Of Ti-mo Alloy Absorbing And Releasing Hydrogen Isotope Effect Studies

For developing a new candidate of hydrogen separation material to replace the currentlyused but expensive material, Palladium, we focus on the properties of Ti-Mo solid solutionalloys, such as adsorption/desorption kinetics, and hydrogen isotope effects.The TiMo_x solid solution alloys (x=0.03,0.13,0.25,0.50,1.00, atomic ratio) are synthesizedusing the electromagnetic levitation melting technique, the composition of the alloys isidentified by ICP-AES, and the phase structure of the alloys is identified by XRD and thethermal stability of the alloy hydrides is investigated using TG-DSC thermal analyticaltechnique. Results reveal that the composition and phase structure of the synthesized alloysmeet their desired values. In 0.02MPa H₂ atmosphere, the saturated hydrides of Ti-Mo alloysat room temperature have a structure of TiH_1.971 (fcc) mixed withβ-Ti (bcc), thecomposition of bcc phase increases as the Mo content increases. When the Mo content is 0.03,the hydride mainly consists of TiH₂; and it contains Ti-Mo hydride when the Mo content isover 0.13. The dehydrogenation temperature drops as the Mo content rises.Their absorption/desorption kinetics, hydrogen capacity are quantified, and the reactionscan be described by the first-order rate law. It is seen that the presence of Mo dramaticallylowers the absorption/desorption activation energy, however, the increased concentration ofMo in the samples has an adverse effect on their absorption kinetics and lowers theirhydrogen storage capacity, which means the energy level of Ti-Mo-H ternary system rises asthe Mo content increases, namely, the increased Mo content lowers the dehydrogenationtemperature of the hydride. The hydrogen storage properties also vary with reactiontemperature. The hydrogen absorption activation energy, E_a, is 11.9kJ.mol^-1, 8.0 kJ.mol^-1, -6.1kJ.mol^-1, -1.4 kJ.mol^-1, 3.3 kJ.mol^-1, respectively, E_a decreases with increasing Mo content andreaches minimum when Mo content is about 0.50, and then increases with more increasingMo content. The activation energy for hydrogen desorption of TiMo_x hydrides(x=0.03,0.12,0.25,0.50,1.00, atomic ratio), E_d, is 37.9 kJ.mol^-1, 30.4kJ.mol^-1, 32.6 kJ.mol^-1,12.5 kJ.mol^-1 and 13.2 kJ.mol^-1, respectively, E_d decreases as the Mo content increases, but thetendency is not obvious when the Mo content is below 0.50. For the dispersed and activatedsamples, where the rates are extremely rapid, consideration of possible slow steps forhydrogen sorption must be given to mass transport control.The thermodynamic isotope effect of Ti-Mo alloys is investigated by H-D separation factorα(H-D) at 20℃and 200℃, which is calculated by its definition, and the initial and equilibrium H-D composition in gas phase is examined by MAT 253 mass spectroscopy. Theresults show that, at room temperature,α(H-D) varies from 0.86 to 1.27, and Ti-Mo alloysshow positive isotope effect when the Mo content is over 0.03. It increases as Mo contentincreases, and keeps unvaried when the Mo content is between 0.50 and 1.00. When the Mocontent is 0.03,α(H-D)≈1.0, where the hydride contains Titanium hydrides with negativeeffect and Ti-Mo alloy hydrides with positive effect. It is estimated that D atom prefers to takeposition of 3Ti-1Mo tetrahedron interstices, whereas H atom prefers to take 4Ti position. At200℃,α(H-D)＜1.0, varying from 0.85 to 0.90, the alloys show negative effect, this adverseeffect has something to do with the preference of position taken by hydrogen isotopes.The deuterium absorption/desorption properties are examined to investigate the kineticisotope effect of the alloys by comparing the protium absorption/desorption kinetics. It is seenthat the protide is more stable than the deuteride when the range of temperature is 250～650℃, and the Mo content is 0.03～1.00. At 450～650℃, it is easier for the alloys to absorb H₂,but harder to absorb H₂ when the temperature range is 250～450℃. Maybe D atom is moresuitable than H atom to take the position of the alloy interstices, as the atomic radium of H isslightly smaller than that of D, and easier for H atom to escape from the interstices. Theobvious drop of Ti-Mo deuteride thermal desorption energy, E_d, happens when the Mo contentis over 0.13; but for protide, Eadecreases by a large value when the Mo content is over 0.50.The difference may also be influenced by the preference of position taken by hydrogenisotopes.