Determining the X-ray absorption signatures of Ga(I) and Ga(III) in molecular complexes and crystals*

Ga-based heterogeneous catalysts are the basis for commercial propane dehydroaromatization and can potentially replace Pt- and Cr-based ones in propane dehydrogenation. Based on experimental and theoretical studies, the proposed Ga active sites have a variety of coordination environments and oxidations states. To understand this diversity, we have studied the Ga K-edge XANES of compounds with well-known structures using StoBe, CASTEP and FEFF simulations. We find that for a series of Ga(III) molecular compounds, the position of the intense white line (WL) is modulated by two main factors: 1) Changing the local coordination environment from O to C atoms shifts the position of the empty p-state involved in the WL to higher energies, and 2) variations in the nearest-neighbor distance change the screening of the core-hole and its ionization energy. We also studied well-defined molecular and crystalline Ga(I) compounds to understand how their much more intense observed WL compares to those of compounds containing Ga(III) sites. We find that the increase in WL intensity originates from the presence of not just one empty p-like state, but either two or three of them, depending on the structure of the compounds. This change in intensity could be used to differentiate between oxidation states under realistic reaction conditions.