Hopping transport on a fractal: ac conductivity of porous silicon

We have measured the frequency dependence of the conductivity and the dielectric constant of various samples of porous Si in the regime 1 Hz-100 kHz at different temperatures. The conductivity data exhibit a strong frequency dependence. When normalized to the dc conductivity, our data obey a universal scaling law, with a well-defined crossover, in which the real part of the conductivity sigma' changes from an sqrt(omega) dependence to being proportional to omega. We explain this in terms of activated hopping in a fractal network. The low-frequency regime is governed by the fractal properties of porous Si, whereas the high-frequency dispersion comes from a broad distribution of activation energies. Calculations using the effective-medium approximation for activated hopping on a percolating lattice give fair agreement with the data.

The steady performance of a porous and compliant aerostatic thrust bearing

Recent developments in aerostatic thrust bearings have included: (a) the porous aerostatic thrust bearing containing a porous pad and (b) the inherently compensated compliant surface aerostatic thrust bearing containing a thin elastomer layer. Both these developments have been reported to improve the bearing load capacity compared to conventional aerostatic thrust bearings with rigid surfaces. This development is carried one stage further in a porous and compliant aerostatic thrust bearing incorporating both a porous pad and an opposing compliant surface. The thin elastomer layer forming the compliant surface is bonded to a rigid backing and is of a soft rubber like material. Such a bearing is studied experimentally and theoretically under steady state operating conditions. A mathematical model is presented to predict the bearing performance. In this model is a simplified solution to the elasticity equations for deflections of the compliant surface. Account is also taken of deflections in the porous pad due to the pressure difference across its thickness. The lubrication equations for flow in the porous pad and bearing clearance are solved by numerical finite difference methods. An iteration procedure is used to couple deflections of the compliant surface and porous pad with solutions to the lubrication equations. Comparisons between experimental results and theoretically predicted bearing performance are in good agreement. However these results show that the porous and compliant aerostatic thrust bearing performance is lower than that of a porous aerostatic thrust bearing with a rigid surface in place of the compliant surface. This discovery is accounted to the recess formed in the bearing clearance by deflections of the compliant surface and its effect on flow through the porous pad.

On the active site in H3PW12O40/SiO2 catalysts for fine chemical synthesis

The surface environment and structural evolution of silica supported phosphotungstic acid (H3PW12O40) catalysts have been investigated as a function of acid loading. H3PW12O40 clusters are deposited intact upon the silica surface, adopting a Stranksi-Krastanov growth mode forming a two-dimensional adlayer which saturates at 45wt% acid. Intimate contact with the silica support perturbs the local chemical environment of three tungstate centres, which become inequivalent with those in the remaining cluster, suggesting an adsorption mode involving three terminal W==O groups. Above the monolayer, H3PW12O40 clusters form three-dimensional crystallites with physico-chemical properties indistinguishable from those in the bulk heteropoly acid. These H3PW12O40/SiO2 materials are efficient for the solventless isomerisation of α-pinene under mild reaction conditions. Activity scales directly with the number of accessible perturbed tungstate sites at the silica interface; these are the active species.

Osmotic flow in porous membranes:effects of electric charge

The electrostatic model for osmotic flow across a porous membrane in our previous study (Akinaga et al. 2008)" was extended to include the streaming potential, for solutes and pores of like charge and fixed surface charge densities. The magnitude of the streaming potential was determined to satisfy zero current condition along the pore axis. It was found that the streaming potential affects the velocity profiles of the pressure driven flow as well as the osmotic flow through the pore, and decreases their flow rates, particularly in the case of large Debye length relative to the pore radius, whereas it has little effect on the reflection coefficients of spherical solutes through cylindrical pores.

Three-dimensional fluid pressure mapping in porous media using magnetic resonance imaging with gas-filled liposomes

This paper presents and demonstrates a method for using magnetic resonance imaging to measure local pressure of a fluid saturating a porous medium. The method is tested both in a static system of packed silica gel and in saturated sintered glass cylinders experiencing fluid flow. The fluid used contains 3% gas in the form of 3-μm average diameter gas filled 1,2-distearoyl-sn-glycero-3-phosphocholine (C18:0, MW: 790.16) liposomes suspended in 5% glycerol and 0.5% Methyl cellulose with water. Preliminary studies at 2.35 T demonstrate relative magnetic resonance signal changes of 20% per bar in bulk fluid for an echo time TE=40 ms, and 6-10% in consolidated porous media for TE=10 ms, over the range 0.8-1.8 bar for a spatial resolution of 0.1 mm3 and a temporal resolution of 30 s. The stability of this solution with relation to applied pressure and methods for improving sensitivity are discussed. © 2007 Elsevier Inc. All rights reserved.

Mesoporous silicas as versatile supports to tune the palladium-catalyzed selective aerobic oxidation of allylic alcohols

Surfactant templating offers a simple route to synthesize high-surface area silicas with ordered, tunable mesopore architectures. The use of these materials as versatile catalyst supports for palladium nanoparticles has been explored in the aerobic selective oxidation (selox) of allylic alcohols under mild conditions. Families of Pd/mesoporous silicas, synthesized through incipient wetness impregnation of SBA-15, SBA-16, and KIT-6, have been characterized by using nitrogen porosimetry, CO chemisorption, diffuse reflection infrared Fourier transform spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and high-resolution TEM and benchmarked in liquid phase allylic alcohol selox against a Pd/amorphous SiO2 standard. The transition from amorphous to two-dimensional parallel and three-dimensional interpenetrating porous silica networks conferred significant selox rate enhancements associated with higher surface densities of active palladium oxide sites. Dissolved oxygen was essential for insitu stabilization of palladium oxide, and thus maintenance of high activity on-stream, whereas selectivity to the desired aldehyde selox product over competing hydrogenolysis pathways was directed by using palladium metal. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Size-controlled TiO2 nanoparticles on porous hosts for enhanced photocatalytic hydrogen production

Nanocystalline TiO2 particles were successfully synthesized on porous hosts (SBA-15 and ZSM-15) via a sol-gel impregnation method. Resulting nanocomposites were characterized by XRD, TEM, BET surface analysis, Raman and UV-vis diffuse reflectance spectroscopy, and their photocatalytic activity for H2 production evaluated. XRD evidences the formation of anatase nanoparticles over both ZSM-5 and SBA-15 porous supports, with TEM highlighting a strong particle size dependence on titania precursor concentration. Photocatalytic activities of TiO2/ZSM-5 and TiO2/SBA-15 composites were significantly enhanced compared to pure TiO2, owing to the smaller TiO2 particle size and higher surface area of the former. TiO2 loadings over the porous supports and concomitant photocatalytic hydrogen production were optimized with respect to light absorption, available surface reaction sites and particle size. 10%TiO2/ZSM-5 and 20%TiO2/SBA-15 proved the most active photocatalysts, exhibiting extraordinary hydrogen evolution rates of 10,000 and 8800μmolgTiO2 -1 h-1 under full arc, associated with high external quantum efficiencies of 12.6% and 5.4% respectively under 365nm irradiation.