Effect of titanium on adsorption of hydrogen in β-cyclodextrin
Graphical abstract
For titanium modifying β-cyclodextrin, hydrogen molecules are absorbed inside cavity of it, the frontier orbitals of β-cyclodextrin are evidently delocalized, and the adsorption energy between β-cyclodextrin and hydrogen are enlarged.
Introduction
Hydrogen has been viewed as a highly appealing energy carrier for renewable energy because of its abundance and environmental friendliness. To achieve economic feasibility, hydrogen storage materials with high gravimetric and volumetric densities must be developed. Furthermore, hydrogen recycling should be performed reversibly under near ambient conditions. Numerous experimental and theoretical works have been performed to investigate the hydrogen adsorption in various materials [1], [2], [3], [4], such as, carbon nanomaterial, metal organic material (MOF) and Mg-based alloy. However, pristine carbon nanostructures are chemically too inert to be useful for practical hydrogen storage [5], [6], [7], [8], MOF materials are synthesized hardly, result in application limited, and alloy is expensive, so it is urgent to find others hydrogen storage material with application potential. Cyclodextrin is a kind of cyclic oligosaccharides, which is consisted of plurality glucopyranose residues through chair conformation of α-1, 4 glycosidic bonds, which exist in nature widely. Lots of reports think that cyclodextrin as host molecule can adsorption gas and organic liquid, even some medicine molecules [9], [10], [11]. In 2009 year, Forgan et al. find that cyclodextrin can adsorption hydrogen at 77 K [12]. Zhu et al. firstly investigate hydrogen storage properties by density functional theory in 2010 year [13]. However, it is still little reports about cyclodextrin hydrogen storage. In order to obtain the higher properties of cyclodextrin hydrogen storage, in present work, the storage hydrogen property of Ti-cyclodextrin is investigated by density functional theory combining with Monte Carlo method. Geometric structures, orbitals populations and hydrogen storage properties of the β-cyclodextrin doped Ti are calculated. The mass densities of hydrogen storage are predicted in 77 K. It is found that the effects of titanium on electronic structure and hydrogen storage properties of the β-cyclodextrin.
Section snippets
Computational method
In present paper, abinitio calculations at the density functional level of theory (DFT) were performed to calculate the structures of β-cyclodextrin and the adsorption energy of H2 molecules. DFT calculations are carried out using a linear combination of atomic orbital density-functional theory (DFT) methods implemented in Gaussian09 [14]. The generalized gradient approximation (GGA) in the Becke Exchange Plus Lee–Yang–Parr (B3LYP) form are chosen for the spin restricted/unrestricted DFT
Result and discussion
Based on DFT/B3LYP/6-31G(d) method, the β-cyclodextrin is optimized, and the adsorption energy between β-cyclodextrin and one hydrogen inside and outside cavity are calculated, respectively. Results show that hydrogen inside cavity of β-cyclodextrin, the adsorption energy only is 0.091 eV. But the adsorption energy is 0.97 eV for the model that hydrogen located on outside cavity. That is, inside cavity of pristine β-cyclodextrin is unfavorable for adsorption hydrogen. It is same results as Zhu et
Conclusion
From the first principle calculation results, metal Ti enhances the hydrogen storage performance of β-cyclodextrin, that is, Ti atom causes the larger delocalization of frontier orbitals and induces the change of adsorption sites and adsorption energy. In addition, the adsorption isotherms of β-cyclodextrin and Ti modifying derivatives are simulated by Monte Carlo (GCMC) calculations. Simulated results verify that, the maximum adsorption capacity of Ti modifying derivatives is increased to 8.31
Acknowledgments
This work was supported by the Specialized Research Fund of Xian yang Normal University (No. 14XSYK013), the 56th China Postdoctoral Science Foundation (No. 2014M560758), and the Natural Science Basic Research Plan in Shaanxi Province (No. 2013JM2013).
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