| SiO2 and its composite aerogels are widely used in various fields due to their special pore structure,extremely low density and large specific surface area.However,the limitation of drying methods failed to achieve large-scale use,and the application of single SiO2 aerogels was also limited.In this paper,SiO2 and TiO2/SiO2 composite aerogels were prepared from ethyl orthosilicate(TEOS)and butyl titanate(TnBT)as materials,deionized water and absolute ethanol as solvent,hydrochloric acid,ammonia and formamide as the catalyst.The SiO2 and TiO2/SiO2 composite alcohol gels were prepared by acid/base two step sol-gel method.After aging,solvent replacement and other subsequent processes,blocky transparent SiO2 aerogel were prepared by subcritical conditions and compared with ambient pressure drying and freeze drying.The aerogels were characterized by SEM,TEM,XRD,BET,TG-DSC,FTIR and TG-DSC.The prepared TiO2/SiO2 composite aerogel was used for ultraviolet photocatalytic degradation of methyl orange.The results show:(1)The SiO2 aerogel which was prepared by ambient pressure drying had an amorphous structure and poor bulk formation.The specific surface area was 446.18-816.34 m2/g,and the density was 0.274-0.453 g/cm3.The porosity was 79.3%-87.5%,and the pore volume was relatively small.The density was large,and the average pore size distribution was between 2-4 nm.(2)The aerogel which was prepared by freeze-drying had an amorphous structure and was poor in bulk.The specific surface area was 623.89-888.88m2/g,and the average pore size distribution was between 8-12 nm.After being calcined at 600oC,the structure collapsed,and the pore diameter occurred in the table,indicating that the thermal stability was relatively poor.(3)The SiO2 aerogel which was prepared by the subcritical method had an amorphous structure with good fastness and high light transmittance.The specific surface area was 867.9l-1049.68m2/g,and the density was 0.170-0.221 g/cm3.The porosity was between 90.0%-92.2%,and the pore volume was 1.20-2.76cm3/g.The aerogel performance was best when the drying temperature was 220? and the constant temperature was 1.5 h.The pore volume can reached 2.08cm3/g,and the aerogel block was 5.7crrm2.After being calcined at 600?,the original bulk structure could be maintained well.The specifie surface area was 850.59m2/g,and the pore volume can reached 1.60cm3/g.In addition,the pore size distribution was unchanged.The analysis of the subcritical drying mechanism showed that under the subcritical condition,the saturated vapor pressure of the ethanol in the drying medium increased,and the surface tension decreased,and the gel shrinkage caused by the concave liquid surface was avoided,which made the skeleton structure maintain well.Therefore,the aerogel had excellent performance.(4)The composite aerogel with different TiO2/SiO2 ratios were prepared by subcritical drying method.After being calcined at 500oC,the specific surface area of the composite aerogel which was composed of amorphous SiO2 and anatase TiO2 was as high as 480.61m2/g.(5)UV-catalyzed degradation of methyl orange showed that the TiO2/SiO2 composite aerogels which were prepared by subcritical drying had a catalytic degradation rate up to 99.6%and exhibited excellent catalytic performance. |
PDF Full Text Request