Preparation Of Mesoporous Zeolite LTA-encapsulated Bimetallic Nim(M=Cr, Mn, Co, Cu, Zn)Schiff-base Complexes And Their Electrocatalytic Performancefor Methanol Oxidation

Direct methanol fuel cells (DMFCs) have been attracted extensive attentions forportable-power applications due to their advantages of high energy efficiency,low operation temperature, ease of fuel storage and distribution, low-to-zeropolluting emissions and environmentally friendly substitute. In theelectrocatalyticoxidation of methanol, the electrode material is obviously animportant factor where a highly efficient electrocatalyst is needed. AlthoughPt-based materials have beenextensively investigated as the electrocatalysts andshow towering anodic activity for the anodic electrooxidation of methanol, theresource scarcity, the high price and the activity loss due to CO poisoningimpeded the development of DMFCs. Bimetallic nickel-based complexes,especially bimetallic nickel-basedSchiff-base complexes, have been moreattractive as the anodic electrocatalysts in alkaline media because they areinexpensive and exhibit good electrocatalytic activity for the methanolelectrooxidation. However, the degradation of the organic ligands and dimerization of the transition metal complexes could occur during thehomogeneous catalysis reactions, resulting in a reduction in the activity andeven an irreversible deactivation, and the difficult recovery and recycling of thecatalysts also limit their reuse. Encapsulation of the bimetallicnickel-basedcomplexes in zeolite gains incomparable advantages for homogeneous andconventional heterogeneous catalysts. Upon encapsulation in zeolites, transitionmetal complex molecules are encaged and site-isolated, making these complexesstable, highly active and selective, while heterogeneous catalysts offer theadvantages of easy catalyst separation, possible catalyst recycle and highselectivity. The mesoporous zeolite LTA not only has nano-cages of3nminterconnected to each other through0.8-1.2nm channels,but also hassuper-cages of1.2nm. This makes the ligands diffusion into the zeolite channelseasier, and as a consequence, promotes the formation of metal complexesincages of zeolite LTA, which brings about a great improvement in theirelectrochemical and electrocatalytic performances.In this paper, a series of NiSchiff-base complexes, bimetallic NiMsalencomplexes, NiMn tetrahydro-Schiff-base complexesand NiMn-X2-salpncomplexes were encapsulated in the nano-cages of mesoporous zeolite LTA bythe flexible ligand method.The modified electrodes were studied for theirelectrochemical redox behavior and electrocatalytic oxidation of methanol inalkaline solution by using the methods of cyclic voltammetery(CV),chronoamperometry(CA) and so on to obtain the best corresponding relation between the steric strcture and electronic effect of the ligand and theelectrocatalytic performance for methanol oxidation, and the effects of thedoping metal elements on methanol electrooxidation activities. The maincontents in this paper are listed as follows:1. Electrochemical behavior and electrocatalytic oxidation of methanol onNiL/LTA/GCEsSeven kinds of NiSchiff-base complexes were encapsulated in mesoporouszeolite LTA (denoted as NiL/LTA with L=salen, salpn, salbn, salphen,tBu4salen,salcn and Jacobsen salen)by using the flexible ligand method. Theseheterogeneous catalystswere characterized by XRD, FT-IR, UV-vis, elementalanalysis, as well as N2adsorption/desorption techniques, and applied tomethanol electrooxidation in alkaline media. The results showed thatNiL/LTAgave a much higher electrocatalytic activity for methanolelectrooxidation than the Ni/LTA due to the interaction between the central ionsNi(II) and the ligands, which speeds up the charge transfer between the metalions and the ligands under external electric field. In addition, differentperformances for methanol oxidation on NiL/LTA/GCEsaredependent on thedifferent types of ligands which exhibitdifferent basicity of ligands, size ofconjugated system, flexibility of skeletons and the steric hindrance.Regardingthe flexible ligand method, the ligand with stronger N-basicity, largerconjugated systems, more flexibility skeletons and smallersteric hindrance givebetter catalytic activities toward the methanol electrooxidation. The Nisalpn/LTA, Nisalcn/LTA, Nisalphen/LTAshowed the better electrocatalyticactivity for the methanol oxidation than the other four composite catalysts.2. Electrochemical behavior and electrocatalytic oxidation of methanol onNiMsalen/LTA/GCEsFive kinds of bimetallic salen complexes NiM (M=Cr, Mn, Co, Cu, Zn)have been simultaneously encapsulated in the nano-supercages of mesoporouszeolite LTA by using the flexible ligand method.These prepared catalystswerecharacterized by XRD, FT-IR, UV-vis, elemental analysis, as well as N2adsorption/desorption techniques. Compared to the Nisalen/LTA,NiMsalen/LTAshowed superior electrocatalytic activity towards the methanol electrooxidationin alkaline solution due to theexistence of a synergetic effectthat could beattributable to the interaction between the partially coordinated Ni(salen) andM(salen) molecules through the zeolite lattice oxygen, which is resulted in aredistribution of d electrons in Ni2+and Mn+ions, corresponding with the DRUV-Vis spectral results. Different performances for methanol oxidation onNiMsalen/LTA/GCEs may be ascribed to the different nature of the dopingmetal elements. The higher activities for the methanol oxidation onNiCrsalen/LTA, NiMnsalen/LTA and NiCosalen/LTA are proposed to be thepre-adsorption of methanol molecules at Cr, Mn, Co species sites by interactionof non-bonded electron pairs of the O-atom of methanol with partially vacantd-orbital of the Cr, Mn, Co species within the above three catalysts.For NiCusalen/LTA and NiZnsalen/LTA, desorption of intermediates and productsbecame easier due to the smaller of vacant d-orbital.3. Electrochemical behavior and electrocatalytic oxidation of methanol onNiMn[H4]-Schiff-base/LTAA series of NiMn tetrahydro-Schiff base complexes (denoted as NiMn[H4]-Lwith L=H2[H4]salpn, H2[H4]salbn,H2[H4]salcn) were encapsulated inmesoporous zeolite LTA by using the flexible ligand method and characterizedby XRD, FT-IR, UV-vis, elemental analysis, as well as N2adsorption/desorption.The NiMn[H4]-L/LTA exhibited higher activity for the methanolelectrooxidation than the corresponding NiMnL/LTA, indicating thathydrogenation of the C=N bondled to the ligands with stronger N-basicityandmore flexibility, and therefore a modification of the coordinationenvironment of the central ions. The value for the catalyticrate constantobtainedfrom the chronoamperometry indicated that the NiMn[H4]-L/LTAmodifiedelectrode can overcome the kinetic limitation for the methanol oxidation to someextent by a catalytic process and candecrease the over-potential for the methanoloxidation reaction.4. Electrochemical behavior and electrocatalytic oxidation of methanol onNiMn-X2-salpn/LTATwo kinds of mesoporous zeolite LTA encapsulated NiMn-X2-salpn (X=NO2and OCH3) were prepared by using the flexible ligand method andinvestigated the electrooxidation of methanol on glassy carbon electrode in0.1 M NaOH solution. The prepared catalysts exhibited high activity and stabilitytowards the electrooxidation of methanol. Substitution of the aromatic hydrogenatom of the ligand with electron withdrawing group (-NO2) dereased the amountof the complex encapsulated in the zeolite cavities, but could improve thecatalytic activity of the encapsulated complex. However, the substitution oftheelectrondonating group (-OCH3) had little influenced on the methanoloxidation. This indicated that the nature of the substituents have significanteffects on the electrocatalytic activities for methanol oxidation.