Mass Spectrometry in the SSITKA Studies

Abstract

Study of the dynamics of isotope transfer under steady state reaction conditions, the so called steady state isotopic transient kinetic analysis ( SSITKA ), is successfully employed in the investigation of different physical and chemical processes that occur on the surface of solids and in their lattice. The theoretical basis of SSITKA was initially developed and used to study the mechanism of catalytic reactions [1 – 4] . SSITKA applications were later extended to the reactions complicated by mass transfer processes as well as for the characterization of the catalyst acidity. Such a technique is based on the analysis of the isotope responses that result from the stepwise change of isotopic composition of the feed gas at equilibrium. Whereas most of methods used for the identifi cation of reaction intermediates or catalyst characterization operate at low temperature and pressure, such measurements can be performed in the conditions of interest that successfully solves the well known problems associated with “ pressure and temperature gaps. ” Therefore, the reliability of measured characteristics is substantially increased. This chapter includes a discussion of the general principles of SSITKA as well as the experimental and theoretical approaches that are used to study the isotopic transient kinetics, reactions of label transfer as applied to the problem to be solved. The dynamic features of the experimental setup that includes the stepwise disturbing system of feed gas fl ows, catalytic reactors, and mass spectrometric analysis are considered in detail. As a result, the reactor and mass spectrometers that are most convenient for dynamic isotopic studies were selected. The theoretical study of isotopic transient kinetics allowed for the separation of fi ve basic reaction pathways, which serve as elementary “ bricks ” to build virtually any reaction mechanism. It is shown that the key features of the reaction mechanism can be revealed directly from the shape of isotope response curves, while the detailed characteristics can be derived from numerical modeling by fi tting the experimental data with calculated ones. By the example of various catalytic reactions (ethylene epoxidation over Ag, deNOx with methane on Co ZSM 5, Fischer – Tropsch synthesis over Co based systems), we show how the reaction mechanism was revealed and the concentrations of key intermediates and reaction rate coeffi cients were estimated using different isotope labels ( 18 O, 15 N, 13 C, etc.). A single instance of transient isotopic kinetic study for the evaluation of mass transfer parameters of NO and water diffusion into the bulk of Pt containing fi berglass catalyst is discussed. Our experience helped to elucidate the mechanism of oxygen transport in oxides with perovskite and fl uorite lattice at high temperatures (up to 900 ° C). All kinetic parameters including the diffusion coeffi cients were obtained. The probability of H/D exchange for the quantifi cation of Br o nsted acidity in fi berglass materials is also discussed.