Effective catalyst layer

An effective catalyst layer must serve multiple functions simultaneously electron and proton conduction, oxygen or hydrogen supply, and water management. The composition and structure of a CL can affecf all fhese functions... 

In this approach, discussed in the section Effective Catalyst Layer Properties from Percolation Theory, Tstat corresponds to the statistical fraction of Pt particles at or near the triple-phase boundary of solid carbon/Pt phase (volume fraction Xptc), ionomer phase (A /), and pore space (Xp = 1 — Xptc — Xei). 

Simulations of physical properties of realistic Pt/support nanoparticle systems can provide interaction parameters that are used by molecular-level simulations of self-organization in CL inks. Coarse-grained MD studies presented in the section Mesoscale Model of Self-Organization in Catalyst Layer Inks provide vital insights on structure formation. Information on agglomerate formation, pore space morphology, ionomer structure and distribution, and wettability of pores serves as input for parameterizations of structure-dependent physical properties, discussed in the section Effective Catalyst Layer Properties From Percolation Theory. CGMD studies can be applied to study the impact of modifications in chemical properties of materials and ink composition on physical properties and stability of CLs. 

The MHM was exploited in optimization studies of CCL composition and thickness, using the relations specified in the section Effective Catalyst Layer Properties... 

Figure 4.6b compares calculated plots of Eceii versus jo with experimental data of Uchida et al. (1995a,b) for CCL with different ionomer content, as specified in the legend. Composition-dependent properties were parameterized using the functions proposed in the section Effective Catalyst Layer Properties from Percolation Theory in Chapter 3. The fuel cell voltage was assumed to be of the form... 

As discussed in the section Ionomer Structure in Catalyst Layers Redefined in Chapter 3, a theory of composition-dependent effective properties that incorporates recent insights into stmcture formation in CCLs is yet to be developed. At present, the relations presented in the section Effective Catalyst Layer Properties from Percolation Theory in Chapter 3 do not account for agglomerate formation and skin-type morphology of the ionomer film at the agglomerate surface. Qualitative trends predicted by the simple structure-based catalyst layer theory should be correct, as confirmed by the results discussed in this section. 

Both reactions were carried out under two-phase conditions with the help of an additional organic solvent (such as iPrOH). The catalyst could be reused with the same activity and enantioselectivity after decantation of the hydrogenation products. A more recent example, again by de Souza and Dupont, has been reported. They made a detailed study of the asymmetric hydrogenation of a-acetamidocin-namic acid and the kinetic resolution of methyl ( )-3-hydroxy-2-methylenebu-tanoate with chiral Rh(I) and Ru(II) complexes in [BMIM][BF4] and [BMIM][PFg] [55]. The authors described the remarkable effects of the molecular hydrogen concentration in the ionic catalyst layer on the conversion and enantioselectivity of these reactions. The solubility of hydrogen in [BMIM][BF4] was found to be almost four times higher than in [BMIM][PFg]. 

Figure 5.18. Schematic representation of the density of states N(E) in the conduction band and of the definitions of work function d>, chemical potential of electrons p, electrochemical potential of electrons or Fermi level p, surface potential x> Galvani (or inner) potential

The obvious question then arises as to whether the effective double layer exists before current or potential application. Both XPS and STM have shown that this is indeed the case due to thermal diffusion during electrode deposition at elevated temperatures. It is important to remember that most solid electrolytes, including YSZ and (3"-Al2C)3, are non-stoichiometric compounds. The non-stoichiometry, 8, is usually small (< 10 4)85 and temperature dependent, but nevertheless sufficiently large to provide enough ions to form an effective double-layer on both electrodes without any significant change in the solid electrolyte non-stoichiometry. This open-circuit effective double layer must, however, be relatively sparse in most circumstances. The effective double layer on the catalyst-electrode becomes dense only upon anodic potential application in the case of anionic conductors and cathodic potential application in the case of cationic conductors. 

As shown in Chapter 4 (section 4.5.9.2), Equation (8.14) can also be derived via a rigorous electrostatic model which takes into account the presence of the effective double layer on the catalyst surface and gives in general ... 

Promotion, electrochemical promotion and metal-support interactions are three, at a first glance, independent phenomena which can affect catalyst activity and selectivity in a dramatic manner. In Chapter 5 we established the (functional) similarities and (operational) differences of promotion and electrochemical promotion. In this chapter we established again the functional similarities and only operational differences of electrochemical promotion and metal-support interactions on ionic and mixed conducting supports. It is therefore clear that promotion, electrochemical promotion and metal-support interactions on ion-conducting and mixed-conducting supports are three different facets of the same phenomenon. They are all three linked via the phenomenon of spillover-backspillover. And they are all three due to the same underlying cause The interaction of adsorbed reactants and intermediates with an effective double layer formed by promoting species at the metal/gas interface (Fig. 11.2). 

On the other hand, as already discussed in Chapter 11 in connection to the effect of metal-support interactions, it appears that a fully dispersed noble metal catalyst on porous YSZ is already at a NEMCA or electroche-mically-promoted state, i.e. it is covered by an effective double layer of promoting backspillover O2 ions. This can explain both the extreme catalytic activity ofZr02- and Ti02- supported commercial catalysts, as well as the difficulty so far to induce NEMCA on fully dispersed noble metal catalysts deposited on YSZ. 

Figure 15.3 Simulated effectiveness factor for porous carbon electrode as a function of the exchange current density jo and DCo for Ip] = 0.4 V for a 10wt% Pt/C catalyst layer with 7= 10, A = 140m g p = 2gcm, Nafion volume fraction 0.6, thickness p,m, and ionic conductivity 0.05 Scm See the text for details. (Reproduced from Gloaguen et al. [1994], with kind permission from Springer Science and Business Media.)...

Steep temperature gradients inside the catalyst layer will enhance the bubble formation and bring about efficient product desorption and effective regeneration of vacant active sites consequently. There irreversible processes are followed by another irreversible act of bubble detachment from the surface. 

Considerable work has been conducted on a water-soluble catalyst using sulfonated phosphine-modified rhodium. Details of this chemistry will be described in Chapter 5. The general concept (Figure 2.3) is to make the catalyst water soluble, then after product formation, decant the product. In order for the water-soluble catalyst to be effective, the alkene must dissolve in the aqueous layer. This has been demonstrated on a commercial basis using propene. The low solubility of higher alkenes in the aqueous catalyst layer has proven problematic. The desirable characteristic of the ligand, water solubility, is needed in the separation step but is a disadvantage in the reaction step. 

Of course, the calculated minimum-energy Ti species supported on different lateral faces of MgC layers can only represent a basis for the description of the real effective catalyst, since the relative distribution, interconversion, and stereospecificity of the various Ti species are sensitively modified by the presence of alkylaluminum compounds and, particularly, by the added Lewis bases. 

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