The C-13 kinetic isotope effects in the oxidation of CO
by oxygen over a 0.5% Pd/γ-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/O2 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)⧧ of various
geometries and force constants was considered. The experimental
data
can be satisfactorily interpreted only with an interbond angle close to
110° and a reaction coordinate described
by an asymmetric normal vibration of an asymmetric transition
state.
The 15N and 18O kinetic isotope effects (KIEs) in the catalytic decomposition of nitrous oxide on NiO powder
were determined in the temperature range of 625−825 K, and the following temperature dependencies were
found: KIE(15N) = (0.821 ± 0.180) + (1445 ± 128)/T and KIE(18O) = (1.384 ± 0.124) + (1450 ± 89)/T.
At initial pressures of N2O between 40 and 60 kPa, the reaction is first order in N2O and has an apparent
activation energy of 131 ± 3 kJ mol-1. According to Bigeleisen's formalism, a bent, product-like NNO
transition state and a fork-type NNOO transition state best fit the experimental results.
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