Catalysts sublayer

The model was also used to explore novel design ideas. It was predicted and subsequently confirmed in experiment that functionally graded layers result in improved performance compared to standard CCLs with uniform composifion.243 244 design, the catalyst layer is fabricated as a sublayer... 

Peacock et al. (37) showed that a bismuth molybdate catalyst can be reduced with propylene and that the oxygen appearing in the gaseous products (acrolein, carbon dioxide, and water) can be quantitatively replaced in the lattice. The amount of oxygen removed during reduction corresponds to the participation of many sublayers of oxide ions. 

The catalysts to be deposited are inserted into the sputter plant as the so-called targets. The shuttle carries the titer-plate and revolves below the metal targets (Figure 3.4). In every rotation up to three sublayers are deposited. For a binary system, the sublayer thickness was 10 nm in each revolution. The thickness gradients were realized by aperture orifices which shaped the particle beam (Figure 3.5). 

The Ha number (Ham) defined here for the mth sublayer of the membrane layer corresponds to the well-known Thiele modulus defined for catalyst particles, while the Peclet number corresponds to the often used Bodenstein number. 

Figure 33 shows the profiles of reaction rates across the catalyst layers in the inlet element (along the white line in Fig. 32). In this example, the reaction rate on the cathode side is almost constant along x, whereas on the anode side it rapidly grows with x. Most of the anode catalyst layer thickness is not used for reaction, except for the thin sublayer near the membrane, with the thickness of the order of 2-3 pm.

Catalysis proceeding directly above a reconstructed sublayer, which then acts as the actual catalyst rather than undisturbed substrate, may be rather common. A basic uncertainty concerning cataljrtic oxidations on metals, for example, is whether the surface is effectively an oxide catalyst or not (366). And confusion of interpretation must result if a catalytically active surface compound contains no atoms at all from reacting molecules, as could happen if a reconstructed top layer were formed by spurious contamination after an induction period. All of these primary basic questions require investigation by LEED. 

In the oxygen depletion regime, jo I, only a thin sublayer with thickness L, adjaeent to the GDL, is active. The remaining sublayer with thickness L — 6eff) L, adjaeent to the membrane, is not used for reactions, due to the starvation in oxygen. For this situation with rather nonuniform reaction rate distribution, catalyst is used very ineffeetively. The inactive part causes overpotential losses due to proton transport in the polymer electrolyte, which could cause limiting current behavior, if the proton conductivity is low. 

An alternative, but related, approach to reduce Ft content and yet increase catalytic activity is the nanostructured core-shell approach taken by Adzic and co-workers [41]. As an example, a cartoon of a core-shell structure is illustrated in Figure 14.5, in which a nanostructured catalyst particle is shown that has a Ft monolayer surface with a Fd sublayer on top of a core material of metal M. This core-shell nanostructure is architected to take into account the surface contraction effects that shift the d-band center and reduce the oxygen binding energy, the natural surface segregation effects, and the stability offered by a contiguous monolayer. Clearly, the amount of Ft is reduced because the core can be made of less costly materials. 

A well-known weakness of the Pt-alloy approach is the leaching out of the base metal from the surface immediately and from the bulk over time. It has been shown by Stamenkovic et al. [64] that almost all the surface base metal is leached off once in contact with electrolyte aPt skeleton structure is formed (or Pt-skin for annealed catalysts) that has higher coordinated Pt atoms and a sublayer enriched in the base metal contributes to the higher activity. Nevertheless, as long as the catalyst activity... 

Early MEA fabrication relied on catalyst deposition directly onto the GDL materials (via spraying, screen printing, or blade application), and subsequent bonding to the membrane. Other MEA designs incorporate mass-produced, catalyst-coated membranes (CCMs) that are separated from the GDL by one or more sublayers. These sublayers can be fabricated from multiwalled carbon nanotubes (Kannan et al, 2009), doped polyaniline (PANl) (Cindrella and Kannan, 2009), or more commonly, carbonaceous particles with polymeric binders. The pore diameters in the resulting MPL can be two orders of magnitude smaller than the corresponding pores in the GDL (e.g., 10 m and 10 m, respectively). 

Transition metals are widely used as catalysts for nucleophilic substitution reactions and there has been summary of their use in amination reactions. Pd(dba)2-ligand A is reported to be an efficient catalyst for the amination of a variety of aryl chlorides. Poly[(j9-cyclopentadienylmethylstyrene)rhodium] + grafted on to a polymeric sublayer has been used as an effective catalyst for alkoxydefluorination reactions of fluoroarenes. The activity is similar to that of the homogeneous catalyst and may be conserved for repeated applications. 

C catalyst support, sublayer and porous substrate Gas-diffusion electrode... 

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