Electronic Tuning of Cobalt Porphyrins Immobilized on Nitrogen

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Electronic Tuning of Cobalt Porphyrins Immobilized on Nitrogen-Doped Graphene for CO2 Reduction Minghui Zhu, Chenxi Cao, Jiacheng Chen, Yang Sun, Ruquan Ye, Jing Xu, and Yi-Fan Han ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00368 • Publication Date (Web): 14 Mar 2019 Downloaded from http://pubs.acs.org on March 17, 2019

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ACS Applied Energy Materials

Electronic Tuning of Cobalt Porphyrins Immobilized on Nitrogen-Doped Graphene for CO2 Reduction Minghui Zhu,[a] Chenxi Cao,[a] Jiacheng Chen,[a] Yang Sun,[a] Ruquan Ye,[b]* Jing Xu[a] and Yi-Fan Han[a,c],* [a]

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai

200237, China [b]

Department of Chemistry, City University of Hong Kong, Hong Kong 999077, China.

[c]

Research Center of Heterogeneous Catalysis and Engineering Sciences, School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, China

Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

ABSTRACT Electroreduction of CO2 into CO can be catalyzed by cobalt porphyrins, and the performance can be improved by peripheral functionalization or heterogenization. Herein, we report a facile approach to electronically tune the metal center. Through the introduction of nitrogen atoms onto the graphene support, the activity of cobalt porphyrin/graphene composite can be increased by a factor of two. DFT calculations and XPS analysis revealed that nitrogen atoms embedded in graphene increase the electron density of cobalt atoms and boost its activity for CO2 electroreduction. Such a methodology has the potential to be extended to other catalytic systems involving molecular catalysts.

Keywords: carbon dioxide; electroreduction; graphene; nitrogen doping; cobalt porphyrin

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Electrochemical CO2 reduction is an attractive technique for energy storage and production of value-added commodity chemicals.1,2 Numerous type of catalysts were developed with high activity, selectivity and stability towards various chemicals.3–5 Among them, molecular complexes with inexpensive metal centers such as Co and Fe usually possess ultra-high selectivity toward CO, which is of industrial importance as the it can be further converted into a wide range of value-added products, including acids, esters, and alcohols via downstream thermal processes.6–9 One of the most common strategies to improve the activity and selectivity of molecular catalysts is peripheral functionalization. The turnover frequency (TOF) of cobalt phthalocyanine (CoPc), which can selectively convert CO2 to CO, was reported to be improved through modification with eight butoxy (BuO) substituents. This was attributed to the electron-donating nature of BuO that facilitates the coordination of CO2 as well as the electron transfer from the catalyst to the adsorbed CO2.10 An electron-withdrawing cyano substituted CoPc(CN)8 catalyst was reported to exhibit higher activity than CoPc molecules as well, and was explained by facilitated CoII/CoI reduction as well as CO desorption.11,12 In addition, perfluorinated cobalt phthalocyanine (CoPcF16) was found to be more active than CoPc. The electron-withdrawing fluorine substituents were considered to protect the catalyst from being poisoned by CO and make the CoI state more accessible.13 Ambre et al.

14

has systematically studied the effect of ester

groups on iron tetraphenylporphyrin (FeTPP) for electrocatalytic CO2 to CO conversion. Both the selectivity and activity were found to depend on the stability of Fe-COOH intermediates that can be electronically tuned by changing the position of ester groups.14 In addition to the throughcharge effect, the peripheral functionalities were also proved to be capable of inducing throughspace interaction. Positively charged substituents on FeTPP was reported to directly interact with and stabilize the Fe-CO2 intermediate and promote the electroreduction of CO2.15 Heterogenization of these catalysts leads to much increased number of available active sites, resulting in high current densities and favors industrial application.16–19 The molecular catalysts were usually immobilized onto carbonaceous supports through π-π interaction due to their high electron conductivity and surface area.20 For example, Zhang et al. immobilized CoPc on carbon black, carbon nanotube and reduced graphene oxide, respectively. The CoPc/CNT composite exhibited the best activity for electroreduction of CO2 compared to unsupported catalyst as well as CoPc on other type of supports.12 Hu et al. has demonstrated that immobilizing CoTPP on carbon significantly changes its activity, selectivity and even reaction pathway.17 The underlying mechanism was not discussed in detail, but likely due to the electronic interaction between the


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support and CoTPP molecules. Similar interaction has been demonstrated by several studies on the molecular complex with metal support as a model system.21–24 Although significant improvements have been achieved by immobilizing the molecular catalysts onto carbonaceous support, the understanding of the correlation between the electronic structure of the support and the CO2 electroreduction activity of molecular catalyst has yet to be studied. In this work, we demonstrate that carbonaceous support itself can serve as a media to tune the electronic structure and catalytic performance of the immobilized molecular catalysts. In the context of cobalt porphyrin, we successfully prepared hybrid catalyst with cobalt porphyrins dispersed on nitrogen-doped reduced graphene oxide (NrGO). Nitrogen was confirmed both experimentally and theoretically to play an important role in increasing the electron density of the catalytically active cobalt center, which results in an increased activity for electrochemical CO2to-CO conversion. This porphyrin-support interaction is extendable to a wide range of molecular complexes and reaction systems. To synthesize the hybrid CoP@NrGO catalyst, cobalt tetra(4-trimethylanilinium) porphine tetrachloride (CoTMAP) was firstly dispersed on the graphene oxide (GO) through electrostatic interaction between the negatively charged functionalities on GO and the cationic porphyrins (figure. 1a). The mixture was then placed in a urea solution and hydrothermally treated to reduce the GO and finally form the CoP@NrGO catalyst. Transmission electron microscopy (TEM) reveals that the as-prepared hybrid catalyst possesses a clean two-dimensional morphology with no aggregations (figure. 1b). This indicates the cobalt porphyrins were evenly distributed on graphene, which is further confirmed by the corresponding energy dispersive X-ray spectroscopy (EDX) maps showing the well dispersed Co signal on graphene structure (figure. 1c). UV-vis spectrum was collected on Co@NrGO and reveals a band at 448 nm, which can be assigned to the Soret band of CoTMAP (figure 1d). The small Q bands observed on Co@NrGO, which is the characteristic of cobalt porphyrin structure, further consolidates the identity of cobalt. This confirms the intact structure of the cobalt porphyrin after hydrothermal treatment.25



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Figure 1. (a) Preparation of CoP@NrGO, (b) TEM images of CoP@NrGO, (c) STEM image of the CoP@NrGO and the corresponding EDX maps of C, N and Co. (d) UV-vis spectrum of NrGO, Co@NrGO and CoTMAP.

To quantify the N content on the support while avoiding the interference from immobilized cobalt porphyrin, NrGO was synthesized separately following the same procedure with the absence of CoTMAP. X-ray photoelectron spectroscopy (XPS) analysis reveals that NrGO possesses N content of 11.25 at.%, C content of 78.60 at.% and O content of 10.10 at.%. We also prepared the hybrid catalyst with the absence of urea during the hydrothermal treatment; the asprepared catalyst, denoted as CoP@rGO-I, contains no N atom on the graphene support. Compared to NrGO, rGO-I contains a slightly higher atomic O content of 14.22 at.%, because the urea solution is a much stronger reducing agent than water. We also prepared CoP@rGO-II by introducing a pre-reduction procedure (see supplementary information) and the atomic content of O in the resulting rGO-II dropped to 12.83 at.%. Inductively coupled plasma (ICP) analysis


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revealed the Co content in CoP@rGO-I, CoP@rGO-II and CoP@NrGO are 0.60 wt.%, 0.8 wt.% and 0.8 wt.%, respectively. The electrochemical performance of CO2 reduction was tested in a customized low volume three-electrode cell.26 Linear sweep voltammetry (LSV) was first performed in a 0.5 M NaHCO3 electrolyte saturated with CO2, which shows prominent reduction current at potentials more negative than -0.4 V vs. RHE (figure. 2a). During the steady-state electrolysis, current densities slightly decreased within the tested potential range (from -0.55 to -0.80 V vs. RHE) (figure 2b). This is likely due to the leaching of immobilized porphyrins during electrolysis. It’s worth mentioned that de-metalation, which has been previously reported in copper complexes, did not take place in our case.27,28 Otherwise, the cobalt metals from de-metalation would result in exclusive formation of H2.29 Faradaic efficiencies for CO (FECO) over all three samples show similar trends, which slightly raises to 80% with the increasing of overpotentials, and then decrease at potentials more negative than -0.7 V vs. RHE (figure 2c). The decrease in FECO is concomitant with the prominent hydrogen evolution reaction (HER) catalyzed by the carbonaceous support, which produces negligible amount of CO (Figure S1).



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Figure 2. (a) Linear sweep voltammograms (LSV) recorded at a sweep rate of 5 mV/s on CoP@rGO-I, CoP@rGO-II and CoP@NrGO, (b) Current densities, (c) Faradaic efficiencies and (d) normalized CO partial current densities of CoP@rGO-I, CoP@rGO-II and CoP@NrGO at various potentials, (e) Faradaic efficiencies and (f) partial current densities of CoP@NrGO with Co loadings of 0.8 wt.%, 1.5 wt.% and 2.0 wt.%. Electrolyte: 0.5 M NaHCO3. We then normalized the CO partial current densities (jCO) against Co loadings. As shown in figure 2d, CoP@NrGO exhibited much higher normalized partial current for CO than CoP@rGOI and CoP@rGO-II over tested potentials, demonstrating the positive effect of N-doped graphene on improving the catalytic activity of immobilized cobalt porphyrins. At -0.8 V vs. RHE, jCO for


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CoP@NrGO reaches 1.7 A/mgCo, which is approximately two folds higher than CoP@rGO. We observed similar CO partial current on CoP@rGO-I and CoP@rGO-II, it confirms that under this circumstance, there is no correlation between oxygen content of support and the catalytic performance. We also calculated the ECSA surface areas for these samples (figure S3), CoP@NrGO possesses a similar area (2.5 mF/cm2) to CoP@rGO-I (2.6 mF/cm2) and CoP@rGOII (2.8 mF/cm2). Therefore, we conclude that the activity improvement on CoP@NrGO results from N-doping on the graphene support. In addition to CoP@NrGO with a Co content of 0.8 wt.%, we also prepared CoP@NrGO with a Co loading of 1.5 wt.% and 2.0 wt.% by varying CoTMAP/GO ratio in the preparation procedure. With an increase in Co loading, we observed an increasing CO partial current density attributed to more active sites. In contrast, H2 partial current densities follow the opposite trend with CoP@NrGO(2.0%) showing the lowest HER activity. This is probably because CoP@NrGO(2.0%) has the highest overall current densities and consumes the greatest number of protons; it leads to a highest local pH on the cathode surface, which in turn, suppresses the acidity sensitive HER reaction. As a result, CoP@NrGO with the highest Co loading exhibited the highest Faradaic efficiency for CO (~90% between -0.8 to -0.6 V vs. RHE). We also calculated the normalized CO partial current densities, which slightly decreased with a higher Co content (figure S4). This is probably because of the existence of diffusion limitation as a result of higher overall current densities and corresponding reactant consumption rate. We use DFT calculations to mechanistically interpret the promotion mechanism of N atoms on the graphene support. As shown in figure S5, the hydrothermal treatment with addition of urea as nitrogen source introduces three type of N atoms to the graphene support including graphitic-N (401.7 eV), pyrrolic-N (399.6 eV) and pyridinic-N (398.7 eV). Quantitative analysis shows the majority of N atoms are pyrrolic-N (54.8%) and pyridinic-N (36.7%). Therefore, we established a DFT model with these two types of hetero atoms, respectively. Schematic configuration of CoP@rGO was presented in figure 3a (top view) and figure 3d (side view). CoP was immobilized on graphene by π-π interaction, resulting in a cobalt-to-support distance of 0.342 nm. Hirshfeld population analysis was performed to quantify the electronegativity of Co atom. The Hirshfeld charge of Co atom in CoP@rGO is 0.187. The Co atom in a free-standing cobalt porphyrin exhibited a Hirshfeld charge of 0.198 (figure S6), it demonstrates a minute electronical interaction by immobilizing onto a nitrogen-free graphene support. In contrast, the addition of N atoms into rGO disturbed the electron distribution of support and cobalt porphyrin. When pyrrolic-N were introduced, the cobalt-to-support distance slightly decreased to 0.324 nm,



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indicating a stronger porphyrin-support interaction. The electron-rich N is capable to increase the electron density of Co atoms in CoP even without direct coordination; consequently, Hirshfeld charge of Co decreases to 0.166 (figure 3b,e). Similarly, analysis on the CoP@NrGO model containing three pyrrolic-N revealed a cobalt-to-support distance of 0.329 nm and Hirshfeld charge of 0.150 on cobalt (figure 3c,f). The increase in Co electron density for CoP@NrGO was further confirmed by XPS analysis, showing that Co 2p bands shifted to a lower binding energy compared to CoP@rGO (figure 4).

0.342 nm

0.324 nm

0.329 nm

Figure 3. Top view of the schematic configuration of (a) CoP@rGO, (b) CoP@NrGO with three pyrrolic-N, (c) CoP@NrGO with three pyridinic-N and side view of (d) CoP@rGO, (e) CoP@NrGO with three pyrrolic-N, (f) CoP@NrGO with three pyridinic-N. The blue, grey and white balls represent N atom, C atom and H atom, respectively. Both DFT calculation (Hirshfeld charge) and experimental result (XPS spectra) have already demonstrated that N atoms doped on the graphene, which presented primary in the form of pyrrolic-N and pyridinic-N, have the ability to tune the electronic structure of the π-π stacked cobalt porphyrins and increase the electron density around Co atoms. As a result, the cobalt sites were more nucleophilic and tend to bind CO2 more strongly. Previous studies have revealed that the active center of cobalt porphyrins for CO2 electroreduction is CoI, and the rate-determining step involves the adsorption of CO2 molecule and the concerted electron transfer step.30 Although the DFT calculation and XPS measurement were based on CoII sites, the trend of electron redistribution originates from the inductive effect and should hold true for CoI sites as well. Therefore, an increased nucleophilicity of cobalt sites can be directly corelated to the electrocatalytic performance for CO2 to CO conversion.


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Co 2p3/2 Co 2p

Intensity (a.u.)

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1/2

CoP@NrGO

CoP@rGO2

CoP@rGO1

805

800

795

790

785

780

775

770

Binding Energy (eV) Figure 4. Co 2p spectra of CoP@rGO-I, CoP@rGO-II and CoP@NrGO.

In conclusion, we have demonstrated a facile approach to tune the electronic structure of cobalt porphyrins through porphyrin-support interaction. By introducing electron-rich N atoms to the graphene support, we were able to increase the electron density of Co atoms on the immobilized porphyrins and furthermore the activity for CO2 electroreduction. This approach has the potential to be extended to other applications such as oxygen evolution reaction (OER) and oxygen reduction reaction (ORR).

ACKNOWLEDGMENTS We gratefully acknowledge the sponsorship by Shanghai Sailing Program (19YF1410600) and Start-up Grant of East China University of Science and Technology (SG1503A003). RY thanks the Start-up Grant of the City University of Hong Kong (7200600) and the CityU New Research Initiatives/Infrastructure Support from Central (APRC 9610426).

ASSOCIATED CONTENT Supporting Information Available: Experimental section, details for DFT calculation and coordination of optimized structures, partial current densities, ECSA results, structure of cobalt porphyrin, XPS results.



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DECLARATION OF INTERESTS The authors declare no competing interests.

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Electronic Tuning of Cobalt Porphyrins Immobilized on Nitrogen

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