Role of Pore Structure

As discussed above, oxygen groups play a crucial role in determining the metal loading and dispersion of carbon-supported metal catalysts. Also, the pore structure is claimed to have an influence on the metal dispersion. We give a short overview of the influence of the pore structure on metal loading and distribution, although the field is rather descriptive. 

Fuente et al. [27] compared two activated carbons of Norit with similar oxygen content and p/f but different pore volumes (i.e., 0.522 mL/g vs. 0.393 mL/g). Loading these supports with either H2PtCl6 or [Pt(NH3)4]Cl2 resulted in 1 wt% Pt on the high-pore-volume support, with a dispersion of 35% irrespective of the precursor. The low-pore-volume material resulted in metal loading of about 

7 wt% with a dispersion of 20% therefore, the authors conclude that high porosity is beneflcial for dispersion. 

Samant et al. [28] prepared a highly mesoporous carbon by condensing 

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At the heart of the force response behavior of the polishing pad is the nature of the pad itself. First the basic mechanical properties of solid polymers will be examined, after which we will review the role of pore structures and finally we briefly discuss the manufacturing issues. 

The Role of Pore Structure The Origin of Shape Selectivity... 

Figure 29 demonstrates the role of pore structure on distribution of retained polymer. The exact shape of the curves in unknown since one has to know the retained polymer minimum at three locations to determine the constants in Equation (1). It should be noted that because of the C t,constant on the right-hand side of Equation (1), a... 

Role of pore structure in salt crystallisation in xmsaturated porous stone, J. Ciyst. Growth 2004, 260 (3-4) 532-544. 

There has been considerable speculation concerning the role of carbide in the iron base catalyst. The carbide was originally depicted as an intermediate in the reaction (7), but more recent work indicates the contrary (3). It now appears more probable that the lattice between carbide crystals or between groupings of carbide and relatively fewer oxide or even free iron crystals offers the form of pore structure required for both high activity and selectivity. 

Although both the laboratory and industrial scale materials science of catalysts requires an integrated approach as already mentioned above, it is customary to classify the characterization methods by their objects and experimental tools used. I will use the object classification and direct the introductory comments to analysis, primarily elemental and molecular surface analysis, determination of geometric structure, approaches toward the determination of electronic structure, characterization by chemisorption and reaction studies, determination of pore structure, morphology, and texture, and, finally, the role of theory in interpreting the often complex characterization data as well as predicting reaction paths. 

A series of CoMo/Alumina-Aluminum Phosphate catalysts with various pore diameters was prepared. These catalysts have a narrow pore size distribution and, therefore, are suitable for studying the effect of pore structure on the deactivation of reaction. Hydrodesulfurization of res id oils over these catalysts was carried out in a trickle bed reactor- The results show that the deactivation of reaction can be masked by pore diffusion in catalyst particle leading to erro neous measurements of deactivation rate constants from experimental data. A theoretical model is developed to calculate the intrinsic rate constant of major reaction. A method developed by Nojcik (1986) was then used to determine the intrinsic deactivation rate constant and deactivation effectiveness factor- The results indicate that the deactivation effectiveness factor is decreased with decreasing pore diameter of the catalyst, indicating that the pore diffusion plays a dominant role in deactivation of catalyst. 

However, while from these general considerations it is clear that both acidity and pore structure of a zeolite affect the rate of formation of carbonaceous compounds (other factors being held constant), it is generally impossible to quantify the effect of each of these parameters because of the difficulty in obtaining zeolite samples with identical acidities and with different pore structures or vice-versa. Having said that, some examples are given below which illustrate within these limitations the respective roles of acidity and of pore structure. 

The acidity and pore structure of zeolites play significant roles in their deactivation by carbonaceous deposits ("coke"). This is not surprising, as the formation of coke involves reactions cataly by acid sites located inside the pores and also requires the retention of coke molecules by adsorption on the acid sites or by condensation or by steric blockage in the pores. Although it is often difficult to estimate quantitatively and separately the impacts of the acidity and of pore structure, it is clear that it is the latter characteristic which plays the greater role. 

Bore, M.T., Pham, H.N., Switzer, E.E., Ward, T.L., Fukuoka, A., and Datye, A.K. 2005. The role of pore size and structure on the thermal stability of gold nanoparticles within mesoporous silica. Journal of Physical Chemistry B 109, 2873-2880. 

The effect of crystal structure may be investigated by preparing catalysts, as described above, at various temperatures which assures a set of catalysts having variable surface areas, pore size distributions, and crystallinity. Measuring the catalytic activity as a function of these physical properties will help to define the role of crystal structure for the particular transition metal sulfide. In general, the HDS is poorly correlated to N2 BET surface area. This non-correlation can be most easily seen by preparing a... 

At this time it had become possible to determine experimentally total surface area and the distribution of sizes and total volume of pores. Wheeler set forth to provide the theoretical development of calculating the role of this pore structure in determining catalyst performance. In a very slow reaction, reactants can diffuse to the center of the catalyst pellet before they react. On the other hand, in the case of a very active catalyst containing small pores, a reactant molecule will react (due to collision with pore walls) before it can diffuse very deeply into the pore structure. Such a fast reaction for which diffusion is slower than reaction will use only the outer pore mouths of a catalyst pellet. An important result of the theory is that when diffusion is slower than reaction, all the important kinetic quantities such as activity, selectivity, temperature coefficient and kinetic reaction order become dependent on the pore size and pellet size with which a pellet is prepared. This is because pore size and pellet size determine the degree to which diffusion affects reaction rates. Wheeler saw that unlike many aspects of heterogeneous catalysis, the effects of pore structure on catalyst behavior can be put on quite a rigorous basis, making predictions from theory relatively accurate and reliable. 

In Chapter 5 we considered the mathematical treatment of complex reactions. It was also shown how some simpler reaction schemes such as parallel, series, and series-parallel reactions are amenable to analytical solution. In this section we consider the role of pore diffusion in these complex reactions. We omit the mathematical details and in Table 7.5 present the salient features of the effect of pore structure, monomodal or bimodal distribution, on yield and conversion... 

Template effects have been studied quite extensively in the synthesis of zeolites. They also play an important role in the formation of pore structures of... 

The role of pore size distribution in structural heterogeneity... 

Figure 8.15. Plot of cell potential vs. fuel cell current density, (/o), indicating the effect of liquid water accumulation in the CCL on performance (soUd hne). The interplay of liquid water accumulation in pores and impeded oxygen transport causes the transition from the ideally wetted state to the fully saturated state (dotted tines), as indicated [51]. (Reprinted from Electrochimica Acta, 53.13, Liu J, Eikerting M. Model of cathode catalyst layers for polymer electrolyte fuel cells The role of porous structure and water accumulation, 4435— 46, 2008, with permission from Elsevier.)...

This section provides a systematic account of proton transport mechanisms in water-based PEMs, presenting studies of proton transport phenomena in systems of increasing complexity. The section on proton transport in water will explore the impact of molecular structure and dynamics of aqueous networks on the basic mechanism of proton transport. The section on proton transport at highly acid-functionalized interfaces elucidates the role of chemical structure, packing density, and fluctuational degrees of freedom of hydrated anionic surface groups on concerted mechanisms and dynamics of protons. The section on proton transport in random networks of water-filled nanopores focuses on the impact of pore geometry, the distinct roles of surface and bulk water, as well as percolation effects. 

The role of structural defects in MOFs has been probed as well. For instance, although the Zn atoms in intact MOF-5 are inaccessible for ligation, catalytic activities have been reported for this material, for instance, for esterification reactions or for para alkylation of large polyaromatic compounds [4, 60]. It is most probable that Zn-OH defects are created inside the pores as a consequence of adsorption of moisture [28]. 

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