 reduction and product selectivity on CeO2(1 1 1) by periodic DFT calculations.13 The results demonstrate that Cu is more helpful for improving CO2 adsorption, especially on Cu/CeO2(1 1 1). Heterogeneous loading is also a common method to improve the photocatalytic CO2 reduction activity. Dai et al reported the composites of CeO2/Bi2MoO6, which can mainly produce methanol and ethanol through the photo-reduction of CO2 saturated in water without sacrificial agent under light (λ ≥ 420 nm) irradiation5. Li et al. synthesized mesostructured CeO2/gC3N4 nanocomposite, which could photo-reduce CO2 to 0.59 μmol CO and 0.694 μmol CH4 in 5 h (50 mg photocatalyst)4. The migration and accumulation of photo-exicited electrons from g-C3N4 to CeO2 induced the generation of Ce3+ and O vacancies, which was beneficial to the performance enhancement of CO2 reduction. Therefore, introducing O vacancies in CeO2 would be a feasible approach to improve its performance of CO2 photo-reduction by the narrowed band gap, promoted CO2 adsorption and prolonged carriers’ lifetime. However, the generated O vacancies would be gradually filled or lost during the photo-reduction process resulting in declined activity. Hence, it is of great importance to introduce and stabilize O vacancies into photocatalysts for achieving and maintaining its high CO2 reductive activity. Unfortunately, few studies on the O vacancy stabilization have been reported till now. In this work, we introduce Cu into CeO2 for generating O vacancies (CeO2x), and have found that the Cu-introduced CeO2 showed markedly enhanced activity and stability in CO2

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photocatalytic reduction. The Cu-addition in CeO2-x can not only increase the concentration of O vacancies, but also protect O vacancies from being occupied. EXPERIMENTAL Preparation of CeO2-x nanoparticles (NPs) The CeO2 NPs were prepared by a hydrothermal method as described in a related literature14. Typically, 0.1 g of Ce(NO3)3 ·6H2O was dissolved in 15 mL of deionized water. 9 mL of oleylamine was added in the above solution. Then the solution was transferred to a Teflon bottle, which then tightly sealed in a stainless steel vessel autoclave, heated and kept at 423 K for 24 h in an oven. After hydrothermal reaction, the obtained product was washed and centrifuged using hot ethanol to remove the oleylamine. Subsequently, the product was further washed with deionized water for several times and freezedried at last. Thus the sample CeO2 was obtained. A certain amount of CeO2 was transferred into a quartz boat and heated up in a tube furnace to 423 K at a ramp of 5 K min-1 and kept at this temperature for 1 h under 5 vol% H2/Ar flow. The obtained samples were labeled as CeO2-x. Preparation of Cu/CeO2-x NPs The Cu/CeO2-x NPs were also prepared by the similar hydrothermal method as CeO2-x NPs. Typically, 0.1 g of Ce(NO3)3 · 6H2O and different amount of Cu(NO3)2 · 5H2O were dissolved in 15 mL of deionized water and treated under the same hydrothermal and thermalreduction conditions as pure CeO2-x NPs. The obtained samples were denoted as Cu/CeO2-x-y, here y represents the mole ratio of Cu(NO3)2·5H2O to Ce(NO3)3·6H2O. Characterizations Rigaku D/Max 2200PC X-ray diffractmeter was used to record X-ray diffraction patterns (XRD) of the samples. The measurement was operated at room temperature under the Cu Kα radiation with scanning rate of 4°min-1. Scanning electron microscopy (SEM) images were collected through a field emission Hitachi S-4800. A JEM2100F field emission TEM was operated at 200 kV to obtain the images of Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) patterns. Ultraviolet visible (UV-Vis) absorption spectra were recorded from 800 to 200 nm by a UV-3101 PC Shimadzu spectroscope (BaSO4 as the reference standard material). With the excitation wavelength of 330 nm, Hitachi F-4500 was used to analyze the photoluminescence (PL) spectra of the samples. X-ray photoelectron spectroscopy (XPS) was obtained on a Thermo Scientific ESCALAB 250 spectrometer with multichannel detector, Al Kα radiation as excitation source and C1s at 284.6 eV as a signal calibrating standard of binding energy values. Time resolved fluorescence spectra were test on the machine of Edinburgh Instruments FLS920 spectrometer under 420 nm excitation of deuterium lamp. DXR Raman microscope was used to obtain Raman spectra under the excitation (532 nm). In-situ Fourier transform infrared

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(FT-IR) spectra were detected in N2 and CO2 by MCT detector (Nicolet iS10). The carbon isotope analysis is performed on a high resolution Nu Evolution Stable Isotope Ratio Mass Sepctrometer with ion source emission current of 2.6 mA. Electrochemical measurements Electrochemical measurements were conducted on a CHI660A electrochemical workstation with a standard three-electrode cell. Fluorine-doped Tin Oxide (FTO) deposited with photocatalysts, graphite carbon paper and Ag/AgCl act as working, counter and reference electrodes, respectively. The electrolyte was 0.2M Na2SO4 aqueous solution. The preparation process of working electrodes by electrophoretic deposition as follows：sample powder (20 mg) and iodine (40 mg) were mixed and well milled in an agate mortar, then well-dispersed in acetone (30 mL). Thus the plating solution was obtained. At a potentiostat bias of 10 V for 10 min, a thin film of the sample was uniformly deposited on FTO and then calcined in a drying oven at 423 K for 2 h. During the electrochemical measurement, the coated area of all the samples was controlled at 1 cm2. Nyquist plots were tested in dark with the bias voltage of -0.4 V versus Ag/AgCl. With irradiation from a 300 W of Xenon-arc lamp, the transient photocurrent densities were obtained at 0.3 V versus Ag/AgCl. Photocatalytic activity evaluation The light source was a 300 W of Xenon-arc lamp from Aulight CEL-HX, Beijing. The gas products (CO, CH4) were analyzed by GC-2060 gas chromatograph (FID detector), which is equipped with a 5A molecular column and a TDX-01 packed column. The photocatalysis test was conducted typically as follows: 50 mg of photo-catalyst was uniformly dispersed on a 2.5 cm×2.5 cm glass, treated at 423 K for 2 h in air and put at the bottom of a 500 mL sealed glass reactor equipped with an optical quartz window at the top. The temperature of reactor was kept at 298 K by cooling water circulation. To exclude the possible influence of contaminants, the sealed reactor was full of N2 and under the irradiation of Xe lamp for 2 h. Subsequently, CO2 (99.99% of purity) passed through a water bubbler was filled in the reactor and stabilized for 30 min,. During the photoreaction, 1 mL of sample gas was continually extracted from the reactor per hour and analyzed on GC based on external standard method. Before each (or cycle) activity test, the photocatalyst was first heat-treated at 423 K for 2 h, removing the organic impurities and adsorbed carbon species on catalysts’ surface. RESULTS AND DISCUSSION Improved photocatalytic activity of Cu/CeO2-x in CO2 reduction reaction In this work, the main gas product of CO2 reduction was CO. Trace CH4 was also detected. The unit of CO yield was μmol g-1. At first, the similar catalytic experiments were carried out under the same conditions without light irradiation or photocatalysts. No products were detected, hinting that the product of CO was

derived from CO2 photoreduction. To eliminate the impact of containments, the carbon isotope analysis (13C) was performed and the result is shown in Figure 1. The relative content of 13CO is about 93%, which indicates that the influence of containments could be negligible. And there was trace 13CH4 having been detected, further implying the main product on Cu/CeO2-x-0.1 is CO. As displayed in

Figure 1 The 13C isotope analysis spectrum of the products on Cu/CeO2-x after 2 h Xenon-light irradiation under humid CO2 atmosphere. Figure 2a, little CO was detected on pure CeO2, and trace CO was produced on CeO2-x. On Cu/CeO2-x, the CO is produced remarkably, indicating that Cu-introduction contributes greatly in CO2 photoreduction reaction. With the increase of Cu content, the CO2 photocatalytic activity decreased. Among the as-obtained catalysts, the sample Cu/CeO2-x-0.1 shows the best CO2 photoreduction activity and the CO yield is about 8.25 μmol g-1 under 5 h irradiation, which is about 26 times higher than that on CeO2-x (Figure 2a). As is shown in Figure S1, the CO generation decreases in per hour irradiation, which possibly results from the untimely conversion of intermediate or delayed desorption of CO from the active sites, resulting in the gradual deactivation of the active sites. After a mild heat treatment at 448 K for 2 h in N2 flowing, the adsorbed CO or intermediates on the active sites was removed and the photocatalytic activity of Cu/CeO2-x-0.1 was recovered. And after several cycles with heat treating after each cycle, Cu/CeO2-x-0.1 still maintained the original high activity, demonstrating that the as-prepared catalysts have good chemical stability (Figure 2b). To understand the relationship between the O vacancies and catalytic stability of Cu/CeO2-x in more detail, the photocatalytic activity of Cu/CeO2-x-0.1 was retested for five cycles, during which the catalyst was just swept by N2 gas at room temperature after each cycle. As displayed in Figure 2c, the yield of CO shows slight decline, implying that the adsorbents on the catalyst surface is the main reason of the decreased CO yield by covering catalytic active sites. The result clearly illustrates that the introduction of Cu can remarkably enhance the photocatalytic activity of CeO2-x in CO2 reduction and also helps to maintain the photocatalytic stability of the catalyst.

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Chemical and physical characterizations XRD patterns of CeO2, CeO2-x, Cu/CeO2, and Cu/CeO2-0.1 are shown in Figure 3, S2 and S3. The XRD peaks of x CeO2 are consistent with the Powder Diffraction Standards data (JCPDS Card No. 043-1002), indicating its fluorite cubic phase. The sample Cu/CeO2 obtained after hydrothermal treatment is composed of separate Cu2O and CeO2 according to the XRD peak locations. Then Cu2O was reduced to Cu0 after H2 reduction at 423 K

Figure 2 Time-dependent CO evolution over CeO2, CeO2-x and Cu/CeO2-x (a), Cycle tests of photocatalytic CO evolution over Cu/CeO2-x-0.1 with mild heat treatment after each cycle (b) and with N2 sweeping at room temperature after each cycle (c).

and 4b). HRTEM image gives the lattice spacing values of 0.28, 0.31 and 0.18 nm, which are indexed to the exposed CeO2 (1 0 0), CeO2 (1 1 1), and (2 0 0) facets of Cu, respectively. The SAED pattern shows clear and highly regular diffraction spots and diffraction rings, indicating the polycrystalline feature of the as-obtained samples. According to the elements mapping of Cu/CeO2-x-0.1 (Figure S11), Cu is uniformly dispersed in the catalyst.

Figure 3 XRD patterns of CeO2-x and Cu/CeO2-x-0.1.

Figure 4 TEM images of CeO2-x (a) and Cu/CeO2-x-0.1 (b),

HRTEM image (c) and SAED pattern (d) of Cu/CeO2-x-0.1.

(denoted as Cu/CeO2-x). TEM iamges show that CeO2-x and Cu/CeO2-x composites are both in the morphology of nanoparticles of below 50 nm in particle size (Figure 4a

XPS spectra were collected to investigate the chemical composition and chemical state on the surface of the asobtained samples. The survey spectrum of Cu/CeO2-x-0.1 (Figure S3) shows that the surface of Cu/CeO2-x-0.1 has Cu, Ce and O atoms. The Ce 3d spectra are rather complicated as shown in Figure 5a. The peaks located at 916.9, 907.4, 901.4 eV and 898.4, 889.1 and 882.7 eV (spinorbit splitting peaks) are corresponding to Ce4+ final states: 3d94f1, 3d94f2 and 3d94f0, respectively15. There are two peaks at 903.4 and 899.4 eV, which attributing to Ce 3d5/2 and 3d3/2 of Ce3+ with 3d94f1 and 3d94f2 final states, respectively5, indicating that Ce species exist with mixed valences of Ce3+ and Ce4+ in Cu/CeO2-x-0.1. The Cu 2p XPS spectrum of Cu/CeO2-x-0.1 is shown in Figure 5b, which contains two peaks at 932.4 and 952.6 eV assigned to Cu 2p3/2 and 2p1/2 of Cu0 (or Cu+), respectively. Furthermore, the O 1s spectra of the samples (Figure 6) show two peaks (529.5, 531.6 eV) corresponding to the O atoms in CeO2 lattice and the adsorbed oxygen on its surface. After Ar ion beam etching, the contents of lattice oxygen and adsorbed oxygen on the subsurface of CeO2 and Cu/CeO2x-0.1 are found to be different from those on their surface. As displayed in Figure 6 and Table S1, the content of adsorbed oxygen on their subsurface is lower than that on their surface, which suggests there are more oxygen vacancies on their surface. And as shown in Figure S9 and Table S2, the atom content of Cu is stable, but the ratio of Cu/(Ce+Cu) decreases with Ar ion etching time.

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Combining the results of XRD and TEM, Cu may not only for clusters on ceria surface, but also been doped in ceria lattice. And the Cu atom content on its surface is higher than that on its subsurface. Compared with CeO2, the content of adsorbed oxygen is much higher on the surface and subsurface of Cu/CeO2-x-0.1, implying that Cu introduction has increased the concentration of O vacancies in CeO2-x via charge transfer and activated adsorbed oxygen16. This phenomenon is similar with Wang’s result12b. From the XPS results, O vacancies are verified to exist on the surface of CeO2-x and Cu/CeO2-x-0.1 but after the introduction of Cu species (Cu0 or Cu+), the change of O vacancy amount cannot been verified. To further confirm the valences of Cu species and the O vacancies, Raman spectrum of Cu/CeO2-x-0.1 was collected and shown in Figure 5c. No peaks appeared at 220 and 290 cm-1 corresponding to Cu2O and CuO, suggesting that Cu exists as Cu 17. In addition, a hump peak is observed in the Raman spectrum, which can be fitted into two peaks. According to

Figure 5 High-resolution XPS spectra of Ce 3d (a), Cu 2p (b) on the surface of Cu/CeO2-x-0.1 and CeO2-x; Raman spectra of Cu/CeO2-x-0.1 and CeO2-x (inset: the fitted Raman spectrum of Cu/CeO2-x-0.1)(c).

Figure 6 High-resolution XPS spectra of O 1s on the surface and subsurface of CeO2-x(c) and Cu/CeO2-x-0.1(b).

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Figure 7 UV-Vis absorption spectra (a), time resolved fluorescence spectra under 420 nm excitation (b), EIS Nyquist plots of the as-obtained samples in dark (c), and transient photocurrent density of CeO2, CeO2-x and Cu/CeO2-x-0.1 (d).

Figure 8 O 2p XPS spectra (a) and Raman spectra (b) of Cu/CeO2-x-0.1 before and after photocatalytic CO2 reduction reaction.

related literatures22, in the UV Raman spectrum, the peaks at 459, 585 and 1179 cm-1 are corresponding to F2g symmetry mode of fluorite phase, defect-induced mode (D band) and second-order longitudinal optical mode (2LO band) of CeO2, respectively. And these peaks shift to 464 cm, 590 and 1184 cm-1 in the visible Raman spectra. The bands at 458 cm-1 and 592 cm-1 are respectively assigned to F2g Raman active mode of cubic CeO2 and O vacancies18. Compared to Raman spectra of CeO2-x, the intensity of peak at 458 cm-1 decreases and the peak at 592 cm-1 is intensified after the introduction of Cu, suggesting that the Cu incorporation has led to the increased disorder of CeO2 and more O vacancies has been produced in CeO2. The half width at half-maximum (HWHM) of the peak at 458 cm-1 is about 30.2 cm-1.

According to the literatures18a, 19, the O vacancy concentration on the surface of Cu/CeO2-x-0.1 can be calculated to be about 4.18×1021 cm-3. UV-Vis diffuse absorbance spectra were recorded and displayed in Figure 7a. Compared with CeO2-x, all the Cu/CeO2-x samples show an enhanced and red-shifted absorption. Among them, the sample Cu/CeO2-x-0.1 shows the strongest UV-Visible light absorption. The result indicates that the introduction of Cu can greatly improve the photo-absorption within UV-Visible spectrum, which favors the Xenon-light-induced photo-catalytic activity. Moreover, there is a shoulder peak at about 430 nm, which may be consistent with the defect energy level. The shoulder peak became broadened and intensified after the addition of Cu, implying the increased defect

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concentration by Cu incorporation in CeO2. Photoluminescence (PL) spectra are displayed in Figure S7. All the catalysts show a broad peak about 460 nm, which extends to 700 nm and a small peak at about 560 nm. The emission intensity might associate with the recombination of photo-generated electrons and holes while its emission photo energy might correspond to the band-gap. The sample Cu/CeO2-x-0.1 shows the lowest PL intensity, which indicates the largely facilitated charge transfer between CeO2 and Cu species and the effectively suppressed electron-hole recombination. This result was further confirmed by the transient photoluminescence lifetime analysis under 420 nm excitation of deuterium lamp (Figure 7b and Table S3). The average carriers’ lifetime in CeO2-x is about 74.87 ns and that in Cu/CeO2-x is about 266.04 ns, indicating that Cu can extremely prolong the carriers’ lifetime in CeO2-x. These findings are in well accordance with the catalytic activity as shown above. EIS Nyquist plots of the as-obtained samples in dark demonstrate that Cu/CeO2-x-0.1 is much higher conductive than CeO2 and CeO2-x (Figure 7c), suggesting that it can efficiently promote photo-excited carriers’ separation and greatly accelerate the transmission of interfacial charger. The photocurrent responses of CeO2-x and Cu/CeO2-x-0.1 were further investigated for 10 on-off cycles. An instantaneous and uniform photocurrent response can be observed accompanying the light on and off process (Figure 7d). Cu/CeO2-x-0.1 shows much higher photocurrent intensity than CeO2 and CeO2-x, indicating its most efficient electron-hole separation /transfer and highest photoreduction activity. These photoelectrochemical results are well in accordance with the catalytic activity test. In order to further certify the role of Cu in stabilizing O vacancies, the O 2p XPS and Raman spectra were used to check the changes of O vacancies. O 2p XPS spectra of CeO2-x and Cu/CeO2-x-0.1 before and after the photocatalytic CO2 reduction reactions are shown in Figure 8a. The peak of the adsorbed O (533.1 eV) on Cu/CeO2-x-0.1 has kept unchanged after the reaction (irradiation for 5 h in wet CO2 atmosphere). Raman shift of O vacancies has changed a little after 5 h CO2 photoreduction process, which indicates that O vacancies are basically stable dur ing the process of photocatalysis (Figure 8b and S6). The half width at half-maximum (HWHM) of the peak at 458 cm-1 is narrowed to about 24.7 cm-1. According to the same calculation method used above, the calculated O vacancy concentration on the surface of used Cu/CeO2-x-0.1 is about 3.40 × 1021 cm-3, which is still in the same magnitude with that on the fresh sample surface. Based on the above all evidences, we can conclude that Cu presence benefits the stabilization of O vacancies in CeO2-x. Catalytic mechanism of CO2 photocatalytic reduction on Cu/CeO2-x

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Figure 9 The scheme of the possible mechanism of CO2 reduction on Cu/CeO2-x (a) and the FT-IR spectra of CeO2-x and Cu/CeO2-x-0.1 swept by N2 after CO2 and H2O adsorption (the spectra of the samples swept by N2 for 1 h as the background) Based on the electron structure analysis on the catalysts, a possible photocatalytic mechanism of photocatalytic CO2reduction on Cu/CeO2-x was proposed and sketched In Figure 9a. Under the irradiation, electrons and holes in CeO2 are generated and separated. Electrons transfer to conduction band (CB) and holes stay on valence band (VB). Then the excited electrons transfer to the defect level created by O vacancies and reduce CO2 to CO. The excited electrons successively transfer to the O vacancies, which may keep Ce3+ and O vacancies stable. The CO2 molecules are adsorbed and activated and accept the trapped electrons. Then the accepted electrons will mainly occupy CO2 molecules’ lowest unoccupied molecular orbital (LUMO), which is characterized by anti-bonding. As electrons transfer to CO2 molecules, the O-C-O stretching is weakened and subsequently broken to generate CO. And the separated holes have two possible paths: one possible way is to transfer to H2O and produce H* and O2, and the partial H* will yield slight CH4 with CO2; the other possible way is to oxidize Ce3+ to Ce4+.23 But these speculations are still very difficult to be proved by experimental methods. To further understand the reaction path of CO2 photocatalytic reduction on the catalysts, in-situ FT-IR spectroscopy was used to investigate the adsorption, activation and conversion process of CO2 on Cu/CeO2-x and CeO2-x. The FT-IR spectra of CeO2-x and Cu/CeO2-x-0.1 swept by N2 after CO2 and H2O adsorption in dark were collected and shown in Figure 9b, which indicates that several carbonates have formed on the samples’ surface after the humid CO2 adsorption. The peak at about 1200 cm-1 corresponds to hydrocarboxylate (HCO3-)24, and those at about 1367 and 1280 cm-1 are corresponding to monodentate carbonate (b-CO32-)25. The others at about 1470 cm-1 and 2000-2350 cm-1 are corresponding to monodentate carbonate (m-CO32-)5, 26 and the asymmetric stretching of CO227, respectively. Additionally, the peaks at about 1963, 1670 and 1640 cm-1 are assigned to bending vibration of carbonxylate (CO2-)21, 28. The existence of CO32- can be attributed to the formation of carbonic acid or/and the adsorbed CO2 on the catalyst surface20. Clearly, compared to CeO2-x, different CO2- species and higher

intensity of the CO2- signals can be found on Cu/CeO2-x0.1. Moreover, the signals of carbonates (b-CO32-, m-CO32-) on the surface of Cu/CeO2-x-0.1 are much more intensive than on CeO2-x. These results imply that the existence of Cu and O vacancies have not only provided more active sites for CO2 adsorption/activation, but also altered the adsorption/activation modes of CO2. Interestingly, on Cu/CeO2-x-0.1, the intermediate HCO3- exhibits a band with high intensity, suggesting the introduction of Cu and the abundant O vacancies further facilitate the reaction of CO2 with H2O21. It should also be noted that the absorption peaks of CO2-, m-CO32- and b-CO32- on Cu/CeO2-x-0.1 have shifted to higher wavenumbers, owing to the intensified surface basicity after Cu introduction.21 Figure 10 shows the in-situ FT-IR spectra of CeO2-x and Cu/CeO2-x-0.1 collected every 5 min under irradiation in CO2 and H2O. Clearly, several positive and negative peaks can be found during the photocatalytic reaction, which correspond to the species formation and consumption, respectively. The negative peak at about 2000-2350 cm-1 is corresponding to the asymmetric stretching of CO229, and those at about 1963, 1670 and 1640 are corresponding to the bending vibration of carbonxylate (CO2-)26,28,29. The positive peaks at about 1604, 1485, 1367 and 1186 cm-1 are attributed to formate (HCOO-), monodentate carbonate (m-CO32-), bidentate carbonate (b-CO32-) and 24,25,28 hydrocarboxylate (HCO3 ), respectively . Almost all the bands of Cu/CeO2-x-0.1 show much stronger intensity and moresignificant changes than those of CeO2-x. These results suggest that during the photocatalytic reduction, CO2 and CO2- were consumed and the intermediates such as carbonate species, HCO3- and HCOO- were generated more quickly on Cu/CeO2-x than CeO2-x. Notably, a new peak at 1735 cm-1 can be observed and corresponding to chelating-bridged carbonate (c-CO32-), which implies that the increased amounts of Ce3+ and O vacancies will provide more adsorption sites for CO2 and influences the chemical bonding of adsorbed CO2 molecules28. Moreover, a clear signal of CO can be found on Cu/CeO2which x-0.1,

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Figure 10, In-situ FT-IR spectra of CeO2-x (a: separated, b: overlapped) and Cu/CeO2-x-0.1(c: separated, d: overlapped) collected every 5 min under the irradiation in the presences of CO2 and H2O (The spectra collected at 0 min of irradiation in CO2 and H2O was used as background. In b and d, the upward and downward arrows indicate the formation and consumption of intermediate species, respectively.)

cannot be observed on CeO2-x, indicating the accelerated conversion of the intermediates to the final product, CO, and the increased yield of CO on catalyst’s surface. And the CO peak is located at about 2160 cm-1, which is in between the absorption peak at about 2127 cm-1 of COCe3+ and the one at about 2110/2092 cm-1 of CO-Cu+/ Cu0.5, 25,27,30,31 Thus CO is supposed to be generated at the interface of Cu and CeO2-x. Therefore, the introduction of Cu has greatly increased amount of active sites and elevated CO2 conversion rate, i.e. enhanced the photocatlaytic activity of the catalyst on CO2 reduction. Compared with the in-situ FT-IR spectra of CeO2-x and Cu/CeO2-x-0.1, the peak intensity of carbonates and HCOO- all increased and accumulated on the surface of CeO2-x and Cu/CeO2-x-0.1 during the photocatalytic reaction, suggesting the possible reaction pathway on these two samples may be similar (CO2→CO2-→HCOO-→ CO). In contrast, the negative peaks of CO2- on the surface of Cu/CeO2-x-0.1 changed more significantly than that on the surface of CeO2-x, indicating that Cu/CeO2-x0.1 might have provided more active sites or lowered the adsorption energy of CO2 thus make CO2 adsorption and reduction much easier. According to the different absorption bands of CO2-, the different adsorption modes of CO2 on the catalysts’ surface is believed to be crucial in CO2 reduction.21,28,31 However, the bands belonging to carbonates on the surface of Cu/CeO2-x-0.1 are also of enhanced intensity, manifesting the continuous

accumulation of carbonates on its surface and the gradual deactivation of the catalyst. CONCLUSION In summary, Cu has been introduced to generate and stabilize O vacancy in CeO2, resulting in greatly enhanced photocatalytic activity of CO2 reduction reaction. Cu introduction favors the photo-absorption of the catalyst within UV-Visible light spectrum, promotes the electronhole separation/transfer, prolongs the carriers’ lifetime, and eventually provides more active sites and alters the configurations of the adsorbed CO2 on CeO2-x. Mixed valences of Ce3+ and Ce4+ are formed in the Cu/CeO2-x nanocomposites and Cu exists as Cu0, accompanying the generation of surface O vacancies due to the Cu introduction. The sample Cu/CeO2-x-0.1 shows the highest photocatalytic activity with a CO yield of about 8.25 μmol g-1 during 5 h of Xe-light irradiation, which is about 26 times higher than that on CeO2-x. The O vacancies in Cu/CeO2-x keep stable after the CO2 photocatalytic reaction owing to the Cu presence, which results in the enhanced photocatalytic stability of Cu/CeO2-x in CO2 reduction reaction.

ASSOCIATED CONTENT Supporting Information. Supplementary additional characterization data and results for CeO2-x and Cu/CeO2-x are shown in Supporting

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Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]; [email protected]

ACKNOWLEDGMENT We acknowledge the financial support from the National Natural Science Foundation of China (51872317, 21835007).

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