Studies On Some POMs Based On Tungsten-clusters And Lanthanum Cobaltite Nanomaterials

In this dissertation, the synthesis and structures of Ployoxometalates （POMs） basedon tungsten-clusters, as well as the preparation and electrochemical properties oflanthanum cobaltite nano-particles were systematically investigated, for the purpose toexplore functional nanomaterials with a novel structure or potential catalytic activities.In the first part of the dissertation, POMs based on tungsten-clusters were chosen asresearch objects. Three POMs compounds based on tungsten-clusters with novel structureswere successfully achieved by traditional aqueous solution method, in which organicligands were introduced into framework of tungsten-clusters, and rare earth elements wereused as linkage. It should be noted here that, Compound （I）[Na₂(C₈H₁₂N₂O₃)₄（C₂H₆SO）₄(W₆O₁₉)]_nis a unusual2D organic-polyoxotungstate and a new example ofhexapolyoxotungstate units with a2-D structure. Compound （II）[（H₂O）Na（-DMSO）₃La（DMSO）₄(W₁₀O₃₂)]_nis the first example that the basic decapolyoxotungstates buildingblock linked up into multi-dimensional structure and the first polyoxotungsten compoundincorporating both rare transition metals and organic ligands with multi-dimension.Compound （Ⅲ）[Na₁₀（H₂O）₁₈(H₂W₁₂O₄₂)]_nis a new packing example of paradodeca-tungstate salt with simple composing but complicated3-D structure.In the second part of the dissertation, the preparation and electrochemical propertiesof lanthanum cobaltite nano-materials and rare transition metal cobaltites were studied.The existing types of Co ion and citric acid in the sol were analysed at molecule levelbased on thermodynamic principle. The coordination modes and the optimal coordinationenvironment of citric acid were also discussed. A new technological process, i.e. anammonium hydroxide improved citric acid sol-gel method, was drawn out and thenlanthanum cobaltites were successfully prepared. This process was stable with goodreproducibility for preparing cobaltite nano-particle.The effects of preparing condition, the calcination temperature and the rare transitionmetals doping on the phase structure and topography of lanthanum cobaltites and raretransition metal cobaltites were systematically studied. It comes to a conclusion that calcination temperature at600℃, using the new sol-gel technological process, gives outpure perfect perovskite structure LaCoO3nanomaterial with particle size25.8nm inrhombohedra symmetry. The LaCoO3particles are in perovskite structure when thecalcination temperature between600℃and900℃, but the particle size grows as thecalcination temperature rises. Then, a series of La and Nd cobaltites （La1-xNdxCoO3, x=0¹） were studied. The results show that perovskite phase can be formed at any ratio of Laand Nd, and rhombohedra structure （mainly LaCoO3） can be transformed into the cubicstructure （mainly NdCoO3） when x0.4. Further, the cobaltites La1-xAxCoO3（A=Ce, Pr,Nd, Sm, Eu, Gd, Tb, Dy, Er） were studied. The results show that La1-xAxCoO3cobaltitesalso possess a perovskite structure when x=0.5except La1-xCexCoO3, and theLa1-xCexCoO3has a perovskite structure only when x0.1.The cobaltites catalytic electrodes were fabricated with Nafion ion-exchange resin,and the electrochemical properties in alkaline solution were studied. It is found that thecatalytic electrode made by LaCoO3particles calcined at600℃has the highest anodecurrent, and the highest catalytic activities. Moreover, the electrochemical properties forseries of binary rare transition metals cobaltites La0.5A0.5CoO3were studies. It is found thatthe light rare earth lanthanum cobaltites show higher catalytic activities than heavy rareearth lanthanum cobaltites, and that the lanthanum cerium cobaltites or heavy raretransition metals lanthanum cobaltites show catalytic inactivities. Furthermore, thePt-LaCoO3/C and Pt/C catalyst （20wt.%Pt） were prepared by glycol reduction process.The results show that the catalyst of Pt-LaCoO3/C has advantageous of large surface area,excellent dispersion of Pt with particle size of ac.2nm, and finally show goodelectrocatalytic properties for alcohol electro-oxidation. The potential at anode currentpeak for methanol electro-oxidation on Pt-LaCoO3/C is lower than that on Pt/C electrode atthe same Pt loadings due to the higher dispersion Pt particles on LaCoO3/C.The perovskite lanthanum cobaltite was also successfully prepared by coprecipitationwith microwave irradiation to compare with the above research results. It has a smalleraverage particle size, higher BET surface area, higher electro-catalytic activities but lesspure in comparison with that prepared by sol-gel route.