The Effect of pH of Hydrothermal Treatmet of Hydrogel for the Structure of Surface and Pore of Silica Gel

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Other Title
  • シリカゲルの表面および細孔構造に対する水熱処理pHの影響
  • シリカゲルの表面および細孔構造に対する水熱処理pHの影響--新しい型の表面水酸基
  • シリカ ゲル ノ ヒョウメン オヨビ サイコウ コウゾウ ニ タイスル スイネ
  • 新しい型の表面水酸基
  • A New Type of Surface Silanol Groups

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The pore and surface structures are studied on silica gels made by the hydrolysis of tetraethoxy silicic acid and by hydrothermal aging of hydrogel thus made at 100°C for 2 hr and at various pH. The materials thus obtained have the BET N2 surface area (SN2) of about 800 m2/g at low pH 900 m2/g at neutral pH and 130 m2/g at pH 11. 2. The density of surface OH groups (N0H/nm2) shows rather.complicated change such as 8, 5 and 18 at low, neutral and high pH, respectively. The infrared spectroscopy gives the species and relative concentration of those hydroxyl groups as follows. At low pH, where the gel is of micropore type, (Hydrogen-bonded OH between the primary silica particles) > (Free OH) >> (Inner OH). At neutral pH, where the gel is of mesopore type, (Free OH), (H-bonded OH between particles) ≅ (Inner OH). At high pH which makes macropore type gels, (Inner OH) ≅ (New type OH) >> (Free OH). The free, H-bonded and inner OH are similar to those found before. The newly found OH groups have the IR absorption maxima at 3,500cm-1 dehydrate in vacuo at 400°C and rehydrate to the former OH very easily. SN2, treated at 400°C is the same with that without heat treatment. The initial monolayer heat of adsorption of water to this material is about half of the heat of condensation of water. These facts suggest that the new type of OH groups found in this study are on the bare surface of silica and are much weaker adsorption sites for water.

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