Primary pore size

The primary pore size distributions of the cases A and B in Table 3.5 are experimentally determined by mercury injection porosimetry (MIP) (Fermeglia and Pricl, 2009) on the instrument of PoreMaster GT 60. The MIP enables the measurements of both the pressure required to force mercury into the pores of CLs and the intruded Hg volume at each pressure. The employed equipment operates from 13 kPa to a pressure of 410 MPa, equivalent to the pores with the diameters, d, ranging from 100 [xm to 0.0036 xm. On the other hand, the 3 V method provides a tool to theoretically calculate the agglomerate volume depending on the probe radius. In analogy to the... 

Figure 3.11. Comparison of the primary pore size distribution (PSD) in the CLs obtained by CG-MD step formation (colored lines) and experiments (grey lines).

The limits of pore size corresponding to each process will, of course, depend both on the pore geometry and the size of the adsorbate molecule. For slit-shaped pores the primary process will be expected to be limited to widths below la, and the secondary to widths between 2a and 5ff. For more complicated shapes such as interstices between small spheres, the equivalent diameter will be somewhat higher, because of the more effective overlap of adsorption fields from neighbouring parts of the pore walls. The tertiary process—the reversible capillary condensation—will not be able to occur at all in slits if the walls are exactly parallel in other pores, this condensation will take place in the region between 5hysteresis loop and in a pore system containing a variety of pore shapes, reversible capillary condensation occurs in such pores as have a suitable shape alongside the irreversible condensation in the main body of pores. 

Permeability is the conductance of the medium and has direct relevance to Darcy s law. Permeability is related to the pore size distribution, since the distribution of the sizes of entrances, exits and lengths of the pore walls constitutes the primary resistance to flow. This parameter reflects the conductance of a given pore structure. 

The retention efficiency of membranes is dependent on particle size and concentration, pore size and length, porosity, and flow rate. Large particles that are smaller than the pore size have sufficient inertial mass to be captured by inertial impaction. In liquids the same mechanisms are at work. Increased velocity, however, diminishes the effects of inertial impaction and diffusion. With interception being the primary retention mechanism, conditions are more favorable for fractionating particles in liquid suspension. 

Zinc chloride was used as a catalyst in the Friedel Crafts benzylation of benzenes in the presence of polar solvents, such as primary alcohols, ketones, and water.639 Friedel-Crafts catalysis has also been carried out using a supported zinc chloride reagent. Mesoporous silicas with zinc chloride incorporated have been synthesized with a high level of available catalyst. Variation in reaction conditions and relation of catalytic activity to pore size and volume were studied.640 Other supported catalytic systems include a zinc bromide catalyst that is fast, efficient, selective, and reusable in the /wa-bromination of aromatic substrates.641... 

Modern SMR plants (Figure 2.5b) incorporate a PSA unit for purifying hydrogen from C02, CO, and CH4 impurities (moisture is preliminarily removed from the process gas). The PSA unit consists of multiple (parallel) adsorption beds, most commonly filled with molecular sieves of suitable pore size it operates at the pressure of about 20 atm. The PSA off-gas is composed of (mol%) C02—55, H2—27, CH4—14, CO—3, N2—0.4, and some water vapor [11] and is burned as a fuel in the primary reformer furnace. Generally, SMR plants with PSA need only a HT-WGS stage, which may somewhat simplify the process. 

Microstructures of CLs vary depending on applicable solvenf, particle sizes of primary carbon powders, ionomer cluster size, temperafure, wetting properties of carbon materials, and composition of the CL ink. These factors determine the complex interactions between Pt/carbon particles, ionomer molecules, and solvent molecules, which control the catalyst layer formation process. The choice of a dispersion medium determines whefher fhe ionomer is to be found in solubilized, colloidal, or precipitated forms. This influences fhe microsfrucfure and fhe pore size disfribution of the CL. i It is vital to understand the conditions under which the ionomer is able to penetrate into primary pores inside agglomerates. Another challenge is to characterize the structure of the ionomer phase in the secondary void spaces between agglomerates and obtain the effective proton conductivity of the layer. 

Eikerling i has demonsfrafed capabilities of this approach. A simple representation of fhe pore space by a bimodal 5-distribution reveals the role of fhe CCL as a "wafershed" in PEFCs. For this case, a full analytical solution could be found. Af fhe same fime, it still captures essential physical processes and major structural features such as typical pore sizes (r, r ), and distinct contributions to porosity from primary and secondary pores (X,Xm). 

The smaller and more uniform the primary particles, and the weaker the agglomerates in the sol are, the smaller the pore size and the sharper its distribution in the membrane will be. The thickness of the layer L, increases linearly with the square root of the dipping time. The process is quantitatively described by Leenaars and Burggraaf (1985). The rate of membrane deposition increases with the slip concentration or with decreasing pore size of the support as shown below. This has been experimentally confirmed for alumina and titania (Leenaars and Burggraaf 198S, Uhlhom et al. 1989). 

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