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44103-G10
First Principles Study of Light Metal Complex Hydrides as Potential Hydrogen Storage Materials

Qingfeng Ge, Southern Illinois University (Carbondale)

Our original proposal is to develop a multiscale approach to model desorption and adsorption of hydrogen in complex metal hydrides. The proposed research direction is now funded by a DOE hydrogen fuel initiative award. We therefore explored new research directions with the support of the PRF grant. The grant was used to support students who worked on two projects: (1) Pt supported anatase TiO₂ surfaces and its catalytic property toward methanol adsorption and dissociation; (2) Adsorption and protonation of CO₂ on partially hydroxylated γ-Al₂O₃ surfaces. The major findings from these two projects were summarized in the following:

 (1)   Effect of Surface Oxygen Vacancy on Pt Cluster Adsorption and Growth on the Defective Anatase TiO₂(101) Surface The effect of surface oxygen vacancies on the adsorption and clustering of the Pt adatoms over the defective anatase TiO₂(101) surface has been studied using density functional theory slab calculations. The surface oxygen vacancy site was found to be the most active site for a single Pt adatom with an adsorption energy of 4.87 eV. As such, this site may act as a nucleation center for particle growth on the defective anatase TiO₂(101) surface. The pathways for forming the two stable Pt₂ adsorption configurations from a single Pt adatom in the oxygen vacancy site and another from the neighboring bridging 2cO sites involve the diffusion of the second Pt adatom out of the bridging 2cO site. The transition states for the dimer formation were located at Pt adatom diffusing out of the bridging 2cO site. Furthermore, we found that the bond-breaking step determines the barrier height for the Pt adatom diffusion, which is ~ 1 eV. Among five stable Pt₃ adsorption structures at the oxygen vacancy site on the defective anatase TiO₂(101), the most stable structure is triangular with all three Pt atoms interacting directly with the surface atoms. The possible pathways for Pt adatom diffusion to form the most stable Pt₃ configuration were analyzed.  The barriers for Pt adatom diffusion in Pt₃ formation were predicted to be similar to that of the Pt₂ formation due to similar transition state structures. The barrier height indicates that clustering will be kinetically hindered at low temperature.

 (2)   Adsorption and Protonation of CO₂ on Partially Hydroxylated γ-Al₂O₃ Surfaces: A Density Functional Theory Study: Adsorption and protonation of CO₂ on the (110) and (100) surfaces of γ-Al₂O₃ have been studied using density functional theory slab calculations. On the dry (110) and (100) surfaces, the O-Al bridge sites were found to be energetically favorable for CO₂ adsorption. The adsorbed CO₂ was bound in a bidentate configuration across the O-Al bridge sites, forming  a carbonate species. The strongest binding with an adsorption energy of 0.80 eV occurs at the O_3c-Al_5c bridge site of the (100) surface. Dissociation of water across the O-Al bridge sites resulted in the partially hydroxylated surfaces and the dissociation is energetically favorable on both surfaces. Water dissociation on the (110) surface has a barrier of 0.42 eV, but the same process on the (100) surface has no barrier with respect to the isolated water molecule. On the partially hydroxylated γ-Al₂O₃ surfaces, a bicarbonate species was formed by protonating the carbonate species with the proton from neighboring hydroxyl groups. The energy difference between the bicarbonate species and the co-adsorbed bidentate carbonate species and hydroxyls is only 0.04 eV on the (110) surface, but the difference reaches 0.97 eV on the (100) surface. The activation barrier for forming the bicarbonate species on the (100) surface, 0.42 eV, is also lower than that on the (110) surface (0.53 eV).

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