RESUMO
Phosphoric acid (PA) confined in a commercial mesoporous silica (CARIACT G) with porous size in the range of 3 to 10 nm was studied in relation to its coordination with the silanol groups on the silica surface as a function of temperature, up to 180 °C, using 31P and 29Si MAS NMR spectroscopy. As the temperature increases, the coordination of Si and P in the mesopores depends on the pore size, that is, on the area/volume ratio of the silica matrix. In the mesoporous silica with the higher pore size (10 nm), a considerable fraction of PA is nonbonded to the silanol groups on the surface, and it seems to be responsible for its higher conductivity at temperatures above 120 °C as compared to the samples with a smaller pore size. The electrical conductivity of the functionalized mesoporous silica was higher than that reported for other silico-phosphoric composites synthesized by sol-gel methods using soft templates, which require high-temperature calcination and high-cost reagents and are close to that of the best PA-doped polybenzimidazole membranes used in high-temperature proton exchange membrane fuel cells (HT-PEMFCs). The rate of PA release from the mesoporous silica matrix when the system is exposed to water has been measured, and it was found to be strongly dependent on the pore size. The low cost and simplicity of the PA-functionalized mesoporous silica preparation method makes this material a promising candidate to be used as an electrolyte in HT-PEMFCs.
RESUMO
Apparent molar volumes, Vphi,2, of aqueous NaCl, NaOH, NaOD, HCl, and DCl in water and heavy water were determined at T = 523 and 573 K and p = 14 MPa with a high-temperature platinum vibrating-tube densimeter in the aquamolality range 0.25 = maq = 2.5 mol. (55.509 mol solvent)-1. The experimental results have been represented with an extended Debye-Hückel equation to describe the concentration dependence of Vphi,2 and to derive standard partial molar volumes of these electrolytes in light and heavy water, V degrees 2,H and V degrees 2,D, respectively. For NaCl and NaOH, the D2O isotope effect at infinite dilution, [V degrees 2,H - V degrees 2,D], increases from 0.2 and 0.8 cm3 mol-1 to 4.5 and 7.1 cm3 mol-1, respectively, when the temperature is increased from 523 to 573 K. For HCl and DCl, the effect is smaller and the sign is reversed, [V degrees 2,H - V degrees 2,D] = -0.7 cm3 mol-1 at 523 K and -1.4 cm3 mol-1 at 573 K. When the effect of ion association is included, the deuterium isotope effect for HCl becomes positive, [V degrees 2,H - V degrees 2,D] approximately 17 cm3 mol-1 at 573 K, consistent with NaCl and NaOH. Two models are proposed to describe the solvent isotope effect on the infinite dilution limit, one based on the Born equation and the other on the dimensionless Krichevskii parameter. The experimental values of V degrees 2,D also have been used to calculate the first reported values for the pressure dependence of the ionization constant of D2O at temperatures higher than 313 K.