Electrocatalytic oxidation of methanol on glassy carbon electrode modified with nickel–manganese salen complexes encapsulated in mesoporous zeolite A

•Two kinds of metal salen complexes are simultaneously encapsulated in zeolite.•NiMnsalenA shows superior electrocatalytic activity for methanol oxidation.•Electrooxidation ability of bimetal-complexes in zeolite is higher than single one.•The rate-determining step for methanol oxidation vary with the potential.•Oxidation of methanol and desorption of intermediates are key factors in oxidation.

Nickel and manganese salen complexes were simultaneously encapsulated in the supercages of the mesoporous zeolite A by using the flexible ligand method. The electrochemical behavior and electrocatalytic activity toward the electrooxidation of methanol on glassy carbon electrode coated with NiMnsalenA (NiMnsalenA/GCE) in 0.1 M NaOH solution were studied by cyclic voltammetry (CV), chronoamperometry (CA) and AC electrochemical impedance spectroscopy (EIS). The results showed that the addition of small amounts of Mn species can give rise to much higher electrocatalytic activity than the pure NisalenA toward the oxidation of methanol, mainly due to a synergetic effect that may originate from the interaction of Ni(salen) and Mn(salen) and/or the formation of dinuclear salen complexes via the lattice oxygen of the zeolitic host. Kinetic parameters such as the electron transfer coefficient α, the rate constant ks of the electrode reaction and the charge transfer coefficient of the catalytic reaction, αn were determined to be 0.40, 1.898 s−1 and 0.43, respectively, and the detection limit was found to be 97.66 μM. The mechanism of electrocatalytic oxidation of methanol on NiMnsalenA/GCE was investigated and proposed to be done by reaction with Ni3+(salen)O(OH) and also direct electro-oxidation reaction. The impedance behaviors showed different patterns at different applied potentials, indicating the changes of the rate-determining step, from the methanol electrooxidation leading to the formation of intermediates on the surface of electrode gradually to the oxidation and removal of adsorbed intermediates. The influence of methanol concentration on the impedance behaviors was also studied and the results demonstrated that the semidiameters of the Nyquist diagrams decreased with the increase of methanol concentration, signifying the acceleration of the reaction rate of methanol electrooxidation.