Quaternary Ammonium Borohydride Adsorption in Mesoporous Silicate MCM-48
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Quaternary Ammonium Borohydride Adsorption in Mesoporous Silicate MCM-48 Michael J. Wolverton1,2, Luke L. Daemen1, Monika A. Hartl1 1. Los Alamos Neutron Science Center, Los Alamos National Laboratory, Los Alamos, NM 87545, USA 2. Dept. of Applied Science, University of Arkansas at Little Rock, Little Rock, AR 72204, USA ABSTRACT Inorganic borohydrides have a high gravimetric hydrogen density but release H2 only under energetically unfavorable conditions. Surface chemistry may help in lowering thermodynamic barriers, but inclusion of inorganic borohydrides in porous silica materials has proved hitherto difficult or impossible. We show that borohydrides with a large organic cation are readily adsorbed inside mesoporous silicates, particularly after surface treatment. Thermal analysis reveals that the decomposition thermodynamics of tetraalkylammonium borohydrides are substantially affected by inclusion in MCM-48. Inelastic neutron scattering (INS) data show that the compounds adsorb on the silica surface. Evidence of pore loading is supplemented by DSC/TGA, XRD, FTIR, and BET isotherm measurements. Mass spectrometry shows significant hydrogen release at lower temperature from adsorbed borohydrides in comparison with the bulk borohydrides. INS data from partially decomposed samples indicates that the decomposition of the cation and anion is likely simultaneous. These data confirm the formation of Si-H bonds on the silica surface upon decomposition of adsorbed tetramethylammonium borohydride. INTRODUCTION Large amounts of hydrogen can be released through the thermal decomposition of hydride complexes [1,2]. As the projected hydrogen density requirements for hydrogen storage media have become more demanding, borohydrides have gained popularity as a potential storage medium [3]. The thermodynamic parameters obtained via ab-initio studies predict that many borohydrides are theoretically capable of thermolysis at very reasonable temperatures [4,5]. In practice, much higher temperatures are required, and the rate of reaction is often extremely sluggish except in the presence of a catalyst [1,2,6]. This implies that the limiting factors are associated with kinetics and the activation energy barrier. Silica has been shown to encourage the decomposition of LiBH4 causing the reaction to occur at a lower temperature, with a faster rate of gas release [1]. Fang et. al. have shown that that dispersing LiBH4 with a carbonaceous mesoporous network encourages thermal decomposition [7]. The degree of inclusion within the pores was somewhat ambiguous in the Fang et. al. study. Conclusive evidence of borohydride encapsulation in a mesoporous silicate has never been reported in the literature. MCM-48 was chosen for this study because of the regularity of its pore size, high specific surface area, and cubic pore structure. Tetraalkylammonium borohydrides (R4NBH4) were selected for MCM-48 loading because common alkali metal borohydrides such as NaBH4 and LiBH4 do not properly enter the pores from solution. This is discussed in
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