Steady-state isotopic transient kinetic analysis catalyst surface

In SSITKA (steady-state isotopic transient kinetic analysis) developed and actively applied by Happel, Biloen and Goodwin, it is common to consider the catalyst surface to be composed of a system of interconnected pools, also termed compartments, where each pool represents a homogeneous or well-mixed subsystem within the reaction pathway. 

The greater activity of Pd for methanol decomposition reaction was also found by using the steady state isotopic transient kinetic analysis (SSITKA) method over noble metal (Pt, Pd, Rh)/ceria catalysts. Their activity increased in the order Rh < Pt < Pd, while the by-products were (i) methane, carbon dioxide, water, methyl formate and formaldehyde in most cases and (ii) ethylene and propylene, formed only over Rh/Ce02, at 553 K. SSITKA measurements indicated that two parallel pools exist for the formation of CO (via formation and decomposition of formaldehyde and methyl formate). The difference in the activity order of noble metal/ceria catalysts seems to correlate with the surface coverage of active carbon containing species, which followed the same order. The latter implies that a part of these species is formed on the ceria surface or/and metal-ceria interface. ... 

Temperature-prograimned reduction, oxidation and desorption (TPR, TPO, TPD), belong probably to the most widely used in situ techiuques for the characterization of oxidation catalysts and are discussed in more detail in Section 19.4. While TPD (with ammonia as the probe molecule) is frequently used to examine surface acid sites, TPR and TPO (with H2 or O2, respectively) provide information on the redox properties of oxide catalysts being crucial for their performance in catalytic oxidation reactions. Important information on reaction mechanisms can be obtained when the catalysts are heated in the presence of reactants combined with mass spectrometric product analysis. This is called temperature-programmed reaction spectroscopy (TPRS). As far as reaction mechanisms and kinetics are concerned, transient techniques which reflect the response of the catalytic system to a sudden change of reactant are inevitable tools. Two such techiuques, namely the temporal analysis of products (TAP) reactor and steady-state isotopic transient kinetic analysis (SSITKA) will be described in more detail in Section 19.5. 

However, the carbonate mechanism is favored by some authors. Using a combination of DRIFTS and steady-state isotopic transient kinetic analysis (SSITKA) Meunier et al. assessed the reactivity of the species formed at the surface of an Au/Ce(La)02 catalyst during the WGS reaction. The analysis revealed that surface formates are not important factors in the WGS reaction mechanism. Their role is ascribed to minor reaction intermediates but not spectators because they nevertheless participate in the formation of reaction product. 

The distribution of the isotopic labels is dependent upon the steady-state transfer rates, between pools. Analysis of SSITKA is rather complicated and is outside of the scope of the present text. It can be summarized, that SSITKA is a powerful technique to determine in situ kinetic information about the catalyst-surface reaction intermediates and mechanisms at steady state without substantial disturbance of the catalyst-surface behavior, contrary to some other transient techniques. 

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