用正电子湮没谱研究煅烧对SBA-15热稳定性的影响

Effect of calcination on the thermal stability of SBA-15 studied by positron annihilation spectroscopy

多孔材料由于具有高比表面积、高渗透性、吸附性和可组装性等优异的物理化学性能,被广泛应用于气体吸附、电化学、药物输送、催化剂和催化剂载体等领域。多孔材料的应用性取决于其宏观性质,尤其是孔隙结构的多样性、孔径的可调性、热稳定性等对其大规模实际应用非常重要。利用正电子湮没谱学(Positron Annihilation Spectroscopy,PAS)观察了SBA-15的热稳定性。以正硅酸四乙酯为硅源,P123为结构导向剂,合成了原生有序介孔二氧化硅(SBA-15)和550℃煅烧后的有序介孔二氧化硅(SBA-15)。将上述两种SBA-15在100~1000℃温度下煅烧处理,观察其孔隙结构的热稳定性变化。采用小角度X射线散射(Small Angle X-ray Scattering,SAXS)、扫描电子显微镜(Scanning Electron Microscopy,SEM)、高分辨透射电子显微镜(Highresolution Transmission Electron Microscope,HRTEM)、N_(2)吸附/脱附(N_(2)adsorption-desorption)和正电子湮没谱学(Positron Annihilation Spectroscopy,PAS)等方法研究了两种SBA-15有序孔隙结构的稳定性。正电子寿命测量结果表明:两种SBA-15均存在4种寿命组分,较长寿命τ_(3)和τ_(4)分别对应于o-Ps在材料中微孔和二维六角的较大孔中的湮灭结果。随着热处理温度的升高,这两种SBA-15中较长寿命τ_(4)逐渐降低,其强度I_(4)也逐渐降低,但煅烧后SBA-15较长寿命及其强度在600℃后下降更为明显。同时,S参数结果与正电子寿命的结果吻合较好,S-W曲线也表明煅烧后SBA-15孔隙类型在100~900℃间几乎未发生变化。结果表明:煅烧后SBA-15表现出较好的热稳定性,同时电子偶素(Ps)是一种非常灵敏的研究孔隙结构的探针。

[Background]Since the 1990s,porous materials have been widely used in the fields of gas adsorption,electrochemistry,drug delivery,catalyst and catalyst carrier because of their excellent physical and chemical properties such as high specific surface area,high permeability,adsorption and assemblability.The application of porous materials largely depends on their macroscopic properties,especially the diversity of pore structure and the adjustability of pore size,especially,the thermal stability of SBA-15 is very important for its large-scale practical application.[Purpose]This study aims to observe the thermal stability of SBA-15 by positron annihilation spectroscopy(PAS).[Methods]First of all,ordered mesoporous silica SBA-15 were synthesized by using TEOS(tetraethyl orthosilicate)as the silicon source and P123 as the structure-directing agent.Part of the obtained SBA-15 was further calcined at 550℃.Then,the above two kinds of SBA-15 were heated at 100~1000℃to check the thermal stability of their pore structure.Small angle X-ray scattering(SAXS),scanning electron microscopy(SEM),high-resolution transmission electron microscopy(HRTEM),N_(2)adsorption/desorption and positron annihilation spectroscopy(PAS)measurements were used to study the ordered pore structure of SBA-15.[Results]Positron lifetime measurements reveal that four life components exist in both SBA-15 whilst the two longer lifetimesτ_(3)andτ_(4)correspond to the annihilation of o-Ps in the micropores and large pores of the material,respectively.With the increase of heat treatment temperature,the longest lifetimeτ_(4)of these two kinds of synthesized silica decrease gradually,as well as the corresponding intensity I_(4).However,the longer life and intensity of calcined SBA-15 decrease obviously after 600℃.Meanwhile,the S parameter of calcined SBA-15 is in good agreement with the results of o-Ps lifetime,and the S-W curve also shows that the pore types of calcined SBA-15 nearly are unchanged during in the heat treatment process at the temperature range of 100~900℃.[Conclusions]All the results indicate that the synthesized SBA-15 calcined at 550℃exhibits relatively better thermal stability,and positronium(Ps)is a very sensitive probe to study pore structure.