Remote Control of Electron Transfer Reaction by Microwave Irradiation

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Remote Control of Electron Transfer Reaction by Microwave Irradiation: Kinetic Demonstration of Reduction of Bipyridine Derivatives on Surface of Nickel Particle Fuminao Kishimoto, Masayuki Matsuhisa, Takashi Imai, Dai Mochizuki, Shuntaro Tsubaki, Masato M. Maitani, Eiichi Suzuki, and Yuji Wada J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00629 • Publication Date (Web): 03 May 2019 Downloaded from http://pubs.acs.org on May 18, 2019

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The Journal of Physical Chemistry Letters

Remote Control of Electron Transfer Reaction by Microwave Irradiation: Kinetic Demonstration of Reduction of Bipyridine Derivatives on Surface of Nickel Particle Fuminao Kishimoto*, Masayuki Matsuhisa, Takashi Imai, Dai Mochizuki, Shuntaro Tsubaki, Masato M. Maitani, Eiichi Suzuki, and Yuji Wada* Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology2-12-1, Ookayama, Meguro-ku, Tokyo 152-8552 (Japan) Corresponding Author *E-mail: [email protected], [email protected] Present addresses Dr. F. Kishimoto: Department of Chemical System Engineering, School of Engineering, The University of Tokyo. 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. Mr. T. Imai: Nippon Kayaku Co., Ltd. Asa Plant, Functional Products Technical Department. 2300 Ouji-gun, Sanyo-onoda-shi, Yamaguchi 757-8686, Japan. Prof. D. Mochizuki: Department of Applied Chemistry, Graduate School of Engineering, Tokyo Denki University. 5 Senju Asahi-cho, Adachi-ku, Tokyo 120-8551, Japan.

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Prof. Masato M. Maitani: Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153-8904, Japan.

ABSTRACT. Microwave irradiation has great potential to control chemical reactions remotely, particularly reactions that involve electron transfer. In this study, we found that the reduction reaction of bipyridine derivatives on metal nickel particles was accelerated or decelerated by 2.45 GHz microwaves without an alteration of the reaction temperature. The order of the extent of the microwave acceleration of the electron transfer reaction coincided with the negativity of the redox potential of the bipyridine derivatives, i.e., the electron transfer with smaller ΔG was significantly enhanced by microwave irradiation. By applying Marcus’ electron transfer theory, we propose two mechanisms of the microwave effect on electron transfer reactions, i.e., vibration of the electrons in Ni particles to make the electron transfer easier and rotation of the water molecules to prevent the reorganization of the hydrated systems after the electron transfer reaction.

TOC GRAPHICS

KEYWORDS. Microwave, Electron transfer, Marcus theory, Heterogeneous catalysis, Solid state.


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In the past two decades, many studies have reported microwave acceleration of molecular conversions using solid catalysts1-6. Under microwave irradiation, temperature of the solid catalysts should be higher than the surroundings, because these catalysts have high-dielectric loss tangent7. The locally formed high temperature region around the solid catalysts has been observed directly by spectroscopic techniques8,9. Such a local high temperature region was considered the origin of microwave acceleration of catalytic reactions. Recently, microwave acceleration of chemical reactions without temperature alteration has been reported. This phenomenon is considered “a special effect of microwaves”10,11. However, due to a lack of physicochemical insight, the existence of such special effects is considered suspicious. Recently, we reported microwave enhancement of electron transfer reactions on the surface of solid catalysts12,13. In the case of photocatalytic reduction of bipyridine derivatives by CdS nanoparticles, the electron transfer rate from CdS to bipyridine derivatives was enhanced 3.5 times by microwaves12. Based on solid-state physics, we proposed a hypothetical mechanism, i.e., the vibration of electrons induced by the alternating electric field of microwaves accelerates the electron transfer reaction. However, the mechanism is not proven because (1) the heterogeneous catalytic reactions comprise many elementally steps and (2) it is difficult to estimate the temperature at the surface of the solid catalyst under microwaves. Thus, it is necessary for us to broaden our understanding of the desorption/adsorption pre-equilibrium of substrates on the catalyst surface. From the equilibrium constant, we can demonstrate the microwave effect on the pre-equilibrium step, and estimate the surface temperature of the solid catalyst during the chemical reaction. In this study, we examined the microwave effects on electron transfer reactions from metal Ni particles to divalent bipyridine derivatives (Bpys2+). We dispersed the Ni particles with a


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diameter of ~5 μm in water. The reaction vessel was purged with Ar, and then heated to the boiling point (373 K) by means of an oil bath or 2.45 GHz microwave resonators. Accurate temperature measurements of the dispersion were obtained using an optical fiber thermometer. Subsequently, the electron transfer reaction was initiated by adding a portion of the aqueous Bpys2+ solution. Note that no plasma or sparks were observed in all experiments. The redox potentials of Ni/Ni(OH)2 and Bpys2+ are −0.72 and −0.56~−0.39 V vs. NHE, respectively, thus, the electron transfer reactions are spontaneous exergonic reactions. The reaction rate can be evaluated by quantitative measurement of the radical reductant Bpys∙+ via in situ UV-Vis spectroscopy using a water-immersible rod shape light guide. The experimental procedure is described in detail in the Supporting Information. Figures 1(a)–(c) shows the numbers of reduced Bpys2+ radical cations generated by a oneelectron transfer reaction from Ni particles dispersed in water, which were estimated from UV-Vis absorption spectra (Figures S1 and S2) of reductant cations of Diquat (DQ·+)14, Methylviologen (MV·+)15, and N,N'-ethylene-2,2'-bipyridinium (3DQ·+)16. In these experiments, we used a microwave cavity with ellipsoidal shape to generate the TM110 mode of 2.45 GHz microwaves, where the sample vessels are positioned at the antinode of the alternating electric field (see Supporting Information). Here the temperature was maintained at 373 K under both microwave and oil bath heating. The products of the one-electron transfer reactions from the Ni particle to 3DQ2+, MV2+, and DQ2+ increased constantly, and the reaction rate under microwave heating was faster than that under conventional heating, which suggests that microwaves can enhance reduction of 3DQ2+, MV2+, and DQ2+.


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Figure 1. Time variation plots of the concentration of (a) 3DQ·+, (b) MV·+, and (c) DQ·+ under microwave heating (MW) or oil bath heating. (d) Relationship between initial reaction rate and redox potential of Bpy.

Corresponding to the reduction of the Bpys2+, the zero valent nickel metal was oxidized. Ni0 and Ni2+ peaks were observed from an XPS spectrum of the pristine Ni particles (Figure S3(a)). Thus, the surface of the pristine Ni powder was partially oxidized. After the reaction with the Bpys2+, the Ni0 peak nearly disappeared (Figure S3(b)), which means that the surface of the Ni particles was fully oxidized by the Bpys2+ reduction reaction. In addition, we observed the generation of a white precipitate after the reaction. The precipitate was identified as nickel hydroxide, Ni(OH)2, by X-ray diffraction measurement (Figure S4). Therefore, the zero valent nickel particles were oxidized to the Ni(OH)2 in parallel with the reduction of Bpys2+.


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The relationship between the redox potentials of Bpys2+/Bpys·+ and the initial reaction rate, estimated from the change of the amount of the product from 0 to 3min, is shown in Figure 1(d)17. Noted that as the negative redox potential of the Bpys2+/Bpys·+ increases, the Gibbs energy change of the electron transfer reaction decreases. Therefore, the plots in Figure 1(d) indicate a tendency for the magnitude of the microwave enhancement to increase significantly with a smaller Gibbs energy change of the reation. Here, it is expected that the acceleration of the electron transfer reaction from Ni0 to Bpys2+ is caused by the vibration of electrons directly induced by microwaves. However, under microwave conditions, confirmation of this effect of microwaves is always interrupted by a change in temperature in the vicinity of the reaction field8,18-20. In the current study, the temperature of the aqueous phase was measured using the optical fiber thermometer, however, due to the ferromagnetic property of the nickel, it was possible for the Ni powder dispersed in water to be heated locally by microwave irradiation. Thus, we must consider the possibility that the temperature of the nickel powder may be higher than the temperature of the aqueous phase. To estimate the surface temperature of the Ni particles during the electron transfer reactions, the Langmuir-Hinshelwood mechanism was applied to the reactions to analyze the adsorption/desorption pre-equilibrium of Bpys2+ on Ni particles. Initially, the activated Ni0 and Bpys2+ complex is formed by adsorption of Bpys2+. Then, the electrons can be transferred from Ni0 to Bpys2+, which results in the generation of oxdized Ni2+ and reduced Bpys·+. Therefore, the overall reaction can be described as follows. Ni0 + 2Bpys2+ ⇄ [Ni0-2Bpys2+] → Ni2+ + 2Bpys·+ (1)


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In the following, the pre-equilibrium constant to form the activated complex is denoted K. The rate constant to form the Ni2+ and 2Bpys·+ from the activated complex is denoted kET, indicating the rate constant of the electron transfer step. Since the formation of the activated complex is considered as the adsorption of Bpys2+ on the surface of Ni particles in water, a coverage factor (θ) is shown in the Langmuir adsorption isotherm at the adsorption-desorption equilibrium state. This coverage factor is expressed as follows. 𝜃 = 𝐾[𝑩𝒑𝒚𝒔𝟐 + ]/(1 + 𝐾[𝑩𝒑𝒚𝒔𝟐 + ] )

(2)

In addition, the reaction rate (v) can be represented as follows. 𝑣 = ―𝑑[𝑩𝒑𝒚𝒔𝟐 + ]/𝑑𝑡 = 𝑘ET𝜃

(3)

Then, eq. (2) is assigned to eq. (3): 𝑣 = (𝑘ET𝐾[𝑩𝒑𝒚𝒔𝟐 + ])/(1 + 𝐾[𝑩𝒑𝒚𝒔𝟐 + ] ) (4) In the initial reaction stage, the initial reaction rate (v0) and the initial concentration ([Bpys2+]0) can be assigned to v and [Bpys2+], respectively. Eq. (4) can be rewritten in follows. 1/𝑣0 = 1/(𝑘ET𝐾[𝑩𝒑𝒚𝒔𝟐 + ]0 ) + 1/𝑘ET (5) Then, we demonstrated the relationship between the inverse of the initial reaction rate (1/v0) and the inverse of the initial concentration of Bpys2+ (Figures 2(a)–(c)). These experiments under microwave conditions were also performed at the antinode of the alternating electric field in TM110 mode. The initial reaction rate was determined from the time variation plots of the concentration of generated reductants in the different initial concentrations of the reactants (Figure S5).


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According to eq. (5), the intercept of fitted line is assigned to an inverse of the electron transfer rate kET, and the equilibrium constant K can be estimated from the slope. The values are summarized in Table 1. The coincidence of the equilibrium constant under microwave heating and oil bath heating suggests that the temperature of the surface of the nickel particles should be nearly the same under both heating methods because the equilibrium is generally shifted to desorption with a higher temperature. On the other hand, the electron transfer rate was drastically increased by microwave heating compared to oil bath heating (1.70 × 10−7 L mol−1 s−1 vs. 5.02 × 10−8 L mol−1 s−1 for 3DQ2+). The electron transfer reaction rate from Ni particles to 3DQ2+ and MV2+ were accelerated 3.39 and 1.46 times due to microwave irradiation, respectively. From these results, we can conclude that the microwaves can accelerate the electron transfer reaction without the changing the reaction temperature.


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Figure 2. Relationship between the initial reaction rate and the initial concentration of (a) 3DQ2+, (b) MV2+, and (c) DQ2+. (d) Marcus plot derived from Langmuir absorption isotherm. Table 1. Results of Langmuir-Hinshelwood analysis and Marcus’ equation.

Microwave

Oil bath

Microwave/Oil bath

kET [L mol−1 s−1]

1.70 × 10−7

5.02 × 10−8

3.39

K

124

142

0.87


2.84 × 10−6

1.95 × 10−6

1.46

K

80.0

77.4

1.03


9.81 × 10−6

9.04 × 10−6

1.08

K

276

288

0.96

λ [kJ mol−1]

41.1 ± 5.13

22.5 ± 2.02

1.83

HAB2 [10−34 J2]

18.4 ± 6.88

3.55 ± 0.41

5.18

3DQ2+

MV2+

DQ2+

To understand the mechanism of the effect of microwaves on the electron transfer reaction, Marcus’ theory for outer-sphere type electron transfer reactions was applied21. In Marcus’ theory, the electron transfer rate constant is expressed as follows: 2 ln𝑘ET = ― (∆𝐺0 + 𝜆) 4𝜆𝑅𝑇 + ln (2𝜋𝐻𝐴𝐵2 ℏ 4𝜋𝜆𝑅𝑇) (6)

where ℏ is the reduced Planck’s constant, HAB2 is the electron coupling of donors and acceptors, λ is the reorganization energy and ΔG0 is the Gibbs free energy change. The ΔG0 value was calculated from the difference between the redox potential of Ni/Ni(OH)2 and Bpys2+/Bpys·+. From the fitting of the kET vs. ΔG0 plots in a quadratic function using the least squares method (Figure 2(d)), λ and HAB2 values were estimated (Table 1). With microwave heating, the HAB2 value


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increased drastically (5.3 times) compared to oil bath heating. Previously, we reported that electronic coupling between CdS nanoparticles and bipyridine derivatives has also been enhanced by microwaves (1.9 times)4. In our previous paper, we considered that the enhancement of electronic coupling should be attributed to the vibration of the electron clouds in CdS that was induced by the alternating electric field of the microwaves. Similar to the previous work, the vibration of the electron clouds in Ni particles can also be induced by microwaves. Under microwave conditions, electronic coupling between Ni particle and Bpys2+ was enhanced. Consequently, reaction speed increased. The reorganization energy under microwave heating was 1.8 times greater than under conventional heating. Reorganization energy in an aqueous solution originates from the energy that occurs when the conformation of hydrated molecules changes prior to and after the electron transfer reaction. Therefore, a large reorganization energy indicates a huge barrier to the conformation change of hydration molecules during the reaction. In fact, the reorganization energy has the following relationship with an activation free energy (ΔG‡). 2

∆𝐺 ‡ = (∆𝐺0 + 𝜆) 𝜆 (7) Thus, the increase of reorganization energy under microwave irradiation indicates that the microwaves should prevent the electron transfer reaction. The reason why the microwaves increase the reorganization energy can potentially be explained by the induction of rotational motion of water molecules under the alternating electric field of the microwaves. Thus, the induced rotational motion should prevent that reorganization of water molecules. In summary, the microwave effect on the electron transfer reaction from Ni particles to Bpys2+ can be classified as the acceleration effect attributed to the vibration of the electrons in Ni, and the deceleration effect caused by the


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rotation of the water molecules (Figure 3). The balance of the contributions of these two effects determines whether the electron transfer reaction is enhanced.

Figure 3. Proposed microwave non-thermal effect on the electron transfer reaction from Ni particle to bipyridine derivatives in aqueous phase.

To reveal that microwave acceleration of the electron transfer reaction can be attributed to the vibration of the electron in the Ni particles, we demonstrated the contribution of an isolated alternating electric field or magnetic field on the reaction by using a waveguide type microwave resonator (Figure S7(c)). The TE103 mode of standing microwaves can be generated in the resonator cavity where the phase of alternating electric and magnetic field is shifted by a quarter of the wavelength (~3.05 cm). For example, when the reaction vessel is positioned at the antinode of the magnetic field, the contribution of the electric field is negligible compared to the contribution of the magnetic field. If the microwave acceleration of the electron transfer reaction


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is based on the vibration mechanism, the acceleration should be induced only by the alternating electric field and should not be induced by the alternating magnetic field. Figure 4 shows the relationship between the initial reaction rate determined by the time courses of the concentration of produced MV·+ (Figure S6) and the electrical power applied to the microwave generator.

Figure 4. Relationship between the initial reaction rates of the electron transfer rate vs. the applied power for the microwave generator. The reaction vessel was positioned at the antinode of the alternating electric field (yellow, diamond) and the alternating magnetic field (red, square).

Under the alternating magnetic field, the rate of the electron transfer did not change significantly even if the microwave generator power was increased. Previously, we reported a microwave enhancement of the dehalogenation of organic compounds using ferromagnetic Fe particles as a catalyst in a decalin solvent22. In that case, Fe particles (diameter 45 μm) were heated locally by the alternating magnetic field of the microwaves due to magnetic loss. It is well known that Ni particles are also ferromagnetic; therefore, it can be expected that the temperature at the


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surface of the Ni particles should be high locally under the alternating magnetic field of the microwaves. However, in the current study, the reaction was not enhanced under the alternating magnetic field, i.e., the local high temperature region cannot be formed at the vicinity of the Ni particles dispersed in water. To determine the reason for this, a coefficient of heat transfer (α) at the interface of Ni(φ = 5 μm)/water or Fe(φ = 45 μm)/decalin was considered. The relationship between Nusselt number (Nu) and α is described as follows; 𝑁𝑢 = 𝛼𝐿 𝜅 (8) where L is the representative length of solid particles which is the quotient of the division of a volume by surface area, and κ is the heat conductivity of liquid. Assuming that the fluid film at the interface of the metal particles and liquids should be sufficiently thick, the Nu can be fixed as 1, which is in the case for natural convection at the interface. Since the heat conductivities of water and hydrocarbons are approximately 650 mW m−1 K−1 and 150 mW m−1 K−1 at 373 K, respectively, the coefficient of heat transfer at the interface of Ni(φ = 5 μm)/water, or Fe(φ = 45 μm)/decalin can be estimated as 783 mW m−2 s−1 K−1 and 20 mW m−2 s−1 K−1, respectively. These calculation results indicate that the diffusion of the heat energy of the Ni powder (φ = 5 μm) into the surrounding water should be approximately 40 times faster than that of the Fe powder (φ = 45 μm) into the surrounding decalin, demonstrating that the local high temperature region cannot be formed at the Ni powder diffused in water under the alternating magnetic field. Therefore, we can conclude that the microwave acceleration of the electron transfer reaction from Ni particles to Bpys2+ was not caused by the change of temperature in the vicinity of the reaction field. On the other hand, irradiation of the alternating electric field enhances the electron transfer reaction. The initial reaction rate under the alternating electric field with applied power of 14.5 W


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was 1.5 times greater than that under oil bath heating. When the applied power was less than 14.5 W, the temperature of the aqueous phase did not increase to 373 K. Interestingly, the initial reaction rate decreased when the applied power was increased to greater than 14.5 W. Compared to the reaction under oil bath heating, the electron transfer reaction rate decreased when the applied power was greater than 35 W. These tendencies well explain the trade-off relationship of the microwave effect on the electron transfer reaction discussed above, i.e., the microwave effects on the electron transfer reaction from Ni particles to Bpys2+ have two aspects, i.e., acceleration by enhancing electronic coupling and deceleration by increasing reorganization energy. In the region where the applied power was less than 20 W, the acceleration effect of microwaves was stronger than the deceleration effect. However, in association with increasing power, the contribution of the deceleration effect of microwaves increased. Under these conditions, with an alternating electric field, the electron transfer reaction rate was less than that for oil bath heating. In conclusion, we successfully demonstrated acceleration of the electron transfer reaction from Ni particles to bipyridine derivatives with different molecular flame and oxidation-reduction potential by irradiation using 2.45 GHz microwaves. The results of two experiments, i.e., back estimation of the reaction field temperature using the Langmuir-Hinshelwood model and the reaction under the alternating magnetic field, confirmed that the effect of microwave irradiation was not caused by a temperature change in the vicinity of the reaction field. Therefore, we concluded that the acceleration of the electron transfer reaction is caused by a “special effect of microwaves”. Applying Marcus’ theory for electron transfer reactions and the experiment under an alternating electric field revealed the mechanism of the effect of microwaves. The alternating electric field of microwaves can induce vibration of the electron clouds in Ni particles and the rotation of water molecules. By the vibration of the electron clouds, electron coupling (HAB2 in


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Marcus’ theory) between the Ni particles and Bpys2+ is increased. Under these conditions, the electron transfer reaction rate increases. However, simultaneously, the rotation of water molecules prevents the reorganization of the surroundings (λ in Marcus’ theory) after the electron transfer reaction from decreasing the electron transfer rate. Such a trade-off was demonstrated experimentally, i.e., the dependence of the reaction rate on the microwave power was confirmed.

ASSOCIATED CONTENT Supporting information for this article is consisting with following information. Experimental section, UV/Vis absorption spectra during the reaction between Ni and Bpys2+, Other characterizations (PDF)

AUTHOR INFORMATION The authors declare no competing financial interests.

ACKNOWLEDGMENT We thank Prof. T. Yano (Tokyo Institute of Technology) for XPS measurement. This study was supported in part by Grant-in-Aid for Scientific Research (A) 25249113, Grant-in-Aid for Scientific Research (C) 18K04882, Grant-in-Aid for JSPS Fellows 15J08370 from JSPS, Research Grants of TEPCO Memorial Foundation, and Clean Device Installing Program, NEDO.

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(22)Mochizuki, D.; Shitara, M.; Maitani, M. M.; Wada, Y. Local Thermal Nonequilibrium on Solid and Liquid Interface Generated in a Microwave Magnetic Field. Chem. Lett. 2012, 41, 1409-1411.


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Remote Control of Electron Transfer Reaction by Microwave Irradiation

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