Catalyst layer pore size distributions

The catalyst activity depends not only on the chemical composition but also on the diffusion properties of the catalyst material and on the size and shape of the catalyst pellets because transport limitations through the gas boundary layer around the pellets and through the porous material reduce the overall reaction rate. The influence of gas film restrictions, which depends on the pellet size and gas velocity, is usually low in sulphuric acid converters. The effective diffusivity in the catalyst depends on the porosity, the pore size distribution, and the tortuosity of the pore system. It may be improved in the design of the carrier by e.g. increasing the porosity or the pore size, but usually such improvements will also lead to a reduction of mechanical strength. The effect of transport restrictions is normally expressed as an effectiveness factor q defined as the ratio between observed reaction rate for a catalyst pellet and the intrinsic reaction rate, i.e. the hypothetical reaction rate if bulk or surface conditions (temperature, pressure, concentrations) prevailed throughout the pellet [11], For particles with the same intrinsic reaction rate and the same pore system, the surface effectiveness factor only depends on an equivalent particle diameter given by... 

For the detailed study of reaction-transport interactions in the porous catalytic layer, the spatially 3D model computer-reconstructed washcoat section can be employed (Koci et al., 2006, 2007a). The structure of porous catalyst support is controlled in the course of washcoat preparation on two levels (i) the level of macropores, influenced by mixing of wet supporting material particles with different sizes followed by specific thermal treatment and (ii) the level of meso-/ micropores, determined by the internal nanostructure of the used materials (e.g. alumina, zeolites) and sizes of noble metal crystallites. Information about the porous structure (pore size distribution, typical sizes of particles, etc.) on the micro- and nanoscale levels can be obtained from scanning electron microscopy (SEM), transmission electron microscopy ( ), or other high-resolution imaging techniques in combination with mercury porosimetry and BET adsorption isotherm data. This information can be used in computer reconstruction of porous catalytic medium. In the reconstructed catalyst, transport (diffusion, permeation, heat conduction) and combined reaction-transport processes can be simulated on detailed level (Kosek et al., 2005). 

PCH materials offer new opportunities for the rational design of heterogeneous catalyst systems, because the pore size distributions are in the supermicropore to small mesopore range (14-25A) and chemical functionality (e.g., acidity) can be introduced by adjusting the composition of the layered silicate host. The approach to designing PCH materials is based on the use of intercalated quaternary ammonium cations and neutral amines as co-surfactants to direct the interlamellar hydrolysis and condensation polymerization of neutral inorganic precursor (for example, tetraethylorthosilicate, TEOS) within the galleries of an ionic lamellar solid. 

Figure 6.15. Influence of GDL pore-former content on cell performance of a H2/02 single cell (0) 0 mg/cm2, (o) 3 mg/cm2, ( ) 5 mg/cm2, (A) 7 mg/cm2, and (V) 10 mg/cm2 pore-former loading 5 mg/cm2 carbon loading in the GDL and 0.4 mg Pt/cm2 in the catalyst layer [15]. (Reprinted from Journal of Power Sources, 108(1-2), Kong CS, Kim DY, Lee HK, Shul YG, Lee TH. Influence of pore-size distribution of diffusion layer on mass-transport problems of proton exchange membrane fuel cells, 185-91, 2002, with permission from Elsevier and the authors.)...

Next, we examined the effects of control of the microstnicture such as pore-size distribution (or porosity) and the ohmic resistance of SDC layers on the improvement of the electrocatalytic activity of the SDC anode with and without nm-sized Ru metal catalyst loading. Fine polymer beads (cross-linked polystyrene, d = 1.2 pm) were added to the SDC paste, resulting in the fonnation of pm-sized pores after sintering, which can improve the gas-diffusion rates in the electrode. 

Gas diffusion electrodes have been characterized with the objective to Imk structural parameters (such as permeability, fraction of hydrophobic pores, pore size distribution and volume, and catalyst layer thickness and composition [26,27,28]) to cell performance. Although this information is valuable to validate existing gas diffusion electrode models [29, 30, 31], the Imk between... 

In Refs. [18,19], the macrohomogeneous theory was extended to include concepts of percolation theory. The resulting structure-based model correlates the performance of the CCL with the volumetric amounts of Pt, C, ionomer, and pores. A detailed review of macroscopic catalyst layer theory can be found in Ref. [17]. A further extension of this theory in Ref. [25] explores the key role of the CCL for the fuel cell water balance. This function is closely linked to the pore size distribution. Major principles of these models will be reproduced here. The details can be found in the literature cited. 

The structural picture of the catalyst layer as a three-phase composite medium with bimodal pore size distribution, outlined in Section 2.6, was used in Ref. [25] to explore the water-handling capabilities of the CCL. 

Finally, the acid- or base-catalyzed reaction of hydrolysis and condensation polymerization of TEOS into a layered silicate gallery could affect the physical properties of silica-pillared magadiite and kenyaite. The samples that were silica-pillared by acid- and base-catalyzed reactions show a large increase in basal spacing. Also, they exhibit relatively narrow pore size distributions and show high surface areas, depending on the type of catalyst and layered silicate. These results indicate that variations in the conditions of gelation contribute to the improvement in the physical properties of silica-pillared molecular sieves. 

Porosity and pore size distribution of catalyst layers... 

It should be mentioned that the mean transport pore model is probably not the best option for SCR catalyst layers, as it assumes a uniform pore size distribution. The actual pore size distribution of the SCR layers is highly bimodal with two distinct maxima micropores and mesopores. The random pore model considers a bi-dispersive washcoat material with two characteristic pore sizes with their respective mean pore sizes and void fractions. The total dififusivity is calculated as a combination of the respective Knudsen dififusivities ... 

Figure 8.9. Bimodal pore size distributions with varying portions of primary and seeondary pores, obtained with Equation 8.24. Shaded areas represent the Uquid water saturation at given operating conditions [50]. (Reprodueed by permission of ECS - The Eleetroehemieal Society, from Eikerling M. Water management in eathode catalyst layers of PEM fuel eeUs.)...

Contamination effects of impurities on a PEMFC may be classified into three categories (1) kinetic effects, caused by adsorbing onto the catalyst surface and poisoning active sites on both the anode and cathode catalyst layers (2) mass transfer effects, due to changes in the structure, pore size, pore size distribution, and hydrophobicity/hydrophilicity of the catalyst layers or gas... 

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