Modified Electrodes With Zeolite-encapsulated Melal Schiff Base Complexes: Preparation, Electrochemistry And Electrocatalysis In Dioxygen Reduction

It is a fact that metal complexes of Schiff-base can mimic the active site of cytocrome P-450 and catalyze a serious of chemical reaction under mild condition. But the catalytic activities of the Schiff-base metal complexes in homogeneous solution will decrease with time due to formation of dimericμ-oxo and/orμ-peroxo complexes. The encapsulation of metal Schiff-base complexes into micropores or cages of zeolites is a good method to isolate the complexes to prevent the dimerization and to obtain a serious of model catalysts mimicking the catalysis of nature enzymes about molecular structure and function. It has been showed from the research reports up to now that these encapsulated complexes are of many advantages of high selectivity, reaction under mild conditions, reusability and separation from products with ease.Now there are lots of reports which focus on preparation, characterization and evaluation of the catalytic activities of these hybrid catalysts. But the details of mechanism for the catalytic oxidation reaction are not yet clear. Zeolite (Y, X, LSX, AlPO4-5)-encapsulated transition metal complexes of Schiff-base have been used as catalysts of oxidation reactions of hydrocarbons with oxidants including dioxygen. But the molecular oxygen as oxidant did not show good activity compared with other oxidants such as TBHP, PhIO and H2O2 in the above processes. It has restricted the use of oxygen as a green and environment-friendly, cheap oxidant.In present work, zeolite-encapsulated M(Salen) [Salen: N, N'-bis(salicylidene)ethyleneediamine and its derivatives] complexes modified glassy carbon electrodes [M(Salen)/Z/GCEs (M=Cr, Mn, Fe, Co, Ni, Cu, Zn etc. and Z= Y, X, LSX, AlPO4-5)] were prepared and studied on their electrochemical behaviors in different pH buffer solution so as to evaluate the catalytical effect of the hybrid materials on the process of activating molecular oxygen. Then these zeolite-modified electrodes were used as electrocatalysts for oxygen reduction reaction (ORR) in aqueous solutions to check the electrocatalysis of the encapsulated complexes for ORR. The electrocatalytic reduction of dioxygen was investigated by cyclic voltammetry (CV) and chronocoulometry (CC) and chronoamperometry (CA) at glassy carbon electrodes (GCEs) modified with transition metal complexes of Salen encapsulated inside NaY, NaX, LSX and AlPO4-5 in a wide range of different pH aqueous solutions (4.0~10.0). The kinetic parameters were obtained for oxygen reduction on the modified electrodes and the elctrocatalytic mechanism of ORR was first proposed in aqueous solutions based on cyclic voltammetry and double-step technique of chronocoulometry and chronoamperometry.The conclusions of this dissertation are listed as follows:1. Zeolite Y encapsulated metal (Cr, Mn, Fe, Co, Ni, Cu, Zn) Salen complexes were prepared by flexible ligand method and characterized by XRD, FT-IR and UV-vis. And the zeolite modified glassy carbon electrodes were prepared with these hybrid catalysts as the major modifier by zeolite/polymer step-by-step coating method and showed well electrochemical stability and reproducibility.2. The M(Salen)/Y/GCEs (M: Cr, Mn, Fe, Co, Ni, Cu, Zn) all behave moderate to good electrocatalytic activities. The Co(Salen)/Y/GCE is showed the best electrocatalytic activity for ORR among these different transition metal complexes, followed by Fe(Salen)/Y, Mn(Salen)/Y, Cu(Salen)/Y about their catalytic activity in neutral electrolyte.3. The neutral electrolyte of pH 6.86 solution or near neutral solution such as pH 7.4 and/or 8.0 is more propitious to increase in electrocatalytic activity of M(Salen)/Y modified electrode.4. The E1CE2 mechanism was first proposed for ORR on the M(Salen)/Y/GCEs according to kinetic parameters and the apparent numbers of electrons transferred obtained from Randles-Sevcik equation of the irreversible and diffusion-controlled reaction. And dioxygen is reduced to water on the M(Salen)/Y in neutral buffer solution.5. The electrocatalytic activity of CoL/Y decreases in the order of Co(salen)>Co(salpn)>Co(salcn)>Co(salbn)>Co(tBu4salen)>Co(salphen), and it is related to that the configuration between square planar and tetrahedral of encapsulated complexes into the cage of zeolite.6. Oxygen was reduced to water on the electrode modified with Co(salen)/Y, Co(salpn)/Y, Co(salcn)/Y and Co(salbn)/Y but to H2O2 on Co(tBu4salen)/Y modified electrode, while oxygen is hard to be reduced on Co(salphen)/Y electrode.7. M(Salen)/AlPO4-5 (M=Fe, Co, Mn) also show good electrocatalysis for ORR and their activity increases in the order: Mn(Salen)/AlPO4-5, Fe(Salen)/AlPO4-5, Co(Salen)/AlPO4-5. So Co(Salen)/AlPO4-5 is best for ORR in neutral buffer solution.8. E1CE2 mechanism is proposed for ORR on the M(Salen)/ AlPO4-5 (M=Fe, Co, Mn) modified electrodes based on the experiment data from CV and CA. The different pH value of electrolyte only change the peak potential and peak current but does change the kinetic mechanism. Also the pH 6.86 solution is the best system in this case.9. The higher the ratio of SiO2:Al2O3, the better the reversibility of the couple MⅢ/MⅡof metal Salen complexes, and then the higher the electrocatalytic activity of metal Salen complexes.10. The reversibility of E1 reaction in E1CE2 mechanism remain crucial for the electrocatalytic activity of M(Salen)/Z (M=Co, Fe, Z=Y, X, LSX) although the E1 reaction is not the speed-controlled step.In conclusion, the nature of metal is the primary factor to determine the electrocatalytic activities of M(Salen) complexes encapsulated into zeolite for ORR. The ligand's structure of complexes which result in the configuration change of metal complexes and ratio of SiO2:Al2O3 are also important impact factors on the electrocatalytic activity for ORR.