Spatial configurations and X-ray absorption of Ti catalyzing on NaAlH4 surfaces: Car-Parrinello molecular dynamics and density functional theory study

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A theoretical study on the spatial configurations of the catalytic dehydrogenation of the pre- and post- Ti-doped NaAlH4(001)2×2×1 supercell surface crystals was performed by using the Car-Parrinello molecular dynamics (CPMD) method at 333 K (60 ℃). It was be found that two of the Al—H bond lengths increased from approximately 1.64 to 1.74 and 1.93  respectively in the AlH4 groups of the Ti-doped alloy. Compared with this change, the four Al—H bond lengths almost kept invariant in the AlH4 group of un-doped alloy, which means that it was easier to dehydrogenate for the Ti-doped alloy than un-doped alloy. There was no bonding tendency between atom Ti and Al observed, which is probably because the temperature in the simulation process is not high enough. Based on the obtained surface crystal configuration, the Ti K-edge x ray absorption near-edge structure (XANES) spectra of the TiAl3, TiH2 crystals and Na8Ti8Al16H64(001) surface crystal have been calculated by using the full-potential linearized augmented plane wave method (FPLAPW). It was also found that the atom Ti may not only exist in the mixture of TiAl3 and TiH2 but also probably partially substitute for the Na atoms in NaAlH4 surface crystal, by comparing the experimental XANES and edge x ray absorption fine structure (EXAFS) spectra.