以離子液體模版溶膠-凝膠法製備孔徑可調之多孔 二氧化鈦光觸媒-無機鹽類的作用

Abstract

本研究中我們成功地開發離子液體(ionic liquid, IL)模版溶膠-凝膠法製備熱穩定性高且孔徑可調的中孔磷摻雜TiO2光觸媒，我們利用水在油相乳化的方式(water-in-oil)使極性離子液體BMIMCl在疏水性苯甲醇溶劑中自組裝導引TiO2孔洞結構，並加入磷酸與四種鹽類包括NH4NO3、NH4Cl、CaCl2 與 NaCl 作為結構穩定物質與擴孔劑，結果顯示當IL以鍛燒550 °C移除時，磷酸能有效維持TiO2的孔徑在4.2 nm，且比表面積可高達164 m2/g，當鹽類參與合成反應時，它們著實擴大TiO2孔徑尺寸，各鹽類對孔徑擴張的影響依序為NH4NO3 (16.8 nm) > NH4Cl (13.7 nm) > CaCl2 (12.4 nm) > NaCl (8.7nm)，孔徑大小與鹽類陰陽離子的離子半徑及價態有關，大離子半徑如NH4+:140 pm 與NO3-: 189 pm 與高電荷密度如Ca2+能產生較大孔徑，多孔TiO2在鹽類作用下比表面積仍可維持在154-199 m2/g。擴大孔徑幫助反應物質克服表面張力並因毛細凝結作用快速進入孔洞內進行光催化反應，明顯地，在相近比表面積下，經NH4Cl與NH4NO3擴孔的二氧化鈦光觸媒其光催化活性為未經擴孔樣品6.7-9.3倍。

We have successfully prepared mesoporous TiO2 photocatalysts with highly thermal stability and tunable pore sizes through an ionic-liquid templated sol-gel method. The mesoporous structure was directed by self assembly of a hydrophilic ionic liquid, butbleyl-3-methylimidazolium chloride (BMIMCl), in a benzyl alcohol (log Kow= 1.2) oil phase. Phosphate ions were incorporated into the TiO2 framework to stabilize the pore structure against thermal induced collapse. Four salts, including NH4NO3, NH4Cl, CaCl2 and NaCl, were used as auxiliary reagents to expand pore sizes of the porous TiO2 samples. After removal of BMIMCl at 550 °C, the porous TiO2 samples exhibited a large specific surface area of 164 m2/g and an arrearage pore size of 4.2 nm. Addition of the salts effectively expand the pore size in the order of NH4NO3 (16.8 nm) > NH4Cl (13.7 nm) > CaCl2 (12.4 nm) > NaCl (8.7 nm), while the surface areas were maintained in the range of 154-199 m2/g. The ionic radii and valence states of the cations/anions (Na+: 116 pm, Ca2+: 114 pm, NH4+: 140 pm, NO3-: 189 pm, Cl-: 181 pm) determined the pore sizes. Expanded pores assist the adsorption of bisphenol A into the pore channels of the porous TiO2 samples because of capillary condensation. Improved adsorption capability promotes photocatalytic performance. Compared to the porous sample having the pore size of 4.2 nm, the samples having the pore size of 13.7-16.8 nm enhanced the photocatalytic activity by 6.7-9.3 times although they have similar surface areas.