Molecular Evidence for the Catalytic Process of Cobalt-Porphyrin

Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives. Chem. Soc. Rev. 2017, 46, 337-365. (7) Han, Y. Z.; Wu,...
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Molecular Evidence for the Catalytic Process of Cobalt-Porphyrin Catalyzed Oxygen Evolution Reaction in Alkaline Solution Xiang Wang, Zhen-Feng Cai, Dong Wang, and Li-Jun Wan J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b01229 • Publication Date (Web): 03 May 2019 Downloaded from http://pubs.acs.org on May 3, 2019

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Molecular Evidence for the Catalytic Process of Cobalt-Porphyrin Catalyzed Oxygen Evolution Reaction in Alkaline Solution Xiang Wang,†‡ Zhen-Feng Cai,†‡ Dong Wang*,†‡ and Li-Jun Wan*,†‡ †CAS

Key Laboratory of Molecular Nanostructure and Nanotechnology, CAS Research/Education Center for Excellence in Molecular Sciences, Beijing National Laboratory for Molecular Science (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡University

of Chinese Academy of Sciences, Beijing 100049, China

Supporting Information Placeholder ABSTRACT: We report an electrochemical scanning tunneling microscopy (ECSTM) study of the 5,10,15,20-tetraphenyl21H,23H-porphyrin cobalt(II) (CoTPP) catalyzed oxygen evolution reaction (OER). A highly ordered self-assembled monolayer of CoTPP is formed on Au(111) electrode. Cyclic voltammetry results show the OER activity of the electrode is enhanced with the increasing alkalinity of the electrolytes. The CoTPP molecules appear as two symmetric bright spots in STM images in alkaline solution, which is in sharp contrast to that in acidic solution. The molecular contour changes are attributed to the formation of the CoTPP-OH- species before OER, which is further confirmed by UV-Vis absorption spectroscopy. In situ ECSTM results reveal the evolution from the CoTPP-OHspecies to CoTPP molecules during OER.

Water splitting is a potential strategy for producing hydrogen to solve energy problems.1-3 However, due to the slow kinetics, the oxygen evolution reaction (OER) is often considered as a limitation for water splitting.4-6 Metalloporphyrins, a model system of the metal-organic catalysts for OER, have attracted considerable attention for the advantages of high efficiency and low cost.7-10 Understanding the catalytic role of the active sites in OER facilitates the design of high performance catalysts to overcome the kinetic barriers.11-14 A variety of experimental techniques have been used to investigate the mechanism of OER, such as X-ray absorption spectroscopy (XAS),15 electron energy loss spectroscopy (EELS),16 surface-enhanced Raman spectroscopy (SERS) 17 and electrochemical atomic force microscopy (EC-AFM).18 Electrochemical scanning tunneling microscopy (ECSTM) is a powerful technique for investigating electrocatalytic reaction at solid/liquid interface at the molecular scale. For instance, the relationships between the catalytic activity towards CO electrooxidation reaction and the step structures on platinum was revealed by ECSTM.19 The distribution of the catalytic active sites on noble metal catalysts could be distinguished by ECSTM current noise analysis.20 Yoshimoto et al. investigated the changes of an iron porphyrin adlayer on Au(111) for the electrocatalytic oxygen reduction reaction (ORR) by ECSTM.21 Gu et al. employed in situ ECSTM to investigate the ORR process

catalyzed by iron phthalocyanine (FePc).22 The reversible transformation between the high contrast FePc-O2 complexes and the low contrast FePc molecules could be observed. The MPc-O2 complex has been probed by tip-enhanced Raman spectroscopy (TERS) under O2 exposure in vacuum enrionment.23 However, there are few studies on the catalytic mechanisms of metalloporphyrin catalyzed OER at the molecular scale. Wurster et al. constructed the porphyrin based bimetallic OER catalysts by coordination interaction on Au(111).24 The ex situ STM images of the catalysts acquired before and after OER show the structure change from ordered networks to clusters, while the catalytic activity was kept. The catalytic performances of the catalysts vary significantly in solutions with different pH, which are related to the various reaction intermediates and routes.6,25 Cobalt-porphyrins are typical catalysts for OER mechanism study, and often used in alkaline environment.26,27 Herein, we investigate the OER performance of 5,10,15,20-tetraphenyl-21H,23H-porphyrin cobalt(II) (CoTPP) modified electrode in solutions with different pH at the molecular scale by ECSTM. An ordered monolayer of CoTPP was prepared on Au(111). The higher catalytic performance of a CoTPP-modified Au(111) electrode in alkaline electrolytes was revealed by cyclic voltammetry (CV). When the alkaline solution was employed, the adsorbed species with two symmetric bright spots can be observed in STM images, which are in sharp contrast to the contour of the molecules in acidic electrolyte and are ascribed to CoTPP-OHspecies. The formation of the CoTPP-OH- species was further confirmed by UV-Vis absorption spectroscopy. In situ ECSTM results reveal the transformation from the CoTPP-OH- species to CoTPP molecules when OER occurs. These results provide molecular evidence for understanding the OER mechanism. Figure 1 shows the CV curves of bare and CoTPP-modified Au(111) electrodes in different electrolytes. The catalytic activity of CoTPP towards OER varies remarkably in solutions with different pH. In 0.1 M KOH, the anode current due to OER commenced at 1.62 V, and the current density continued to increase and reached 3.5 mA/cm2 at 1.8 V. However, in 0.1 M NaClO4 and 0.1 M HClO4, the anode current density was much smaller. On the bare Au(111) electrode, the anode current commenced at 1.67 V, and the current density was also much smaller. 1

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catalytic property of cobalt phthalocyanine (CoPc) and 5,10,15,20-tetraphenyl- 21H,23H-porphyrin (H2TPP) was also examined, and almost no anodecurrent could bemeasured (Figure S2a). The catalytic activity of CoPc is much lower than that of CoTPP (Figure S2b), which is consistent with the literature.28

Figure 1. Cyclic voltammograms of bare and CoTPP-modified Au(111) electrodes in different electrolytes. Scan rate is 50 mV/s.

Figure 2 (a) Large-scale and (b) high-resolution STM images of a CoTPP adlayer on Au(111) in 0.1 M HClO4. (c-d) STM images of a CoTPP adlayer in (c) 0.1 M KOH and (d) 0.1 M NaClO4. (e) Cross-section profiles along the white dashed line in (b)-(d) (from top to bottom). (f) Proposed structural model for the CoTPP adlayer. Image conditions: (a) E = 600 mV, Ebias = -194.0 mV, It = 4.0 nA; (b) E = 600 mV, Ebias = -228.0 mV, It = 3.0 nA; (c) E = 1.30 V, Ebias = -199.0 mV, It = 1.0 nA; (d) E = 950 mV, Ebias = -200.0 mV, It = 1.0 nA. The CV results indicates CoTPP has better catalytic activity in alkaline solution. The onset potential in this condition is slightly positive than that of CoTPP on glassy carbon electrode,10 which may be due to the coupling effect between CoTPP and the substrate. The durability of the modified electrode was measured at 1.8 V in KOH by chronoamperometry (Figure S1), and the current density remained stable at around 1.5 mA/cm2 for several hours. The

Figure 2a shows a typical large-scale STM image of a CoTPP adlayer on Au(111) in 0.1 M HClO4. It can be clearly seen that the atomically flat Au(111) terrace was completely covered by CoTPP molecules and a tetragonal structure was formed. Figure 2b shows the high-resolution STM image which reveals the internal molecular structure of CoTPP. Each CoTPP molecule can be distinguished as a center bright spot, corresponding to the cobalt ion. The high contrast at the central position of CoTPP is due to the tunneling current increased by the dz2 orbital of cobalt.29 The unit cell is outlined with the white box in Figure 2b with the measured parameters of a = b = 1.51 ± 0.1 nm and α = 95 ± 1°, which are in good agreement with the previous study.29,30 The structure model for the CoTPP adlayer is proposed in Figure 2f. We carried out ECSTM investigation of the adlayer of CoTPP in solutions of different pH. The substrate potentials were held below 1.5 V, where the OER could not occur. Figure 2c and 2d shows the STM images of the CoTPP adlayer in 0.1 M KOH and 0.1 M NaClO4, respectively. In alkaline electrolyte, the specie exhibits a two-fold symmetry and consists of two bright spots with a nodal plane in the middle. Intriguingly, the coexistence of two species with different STM appearance is observed in neutral electrolyte. As shown in Figure 2d, most of the molecules show as one bright spot as that in acidic environment and some species shows similar appearance as that in alkaline solution as highlighted by red circles. The contrast difference of the adsorbed species in acidic and alkaline solution, and the coexistence of two species in neutral solution can be seen more clearly in the cross-section profiles in Figure 2e. The contrast of the adsorbed species in STM images is a comprehensive reflection of the height and electronic effect. Considering that the morphology of the adsorbed species is related to the concentration of OH-, the adsorbed species in alkaline electrolyte are ascribed to the CoTPP-OH- species. The ECSTM experiment was also performed in 10-4 M KOH, and the proportion of the CoTPP-OHspecies on Au(111) is between that in neutral and alkaline solution (Figure S3). According to the previous study, the adsorption of OH- on metal center is the initial

Figure 3. UV-Vis absorption spectra of CoTPP in mixed solutions (DMF : H2O = 9 : 1) with different electrolytes (KOH, NaClO4 and HClO4). 2

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Figure 4. Sequential STM images and cross-section profiles of the CoTPP adlayer on Au(111) in 0.1 M KOH at different potentials. The white insets (15 × 15 nm2) in (a)-(c) show high-resolution STM images of CoTPP. (d-f) Cross-section profiles along the white dashed line in (a)-(c). Image conditions: (a) E = 1.30 V, Ebias = -200.0 mV, It = 3.5 nA; (b) E = 1.80 V, Ebias = -685.0 mV, It = 3.5 nA; (c) E = 1.30 V, Ebias = -200.0 mV, It = 3.5 nA. step for OER in alkaline environment.31,32 As expected, the CoTPP-OH- species are more easily formed in electrolytes with higher OH- concentration. As the concentration of OH- is lower in neutral and acidic solution, the probability of forming the CoTPP-OH- species is also less. The STM images of catalytically inactive mole cules show no morphology distinctions in different solutions (Figure S4, 5). To further confirm the formation of the CoTPP-OH- species, UVVis absorption spectra was measured. The absorption spectra of CoTPP in mixed solutions of different OH- concentration are shown in Figure 3. With the increasing of OH- concentration, the intensity of the peak at 430 nm decreases, while a new peak with an increasing peak intensity appears at ~410 nm. Combined with the previous study, the peaks at 430 nm and ~410 nm are ascribed to the CoTPP-H2O complex and the CoTPP-OH- species, respectively, which are different from that of CoTPP in pure DMF (Figure S6).33-35 The complexation of OHto CoTPP leads to a blue shift in the absorption peak in Soret band and Q-band. However, the absorption peak position of the CoTPP-OH- species is not fixed in solutions of different pH, which is due to the formation of the six-coordinate complexes at high OH- concentration.36 Itis more difficult to form a sixcoordinate complex with OH- on Au(111). The equilibrium constant of the coordination reaction is about 106 L/mol,36 so the CoTPP molecules and CoTPP-OH- species can be observed simultaneously in neutral solution, as disclosed in Figure 2d. In situ ECSTM experiments were performed to monitor the CoTPP during OER. Figure 4 shows sequential STM images of the adsorbed species on Au(111) in KOH at different potentials. As shown in Figure 4a and d, when the substrate potential is 1.3 V (the OER does not occur at this potential), the adsorbed species appear as the CoTPP-OH- species. When the substrate potential changes from 1.3 V to 1.8 V (the OER occurs at this potential), the CoTPP-OH- species disappear and the adlayer consists of pristine CoTPP molecules (one bright spot) and some high-contrast species (Figure 4b). According to the previous study of CoTPP catalyzed ORR, the high-contrast species is ascribed to the CoTPP-O2 complex.30 This indicates that some oxygen molecules produced from OER adsorb on

CoTPP, so the number of the high-contrast species increases. The cross-section profiles in Figure 4e clearly show the height difference between CoTPP (green dashed line) and the CoTPPO2 complex (red dashed line). When the substrate potential turns back to 1.3 V, the CoTPP-OH- species (two bright spots) form again, and the number of the high-contrast species decreases due to the desorption of oxygen (Figure 4c, f). These results indicate that the transformation between the CoTPP molecules and CoTPP-OH- species is reversible at different potentials. According to the literature, the metalloporphyrinOH- species undergo a series of electron transfer processes and combine with H2O or OH- in the solution to form O2.35,37 When the in situ experiments were carried out in acidic environment, the adsorbed species appear as CoTPP molecules, and no morphology changes can be observed at different potentials (Figure S7). Notably, in our experiments, CoTPP returned to its original state after the catalytic process after 10 cycles (Figure S8) and no cobalt oxides could be observed. X-ray photoelectron spectroscopy (XPS) also reveal that there are no changes in CoTPP before and after OER (Figure S9). So the real catalytic active species in this condition are CoTPP molecules. Some previous work showed that cobalt porphyrins can be oxidized to CoOx when OER is triggered on.38 The different reaction processes caused by different solution environments and/or the substrate effect may contribute to this distinction. In summary, CV, ECSTM and UV-Vis absorption spectroscopy have been performed to investigate the CoTPP catalyzed OER in solutions with different pH. The CV results indicate that the electrocatalytic activity of the CoTPP for OER in alkaline solution is significantly higher than that in acidic and neutral solutions. STM results indicate that the CoTPP molecules can spontaneously form a highly ordered adlayer on Au(111). The uniform adsorbed species with two symmetric bright spots are observed in alkaline solution before OER, which are ascribed to be the CoTPP-OH- species. The formation of the CoTPP-OHspecies is supported by the UV-Vis absorption spectra in solutions of different pH. In situ ECSTM experiments show the reversible transformation between the CoTPP-OH- species and CoTPP molecules at different potentials during OER. This work 3

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provides a molecular evidence for the OER mechanism of the CoTPP modified electrode and expands the potential application of ECSTM in the investigations of electrocatalytic reactions.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental methods, additional electrochemistry, ECSTM and spectra data (PDF)

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

ORCID Dong Wang: 0000-0002-1649-942X Lijun Wan: 0000-0002-0656-0936

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (Grant No. 21725306, 21433011 and 91527303), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12020100), and the National Key R&D Program of China (2017YFA0204702).

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