Calculation Of Surface Acid On Different Crystals Of Alumina

In the context of alumina supported catalysts applied on hydrodesulfurization,the bulk and surface properties of different crystals of alumina were investigated by calculation method in this thesis.Calculation of quantum chemistry was the main computational approach.First,bulk structures ofγ-Al₂O₃,θ-Al₂O₃ andα-Al₂O₃ were determined and verified by comparison between XRD patterns from simulation and from experiment.The analyses include their energies,bond lengths and electronic states.Second,surface models were established based on the bulk structures.Optimum surfaces were selected through energy comparison.Structure transformations on the surfaces were described in the thesis.Third,the distribution of Lewis acid onγ-Al₂O₃（100）,（110）and（111）surface as well as onθ-Al₂O₃（-102）,（120）and（221）surface was explored by molecule NH₃.Bronsted acid provided by adsorbed H₂O on Lewis acid positions were also investigated by NH₃.By computation,some conclusions were obtained.In alumina,bond of Al-O for hexa-coordinated Al is longer than that for tetra-coordinated Al.Ionicity and conductivity decrease in the orderγ-Al₂O₃,θ-Al₂O₃ andα-Al₂O₃.Concluded from the calculation results,Lewis acid is determined by the surrounding environment.Generally,the less the coordinate bonds,the stronger the acid.Based on the distribution of Al with different coordinations,strong acid positions are mostly on the（100）surface ofγ-Al₂O₃.The（110）surface has only medium strong acid positions,while the（111）surface shows no acid position.Onθ-Al₂O₃,acid positions are dispersed.（-102）surface are mostly medium strong acid positions.（120）surface shows more strong acid positions while（221）surface shows less strong acid positions.For the Bronsted acid transferred from adsorbed H₂O on Lewis acid positions,it is remarkable on（100）surface ofγ-Al₂O₃ and（120）surface ofθ-Al₂O₃.