Unveiling Active Sites of CO2 Reduction on Nitrogen-Coordinated and

Mar 8, 2018 - Herein, we report the exploration of understanding the reactivity and structure of atomically dispersed M–N4 (M = Fe and Co) sites for...
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Unveiling Active Sites of CO2 Reduction on Nitrogen Coordinated and Atomically Dispersed Iron and Cobalt Catalysts Fuping Pan, Hanguang Zhang, Kexi Liu, David A. Cullen, Karren L More, Maoyu Wang, Zhenxing Feng, Guofeng Wang, Gang Wu, and Ying Li ACS Catal., Just Accepted Manuscript • Publication Date (Web): 08 Mar 2018 Downloaded from http://pubs.acs.org on March 8, 2018

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Unveiling Active Sites of CO2 Reduction on Nitrogen Coordinated and Atomically Dispersed Iron and Cobalt Catalysts Fuping Pan,†,1 Hanguang Zhang,‡,1 Kexi Liu,⊥ David Cullen,§ Karren More,# Maoyu Wang∥, Zhenxing Feng∥, Guofeng Wang,⊥,* Gang Wu,‡,* and Ying Li†,* †

Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843,

United States ‡

Department of Chemical and Biological Engineering, University at Buffalo, The State University of

New York, Buffalo, New York 14260, United States ⊥ Department

of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh,

Pennsylvania 15261, United States §

Materials Science and Technology Division and #Center for Nanophase Materials Sciences, Oak

Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ∥ School

of Chemical, Biological, and Environmental Engineering, Oregon State University, Corvallis,

Oregon 97331, United States *Corresponding authors: [email protected] (Y. Li); [email protected] (G. Wu); [email protected] (G. Wang) 1

These two authors contribute equally

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ABSTRACT: Herein we report the exploration of understanding the reactivity and active sites of atomically dispersed M−N4 (M = Fe and Co) sites for CO2 reduction reaction (CO2RR). Nitrogen coordinated Fe or Co site atomically dispersed into carbons (M-N-C) containing bulk- and edge-hosted M−N4 coordination were prepared by using Fe- or Co- doped metal-organic framework precursors, respectively, which were further studied as ideal model catalysts. Fe is intrinsically more active than Co in M−N4 for the reduction of CO2 to CO in terms of a larger current density and a higher CO Faradaic efficiency (FE) (93% vs. 45%). First principle computations elucidated that the edge-hosted M−N2+2-C8 moieties bridging two adjacent armchair-like graphitic layers is the active sites for the CO2RR. They are much more efficient than traditionally proposed bulk-hosted M−N4-C10 moieties embedded compactly in a graphitic layer. During the CO2RR, when the dissociation of *COOH occurs on the M−N2+2-C8, the metal atom is the site for the adsorption of *CO and the carbon atom with dangling bond next to adjacent N is the other active center to bond *OH. In particular, on the Fe−N2+2-C8 sites, the CO2RR is more favorable over the hydrogen evolution reaction, thus resulting in remarkable FE.

Keywords: CO2 reduction; single atomic catalyst; active sites; DFT calculation; electrocatalysis

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Electrochemical reduction of CO2 using water as the medium is a sustainable solution for both CO2 remediation and fuels generation, because it can convert CO2 and store intermittent renewable energy into transportable fuels simultaneously.1-3 This desired technology, however, faces grand challenges owing to the sluggish rate of CO2 reduction reaction (CO2RR) and the inevitable competition over hydrogen evolution reaction (HER) in aqueous media.4-5 Among explored catalyst formulations, metal (Fe, Co, and Ni) and nitrogen co-doped carbon (M-N-C) materials have been emerging as promising electrocatalysts due to their low cost and appealing CO2RR selectivity.2,6-10 Especially, nitrogen coordinated metal (M−NX) moieties were proposed to be likely catalytic sites, because of their optimal binding strength with the chemical species involved in CO2RR.6,9,11-12 In spite of recent progress reported in the literature,2,6-9 the M-N-C catalysts still suffer from poor selectivity and low current densities associated with CO2 reduction to CO. Importantly, the nature of active site remain a puzzle, which make rational design impossible for advanced catalysts with high density of active sites for the CO2RR, while free of the sites active for the parasitic HER. The obstacle hindering the exploration of the active M−Nx sites for the CO2RR mainly lies in the lack of ideal model catalysts. Due to high mobility and diffusivity of isolated metal atoms, M−Nx species will undergo uncontrolled sintering during the synthesis of M-N-C by pyrolyzing the mixed metal salts, nitrogen, and carbon precursors at high temperature (>900oC).13-17 This eventually leads to the formation of mixed phases of M−Nx and metallic aggregates in traditional M-N-C catalysts,9,18-24 which inevitably leads to a reduction of densities of active M−Nx sites and cannot accurately link to 3 ACS Paragon Plus Environment

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catalytic reactivity measured for CO2RR. Even worse, the metallic species often prefers to catalyze the HER rather than the CO2RR.8,25-26 Thus the overall CO2RR activity of M−Nx in traditional M-N-C catalysts will be significantly underestimated. Recently, we developed an effective synthesis approach to preparing uniform atomic Fe− and Co−N4 site catalysts without containing metallic phases,27-30 which can be used as the ideal model systems to study the nature of active sites for the CO2RR. As for the subtle structures of M−N4, there are two possible forms either embedded at edge or in bulk of the carbon matrix.20,31-33 However, distinguishing their catalytic activity and selectivity for the CO2RR are not elucidated yet due to the limitations in experimentally synthesizing M-N-C exclusively with either edge- or bulk-hosted M−N4 sites. Theoretical computation thus would be effective to identify the real active M−N4 sites as it can predict favorable CO2RR pathway on these two different types of M−N4 sites.33 Herein, we report a comprehensive investigation on reactivity and catalytic sites of M−N4 (M=Fe or Co) for CO2 reduction through a combination of the synthesis of atomically dispersed Fe and Co-based M-N-C model catalysts, the verification of Fe−N4 or Co−N4 structures, the measurements of activity and selectivity for the CO2RR, and the theoretical understanding of the active M−N4 sites by first-principles density functional theory (DFT) calculations. The atomically dispersed M-N-C catalysts with well-defined M−N4 sites were prepared by using chemically Fe or Co-doped zeolitic imidazolate frameworks-8 (ZIF-8), a zinc-rich meal-organic framework (MOF), respectively, followed by a facile thermal activation (Figure 1a). Rather than physically absorbed in the pores of ZIF-8, Fe (or Co) ions are nodes that chemically bond with 2-methylimidazole, thus stabilizing atomic Fe (or Co) dispersion in precursors through covalence bonds, showing similar crystal structure and rhombic dodecahedron 4 ACS Paragon Plus Environment

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shapes to Fe and Co-free ZIF-8 (Figure S1 and S2). During high-temperature activation to produce M-N-C catalysts, Zn also acts as spacers to disperse Fe (or Co) to avoid the formation of metallic clusters. We observed that all the catalysts preserved their dodecahedron shape with the size of 20-30 nm (Figure 1b-1d for Fe-N-C and Figure 1f-1h for Co-N-C). X-ray diffraction (XRD) patterns (Figure S3) show that they are partially graphitized carbon without metallic phases, and Raman spectroscopy demonstrate the similar disordered structure of the carbon phases for these catalysts (Figure S4).

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Figure 1. (a) Schematic illustration of the synthesis of M-N-C catalysts. The HAADF-STEM images of Fe-N-C (b-e) and Co-N-C catalysts (f-i).

To reveal the atomic structure of Fe and Co in M-N-C, high-resolution high-angle angular dark-field scanning transmission electron microscopy (HAADF-STEM) was performed. As depicted in Figure 1e and 1i, isolated and well-dispersed Fe and Co atomic sites were directly observed, which both located at edge sites and embedded in the carbon matrix. We further carried out X-ray absorption spectra to verify the evolution of the M−N bonds and their local coordination. The Co K-edge X-ray absorption near edge structure (XANES) in Figure 2a shows that Co-N-C has the oxidation state close to 2+ compared to CoO standard. In addition, the Fe K-edge XANES in Figure 2d shows that Fe-N-C possesses the oxidation state in-between 2+ and 3+ compared to FeO and Fe2O3 standards. More detailed comparison can be seen in the derivative of corresponding XANES spectra. As shown in Figure 2b, the major peak is aligned with the one of CoO, supporting the oxidation state of 2+ for our Co-N-C catalyst, while 3+ dominates in the Fe-N-C catalyst (Figure 2e). To further determine the local coordination of Co and Fe, their L-edge XANES and K-edge EXAFS were analyzed. The lack of clear splitting (708 eV in Figure S5) in Fe L3 XANES rules out the FeO6 octahedral configuration,34 and suggests a possible FeO4 or FeN4 coordination. Similarly, the loss of the feature peak (776 eV in Figure S6) of octahedral structure in Co L3 XANES suggests tetrahedral coordination (e.g., CoN4) of Co catalyst as well. Therefore, the EXAFS fitting (Figure 2c, f) of the first shell was performed on M-N-C by using the M−N scattering paths from standard CoPc and FePc, respectively,27-28,35-36 showing that both Co and Fe are coordinated with four N in the form of M−N4 complexes embedded in the carbon 6 ACS Paragon Plus Environment

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(Table S1). This is consistent with soft XAS results and reported atomic structures of Fe-N4 and Co-N4.37-38 Note that Co−O and Fe−O, as well as mixed Co−O/Co−N and Fe−O/Fe−N scattering paths were also used to fit our catalysts, but none of these fits gives reasonable tetrahedral structures that are consistent with soft XAS and XPS results (Figure S7-S9).

Figure 2. (a) Co K-edge XANES, (b) derivation of Co K-edge XANES, and (c) Co K-edge EXAFS fitting of Co-N-C. (d) Fe K-edge XANES, (e) derivation of Fe K-edge XANES, and (f) Fe K-edge EXAFS fitting of Fe-N-C.

As for the structural configurations of Fe (or Co) with four N coordination, previous findings have experimentally revealed that two kinds of Fe−N4 could be generally formed in Fe-N-C synthesized by heating treatment of Fe, N, and C-containing precursors.39-41 One is the bulk-hosted Fe−N4 moieties, 7 ACS Paragon Plus Environment

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which were embedded in the bulk of a graphitic layer and is fully encapsulated by carbon atoms.33 Another is the edge-hosted Fe−N2+2, where two N-doped graphitic layers are connected by an Fe atom bonded with two N atoms at the edges of each graphitic layer.32 The Fe−N2+2 also leads to a distortion of carbon plane environment that cause the out-of-plane position of iron, while the Fe−N4 maintains the in-plane structure. Mossbauer spectroscopy further verifies the co-existence of both Fe−N2+2-C8 and Fe−N4-C10 in a Fe-N-C catalyst that was synthesized via a similar high-temperature approach (Figure S10).41-42 The Brunauer-Emmett-Teller (BET) surface area of these carbon-based catalysts were measured via N2 isothermal adsorption and desorption (Figure S11). The similar BET surface area of three model catalysts (N-C: 768 m2 g−1, Co-N-C: 702 m2 g−1, and Fe-N-C: 646 m2 g−1) indicates that the morphology difference of these catalysts are negligible on their CO2 reduction activity, taking advantages of the well-defined ZIF precursors. The element compositions measured by X-ray photoelectron spectroscopy (XPS) indicate that these three catalysts had the similar C, N, and O contents (Table S2). It should be noted that the Fe content is close to that of Co in the final optimized M-N-C catalysts via the controlled synthesis. In addition, these three catalysts possessed similar types of nitrogen-induced groups including pyridinic N, graphitic N, and oxidized N. The presence of the M−N complexes is also confirmed from fitted N 1s spectra (Figure S12 and Table S3). The oxidation states of Co and Fe in model catalysts were confirmed as 2+ for Co as well as between 2+ and 3+ for Fe by XPS Co 2p and Fe 2p spectra (Figure S13), respectively, which are constant with XANES results. Electrochemical reduction of CO2 on these model catalysts were measured systematically in KHCO3 solution. Linear sweep voltammetry (LSV) curves of the optimized catalysts (Figure S14 and 8 ACS Paragon Plus Environment

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S15) exhibited much larger current density in CO2-saturated solution compared to those obtained in the Ar-saturated electrolyte revealing the occurrence of CO2 reduction. Fe-N-C exhibited a more positive onset potential (−0.29 V) toward CO2RR than those of Co-N-C (−0.38 V) and N-C (−0.67 V), indicating that intrinsic activity for the CO2RR follows the order of Fe-N-C > Co-N-C > N-C. We further conducted chronoamperometric electrolysis to evaluate the selectivity of products in CO2 reduction (Figure S16). Gaseous CO and H2 were detected as main products with trace amounts of CH4 (Figure 3a and Figure S17); no liquid hydrocarbon products were detected (Figure S18). Noted that FEs of CO and H2 are largely dependent on applied potentials, suggesting the competitive nature of CO2RR and HER. “Fe- and Co-free” N-C exhibited a maximum FE of 20% for CO generation at – 0.79 V vs. RHE, corresponding to an overpotential of 0.68 V against the standard equilibrium potential of CO2/CO at –0.11 V.43 Upon atomic dispersion of Co sites coordinated with N and embedded into carbon, a maximum CO FE increased to 45% at an overpotential of 0.48 V on optimized Co-N-C, which is more than two times higher than that of N-C, along with 200 mV reduction in overpotential. As for the Fe-N-C, the highest FE for the CO reaches 93% at an overpotential of 0.47 V. To our best knowledge, this newly achieved FE is among the highest value compared with other Fe-N-C based catalysts reported in literature (Figure S19).8-9 Accordingly, maximum FEs of H2 are in the order of N-C (79%) > Co-N-C (55%) > Fe-N-C (6%). These results indicate that the Fe−N4 and Co−N4 sites are more effective in suppressing the HER compared to C−NX sites. Obviously, larger CO current densities on Fe-N-C than those on Co-N-C in overall potentials further directly manifest that Fe−N4 sites are inherently more active than Co−N4 for CO2RR reduction to CO (Figure 3b). Electrochemical stability of the best performing Fe-N-C was further tested. As shown in Figure 3c and Figure S20, a 9 ACS Paragon Plus Environment

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20-hour test reveals that Fe-N-C exhibited stable CO FEs of above 93% and current density of −2.8 mA cm–2, corresponding to a total CO production of 1.0 mmol cm–2 at a low overpotential of 0.47 V. This implies excellent stability of atomic Fe−N4 sites during the CO2RR in aqueous media.

Figure 3. (a) CO and H2 FEs of N-C, Co-N-C, and Fe-N-C. (b) CO and H2 partial current densities of N-C, Co-N-C, and Fe-N-C. (c) Electrocatalytic CO2RR stability of Fe-N-C at –0.58 V for 20 hours.

To elucidate the reactivity nature and catalytic sites of such N coordinated and atomically dispersed Fe and Co site catalysts for the CO2RR, the first-principles DFT calculations were performed. Following previous studies20,31,33,39-40,44 and structural characterization results attained in this work, bulk-hosted M−N4 moiety compactly embedded in a graphitic layer (denoted as M−N4-C10) and 10 ACS Paragon Plus Environment

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edge-hosted M−N4 moiety bridging two adjacent armchair-like graphitic edges (denoted as M−N2+2-C8) are proposed as possible site candidates (Figure 4a). One of critical difference between these two types of sites is that M−N2+2-C8 site contains adjacent carbon atoms with dangling bonds, while M−N4-C10 does not. Examining the reaction thermodynamics, we calculated the free energy evolution of CO2 reduction to CO on these two sites assuming a reaction pathway involving the formation of COOH* and CO* intermediates via the coupled proton-electron transfer process (Figure S21).2,4,9 We found that it always requires an increase of free energy from CO2 gas phase transition to COOH* adsorbed on both the active sites under an electrode potential (U) of 0 V, indicating nonspontaneous nature of CO2RR. However, with a further decrease of U below a critical limiting potential, the elemental step of CO2(g) to COOH* could become thermodynamically favorable, as shown in Figure 4b, and hence, the CO2RR can proceed on both the M−N4-C10 and M−N2+2-C8 sites. Consequently, this limiting potential defines the onset potentials of CO2RR on these active sites, which were also used for predicting the HER (Figure S22). By correlating catalyst activities with theoretical predications, we found the obtained DFT results (Table S4) well support our experimental measurements. For example, the CO2RR starts at −0.10 V on Fe−N2+2-C8 and −0.52 V on Co−N2+2-C8, in good agreement with the experimental trend that onset potential of CO2RR on Fe-N-C (−0.29 V) is more positive than that of Co-N-C (−0.38 V). The HER can start on Co−N4-C10 site at −0.36 V, which is more positive than −0.52 V for CO2RR on Co−N2+2-C8 site, explaining the low selectivity measured with the Co-N-C catalysts for CO2RR owing to strong competition by HER. The potentials of HER started on both Fe−N4-C10 and Fe−N2+2-C8 sites are more negative than those for the CO2RR, validating the observed 11 ACS Paragon Plus Environment

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high selectivity of Fe-N-C catalysts for CO generation during the CO2RR because of the thermodynamic suppress of HER.

Figure 4. (a) Atomic structure of M−N4-C10 and M−N2+2-C8 (M=Fe or Co) active sites. (b) Calculated free energy evolution of CO2 reduction to CO on M−N2+2-C8 sites under applied electrode potential (U) of 0 V and −0.6 V. (c) The initial and final state for the COOH dissociation reaction on M−N4-C10 and M−N2+2-C8 sites. In the figure, the gray, blue, yellow, red, and white balls represent C, N, M, O, and H atoms, respectively.

Furthermore, we calculated the heat of reaction for non-electrochemical reaction step of COOH* → CO* + OH*, through which one of C=O bonds is broken. As illuminated in Figure 4c, the initial state of this elementary step is COOH adsorbed on the central M atom, whereas the final state is that CO is adsorbed on the M site and OH is adsorbed on an adjacent C atom with dangling bonds after cleavage 12 ACS Paragon Plus Environment

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of the C-O bond. Our DFT results in Table S4 show that this COOH dissociation step is an endothermic process requiring more than 1.0 eV external heat on both Fe−N4-C10 and Co−N4-C10 sites, but is a desirable exothermic process on Fe−N2+2-C8 and Co−N2+2-C8 sites. These results suggest that it requires insurmountable activation energy for the COOH dissociation on M−N4-C10 sties, but only requires reasonable activation energy on M−N2+2-C8 sites. We also calculated OH adsorption on saturated carbon atoms of M−N2+2-C8 (Figure S23), showing positive heat of reaction similar to that on M−N4-C10. This highlights that the carbon atoms with dangling bonds play key roles in the COOH dissociation. Note that although the dangling bond on the edge carbon may be compensated by the H and OH from water, the dissociation of OH from COOH may react with the H atom and generate H2O, and the OH could possibly acquire an electron to generate OH− over the reduction potential range, thus maintaining the regeneration and stability of active carbon atoms with dangling bond. Previous experimental results also indicated that OH− is more favorable for adsorption on edge sites of carbon in aqueous solution,45 thus, the dissociated OH after the cleavage of C=O bond might have strong adsorption onto the C atoms with dangling bonds next to M−N2+2 moiety. Therefore, we conclude that only the M−N2+2-C8 site having active carbon atoms with dangling bonds is thermodynamically and kinetically active for the CO2RR. This finding will provide guidance for advanced catalyst design and synthesis by creating more defects (e.g., carbon with dangling bonds) and micropores containing two graphitic flakes for M−N2+2-C8 structures. In summary, using well-defined nitrogen coordinated and atomically dispersed Fe and Co catalysts, we experimentally discovered that Fe−N4 sites are intrinsically more active than Co−N4 sites in M-N-C catalysts for CO2 reduction to CO with much higher FE, more positive onset potential, and larger 13 ACS Paragon Plus Environment

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partial current densities. The experimental observation was fully supported by theoretical prediction. Furthermore, the edge-hosted M−N2+2-C8 sties bridging two armchair-like graphitic layers was computationally identified to be the active moiety for the CO2RR. More specifically, the M centers and C atoms with dangling bonds next to N are the active sites to adsorb *CO and *OH in the cleavage of C−O bond during the CO2RR, respectively. We believe that these new findings regarding the champion Fe−N2+2-C8 sites will provide guidelines on the design of advanced M-N-C catalysts for developing an efficient and cost-effective electrochemical CO2-to-fuels process.

ACKNOWLEDGMENTS G. Wu thanks the financial support from the Research and Education in eNergy, Environment and Water (RENEW) Institute at the University at Buffalo, SUNY. Y. Li acknowledges the support from American Chemical Society Petroleum Research Fund (ACS-PRF, grant no. 58167-ND10). G. Wang gratefully acknowledges the support from National Science Foundation grant number ACI-1053575. Electron microscopy research was conducted at the Center for Nanophase Materials Sciences of Oak Ridge National Laboratory, DOE Office of Science User Facilities. Z. Feng thanks Callahan Scholar Fund of Oregon State and Department of Energy (No. DE-AC02-06CH11357) for use of 4-ID, 5-BM and 9-BM at APS of ANL. We thank Dr. Ye Lin for the XPS analysis.

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