The Synthesis Of Mesoporous Silicas And Their Applications As The Loader And Template For The Preparation Of Nanocrystalline Co₃O₄

The research of this thesis focused on the pore size modulation, morphology control of mesoporous silicas, and their applications in pore channels loading and preparing nanocrystalline Co₃O₄. The main contents are shown as following.In chapter 2, the pore expansion effect of gelatin, a common amphoteric biological protein, on the hexagonal mesoporous silica materials is reported. Tetraethyl orthosilicate (TEOS) is used as silica source and the nonionic surfactant P123 (EO20PO70EO20) as template. It is revealed that the presence of gelatin dose not change the intact 2D-hexagonal mesoporous structure of materials. The products prepared with gelatin have the mild expansion ratios of (29-39)% and (5-22)% in pore diameter and pore volume, respectively. The specific surface area of products ranges from 445 m2·g-1 to 590 m2·g-1. The analysis of the reaction solutions for preparing materials by UV-Vis reveals that gelatin can interact with the hydrophilic sides of P123 micelles via hydrogen bonds interaction, which results to the pore expansion effect. The pore expansion mechanism is different from the reported those for other systems.In chapter 3, mesoporous silica nanospheres are prepared using cetyl trimethylammonium bromide (CTAB) as template and gelatin as suppressant in the alkaline solution. The findings reveal that the growth of the silica particles was effectively suppressed by gelatin. When the mass ratio of gelatin to CTAB increases from 0 to 1.0, the particle size of material gradually decreases from a wide micrometer range of about (0.25-0.8)μm to a narrow nanometer range of about (35-100) nm without destroy of the mesostructure. More significantly, low-angle X-ray diffraction (LAXRD) and N2 adsorption-desorption investigations disclose that the pore size of materials increases from around 2 nm to around 3.7 nm after gelatin is introduced into the preparation. The particle size control mechanism of gelatin is discussed in this paper.In chapter 4, Co₃O₄/MCM-41-x (x is the mass ratio of Co₃O₄ to MCM-41) composites are obtained by calcining the precursors that were prepared by impregnating Co(NO3)2·6H2O into the pore channels of mesoporous silica MCM-41. The composites have the large specific surface areas and pore volumes. The electrochemical properties of materials are investigated by cyclic voltammetry and electrochemical impedance spectroscopy measurements. The findings show that the loading of Co₃O₄, sweep rate, and concentration of electrolyte had significantly influenced on the electrochemical performances. The composites have good electrochemical performances including the high onset frequency, low charge transfer resistance Rct and inner resistance Ri. When x=1.6, this material can provide a highest specific capacitance of 308 F·g-1. The capacity retention was 91.3% after 500 continuous cycles.In chapter 5, mesoporous Co₃O₄ craterlike microspheres (Co₃O₄-CM) with mesoporous structure are obtained by utilizing MCM-41 as template. The material has a large specific surface area of 60 m2·g-1 and a narrow pore size distribution of centering around 3.7 nm. The findings reveal that Co₃O₄-CM has the higher onset frequency, the smaller charge transfer resistance Rct and inner resistance Ri. This material can provide a high specific capacitance of 102 F·g-1 and a large capacity retention of 74% in 500 continuous cycles test at the sweep rate of 3 mV·s-1. This is the first time that the mesoporous Co₃O₄ with the novel novel morphology is prepared and studied as supercapacitor electrode materials.In chapter 6, Co₃O₄ nanowires with regular morphology and uniform size have been prepared by utilizing the bandlike SBA-15 as template. It has the lowest Rct and Ri, highest onset frequency and specific capacitance in 6 mol·L-1 of KOH solution. The maximal specific capacitance value is 373 F·g-1. After 500 continuous cycles, the specific capacitance value is 90% of the original value.Co₃O₄ nanorods with 2D-hexagonal mesoporous structure have been obtained by nanocasting from rodlike SBA-15. Its average pore size is 6.3 nm. The nanorods have a uniform width of about 6 nm and a narrow length range of about (15-50) nm, which are much smaller than the Co₃O₄ nanorods prepared by a hydrothermal approach. In 2 mol·L-1 of KOH solution, the Rct and Ri are 1.74Ωand 0.17Ω, respectively. Co₃O₄ nanorods exhibit a greatest specific capacitance of 321 F·g-1 and a high capacity retention of 88% after 500 continuous cycles. Both Co₃O₄ nanowires and nanorods are excellent supercapacitor electrode materials.In chapter 7, the selective catalytic properties of MCM-41, Co₃O₄/MCM41-x, Co₃O₄-CM, and Co₃O₄ nanowires in cyclohexane oxidation mainly are investigated. The results show that the main product is cyclohexanone without any catalyst. The main product is cyclohexanol when MCM-41 is used as catalyst. When Co₃O₄/MCM-41-x is applied as catalyst, the selectivity of cyclohexanone increases with the growth of the loading amount of Co₃O₄. The selectivities of Co₃O₄ nanowires and Co₃O₄-CM for product are quite different. The selectivity of former for cyclohexanone is as high as 97.5%, while the selectivity of latter for cyclohexanol is 79.3%. The reaction mechanism of cyclohexane oxidation and action mechanisms of catalysts were discussed in the paper.