Mesoporous Skeleton Research Articles

Concentration polarization and nonequilibrium electroosmotic slip in hierarchical monolithic structures

This article illustrates the appearance and electrohydrodynamic consequences of concentration polarization (CP) in hierarchically structured monolithic fixed beds used as stationary phases in CEC and related electrical-field-assisted separation techniques. Subject of the investigation are silica-based monoliths in capillary format with a bimodal pore size distribution. Ion-permselectivity in the intraskeleton pore space together with diffusive and electrokinetic transport induces depleted and enriched CP zones at the anodic and cathodic interfaces, respectively, of the cation-selective mesoporous skeleton. The extent of electrical-field-induced CP is shown to be governed by the fluid phase ionic strength, which tunes the ion-permselectivity of the mesoporous monolith skeleton via local electrical double layer overlap, and by the applied electrical field strength, which determines local transport. The analysis of quantitative confocal laser scanning microscopy data, resolving CP on the local skeleton scale, indicates that at sufficiently high field strength a transition from intraskeleton to interskeleton boundary-layer-dominated transport of charged species occurs. This transition is correlated to the onset of macroscopically measured, nonlinear EOF velocities, whose occurrence is explained in the framework of a nonequilibrium electroosmotic slip. It is shown that the onset of nonlinear electrokinetics in the system can be tuned by properties of the BGE, particularly buffer pH, which modulates the pH-dependent surface charge density and consequently the ion-permselective skeleton's charge selectivity. Finally, the CP dynamics of monolithic and particulate fixed beds are compared, and the observed differences are related to the specific morphologies of the two hierarchical fixed bed structures.

Hierarchically Porous Oxides, Hybrids and Polymers via Sol-gel Accompanied by Phase Separation

ABSTRACTIn various crosslinking systems containing metal oxides, organo-siloxane polymers and pure hydrocarbons, monolithic materials with hierarchical well-defined macropores and controlled mesopores have been synthesized. Synthetic progress in alkoxy-derived macroporous silica lead to the preparation of long-range ordered mesoporous skeletons in well-defined macroporous framework. Alkylene-bridged silicon alkoxides can also be prepared into similarly hierarchical porous structures with broadened variations in framework morphology. Macro-mesoporous alkoxy-derived pure titania and zirconia have been prepared using hydrochloric acid – mediated processes. Compared with those prepared from colloidal dispersions, alkoxy-derived macroporous titania exhibited much higher mechanical strength. Titania monolith is a promising candidate as a separation medium to discriminate phosphorylated compounds in a liquid chromatography mode. Pure alumina macroporous monolith has been first synthesized from aluminum salt using propylene glycol as a proton scavenger to thrust the solution pH from acidic into neutral conditions. Alumina-based complex oxides such as garnets and spinels can also be prepared in pure phases. Polymerization and phase separation in organic crosslinker system was also controlled to obtain well-defined co-continuous macro-frameworks instead of those composed of aggregated particles. These examples demonstrate the versatility of using phase-separation in gelling systems to obtain well-defined macroporous structures.

Synthesis and Mechanism Study of Mesoporous SnO<SUB>2</SUB>/SiO<SUB>2</SUB> Composites

Mesoporous SnO2/SiO2 composite particles (Si/Sn < or = 0.25) sustainable to calcination up to 600 degrees C have been fabricated using a stepwise sol-gel technique on nonionic surfactant template (tetradecylamine, TDA). The newly designed preparation method involved the pre-formation of SnO2 sol solution from SnCl4. Subsequently, SnO2 nanocrystals were covered by the silicate species (from the hydrolysis of tetraethylorthosilicate, TEOS) in a pH controlled colloidal solution. Upon mixing with the surfactant solution, mesophase composite was obtained. After the removal of templates at various temperatures (400 to 600 degrees C), worm-like mesoporous SnO2/SiO2 with large specific surface area and pore volume as high as 362 m2/g and 0.33 cc/g were obtained, respectively. High thermal stability is mainly due to the effective inhibition of SnO2 crystal growth (mean crystallite size <30 A) by the amorphous SiO2 species at the grain boundaries. Formation of mesoporous silicate skeleton such as M41S family material was prevented. The obtained materials maintain the relatively narrow pore size distribution typically in the range of 30 to 70 A. Relations between material properties and key synthesis parameters (i.e. TDA/Si/Sn molar ratio and calcination temperature) were investigated by TGA, wide/small-angle X-ray scattering, (HR)TEM, BET, and FTIR techniques. Mechanisms on the mesostructure formation and crystal growth inhibition were also proposed with detailed discussion.

Fabrication of a Bimodal Ferromagnetic Nanosystem in an Etched Silicon Structure and its Magnetic and Magneto-Optic Characterization

AbstractNi-nanowires are fabricated in a two-step electrochemical process. In the first step a porous silicon template with oriented pores perpendicular to the surface is produced. The electrochemical parameters for this etching procedure, like HF-concentration, current-density, etching-time and bath-temperature have to be chosen in a very small regime to obtain the favored structure in a good quality. This mesoporous silicon skeleton with highly oriented pores and homogeneous spatial distribution is filled in a further electrochemical step by a ferromagnetic metal, like Ni. This selforganized Ni-nanowire array is characterized by Auger-spectroscopy to evidence the loading of the pores over the full length. SEM and BSE are used to reveal the orientation of the pores and the homogeneous Ni-filling. By Fourier Transform image processing a predominant quadratic self-organized grouping of the (100) grown pores is identified. An additional investigation method is EDXS to show the element distribution in the sample. Furthermore magnetization measurements are used to generate a model for the Ni-loading in the channels. Not only wires but also granules in the size up to 200 nm are present. IR-spectroscopy investigations are used to compare the bare silicon wafer, the porous silicon (PS) sample and the PS with incorporated Ni at zero and finite magnetic fields.

Control of mesoporous structure of carbons synthesised using a mesostructured silica as template

Mesoporous skeleton carbons with controllable pore size distributions (PSDs) were synthesised by modifying the extent of which the carbon precursor infiltrated the pores of the mesostructured silica, which was used as a template. When the silica porosity was fully infiltrated, carbons with a unimodal narrow PSD (∼3 nm) were obtained. This porosity was an inverse replica of the silica framework. If the silica pores were not completely filled, carbons with bimodal PSDs were synthesised. The carbons exhibited two kinds of mesopores: pores derived from the silica walls (∼3 nm) and larger pores, with sizes of up to 18 nm, formed from the coalescence of unfilled silica pores when the silica walls were dissolved. These results show that the synthesis of mesoporous carbons can be accurately controlled to obtain carbon materials with different PSDs using only one type of silica template.