(594a) Rigorous Oxidation State Assignment for Ga Catalysts Using Theory-Informed X-Ray Absorption Spectroscopy Signatures from Well-Defined Ga(I) and Ga(III) Compounds

Supported gallium oxides have been investigated for their ability to catalyze a variety of energy-related chemical transformations. In particular, their ability to catalyze the selective dehydrogenation of propane has inspired much interest. Under reaction conditions, the Ga species proposed to exist or co-exist include [GaO₄]^-, [Ga₂(OH)₄], [GaO]⁺, [Ga₂O₂]²⁺, [GaH]²⁺, [GaH₂]⁺, and [Ga]⁺. Identification of Ga(I) sites in catalysts under reaction conditions relies primarily on X-ray absorption spectroscopy (XAS). However, the observation of a low energy absorption edge is necessary but not sufficient evidence for [Ga]⁺. There are significant variations in edge position for the XANES of Ga(III) compounds. We develop a rigorous set of criteria for characterizing the white line behavior, derived from the experimentally observed and computed properties of several Ga(I) molecular complexes and Ga⁺-β´´-Al₂O₃, together with an investigation of the origin of their distinctive features. We report an analysis of the XAS of Ga/HZSM-5 and Ga/g-Al₂O₃, both of which exhibit low edge energy peaks.

Ga K-edge XAS were recorded for several well-defined molecular and crystalline Ga(I) compounds. The XANES is essential to establishing the presence of Ga(I), despite the overlap in edge energies with organoGa(III) compounds, because Ga(I)-containing oxide materials show very weak EXAFS scattering. Compared to the XANES of trigonal Ga(III)-containing materials, Ga(I) spectra display a significantly more intense white line feature. Theoretical simulations reveal that the strong XANES intensity originates from the superposition of transitions to several empty, nearly degenerate p-like states. These signatures provide compelling evidence for assigning the intense white line in Ga/HZSM-5 and dramatic loss of EXAFS intensity to the near-quantitative reduction of Ga(III) to Ga(I), while the weaker white line and conventional EXAFS signal of Ga/g-Al₂O₃ point to, at most, a minor fraction of Ga(I) sites. This study provides guidelines to ensure that Ga K-edge XAS data of catalysts are interpreted properly.