Catalyst bimodal pore structure

As most catalyst supports are manufactured by pelletizing microporous powder one gets a bimodal pore structure with two distinct pore networks, a macro- and a micro-pore network, but where molecules diffuse from one network into the other. 

Porosity measurements by mercury porosimetry show a bimodal porous structure of the industrial catalyst, with predominant pores radius 22 and 30 nm. During overheating of a catalyst, the bimodal pore structure is still observed, whereas their average size decreases (fig. 4). As an example, for the unpressed catalysts, pore s fraction (measured by the mercury porosimetry) up from 50 nm covers 73% of total volume. After pressing the catalyst under a... 

Takahashi R, Sato S., Sodesawa T., Yabuki M. SiUca-alumina catalyst with bimodal pore structure prepared by phase separation in sol-gel process. J. Catal. 2001 200 197-202 Tamagawa H., Oyama K., Yamaguchi T., Tanaka H., Tsuiki H., Ueno A. Control ofNi metal particle size in Ni/Si02 catalysts by calcinations and reduction temperatures. J. Chem. Soc., Faraday Trans. 1 1987 83 3189-3197... 

The relation between the dusty gas model and the physical structure of a real porous medium is rather obscure. Since the dusty gas model does not even contain any explicit representation of the void fraction, it certainly cannot be adjusted to reflect features of the pore size distributions of different porous media. For example, porous catalysts often show a strongly bimodal pore size distribution, and their flux relations might be expected to reflect this, but the dusty gas model can respond only to changes in the... 

FIGURE 10.1 Diagram of bimodal catalyst pore structure. 

The silica carrier of a sulphuric acid catalyst, which has a relatively low surface area, serves as an inert support for the melt. It must be chemically resistant to the very corrosive pyrosulphate melt and the pore structure of the carrier should be designed for optimum melt distribution and minimum pore diffusion restriction. Diatomaceous earth or synthetic silica may be used as the silica raw material for carrier production. The diatomaceous earth, which is also referred to as diatomite or kieselguhr, is a siliceous, sedimentary rock consisting principally of the fossilised skeletal remains of the diatom, which is a unicellular aquatic plant related to the algae. The supports made from diatomaceous earth, which may be pretreated by calcination or flux-calcination, exhibit bimodal pore size distributions due to the microstructure of the skeletons, cf. Fig. 5. 

Unsteady state diffusion in monodisperse porous solids using a Wicke-Kallenbach cell have shown that non-equimolal diffusion fluxes can induce total pressure gradients which require a non-isobaric model to interpret the data. The values obtained from this analysis are then suitable for use in predicting effectiveness factors. There is evidence that adsorption of the non-tracer component can have a considerable influence on the diffusional flux of the tracer and hence on the estimation of the effective diffusion coefficient. For the simple porous structures used in these tests, it is shown that a consistent definition of the effective diffusion coefficient can be obtained which applies to both the steady and unsteady state and so can be used as a basis of examining the more complex bimodal pore size distributions found in many catalysts. 

Figure 2.1. Structure and composition of catalyst layers at three different scales At the nanoparticle level, anode and cathode processes are depicted, including possible anode poisoning by CO. At the agglomerate level, ionomer functions as binder and proton-conducting medium are indicated, and points with distinct electrochemical environments are shown (double- and triple-phase boundary). At the macroscopic scale, the interpenetrating percolating phases of ionomer, gas pores, and solid Pt/Carbon are shown, and the bimodal porous structure is indicated.

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. 

Hierarchically ordered mesoporous carbons (HOMC) are attractive as a support for fuel cell applications because of their interconnected bimodal pore-size distribution. Both pore systems can be mesoporous or one can be mesoporous while other can be macroporous. While a mesoporous pore structure imparts high surface area and uniform distribution of catalyst particles, macropores provide efficient mass transfer. Of course, the interconnectivity between pores has a significant role in realizing the advantages of both pore stmctures. Also, a novel feature about these structures is that the two pore structures can be adjusted independently, allowing for good control over their porosity [73, 74]. Like OMC, controllable pore structure, and carbon microstracture and surface chemistry, makes them an attractive support for fuel cell catalysis. Fang et al. have shown that Pt on hollow... 

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