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Tuning Metal Catalyst with Metal-CN Interaction for Efficient CO Electroreduction 2
Le Zhang, Fangxin Mao, Lirong Zheng, Haifeng Wang, Xiaohua Yang, and Hua Gui Yang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03789 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 1, 2018
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ACS Catalysis
Tuning Metal Catalyst with Metal-C3N4 Interaction for Efficient CO2 Electroreduction Le Zhang,† Fangxin Mao,† Li Rong Zheng,‡ Hai Feng Wang,*,§ Xiao Hua Yang,*,† and Hua Gui Yang*,† †
Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China ‡
Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China §
Key Laboratory for Advanced Materials, Center for Computational Chemistry and Research Institute of Industrial Catalysis, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China
ABSTRACT: Electrochemical CO2 reduction reaction (CO2RR) has attracted significant interest in the storage of renewable solar energy and mitigation of environmental issues. We report here a strategy to tune metal electrocatalyst by means of metal-substrate interactions to boost CO2RR process. It was found that carbon nitride (C3N4) supported Au nanoparticles (Au/C3N4) exhibit a better performance than carbon supported Au nanoparticles (Au/C). The combined experimental and theoretical results evidence that Au-C3N4 interaction induces the formation of negatively charged Au surface, which could stabilize the key intermediate *COOH. Similar enhanced CO2RR performance is also observed on C3N4 supported Ag nanoparticles (Ag/C3N4), demonstrating the universality of this strategy for enhancing CO2RR. KEYWORDS: CO2 electroreduction, electrocatalysis, Au-C3N4 interaction, electron transfer
Converting CO2 into value-added carbon-based fuels and feedstocks via electrocatalytic reduction in aqueous medium is considered as a promising route to store intermittent electricity provided by photovoltaics and mitigate rising greenhouse gas emissions.1,2 However, this process is usually limited by the large overpotential, low current density, and poor selectivity toward desired product due to the complicated reaction steps.3,4 Despite recent breakthroughs,4-9 more efficient electrocatalysts are still required for this sustainable solar-to-fuels conversion. Recently, several strategies have been utilized to accelerate the CO2RR by tailoring the electronic structure on catalyst surface. For example, the concentrated negative charges at high-curvature structure generate high local negative electric field, which can attract CO2 to the reactive sites to facilitate the CO2RR.5,6 Increasing the electron density on the catalyst surface via introducing oxygen vacancies promotes the activation of CO2.7 Making metal surface highly electron rich by modifying metal nanoparticles with N-heterocyclic carbine contributes a fast electron transfer to CO2.8 According to these findings, rationally designing a negatively charged catalyst surface seems feasible to enhance the CO2RR. It has been reported that graphitic carbon nitride (C3N4) framework, which contains particular electron-rich
N atoms in triazine/heptazine heterorings, could act as a molecular scaffold for appropriately engineering the electronic structures of metal centers to introduce excellent electrocatalytic activities in CO2RR and oxygen reduction/evolution reactions.9,10 Motivated by these studies, taking the widely investigated Au as a module, we find that C3N4 supported Au nanoparticles (Au/C3N4) exhibits a higher CO2RR performance than that of carbon black supported Au nanoparticles (Au/C). The selectivity toward CO2RR reaches 90% over Au/C3N4 at a low potential of -0.45 V vs RHE (overpotential of -340 mV). The combination of experiment and theoretical calculation reveals that the interaction between Au and C3N4 makes the Au surface highly electron rich, thus strengthening the adsorption of key reaction intermediate *COOH. Similar enhanced CO2RR performance is observed on C3N4 supported Ag nanoparticles (Ag/C3N4), which also possess electron-enriched Ag surface. Both of Au/C3N4 and Ag/C3N4 indicate the successful engineering of metal catalysts to improve their CO2RR activity. Au/C3N4 and Au/C were prepared by using C3N4 and carbon black as support, respectively (see details in Experimental Section, Supporting Information). X-ray diffraction (XRD) patterns (Figure 1a) demonstrate that the resulting Au is cubic crystal structure. The diffraction peaks at 2θ = 38.2°, 44.4°, 64.6°, and 77.6° can be assigned to the (111), (200), (220), and (311) crystal planes of Au
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(JCPDS No. 65-2870). The additional diffraction peak at 2θ = 27.6° in Au/C3N4 pattern is the characteristic peak of C3N4 (see details in Figure S1).11 Fourier transform infrared (FTIR) spectra of both Au/C3N4 and C3N4 (Figure 1b) show broad peaks attributed to amine groups or water from 3000 to 3600 cm-1, characteristic stretching modes of CN heterocycles between 1000 and 1700 cm-1, and a typical breathing mode of triazine units at 810 cm-1.12,13 Figure 1c-f present the transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of Au/C3N4 and Au/C. The Au nanoparticles are uniformly deposited on C3N4 and carbon black support (Figure 1c, e), and typical Au (111) plane with a characteristic lattice spacing of 0.235 nm is observed (Figure 1d, f),14 respectively. The inductively coupled plasma atomic emission spectroscopy (ICPAES) analyses confirm that the Au loading in Au/C3N4 and Au/C is 37 wt.% and 34 wt.% (Table S1), respectively.
Figure 1. (a) XRD patterns and (b) FTIR spectra for Au/C3N4, Au/C, and C3N4. (c) TEM and (d) HRTEM images of Au/C3N4. (e) TEM and (f) HRTEM images of Au/C.
The CO2RR electrocatalytic activity of Au/C3N4 and Au/C were evaluated in 0.5 M KHCO3 using a gas-tight two-compartment cell (see details in Experimental Section, Supporting Information). Figure S3 shows the linear sweep voltammetric (LSV) curves of Au/C3N4 and Au/C in Ar- and CO2-saturated 0.5 M KHCO3. Both samples show a larger catalytic current density under CO2 atmosphere than that under Ar atmosphere, while Au/C3N4 exhibits a noticeably improved current density relative to Au/C in CO2-saturated 0.5 M KHCO3, suggesting the higher CO2RR activity over Au/C3N4. Potentiostatic electrolysis was then conducted over the potential range of -0.4 to 1.0 V vs RHE in CO2-saturated 0.5 M KHCO3. Products were quantified via on-line gas chromatography and CO
and H2 were found to be the dominating products. The total current density (jtotal), Faradaic efficiency of CO (FECO) and H2 (FEH2) vs applied potential are presented in Figure 2a, 2b, and Figure S4. Notably, compared with Au/C, Au/C3N4 exhibits a higher jtotal and FECO in the whole potential range. Au/C3N4 only requires a potential of -0.45 V vs RHE (corresponding to the overpotential of 340 mV) to reach FECO of 90% with jtotal of -2.84 mA cm-2, and the FECO is maintained above 90% over a wide potential window of -0.45 to -0.85 V vs RHE. These results demonstrate that after being loaded to C3N4, the CO2RR activity of Au nanoparticles could be significantly enhanced.
Figure 2. (a) Total current densities, (b) CO Faradaic efficiencies, and (c) CO partial current densities for Au/C3N4, Au/C, and C3N4 at different potentials. (d) Nyquist plots for Au/C3N4 and Au/C at -0.5 V vs RHE. The inset is the corresponding Randles equivalent circuit. (e) Specific CO partial current densities for Au/C3N4 and Au/C at different potentials. (f) Stability test for Au/C3N4 at -0.7 V vs RHE. All measurements were performed in CO2-saturated 0.5 M KHCO3.
Figure 2c presents the CO partial current density (jCO), which was calculated by multiplying jtotal by FECO. At all applied potentials, the jCO of Au/C3N4 is much higher than that of Au/C, indicating a better performance of Au/C3N4. Besides, it can be observed that the jCO of Au/C3N4 at high potential region increases more slowly than that at low potential region, which may be caused by the limited mass-transport process at high potential due to the low concentration of CO2 in aqueous media.15 This is further supported by the electrochemical impedance spectroscopy (EIS) (Figure 2d). The EIS was conducted at -0.5 V vs RHE and the collected Nyquist plot was then fitted using
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ACS Catalysis a simple equivalent circuit (the inset in Figure 2d and Table S2), which consists of an electrolyte solution resistance (Rs), charge-transfer impedance (Rct), Warburg impedance (Zw), and constant phase element (CPE). The existence of Zw in the low-frequency region in the plot of Au/C3N4 implies that the mass transport is likely playing a significant role in CO2RR. More importantly, the fitted Rct of Au/C3N4 (19.96 Ω) is much lower than that of Au/C (93.71 Ω), suggesting a faster electron transfer between Au/C3N4 and CO2. To better understand the high activity of Au/C3N4, the specific activity of Au/C3N4 and Au/C, which were obtained by normalizing the jCO to the corresponding electrochemically active surface area (ECSA), were then compared to exclude the influence of ECSA. The ECSA was measured through underpotential deposition (UPD) method (see details in Experimental Section, Supporting Information).5 As seen in Figure 2e, Au/C3N4 indeed exhibits a higher specific jCO than that of Au/C at each potential, demonstrating that Au/C3N4 is intrinsically more active than Au/C. Stability is another important criterion to evaluate the catalyst performance. As shown in Figure 2f, at the constant potential of -0.7 V vs RHE, the jtotal of Au/C3N4 keeps at -13 mA cm-2 and FECO maintains ≥ 85% during a 15 h operation, suggesting the excellent stability of Au/C3N4. In contrast, Au/C has a poor stability at the same potential (Figure S6).
the Au 4f5/2 and Au 4f7/2 peaks of Au/C3N4 both shift to lower binding energies. Such a shift of binding energy indicates that the Au surface in Au/C3N4 is enriched with electrons. This can be further substantiated by the medium-energy XANES at Au M5-edge. The Au M5-edge XANES spectrum was collected in total electron yield (TEY) mode, which characterizes the surface information of material.17 As shown in Figure 3b, the Au M5-edge XANES spectrum of Au/C3N4 clearly exhibits a decrease in photon energy of absorption edge relative to Au/C, again confirming that the Au surface in Au/C3N4 is negatively charged. The hard XANES at Au L3-edge was also measured to study the bulk electron structure. Interestingly, the Au L3-edge XANES (Figure S8) verifies that there are no obvious differences for the bulk electronic structures of Au in these two samples. Based on these results, we speculate that in Au/C3N4 system, the C3N4 support could transfer electrons to Au surface, while Au bulk electronic structure remains initial state like that in Au/C. Highly sensitive soft XANES at N and C K-edge were then conducted in TEY mode to further clarify the electron transfer process between Au and C3N4. In N K-edge XANES spectrum (Figure 3c), C3N4 shows two typical π* resonances at 399.86 and 402.72 eV corresponding to aromatic C-N-C coordination in one tri-s-triazine heteroring and N-3C bridging among three tri-s-triazine moieties.18 As compared with pure C3N4, these two π* resonance peaks in Au/C3N4 spectrum have a slightly positive shift in photon energy. Such a shift is presumably caused by the loss of some electrons from N atoms in Au/C3N4. In C K-edge XANES spectra (Figure 3d), both Au/C3N4 and C3N4 display two characteristic resonances including π* (C=C) and π* (C-N-C).18 No obvious peak shifts in C Kedge XANES spectrum of Au/C3N4 are found relative to C3N4, indicating that C atoms in Au/C3N4 are not easy to lose or accept electrons. Combining the XPS and XANES results, we propose that in Au/C3N4 system, N atoms in C3N4 could donate some electrons to Au surface, because C3N4 possesses abundant N atoms with unpaired electrons.10,19,20 The negatively-charged Au surface may strengthen the adsorption of reaction intermediates and thus facilitate the CO2RR.
Figure 3. (a) Au 4f XPS spectra for Au/C3N4 and Au/C. (b) Au M5-edge medium-energy XANES spectra for Au/C3N4 and Au/C. (c) N K-edge and (d) C K-edge soft XANES spectra for Au/C3N4 and C3N4.
To identify the origin of the high CO2RR electrocatalytic activity of Au/C3N4, both X-ray photoelectron spectroscopy (XPS) and synchrotron-based X-ray absorption near edge structure (XANES) were carried out to probe the electronic structures of Au/C3N4 and Au/C. The highresolution Au 4f XPS spectra are shown in Figure 3a. For Au/C, the peaks at 87.9 and 84.2 eV can be attributed to the metallic Au0 4f5/2 and Au0 4f7/2, respectively.16 More importantly, compared with those of Au/C counterpart,
To elucidate the mechanism of high CO2RR performance over Au/C3N4 relative to Au/C in more depth, density functional theory (DFT) calculations were performed to confirm the electronic interaction between metal and support and how it alters the catalytic property of Au. Firstly, we optimized the structure of Au supported on C3N4 and graphene-carbon substrate, respectively, and examined their charge transfer in between, in which a Au8 cluster was used to mimic the Au nanoparticle (see computational details in Experimental Section, Supporting Information). As shown in Figure 4a and 4b, Au8 can interact much stronger with C3N4 at the hollow site among three tri-s-triazine units, forming the evident N-Au bond, in comparison with the C support where only weak van der Waals interaction exist (with a large Au-C interlayer distance of ~3.57 Å). Quantitatively, the Bader charge analysis shows that Au8 captures a charge of -0.06 |e|
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mainly denoted from the interface N-heterocyclic edgesite, while no evident charge transfer occurs for Au/carbon interface (~0.00 |e| for Au8 on average). This further demonstrates the above experimental observations for the electronic structure of Au/C3N4 and Au/C on the DFT computational level.
the key *COOH intermediate relative to Au0/C, and thus enhance the CO2RR. In order to explore the enhancing effect of C3N4 toward other metals, Ag/C3N4 and Ag/C were synthesized (see details in Experimental Section, Supporting Information). XRD, FTIR, and TEM (Figure S10 and S11) confirm the successful preparations for these two samples. XPS also reveal that the Ag surface in Ag/C3N4 is negatively charged (Figure S12a). As expected, Ag/C3N4 indeed exhibits a higher jtotal and FECO in the whole potential range than those of Ag/C (Figure S13). In addition, Ag/C3N4 has a good stability as well (Figure S14). These results demonstrate the wider applicability of this strategy for enhancing CO2RR. In summary, we successfully tune Au nanoparticle via Au-C3N4 interaction to enhance its CO2RR activity. The Au-C3N4 interaction induces the formation of negatively charged Au surface, which favors the CO2RR through stabilizing the key intermediate *COOH. Similar enhanced CO2RR performance could also be observed on C3N4 supported Ag nanoparticles. Inspired by this investigation, our future work will be focused on applying this strategy to designing more efficient non-precious metal-based CO2RR electrocatalysts.
ASSOCIATED CONTENT Figure 4. Top and side (inserted) views of the optimized Au8 cluster adsorbed on C3N4 at the hollow site (a) and graphenecarbon support (b). The key charge transfers are labeled. (c) The calculated Gibbs free energy profile of CO2RR into CO δ0 catalyzed by Au /C3N4 and Au /C, respectively. Yellow, while, red, grey, and blue balls represent Au, H, O, C, and N atoms, respectively.
Secondly, to demonstrate the relative activities of the negatively charged Auδ- on C3N4 and neutral Au0 on carbon in catalyzing CO2RR into CO, we calculated the thermodynamics (in the Gibbs free energy landscape, see computational details in Experimental Section, Supporting Information) of each elementary reaction step involved in the CO2RR process as an approximated measure. Following previous studies on CO2RR to CO, we considered the following reaction mechanism:21,22 CO2(g) + * + H+(aq) + e- → *COOH (1) *COOH + H+(aq) +e- → *CO + H2O(l) (2) *CO → CO(g) + * (3) where * denotes an adsorption site and e- is an electron. The corresponding free energy profiles under the experimental condition (Uwork = -0.45 V vs RHE, T = 298 K) were given and compared in Figure 4c. Overall, one can see that Auδ-/C3N4 exhibits a much improved catalytic activity for CO2RR into CO relative to Au0/C, embodied in a gentler energy requirement to accomplish the whole reaction. In particular, in comparison with Au0/C, one can evidently see that Auδ-/C3N4 largely facilitates the electroreduction of CO2 into *COOH, corresponding to a lower free energy change of 0.58 eV (vs 1.26 eV for Au0/C), rationalized by the improved binding ability of Auδ-/C3N4 toward
Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Experimental procedures, DFT calculations, supported physical characterizations, and additional electrochemical data.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected] *E-mail:
[email protected] *E-mail:
[email protected] Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT This work was financially supported by National Natural Science Foundation of China (21573068), National Natural Science Funds for Distinguished Young Scholar (51725201) and Fundamental Research Funds for the Central Universities (222201718002).
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