Catalyst layer three-dimensional

Fig. 21. (a) The nature of the glide shear plane defects in three-dimensional projection and (b) in one layer of idealized structure, showing the novel glide shear process and the formation of glide shear plane defects. Filled circles are anion vacancies, (c) Schematic of glide shear. Glide defects accommodate the misfit at the interface between catalyst surface layers with anion vacancies (filled circles) and the underlying bulk (85,89). 

The structure of skeletal catalysts is so fine that electron microscopes are required for sufficient resolution. The use of a focussed ion beam (FIB) miller has enabled a skeletal copper catalyst to be sliced open under vacuum and the internal structure to be imaged directly [61], Slicing the catalyst enabled viewing beyond the obscuring oxide layer on the surface. A uniform, three-dimensional structure of fine copper ligaments was observed [61], which differed from the leading inferred structure at the time of parallel curved rods [54],... 

An active, catalytic layer, comprising a three-dimensional porous structure composed of a mixture of hydrophilic carbon particles (Vulcan XC-72) supporting a finely dispersed catalyst, and a hydrophobic binder (PTFE). This layer faces the liquid side and can be visualised as being formed from many hydro-phobic channels (the route of the oxygen supply) and hydrophilic channels, required for the rapid removal of caustic released into the gap between the membrane and GDE. 

Nanowire-Based Three-Dimensional Hierarchical Core/Shell Catalyst Layer... 

Equations (6.59)-(6.61) represent a highly simplified scheme for evaluating various catalyst layer designs. Refinements of this crude framework for evaluating catalyst layer performance should address all transport limitations, account for water accumulation, and include two- and three-dimensional effects. 

A recent investigation has demonstrated the usefulness of ultrasonic irradiation in the preparation of delaminated zeolites, which are a particular type of modified oxides - microporous crystalline aluminosilicates with three-dimensional structures - having a greater catalytic activity than the layered structures (clays) and mesoporous catalysts. In an attempt to increase the pore size of zeolites, a layered zeolite precursor was... 

Figure 36. Cathode voltage loss as predicted by direct numerical simulation of proton, oxygen, and water transport in a catalyst layer at the pore level (left), and three-dimensional oxygen concentration contours in a random microstructure of the catalyst layer (right).

In principle, all the kinetic concepts of intercalation introduced for layer-structured silicates hold for zeolites as well. Swelling, of course, is not found because of the rigidity of the three dimensional frame. The practical importance of zeolites as molecular sieves, cation exchangers, and catalysts (cracking and hydrocracking in petroleum industry) is enormous. Molecular shape-selective transport (large differences in diffusivities) and micro-environmental catalysis (in cages and channels)... 

Jiang et al.16 developed a multiphase, three-dimensional model to describe non-isothermal cold start and to study the effect of temperature rise. Due to the temperature rise during cold start, more water was transported into the membrane and less ice formation occurred in the catalyst layer. It was also found that a lumped thermal analysis significantly overestimated the overall thermal requirement for successful self-cold start. In addition, pre-startup conditions such as gas purge had significant impact on cold start that implied the importance of the shutdown process. 

As a whole, considerable experimental data have been accumulated in the literature concerning the influence of the gas phase on the surface state of adsorbents and catalysts. The surface state is used specially to preliminarily produce the required surface composition of a catalyst. At the same time, quantitative characteristics of these changes are mainly available only for binary alloys contacting with molecules of H2, CO, and 02 [41,42]. Investigations conducted in recent years reveal that the influence of adsorbed particles on the state of a solid surface is apparently more significant than is customarily considered at present. The structural transformations in the surface layers (an example is the rearrangement of the surface layer of platinum in the adsorption of CO and 02 molecules [43]) and the processes of formation of new phases in them, which are similar to three-dimensional topochemical processes [44], may be of a major significance. 

Nafion content in the catalyst layer plays an important role in electrode performance. Incorporation of Nafion ionomer into carbon-supported catalyst particles to form the catalyst layer for the gas diffusion electrode can establish a three-dimensional reaction zone, which has been proven by cyclic voltammetric measurements. An optimal Nafion content in the catalyst layer of the electrode may minimize the performance loss that arises from ohmic resistance and mass transport limitations of the electrode [6],... 

Oxides with layered stmcture 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 andZeolites). 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). 

The conversion of a iayered material like OL-1 into a 1-dimensional porous material like OMS-1 is also not surprising. It is well known that certain fluid cracking three dimensional zeolite catalysts can be grown from layered clays. In addition, it is noted that birnessite is the most common manganese mineral. Its abundance may be important in geological transformation of birnessite into todorokite.68.69... 

The growth of Ru islands on Pt(hkl) was found to be substrate-dependent, so that the Ru layer is almost completely in the fonn of a monolayer on Pt(llO), whereas the two- and three-dimensional growth is facilitated on the other two low-index Pt surfaces, especially on Pt(lll). The Ru-Pt(lll) is more effective catalyst for methanol oxidation " than the other two surfaces decorated with Ru, because the edge of a Ru island is the active site in methanol oxidation therefore, controlling the extent of the multidimensional islands is of a particular importance for fuel cell catalysis. 2 ... 

Finally, three different sorts of compound which can insert and exchange cations and ions in their structures will be discussed. Of these, the zeolites have been developed from minerals, and are used as ion exchangers, catalysts and sorbents. The use of their framework structures for shape-selective catalysis will be discussed. The unique properties of both layered and three-dimensional compounds which can accept extra ions, such as graphite and will also be examined. 

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