Oxygen pore structure

Zeolites are crystalline alumina-silicates having a regular pore structure. Their basic building blocks are silica and alumina tetrahedra. Each tetrahedron consists of silicon or aluminum atoms at the center of the tetrahedron with oxygen atoms at the comers. Because silicon and aluminum are in a +4 and +3 oxidation state, respectively, a net charge of -1 must be balanced by a cation to maintain electrical neutrality. 

The microstructure of a catalyst layer is mainly determined by its composition and the fabrication method. Many attempts have been made to optimize pore size, pore distribution, and pore structure for better mass transport. Liu and Wang [141] found that a CL structure with a higher porosity near the GDL was beneficial for O2 transport and water removal. A CL with a stepwise porosity distribution, a higher porosity near the GDL, and a lower porosity near the membrane could perform better than one with a uniform porosity distribution. This pore structure led to better O2 distribution in the GL and extended the reaction zone toward the GDL side. The position of macropores also played an important role in proton conduction and oxygen transport within the CL, due to favorable proton and oxygen concentration conduction profiles. 

Carbon has been used in several forms. Activated charcoal found limited usage because of significant batch-to-batch variations. Kaiser (16) developed a carbonaceous material with pore structure similar to zeolites and referred to it as carbon molecular sieve. It is useful only for gases and very short chain compounds. It is unique in that it can be used for the analysis of oxygen, nitrogen, and carbon dioxide as shown in Figure 3.7. 

The following review is concerned with the synthetic and structural chemistry of molecular alumo-siloxanes, which combine in a molecular entity the elements aluminum and silicon connected by oxygen. They may be regarded as molecular counterparts of alumo-silicates, which have attracted considerable attention owing to their solid-state cage structures (see for example zeolites).1 3 Numerous applications have been found for these solid-state materials for instance the holes and pores can be used in different separation techniques.4,5 Recently the channel and pore structures of zeolites and other porous materials have been used as templates for nano-structured materials and for catalytical purposes.6 9... 

The dependence of U2 app on the oxygen content of extracted and unextracted coal samples (on a daf basis) is presented in Figure 4. The same Figure includes the data of Sanada and Honda (8) and Kirov et al (9) for comparative reasons. The unextracted coal samples do, for the most part, show more "apparent swelling, i.e. smaller values of U2 app than the extracted ones. This behavior may be the result of collapse of the pore structure due to extraction. This speculation is presently under investigation. 

Adsorption. The adsorption properties for selected AlP0,-based molecular sieves are summarized in Table VIII. The adsorption data are arranged for each structure-type in order of increasing adsorbate size. The large pore structure-types (5, 36, 46) with pores defined by 12-rings of oxygen readily adsorb neopentane (kinetic dia. 0.62 nm>. The 5 and 46 structure-types have been... 

The small pore (8-ring) structures all adsorb oxygen but only the chabazite-types and levynite adsorb n-butane or n-hexane and the rate of adsorption is very strongly dependent on particle size. Larger adsorbates are completely excluded. The very small pore structures (6-ring) adsorb only water and exclude oxygen. 

These results reveal that at low temperature the rate of diffusion of Ar and N2 into the intracrystalline pore structure is extremely slow. The increase in the adsorption with temperature is not thermodynamically controlled but is instead dependent on the molecules gaining enough kinetic energy to allow their passage through some of the 4A apertures. This process is probably assisted by enhanced vibrational amplitude of the oxygen ring structure. 

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