Synthesis And Catalytic Properties Of Transition Metal Oxides

The catalysts of transition metal oxides have been studied extensively. Among them, perovskite-type composite oxides are of the broad application due to their thermal stability, variability in structure and composition, low cost of production, and excellent catalytic activity for the destruction of common exhaust pollutants such as CO, hydrocarbons and nitrogen oxides. Moreover, perovskite-type composite oxides exhibit a high photo-catalytic activity in the degradation of dyes and the decomposition of water to obtain hydrogen. It is important to improve the catalytic activity of catalysts by controlling the size, the morphology, the aggregation, and increasing the BET in preparation. In addition, compared with the catalysts of the single component, the response range to light and the effect of electron-hole separation are well improved in the catalysts with heterostructure. In fact, the effective connection between the different kinds of particles is essential for the effective separation of electron-hole pair.1. It is important and difficult to obtain the nanoparticles with high BET surface area, uniform size and good dispersivity. It is also difficult to obtain the nanocrystal clusters with secondary structures. So it is more challenging to synthesize transition metal composite oxide catalysts meeting the above conditions. In this paper, we have made some breakthroughs in preparation catalysts of ZnSnO₃ and LaCo_0.9Mg_0.1O₃ with a perovskite structure.A simple method is applied to synthesize ZnSnO₃ nanoparticles. With the aid of various Amino acids, ZnSnO₃ nanocrystal clusters and nanocubes are obtained. When the reaction time is 2 h and the concentration of reactants is 0.04 mol·dm^-3, the morphology and size of samples obtained with the aid of various Amino acids of 0.25 g are different. The ZnSnO₃ nanocrystal obtained with the aid of glutamic acid and aspartic acid are of high BET due to good dispersivity and the small size existing in the secondary structure. While the ZnSnO₃ nanocubes obtained with the aid of histidine and lysine are of low BET surface area due to serious aggregation and larger size of paticles. But with the aid of histidine, the ZnSnO₃ nanocrystal clusters with good dispersivity and large BET surface area can also be prepared by changing the concentration of reactants and the amount of histidine. The ZnSnO₃ nanocrystal clusters with good dispersivity exihibit high photo-catalytic activity for the degradation of Congo red. While the photo-catalytic activity of the as-prepared ZnSnO₃ nanocubes is lower due to the aggregation and low BET of particles.The mechanisms of the formation of ZnSnO₃ nanocrystal clusters and nanocubes need to be further studied.The synthesis temperature of LaCoO₃ oxide is above 600oC in the sol-gel method. The heat treatment will lead to the growth, aggregation, and low BET of the particles. In the preparation of LaCo_0.9Mg_0.1O₃ nanoparticles, the traditional sol-gel method is adjusted by adding excessive Mg²⁺ into the reaction system. The excessive MgO generated during the reaction plays a role in inhibiting growth and aggregation of LaCo_0.9Mg_0.1O₃ nanoparticles in the final heat treatment. Moreover, by introducing different amounts of magnesium, the size and specific surface area of the LaCo_0.9Mg_0.1O₃ product can be well controlled in the range of 13.9 31.6 nm and 64.1 13.9 m²/g. The results of XPS, TPR, TPD and the results of catalytic oxidation of CO confirm that the LaCo_0.9Mg_0.1O₃ samples with large BET surface area exihibit high activation. The catalytic activity is related to the amount of oxygen adsorbed on the surface, which is related to the BET surface area.2. The low solar light utilization efficiency exists in the UV-light driven catalysts, and the quantum efficiency is low in the catalysts of single component. So a general method to get efficient visible-light driven catalysts is to prepare composite catalysts, in which the effective combination of heterogeneous particles is essential. ZnO is a UV-light driven catalyst and CdO is a visible-light driven catalyst. A simple method is developed to prepare ZnO/CdO composite catalyst with heterostructure. Firstly, a sponge-like and porous metal zinc is obtained by reacting Zn with dilute sulfuric acid, and then, a homogeneous CdCO3, which is obtained by reacting Cd²⁺ with CO3^2-, deposits on this sponge-like and porous surface of the metal zinc. Finally, a heat treatment is needed to obtain ZnO/CdO composite oxide with conjunctive structure between the ZnO and CdO particles. The experiments show that the size of the sample can be controlled by the heating temperature and the reaction time.The as-prepared ZnO/CdO composite oxide samples are driven by visible light, and exhibit higher photo-catalytic activity. This shows that a effective combination has forms between the ZnO and CdO particles.