Electrochemical and quantum chemical studies of peculiarities of 2,5-di-Me-pyrazine-di-N-oxide oxidation in the presence of methanol at single-walled and multi-walled carbon nanotube paper electrodes

Abstract

The use of methanol (MeOH) in direct methanol fuel cells has increased the interest in the search for new electrode materials and catalysts that allow the oxidation of MeOH to be carried out under conditions that satisfy their practical applications: low cost, stability, and high catalytic activity. In this work, electrochemical and quantum chemical methods were used to study peculiarities of the electrocatalytic system 2,5-di-Me-pyrazine-di-N-oxide–methanol at single-walled and multi-walled carbon nanotube (CNT) paper electrodes in comparison with a glassy carbon (GC) electrode in 0.1 M Bu₄NClO₄ solution in acetonitrile (MeCN). The adsorption energies of MeOH, CH₃COOH and H₂O on the CNT surface were determined by quantum chemical modeling; this opened the door for the explanation of effects found in the Pyr₁–MeOH catalytic system and for the ascertainment of factors affecting the catalytic efficiency of the process at CNT electrodes. We believe that this research will be helpful in using this process in electrocatalysis, sensors and direct methanol fuel cells, since the deactivation of aromatic di-N-oxides (as opposed to processes involving noble metals or noble metal oxides) is insignificant.

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1. Introduction

The study of methanol oxidation is of particular interest in connection with its use in direct methanol fuel cells (DMFCs).^1,2 In DMFCs, the most commonly used catalysts for methanol oxidation are noble metals and noble metal oxides. However, the high cost of noble metals and the poisoning of catalysts by irreversibly adsorbed carbon monoxide (CO) generated during methanol electrooxidation are driving the search for non-noble metals, metal free materials, and catalysts that are inexpensive, stable and active in catalysis.

Carbon nanomaterials made of reduced graphene oxide, single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs), modified with additives of noble metals (Pt, Ir, Ru, and Pd) or their oxides (IrO₂ and RuO₂), are used in the electrocatalytic processes of methanol oxidation.^3–8 An alternative approach could be the use of metal-free materials and organic mediators.

It was shown^9–15 that the aromatic di-N-oxide radical cations of phenazine-di-N-oxide, pyrazine-di-N-oxide (Pyr₀) and its substituted derivatives, electrochemically generated at GC and Pt electrodes, are carriers of active oxygen that is capable of activating the C–H bond of substrates: alcohols, ethers and cyclohexane. It was assumed that the activation process is accompanied by the formation of a complex of radical cations with a substrate that was confirmed by the registration of radical intermediates by EPR electrolysis at oxidation of phenazine-di-N-oxide in MeOH and its deuterated derivatives.¹¹ The detection of the same radical intermediate in CH₃OH and CH₃OD proved the participation of the CH₃ group of alcohol in the formation of the intermediate.

In previous studies,^16–19 in the presence of aromatic di-N-oxide, a several-fold increase in the catalytic efficiency of oxidation of organic compounds at SWCNT or MWCNT paper electrodes in comparison with the GC electrode was found. It was noted^16–19 that in the absence of the substrate the oxidation current of phenazine-di-N-oxide, 2,3,5,6-tetra-Me-pyrazine-di-N-oxide, at SWCNT or MWCNT paper electrodes is by several times higher than the oxidation current of the ferrocene (Fc) reference. Quantum chemical modeling of the adsorption energies of Pyr₁, Fc and MeCN both on the CNT surface and in grooves between two CNTs using the cluster model simulating the surface of conducting and non-conducting carbon nanotubes²⁰ can explain this effect.

In this work the peculiarities of oxidation of Pyr₁ in the presence of MeOH were studied in 0.1 M Bu₄NClO₄ solution in MeCN at SWCNT and MWCNT paper electrodes in comparison with the GC electrode using cyclic voltammetry, EPR electrolysis, differential capacitance and quantum chemical modeling. The study of electrocatalytic oxidation of methanol at the electrodes of CNTs in the presence of aromatic di-N-oxides as mediators is of interest for using this process in electrocatalysis, sensors and direct methanol fuel cells.

2. Materials and methods

Materials and experimental techniques for obtaining cyclic voltammograms, EPR spectra during electrolysis at controlled potentials and dependence of the differential capacitance of the electrical double layer on potentials were described in previous studies.^18,20 The quantum chemical modeling technique was described in detail in our previous work.²⁰

3. Results and discussion

3.1. Quantum-chemical modeling of the adsorption of CH₃OH, CH₃COOH and H₂O on the surface of CNTs

In this paper simulation of the adsorption of MeOH, CH₃COOH and H₂O at bundles of CNTs with a diameter of 1.4 nm was carried out. The C₅₄H_18(c) cluster (structure 1, Fig. 1) was chosen to simulate single-walled conductive (10, 10) CNTs because in our previous work²⁰ it was shown to be a suitable model. For the modeling of the adsorption of CH₃OH, CH₃COOH and H₂O, on the surface of CNTs, the functional wB97XD²¹ was used in the software package GAUSSIAN-09.²²

Fig. 1 Structure 1 of the cluster C₅₄H_18(c) modeling surface of single-walled conductive (10,10) CNTs; structures of clusters C₅₄H_18(c)*X, X = H₂O, CH₃OH and CH₃COOH (structures 2–4). Adsorption energies (in kcal mol⁻¹) calculated at different basis sets are given: ** – 6-311G(d,p), *** – 6-311G(d,p) (BSSE).

Structures and adsorption energies. The calculated most stable isomers of H₂O, CH₃OH and CH₃COOH with the carbon cluster C₅₄H_18(c) modeling surface of the isolated conducting carbon nanotubes are shown in Fig. 1 (structures 2, 3, and 4). In complexes C₅₄H₁₈*X (X = H₂O, CH₃OH, CH₃COOH) the shortest distance from the C atoms of the CNT to H atoms of the X molecule is 2.6–3.1 Å, and the distance to C or O atoms of the X molecule is 3.2–3.4 Å.

The obtained values of the adsorption energies of H₂O, CH₃OH and CH₃COOH on the carbon cluster C₅₄H_18(c), modeling surfaces of the isolated conductive (10, 10) carbon nanotubes, are presented in Fig. 1 and Table 1. The adsorption energy decreases from CH₃COOH (6.3 kcal mol⁻¹) to MeCN (5.9 kcal mol⁻¹²⁰), CH₃OH (5.1 kcal mol⁻¹) and H₂O (3.0 kcal mol⁻¹). An insignificant difference between the obtained values of the adsorption energies of MeCN and MeOH on the CNT surface indicates the possibility of their simultaneous adsorption on the CNT surface.

Table 1 Estimation of the energies of adsorption (E_ad, kcal mol⁻¹) of CH₃OH, CH₃COOH, H₂O (this work) and Pyr₁, Fc и MeCN²¹ on the structures modeling single carbon nanotubes. Conducting (c) CNT (10, 10) nanotubes were imitated by the cluster C₅₄H_18(c). BSSE stands for the basis superposition effect

N	Structure C54H18(c)*X, X =	E ad	E ad – BSSE
8	H2O	5.0	3.0
9	CH3OH	6.4	5.1
10	MeCN	6.8	5.9
11	CH3COOH	8.2	6.3
12	Fc	12.6	10.9
13	Pyr1	18.2	14.5

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3.2. Oxidation of Pyr₁ at GC, SWCNT and MWCNT paper electrodes in MeCN

In the absence of MeOH. The oxidation of Pyr₁ at GC, SWCNT and MWCNT paper electrodes was studied in a previous study²⁰ in 0.1 M Bu₄NClO₄ solution in MeCN with the use of the methods of quantum-chemical modeling, cyclic voltammetry, EPR-electrolysis and differential capacitance. It was shown²⁰ that the process of Pyr₁ oxidation at the GC electrode (E_ox = 1.56 V) in 0.1 M Bu₄NClO₄ solution in MeCN is one-electron, irreversible, and diffusion-controlled and the charge transfer is followed by an irreversible chemical reaction (an EC mechanism) with a rate constant equal to 0.6 s⁻¹.

Oxidation of Pyr₁ at MWCNT and SWCNT paper electrodes in 0.1 M Bu₄NClO₄ solution in MeCN occurs at a potential of 1.78 V,²⁰ that is, at higher by 220 mV as compared with oxidation at the GC electrode. The process is irreversible and not diffusion-controlled. In contrast to the GC electrode, the oxidation currents of 1 mM Pyr₁ at MWCNT and SWCNT paper electrodes are several times higher than the diffusion oxidation current of 1 mM Fc (as the reference).

In the presence of methanol. It should be noted that the data on Pyr₁ oxidation in the absence²⁰ and presence of MeOH (this paper) at MWCNT and SWCNT paper electrodes were obtained for the first time. Previously,¹⁹ the oxidation of 2,3,5,6-tetra-Me-pyrazine-di-N-oxide (Pyr₂) in the presence of methanol at GC and MWCNT paper electrodes in 0.1 M solution of LiClO₄ in MeCN was studied. It is important that the absence of two methyl groups in the 2,5-di-Me-pyrazine-di-N-oxide (Pyr₁) molecule in comparison with Pyr₂ leads to an increase in its oxidation potential by ≈120 mV^18,19 and, consequently, to an increase in the reactivity of its radical cation. In this work the study was carried out, in contrast,¹⁹ at both the MWCNT and SWCNT paper electrodes, and a solution of 0.1 M Bu₄NClO₄ in MeCN was used as a background electrolyte. LiClO₄ is hygroscopic, and forms several crystalline hydrates, the most stable of which is LiClO₄·3H₂O. The presence of a trace amount of water as a base in the solvent facilitates the oxidation of organic compounds by the electrochemically generated radical cations of aromatic di-N-oxides, proceeding with the elimination of a proton. Therefore, Bu₄NClO₄ was used as a supporting electrolyte.

In the absence of Pyr₁ no oxidation of MeOH is observed on the CV curves obtained at the GC electrode in 0.1 M Bu₄NClO₄ solution in MeCN (Fig. 2a). In the presence of 1 mM Pyr₁ with the addition of MeOH a second anodic peak appears at higher anodic potentials (Fig. 2b). The current of the second anodic peak increases with an increase in the MeOH concentration and shifts to the first anodic peak. At a concentration of 0.5 M MeOH, anodic peaks merge.

Fig. 2 CVs of 0.1 M Bu₄NClO₄ solution in MeCN at the GC electrode at a potential scan rate of 20 mV s⁻¹ in the presence of MeOH: (a) (1) 0, (2) 0.01, (3) 0.05, (4) 0.1, (5) 0.5, and (6) 1.0 M; (b) (1) 1 mM Pyr₁, (2) 1 mM Pyr₁ and 0.01 M MeOH, and (3) 0.01 M MeOH; (c) 1 mM Pyr₁ and MeOH: (1) 0, (2) 0.01, (3) 0.05, (4) 0.1, (5) 0.5, and (6) 1.0 M; and (d) dependence of the anodic peak current on the square root of the MeOH concentration.

The study of the dependence of the currents of these peaks on the potential scan rate at a MeOH concentration below 0.1 M shows that, as in 0.1 M LiClO₄ solution in MeCN,¹⁵ the first anodic peak is diffusion (increases directly proportional to the square root of the scan rates; Fig. 3a), and the second anodic peak has a kinetic nature (decreases with an increase of the potential scan rates; Fig. 3a). This dependence of anodic peak current on the scan rate corresponds to the E₁C₁E₂ mechanism of the electrode process when two stages of electron transfer are separated by a chemical stage C₁.^23,24

Fig. 3 CVs of 0.1 M Bu₄NClO₄ solution in MeCN at the GC electrode in the presence of : (a) 1 mM Pyr₁ and 0.01 M MeOH at potential scan rates of: (1) 20, (2) 50, (3) 80, and (4) 200 mV s⁻¹; (b) 1 mM Pyr₁ and 0.5 M MeOH at potential scan rates of : (1) 20, (2) 50, (3) 80, and (4) 200 mV s⁻¹; and (c) 0.5 M MeOH and Pyr₁: (1) 0.5 mM and (2) 1 mM at a potential scan rate of 20 mV s⁻¹.

The second anodic peak at a MeOH concentration above 0.1 M is proportional to the square root of its concentration (Fig. 2d), weakly depends on the potential scan rate (Fig. 3b) and is proportional to the Pyr₁ concentration (Fig. 3c). According to a previous study,²⁵ this indicates the catalytic nature of the second anodic peak. The ratio of the catalytic oxidation current of Pyr₁ in the presence of 0.5 M MeOH to the current recorded in the absence of MeOH (the catalytic efficiency of the process) is 6.0. It should be noted that in the presence of 2,3,5,6-tetra-Me-pyrazine-di-N-oxide (Pyr₂) the catalytic efficiency of MeOH oxidation was 4.0.¹⁹ Thus, the catalytic efficiency of the methanol oxidation increased by 1.5 times with an increase of the oxidation potential of di-N-oxide.

It was shown that the dependence of the CV curves of Pyr₁ oxidation at the GC electrode in 0.1 M Bu₄NClO₄ solution in MeCN on the MeOH concentration and the potential scan rate are identical to those obtained in ref. 15 in 0.1 M LiClO₄ solution in MeCN. Therefore, the nature of the background cation does not affect the electrocatalytic oxidation of MeOH in the presence of Pyr₁ at the GC electrode.

Previously,¹⁵ it was established that the EPR spectra of radical cations pyrazine-di-N-oxide and its substituted derivatives were not found in the presence of MeOH in 0.1 M LiClO₄ solution in MeCN. The EPR spectra of radical cations could have been detected during electrolysis at the Au electrode in deuterated methanol at a temperature close to the freezing point of the solvent at −85 °C.¹⁵ The EPR spectrum of the radical cation of Pyr₁ in 0.1 M LiClO₄ solution in CD₃OD is presented in Fig. 4b and c. The registration of the EPR spectrum of the radical cation of Pyr₁ in 0.1 M LiClO₄ solution in CD₃OD confirms that the Pyr₁ structure remains unchanged during the alcohol catalytic oxidation. The registration of the EPR spectra of the Pyr₁ radical anion at −35 °C during electrolysis at a controlled potential after alcohol catalytic oxidation at positive potentials (Fig. 4d–f) is further evidence that the structure and concentration of Pyr₁ remain unchanged after the catalytic process. Since the nature of the background cation does not affect the electrocatalytic oxidation of MeOH in the presence of Pyr₁, these data can be used as evidence for the invariability of the structure and concentration of Pyr₁ in the catalytic process in 0.1 M Bu₄NClO₄ solution in MeCN in the presence of MeOH.

Fig. 4 EPR spectra recorded during electrolysis of 1 mM Pyr₁ solution in the presence of 0.1 M LiClO₄ at different temperatures and potentials.¹⁵ (a) In MeCN at −45 °C and +1.6 V, the dotted line is simulation using a Bruker software Symfonia taking into account hyperfine splitting on all nuclei of Pyr₁: a_N = 0.047 mT (2), a_H1 = 0.042 mT (2), and a_H2 = 0.113 mT (6); the numbers in parentheses are the numbers of equivalent nuclei, the half-width at half-height of the Lorentz line is 0.012 mT, (b) in CD₃OD at −97 °C and +1.8 V, (c) in CD₃OD at −85 °C and +1.7 V, (d) in CH₃OH at −35 °C and −0.9 V, (e) in CH₃OD at −35 °C and −0.9 V, and (f) in CD₃OD at −35 °C and −0.8 V. The spectrum (a) was recorded at the Pt electrode, and the other spectra were obtained at the Au electrode.

In the absence of Pyr₁ at CV curves recorded at SWCNT and MWCNT paper electrodes in 0.1 M Bu₄NClO₄ solution in MeCN in the presence of MeOH from 0.01 to 1.0 M, as in the case of the GC electrode, the MeOH oxidation is not observed (Fig. 5a and b). The oxidation current of Pyr₁ at SWCNT and MWCNT paper electrodes in 0.1 M Bu₄NClO₄ solution in MeCN increases with the addition of MeOH from 0.01 to 1.0 M, and an increase of the anodic current shifts to lower positive potentials (Fig. 5c and d).

Fig. 5 CVs of 0.1 M Bu₄NClO₄ solution in MeCN at a potential scan rate of 20 mV s⁻¹ in the presence of MeOH: (1) 0, (2) 0.01, (3) 0.05, (4) 0.1, and (5) 0.5 M at (a) SWCNT and (b) MWCNT paper electrodes, (c) at the SWCNT paper electrode in the presence of 1 mM Pyr₁ and MeOH: (1) 0, (2) 0.01, (3) 0.05, (4) 0.5, and (5) 1.0, (d) at the MWCNT paper electrode in the presence of 1 mM Pyr₁ and MeOH: (1) 0, (2) 0.01, (3) 0.05, (4) 0.1, (5) 0.5, and (6) 1.0 M; and dependence of the anodic peak current on the square root of the MeOH concentration at (e) SWCNT and (f) MWCNT paper electrodes.

Unlike oxidation at the GC electrode, at the CV curves of Pyr₁ oxidation at SWCNT and MWCNT paper electrodes in 0.1 M Bu₄NClO₄ solution in MeCN in the presence of MeOH, one anodic wave is observed. This can be explained by the adsorption of Pyr₁ molecules on the surface of SWCNT and MWCNT paper electrodes, which leads to a 220 mV shift towards higher positive potentials of the Pyr₁ oxidation potential as compared to the oxidation potential of the unadsorbed Pyr₁ molecules on the GC electrode. This shift results in the merging of the first and second anodic waves of Pyr₁ oxidation at SWCNT and MWCNT paper electrodes in the presence of MeOH.

The linear dependence of the anodic peak of Pyr₁ oxidation on the square root of the concentration of MeOH at SWCNT and MWCNT paper electrodes (Fig. 5e and f) at a MeOH concentration above 0.05 M corresponds to the catalytic nature of this peak.²⁵ The ratio of the catalytic current at 0.5 M MeOH to the diffusion current of ferrocene (as the reference) is 16. Thus the catalytic efficiency of MeOH oxidation at SWCNT and MWCNT paper electrodes is ∼2.7 times higher than at the GC electrode.

It should be noted that in contrast to the GC electrode, on which the catalytic current (I_a) increases proportionally to the concentration of Pyr₁ (Fig. 3c), at SWCNT and MWCNT paper electrodes, it increases by a factor of 1.3 (Fig. 6a and b) with a two-fold magnification of the Pyr₁ concentration. This can be explained by the fact that in the absence of MeOH the oxidation current of Pyr₁ is several times higher than the oxidation current of Fc as a reference and weakly depends on the Pyr₁ concentration.²⁰ Since the adsorption energy of Pyr₁ (14.5 kcal mol⁻¹, Table 1) on the CNT surface significantly exceeds the adsorption energies of MeCN and MeOH, the presence of MeOH in solution will not affect the adsorption of Pyr₁ on the CNT surface and retains a weakly dependent anodic current of Pyr₁ oxidation on its concentration.

Fig. 6 CVs of 0.1 M Bu₄NClO₄ solution in MeCN in the presence of 0.5 M MeOH and Pyr₁: (1) 0.5 mM and (2) 1 mM at a potential scan rate of 20 mV s⁻¹ at (a) SWCNT and (b) MWCNT paper electrodes and in the presence of 1 mM Pyr₁ and 0.5 M MeOH at potential scan rates of: (1) 20, (2) 50, and (3) 80 mV s⁻¹ at (c) SWCNT and (d) MWCNT paper electrodes.

The adsorption of Pyr₁ at the MWCNT paper electrode in 0.1 M Bu₄ClO₄ solution in MeCN was studied in a previous study²⁰ by measuring the C,E-dependence of the differential double layer capacitance of the electrode on potential. It was found that the differential double layer capacitance of the electrode recorded in 0.1 M Bu₄ClO₄ solution in MeCN at the MWCNT paper electrode is not changed when 1 mM Pyr₁ is added to the solution (Fig. 7). It has been established by quantum chemical modeling²⁰ that the preferred arrangement is the parallel arrangement of the planar part of the Pyr₁ molecule adsorbed on the surface of carbon nanotubes. It is known²⁶ that the differential double layer capacitance of the electrode does not decrease at the flat arrangement of the adsorbed aromatic molecules on the electrode surface. With the addition of 0.5 M MeOH to the solution of 1 mM Pyr₁ in 0.1 M Bu₄ClO₄ solution in MeCN the differential double layer capacitance of the electrode decreases. This indicates the formation of the adsorption layer on the surface of the MWCNT paper electrode, including Pyr₁, MeOH and MeCN molecules. This conclusion is confirmed by quantum chemical modeling (Fig. 1 and Table 1).

Fig. 7 Dependences of the differential capacitance C of the MWCNT paper electrode at a potential E of 0.1 M Bu₄ClO₄ solution in MeCN in the presence of Pyr₁: 0 (curves 1 and 2) and 1 mM (curves 3–5), 1 mM Pyr₁ and 0.5 M MeOH (curve 6–8). Curves 1 and 2 were recorded just after recording the cyclic voltammetry curves, and curves 3, 4 and 5 are settled curves.

Studying the influence of CH₃COOH and water (as a basis) on the potential and current of the catalytic wave of Pyr₁ oxidation at GC, SWCNT, and MWCNT paper electrodes in the presence of MeOH makes it possible to establish the mechanism of the electrode process. A decrease of the catalytic current and the shifts of the catalytic wave and the first oxidation wave of Pyr₁ towards higher positive potentials (Fig. 8a–c) are observed at the GC electrode with the addition of 1.0 and 2.0 M CH₃COOH to solution 1.0 mM Pyr₁ and 0.5 M MeOH in 0.1 M Bu₄ClO₄ solution in MeCN. It should be noted that with the addition of 2.0 M CH₃COOH, the catalytic current at the GC electrode decreases by 1.4 times and at SWCNT and MWCNT paper electrodes by 3.8 and 3.1 times, respectively (Fig. 9).

Fig. 8 CVs of 1.0 mM Pyr₁ and 0.5 M MeOH in 0.1 M Bu₄NClO₄ solution in MeCN in the presence of CH₃COOH: (1) 0, (2) 1.0, and (3) 2.0 M at (a) (GC), (b) SWCNT and (c) MWCNT paper electrodes at a potential scan rate of 50 mV s⁻¹.

Fig. 9 CVs of 0.5 mM Pyr₁ and 0.5 M MeOH in 0.1 M Bu₄NClO₄ solution in MeCN in the presence of H₂O: (1) 0, (2) 0.5, and (3) 2.0 M at (a) (GC), (b) SWCNT and (c) MWCNT paper electrodes at a potential scan rate of 20 mV s⁻¹.

The shift of the catalytic wave at the GC electrode to potentials of the anodic wave of Pyr₁ oxidation (Fig. 8a) is recorded with the addition of 0.5 and 2.0 M H₂O to a solution of 0.5 mM Pyr₁ and 0.5 M MeOH in 0.1 M Bu₄ClO₄ solution in MeCN (Fig. 8a). This indicates that the oxidation potentials of the reaction product of the radical cation with MeOH and the initial di-N-oxide are close. At SWCNT and MWCNT paper electrodes the influence of the addition of H₂O is less pronounced.

The difference in the manifestation of the inhibiting effect of CH₃COOH and of the catalytic effect of H₂O at SWCNT and MWCNT paper electrodes as compared with the GC electrode can be explained by the performed quantum chemical modeling and the determination of the adsorption energies of MeCN²⁰ and MeOH, CH₃COOH and H₂O (this work) on the CNT surface (Table 1). The adsorption energy of CH₃COOH (6.3 kcal mol⁻¹) on the CNT surface is sufficient to displace MeCN with an adsorption energy of 5.9 kcal mol⁻¹ from the electrode surface. As a result of the adsorption of CH₃COOH, its concentration on the CNT surface increases and the inhibiting effect is more pronounced compared to the GC electrode. The adsorption energy of H₂O (3 kcal mol⁻¹) on the CNT surface (Table 1) is much lower than the adsorption energy of MeCN and therefore its catalytic effect is less pronounced compared to the GC electrode. A similar difference in the manifestation of the inhibiting effect of CH₃COOH and of the catalytic effect of H₂O at SWCNT or MWCNT paper electrodes as compared with the GC electrode was observed in ref. 17–19. Quantum chemical modeling has provided an explanation for this effect. The effect of acid and water additions on the catalytic process indicates that the catalytic process is accompanied by proton elimination.

Mechanism of Pyr₁ oxidation in the presence of MeOH at GC, SWCNT and MWCNT paper electrodes in MeCN. Previously¹⁵ quantum chemical modeling of the reaction of the radical cation of unsubstituted pyrazine-di-N-oxide (PyrDNO⁺˙) with MeOH by the mechanism of hydrogen atom abstraction with the formation of the protonated pyrazine-di-N-oxide (PyrDNOH⁺) and the HOCH₂˙ radical,

PyrDNO⁺˙ + CH₃OH → PyrDNOH⁺ + HOCH₂˙

was performed. It was found that the adduct [(CH₃OH)PyrDNO⁺˙] was formed with a noticeable gain in energy equal to 16.495 kcal mol⁻¹, and the equilibrium constant of its formation was 7 × 10⁶ M⁻¹. H-atom abstraction from MeOH by the PyrDNO⁺˙ radical cation is impossible because the product, the complex of the HOCH₂˙ radical and the PyrDNOH⁺ cation, has a negligibly small lifetime and virtually zero activation energy for its transformation to the original adduct. It should be noted that the EPR spectrum of the HOCH₂˙ radical was not found. Thus the oxidation of aromatic di-N-oxides in the presence of methanol at the GC electrode was interpreted by the E₁C₁E₂C₂ mechanism of a two stage electrode process with the catalytic current recorded at the second electrode stage.

Based on the obtained experimental data and quantum chemical modeling the following mechanism of Pyr₁ oxidation at SWCNT and MWCNT paper electrodes in MeCN in the presence of MeOH is proposed.

E₁ – first electrode stage.
At the GC electrode. At the first electrode stage (E₁), Pyr₁ is oxidized to the radical cation. This process is one-electron, irreversible, and diffusion-controlled. The charge transfer is followed by an irreversible chemical reaction.
At SWCNT and MWCNT paper electrodes. At the first electrode stage (E₁), Pyr₁, adsorbed on the CNT surface, is oxidized to the radical cation. This process is irreversible and not controlled by diffusion.

C₁ – first chemical stage.
At GC, SWCNT and MWCNT paper electrodes. At the first chemical stage (C₁), two competitive chemical reactions of the electrophilic addition of the oxygen atom of Pyr₁ radical cations to the C–H bonds of MeCN and MeOH occur. The reactions are accompanied by proton elimination and the formation of radical intermediates (complexes with an N–O–C structure). Compounds of this structure and methods for their preparation are known.^27–33
At SWCNT and MWCNT paper electrodes. The chemical stage proceeds with the participation of the Pyr₁ radical cations, as well as MeCN and MeOH, adsorbed on the surface of the SWCNT and MWCNT paper electrodes.

E₂ – second electrode stage.
At the GC electrode. At the second electrode stage (E₂), the radical intermediates are oxidized to the cations.
At SWCNT and MWCNT paper electrodes. The radical intermediates adsorbed on the surface of the SWCNT and MWCNT paper electrodes are oxidized to the cations of (A) and (B).

C₂ – second chemical stage.
At GC, SWCNT and MWCNT paper electrodes. The second chemical stage (C₂) corresponds to the interaction of the cations of (A) and (B) with a nucleophile or a base (a water admixture in the solution). It is assumed that the cation (A) decomposes at the stage with the formation of the initial aromatic di-N-oxide and the product of two-electron oxidation of MeOH (formaldehyde). The regenerated aromatic di-N-oxide is immediately oxidized at the electrode to a radical cation, and the cycle is repeated; the catalytic current of the total two-electron oxidation of MeOH is recorded. In a competitive chemical stage , the cation (B) is attached to a base or a nucleophile to form an adduct.²⁰

4. Conclusions

In this work, electrochemical and quantum chemical methods were used to study the peculiarities of the behavior of the electrocatalytic system of 2,5-di-Me-pyrazine-di-N-oxide – methanol at single-walled and multi-walled carbon nanotube paper electrodes in comparison with a glassy carbon (GC) electrode. The cluster model describing the surface of conducting carbon nanotubes (10, 10) was used to simulate the adsorption of MeOH, CH₃COOH, and H₂O on the surface of CNTs. The observed effects in the catalytic system are explained by quantum chemical modeling of the non-covalent interaction of the components of the studied system with the CNT surface. It is concluded that, in the course of the catalytic process at SWCNT and MWCNT electrodes, in contrast to the GC electrode, the adsorption of the components of the catalytic system on their surface is of decisive importance. This research may be helpful in using this process in electrocatalysis, sensors and direct methanol fuel cells.

This work was performed in accordance with the state task, state registration no. AAAA-A19-119061890019-5.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

Calculations were performed at the Computer Center, Institute of Problems of Chemical Physics, RAS.

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