Carbon-13 kinetic isotope effects in the catalytic oxidation of carbon monoxide over Pd/Al2O3

The C-13 kinetic isotope effects in the oxidation of CO by oxygen over a 0.5% Pd/gamma-Al2O3 catalyst were experimentally determined in a temperature range of 323-413 K, and the following temperature dependence was found: 100 ln(k(12)/k(13)) = 3.18-831/T (+/-0.15). A reaction gas mixture of CO/O-2 at a ratio of 1:2 and an initial pressure in a static system ranging from 5 to 10 kPa were used. The reaction kinetics were found to be of order +1 in CO, order 0 in oxygen, and order -1 in CO2. Under these experimental conditions, an activation energy of 56 +/- 3 kJ mol(-1) was obtained. Using Bigeleisen's formalism based on the absolute rate theory of chemical reactions, kinetic isotope effects were calculated. For the transition state of the rate-determining step, a (CO2)double dagger of various geometries and force constants was considered. The experimental data can be satisfactorily interpreted only with an interbond angle close to 110 degrees and a reaction coordinate described by an asymmetric normal vibration of an asymmetric transition state.