Structural models of activated gamma-alumina surfaces revisited: Thermodynamics, NMR and IR spectroscopies from ab initio calculations
The activation of highly catalytic gamma-alumina surfaces by thermal treatment and the description of the related chemical processes at atomic scale is a topical issue. According to a recent study [J. Am. Chem. Soc. 134 (2012) 14430], the enhanced reactivity of gamma-alumina has been associated to tri-coordinated aluminum sites which supposedly are exposed exclusively on the (110) surfaces of this oxide. In this work, we explore this possibility by modeling the (100) and (110) terminations using Krokidis et al. [J. Phys. Chem. B 105 (2001) 5121] bulk structure and performing an extensive search of the most stable hydrated surface models at conditions consistent with experiment. Among the 156 structures analyzed, we identify several "metastable'' models for the (110) surface with a considerable probability of containing the Al-m centers at OH coverages of 9.0 and 6.0 OH/nm(2). We then test the reactivity of these sites through their Lewis acidity by simulating the CO adsorbtion on the surface and our results confirm the high reactivity of Al-m centers. Based on the Gibbs free energy of the explored structures, we carry on a thermodynamical analysis at varying hydroxylation degrees and pretreatment temperatures and simulate the experimental volcano-type behavior reported in [J. Am. Chem. Soc. 134 (2012) 14430] and predict the optimum pretreatment temperature as 700 degrees C, in very good agreement with experimental findings. We further use infrared and solid state MAS NMR spectroscopies and reproduce the H-1 MAS NMR spectra under high vacuum conditions (10(-5) Torr). The strong resemblance of spectra to the experimental ones in the literature [J. Phys. Chem. C 116 (2012) 834] validate further the structural models we have generated in this study. (C) 2013 Published by Elsevier B.V.