Electronic charge distribution and Lewis acidity of surface aluminium atoms in γ-Al₂O₃: a quantum-chemical model

Abstract

A quantum-chemical cluster model has been proposed which provides an adequate description of electronic charge on differently coordinated aluminium atoms both in the bulk and on the surface of y-Al,O,.It also describes Lewis acidity of the surface aluminium atoms characterized by (a) the energy of interaction with electron donor molecules, (b) position of the local acceptor energy levels and (c) atomic charge. The model employs the semi-empirical MO CNDO/2 method in a standard parametrisation. Geometrical optimisation of surface atoms is shown to be important for the electronic modelling properties of Lewis-acid sites.Aluminium ions in the bulk of y-Al,O, crystal are known to occupy octahedral and tetrahedral interstices between the surrounding densely packed oxygen ions. Accordingly, two types of coordinatively unsaturated (cus) aluminium ions are expected to appear on the surface following crystal splitting: Al$' with coordination number 5 and AlP' with coordination number 3.' These ions are responsible for the local acceptor states in the band gap of the crystal energy scheme and, provided they are sterically accessible to the gas-phase molecules, display themselves in chemical experiments as Lewis acids.' The three main parameters usually employed to measure Lewis acidity of the surface metal ions in oxides are (a) the energy of bond formation with electron-donating molecules, (b) the position (depth) of acceptor energy level localized on the metal ion and (c) the value of the positive atomic charge. The stronger the bond with the electron-donating molecule, the deeper the local acceptor energy level will be and the greater the positive atomic charge i.e. the stronger Lewis acid is the metal ion on the solid surface.

XPS and X-ray AES(XAES) Study of Various Aluminate Compounds.

“…Same tendency was also observed in sili-cate group, but the observed Al 2 p energies were more widely distributed than those of aluminum oxide group. These results were in good agreement with those report that in the [6] Al has smaller positive atomic charge than [4] Al 8,9 . On the other hand, a clear chemical shift was observed in aluminum hydroxide group.…”

“…Hughes et al 7 examined hydrous aluminas by XPS and reported that the XPS and XAES data of six coordinated aluminum ( [6] Al) were different from those of four coordinated aluminum ( [4] Al) in two dimensional chemical state plot. Fleisher et al 8 calculated atomic charges of octahedral and tetrahedral clusters in γ -Al2 O3. Then, they reported that the atomic charge of [4] Al was dominantly greater than that of [6] Al.…”

Interactions of methane, ethane and pentane with the (110C) surface of γ-alumina

“…The large cluster size is important because second-nearest-neighbor effects have been shown to heavily influence surface chemistry in cubic aluminas [18]. Semi-empirical models have been used effectively in theoretical investigations of similar systems [8,15,19,20] and have been shown to lead to qualitative conclusions and energetic behavior consistent with first-principles calculations.…”

“…Such chemisorption would produce the surface species CD 3 ␦− and D ␦+ , with the CD 3 ␦− moiety bound to a valence unsaturated Al atom of Lewis acid character. The Lewis acidity of surface Al sites on ␥-alumina has been the subject of several previous investigations [7][8][9], as has the reactivity of these sites with water [10][11][12], hydrogen sulfide [10], carbon monoxide [10], ammonia [11], pyridine [11], and methanol [13]. Previously we applied theoretical calculations to gain insight into the interaction of 1-hexene with ␥-alumina, and found the interaction to be purely repulsive at valence unsaturated surface Al sites [14].…”

“…Under certain conditions these reactions can be exothermic. The lowest energy barriers to these reactions are around 60-70 kcal/mol on H 8 …”