Proton catalyst loading

A very reactive Lewis acid is obtained when the complex [(EBTHI)Zr(Me)2] is converted in situ to a dicationic species by protonation with the acid H-BARF (BARF = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) in the presence of the Diels—Alder substrate oxazolidi-none [88] (Scheme 8.48). The dicationic species is stabilized through coordination by the oxazolidinone and by diethyl ether (derived from the acid etherate employed). The catalyst loading in the Diels—Alder reaction could be lowered to 1 mol% (Zr) and the reaction still... 

It is well known that Nafion ionomer contains both hydrophobic and hydrophilic domains. The former domain can facilitate gas transport through permeation, and the latter can facilitate proton transfer in the CL. In this new design, the catalyst loading can be further reduced to 0.04 mg/cm in an MEA [10,11]. However, an extra hydrophobic support layer is required. This thin, microporous GDL facilitates gas transport to the CL and prevents catalyst ink bleed into the GDL during applications. It contains both carbon and PTFE and functions as an electron conductor, a heat exchanger, a water removal wick, and a CL support. 

The catalyst layer is composed of multiple components, primarily Nafion ion-omer and carbon-supported catalyst particles. The composition governs the macro- and mesostructures of the CL, which in turn have a significant influence on the effective properties of the CL and consequently the overall fuel cell performance. There is a trade-off between ionomer and catalyst loadings for optimum performance. For example, increased Nafion ionomer confenf can improve proton conduction, but the porous channels for reactanf gas fransfer and water removal are reduced. On the other hand, increased Pt loading can enhance the electrochemical reaction rate, and also increase the catalyst layer thickness. 

The catalyst layers (the cathode catalyst layer in particular) are the powerhouses of the cell. They are responsible for the electrocatalytic conversion of reactant fluxes into separate fluxes of electrons and protons (anode) and the recombination of these species with oxygen to form water (cathode). Catalyst layers include all species and all components that are relevant for fuel cell operation. They constitute the most competitive space in a PEFC. Fuel cell reactions are surface processes. A primary requirement is to provide a large, accessible surface area of the active catalyst, the so-called electrochemically active surface area (ECSA), with a minimal mass of the catalyst loaded into the structure. 

The evolving structural characteristics of CLs are particularly important for further analysis of transport of protons, electrons, reactant molecules (O2), and water as well as for the distribution of electrocatalytic activity at Pt-water interfaces. In principle, the mesoscale simulations allow relating these properties to the choices of solvent, ionomer, carbon particles (sizes and wettability), catalyst loading, and hydration level. Explicit experimental data with which these results could be compared are still lacking. Versatile experimental techniques have to be employed to study particle-particle interactions, structural characteristics of phases and interfaces, and phase correlations of carbon, ionomer, and water in pores. 

Cycloheptanones attained better enantioselectivity values than their six-membered analogs and the use of alkyl-substituted silyl enol ethers resulted in only moderate enantioselectivities. Indeed, replacement of P=0 by P=S or P=Se in the phospho-ramide catalyst led to improved results in terms of reactivity as well as enantioselectivity. The catalyst loading could be decreased to 0.05 mol% without a deleterious effect on the enantioselectivity (one example). Optimization experiments revealed the critical influence of the achiral proton source on the reactivity and enantioselectivity. This observation suggests a two-step mechanism for the protonation reaction (Scheme 71). 

The Rouden group utilized 121 and 124 as organic bases for the asymmetric decarboxylative protonation of cyclic, acyclic, and bicyclic N-acylated a-amino hemimalonates [290]. The introduced protocol suffered from high catalyst loading... 

The heterobimetallic multifunctional complexes LnSB developed by Shibasaki and Sasai described above are excellent catalysts for the Michael addition of thiols [40]. Thus, phenyl-methanethiol reacted with cycloalkenones in the presence of (R)-LSB (LaNa3tris(binaphthox-ide)) (10 mol %) in toluene-THF (60 1) at -40°C, to give the adduct with up to 90% ee. A proposed catalytic cycle for this reaction is shown in Figure 8D.9. Because the multifunctional catalyst still has the internal naphthol proton after deprotonation of the thiol (bold-H in I and II), this acidic proton in the chiral environment can serve as the source of asymmetric protonation of the intermediary enolate, which is coordinated to the catalyst II. In fact, the Michael addition of 4-/en-butylbenzcnethiol to ethyl thiomethacrylate afforded the product with up to 93% ee using (R)-SmSB as catalyst. The catalyst loading could be reduced to 2 mol % without affecting enantioselectivity of the reaction. 

This transformation was further studied and the catalyst load could be decreased to 0.2 equiv.35 33 A mechanism was proposed through deuterium incorporation experiments, and the conclusion was that there was no 1,2 shift of the deuterium present in the starting material (A, Scheme 5.3) since exclusive formation of furans D deuterated on position 3 could be explained by the presence of an external source of deuterium (such as D20). Therefore, it is believed that after silver(I) coordination to the allenyl system (A, Scheme 5.3), the attack by the carbonyl oxygen may lead to an oxo cation intermediate B. Finally, proton lost would generate silver furan C that would lead to furan D after silver release (Scheme 5.3).39... 

The theoretical cell voltage of a DMFC at standard conditions is 1.20 V. The materials used in DMFCs are similar to those in PEMFCs. Pt, PtRu, and Nafion membrane are used as cathode catalyst, anode catalyst, and proton transfer membranes, respectively. However, the catalyst loading in a DMFC is much higher than the loading used in H2/air fuel cells, because both side reactions are slow (Pt loadings 4 mg/cm2 for a DMFC, 0.8 mg/cm2 for a H2/air fuel cell). 

Figure 6.11. Nyquist plots for MEAs containing different proton-conducting ionomers at 0.85 V without external humidification catalyst loading = 0.4, 0.7 mg Pt/cm2 for anode and cathode, respectively TceU = 25°C Pressure = 1 atm and H2/02 flow = 400 cmVmin [8]. (Reprinted from Electrochimica Acta, 50(2-3), Ahn SY, Lee YC, Ha HY, Hong SA, Oh IH. Effect of the ionomers in the electrode on the performance of PEMFC under non-humidifying conditions, 673-6, 2004, with permission from Elsevier.)...

Chelating ferrocene phosphine L18 was reported by Hartwig to efficiently catalyze the amination of most aryl chlorides with any type of primary aliphatic amine, imine, or hydrazine at 80-100 °C with NaOf-Bu in DME. Base sensitive aryl chlorides, or those containing acidic protons, may be aminated using LiHMDS as the stoichiometric base. Impressively, catalyst loadings as low as 0.005 mol% can be used. 

Tin Coulombel and coworkers have used tin(IV) triflate as catalyst in the hydroalkoxylation of unsaturated alcohols (Scheme 9a) [51]. The substrate reactivity decreases along the order trisubstituted olefins 1,1-disubstituted olefins > 1,3-disubstituted > monosubstituted olefin. Incidentally, this is a typical reactivity profile for most Lewis acid catalysts discussed in this section. The catalyst loading could be reduced down to 0.1% in favorable cases and in the absence of a solvent. As trifiic acid alone (5%) also catalyzed the reaction in Scheme 9 efficiently, and because Sn(OTf)4 is readily hydrolyzed, a control experiment with cocatalytic amounts (5% each) of Sn(OTf)4 and 2,6-lutidine as proton quencher was performed, in which catalytic activity was retained. We do not believe that this experiment is sufficient proof of tin catalysis, as Sn(OTf)4 may release more than a single equivalent of triflic acid upon hydrolysis. In any case, the selectivity profile of the tin-catalyzed reaction matches that of the trifiic acid-induced hydroalkoxylation reactions studied earlier in the same research group [45]. 

Min et al. [35] experimented on high-catalyst loading with 60% carbon and 40% Teflon backing claimed to be the most efficient electrode for direct methanol/proton exchange membrane fuel cell (PEMFC). The catalysts used were platinum and ruthenium which formed an alloy at an atomic ratio 1 1. The formation of the alloy was seen in XRD as there were no pure metal peaks found. The alloy formation of Pt and Ru promotes oxidation of methanol at lower temperatures. The 60% carbon backing makes it evident that the lower the percentage of carbon increases the efficiency. 

H+) Produced by the dissociation of Hj at the anode to the cathode, (3) Prevention of the associated electron flow through the membranes forcing them to flow in the external circuit to the cathode to produce DC current, and (4) Support for the catalyst loaded on the electrodes. When Hj is replaced by methanol as a fuel in liquid form in direct methanol fuel cell (DMFC), the dissociation of methanol solution at the anode produces protons that are transported through the hydrated PEM to the cathode, where a reduction of O2 produces water in the presence of the protons. To qualify PEM for commercial application in PEMFC and DMFC, it should have a combination of properties including (Maiyalagan and Pasupathi 2010 Neburchilov et al. 2007 Nagarale et al. 2010) ... 

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