Catalyst layer structure formation

Ionomer in Catalyst Layers Structure Formation and Performance... 

For the amounts of Fe below x=l, the sheath-like structures form mostly (Fig. 2d). This proceeds likely so when the Fe amount is low enough, the catalyst does not get to deeper layers of onions and pyrrole polymerises already in outer layers, which hinders the access of further monomer molecules to the onions inside. Use of still smaller amounts of the Fe catalyst results in formation of carbon (e.g., OCM-.NO.25) consisting of both foam- and sheath-like structures (Fig. 2c). The XPS analysis reveals that 0.43 wt.% Si and 0.5 wt.% Fe remain in the surface layer of OCM-.NO.25. This sample as well as CMK-3N1.25 and CMK-3N2.00 do not bum up totally (Table 1, Fig. 3). 

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. 

At macroscopic level, the overall relations between structure and performance are strongly affected by the formation of liquid water. Solution of such a model that accounts for these effects provides full relations among structure, properties, and performance, which in turn allow predicting architectures of materials and operating conditions that optimize fuel cell operation. For stationary operation at the macroscopic device level, one can establish material balance equations on the basis of fundamental conservation laws. The general ingredients of a so-called "macrohomogeneous model" of catalyst layer operation include ... 

Find et al. [42] developed a nickel-based catalyst for methane steam reforming. As material for the micro structured plates, AluchromY steel, which is an FeCrAlloy (see Section 2.10.7) was applied. This steel forms a thin layer of alumina on its surface, which is less than 1 pm thick. This layer was used as an adhesion interface for the catalyst. I ts formation was achieved by thermal treatment of micro structured plates for 4 h at 1 000 °C. 

In epoxidation, the propene-to-CHP molar ratio is 10 1, the reaction temperature is 60 °C and the pressure is sufficient to maintain propene in the liquid phase. The feed to the epoxidation reactor must contain less than 1% water in order to limit the hydrolysis of PO to glycol. The reaction is catalyzed by a proprietary, silylated, titanium-containing silicon oxide catalyst. The conversion of CHP is greater than 95%. Selectivity for PO based on hydroperoxide is 95%, whereas selectivity based on propene is around 99%. By-products of the reaction are aldehydes, such as acetaldehyde and propionaldehyde, alcohols (methanol and propene glycol), ketones and esters (e.g., acetone and methyl formate). The catalyst fixed-bed is structured into multiple catalyst layers, with heat exchangers in between the layers. This prevents excessive increases in temperature due to the exothermal reaction that would cause both thermal decomposition of the hydroperoxide and consecutive reactions of PO. 

On the contrary, in the present work, samples show the same and nearly stoichiometric P/V bulk atomic ratio. The procedure of the development of the VOHPO4.I/2 H2O (Precursor B) was then controlled principally by the amount of water added during preparation of precursors. The prepared samples then differed by the amount of intercalated organic material as shown by the changes of the C/V values. This results in a distortion of the layered structure of the precursor and consequently in the VPO catalyst (XRD emd FTIR). Our results confirm the fact the selectivity in the formation of MA and PA is directly related to the structural characteristic of the prepared VPO catalysts. 

Oxides with layered structure or those whose structures contain large tunnels or cavities may display abnormal ion movement or serve as templates for heterogeneous catalysis see Ionic Conductors, Intercalation Chemistry, Oxide Catalysts in Solid-state Chemistry, and Zeohtei). Many oxides are stabilized by the formation of structures that are highly defective nature and have similar properties to those listed above see Defects in Solids). The strong bonds, which result in three-dimensional cross-linked structures, give rise to inert, refractory materials that have a variety of uses (see Section 5.3.6 and Ceramics). 

Delamination of the MPL from the GDL substrate has not been widely reported but may occur during freeze-thaw cycles, as occurs with catalyst-layer delamination from the membrane [131, 132]. A different situation occurs in the GDL/MPL, where the pore diameters are on the order of a micron or larger and the water is not hydrating the sulfraiic acid of the ionomer. The volume expansion caused by ice formation can produce large isotropic stresses that can damage the structure of the catalyst layer, the MPL, or the GDL. 

Hydrogen addition across the C=C bond of dimethyl itaconate was performed with sodium formate in aqueous solution as an H-transfer reagent (Scheme 9.4) [54]. In this reaction, the substrate is in the organic layer whereas the catalyst and the formate are in the aqueous phase where the reaction takes place. In such a liquid-liquid biphasic system, mass transfer may be an issue. A 43% conversion was obtained in the micro-structured reactor while benchmarking with a traditional batch vessel afforded quantitative conversion (>99%). This was attributed to the poor mass transfer coefficient in the microdevice, which was originally designed to resolve heat transfer issues. 

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