Majority Ga(I) Sites in H2–Activated Ga/γ-Al2O3 and Ga/ZSM5 Catalysts Exhibit Distinctive XANES and Anomalous EXAFS Behavior

Ga-containing oxides are selective catalysts for high-temperature hydrocarbon reactions such as alkane dehydrogenation. The active sites are proposed to have an oxidation state of either +1 or +3, and bear a combination of hydride and alkyl ligands, in addition to oxygen-donor ligands associated with the support. Activating a Ga/γ-Al2O3 or Ga/ZSM5 catalyst in H2 at or above 473 K causes the Ga K-edge to (1) shift decisively to lower energy, and (2) increase significantly in its white line intensity. Taken together, these behaviors are shown to be signatures for Ga(III) reduction. The extent of Ga(I) formation, assessed by quantitative XANES analysis, varies with temperature, H2 pressure, and the nature of the support. H2 also causes strong suppression of the EXAFS amplitude. The origin of the latter phenomenon was investigated by analyzing Ga(I) EXAFS signatures in two well-defined model compounds: mixed-valent Ga2Cl4 and Ga(I)-β″-Al2O3. Theoretical simulations show the EXAFS “invisibility” of Ga(I)at noncryogenic temperatures is a direct result of very large vibrational amplitudes. Due to its valence electron configuration and largely unhybridized 4p orbitals, Ga(I) forms very long, labile bonds whose mean-squared relative displacements (σ2) are an order of magnitude higher compared to those associated with the much shorter, stronger bonds of Ga(III). Consequently, EXAFS signals for materials with a mixture of Ga oxidation states are virtually devoid of structural information about the Ga(I) sites, being dominated instead by scattering paths involving residual Ga(III) sites. At elevated reaction temperatures in the H2-activated catalysts, hydrides are minority species, accompanying the majority Ga(I) sites. These insights set the stage for quantitative analysis of Ga speciation during alkane dehydrogenation, and elucidation of the structure(s) of the active sites. Furthermore, they have broad implications for the use of X-ray absorption spectroscopy to extract structure–property relations for catalysts and other materials containing Ga(I) or other weakly bonded, highly dynamic metal ions.