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Enhancing CO2 Electroreduction with the Metal-Oxide Interface Dunfeng Gao, Yi Zhang, Zhiwen Zhou, Fan Cai, Xinfei Zhao, Wugen Huang, Yangsheng Li, Junfa Zhu, Ping Liu, Fan Yang, Guoxiong Wang, and Xinhe Bao J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b00102 • Publication Date (Web): 09 Apr 2017 Downloaded from http://pubs.acs.org on April 9, 2017
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Enhancing CO2 Electroreduction with the Metal-Oxide Interface Dunfeng Gao,‡a Yi Zhang,‡a, b Zhiwen Zhou, ‡a, b Fan Cai, a, b Xinfei Zhao, a, b Wugen Huang,a, b Yangsheng Li,a, b Junfa Zhu,c Ping Liu,d Fan Yang,*, a Guoxiong Wang,*, a Xinhe Bao*, a a
State Key Laboratory of Catalysis, CAS Center for Excellence in Nanoscience, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China b University of Chinese Academy of Sciences, Beijing, 100039, China c National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, 230029, China d Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States Supporting Information Placeholder ABSTRACT: The electrochemical CO2 reduction reaction (CO2RR) typically uses transition metals as the catalysts. To improve the efficiency, tremendous efforts have been dedicated to tuning the morphology, size and structure of metal catalysts and employing electrolytes that enhance the adsorption of CO2. We report here a strategy to enhance CO2RR by constructing the metal-oxide interface. We demonstrate that Au-CeOx shows much higher activity and Faradaic efficiency than Au or CeOx alone for CO2RR. In situ scanning tunnelling microscopy and synchrotronradiation photoemission spectroscopy show that the Au-CeOx interface is dominant in enhancing CO2 adsorption and activation, which can be further promoted by the presence of hydroxyl groups. Density functional theory calculations indicate that the Au-CeOx interface is the active site for CO2 activation and the reduction to CO, where the synergy between Au and CeOx promotes the stability of key carboxyl intermediate (*COOH) and thus facilitates CO2RR. Similar interface-enhanced CO2RR is further observed on Ag-CeOx, demonstrating the generality of the strategy for enhancing CO2RR.
The electrochemical CO2 reduction reaction (CO2RR) is a promising approach to convert CO2 to fuel and chemicals powered by renewable electricity.1 However, the process has been hampered by the large overpotential in reducing CO2, which leads to a sharp decrease in product selectivity and energy efficiency.1,2 Tremendous efforts have been dedicated to develop and improve the performance of metal catalysts for CO2RR,3,4 among which controlling the size, shape and morphology of nanostructured metal catalysts has been shown as an effective strategy.4 However, a major problem for metal catalysts in CO2RR remains in the very weak adsorption of CO2 on catalyst surfaces.5 To enhance the solubility and adsorption of CO2, ionic liquid electrolytes,6 nitrogen-containing organic molecules7 and polymers8 have been employed as sorbents or co-catalysts. But, these methods also suffer a few drawbacks, such as expensive, complicated preparation procedures and difficulties in liquid product separation. We propose here a strategy to facilitate CO2RR through the construction of the metal-oxide interface. The remarkable catalytic properties of the metal-oxide interface have been increasingly recognized in heterogeneous catalytic processes, such as CO oxidation,9 water-gas shift reaction10 and methanol synthesis.11 The strong interfacial interaction, particularly the interface con-
finement effect in supported nanostructures, has been suggested to provide and stabilize highly active sites for molecular activation.12 Such interfacial interaction or catalytic chemistry has rarely been noticed or utilized for electrocatalysis, especially for CO2RR. In this study, using Au and Ag catalysts13, 14 as an example, we show that CO2RR could be significantly enhanced by the construction of the metal-CeOx interfaces, which introduce catalytic properties, much different from those of metals or oxides alone. The sizes and loadings of Au nanoparticles (NPs) were controlled in the Au/C and Au-CeOx/C catalysts to show a similar size distribution and average size with similar metal loadings, as confirmed by X-ray diffraction (XRD, Figure S1), transmission electron microscopy (TEM, Figure S2) and inductively coupled plasma optical emission spectroscopy (ICP-OES, Table S1). Figure 1a shows, in Au-CeOx/C, Au NPs sit on CeOx NPs, forming Au-CeOx interfaces. The lattice distances of Au and CeOx NPs are 0.24 and 0.31 nm, corresponding to those of Au(111) and CeO2(111), respectively. X-ray photoelectron spectroscopy (XPS) also confirms that Au NPs in Au-CeOx/C and Au/C catalysts remain the metallic state (Figures S3). Meanwhile, Ce 3d XPS spectra show the mixture of Ce4+ and Ce3+ in both Au-CeOx/C (Figure S3b) and CeOx/C (Figure S3d) catalysts. The atomic fraction of Ce3+, obtained by fitting the Ce 3d peaks,15 increases from 21.6% in CeOx/C to 30.9% in Au-CeOx/C (Table S1). The increase in Ce3+ concentration is not influenced by the reducing agent during synthesis (Figures S3), but rather induced by the strong interfacial interaction between Au and ceria, which facilitates the removal/desorption of lattice oxygen and thus the reduction of CeOx.16-18 The reactivity of the above catalysts for CO2RR was measured in an H-cell filled with 0.1 M KHCO3 (pH 6.8) solution.19 CO and H2 are the only two gas products, and no liquid products are detected by nuclear magnetic resonance (NMR). The CO Faradaic efficiency reaches 89.1% over Au-CeOx/C at −0.89 V vs. reversible hydrogen electrode (RHE), which is significantly higher than 59.0% and 9.8% over Au/C and CeOx/C at the same potential (Figure 1b). The CO geometric current density over Au-CeOx/C (12.9 mA cm−2) is about 1.6 times of that over Au/C (8.3 mA cm−2) at −0.89 V vs. RHE (Figure 1c). The onset potential of CO2RR shifts positively by ~ 0.1 V over Au-CeOx/C, when compared with Au/C. Figure S4a shows the mass activity reaches 32.4 A g−1Au at −0.89 V vs. RHE over Au-CeOx/C, which is 1.5 times of that over Au/C (21.7 A g−1Au). In terms of the specific activity by normalizing the current for CO production with electrochemi-
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cal surface area (ECSA),20 Au-CeOx/C exhibits an intrinsic activity ~2.2 times of that over Au/C at −0.89 V vs. RHE (Figure 1d), while the ESCA of Au-CeOx/C, 9.6 m2 g−1Au, is lower than that of Au/C, 13.3 m2 g−1Au (Figure S5). On the other hand, although CeOx/C shows an appreciable capacity for CO2 adsorption, CO current densities are almost equal over CeOx/C and Vulcan XC72R (Figure 1c), suggesting that CeOx cannot reduce CO2 to CO by itself. Further, the Au-CeOx interface remains highly stable and Au-CeOx/C exhibits a CO Faradaic efficiency of ~91% at −0.89 V vs. RHE throughout the 620-min stability test (Figure S6). We also did not detect any dissolved Ce ions in the electrolyte solution after the CO2RR stability test using ICP-OES. Au NPs were also found stable in Au/C and Au-CeOx/C catalysts after CO2RR (Figure S6).
Figure 1. Structure and performance of Au-CeOx/C catalyst. (a) HRTEM image of Au-CeOx/C catalyst. (b) Faradaic efficiency, (c) geometric partial current density, and (d) specific activity for CO production over Au/C, CeOx/C and Au-CeOx/C catalysts in CO2-saturated 0.1 M KHCO3 solution and their dependence on the applied potentials. Current density for CO production over Vulcan XC-72R is also shown in (c). To understand the role of the Au-CeOx interface in CO2RR, we prepared CeOx islands on Au(111) to study the interaction between the Au-CeOx interface and CO2/H2O (Figure S7). The asgrown CeOx islands expose the CeO2(111) surface and their step edges expose exclusively under-coordinated Ce3+ ions, rendering a higher apparent height in scanning tunnelling microscopy (STM, Figure 2). When the CeOx/Au(111) surface was exposed to CO2 at 78 K, in situ STM shows CO2 adsorbs first at the interfacial boundaries of CeOx, where the addition of an adsorbate row could be clearly resolved along the step edge of ceria (Figures 2c-d). Further exposure led to CO2 adsorption on the surface of CeOx. Unlike the conventionally assumed random adsorption model, CO2 adsorbates prefer to attach to prior adsorbed CO2 species (Figures 2e and S8), forming an adsorbate ring around the periphery of the CeOx island and propagating gradually towards the island center (Figure 2f). The chemical nature of adsorbed species was probed by synchrotron-radiation photoemission spectroscopy (SRPES), which gave a C 1s peak at 289.7 eV and suggested the formation of CO2δ- species on ceria and at the CeOx-Au(111) interface.21 In contrast, CO2 does not adsorb on Au(111) upon extended CO2 exposure. On the CeO2(111) film, CO2 adsorbs primarily at surface defects at 78 K and appears diffusive on flat
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terraces (Figure S9), indicating a weak physisorption.22 Our results demonstrate that the CeOx-Au(111) interface cannot only activate CO2, but also facilitate the subsequent adsorption and activation of CO2 on ceria. Thus, the interfacial effect is no longer local, but a global effect. Accordingly, temperature programmed desorption (TPD) studies of CO2 adsorption over the powder catalysts (Figure S10) show that Au/C only produced a very weak CO2 desorption peak,5 whereas Au-CeOx/C showed a desorption peak area, much larger than the sum of the desorption peaks from Au/C and CeOx/C.
Figure 2. The interaction between CeOx/Au(111) and CO2. (ad) STM images of the CeOx-Au(111) interface before (a, b) and after (c, d) CO2 adsorption at 78 K. The squared areas in (a) and (c) are magnified in (b) and (d), respectively. (e-f) Sequential STM images of the CeOx island upon extended CO2 exposure at 78 K. CO2 adsorbates propagate from the interface to the surface of CeOx, until the entire ceria terrace is covered with CO2. Scanning parameters: (b, d) Vs = 600 mV; It = 0.2 nA. In the presence of water, hydroxyl groups formed by the spontaneous dissociation of water at the CeOx-Au interface23 (Figure S11, S12) is vital to CO2RR. Figure 3a shows, on CeOx/Au(111), adsorbed CO2δ- species desorb at 300 K in ultra-high vacuum (UHV). However, the co-deposition of H2O would enhance the formation of CO2δ- species on CeOx/Au(111) and stabilize CO2δspecies when H2O and CO2 were dosed together at 300 K (Figure 3a, S13). Accordingly, STM results show the formation of surface adsorbates on CeOx surfaces at 300 K, which were not detected when only CO2 was dosed (Figure S14). Simultaneous measurements of resonant photoemission spectroscopy (RPES) of CeOx/Au(111) (see Supporting Information) show an increased reduction of CeOx accompanying the enhanced formation and stabilization of CO2δ- species (Figure 3b). The adsorption of CO2 alone would cause the oxidation of ceria (Figure 3b), whereas OH groups facilitated the reduction of ceria to produce surface Ce3+ sites (Figure S12). Note that, the reduction of CeOx islands in water vapour is more drastic than annealing in UHV or in a CO environment (Figure S12), consistent with previous reports on the reduction of CeO2-x by water at 300 K,24,25 or elevated tempera-
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tures.26 The facilitated reduction of CeOx in H2O was attributed to the H2O/OH promoted redistribution of oxygen vacancies from bulk to surface, by stabilizing Ce3+ sites at the surface.26 In our case, the facile dissociation of water at interfacial Ce3+ sites promotes the formation of surface hydroxyl groups, which subsequently facilitates the removal of lattice oxygen upon annealing. By enhancing ceria reduction to form high-concentration Ce3+ sites, the presence of water also improves the adsorption and stability of CO2δ- species on CeOx/Au(111), which subsequently enables the hydrogenation process.
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Figure 3. The interaction between CeOx/Au(111) and CO2/H2O. (a) C 1s spectra (hν = 400 eV) of CeOx/Au(111) taken after the exposure of CO2 and H2O (1: the exposure to 15 L CO2 at 110 K, followed by the annealing to 200 K (1a) and 300 K (1b) in UHV. 2: the exposure to 15 L CO2 and 3 L H2O at 110 K, followed by the annealing to 200 K (2a) and 300 K (2b) in UHV. 3: the exposure to 500 L CO2 and 50 L H2O at 300 K). Changes in the oxidation states of CeOx were compared in (b), which plots density ratios of D(Ce3+)/D(Ce4+) before and after the above treatments of CO2 and H2O. D(Ce3+)/D(Ce4+) were calculated from the intensity of RPES spectra of Ce3+ and Ce4+, and termed as resonant enhancement ratio (RER). To understand the reaction mechanism, we conducted density functional theory (DFT) calculations using Ce3O7/Au(111) to describe the Au-CeOx interface. In addition, the full hydroxylation of Ce3O7 to Ce3O7H7 was included to account for the effect of electrochemical environment for CO2RR.27 Following previous studies on CO2RR to CO, we considered the following reaction mechanism:28,29 CO g ∗ H aq e → ∗ COOH 1 ∗ COOH H aq e → ∗ CO H Ol 2 ∗ CO → COg ∗ 3 Figure 4 shows the calculated free energy diagram of CO2RR on Ce3O7H7/Au(111). The formation of carboxyl (*COOH) species via protonation (1) is found as the potential limiting step on both Ce3O7H7/Au(111) and Au(111), while the corresponding energy cost on Ce3O7H7 /Au(111) is 0.33 eV lower than that on Au(111). Subsequently, the protonated decomposition of *COOH to *CO (2) and the desorption of *CO (3) are more favourable at the Au-CeOx interface. The promotional effect could also be observed for CeOx on Au(100) and Au(110), indicating the dominance of interface-enhanced CO2RR (Figure S15). Bader analysis of Ce3O7, Ce3O7/Au(111) and Ce3O7H7/Au(111) shows that the formation of interface and the hydroxylation result in the reduction of Ce ions and decrease the oxidation states of Ce to +3.60 and +3.28, respectively (see Supporting information). That means, the key *COOH intermediate is stabilized by reduced Ce site at the interface, via the direct interaction with one of terminal oxygen and thus enhance CO2RR.
Figure 4. DFT calculations of CO2RR at 0 V vs. RHE on Au(111) and Ce3O7H7/Au(111) surfaces. (a) Optimized structures for the main intermediates; (b) Calculated free energy diagram. “*X” indicates an adsorbed surface species. Having understood the catalytic nature of CO2RR at the AuCeOx interface, we realize that these insights might enable us to improve further the catalytic performance of other metals. We thus synthesized Ag-CeOx/C catalyst, which displays a CO geometric current density over 4 times of that over Ag/C at −0.89 V vs. RHE and a CO Faradaic efficiency of 92.6% (Figure S16, S17). In summary, we demonstrate that the construction of the metaloxide interface could significantly enhance the activity and selectivity of CO2RR. Au-CeOx/C shows a CO geometric current density 1.6 times of that over Au/C at −0.89 V vs. RHE, which leads to a CO Faradaic efficiency of 89.1%. The enhancement is attributed to the drastically enhanced CO2 adsorption and activation by the Au-CeOx interface, which cannot only activate CO2 at interfacial sites, but also facilitate the subsequent adsorption of CO2 on ceria terraces. Hydroxyl groups from water dissociation further enhance the reduction of CeOx and the stability of CO2δspecies. The subsequent formation of carboxyl species (*COOH), i.e. the potential limiting step, gives a free energy 0.33 eV lower at the CeOx/Au(111) interface than on Au(111). Thus, by strengthening the adsorption of *COOH at the interface, CO2RR is greatly facilitated and similar interface-enhanced CO2RR could also be observed on Ag-CeOx/C. Therefore, our study demonstrates the unique catalytic properties of the metal-oxide interface in enhancing CO2RR and provides a new route for the design of efficient electrocatalysts.
ASSOCIATED CONTENT Supporting Information Details of experimental procedures, data analysis and DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author
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Author Contributions ‡These authors contributed equally.
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT This work was financially supported by Natural Science Foundation of China (21573222, 21473191, 21303195) and Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17020200). P. L. would like to thank the support from the US Department of Energy, Division of Chemical Sciences under contract DE-SC0012704. G. X. Wang would like to thank the support from CAS Youth Innovation Promotion. We thank Dr. Huanxin Ju, Dr. Shanwei Hu and Dr. Qian Xu from the BL11U beamline in NSRL for assistance with SRPES measurements.
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