Homogeneous pore structure

In recent years, extensive studies have been undertaken by Kaneko and his coworkers of the properties of activated carbon fibres (ACFs) produced from cellulose, polyacrylonitrile (PAN) and pitch. X-ray diffraction and electron microscopy revealed that the PAN-based and pitch-based fibres had a more homogeneous pore structure than that of the cellulose-based material, although the latter had the largest surface area and pore volume (Kakei et al., 1990). 

Changes in the particle structure have a strong effect on combustion behavior, influencing the particle temperature, mass transfer and pore diffusion rates, and consequently the rate-control regime of the process ( 5 ) Tlie changes in size and density of particles that have a homogeneous pore structure and small pore sizes (relative to particle size) are related to fractional burn-off, u, by... 

The internal structure of the catalyst particle is often of a complex labyrinth-like nature, with interconnected pores of a multiplicity of shapes and sizes, In some cases, the pore size may be less than the mean free path of the molecules, and both molecular and Knudsen diffusion may occur simultaneously. Furthermore, the average length of the diffusion path will be extended as a result of the tortuousity of the channels. In view of the difficulty of precisely defining the pore structure, the particle is assumed to be pseudo-homogeneous in composition, and the diffusion process is characterised by an effective diffusivity D, (equation 10.8). 

Other key features in the analysis of pore structure are the length scales associated with the various micro- (nano)-scale obstacles and pores, the possible larger-scale variations in structure, and the averaging domain over which information is needed [6,341,436], The hterature refers to analysis of homogeneous and heterogeneous porous media, where homogeneous refers to media with no variation in physical properties (e.g., porosity, diffu-... 

The pore size and distribution in the porous particles play essential roles in NPS synthesis. For example, only hollow capsules are obtained when MS spheres with only small mesopores (<3 nm) are used as the templates [69]. This suggests that the PE has difficulty infiltrating mesopores in this size range, and is primarily restricted to the surface of the spheres. The density and homogeneity of the pores in the sacrificial particles is also important to prepare intact NPSs. In a separate study, employing CaC03 microparticles with radial channel-like pore structures (surface area 8.8 m2 g 1) as sacrificial templates resulted in PE microcapsules that collapse when dried, which is in stark contrast to the free-standing NPSs described above [64]. 

Figure 2.1. Schematic representation of main types of pore structures and membranes. A and B homogeneous unsupported straight pores C supported asymmetric, interconnected pores D a photograph of a membrane of the type (c). (SCT-support+y-AljOj top layer UT Twente)...

Assuming that Ti(IV) is distributed statistically in all tetrahedral positions, it can be easily seen that even for crystallite sizes of 0,2 m the great majority of T1(IV) is located inside the pore structure. Assuming that every Ti(IV) is a catalytic centre with equal activity, diffusion limitations for molecules of different sizes should be observed. This is in fact the case. It has been shown [27] that the rate of oxidation of primary alcohols decreases regularly as the chain length increases, while for iso-butyl alcohol a sudden drop in the rate is observed. Also the reactivity order of olefins on TS-1 is different from the order observed with homogeneous electrophilic catalysts, while as already indicated very bulky molecules are unreactive when TS-1 is used as the catalyst. All these facts can only be interpreted as due to diffusion limitations of the larger molecules, which means that the catalytic sites are located inside the pore structure of the solid. 

Examination of ultrathin tactions of coal in tko aloctron microscope hat revealed that one type of vitrinite (vitrinite A) it homogeneous, while the remaining vitrinite (vitrinite B) it a two-component material, the components having similar properties to vitrinite A and exinite, respectively. The material similar to exinite occurs in sheets no more than 1000 A. thick and is responsible for the lower reflectance and higher volatile matter yield of vitrinite B. Exinite, micrinite, and semifusinite have been identified in ultrathin sections. By using a technique of impregnation with a lead salt the ultrafine pore structure of vitrinite has been made visible. 

R is the molar ratio of decane/TMB No homogeneous pore size distribution was obtained a Structure of these materials was not yet investigated... 

For nonane even though a series of trials were further made, the sharp increase due to the capillary condensation is not observed. The material is not a structured compound and no homogeneous pore size distribution is obtained (Figure 7e) Some complementary studies will be done to explain this phenomena. This compound will not be taken into account in following analyses. 

Crossflow technology is increasing, as it proves practical. Micioliltration membranes are of an isotropic and homogeneous morphology, i.e., the pore structure is consistent throughout. There is some movement, however, toward ihe use of "skinned" anisotropic membranes. Microliltration membranes are available in a wide variety ol polymers, including some that arc quite chemically inert. They also tire available as tubular, hollow fiber, or capillary fiber elements. 

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