Proton exchange catalysis

Proton electrochemical gradient. 6,714 Proton exchange amine ligands, 2, 24 Proton loss catalysis... 

Ambient temperature catalysis of O2 reduction at low overpotentials is a challenge in development of conventional proton exchange membrane fuel cells (pol5mer electrolyte membrane fuel cells, PEMFCs) [Ralph and Hogarth, 2002]. In this chapter, we discuss two classes of enz5mes that catalyze the complete reduction of O2 to H2O multi-copper oxidases and heme iron-containing quinol oxidases. 

Proton transfer reactions on the aqua oxo complex are described by Eq. (8) (acid catalysis or protolysis), Eq. (9) (base catalysis or hydrolysis), and Eq. (10) (direct proton exchange). 

In the plots of the catalytic activity per Cu2+ ion, to which an NO molecule can be accessible, against the A1 content, Al/(Si+Al), we obtain a good correlation as depicted in Fig. 5. It is worth noting that not only the acid-base catalysis of proton-exchanged zeolites but also other kinds of catalytic reaction are controlled by the A1 content. In the present NO-Cu zeolite system, the zeolite structure would be the factor determining the effectiveness of Cu2+ ions, and the catalytic activity of the effective Cu2+ ion is probably controlled by the A1 content ... 

Washabaugh, M. W. Jencks, W. P. Thiazolium C(2)-proton exchange general base catalysis, direct proton transfer, and add inhibition. 

Sometimes the observation of general acid catalysis may be sufficient to identify the mechanism with considerable confidence. For example, in aromatic proton exchange catalyzed by ammonium salts, the only structurally attractive A-2 mechanism, shown in equations (4) and (5), violates the principles of microscopic reversibility. Thus the elimination of the A-l mechanism leaves only the A-SE2 mechanism. In a more... 

Fundamental Research that Underlay Development of this Cell. Three U.S. universities were involved in the work that culminated in manufacture of the proton-exchange membrane by Ballard Power Systems. First, Case-Western Reserve University must be recognized because of the sustained investigations there (Yeager et al., 1961-1983) on the mechanism and catalysis of the reduction of02, the reaction that causes most of the energy losses in the fuel cell. The Electrochemistry of... 

As discussed, AdoMet is a high energy compound, and therefore, AdoMet-dependent methylation reactions are known to be irreversible. However, enzymatic catalysis is often required to enhance the rate of the reaction. Enzymes employ a variety of ways to enhance the nucleophilicity on the attacking atom in a substrate. Often, the reaction results in a proton exchange for the methyl group. The proton can be removed before, in concert with, or after the methyl transfer this step usually requires the presence of a general base in the active site. 

Figure 3.5. Overall impedance response of a proton exchange membrane (PEM) fuel cell for different cell temperatures, depicted as corresponding values of the real and imaginary parts of the complex impedance (sometimes denoted a Nyquist plot). Each sequence of points represents frequencies ranging from 10 to 10 Hz, with the highest values corresponding to the leftmost points. From M. Ciureanu, S. Mik-hailenko, S. Kaliaguine (2003). PEM fuel cells as membrane reactors kinetic cinalysis by impedance spectroscopy. Catalysis Today 82, 195-206. Used with permission from Elsevier).

FIGURE 2.51 A mechanism for sodium chloride uptake, as explained in four steps of increasing complexity (a) The sodium/proton exchanger, (b) The sodium/proton exchanger with catalysis of add production by carbonic anhydrase. (c) The sodium/proton exchanger with catalysis of acid production and utilization of bicarbonate by the bicarbonale/chioride exchanger, (d) The finished model for transport of sodium and chloride ions. 

The evidence for diazonium-ion formation in neutral or basic solutions is strong. Nonetheless, a number of serious problems remain. One difficulty is the high reactivity that must be attributed to the diazocompounds. Although aliphatic diazoalkanes can be expected to be particularly reactive towards protonation, the difference between, on the one hand, diazomethane, which requires the presence of a carboxylic acid for the observation of proton exchange at room temperature (van der Merwe et al., 1964) and, on the other hand, diazobutane, which undergoes protonation in methanolic sodium methoxide at —64° (Kirmse and Rinkler, 1962) is somewhat surprising. One would wish to see the acidic character of the solvent catalysis corroborated by a Bronsted relation within which the rate constant for the solvent reaction is compared with that for other molecular acids. 

Heterobimetallic catalysis mediated by LnMB complexes (Structures 2 and 22) represents the first highly efficient asymmetric catalytic approach to both a-hydro and c-amino phosphonates [112], The highly enantioselective hydrophosphonylation of aldehydes [170] and acyclic and cyclic imines [171] has been achieved. The proposed catalytic cycle for the hydrophosphonylation of acyclic imines is shown representatively in Scheme 10. Potassium dimethyl phosphite is initially generated by the deprotonation of dimethyl phosphite with LnPB and immediately coordinates to the rare earth metal center via the oxygen. This adduct then produces with the incoming imine an optically active potassium salt of the a-amino phosphonate, which leads via proton-exchange reaction to an a-amino phosphonate and LnPB. 

Hooger, G. Thompsett, D. Catalysis in proton exchange membrane fuel cell technology. CATTECH 2000, 3 (2), 106-124. 

The surfaces were also characterized by their proton exchange or prototropic catalysis, by their shifts of the diphenyl... 

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