Effect of partial fluorination on the Lewis sites of microcrystalline γ-alumina studied by adsorption of pyridine as a probe molecule: A quantum chemical cluster model study

• Published in: Surface science Vol. 605; no. 15; pp. 1522 - 1533
• Main Authors: Nastova, Irena, Skapin, Tomaž, Pejov, Ljupčo
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier B.V 01-08-2011; Elsevier
• Subjects: Adsorption; Clusters; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Density; Electronics; Exact sciences and technology; Fluorinated γ-Al 2O 3; Fluorination; Gamma-alumina (γ-Al 2O 3); Heterogeneous catalysis; Lewis acid adsorption sites; Non-covalent interactions; Physics; Pyridine; Pyridines; Quantum chemistry; Surface chemistry; Vibrational frequency shifts; Heterogeneous catalysis; Lewis acid adsorption sites; Pyridine; Adsorption; Gamma-alumina (γ-Al 2O 3); Fluorinated γ-Al 2O 3; Vibrational frequency shifts; Non-covalent interactions; Aluminium oxide; Inorganic compounds; Gamma-alumina (γ-Al; Fluorinated γ-Al; Lewis acids; ); Spectral line shift; Cluster model; Surface structure; O; Frequency shift; Adsorption structure; Catalysis; Fluorination
• ISSN: 0039-6028; 1879-2758
• DOI: 10.1016/j.susc.2011.05.023

The influence of fluorination of microcrystalline γ-alumina on the acidity of Lewis sites was studied by quantum chemical cluster model approach. B3LYP and HF/6-31++G(
d,
p) levels of theory were employed, considering the standard and counterpoise-corrected potential energy surfaces (PESs). Explicit inclusion of dynamical electron correlation effects along with the elimination of the basis set superposition effects in geometry optimization and harmonic vibrational analysis were found to be crucial to reproduce the experimental trends in the shifts of the pyridine ν
19b and ν
8a modes upon fluorination. The acidity of surface-exposed Al-sites was found to increase upon fluorination, which is manifested as a ~
10% increase of the interaction energies, and also in the characteristics of the electronic density and density Laplacian at the intermolecular bond critical point. Bader analysis of the electronic density has shown that pyridine adsorption on pure and fluorinated γ-alumina can be classified as non-covalent interaction.
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► We studied the effects of fluorination of γ-alumina on the Lewis sites' acidity. ► Partial fluorination of the surface increases the acidity of the exposed Al-sites. ► This is manifested as a ~
10% increase of the interaction energies. ► Shifts of adsorbed pyridine ν
19b and ν
8a modes upon fluorination were calculated. ► Counterpoise-corrected PESs have to be explored to reproduce the experimental data.