Heterolytic Splitting of H2 and CH4 on γ-Alumina as a Structural Probe for Defect Sites

Abstract

A combined use of DFT periodic calculations and spectroscopic studies (IR and solid-state NMR) shows that a γ-alumina treated at 500 °C under high vacuum contains surface defects, which are very reactive toward H₂ or CH₄. The reaction of H₂ on defect sites occurs at low temperature (ca. 25 °C) on two types of Al atoms of low coordination numbers, Al_III or Al_IV, to give Al_IV−H and Al_V−H, respectively. The amount of defects as titrated by H₂ at 25 and 150 °C is 0.043 and 0.069 site/nm², respectively, in comparison with 4 OH/nm²). In contrast, CH₄ reacts selectively at 100−150 °C on the most reactive Al_III sites to form the corresponding Al_IV−CH₃ (0.030 site/nm²). The difference of reactivity of H₂ and CH₄ is fully consistent with calculations (reaction and activitation energy, ΔE and ΔE^⧧).