Isotopic Study of Water Gas Reaction Catalyzed by Platinum

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By the use of 13C- and 18O-labelled carbon dioxide and D2 gas, water gas reaction over an evaporated Pt film was studied at temperatures 380–450 °C under constant elimination of water vapor with cold traps. The reaction of CO2 and H2 was as rapid as that of CO2 and D2, the mixture of H2 and D2 changing rapidly into an equilibrium mixture of H2, HD, and D2 in the course of the reaction with CO2. The mixture of C18O2 and CO2 was also rapidly equilibrated by the reaction with H2. The exchange reactions of 18O and 13C between CO2and CO in the course of the water gas reaction were similarly observed. The theory of the stoichiometric number on the rate-determining step was applied to the results. From the results and the observed rate equation of the water gas reaction, V=k[CO2]0.6[H2]−0.5, the reaction was supposed to be caused by the sequence of steps, CO2\rightleftharpoonsCO2(a), H2\rightleftharpoons2H(a), CO2(a)+H(a)\rightleftharpoonsX(a), X(a)\rightleftharpoonsCO(a)+OH(a), CO(a)\rightleftharpoonsCO, and OH(a)+H(a)\rightleftharpoonsH2O, and to be rate-controlled by the third and fourth steps, where (a) denotes the adsorbed state and X(a) is an intermediate to rule out the direct conversion of CO2(a)+H(a) into CO(a)+OH(a).
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