Zinc oxide hydrogen-deuterium exchange

Thus, Kohn and Taylor (40) point out that the y irradiation of zinc oxide which speeds up the reaction of hydrogen-deuterium exchange lowers the magnitude of the effect when a donor impurity is introduced into the specimen. 

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. 

The introduction of an impurity into a specimen (accompanied by a change in tv and es ) will transfer us from one point to another in Fig. 9. Suppose that when a donor impurity is introduced into the specimen (decrease in v and e8 ), we are transferred from the point A to the point B. This involves a decrease in K, as can be seen from Fig. 9. Such a decrease in the photocatalytic effect caused by the addition of donor impurities has been observed by Kohn and Taylor (40) who studied the photoreaction of hydrogen-deuterium exchange on zinc oxide exposed to y radiation. 

Reversible chemisorption and hydrogen-deuterium exchange on zinc oxide have been observed (Taylor et al., 136,137 , Harrison and McDowell,... 

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. ... 

III. The Hydrogen-Deuterium Exchange on Defect Zinc Oxide. 56... 

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. 

The complexity of the adsorption process, in particular its duality as illustrated above as well as in more recent data of Wicke (12), also shows in the irregular behavior of zinc oxide as a catalyst for the hydrogen-deuterium exchange (13). Thus this reaction proceeds at measurable rates at temperature as low as 14O K., indicating that at least part of the low-temperature adsorption is of the dissociating type. The apparent activation energy at low temperatures is low, but in the temperature... 

Fig. 3. Relation between frequency factors and heats of activation for the hydrogen-deuterium exchange reaction on zinc oxide catalysts (ref. 28a).

In this connection, the dependence of the rate of the hydrogen-deuterium exchange reaction catalyzed by zinc oxide, upon the concentration of a foreign oxide in ZnO is interesting. This system was investigated by Molinari and Parravano (94). As shown in Fig. 16, the rate of... 

Smith and Taylor (50) studied the hydrogen-deuterium exchange on zinc oxide in the temperature range where Type B is predominant. They found that the rate of the exchange corresponds to Type A adsorption. This is readily explained by our model, as the exchange rate will depend on adsorption sites (O ions) already prepared, hence on Type A reactions,... 

In considering the properties of the solid surface and its influence on the chemistry of the reactants, I should like to recall to your attention papers by Harrison and McDowell (9) which merit, I believe, a measure of careful consideration. The authors were principally concerned with a detailed and quantitative examination of the phenomenon published in 1941 by Turkevich and Selwood. These authors had found that a mixture of zinc oxide and a,a-diphenyl- 3-picrylhydrazyl was much more powerfully a converter of para- to orthohydrogen than would be concluded on the basis of the mixture law and their separate activities in the conversion process. This phenomenon can be rationalized on the basis of concepts developed by Wigner. The more recent paper of Harrison and McDowell demonstrates, however, that, whereas neither the hydrazyl nor zinc oxide has any marked ability to produce the hydrogen-deuterium exchange reaction at 77° K, the reaction proceeds on the mixture at a rapid and reproducible rate, 2.4 times faster than the parahydrogen conversion on the mixture at the same temperature ( ) and 81 times faster than it would have occurred on the zinc oxide constituent. 

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). 

Dowden et al. (192) investigated the deuterium exchange of hydrogen on zinc oxide. 

The great advantage of the study of exchange reactions of isotopic molecules on catalysts is that only one molecular species is involved both as reactants and products. One is freed from the restrictions imposed with two reactants where the displacement of one reactant by another or by a reaction product must steadily be taken into consideration. The catalysts which reveal heterogeneity by desorption-readsorption studies should show the same variation in activation energy of reaction with temperature that Taylor and Smith found with zinc oxide in the hydrogen-deuterium reaction. Research in this direction is under way. 

There are few reports of alkene-deuterium reactions on bimetallic catalysts, but those few contain some points of interest. On very dilute solutions of nickel in copper (as foil), the only product of the reaction with ethene was ethene-di it is not clear whether the scarcity of deuterium atoms close to the presumably isolated nickels inhibits ethane formation, so that alkyl reversal is the only option, or whether (as with nickel film, see above) the exchange occurs by dissociative adsorption of the ethene. Problems also arise in the use of bimetallic powders containing copper plus either nickel, palladium or platinum. Activation energies for the exchange of propene were similar to those for the pure metals (33-43 kJ mol ) and rates were faster than for copper, but the distribution of deuterium atoms in the propene-di clearly resembled that shown by copper. It was suggested that the active centre comprised atoms of both kinds. On Cu/ZnO, the reaction of ethene with deuterium gave only ethane-d2. as hydrogens in the hydroxylated zinc oxide surface did not participate by reverse spillover. ... 

---