Kinetic Behavior of Catalytic Active Sites Connected with a Conducting

Article pubs.acs.org/JPCC

Kinetic Behavior of Catalytic Active Sites Connected with a Conducting Surface through Various Electronic Coupling Shino Sato,†,‡,§ Kotaro Namba,† Kenji Hara,∥ Atsushi Fukuoka,⊥ Kei Murakoshi,† Kohei Uosaki,†,‡,# and Katsuyoshi Ikeda*,‡,§,○ †

Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-0810, Japan Global Research Center for Environment and Energy Based on Nanomaterials Science (GREEN), National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan § Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466-8555, Japan ∥ Department of Applied Chemistry, School of Engineering, Tokyo University of Technology, 1404-1 Katakura, Hachioji, Tokyo 060-0810, Japan ⊥ Catalysis Research Center, Hokkaido University, Sapporo 001-0021, Japan # International Center for Materials Nanoarchitectonics (WPI-MANA), NIMS, Tsukuba 305-0044, Japan ○ PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ‡

S Supporting Information *

ABSTRACT: Electrocatalytic model surfaces consisting of catalytically active local surface sites and conducting electrode were formed by layerby-layer assembly of functional molecules. The strength of electronic coupling between the catalytic surface sites and the electrode was tuned by changing the length and structure of wire molecules. When cobalt(II)−tetraphenylporphyrin was employed as a model surface site, the induced activity for oxygen reduction reaction was critically dependent on the coupling strength through the wire molecules. The overall reaction kinetics was disentangled to contributions of the internal electron tunneling and the intermolecular electron transfer. In the weak coupling condition with nanometer separation between the porphyrin and electrode, the overall reaction rate was limited by coherent quantum tunneling rate inside the catalytic system. In the strong coupling condition with the closer separation, on the other hand, the activity of the catalytic sites was significantly suppressed, and the electrochemical charge transfer rate was thus rate-limiting. This result provides a detailed insight for molecular-scale design methodology of highly efficient electrocatalysts.

1. INTRODUCTION

connection between the substrate and surface sites as well as the catalytic performance of the surface sites. For the electronic connection between the substrate and surface sites, two theoretical approximation conditions can be assumed, depending on the coupling strength. In the approximation of the strong coupling limit, the activity of the catalytic sites is not independent of the electronic structure of the substrate, and thus theoretical design of catalytic sites is rather complicated.11 For example, nitrogen-doped graphene is a typical case for this limit; although nitrogen atoms embedded into the conducting carbon matrix are expected to function as active sites for oxygen reduction reaction (ORR),12 the detailed mechanism of the activity enhancement still remains under discussion. The direct attachment of molecular catalysts onto a conducting surface may reduce the activity of the catalytic sites

Catalytic activity of an electrocatalyst is sensitively affected by its geometric and electronic surface properties.1,2 It is therefore of great interest to understand the relation between catalytic activity and atomic- or molecular-scale surface structures.3−7 Electrocatalytic reactions take place along with electron transfer (ET) between delocalized electronic states of an electrode and localized molecular orbitals of reactants or products. In a practical electrocatalyst with various surface sites exposed, such ET is thought to proceed preferentially at specific local surface sites with catalytically active surface structures. Therefore, a simple model for electrocatalysts may consist of a catalytically inactive conducting substrate and catalytically active local surface sites. Indeed, many attempts have been made to fabricate efficient electrocatalysts by direct attachment of molecular catalysts onto a conducting electrode such as a carbon, graphite, or metal substrate.8−10 The overall kinetic behavior of such a system should be dependent on electronic © XXXX American Chemical Society

Received: September 25, 2015 Revised: November 10, 2015

A

DOI: 10.1021/acs.jpcc.5b09364 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. Schematic illustration of layer-by-layer stepwise assembly of catalytic chemical layers on a gold electrode. The catalytic local surface sites are created using CoTPP molecules with the catalytic activity for ORR, which are adsorbed on top of SAMs of wire molecules. The electronic connection between the catalytic sites and electrode can be varied by alternating the wire molecule.

Figure 2. Flowchart of the analytical procedure for the ORR kinetics of CoTPP/ImC9SH-SAM/Au. (left) ORR polarization curves, iORR(ω, E), measured in O2-saturated 0.5 M H2SO4 solution at the sweep rate of 10 mV·s−1 with different rotation rates. (middle) Koutecky−Levich plots of iORR(ω, E) for various electrochemical potentials. (right) Tafel plots obtained from iac(E). Finally, the ω-independent k0 and α can be obtained from iORR(ω, E).

molecular design of catalytic active groups to increase EToutside.8−10 These studies have usually been carried out on thin films of catalytic molecular layers directly attached on a conducting electrode, where the catalyst−electrode interface is uncontrolled. For further improvement in molecular-scale design of electrocatalysts, therefore, it is necessary to understand how the electronic coupling among the conducting electrode and catalytic active sites affects the overall kinetics. In this work, a comprehensive study of heterogeneous ET kinetics of molecular catalyst-modified electrodes is presented in terms of the contribution of the electronic coupling strength between molecular catalysts and electrode. A well-defined catalyst/wire/electrode system was fabricated by layer-by-layer assembly of functional molecular components on an Au electrode, which enables tuning of the strength of the interface coupling in the layered system by alternating wire molecules. The molecular catalyst was fixed to cobalt(II)−tetraphenylporphyrin (CoTPP) with the activity for ORR.10 Kinetic behavior of ORR on the CoTPP adlayer was critically dependent on the length and structure of the wire molecules. In particular, the contributions of ETinside and EToutside were disentangled in the overall kinetics, and thus the breaking of the weak coupling limit in the kinetic behavior was systematically investigated near the electrode surface. This result provides how chemically

due to the strong coupling. In contrast, when the weak coupling limit is assumed, the overall reaction rate may be governed by the internal ET between the electrode and the catalytic sites (ET inside the catalyst−electrode system, ETinside) rather than the catalytically assisted ET between the catalytic sites and reactant molecules (ET outside the catalyst, EToutside). Clearly, the coupling strength needs to be optimized between these two limits in order to maximize the overall reaction rate. In this sense, a molecular catalyst-modified electrode seems to be a good example when catalytic functional groups are immobilized onto a conducting surface via molecular wires such as a long alkyl chain.13 Electrochemical behavior of molecule-modified electrodes has been extensively studied by many researchers.14−18 For example, a ferrocene-terminated alkanethiol monolayer on a metal electrode exhibits characteristic redox behavior of ferrocene moieties. The kinetics of ETinside between the ferrocene and electrode can be well explained by assuming a typical quantum electron tunneling process when the alkyl chain is sufficiently long.17,19 In contrast, for heterogeneous electrochemical reactions involving both ETinside and EToutside, limited information is currently available for each contribution to the overall reaction rate.20,21 Most of the studies regarding molecular catalyst-modified electrodes have focused on B


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behavior of the overall reaction on CoTPP/ImC9SH-SAM/Au is described by the ω-independent parameters, k0 = 49 ± 5 cm· s−1 and α = 0.31 ± 0.01, although the contributions of both ETinside and EToutside are not disentangled.

functionalized electrodes can be constructed at the molecular scale in order to improve their performance.

2. METHODS 2.1. Layer-by-Layer Assembly of CoTPP-Modified Electrodes. In Figure 1, the upper left panel shows a schematic illustration of a series of wire molecules utilized to connect CoTPP molecules and Au electrode in this study: imidazole tagged alkanethiols (ImCmSH, m = 3, 5, 7, 8, 9, 10, 11, 12), 4-pyridinethiols (PySH), 4,4′-bipyridine (biPy), hexane diisocyanide (C6DI), 1,4-phenylene diisocyanide (PDI), 4,4′terphenyl diisocyanide (TPDI), and 1,4-bis[2-(4isocyanophenyl)ethynyl]benzene (OPEDI). Since these molecules have an anchor group such as thiol or isocyanide, wellorganized self-assembled monolayers (SAMs) of these molecules can be formed on a mirror-finished Au disk electrode.22−26 CoTPP is then attached on top of the SAMs through axial coordination of the central Co ion with another terminal group of the wire molecule.9,27−30 Details for this layer-by-layer formation of CoTPP/wire/Au electrodes are described in the Supporting Information. The lengths of the wire molecules were estimated using a Hartree−Fock calculation with 6-31G* basis sets (Gaussian 09, revision A02). 2.2. Kinetic Analysis of ORR on CoTPP-Modified Electrode. Figure 2 shows a case of the analytical procedure for the kinetics of ORR on the CoTPP/ImC9SH-SAM/Au, measured in O2-saturated 0.5 M H2SO4 solution using the rotating disk electrode (RDE) method.31 In this method, the ORR current density, iORR, is obtained as a function of both ω and E, where ω is the angular velocity of the RDE in rad·s−1 and E is the potential in V vs RHE (reversible hydrogen electrode), as shown in the left panel of Figure 2. The measured iORR(ω, E) can be then analyzed according to the Koutecky− Levich equation,32

3. RESULTS AND DISCUSSION 3.1. ORR Activity of CoTPP/Wire/Au. Figure 3 shows typical RDE polarization curves of ORR measured for ImC9SH-

Figure 3. ORR polarization curves for ImC9SH-SAM/Au and CoTPP/ImC9SH-SAM/Au, measured in O2-saturated 0.5 M H2SO4 solution using the RDE method at the sweep rate of 10 mV·s−1 with the rotation rate of 1000 rpm.

SAM/Au and CoTPP/ImC9SH-SAM/Au. In the absence of CoTPP, the ORR current was quite small in the entire region of electrochemical potentials. On the other hand, a significant increase of the ORR current was observed in the presence of CoTPP, indicating that CoTPP is indeed capable of catalyzing ORR. It is here noted that this confirmation was repeated for all of the wire molecules used in this work, prior to the kinetic analysis of the ORR polarization curves. In the Koutecky−Levich analysis, n(E) approached 2 under sufficient negative overpotential application, indicating that the reaction on CoTPP involves the exchange of two electrons per O2 molecule: O2 + 2e− + 2H+ → H2O2. This is in good agreement with the previous reports for CoTPP.6,10,37 Incidentally, a two-electron reduction of O2 is believed to be much simpler, compared with a four-electron reduction of O2.10,38 Therefore, CoTPP was chosen in this study as a model catalyst to investigate the contributions of ETinside and EToutside. The overall kinetic behavior of the modified electrode may be dependent on the surface density of the catalytic active CoTPP immobilized on the electrode. In the present system, the surface density was able to be measured using the redox response of the central Co ions. The CoTPP density estimated for various wire molecules was in the range of (6.0 ± 1.5) × 10−12 mol·cm−2. Although this density distribution was not so small, the influence to the ORR kinetics was still negligible for the relatively low current density in the present experiments (for more details, see the Supporting Information). 3.2. Wire Length Dependence of k0 and α for a Series of CoTPP/Wire/Au. When the wire length, r, is changed in the layered system, ETinside is different as a result of the change in the electronic coupling strength between CoTPP and Au. According to many reports on molecular conductance in metal−molecule−metal junctions, when ETinside can be treated as a coherent quantum tunneling in the weak coupling limit, the rate of ETinside decays exponentially with increasing r.39,40

1/iORR (ω , E) = 1/iac(E) + 1/iL(ω , E) = 1/n(E)Fk ORR (E)cO2 + 1/0.2n(E)FD2/3ν−1/6cO2 2ω1/2

(1)

where iac and iL denote the activation-controlled current density and the mass-transport-limited current density, respectively, and n(E) is the number of electrons transferred per O2 molecule, F the Faraday constant, kORR(E) the reaction rate constant, cO2 the concentration of dissolved O2, D the diffusion coefficient of O2, and ν the viscosity of solvent. For these parameters, the following values were utilized in the analysis: cO2 = 1.2 × 10−6 mol·cm−3, D = 1.9 × 10−5 cm2·s−1, ν = 1.1 × 10−2 cm2·s−1.33−36 As shown in the middle panel of Figure 2, iac(E) and iL(E), i.e., kORR(E) and n(E), are obtained using the linear fitting at each potential. Further analysis for iac(E) can be made according to the Tafel equation, η = a + b log|iac|

(2)

where η = E − E0 denotes the overpotential for the redox reaction of O2/H2O2. Here, the Tafel slope and intercept are b = 2.3RT/(αnF) and −a/b = log|i0| = log|nFk0cO2|, respectively. The transfer number, α, is assumed to be potentialindependent. k0 is the rate constant for the exchange current at the equilibrium, k0 = kORR(η = 0 V). As shown in the right panel of Figure 2, log|iac| is proportional to η. Thus, the kinetic C


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wire length region. Thus, the decrease in k0 may be ascribed to a decrease in EToutside as a result of the change in the activity of CoTPP, which will be discussed later. The wire length dependence of k0 is more clearly presented in Figure 4b, where k0 obtained for other types of wire molecules are plotted along with those for the series of ImCmSH. It is noted that biPy, PDI, TPDI, and OPEDI have a π-conjugated chain, while C6DI and ImCmSH have a saturated alkyl chain. PySH is the shortest wire molecule in the present study. In the longer wire length region (m > 8 or r > 13 Å), k0 for TPDI and OPEDI was much larger than the values expected from the exponential relation of ImCmSH, indicated by the red line. It is widely accepted that molecular conductance of πconjugated molecules is larger than that of saturated molecules (for example, the decay coefficient, β, is estimated to be ∼0.27 Å−1 for the chain body of OPEDI and 0.78 Å−1 for the alkyl chain).39−43 Therefore, this tendency is readily acceptable when ETinside is rate-limiting. For the shorter wire length region, on the other hand, k0 for PySH, biPy, PDI, and C6DI showed a similar tendency with the series of ImCmSH; their deviation from the red line was on the same trend with ImCmSH. This result shows that k0 is not affected much by β of wire molecules in this region, suggesting that EToutside is rate-limiting. Indeed, PDI and C6DI are good examples for this conclusion because they have the same anchor groups but the different chain structure; k0 for PDI was smaller than that for C6DI whereas PDI should be more conductive than C6DI. In addition to k0, the transfer number, α, is also dependent on r. As shown in Figure 5, α obtained for various types of

However, the relation between ETinside and k0 may not be straightforward because EToutside is involved in the heterogeneous reaction. Figure 4a shows a semilog plot of k0 obtained

Figure 4. (a) Semilog plot of k0 obtained for a series of CoTPP/ ImCmSH-SAM/Au as a function of the wire length of ImCmSH. (b) Wire length dependence of k0 for other wire molecules in addition to (a).

for a series of CoTPP/ImCmSH-SAM/Au as a function of the wire length of ImCmSH. For m > 8, i.e., for the relatively long wire length (r > ∼13 Å), an exponential relation is clearly observed; the exponential decay coefficient, β, in k0 ∝ e−βr was estimated to be 1.0 ± 0.1 Å−1. This is close to the value reported for electron tunneling through alkyl chains.17,39−41 When r is sufficiently long, it is naturally expected that the electronic coupling can be treated under the approximation of the weak coupling limit. Under this circumstance, the activity of CoTPP remains the same for changing r, so that the observed length dependence of k0 can be ascribed to the change in the rate of ETinside. Therefore, the exponential decay observed for m > 8 indicates that ETinside is rate-limiting in this wire length region. On the other hand, for m ≤ 8, i.e., for the relatively short wire length (r < ∼13 Å), k0 is clearly apart from the exponential relation. Notably, this deviation becomes larger with decreasing r; the maximum k0 was observed when r was at ∼13 Å. It is expected that ETinside became comparable with EToutside in this

Figure 5. Wire length dependence of α obtained for a series of CoTPP/wire/Au.

CoTPP/wire/Au increases with decreasing r; this trend becomes pronounced especially for m ≤ 8. Clearly, the wire length dependence of α closely correlates with that of k0. In a heterogeneous electrochemical reaction, α denotes the fraction of the interfacial potential that can lower the free energy barrier for the reaction, and thus represents sensitivity of the reaction rate toward the potential application. Therefore, the rapid increase in α in the shorter wire length region indicates that the influence of the applied potential to CoTPP becomes more significant near the Au surface; the electronic coupling between CoTPP and Au is stronger in this region. Consequently, the correlated behavior between k0 and α indicates that the ORR activity of CoTPP is diminished near the Au surface due to the electronic coupling between CoTPP and Au electrode. Importantly, the remarkable features of k0 and α shown in D


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bridge−acceptor model.39,40,44−47 The rate of the electron transfer from the donor to acceptor, kD→A, is given by

Figures 4 and 5 are not limited to CoTPP. A similar correlation was observed even when catalytic surface sites are formed using cobalt(II)−phthalocyanine (CoPC), which also functions as a two-electron ORR catalyst9,10 (see the Supporting Information). As shown in Figure S4, the overall tendency of the wire length dependences was similar between CoPC and CoTPP with different activity. On the other hand, the degree of the change in both k0 and α near the Au surface was smaller in CoPC than in CoTPP. This implies that the coupling strength between the catalyst molecules and Au electrode is affected by the electronic nature of the macrocyclic ligand around the central Co ion. 3.3. Temperature Dependence of k0. The characteristic wire length dependence of the ORR kinetics near Au surface is thought to be due to the change of the rate-limiting step between ETinside and EToutside. The former is characterized by the coherent quantum electron tunneling through the wire molecules. The latter is related to the chemical ET catalyzed by CoTPP. These two different processes are expected to exhibit different temperature dependence. According to the Arrhenius equation, the temperature dependence of the reaction rate can be expressed as ln k 0 = ln A − ΔG⧧/(RT )

kD → A = (2π /ℏ)|VDA|2 DFC

(4)

where VDA is the coupling between the donor and acceptor electronic states and DFC is the thermally averaged, Franck− Condon-weighted density of nuclear states. The coupling, VDA, is dependent on the bridge length. In the present heterogeneous electrochemical system involving the catalytically assisted EToutside, the donor and acceptor correspond to the anchor group chemisorbed to the Au electrode and CoTPP interacting with O2, respectively, and thus kD→A corresponds to the rate of ETinside. When the coupling is weak, the internal ET can be treated as a coherent quantum tunneling through the virtual electronic states. Then, the length dependence of the rate constant is kD→A ∝ e−βr, as already mentioned. Indeed, k0 decayed exponentially with increasing r when m > 8 (r > ∼13 Å) in the present study. Moreover, k0 was dependent on β in the same wire length region. It is evident that ETinside is ratelimiting for k0 when r > ∼13 Å. On the other hand, in the strong coupling limit for eq 4, the nearest neighbor interactions may not be dominant in the bridge or the electron may be delocalized over the entire donor−bridge−acceptor system, resulting that the length dependence of kD→A is inversely proportional to r. In the present electrochemical system, a deviation from the quantum tunneling behavior should be expected for ETinside in a shorter wire length region. However, such Ohmic behavior cannot explain the significant decrease in k0 observed for m ≤ 8 (r < ∼13 Å). Instead, the alternation of the rate-limiting process from ETinside to EToutside was strongly suggested by the experimental results: the β-independence of k0 and the larger temperature dependence for the shorter wire molecules. Therefore, the decrease in k0 near the Au surface is mainly explained by the decrease in EToutside as a result of a drop in the ORR activity of CoTPP. It is here noted that a slight deviation from the quantum tunneling behavior has also been reported in ETinside for the ferrocene-terminated alkanethiol monolayers of m < 8, which does not include EToutside.16 Notably, this deviation was not able to be explained by a decrease in the reorganization energy in Marcus theory,16 which is induced by the electromagnetic interaction with a mirror dipole in the metal. Redox responses of metalloprotein such as azurin at alkanethiol-covered Au(111) also showed a deviation from the quantum tunneling behavior for m ≤ 8, which was explained by contribution of configurational rearrangement of adsorbed azurin.48−51 According to the similar length dependence with the present ORR system, it is thought that the electrochemical natures of functional groups with metal ion centers such as ferrocene, azurin, or CoTPP are affected by the delocalized electronic states of the conducting electrode. Indeed, the ORR behavior observed for CoTPP on the shortest PySH (e.g., the transfer number of 0.42 ± 0.02 and the Tafel slope of −0.14 V/dec) was rather similar to that for graphite electrodes without surface modification,10 indicating the loss of the characteristic nature of CoTPP. Of course, detailed modeling of the electronic coupling in the complete system including larger gold contacts and even the medium will be needed to support this explanation for the experimentally observed wire length dependence of the ET kinetics.

(3) ⧧

where A is pre-exponential factor, ΔG is standard free energy of activation, and T stands for temperature in kelvins. In Figure 6, the temperature dependence of k0 is presented as ln k0 vs 1/

Figure 6. Arrhenius plots of k0, measured for CoTPP/PySH/Au and CoTPP/ImC10SH/Au.

T for two wire molecules: the shortest PySH and much longer ImC10SH. As expected, the rate of ORR on CoTPP/wire/Au exhibited different temperature dependence between these wire molecules. In the case of the shortest PySH, k0 was significantly changed against temperature, which agreed with the assumption that EToutside was rate-limiting. The activation energy, ΔG⧧, for ORR was estimated to be 89 ± 2 kJ/mol by fitting the observed linear relation; the relatively large activation energy may be due to the suppression of the activity near the Au surface. In contrast, the temperature dependence of k0 was much smaller for ImC10SH, suggesting that ETinside is indeed rate-limiting. 3.4. Influence of Electronic Coupling Strength between Catalytic Local Surface Sites and Conducting Electrode. Intramolecular ET behavior has been extensively studied theoretically and experimentally using a donor− E


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Innovative Areas “Nano Informatics” (No. 25106010) from JSPS, World Premier International Research Center (WPI) Initiative on Materials Nanoarchitechtonics and program for Development of Environmental Technology using Nanotechnology from Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Dr. G. Yu (Catalysis Research Center, Hokkaido University) is acknowledged for experimental help.

4. CONCLUSIONS The degree of the electronic coupling between catalytic active sites and conducting electrode was systematically tuned by using various types of wire molecules. In the weak coupling condition with the relatively large catalyst−electrode separation, the electronic states of catalytic local sites are rather isolated from the conducting electronic states. Consequently, the reaction rate is limited by the electron transport rate at the catalyst−electrode interface while the activity of the catalytic sites is invariant by immobilization to the surface. Under this circumstance, the molecular design for activity enhancement of surface sites is not essential in the overall reaction kinetics. In the strong coupling condition with the relatively short catalyst− electrode separation, the electronic states of the catalytic sites are no longer independent from the conducting electronic states. Then, the catalytic activity is significantly lowered while the electron transport at the interface becomes fast enough. This takes place even when the catalytic molecules are not directly attached on the electrode. To develop highly efficient electrocatalysts, therefore, it is necessary to tune the electronic coupling between catalytic sites and a conducting electrode so that both of the internal electron transport and the catalytic activity are properly balanced. Indeed, it has recently been reported that the ORR activity is substantially enhanced when FePC is connected with carbon nanotube surfaces through molecular wires, compared with the direct attachment of FePC onto carbon nanotubes.52,53 In the present study, the maximum k0 was obtained when the wire length was at around 13 Å. In a practical electrocatalyst, however, α must also be considered to achieve a higher current density under relatively low overpotential. The efficient electrochemical lowering of the free energy barrier is obtained when α is close to 0.5. Therefore, the optimum coupling will be achieved when the catalyst−electrode separation is slightly shorter than the distance for the maximum k0. Suppression of the activity is of course unavoidable in this region. However, this could be adjusted by molecular design of the catalytic sites, because the structural difference of the macrocyclic ligand between CoTPP and CoPC clearly affected the degree of the decrease in the activity near the Au surface. The molecular-scale design for efficient electrocatalysts will be advanced by considering the activity of catalytic local sites and the internal electronic connection to a conducting surface separately.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b09364. SAM formation of wire molecules, surface density of catalytic active sites, and comparison of ORR activity (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81.52.735.5484. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially supported by Grant-in-Aid for Young Scientists (A) (No. 24681018), Scientific Research on F


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