On the fate of the Ti catalyst during hydrogen cycling of sodium alanate

The state of the titanium catalyst species formed upon doping and hydrogen cycling of NaAlH4 with Ti-catalysts was studied by X-ray diffraction analysis. After doping with TiCl3, Ti is present as an hcp-Ti(Al) solid solution, while for the Ti(OBu)4 doped system, Ti exists as an XRD-amorphous phase. Cycling of hydrogen for 1.5 times results in transformation of these initially different Ti compounds into similar compounds, whose composition is dependent on the temperature. Thus, if low temperatures were used during the cycling process (up to 175 °C) an amorphous Al-Ti alloy formed, while upon using high temperatures (200 °C and higher) Ti was present as an AlxTi intermetallic. Correlation of the XRD and hydrogen desorption curves shows that the most active catalyst species in the present system is the amorphous Al-Ti alloy, whereas a decreased catalytic activity is found for the AlxTi intermetallic.