Transient methods in heterogeneous catalysis: Experimental features and application to study mechanistic aspects of the CH4/O-2 (OCM), NH3/O-2 and NO/He reactions

Experimental features of the transient method using concentration forcing functions to study the surface dynamics of a heterogeneous catalytic reaction at 1 atm and with on line mass spectroscopy are discussed. Emphasis is given in the presentation and discussion of two important transient techniques, namely the 'steady state tracing' or 'steady state isotopic transient kinetic analysis (SSITKA)' and the 'transient isothermal hydrogenation' of adsorbed surface reaction intermediate species. The SSITKA technique allows to measure the concentration of the active reaction intermediate species, the intrinsic reactivity of the site associated with a given adsorbed reaction intermediate species and elementary step, and also velocities of elementary steps in the reaction network. The 'transient isothermal hydrogenation' technique, when applied in an appropriate reaction system and coupled with the SSITKA technique, allows for the measurement of concentration of inactive (spectator) reaction intermediate species. in addition, important intrinsic kinetic parameters and other mechanistic information related to the hydrogenation process can be obtained. The bulk of this paper concerns the application of transient methods (various kinds of transient experiments) in order to elucidate mechanistic aspects of: (a) the oxidative coupling of methane (OCM) reaction to C-2-hydrocarbons over mixed metal oxide catalysts, where a synergy phenomenon between the two components of the mixed metal oxide catalyst occurs, (b) the ammonia oxidation reaction over V2O5/TiO2 catalysts, and (c) the NO decomposition reaction over Rh supported on TiO2 and W6+-doped TiO2 carriers.