Bi-metallic Catalyst for Hydrogen Sorption of Magnesium Hydride

This thesis is focused on the design of a series of bi-metallic catalyst for enhancing the hydrogen sorption kinetics of magnesium hydride. We have utilized thin films deposited by magnetron sputtering technique as model systems to study various alloys and compositions. It has been known that the transition metals additions can catalyze the sorption reaction. We hypothesized that the addition of bi-metallic catalysts has superior effect over the single metal additions. In order to obtain baselines, we have examined the effect of single transition elements on the kinetics of the transformation. In our first attempt, we showed that the sorption behavior of the ternary Mg-Fe-Ti alloy is significantly improved compared to its binary alloy counterparts. Using this ternary system, we were able to perform absorption/desorption tests up to 100 cycles at a low temperature of 200°C. We further investigated the validity of our hypothesis by performing the similar cycling measurements on two more ternary systems, being Mg-Fe-V and Mg-Cr-Ti. We showed that both systems exhibit remarkably enhanced sorption characteristics over the binary alloys.;In our last attempt, we examined the effect of Cr-V bi-metallic catalyst on the hydrogen sorption behavior of MgH2. The catalyst was so potent that we were able to absorb the activated samples at room temperature and a low hydrogen pressure of 2 bar. We also performed cycling tests on this systems at 300°C with the desorption pressure of 1 bar. In order to explore the microstructural origins of such performance, we utilized transmission electron microscopy (TEM) and X-ray diffraction techniques. Through a systematic grain size measurement, we found that the MgH2 in ternary systems is more resistant to grain coarsening compared to binary alloys. The cryo-stage TEM analysis of the partially absorbed sample shed light on the transformation mechanism of Mg to MgH2. It revealed the absence of a core-shell structure which is mostly assumed as the absorption mechanism for MgH 2. The cryo-stage TEM results also showed the presence of twins in the hydride microstructure which is most likely due to the fast rate of the absorption transformation. We also performed a detailed kinetics analysis in the framework of Johnson-Mehl-Avrami (JMA) model. We found that the activation energy value has a strong dependency on the driving force for the reaction. Using the value of activation energy and the calculated Avrami exponent, the possible rate limiting step for the absorption and desorption reactions was proposed. By combining the microstructural observations and the kinetics analysis we proposed a mechanism for the Mg to MgH2 transformation.