Synchrotron X-ray Absorption Spectroscopy (XAS) for Understanding Dopant Effects in Ti-doped NaAlH_4

摘要

We report studies concerning the time frame for catalytic reactions between NaAlH_4 (sodium alanate) and TiCl_3 during high energy milling. Spectral features in x-ray absorption spectra were analyzed for samples milled for various times (0 minutes, 1 minute, 5 minutes, 25 minutes, and 125 minutes). A structural transition from Ti~(3+) to Ti~0 is observed within the first 5 minutes of milling. The Ti~0 structure persists for samples milled for times longer than 5 minutes—even after those samples underwent a subsequent single hydrogen desorption/absorption cycle. Samples milled for less than 5 minutes retain Ti~(3+). For samples milled for 1 minute, the Ti-K edge position shifts from Ti~(3+) to Ti~0 after a single desorption/absorption cycle. However, the first coordination sphere around the Ti~0 absorber occurs at longer distances for 1 minute milled sample. These results hint at the formation of Ti~0 pure metallic clusters upon desorption/adsorption cycling of 1 minute milled sample. Previously reported x-ray absorption spectroscopy studies have demonstrated the structural transition from Ti~(3+) (in TiCl_3) to Ti~0 (determined to be TiAl_3 found at the surface of the alanate powder). Aluminum deficiency on the NaAlH_4 lattice is responsible for controlling hydrogen absorption capacity. These time-resolved spectroscopy studies suggest the possibility for controlling aluminum deficiency, while still maintaining titanium activation of the alanate powder, by manipulating the coordination environment of the titanium dopant during the first five minutes of high energy milling. Specifically, a possible approach to mitigating aluminum deficiency during doping would be to introduce an alternative species to which the to Ti~(3+) catalyst would form a metallic bond after the has attached at the surface of the sodium alanate powder and before the Ti~(3+) forms the intermetallic species.