Boreskov Institute of Catalysis of the Siberian Branch of Russian ...

KN-5ISOTOPIC TRANSIENT KINETICS STUDYTO IDENTIFY REACTION MECHANISMSB.S. Bal’zhinimaev, E.M. Sadovskaya, A.P. SuknevBoreskov Institute of Catalysis, pr. Lavrentieva, 5, 630090, Novosibirsk, RussiaFax: 383 330 80 56, e-mail: balzh@catalysis.ruSteady State Isotopic Transient Kinetic Analysis (SSITKA) is one of the most promisingfor study of kinetics and reactions mechanism in situ. Being properly unsteady-state, it allowsto study kinetics under the steady state where the rates of the reaction steps, as well asconcentrations of adsorbed species remain unchanged. Experimentally the dynamics of labeltransfer from feed molecules to reaction products is registrated after stepwise change ofisotope composition in feed gas. Qualitative and quantitative analysis of isotope responsesallows to reveal the key elementary steps involving the labeled molecules, to determineconcentration of intermediate species, as well as the rate and sequence of their transformationon the catalyst surface [1].This overview includes discussions of the experimental features, the mathematicalformalism used in transient analysis and for evaluation of kinetic parameters. The results ofSSITKA study in ethylene epoxidation over silver, selective NO reduction with methane overCo-ZSM-5 and fiberglass based catalysts are also represented.Investigation of the 18 O isotope transfer dynamics showed that different surface oxygenspecies are involving in epoxidation and deep oxidation of ethylene. Formation of epoxidizingoxygen [O e ] proceeds via two pathways: from nucleophilic [O n ] and through sub-surfaceones. Both concentrations of active oxygen, and the rates of ethylene epoxidation and deepoxidation were evaluated. The corresponding reaction rate coefficients were estimated to beabout 10 6 -10 7 s –1 . This example clearly demonstrates that, despite of relatively low timeresolution of the SSITKA (~1 s), it allows to estimate the rates of very fast (microsecondscale) processes [1].More detailed SSITKA study was performed for selective NO reduction with methaneover Co-ZSM-5, where three isotopic labels ( 15 N, 13 С and 18 O) were used. It was found thattwo parallel reaction pathways on cobalt cations and Co 2+ − OH pairs takes place. It wasshown that reaction turnover number (TON) over Co 2+ − OH sites was more than an order ofmagnitude higher compared with that of Co cations. The isotope studies were carried out atrather wide feed gas composition and temperatures. It allowed to find kinetic equations forkey reaction steps and evaluate their rate coefficients and activation energies [2-4].23