| It has attracted intensive attention in the field of renewable energy materials to catalyticallyenhance the hydrogen storage properties of light metal complex hydrides. The extensiveexperimental investigations call for detailed theoretical studies on the catalytic mechanism. Weuse the first-principles method and the improved nudged elastic band technique in studying thecatalytic mechanism of the early transition metal doped aluminum. We have discussed in detailthe minimum energy path for the H2dissociation and the produced H atom diffusion onaluminum surface doped with the transition metals. Combining the analyses on structural andelectronic properties, we conclude the catalytic mechanism for improving the hydrogen storageperformance. Our theoretical studies can facilitate new materials design by providing predictionsand guidance to the future experimental studies. The main results of this thesis can besummarized below:1．The Ti catalyst doped in the vicinity of surface step could split H2with0.37eVactivation energy and the dissociated hydrogen atom could diffuse by overcoming0.45eV barrier.Our analysis on the energies of the Ti doping and the surface etching phenomenon suggest thatTi could remain as recycling active catalyst during the aluminum hydrogenation. The electronicproperties of the intermediate state could account for the enhanced splitting properties. Thesestudies on the role of surface step could contribute to understanding the catalytic mechanism oftransition metal catalyzed hydrogen uptake in aluminum.2. With the purpose to present a systematic investigation on the catalytic mechanism, wehave also carefully analyzed the H2splitting processes catalyzed by the early transition metals(Sc, Ti, V, Cr and Mn), which are substitutionally doped in the top layer and the subsurface of anideal flat Al surface and at the edge site of surface step. The transition metal doped in the topsurface can provide3d orbitals to develop the well-known donation and back-donation Kubasinteraction with σ-type H-H bond, which could significantly reduce the activation energy of H2splitting. The catalyst doped in the subsurface could not develop Kubas interaction with H2because of the screening from the charge distributed on the top surface, whose role could beunderstood by combining the structural deformation induced by the doping, the attraction of thedopant to the electrons distributed around Al atoms in the top layer, and the d orbital attendancein the reaction. For the sake of recycling perspectives of the doped catalyst, the diffusion of thedissociated H atoms has also been studied. Thus, the Sc and Ti doping at the lower edge site ofthe stepped surface are better for their low activation energies. The atomic size andelectronegativity could be used to aid new catalyst design for enhancing the hydrogen recharge properties of alanate hydrides. Accordingly, the near-surface alloying of Sc, Ti, Zr, Nb, Hf, andTa in the aluminum surface could be expected to have superior catalytic properties.3. Though the thermodynamics stability of Al(100) is lower than Al(111), it is usuallyinvestigated in experimental studies. Also, the local structure of Ti-doped Al(100) has analogousstructural characteristics of the TiAl-terminated TiAl3(001) surface (the TiAl3is found inexperiment to be catalytically active for aluminum hydrogenation). So, we have also studied indetail the enhanced hydrogen interaction with transition-metal (Sc, Ti, V, Cr and Mn) dopedAl(100) stepped surface. Judged from the calculated total energies, the early transition metalsprefer to dope at the lower edge sites of surface step. The Sc, Ti, and V donate electrons whilethe Cr and Mn gain electrons. The low energy costs for activating both the H2splitting and the Hatomic diffusion show improved catalytic performances. In the transition states, hydrogen wouldbond to both transition metal and Al atoms for H2splitting on Sc-and Ti-doped surfaces, while itwould only develop rather weak interaction with the metals in the other studied materials. Thecharge transfer results in0.8e charge gain and0.4increase in bond length of H2, facilitatingH2dissociation on Sc-and Ti-doped surfaces. However, in the other studied materials, thepresence of hydrogen only induces charge re-distribution, resulting in a rather small charge gainof H2(<0.1e). The insights into catalytic mechanism, on the basis of our detailed analysis onstructural and electronic properties, could benefit the experimental investigations in pursuingmoderate hydrogen storage medium.4. Based on the detailed first-principles studies, we have investigated the catalyticperformances of near-surface alloy of Ti in Al(100) along with the analysis on catalyticmechanism. The doping of single Ti atom, Ti-Ti pair in next-nearest neighbor configuration (0,2),and the local Ti domain [0,2] in the next-nearest neighbor arrangement have better catalyticperformances. In top surface, they need to cost~0.6eV energy to complete a whole catalyticcycle. The main obstacle comes from the strong Ti-H bond hindering the dissociated H atoms todiffuse. They, when doped in subsurface, can also enhance hydrogen interaction with aluminumsurface to catalyze H2splitting. The calculated activation energies are0.80,0.68, and0.48eV forsingle Ti atom,(0,2) pair, and [0,2] domain, respectively. Due to less of the strong Ti-H bond, thedissociated H atoms could diffuse quickly with small activation energies. The charge transferbetween metal and dihydrogen plays crucial role. In the top surface, the Ti could provide3doribitals to develop the Kubas type σ back-bonding interaction with H2, resulting in the chargegain and bond elongation of dihydrogen. However, the Ti doped in subsurface could not bedirectly approached by H2molecule. The slight structural expansion in doping domain, the lowerelectronegativity of Ti, and the fact of more valence electrons of Ti could cooperatively facilitate the charge transfer from the above Al atoms to H2molecule, accounting for the enhancedsplitting properties. |
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