Titanium-containing species are well-known additives that accelerate the kinetics of H2 desorption and activate the reversibility of complex metal hydrides. However, in spite of more than 2000 papers in the literature concerning the mechanism of Ti catalysis in NaAlH4 alone, there is no consensus regarding the mechanism by which this occurs. Although it is likely that Ti has multiple roles in this process, recent work by our group suggests that an important one is the activation of H2 in the rehydriding process. This presentation will describe our experimental and theoretical investigation of Ti-doped NaAlH4 using a suite of laboratory- and synchrotron-based tools that probe all relevant length scales in this process, including the top atomic monolayer, the near-surface region (0 – 5 nm), and the bulk. In addition, we employed nanoporous Metal-Organic Frameworks (MOFs) as highly ordered, defect-free platforms for probing single-atom catalytic sites for H2 activation of Ti-doped NaAlH4 nanoparticles. Together, our results show that Ti is uniformly distributed throughout the bulk prior to H2 desorption, but no Ti is detected on the surface. This implies that Ti-containing surface species have no direct role in the desorption of H2 NaAlH4. However, surface Ti reappears once H2 desorption is complete, suggesting that it has a role in accelerating H2 readsorption. Importantly, these results have implications for complex hydrides other than NaAlH4. In particular, the MOF results provide for the first time a direct line-of-sight connection between nanoscale features and the bulk, indicating that Mg ions with incomplete coordination spheres can activate H2 and initiate the initial stages of H2 reaction with MgB2.

Unravelling the mechanism of complex metal hydride reversibility by catalytic metal sites

255th National Meeting (2018)

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