| With the fast development of the global economy,the deficiency of fossil fuel resources and the deterioration of ecological environment present a tremendous challenge to long-term development of human society.The research and development of new green energy and energy-saving materials are of great significance.On the one hand,hydrogen has attracted great attention all over the world as a kind of clean fuel sources with great development potential.Especially,alanate has recently become one of the most promising hydrogen storage solid materials due to the advantages of high hydrogen storage capacity and low cost.Based on the review of the research and development of alanates,the hydrogenation process of sodium alanate(NaAlH4) was selected as the object of the study.The TiF3-doped and Ti(OBun)4-doped Na3AlH6 complex hydrides were prepared by hydrogenation of the ball-milled(NaH/Al+TiF3) and(NaH/Al+Ti(OBun)4) mixture.Combined with the first-principles calculations of the adsorption and dissociation of hydrogen on the Al(001) surface and Ti-doped Al(001) surface,the local structure around Ti atoms before and after hydrogenation was systematically investigated by means of XAFS,XRD,SEM and DSC-TG analysis to understand the catalytic process and deduce catalytic mechanism.On the other hand, comprehensive utilization of novel energy-saving materials and technology is a subject of great interest in the domain of energy efficiency in buildings industry.For example, optical switchable materials have a wide application prospect as the smart window, which integrates the characters of energy-saving,decoration and insulation.Based on the review,the hydrogen-induced optical and electric changes in Mg-based alloys thin films,which were treated as a new type of optical switchable materials,were studied in detail.The adsorption,dissociation and diffusion of hydrogen on the pure Mg(0001) surface and Ti-,Ni-and Pd-doped Mg(0001) surface were calculated by the first-Principles method.And then,Ni-or Ti-coated pure Mg thin film and LaMg2Ni alloy thin film were prepared by thermal evaporation and electron beam evaporation. Upon hydrogen loading/unloading,the optical and electric properties were studied by XRD,SEM,AFM,transmission and reflection spectra.The main results achieved in this paper are given as below: 1.Catalytic process and mechanism of hydrogenation for Ti-containing catalyst doped NaH/Al powderThe chemical valence and local structure of Ti atoms before and after hydrogenation of TiF3-and Ti(OBun)4-doped NaH/Al powder were investigated by XANES and EXAFS.The results showed that the chemical valence of Ti atoms was reduced,and the reduced Ti atoms combined with Al atoms to form Ti-Al clusters in both TiF3-and Ti(OBun)4-doped samples.The formed Ti-Al clusters were with 1-2 shells from a Ti atom,as the low coordination shells in bulk TiAl3.And the coordination numbers of Ti-Al clusters increased as the rising of hydrogenation temperature.Therefore,it can be speculated that Ti-Al clusters with low coordination shell of TiAl3 were the catalyzing agent for hydrogenation.The phase structure of TiF3-and Ti(OBun)4-doped NaH/Al powder before and after hydrogenation was studied by XRD and the diffraction patterns were analyzed by the Rietveld refinement method to obtain the phase composition.The results showed that:ⅰ) before hydrogenation,the samples consisted of NaH and Al phases.Moreover,the weight ratio of NaH to Al was about 41:59 in the ball-milled TiF3-doped NaH/Al powder,and 46:54 in the ball-milled Ti(OBun)4-doped one;ⅱ) after hydrogenation,the Na3AlH6 phase formed in TiF3-doped samples was more than the Na3AlH6 phase in Ti(OBun)4-doped samples.For TiF3-doped samples,the Na3AlH6 phase was about 44 wt.%at 100℃,increasing to 56 wt.%at 120℃and 55 wt.%at 140℃.However, these values were appreciably larger than 29 wt.%,38 wt.%and 44 wt.%for Ti(OBun)4-doped samples at each corresponding temperature;ⅲ) the NaH phase, existed only at 100℃(about 5 wt.%) and vanished at 120 and 140℃in TiF3-doped samples,but it remained in Ti(OBun)4-doped samples at all three temperatures(15-23 wt.%).These indicated that hydrogenation conditions should be adjusted when different Ti-containing catalysts are used.In order to understand the catalytic character of various Ti-containing catalysts,the thermal stability of the samples hydrogenated at 100-140℃was investigated by DSC-TG.The results of DSC showed that the peaks of phase transition and decomposition for Na3AlH6 were observed in all samples after hydrogenation;the peak of Na3AlH6 decomposition shifted to high temperature in Ti(OBun)4-doped samples, due to particles reunion arised from the hydrocarbon byproducts generated by the Ti(OBun)4 decomposition.The results of TG showed that the weight loss percentage of the TiF3-doped NaH/Al samples hydrogenated at 100,120 and 140℃are 1.0 wt.%,1.3 wt.%and 1.1 wt.%,and that of Ti(OBun)4-doped ones are 2.7 wt.%,3.3 wt.%and 3.5 wt.%,respectively.It is obvious that the weight loss in TiF3-doped NaH/Al samples is smaller than that in Ti(OBun)4-doped ones at corresponding hydrogenation temperature, because of the large weight loss when the Ti(OBun)4 decomposed.First-principles pseudopotential plane wave calculations and the Nudged Elastic Band method based on density functional theory(DFT) were used to study the dissociation of molecular hydrogen onto a Al(001) surface and Ti-doped Al(001) surface.The results showed that the hydrogen molecules were able to dissociate onto a Al(001) surface and Ti-doped Al(001) surface with activation barrier-around 1.06 eV and 0.2 eV,respectively.According to the calculated result and experimental data,the catalytic mechanism for hydrogenation of Ti-containing catalyst doped NaH/Al can be speculated that regardless of the kind of Ti-containing catalyst,Ti was deoxidized and combined with Al to form Ti-Al clusters with low coordination shells.After that the molecular hydrogen dissociated into H atom on the surface of Ti-Al clusters and formed Ti-H-Al pairs.Upon diffusion and disturbance,Ti-H-Al pairs reacted with NaH to generate Na3AlH6 or NaAlH4.Therefore,it is reasonable to deduce that the catalytic activity of a Ti-containing catalyst may depend on the quantity and the structure of Ti-Al pairs suggesting that hydrogenation conditions should be adjusted according to the type of Ti-containing catalysts are used.For a practical application,an ideal Ti-containing catalyst should be able to form more Ti-Al pairs with low coordination shells at a lower temperature after it is doped to the NaH/Al mixture.2.Hydrogen-induced optical and electric changes in Mg-based alloy thin filmsFirst-principles pseudopotential plane wave calculations and the Nudged Elastic Band method based on density functional theory(DFT) were used to study the dissociation of molecular hydrogen onto a Mg(0001) surface and Pd-,Ti-and Ni-doped Mg(0001) surface.The results showed that:(a) the activation barrier for the dissociative chemisorption of molecular hydrogen onto a clean magnesium surface was 1.05 eV,and the surface effect which severely affected the hydrogen diffusion is observed;(b) the hydrogen molecules were able to dissociate onto Pd-,Ti-and Ni-doped Mg(0001) surface with a small activation barrier-about 0.33 eV,0.12 eV and 0.078 eV,respectively.It is obvious that the doping of catalytic atom greatly reduced the activation barrier for the dissociation of molecular hydrogen.Based on the calculated results,the hydrogen-induced optical and electric changes in Ti-and Ni-coated Mg thin film were studied.The results showed that compared with Ti,Ni was more conducive to preparation of films and the hydrogen loading/unloading due to its good oxidation resistance.Moreover,a 375-nm-thick LaMg2Ni alloy thin film was prepared by electron beam evaporation using LaNi,Mg and Ni targets,and followed by vacuum-annealing at 350℃.Upon hydrogen loading/unloading,the reversible conversion from a metallic,reflecting state to a semiconducting, color-neutral transparent one was observed in the as-prepared film.After hydrogenation,the average transmission(400-900 nm) increased significantly from 0.03%to 46.4%,while the resistivity also increased from 0.2 mΩ·cm to 10Ω·cm.The contrast ratios in both the transmission(400-900 nm) and the resistivity between the reflecting state and the transparent one are over four orders of magnitude.The surface and the cross-section of the alloy films before and after hydrogenation were observed by scanning electron microscope and atomic force microscope,showing that the lower reflection as observed in the dehydrogenated film is attributed to the rough surface originated from the first hydrogenation.
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