A Study on Edge Surface Structures and Reactivity for Clay Minerals by Molecular Orbital Method

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  • 分子軌道法を用いた粘土鉱物エッジ表面の構造および反応性に関する研究
  • ブンシ キドウホウ オ モチイタ ネンド コウブツ エッジ ヒョウメン ノ コウゾウ オヨビ ハンノウセイ ニ カンスル ケンキュウ

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Abstract

  To clarify the dissolution mechanism of clay minerals, the basal and edge surface structures of dioctahedral 2:1 phyllosilicate were optimized using the semiempirical molecular orbital method. The stability of the surface structure was estimated from the bond length, bond strength and bond energy of bonds of interest along the basal and edge surfaces. The basal surface structure of montmorillonite was optimized using the periodic boundary condition. The optimized basal surface structure resides at a local minimum on the potential energy surface, because each normal mode for the optimized structure has a real vibrational frequency. The ideal (010)-type edge surface of pyrophyllite was initialized using crystal chemical methods. The edge surface structure that was protonated allowing for pH dependence was optimized. On the optimized edge surfaces, structural relaxation occurred under all pH conditions, then it was found that the Si-O bonds in the silanol groups were stronger and the Al-O bonds in the aluminol groups were weaker than the corresponding bonds in the bulk structures. The rate-determining step is thought to be governed by the hydrolysis of the outer Si-O bonds on edge surfaces during the alkaline dissolution of pyrophyllite and by the hydrolysis of Al-O bonds in bridging the Si-O-Al bonds on edge surfaces during the acid dissolution.<br>

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