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Metal-Oxide Interfaces for Selective Electrochemical C-C Coupling Reactions Chan Woo Lee, Seung-Jae Shin, Hyejin Jung, Dang Le Tri Nguyen, Si Young Lee, Woong Hee Lee, Da Hye Won, Min-Gyu Kim, Hyung-Suk Oh, Taehwan Jang, Hyungjun Kim, Byoung Koun Min, and Yun Jeong Hwang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b01721 • Publication Date (Web): 23 Aug 2019 Downloaded from pubs.acs.org on August 23, 2019
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ACS Energy Letters
Metal-Oxide Interfaces for Selective Electrochemical C-C Coupling Reactions Chan Woo Lee,†,‡,‖ Seung-Jae Shin,§,‖ Hyejin Jung,†, Dang Le Tri Nguyen,†,,# Si Young Lee,†, Woong Hee Lee,† Da Hye Won,† Min Gyu Kim,¶ Hyung-Suk Oh,† Taehwan Jang,§ Hyungjun Kim,*,§ Byoung Koun Min,†, and Yun Jeong Hwang*,†,,&
†Clean
Energy Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea
‡Department
of Chemistry, Kookmin University, Seoul 02707, Republic of Korea
§Department
of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea
Division
of Energy and Environmental Technology, KIST School, Korea University of Science and Technology, Seoul 02792, Republic of Korea
#Institute
of Research and Development, Duy Tan University, Da Nang City 550000, Vietnam
¶Beamline
Research Division, Pohang Accelerator Laboratory (PAL), Pohang University of Science and Technology, Pohang 37673, Republic of Korea
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Green &Department
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School, Korea University, Seoul 02841, Republic of Korea
of Chemical and Biomolecular Engineering, Yonsei University, Seoul 03722, Republic of Korea
AUTHOR INFORMATION
Corresponding Author *E-mail:
[email protected],
[email protected] ABSTRACT: Metal-oxide interfaces provide a new opportunity to improve catalytic activity based on electronic and chemical interactions at the interface. Constructing high density of interfaces is essential in maximizing synergistic interactions. Here, we demonstrate that Cu/ceria interfaces made by sintering nanocrystals facilitate C-C coupling reactions in electrochemical reduction of CO2. The Cu/ceria catalyst enhances the selectivity of ethylene and ethanol production with suppressing H2 evolution in comparison with Cu catalysts. The intrinsic activity for ethylene production is enhanced by decreasing the atomic ratio of Cu/Ce, revealing the Cu atoms near ceria are an active site for C-C coupling reactions. The ceria is proposed to weaken hydrogen binding energy of adjacent Cu sites and stabilize an *OCCO intermediate via an additional chemical interaction with
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ACS Energy Letters
an oxygen atom of the *OCCO. This work offers new insights into the role of the metaloxide interface in the electrochemical reduction of CO2 to high-value chemicals.
TOC GRAPHICS
Anthropogenic warming and related environmental issues caused by the indiscriminate use of fossil fuels have raised concerns around the world.1 Electrochemically reducing CO2 in conjunction with using renewable sources of energy, such as solar, is a promising strategy for developing a carbon-neutral cycle.2 Among transition metals, Cu-based catalysts have been intensively studied due to their unique ability to produce high-value C2+ chemicals, such as C2H4, C2H5OH, and C3H7OH from CO2 reduction reactions
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(CO2RR).35 Copper metal has an intermediate binding energy for a CO intermediate (*CO),4,6 which enables to be further reduced to C2+ chemicals through CO dimerization to form an *OCCO adsorbate and subsequent hydrogenations.7 However, a critical challenge that remains unresolved is to further promote the pathway of CO dimerization over other competing reactions, such as the evolution of H2 and CH4, to improve the Faradaic efficiency (FE) of C2+ products. On the surface of energetically stable Cu(111), the FEs of H2 and CH4 production account for approximately 30% and 40%, respectively.8 Recently, various Cu materials with oxidized Cu states have been reported to improve the C2+ production, such as Cu/Cu3N,9 O2 plasma-treated Cu,10,11 reduced Cu oxychloride,12 and oxide-derived Cu.13,14 The oxidized Cu states are known to facilitate the CO dimerization pathway by tuning the binding strength of *CO and *OCCO. Similarly, a Cu(100) facet is known to have low CO dimerization barrier with favorable intermediate binding strength compared to Cu(111) and Cu(110), showing high C2 production activity.8,15 Furthermore, it is also reported that a larger difference in the binding energies of two adsorbed *CO molecules is another key factor that reduces the energy barrier of
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ACS Energy Letters
C-C coupling.16,17 An active site with different binding energies of *CO was empirically demonstrated by stabilizing both Cu+ and Cu0 sites via boron doping.17 Metal-oxide support interaction has been extensively utilized to control the reactivity in catalysis. Its effects include the stabilization of certain metal size/shape,18 electronic interaction,19 oxygen spillover,20 and participation of support in catalysis.21 Especially, Cu/ceria interfaces have shown interesting features. CeO2 islands supported on Cu(111) surface functioned as a channel for supplying oxygens to neighbor Cu.22 In addition, the charge transfer process between Cu and ceria was reported to be spontaneous, resulting in an electron loss in Cu atom.23 Furthermore, gas-phase methanol synthesis was energetically facilitated at the Cu-ceria interface because Ce3+ centers can bind with the oxygens of a CO2 molecule and other intermediates.21 Such electronic and chemical interactions can give a new avenue to tune Cu-based catalysts for C-C coupling reaction. Here, we propose the Cu/ceria interface as an active site for C-C coupling. The Cu/ceria interface was constructed by sintering sub-10 nm crystals to maximize interfacial density and synergistic interaction at the interface. This kind of interfacial structure has never been designed for the C-C coupling reaction from CO2RR. Under the CO2RR condition,
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spectroscopic analysis shows that metallic Cu sites form an interface with the reduced cerium hydroxide. DFT calculations suggest that the interfacial active sites stabilizes the dimerized *OCCO intermediate to promote the reaction pathway specific to the C-C coupling reaction with the suppression of HER. In comparison with control Cu catalysts, by constructing the interface with ceria, HER selectivity was reduced from 40.0% to 14.6%, whereas the FE of C2H4 and C2H5OH production increased from 38.8% to 64.7%. The detailed tuning of the atomic ratio of Cu to Ce elements further revealed that Cu atoms near ceria are closely related to the enhanced intrinsic activity for ethylene production. Cu/ceria catalysts with high density of interfaces were synthesized by the impregnation of a copper precursor into hydrothermally prepared ceria/C and subsequent calcination at 400 C in air as shown in Figure 1a. In the synthetic procedure, functionalized carbon black acted as host material to impregnate both Ce and Cu (Figure S1), where the input ratio of Cu/Ce was 0.95. The calcination process induced carbon removal, interdiffusion, crystal growth and segregation, finally yielding an interconnected structure of CuO and CeO2 nanocrystals. The Cu catalyst, a control sample, was prepared with an alcothermal
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ACS Energy Letters
method in which a copper precursor reacted with NaOH in boiling ethanol to form CuO rather than Cu(OH)2.24 The low-temperature alcothermal process was used to obtain a similar size of NPs with the Cu/ceria catalysts. Figure 1bd show transmission electron microscopy (TEM) images and corresponding scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) elemental mappings of the Cu, Cu/ceria and ceria/C catalysts. Cu and Ce were found to have a spatially inhomogeneous distribution in the Cu/ceria catalysts. The STEM image clearly showed the Cu-rich and Ce-rich regions denoted by yellow and red, respectively (Figure 1c), and the average elemental ratio of Cu:Ce was 0.86:0.14 and 0.12:0.88 in the Cu-rich and Ce-rich regions, respectively (Figure S2). A high-resolution TEM analysis (Figure 1e) revealed that CuO crystals with the size of approximately
