Catalytic enhancement

Four general mechanisms account for the ability of enzymes to achieve dramatic catalytic enhancement of the rates of chemical reactions. 

In the presence of Bi or Te, the C=0 bond is weakened, as concluded from the displacement of the CO stretching band to lower wavenumbers. There is also a change in the dependence of the band frequency on electrode potential, with the slope dv/dE increasing for the adatom-modified surfaces. These changes indicate that the adatom alters the electronic properties of the surface, increasing the amount of electronic backdonation and stabilizing the adsorbed CO molecule. No catalytic enhancement is expected from this effect. 

Figure 3.55(b) shows the first sweep of the voltammogram obtained in the presence of C02 and also in Ar saturated solution for comparison. The authors concluded that the reductive scan showed that little or no catalytic enhancement occurs at the Bipy-based reduction potential and that the onset of the catalytic current occurs at -1.4V continuing through the region characteristic of the metal-based reduction process (and the 2e reduction of the dimer) in direct contrast to the results obtained by the majority of other workers (see below). 

Adsorbed nitriles are able to block CO from entering its linear configuration when the adsorption step is performed at 0.05 V(SHE). Adsorbed Sn atoms behave in a manner which supports previous models for their catalytic enhancement of CO oxidation rates. 

Mandelate racemase, another pertinent example, catalyzes the kinetically and thermodynamically unfavorable a-carbon proton abstraction. Bearne and Wolfenden measured deuterium incorporation rates into the a-posi-tion of mandelate and the rate of (i )-mandelate racemi-zation upon incubation at elevated temperatures. From an Arrhenius plot, they obtained a for racemization and deuterium exchange rate was estimated to be around 35 kcal/mol at 25°C under neutral conditions. The magnitude of the latter indicated mandelate racemase achieves the remarkable rate enhancement of 1.7 X 10, and a level of transition state affinity (K x = 2 X 10 M). These investigators also estimated the effective concentrations of the catalytic side chains in the native protein for Lys-166, the effective concentration was 622 M for His-297, they obtained a value 3 X 10 M and for Glu-317, the value was 3 X 10 M. The authors state that their observations are consistent with the idea that general acid-general base catalysis is efficient mode of catalysis when enzyme s structure is optimally complementary with their substrates in the transition-state. See Reference Reaction Catalytic Enhancement... 

MULTIENZYME POLYPEPTIDE AUTONOMOUS CATALYTIC DOMAIN CATALYTIC ENHANCEMENT CATALYTIC PROFICIENCY CATALYTIC EFFICIENCY REFERENCE REACTION Catalytic rate acceleration,... 

These two reaction types are anodic examples of the heterogeneous catalytic enhancement of the rates of sufficiently reactive intermediates, brought about by increasing the local concentration of a reaction partner due to preferential physisorption of the respective moiety. 

Catalytic enhancement The reduction of diffusion time of an intermediate from one enzyme to the next. 

The effects of adsorption on an electrochemical response signal are varied, ranging from complete passivation of the electrode, to catalytic enhancement of a redox process, to the appearance of new electrochemical response signals. Commonly encountered situations are dealt with briefly below. The reader is referred to the works by Bauer [13] and Reilley and Stumm [14] for a more detailed discussion. 

Electronic and structural changes of the semiconductor surfaces may influence light energy conversion yields or catalytic enhancements of desired reactions to chemical fuels. The involved processes are up till now not really understood on a molecular level. 

As part of the early work to find alloys ofplatinum with higher reactivity for oxygen reduction than platinum alone, International Fuel Cells (now UTC Fuel Cells, LLC.) developed some platinum-refractory-metal binary-alloy electrocatalysts. The preferred alloy was a platinum-vanadium combination that had higher specific activity than platinum alone.25 The mechanism for this catalytic enhancement was not understood, and posttest analyses26 at Los Alamos National Laboratory showed that for this binary-alloy, the vanadium component was rapidly leached out, leaving behind only the platinum. The fuel- cell also manifested this catalyst degradation as a loss of performance with time. In this instance, as the vanadium was lost from the alloy, so the performance of the catalyst reverted to that of the platinum catalyst in the absence of vanadium. This process occurs fairly rapidly in terms of the fuel-cell lifetime, i.e., within 1-2000 hours. Such a performance loss means that this Pt-V alloy combination may not be important commercially but it does pose the question, why does the electrocatalytic enhancement for oxygen reduction occur ... 

Ross,33 and Beard and Ross34 had also been interested in electrocatalytic properties of Pt-3d transition metal binary-alloys, with a view that stable intermetallics could be formed. It was also their view that the catalytic enhancement shown by Pt-V, Pt-Cr, and latterly Pt-Co was due to the surface roughening of the platinum crystallites caused by leaching of the non-platinum elements from the surface. In the case of the Pt-Co alloy, they believed that a more stable alloy is formed that protects against further alloy degradation. 

The paradoxical electrochemical behavior of PtSn based catalysts makes its stndy more controversial. Nevertheless, the catalytic enhancement of the electro-oxidation of adsorbed CO on PtSn catalysts is generally accepted. To explain the difference in activi-... 

Control of the temperature throughout the reforming catalyst bed can be established by use of a monolithic catalyst. The heat transfer control can be accomplished by combining three effects that monolithic catalyst beds can impact significantly (1) direct, uniform contact of the catalyst bed with the reactor wall will enhance conductive heat transfer (2) uniformity of catalyst availability to the reactants over the length of the flow will provide continuity of reaction and (3) coordination of void-to-catalyst ratio with respect to the rate of reaction will moderate gas-phase cracking relative to catalytically enhanced hydrocarbon-steam reactions. This combination provides conditions for a more uniform reaction over the catalyst bed length. 

Finally, dynamic Monte Carlo simulations are very useful in assessing the overall reactivity of a catalytic surface, which must include the effects of lateral interactions between adsorbates and the mobility of adsorbates on the surface in reaching the active sites. The importance of treating lateral interactions was demonstrated in detailed ab initio-based dynamic Monte Carlo simulations of ethylene hydrogenation on palladium and PdAu alloys. Surface diffusion of CO on PtRu alloy surfaces was shown to be essential to explain the qualititative features of the experimental CO stripping voltammetry. Without adsorbate mobility, these bifunctional surfaces do not show any catalytic enhancement with respect to the pure metals. 

As a second possibility, one may assume that at low potentials OH adsorption occurs exclusively on Ru sites, in agreement with the bifunctional mechanism for CO oxidation on PtRu alloy surfaces. The observed depletion of CO on the Pt sites can then only be explained by rapid surface diffusion of the adsorbed CO molecules to the reactive Ru islands. Because the separation of the active Ru islands is on the nm scale, even a low mobility of the adsorbed CO can account for the observed catalytic enhancement. 

Four chronoamperometric methanol oxidation curves are displayed in Fig. 25. Curve A was obtained with clean Pt(lll), i.e., with no ruthenium on the surface. The low current at the end of the decay (after 30 min)68 is due to Pt site blocking by adsorbed CO from methanol dehydrogenation.67 The Ru/Pt(l 11) surface obtained after the first Ru deposition with 18 3% ruthenium coverage (see Fig. 16a) exhibited a much higher methanol oxidation current (curve B), showing the large catalytic enhancement of the Ru/Pt( 111) electrode in comparison to clean Pt(l 11). The current measured for the Ru/Pt(lll) surface obtained after two consecutive Ru depositions (see Fig. 16b) was 1.6 times higher than that obtained after... 

Figure 13 Catalytic enhancement of reaction rate in THF-CHClj mixtures. Each bar shows the rate enhancement of the reaction between (4) and the 2,4-dinitrophenyl ester of (12) in the presence of 50% product (13). The system was examined under dilute conditions (0.07 mM ester 0.04 mM amine, 0.035 mM template, 8mM TEA) by monitoring the release of 2,4-dinitropheno using UV-visible spectroscopy...

Synergism via deposition of metal atoms on the surface has been postulated for bimetallic clusters of Pt or Rh with Pb, Tl, Bi, and Cd for HCOOH electrooxidation (278). The catalytic enhancement by the nonnoble surface... 

The TON values of 2/8 MSN-AEP/UDP catalysts were higher than those of the monofunctionalized ones. These unusual catalytic enhancements are strong... 

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