Transient isotopic kinetics study to investigate reaction mechanisms

This overview includes discussion of the principles, experimental and theoretical features of the isotopic transient kinetics for the study of reaction mechanism and evaluation of kinetic parameters. The results of the mechanistic study of ethylene epoxidation over silver, selective NO reduction with methane over Co-ZSM-5 and fiberglass based catalysts are presented. Investigation of 18O isotope transfer dynamics allowed to reveal the reaction pathways towards ethylene oxide and CO2 formation as well as to estimate the concentrations of active oxygen species and reaction rate coefficients of key steps. It was found that both ethylene epoxidation and deep oxidation proceed very fast in the microsecond range. A detailed SSITKA study of the NO reduction with methane using 15N, 13C and 18O labels showed that reaction occurred via two parallel pathways involving in nitrite–nitrate and NO2δ+ species, the last ones being 20 times more reactive. For the first time a transient isotopic kinetic study was used for evaluation of mass transfer parameters by an example of NO and water diffusion into the bulk of Pt-containing fiberglass catalyst.