No abstract

To examine the mechanism of the thermal hysteresis between freezing and melting for water confined in a network of the ink-bottle pores of ordered silica, we measured the freezing and melting behavior of pore water in seven kinds of KIT-5 samples with various neck and cavity sizes by means of differential scanning calorimetry and X-ray diffractometry. The melting temperature of pore ice depended on the cavity size, whereas the freezing behavior of pore water depended on the neck size. When the neck size is smaller than ∼4 nm in diameter, the pore water in the spherical cavities freezes via homogeneous nucleation on cooling. On the other hand, the freezing behavior of the pore water changed drastically when the diameter of the necks was increased beyond this critical size: the freezing temperature was increased remarkably and the freezing became very broad. Such a freezing behavior may be accounted for by a gradual percolation of the ice front that grows by traveling through the connected pore network.

The mass transfer kinetics of toluene and polystyrenes (of which the M(w) varies from 162 to 1.85 x 10(6) g mol(-1)) through columns filled with silica porous spheres were studied by inverse size exclusion chromatography. The mass transfer parameters were measured by modeling the band broadening of the chromatograms. The experimental height equivalent to a theoretical plate (HETP) data were analyzed using the general rate model in order to determine the effective diffusion coefficient in porous particles as a function of molecular size. The bulk molecular diffusion coefficients were experimentally determined by dynamic light scattering (DLS) and Taylor dispersion analysis (TDA). The topological tortuosity of the porous particles was determined by electrical measurements. The effective molecular diffusion coefficient through porous particles was modeled taking into account exclusion, friction, and at last tortuosity effects. A phenomenological law is proposed to model the evolution of the tortuosity experienced by a molecule in a porous particle as a function of its size. It gives a good prediction of the evolution of effective diffusion coefficient with the molecule/pore size ratio.

The discovery of oxygen and carbon monoxide tolerant [NiFe] hydrogenases was the first necessary step toward the definition of a novel generation of hydrogen fed biofuel cells. The next important milestone is now to identify and overcome bottlenecks limiting the current densities, hence the power densities. In the present work we report for the first time a comprehensive study of herringbone carbon nanofiber mesoporous films as platforms for enhanced biooxidation of hydrogen. The 3D network allows mediatorless hydrogen oxidation by the membrane-bound hydrogenase from the hyperthermophilic bacterium Aquifex aeolicus. We investigate the key physico-chemical parameters that enhance the catalytic efficiency, including surface chemistry and hierarchical porosity of the biohybrid film. We also emphasize that the catalytic current is limited by mass transport inside the mesoporous carbon nanofiber film. Provided hydrogen is supplied inside the carbon film, the combination of the hierarchical porosity of the carbon nanofiber film with the hydrophobicity of the treated carbon material results in very high efficiency of the bioelectrode. By optimization of the whole procedure, current densities as high as 4.5 mA cm(-2) are reached with a turnover frequency of 48 s(-1). This current density is almost 100 times higher than when hydrogenase is simply adsorbed at a bare graphite electrode, and more than 5 times higher than the average of the previous reported current densities at carbon nanotube modified electrodes, suggesting that carbon nanofibers can be efficiently used in future sustainable H2/O2 biofuel cells.