ORGANICALLY FUNCTIONALISED MESOPOROUS SILICA FOR HYDROGEN STORAGE APPLICATION.

The H2 adsorption properties of prepared functionalized porous silica with different compositions have been studied. Mesoporous silica particles were synthesized by using the sol-gel method, with tetradecyltrimethyl ammonium bromide (C14) or dodecyl trimethyl ammonium bromide (C12) as structure directing agents from tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) precursors, by varying the molar ratio of the silica precursors. The porosity and microstructure of the materials in function of the MTES/TEOS ratio were evaluated by using nitrogen adsorption, and the transmission electron microscopy method. The textural characterization was performed in order to connect their properties to possible applications as hydrogen adsorptive materials. The ability of a material to store gas molecular species may not only be governed by on the specific surface area but also by the porosity and to the possibility to form more than one monolayer of adsorbed gas molecules. These are the reasons why the evaluation of the adsorbed hydrogen molecules per nm2 could be useful to indicate the efficacy of the samples to create more than one monolayer [1]. Improving the interaction between adsorbent and adsorbate is the breakthrough for synthesizing an optimum hydrogen storage material [2]. Thus, studies on physisorption materials aim to increase surface area, optimize the pore diameter, and increase the affinity toward incoming hydrogen molecules through the introduction of atoms and/or molecular groups into the porous material. A previous study had been demonstrated that the amorphous structure of the adsorbent and the use of shorter surfactant molecules in the synthesis increased both the storage capacity and the strength of the interaction between the surface and the adsorbate [1]. In our previous study the data analysis shows that the methyl functionalized sample, according to its structural parameters is the best material either from the point of view of maximum adsorption capacity showed for H2 [3]. The higher adsorption capacity of the samples synthesized with no MTES and with C12 directing agent can be debited to a different and stronger sorbent-adsorbent interaction beyond that slightly differences in the textural properties. In the C14 directing agent series of samples, the sample with no MTES and the samples with low amount of MTES, appears the most performing at 77K immediately followed by the samples synthesized with increase amount of MTES. In each series of samples, C12 and respectively C14, the samples show different grade of interaction with the incoming H2 molecules (K) and different values of homogeneity (t). The distance between two pore centers of parallel pore channels was estimated from TEM at: 3.3 nm for all samples from C12 series. Therefore, the inter-pore distanced is almost the same with the addition of small amounts of MTES, in the present synthesis conditions. [ABSTRACT FROM AUTHOR]