Deuterium exchange catalytic

Recently, other authors when studying the activation of hydrogen by nickel and nickel-copper catalysts in the hydrogen-deuterium exchange reaction concentrated for example only on the role of nickel in these alloys (56) or on a correlation between the true nickel concentration in the surface layer of an alloy, as stated by the Auger electron spectroscopy, and the catalytic activity (57). 

The catalytic activity in relation to a given reaction occurring on the surface is characterized by the rate g of this reaction, i.e., by the amount of reaction products formed under the given external conditions per unit time on unit surface area. An expression for g has different forms for different reactions. For the reactions of hydrogen-deuterium exchange, oxidation of CO, and synthesis of H2O2, this expression will be derived in Sections III, IV, and V, respectively. 

The growth of the catalytic activity of Si02 with respect to the hydrogen-deuterium exchange reaction upon addition of a donor impurity to specimens has also been observed by Taylor and his colloborators (31). 

A number of works have been devoted to the effect of preadsorbed foreign gases on the catalytic activity of a semiconductor in relation to the hydrogen-deuterium exchange reaction. 

Freund (44) studied the influence of ultraviolet light on the catalytic activity of zinc oxide in relation to the reaction of hydrogen-deuterium exchange. The author noted that the photocatalytic effect was positive and that it decreased with rising temperature. 

Boreskov and co-workers (45) point out that on y irradiation the specific catalytic activity of silica gel with respect to the hydrogen-deuterium exchange reaction first increases with increasing radiation dose and then attains saturation at a sufficiently large dose. 

The great majority of experimental data (see Section III.A) indicate that the hydrogen-deuterium exchange reaction belongs to the class of acceptor reactions (i.e., reactions that are accelerated by electrons and decelerated by holes). This means that the experimenter, as a rule, remains on the acceptor branch of the thick curve in Fig. 8a, on which the chemisorbed hydrogen and deuterium atoms act as donors. Here a donor impurity must enhance the catalytic activity, while an acceptor impurity must decrease it. This is what actually occurs, as we have already seen (see Section III.A). 

We see that the correlation between the electrical conductivity of a specimen and its catalytic activity established by the electronic theory (1) must show up distinctly and in fact reveals itself in the case of the hydrogen-deuterium exchange reaction. 

There is considerable evidence available which indicates that these surface Voh centers are the active sites for the irradiation induced catalytic activity of MgO for the hydrogen-deuterium exchange reaction (143). In particular, a correlation exists between the Voh center concentration and the induced catalytic activity (1) for samples degassed at different tem-... 

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... 

Harrison and McDowell 138) observed that while neither zinc oxide nor a, o-dip enyl-/3-picryl hydrazyl (a solid free radical) alone, catalyze hydrogen-deuterium exchange measurably at 77° K, a mixture of the two solids possesses considerable catalytic activity. They suggested that the effective catalyst in this mixture is the zinc oxide and that its catalytic activity is enhanced by electron transfer to the a,a-diphenyl-/3-picryl hydrazyl. It should be noted, however, that this implies a dependence of catalytic activity on electron concentration which is opposite to that observed by Molinari and Parravano 134). To bring the results for this system into line with those obtained by the latter workers, it would be necessary to postulate electron transfer in the opposite direction, i.e., from the a, a-diphenyl-/8-picryl hydrazyl to the zinc oxide. ... 

Voltz and Weller 14S) measured the activity of chromic oxide for hydro-gen-deuterium exchange at —78° and —195°, after pretreatment at 5(X)° in an atmosphere of oxygen or hydrogen. They found the catalytic activity of the reduced state to be higher than that of the oxidized state, although the latter had a higher concentration of defects (positive holes) responsible for electrical conductivity. The relation between catalytic activity and conductivity is thus opposite to that for zinc oxide, although in both cases the activity appears to increase with the electron concentration. The interpretation advanced earlier for zinc oxide has also been extended to chromic oxide (Baker and Jenkins, 14S). 

Because zinc oxide is a relatively well-understood oxide semiconductor, we shall first review its properties as a hydrogenation catalyst in the catalytic hydrogen-deuterium exchange reaction. Since the latter essentially measures the rate of reversible chemisorption of hydrogen at equilibrium, data on the hydrogen chemisorption will be included in this survey. Any theory of hydrogen chemisorption on zinc oxide must explain all the following well-established facts. 

H. J. Koch and R. S. Stuart, A novel method for specific labelling of carbohydrates with deuterium by catalytic exchange, Carbohydr. Res., 59 (1977) C1-C6. 

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