Transient isotopic kinetic studies

The utility of the isotopic transient kinetic studies can be illustrated by considering a simple reaction of A giving product B through a single adsorbed intermediate I ... 

Since the isotopic transient technique involves the number and type of intermediates on the catalyst surface, independent transient experiments (with or without the use of isotopes) have also been used to determine these parameters. The simplest reaction for analysis by the isotopic transient kinetic technique for the conversion of syngas is the production of methane. Studies of methanation provide a background to the isotopic transient kinetic studies and independent justification for the number and type of adsorbed species involved in FTS. Furthermore, the production of methane is undesirable for FTS and an understanding of the mode of its production will aid in FTS catalyst and process design. 

Isotopic Transient Kinetic Studies of the FTS. - Unlike methanation, the Fischer-Tropsch reaction produces a variety of hydrocarbon products having multiple carbon... 

The results for nickel and iron catalysts imply that the reaction scheme given in Figures 48 and 49 do not represent the FTS adequately. A hybrid mechanism of both CO and CH insertion into growing chains has been proposed for nickel catalysts. On iron catalysts the monomer building blocks are proposed to be heterogeneous. Thus, isotopic transient kinetic studies have provided a more detailed understanding of the Fischer-Tropsch reaction. 

Rothaemel, M., Hanssen, K.F., Blekkan, E.A., Schanke, D., and Holmen, A. 1997. The effect of water on cobalt Fischer-Tropsch catalysts studied by steady-state isotopic transient kinetic analysis (SSITKA). Catal. Today 38 79-84. 

Other types of discontinuous flow reactors may be used for transient kinetic studies in which, for example, isotopically labelled reactants are widely used in kinetic studies. This kind of investigation has been particularly developed in the case of discontinuous flow reactors. A different instrument called TAP (for Temporal Analysis of Products) permits extremely sensitive detection of intermediates or products. The results obtained with these various instruments can provide information about reaction mechanisms. 

In a recent review work (117) on the chemical and nano-structural characterization of NM/CeO catalysts, a detailed study of the H interaction with a Pt/CeO catalyst reduced at temperatures ranging fh)m 473 K to 773 K is reported. The experimental techniques used in this work were TPD-MS and Isotopic Transient Kinetics (ITK) of the H2/D2 exchange at 298 K. The catalyst sample was carefully selected in order to minimise the Pt and support sintering effects in the investigated range of reduction temperatures. Likewise, a chlorine-free metal precursor, [Pt(NH3)4](OH)2, was used in the preparation of the catalyst. 

Detailed Isotope Transient Kinetics (ITK) studies of the effects of ageing on Pt/Rh and Pd catalysts have revealed a number of interesting general features about thermal ageing and sulphur poisoning effects within autocatalysts. The major observations can be summarised as follows ... 

A large amount of N2O was formed from the initial stage over LaM03 (M = Co, Mn, Fe, Cr, Ni) at 573 K. The time course of the NO+CO reaction (performed in a batch recirculation system) reflects this situation. These results support a two-step reaction pathway in which N2O is an intermediate for nitrogen formation, deal et al. (1994) confirm the role of N2O as intermediate in this reaction over perovskite oxides. They used steady-state isotopic transient kinetic analysis to study the mechanism of NO + CO reaction over LaCo03. They concluded that N2O was an intermediate in the formation of N2 at T < 873 K. They also concluded that at high temperature CO2 desorption became the rate-limiting step of the overall reaction. This is likely due to the rapid formation and slow decomposition of very stable carbonates on the perovskite surface as reported by Milt et al. (1996). 

The effect of water on the Fischer-Tropsch synthesis over alumina-supported cobalt catalysts has been studied using isotopic transient kinetic methods (SSITKA) in combination with steady-state measurements. Water has been introduced to the catalytic system as a pretreatment procedure as well as under reaction conditions. The SSITKA results showed a decrease in the number of active surface sites, but no change in the specific site activity. 

In the present communication we report on the influence of water on the FT synthesis studied by SSITKA and conventional kinetic experiments. Steady-state isotopic transient kinetic analysis (SSITKA) has proved to be a powerful technique for this work. The technique involves switching between CO and " CO in the feed gas and analyzing the transients with respect to the formation of products containing C and C. This technique allows the determination of the true turnover frequency of the active site, decoupled from site coverage. Applied to the FTS over metal promoted cobalt catalysts SSITKA has shown that the true turnover frequency of cobalt always remains the same, regardless of the second metal [6-8]. 

The use of transient kinetic studies using isotopes of the reactants provides a rich source of information about the mechanism, rates of elementary steps, and concentration of adsorbed reaction intermediates of the FTS. Reviews of the isotopic transient kinetic technique and its application for a number of reactions have been given by Bennett, Mirodatos " and Happel some work was reviewed in an earlier volume of this series. 

Transient Isotopic Kinetic Studies of Methanation. - The Fischer-Tropsch reaction results in the formation of a wide distribution of hydrocarbons containing different numbers of carbon atoms. In contrast, the related reaction of methanation of CO/H2 mixtures involves only one product and is easier to study using isotope transient kinetic techniques. The results of the methanation reaction have a direct relevance to the Fischer-Tropsch reaction and are reviewed below. 

In some cases the so-called "Steady-State Isotopic Transient Kinetic Analysis" (SSITKA) was used for detailed investigations of reaction mechanisms. Shannon and Goodman [123] present an extensive review of this subject. Hinrichsen et al. [124] employed temperature programmed desorption to study the ammonia synthesis on ruthenium catalysts. 

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... 

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 flows, 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... 

ISOTOPE TRANSIENT KINETICS APPLICATION EOR THE STUDY OF MASS TRANSFER PROCESSES... 

---