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

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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, AlIII or AlIV, to give AlIV−H and AlV−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 AlIII sites to form the corresponding AlIV−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⧧).