Single-Atomic Cu with Multiple Oxygen Vacancies on Ceria for

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Single-Atomic Cu with Multiple Oxygen Vacancies on Ceria for Electrocatalytic CO2 Reduction to CH4 Yifei Wang, Zheng Chen, Peng Han, Yonghua Du, Zhengxiang Gu, Xin Xu, and Gengfeng Zheng ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01014 • Publication Date (Web): 22 Jun 2018 Downloaded from http://pubs.acs.org on June 23, 2018

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Single-Atomic Cu with Multiple Oxygen Vacancies on Ceria for Electrocatalytic CO2 Reduction to CH4 Yifei Wang,1,† Zheng Chen,1,† Peng Han,1,† Yonghua Du,2 Zhengxiang Gu,1 Xin Xu,1,* and Gengfeng Zheng1,* 1

Department of Chemistry, Laboratory of Advanced Materials, Shanghai Key Laboratory of

Molecular Catalysis and Innovative Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, Shanghai, 200438, China. 2

Institute of Chemical and Engineering Sciences, A*STAR, 1 Pesek Road, Jurong Island,

627833, Singapore *Address correspondence to: [email protected] (X.X.) and [email protected] (G.Z.) † Y.W., Z.C. and P.H. contributed equally to this work.

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Abstract The electrocatalytic reduction of CO2 into value-added chemicals such as hydrocarbons is potential for relieving the fuel energy need and environmental hazards, while the accurate tuning of electrocatalysts at the ultimate single-atomic level remains extremely challenging. In this work, we demonstrate an atomic design of multiple oxygen vacancybound, single-atomic Cu-substituted CeO2 to optimize the CO2 electrocatalytic reduction to CH4. We carried out theoretical calculations to predict that the single-atomic Cu substitution in CeO2(110) surface can stably enrich up to three oxygen vacancies around each Cu site, yielding a highly effective catalytic center for CO2 adsorption and activation. This theoretical prediction is well consistent with our controlled synthesis of the Cu-doped, mesoporous CeO2 nanorods. Structural characterizations indicate that the low concentration (< 5%) Cu species in CeO2 nanorods are highly dispersed at single-atomic level with an unconventionally low coordination number ~ 5, suggesting the direct association of 3 oxygen vacancies with each Cu ion on surfaces. This multiple oxygen vacancy-bound, single atomic Cu-substituted CeO2 enables an excellent electrocatalytic selectivity in reducing CO2 to methane with a faradaic efficiency as high as 58%, suggesting strong capabilities of rational design of electrocatalyst active centers for boosting activity and selectivity.

Keywords: CO2 reduction; electrocatalyst; CeO2; copper; oxygen vacancy


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Introduction

The continuous acceleration of carbon dioxide (CO2) emission has been driving a global concern for imminent climatic hazards.[1] The electrocatalytic conversion of CO2 into value-added chemicals or fuels, ideally via a clean and economic approach,[2] can lead to significant advances for both environmental merits and energy utilization infrastructures.[3-5] Copper (Cu) based materials are the only known group of heterogeneous catalysts that allow room-temperature electrocatalytic CO2 reduction (ECR) into not only 2-electron transferred products (carbon monoxide[5-8] or formic acid[7]), but also deep reduction products (DRP, such as methane,[9-12] ethylene,[5,12,13] and ethanol[14,15]), with relatively high faradaic efficiencies (FEs).[4] Substantial research efforts have been dedicating to improve the selectivity of Cu-based catalysts by nanostructuring Cu and its derivatives (e.g. copper oxides),[10,11,13] or incorporating Cu with other metals[14,16-18] or oxides.[19] For instance, Yang and coworkers reported that the ECR selectivity was sensitive to the Cu nanowire morphologies, among which the 5-fold twinned copper nanowires exhibited the highest selectivity toward CH4 (FE ~ 55%) compared to other hydrocarbons.[10] The Kanan group reported an oxide-derived approach, by which thick Cu2O film was in situ reduced to Cu during the ECR process, exhibiting higher stability and selectivity than bulk Cu electrodes.[13] Kenis et al. discovered that ordered Cu-Pd bimetallic catalyst presented better selectivity to C1 and C2 products than disordered ones.[14] Nonetheless, Cu atoms/ions are susceptible to oxidation or aggregation,[20] making the precise control of Cu active sites highly challenging. Ceria (CeO2) is known to generate strong metal-support interactions[21] and enhance the dispersion of loading metals even to atomic levels.[22] The metal-loaded CeO2 composites have been used in high temperature catalysis of CO oxidation,[23] CO2 reduction[24] and hydrogenation[25] with high selectivity, and have also recently been investigated for


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electrocatalytic CO2 reduction.[26] For instance, Bao and co-workers reported that the interface between Au or Ag nanoparticles and CeO2 nanoparticles enhanced the CO2 adsorption and activation, and thus increased the FE of CO production.[26] In spite of these progresses, the ultimate control of Cu-based electrocatalysts at the single atomic/ionic level, which can enable high selectivity of certain deep reduction products, has yet to be realized.

Results and Discussion

To achieve this goal, we first carried out a theoretical prediction of the catalyst structure at the atomic level. For an ECR process, one of the rate-determining steps is known to be the adsorption of CO2 molecules on catalyst surfaces, followed by obtaining an electron to form CO2·–.[27] Previous studies have been conducted on different facets of ceria and shown that CeO2 is capable of generating oxygen vacancies with low formation energy on its (110) surface.[28,29] Here we propose that the Cu-substituted CeO2 can enable a highly cooperative effect for enriching oxygen vacancies toward CO2 adsorption/activation. Firstprinciple calculations were carried out to investigate such an effect,[30,31] based on a Cu-ion substituted CeO2 (110) surface (Fig. 1, please see detailed Computational Description in the Supporting Information). The generation of the first oxygen vacancy (VO) is spontaneous,[32] as one pair of Ce4+– O2- on the (110) surface is replaced by a pair of Cu2+– VO to maintain the charge balance (Fig. 1a). Interestingly, the formation energies are found to be favorable to generate the second and the third Vo’s on the Cu-doped CeO2(110) surfaces. Fig. 1b and 1c depict the most stable structures, where the as-formed second and third VO’s prefer to be the nearest neighbors to the Cu site, respectively. Other structures with different oxygen vacancy numbers were also calculated and displayed in Fig. S1 and Fig. S2. Further increasing the VO number to 4 around the Cu site is found to destabilize this structure significantly. Thus, Cu-


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doping on CeO2 favors the formation of single-atomic Cu sites with an unconventionally high Vo number of 3. In addition, charge analysis suggests that the Cu site is reduced from a Cu2+ with two Vo’s to a Cu1+ (nominally) with three Vo’s. Aggregation of Cu atoms/ions or changing the VO numbers around this Cu site would destabilize this whole structure. In order to reveal the ECR catalytic activity of this Cu-substituted CeO2, the energy and the equilibrium state of the CO2 adsorption and activation were further calculated. The Cu sites associated with two and three Vo’s are liable to the CO2 adsorption. While the former possesses a CO2 adsorption energy (Ead) of –0.54 eV, the latter has Ead = –0.39 eV. Notably, the adsorbed CO2 on the Cu site with three Vo’s exhibits a bended structure (Fig. 1e), suggesting efficient activation toward CO2·–. In contrast, CO2 adsorbed on the Cu site with one or two Vo’s (Fig. 1d and Fig. S3), as well as the undoped CeO2, still presents its original linear molecular structure. If CO2 were forced to bend over the Cu site with one or two Vo’s (e.g. on CuCex-1O2x-2), an unstable state would occur with a calculated positive adsorption energy of 0.13 eV (Fig. S3g). Thus, it can be concluded that this 3 VO-bound Cu site should enable a highly efficient catalytic center for CO2 adsorption and activation, due to its unconventionally high associated Vo number and its reduced oxidation state. Enlightened by this theoretical model, we then conducted experimental design of synthesizing the aforementioned Cu-doped CeO2 structure. CeO2 nanorods with abundant mesopores were first hydrothermally synthesized, followed by wet impregnation of Cudoping in a low concentration of Cu2+ solution (Experimental Section in the Supporting Information). N2 adsorption isotherms of both undoped and Cu-doped CeO2 nanorods indicate the existence of mesopores (Fig. 2a). A high Brunner−Emmet−Teller (BET) surface area of 86.7 m2·g-1 was obtained for the undoped CeO2, allowing for efficient doping of Cu ions in the CeO2 surface, which is designated as Cu-CeO2-x% (where x% represents the percentage of Cu/Ce in the composite and was characterized by inductively coupled plasma-


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atomic emission spectrometry, Table S1). The Cu-doped CeO2 nanorods were then annealed in 5% H2 and Ar mixture to generate abundant VO’s. The annealing temperature was carefully controlled to promote VO migration and redistribution around Cu sites, but also to avoid aggregation of Cu species.[20] X-ray diffraction (XRD) patterns of all Cu-CeO2 nanorod samples (with Cu less than 10%) do not show any peaks of Cu or CuO (Fig. 2b, red and blue curves; and Fig. S4), suggesting highly dispersed Cu species. The Cu agglomeration signal starts to appear for samples with Cu percentage around and higher than 10% (Fig. 2b inset, purple curve with a small peak at 2θ ~ 43°). The diffraction peaks are all indexed to a fluorite structure of CeO2 (JCPDS No. 34-0394). The absence of CeO2(100) and (110) planes in the XRD spectra is also consistent with the standard PDF pattern of CeO2, due to the low intensity of (110) signal in XRD measurement. As the (110) and (220) planes are parallel to each other and belong to the same family of planes,[33] our theoretical models on CeO2(110) can well represent the reactivity of the CeO2(220) planes. Transmission electron microscopy (TEM, Fig. 2c) images show that most of the Cu-CeO2 nanorods are about 20 nm wide and 50–100 nm long, with abundant mesopores inside the rods. High-resolution TEM (HRTEM, Fig. 2d) images show the lattice fringes of 0.31, 0.28 and 0.19 nm, corresponding to the (111), (200) and (220) planes of CeO2, respectively, in good accord with the XRD results. Both the HRTEM and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images (Fig. 2e) reveal that the samples (with Cu < 5%) do not present lattice distortion or Cu aggregation, confirming the highly dispersive feature of Cu species. Energy dispersive spectroscopy (EDS) mapping images (Fig. S5) also show that Cu is well distributed in CeO2 nanorods without aggregation. X-ray photoelectron spectroscopy (XPS) was carried out to characterize the valance states of Ce and Cu. The XPS spectra of Ce 3d (Fig. 3a and Fig. S6) were fitted into 10 peaks


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corresponding to the Ce 3d5/2 (νo, 880.5 eV; ν, 882.7 eV; ν′, 885.3 eV; ν′′, 888.6 eV; ν′′′, 898.3 eV) and Ce 3d3/2 states (µo, 899.1 eV; µ, 900.8 eV; µ′, 903.7 eV; µ′′, 907.3 eV; µ′′′, 916.7 eV).[33,34] The νo, ν′, µo, and µ′′′ peaks are assigned to Ce3+ species, and the rest six peaks are Ce4+ species.[31] The ratios between Ce3+ and Ce4+ species of different samples were calculated and summarized (Table S2). Although the decrease of Ce3+/Ce4+ ratio is often ascribed to the decrease of oxygen vacancy density,[33] our calculations suggest that this is not necessary so upon the Cu2+ substitution (please see detailed Computational Description in the Supporting Information). For the charge balance, the replacement of one Ce3+ next to a VO by a Cu2+ should be viewed as being accompanied by a conversion of the other Ce3+ to Ce4+, and thus the increase of Cu2+ doping level will lead to a decrease of Ce3+/Ce4+ ratio from the original undoped CeO2, in good accord with our XPS results with low Cu contents. The spectra of Cu 2p3/2 (Fig. 3b and Fig. S7) were deconvoluted into two peaks at 933.6 and 932.5 eV, corresponding to Cu2+ and [Cu0 + Cu1+], respectively.[34,35] The low oxidation states of [Cu0 + Cu1+] indicate the existence of multiple oxygen vacancies around the corresponding Cu doping sites, based on our calculations. The Cu K-edge X-ray adsorption fine structure (XAFS) experiments were further performed to investigate the valence and coordination states of Cu in samples with different Cu contents (Fig. 3c). Compared to a standard CuO sample (Fig. 3c, green curve) with a characteristic peak at ~ 9000 eV, all the Cu-doped CeO2 nanorod samples (that is, with 2%, 4% and 10% Cu doping levels) show a clear shift of the peak toward lower energy side. This peak shift suggests the appearance of lower oxidation states of Cu0 or Cu1+, and the magnitude of the shift is associated with the Cu doping level. The atomic structures of the samples were further interrogated by the Fourier-Transform (FT) of extended X-ray adsorption fine structure (EXAFS) spectra in R-space at the Cu K-edge (Fig. 3d). Peaks A and B (positions indicated by arrows) represent Cu–O and Cu–Cu bonds, respectively. Both


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the Cu-CeO2-2% (blue curve) and Cu-CeO2-4% (red curve) nanorod samples only present Peak A but not Peak B, indicating most of the Cu species in these samples should exist as Cu–O without any Cu–Cu aggregation. The emergence of peak B (Cu–Cu) in the Cu-CeO210% sample (black curve) suggests the existence of Cu aggregation at this doping level. In addition, the EXAFS spectrum of the Cu-CeO2-4% sample was fitted using parameters of Cu–O bond, which yields an unconventionally low coordination number (CN) of Cu as 4.8 ± 0.5 (Fig. S8, Table S3). Considering the original CeO2(110) surface, each Ce4+ ion is associated with 6 coordinated oxygens on the surface or 8 coordinated oxygens in the bulk, this reduced CN value suggests the appearance of multiple (up to 3) Vo’s around the substituted Cu site, in good accord with our aforementioned theoretical calculation of the most feasible Vo-bounded Cu site on CeO2(110) (Fig. 1c). Furthermore, the patterns of the temperature programmed desorption (TPD, Fig. S9) of CO2 present one peak around 100 oC, and two additional peaks at ~ 200 and ~ 300 oC ascribed to the chemisorption of CO2.[36,37] The much-enhanced peaks of the Cu-CeO2-4% sample at higher temperatures indicate that Cu substitution can significantly enhance the chemisorption capability of CO2. The electrocatalytic performances of those Cu-CeO2 nanorod samples were evaluated in a CO2-saturated 0.1 M KHCO3 (pH 6.8, Experimental Section in the Supporting Information). Cyclic voltammetry (CV) curves of Cu-CeO2-4% nanorods, undoped CeO2 nanorods, and Cu nanoparticles (synthesized by hydrogen reduction of Cu2+ salt, Experimental Section in the Supporting Information, TEM and HRTEM images in Fig. S10) were measured in a potential window between –0.2 and –1.8 V vs. reversible hydrogen electrode (RHE, Fig. 4a). Due to the lower electrical conductivity of CeO2 than Cu, the CuCeO2-4% nanorods (red curve) present a lower total current density (Jtol) than that of pure Cu (blue curve), but still significantly higher than that of CeO2 (black curve). The electrocatalytic reduction products of these three catalysts were examined by both in-line gas


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chromatography (GC) and 1H nuclear magnetic resonance (NMR), and plotted in Fig. 4b-d. Representative GC (Fig. S11) and NMR data (Fig. S12) of the Cu-CeO2-4% sample at –1.8 V vs. RHE were also displayed. Compared to Cu nanoparticles and undoped CeO2 nanorods, the Cu-CeO2 nanorods exhibit a clearly reduced amount of H2 side product from the water reduction (grey columns in Fig. 4b-d), and significant increase of the ECR products, indicating the high activity of CO2 reduction on the Cu-doped CeO2 electrocatalysts. By multiplying jtotal and the corresponding FEs for all deep reduction products (i.e.; CO2 reduction products excluding CO and HCOO–), the current densities for all deep reduction products (jdrp) were also calculated and plotted (Fig. 4b-d, red y-axis on the right). It can be seen that jdrp of the Cu-CeO2-4% nanorods gets to ~ 40, 70 and 90 mA cm-2 (based on geometric surface area) at –1.6, –1.8 and –2.0 V vs. RHE, which are several folds higher than those pure Cu and undoped CeO2 samples. More significantly, for the Cu-CeO2-4% nanorods, the FE of methane (CH4, red columns in Fig. 4b) reaches a peak at ~ 58% at –1.8 V vs. RHE, which to the best of our knowledge, is the highest reported efficiency of CH4 production by electrocatalytic CO2 reduction using H-shaped electrochemical cells (Table S4). Our Cu-CeO2-4% nanorods also show the best performances in all the Cu-doped CeO2 nanorod samples with different Cu percentages (Cu% from 0–10%, Fig. 4e), which can be explained by the fact that more active Cu sites (i.e., multiple Vo-bounded single-atomic Cu) exist in Cu-CeO2-4% than Cu-CeO2-2% for the activity increase, while the Cu aggregation in higher Cu content (e.g. Cu-CeO2-10%) also decreases the number of active Cu sites. The Cu-CeO2-4% also presents an excellent ECR stability, with FECH4 was retained > 40% after > 8,000 s of continuous test at –1.8 V vs. RHE (Fig. 4f). It also shows that our catalysts do not favor the production of methanol (CH3OH), which is different from the CO2 hydrogenation at elevated temperatures and additional H2


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feedstock.[38] Although many transition metals are potential of producing both methane and methanol, however, the oxophilicity of the surface, as measured by the Oads binding energy, plays a critical role in determining the selectivity between these two products.[39] The Oads binding energy was previously calculated as −4.14 and −4.71 eV on Cu(111) and Cu(100), respectively.[40] This is in accordance with many experimental observations that the Cu-based electrocatalysts are oxophilic, leading to methane instead of methanol.[9-11] In our study, we calculated the oxophilicity of the CuCex-1O2x-3 surface. The Oads binding energy was found to be −4.89 eV, giving a strong indication that this multiple oxygen vacancy-bound, single atomic Cu-substituted CeO2 is oxophilic, in favor of the CH4 formation. Taken together, the high ECR activity and CH4 selectivity are attributed to the synergetic effect of both active Cu sites and CeO2, as indicated in our aforementioned theoretical models (Fig. 1). Each catalytic active center is a multiple (~ 3) VO-bound, singleatomic Cu site, where a single CO2 molecule is strongly adsorbed and activated, and then stepwise reduced to different ECR products. Furthermore, as the Cu mainly exists as singleatomically dispersed catalytic center, the C–C coupling pathway between two C1 species into one C2 product is substantially inhibited, leading to a dramatic enhancement of the CH4 formation (i.e.; the deepest C1 product).

Conclusion

In summary, we developed a Cu-doped CeO2 electrocatalyst for selective CO2 reduction to CH4. The strong interaction between CeO2 and Cu leads to single-atomically dispersed Cu species, which further enriches multiple (~ 3) oxygen vacancies into the neighboring positions, thus yielding a highly effective catalytic site for electroreduction of a single CO2 molecule to CH4. Thus, the excellent CH4 selectivity is attributed to both the


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atomic dispersion of the electrocatalytic Cu sites and the surrounded multiple Vo’s, as well the cooperative effect from the CeO2 framework. This study features an exquisite example of rational design of highly dispersed metal catalytic centers at single atomic level, and may inspire a vast of opportunities for optimizing different CO2 electroreduction products with high activity and selectivity.


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Supporting Information The computational description, experimental section, and additional figures and tables are provided. This material is available free of charge via the Internet at http://pubs.acs.org

Acknowledgements We thank the following funding agencies for supporting this work: the National Key Research and Development Program of China (2017YFA0206901, 2018YFA0209401), the Natural Science Foundation of China (21688102, 21473038, 21773036), the Key Basic Research Program of Science and Technology Commission of Shanghai Municipality (17JC1400100), and the Collaborative Innovation Center of Chemistry for Energy Materials. Y.W. acknowledges the support of Hui-Chun Chin and Tsung-Dao Lee Chinese Undergraduate Research Endowment. Thanks to XAFCA beamline of Singapore Synchrotron Light source for supporting this project.

Notes The authors declare no competing financial interest.


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Figures

Figure 1. Theoretical calculations of the most stable structures of Cu-doped CeO2(110) and their effects on CO2 activation. (a-c) Structure models of CeO2(110) doped with one single Cu site, with (a) 1 oxygen vacancy (Vo); (b) 2 Vo’s; and (c) 3 Vo’s. (d, e) Structure models of these Vo-bound, single-atomic Cu site on CeO2 for CO2 adsorption and activation. The energy differences associated with each structure change were also specified. The formation energy for 1 Vo on a pure CeO2(110) was calculated to be 1.74 eV, indicating that the 3 Vo-bound structure (c) is the most stable for the single-atomic Cu site on CeO2.


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ACS Catalysis

Figure 2. Structural characterizations of CeO2 and Cu-doped CeO2 nanorods. (a) Nitrogen isotherms of CeO2 and Cu-CeO2-4% nanorods. (b) XRD patterns of Cu, CeO2, CuCeO2-4% and Cu-CeO2-10% nanorods. (c) TEM images of CeO2 nanorods. (d) HRTEM and (e) HAADF-STEM images of Cu-CeO2-4% nanorods.


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Figure 3. X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) characterization of Cu-CeO2 nanorods. (a, b) XPS spectra of (a) Ce 3d and (b) Cu 2p3/2 in Cu-CeO2-4%. (c) XANES spectra at the Cu K-edge. (d) The corresponding K3weighted Fourier-Transform (FT) spectra in R-space.


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ACS Catalysis

Figure 4. Electrochemical CO2 reduction performances. (a) Cyclic voltammetry curves for Cu-CeO2, CeO2 and Cu. (b-d) Faradaic efficiencies (bars, left y-axis) and deep reduction products current density (jdrp, red curves, right y-axis) of (b) Cu-CeO2-4%, (c) pure Cu, and (d) undoped CeO2 at different overpotentials. The deep reduction products were the first five products in the legends at the bottom, marked with a red line. (e) Faradaic efficiency comparison of samples with different Cu doping levels. (f) Left y-axis: stability of FECH4 (blue squares) and FEH2 (black squares). Right y-axis: total current density (jtotal) of Cu-CeO24% at −1.8 V (red curves, right y-axis).


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Table of Content Graphics:


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Single-Atomic Cu with Multiple Oxygen Vacancies on Ceria for

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