Nano-Structures and Interactions of Alkali Metals within Silica Gel

Abstract

Liquid alkali metals and their alloys can be absorbed into the 15 nm diameter pores of nanostructured silica gel (SG) at loadings up to 40 wt %. Characterization of the resultant materials was done by measuring the amount of hydrogen produced by addition of alcohols and/ or water, by differential scanning calorimetry, and by static 23 Na NMR spectroscopy. While sodium must be heated above 100 °C to form Na in SG, absorption of NaÀK alloys at ambient temperatures yields nanoscale metallic clusters in the pores (Stage 0). In both cases, some of the encapsulated metal may dissolve in the silica and ionize to produce highly mobile M þ ions with reversible electron transfer to the silica. This presumption is consistent with literature observations that sodium is soluble in silica films at ambient temperatures at levels up to 35 mol percent and is presumed to form highly mobile Na þ -O À species (Lagarde, P.; Flank, A.-M.; Mazzara, C.; Jupille, J. Surf. Sci. 2001, 482À485, 376À380). Heating M-SG materials leads to reduction of SiO 2 to form material that contains localized M þ -O À species and SiÀSi bonds in increased proportion as the temperature and time of heating are increased. Above 350 °C, elemental silicon, alkali metal silicides, and alkali metal silicates are formed. The dissolution and reaction of potassium from encapsulated NaÀK alloys occurs preferentially over that of sodium. Irreversible reaction even occurs at room temperature over a period of months. The temperature dependence of solid state 23 Na NMR spectra indicates the presence of rapid exchange between Na 0 in the metal clusters and Na þ at the walls (and perhaps within the silica).

Continuous melting and thermal-history-dependent freezing in the confined Na-K eutectic alloy

“…The composite thus obtained remains stable when kept in the nitrogen atmosphere or in oil contrary to the case of pure Na metal when it is introduced at elevated temperature [25]. Fig.1 shows variations of the Knight shift during two different cooling-warming cycles from room temperature down to 150 K and up to 380 K. The Knight shift varies weakly when the confined alloy remains unfrozen.…”

Impact of opal nanoconfinement on electronic properties of sodium particles: NMR studies

“…The detection of NMR signals from confined solid metals mentioned above was not achieved because of strong broadening of lines due to the anisotropy of the Knight shift. The NMR signals for confined solid metallic particles were found for the Na-K eutectic alloy embedded into a porous glass [8] and for sodium within silica gel [19].…”

“…For instance, the linewidth at half high at room temperature was only about 8 ppm. Much broader 23 Na NMR lines were observed in [19] for metallic sodium introduced into pores of silica gel. The narrow lines for sodium embedded into the opal matrix made it possible for the first time to observe clear the decrease in the Knight shift caused by nanoconfinement and anomalous splitting of the 23 Na NMR line at higher temperature.…”

Mechanism of CO2 capture in nanostructured sodium amide encapsulated in porous silica

“…[16] The commercially-available starting material (sodium encapsulated in a porous silica gel, denoted as Na-SG, 37 wt. % Na), [16][17][18] is produced by agitation of a mixture of molten sodium and silica gel. As synthesized, the Na-SG is a coarse black powder with nano-sized sodium particles dispersed within the pores of a silica matrix, the pore diameters being up to 150 nm [17].…”