Micro-sized pores

Figure 6. Schematic illustration of the array-unit with a single micro-sized-pore (top view). R and r are the radii of PS spheres in the template and the micro-pores at the film surface, respectively.

Baby food decomposition (as model biomass for protein and carbohydrate containing biomass) in near critical water (i.e., 375°C and 24 MPa) was studied with K COj, ZSM-5 and Ni by Sinag et al. (2010). The SEM images of char produced without Ni showed macropores, whereas biochars produced with Ni on silica and ZSM-5 gave both macro and micro-sized pores. However, K COj resulted in only microporous chars. In presence of Ni on SiO catalyst, the amount of hydroxyImethyl furfural increased. The amounts of acetic acid and acetaldehyde were considerably higher under acidic conditions, whereas under basic condition (in the presence of K COj), formic acid and glycolic acid concentrations increased in the aqueous phase. 

From Fig. 6.5a-d and Tables 5.15, 5.16, 5.17 and 5.18 it can be noticed that, SSA increases with the increase in step of recycling of the AAF. bicidentally, contrary to all other AAFs, there is significant increase in the SSA, in the residue, 1.5-PT-36 (refer Fig. 6.5c). This can be attributed to an intensive etching of the ash particles, leading to its dissolution, which helps the development of micro-sized pores in AAFs. In addition, treatment steps of 36 h exhibits trends, contradictory to other trends. This can be attributed to creation of extra fine particles which may be amorphous in nature and nano-sized zeolite crystals. 

The micro-, meso- and macro-sized pores of a zeolite impact the catalyhc and separation properhes. Based on lUPAC terminology micropores are defined by pore sizes smaller than 2nm, mesopores are between 2 and 50 nm and pores greater than 50 nm are referred to as macropores. 

The filling of these molecular-sized pores, which is associated with the isotherm distortion at very low pip0, has been called primary micropore filling . Wider micro-pores are filled by a secondary , or co-operative, process over a range of higher p/p0 (Sing, 1979). These processes are discussed in some detail in subsequent chapters. 

Ion track formation in dielectric materials is becoming more and more interesting, since there are promising applications in sub micro and nanotechnology [1-3], Some papers have already reported experiments concerning the formation of nanometer-sized pores in silicon dioxide by means of MeV ion beams, and subsequent selective etching of ion tracks by liquid or vapor hydrofluoric acid [4-6],... 

The equations and plots presented in the foregoing sections largely pertain to the diffusion of a single component followed by reaction. There are several other situations of industrial importance on which considerable information is available. They include biomolecular reactions in which the diffusion-reaction problem must be extended to two molecular species, reactions in the liquid phase, reactions in zeolites, reactions in immobilized catalysts, and extension to complex reactions (see Aris, 1975 Doraiswamy, 2001). Several factors influence the effectiveness factor, such as pore shape and constriction, particle size distribution, micro-macro pore structure, flow regime (bulk or Knudsen), transverse diffusion, gross external surface area of catalyst (as distinct from the total pore area), and volume change upon reaction. Table 11.8 lists the major effects of all these situations and factors. 

A very interesting method to analyze total carbon dioxide and ammonium in pore water was introduced by Hall and Aller (1992). In this method, a sample carrier stream and a gas receiver stream flow past one another, separated only by a gas-permeable PTFE (Teflon ) membrane. For determining the total COj, the sample carrier stream consists of 10-30 mM HCl. To this stream, the sample in a volume of about 20 [tl is added via a HPLC injection valve. The carbon dioxide traverses the PFTE membrane and enters the gas receiver stream which, in this case, consists of 10 mM NaOH. The CO taken up by the gas receiver stream causes an electrical conductivity change that can be determined exactly in a micro-sized continous flow cell. 

The specific surface area (Sbet) was evaluated by 2-parameters linear BET plot in the range p/p° 0.01-0.2. The total pore volume (Vj) was evaluated by Gurvitsch rule. Mean pore size (doFi) and pore size distributions were calculated using DFT method, based on molecular statistical approach. It was applied over the complete range of the isotherm and was not restricted to a confined range of relative pressure or pore sizes. Pore size distribution was calculated by fitting the theoretical set of adsorption isotherms, evaluated for different pore sizes, to the experimental results. DFT model was particularly effective for evaluation in the borderline range between micro and mesopores [14]. 

A classification of pores based on pore sizes was proposed by the International Union for Pure and Applied Chemistry (lUPAC). As illustrated in Fig. 1, pores are usually classified into three classes macropores (>50 nm), mesopores (2-50 nm) and micropores (<2 nm) [1], Micropores can be further divided into supermicropores (with a size of 0.7-2 nm) and ultramicropores (<0.7 nm in size), Since nanotechnology attracted the attention of many scientists recently, the pore structure has been required to be controlled closely, a part of which will be explained in Section 5. Wlten scientists wanted to express that they are controlling pores in the nanometer scale, some of them preferred to call the smallest pores nano-sized pores, instead of micro/mesopores. 

Aerogels are solid materials that consist of a highly porous network of micro-sized and meso-sized pores. The pores of an aerogel can frequently account for over 90% of the volume when the density of the aerogel is about 0.05 gcm. In general, aerogels are prepared by a supercritical drying technique, i.e., a sol-gel process, to remove the solvent from a gel. The process is conducted in a way that no solvent evaporation can occur. Thus no contraction occurs. 

From Table 9.1, another important conclusion could be drawn regarding the thickness of the studied fibrils. The microfibrils have diameters around 1 pm and that of nanofibrils is between 50 and 150 nm. The specific surface is also different for the two types of fibrils. BET analysis of the scaffolds show that the nanofibril-lar material (PET-Nano) is characterized by the highest surface area, 18.8 m g (Table 9.1). The adsorption-desorption behavior of this sample corresponds to pores of meso- and micro-size (>10nm). In contrast to the PET-Nano sample, the material comprising microfibrils possesses five times less surface area, namely, of 4m g (Table 9.1, sample PET-Micro 2), and the size of the cavities formed is in the macro-range (>50 nm). The microfibrillar PGA scaffold, which possesses larger surface area (Table 9.1) and similar pore size distribution, shows comparable behavior. 

Sintering materials (metals, polymers, ceramics and their mixtures) solvent free easy and rapid precise control on macro- and micro-porosity pore interconnectivity laser spot and particle size Powder material No biomolecules or cells can be incorporated due to high temperature for particle sintering... 

Miscellaneous effects A number of factors can influence the effectiveness factor, some of which are particle size distribution in a mixture of particles/pellets, change in volume upon reaction, pore shape and constriction (such as ink-bottle-type pores), radial and length dispersion of pores, micro-macro pore structure, flow regime (such as bulk or Knudsen), surface diffusion, nonuniform environment around a pellet, dilution of catalyst bed or pellet, distribution of catalyst... 

Nguyen and Do s method [7] to evaluate micro-/meso-pore size distribution was applied in this work. According to the method the Langmuir adsorption isotherm is used to describe the behavior of the adsorbent molecules in the pores. Different isotherms are considered for different pore sizes. Hence, the adsorption isotherm parameters are the function of the pore radius. Thus, the fractional coverage, 9, becomes [5]. 

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