Zeolite-encapsulated M(Co, Fe, Mn)(SALEN) complexes modified glassy carbon electrodes and their application in oxygen reduction

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Abstract

Zeolite-encapsulated transition metal complexes of SALEN [N, N′-bis(salicylidene) ethylenediamine] have been used as catalysts of oxidation reactions of hydrocarbons with oxidants including dioxygen. But in these processes molecular oxygen as oxidant did not show good activity compared with other oxidants such as TBHP, PhIO and H2O2. In order to evaluate the catalytical effect of the hybrid materials on the process of activating molecular oxygen, zeolite-encapsulated M(Co, Fe, Mn)(SALEN) complexes modified glassy carbon electrodes [M(SALEN)/Y/GCEs (M = Mn, Fe, Co)] were prepared and used as electrocatalysts of oxygen reduction reaction (ORR). The electrocatalytic reduction of dioxygen, thus, was investigated by cyclic voltammetry (CV) and chronocoulometry (CC) at glassy carbon electrodes (GCEs) modified with metal (Co, Fe, Mn) complexes of SALEN encapsulated inside NaY in pH 6.90 aqueous solutions. The results have shown that the M(SALEN)/Y/GCEs (M = Mn, Fe, Co) exhibited efficient electrocatalytic activity towards dioxygen reduction with reduced overpotentials of about 505 mV, 393 mV and 397 mV for Co(SALEN)/Y, Fe(SALEN)/Y and Mn(SALEN)/Y, respectively, lower than bare GC electrode and enhanced peak currents. The electroreduction of O2 on these modified GCEs is an irreversible and diffusion-controlled process. The transferred number of electrons and the transfer coefficient for dioxygen reduction reaction were determined by CV and CC. These results suggest that zeolite-encapsulated M(Co, Fe, Mn)SALEN complexes can efficiently activate molecular oxygen by decreasing the overpotential and increasing current of oxygen reduction reaction. And dioxygen is reduced to form water in the process. The significance of this work lies in evaluating the catalysis of the hybrid catalysts for oxidation reaction by electrochemical techniques.

Introduction

Up to the present, there have been many reports on the preparation and characterization of zeolite-encapsulated, ship-in-a-bottle metal complexes of SALEN [N, N′-bis(salicylidene) ethylenediamine] [1], [2], [3], [4], [5], [6]. The encapsulation of metallosalen complexes is usually carried out by the flexible ligand method, in which a flexible ligand, able to freely diffuse through the zeolite pores, complexes with a pre-exchanged metal ion. The resultant complex becomes too large and rigid to leave the cages. It is well known that cytochrome P-450, containing iron metal in the prosthetic active site, is capable of activating dioxygen, forming active species capable of oxidizing alkanes [7]. Therefore, the zeolite-encapsulated metallosalen complexes, a kind of biomimetic system, have been proposed as functional models of cytochrome P-450 [1], [5], [8], [9]. They have been studied extensively as biomimetic catalysts for hydrocarbon hydroxylation/epoxidation and alcohol carbonylation with a variety of oxidants including hydrogen peroxide [1], [3], [4], [10], tert-butylhydroperoxide (TBHP) [1], [10], [11], [12], [6], and iodosylbenzenes (PhIO) [9], [10], and with less frequent use of O2 as oxidant [2], [5], [8]. Molecular oxygen was proven not to be a good oxidant for oxidation reactions of organic compound compared with the other oxidants mentioned above [10], [11] because of its higher chemical stability. Can or to what extent the zeolite-encapsulated metallosalen complexes activate molecular oxygen as cytochrome P-450 in biological process? How does the oxygen reduction reaction (ORR) conduct in the presence of these hybrid catalysts of zeolite-encapsulated metallosalen complexes as electrocatalysts? We would like to know if the zeolite-encapsulated metallosalen complexes modified glassy carbon electrodes are capable of promoting the oxygen reduction reaction in aqueous solution.

On the other hand, oxygen reduction has been of great theoretical and practical importance in power sources, biological processes and chemical syntheses. In particular, the design and development of new catalysts for the multielectron reduction of dioxygen has received a great deal of attention for practical applications such as biological reactions and fuel cells [13], [14], [15], [16]. There have been extensive investigations on electrochemical reduction of O2 with electrocatalysts such as noble-metal materials such as Pt [17], [18], [19], [20], Au [21], Ru [22], non-noble materials including inorganic oxide compounds [23], [24], organic compounds such as anthraquinone and its derivatives [25], [26], [27], [28], biological enzyme such as catalase [29], [30], vitamin B [31], [32] and riboflavin [33], and metal macrocyclic complexes of phthalocyanines [34], [35] and porphyrins [13], [36], [37], [38], [39], [40]. In addition, transition metal complexes of Schiff-base ligands, especially SALEN complexes [8], [41], [42], [43] which may mimic the catalytic cycle of cytochrome P-450, have attracted much research interest with their easy preparation in experimental lab. As electrocatalysts for the ORR process, the complexes are often coated on the surface of glassy carbon electrode, forming modified electrodes by either electropolymerizing the complexes to obtain conducting polymer film [39], [44], [45] on the surface of glassy carbon electrodes, or immobilizing them into some polymer compounds, such as polypyrrole [27], polyacrylamide [30], Nafion [40], [46], [47] and polyaniline [48], followed by the absorption of these polymer compound on glassy carbon electrodes. In fact, chemically modified electrodes (CMEs) have continued to be of major concern during the last decade. And a relatively large amount of electrochemical research has been devoted to the development and application of different types of CMEs [49]. Zeolite-modified electrodes are a kind of important chemically modified electrodes in studying the redox behavior of the metal complexes encapsulated in zeolite. The electrochemical properties of the CMEs of zeolite-immobilized metallosalen complexes were much investigated in 1990s [50], [51], [52]. But there are only a few papers on ORR using zeolite-encapsulated metallosalen complexes as electrocatalysts [53].

In the present contribution, the NaY zeolite-encapsulated M(SALEN) modified electrodes, denoted as M(SALEN)/Y/GCEs (M = Mn, Fe, Co), were prepared and used as electrocatalysts for the ORR process. The electrochemical behavior and catalytic efficiency of these modified electrodes for ORR were examined by CV and CC.

Section snippets

Reagents and chemicals

All chemicals and reagents used in present work were of analytical grade and used as received without further purification. All aqueous solutions for electrochemical experiments were prepared using doubly deionized water. The supporting electrolyte in organic system used for electrochemical experiments, tetra-n-butylammonium perchlorate (TBAP), was prepared according to literature procedure [54] and recrystallized three times, 0.025 mol l−1 phosphate buffer solution (pH 6.90) was purchased from

Electrochemical behavior and stability of the M(SALEN)/Y/GCEs

The electrochemical behavior of M(SALEN)/Y/GCEs (M = Fe, Mn, Co) was studied by using cyclic voltammetry. Fig. 1 shows the cyclic voltammograms corresponding to the response of Mn(SALEN)/Y modified electrodes in a DMSO N2-saturated solution containing 0.1 mol l−1 TBAP as supporting electrolyte. The curves of CV exhibit nearly symmetric anodic and cathodic peaks, corresponding to the Mn(III)/Mn(II) couple. The ΔEp (Epa  Epc) values increase with increasing scan rate, but the formal potential ( = 1/2(E

Conclusion

In this study, we prepared a stable modified electrode using the NaY-encapsulated metal (Mn, Fe, Co) SALEN complexes. These M(SALEN)/Y modified electrodes exhibited good electrochemical reproducibility and efficient electrocatalytic activity towards dioxygen reduction in present experimental conditions. The results clearly show that the Co(SALEN)/Y is the most active catalyst for oxygen reduction reaction. It is also demonstrated from cyclic voltammetric and chronocoulometric experiments that

Acknowledgement

This work was supported by the National Natural Science Foundation of China (No. 50472083).

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