Carbon monoxide oxidation— transient kinetics

The purpose of this article is to review the results of transient low pressure studies of carbon monoxide oxidation over transition metal substrates. Particular emphasis is given to the use of in-situ electron spectroscopy, flash desorption, modulated beam and titration techniques. The strengths and weaknesses of these will be assessed with regard to kinetic insight and quantification. An attempt will be made to identify questions that are ripe for investigation. Although not limited to it, the presentation emphasizes our own work. A very recent review of the carbon monoxide oxidation reaction C l) will be useful to readers who are interested in a more comprehensive view. 

That carbon monoxide could be oxidised in a facile reaction at cryogenic temperature (100 K) was first established in 1987 by XPS at an aluminium surface.21 The participation of reactive oxygen transients O 1 (s) was central to the mechanism proposed, whereas the chemisorbed oxide O2 state present at 295 K was unreactive. This provided a further impetus for the transient concept that was suggested for the mechanism of the oxidation of ammonia at a magnesium surface (see Chapter 2). Of particular relevance, and of crucial significance, was Ertl s observation by STM in 1992 that oxygen chemisorption at Al(lll) resulted in kinetically hot adatoms (Figures 4.1 and 4.7). 

Ertl, G. (1989). The oscillatory catalytic oxidation of carbon monoxide on platinum surfaces. In Spatial inhomogeneities and transient behaviour in chemical kinetics, (ed. P. Gray, G. Nicolis, F. Baras, P. Borckmans, and S. K. Scott), ch. 37, pp. 563—76. Manchester University Press. 

Rival Kinetic Models in the Oxidation of Carbon Monoxide over a Silver Catalyst by the Transient Response Method... 

The platinum catalyzed CO oxidation of carbon monoxide was studied using an advanced transient reactor system, referred to as Multitrack. The experiments indicate, that the reaction is taking place according to the Langmuir-Hinshelwood reaction model. The desorption of CO from the catalyst surface was shown to be a kinetically relevant step in the reaction. From experiments performed using it was shown that isotopic mixing occurs on the catalyst surface due to the decomposition of CO present on the catalyst surface. 

The dynamic phenomena associated with the rhodium-catalyzed oxidation of carbon monoxide, methane and propane have been studied by in-situ infrared thermography. High-resolution temperature maps of the reacting catalyst revealed the mobility of the reaction front during ignition and extinction of the CO oxidation, and the development of thermokinetic oscillations. The catalytic oxidation of methane and propane produced weaker dynamics. Chemisorption and kinetic experiments suggest that the competitive adsorption of the reactants and the occurrence of self-inhibition, represent key factors in the development of the observed transient effects. 

J. Assmann, V. Narkhede, L. Khodeir, E. Loffler, O. Hinrichsen, A. Birkner, H. Over, and M. Muhler, On the nature of the active state of supported ruthenium catalysts used for the oxidation of carbon monoxide Steady state and transient kinetics combined with in situ infrared spectroscopy, J. Phys. Chem. B 108, 14634 (2004). 

Liu W. and Flytzani-Stephanopoulos M. 1995. Total oxidation of carbon monoxide and methane over transition metal-fluorite oxide composite catalysts, J. Catal., 153, 304-316. Sedmak G., HoCevar S. and Levee J. 2004. Transient kinetic model of CO oxidation over a... 

Detailed studies of the coadsorption of oxygen and carbon monoxide, hysteresis phenomena, and oscillatory reaction of CO oxidation on Pt(l 0 0) and Pd(l 1 0) single crystals, Pt- and Pd-tip surfaces have been carried out with the MB, FEM, TPR, XPS, and HREELS techniques. It has been found that the Pt(l 0 0) nanoplane under self-osciUation conditions passes reversibly from a catalytically inactive state (hex) into ahighly active state (1 x 1). The occurrence of kinetic oscillations over Pd nanosurfaces is associated with periodic formation and depletion of subsurface oxygen (Osub)- Transient kinetic experiments show that CO does not react chemically with subsurface oxygen to form CO2 below 300 K. It has been found that CO reacts with an atomic Oads/Osub state beginning at temperature 150 K. Analysis of Pd- and Pt-tip surfaces with a local resolution of 20 A shows the availability of a sharp boundary between the mobile COads and Oads fronts. The study of CO oxidation on Pt(l 0 0) and Pd(l 1 0) nanosurfaces by FEM has shown that the surface phase transition and oxygen penetration into the subsurface can lead to critical phenomena such as hysteresis, self-oscillations, and chemical waves. 

The nickel compounds NiRa(bipy) react with a variety of alkenes, including ethylene, norbomadiene, maleic anhydride, and tetracyanoethylene, to produce Ni(bipy)(alkene) or Ni(bipy)(alkene)2. A kinetic study, supported by characterization of intermediates from the reactions with aorolein and with acrylonitrile, indicates that the reaction mechanism involves the intermediacy of transient complexes NiR2(bipyXalkene), containing the alkene TT-bonded to the nickel as in the products. There is a correlation between the rates of these reactions and the stabilities of the respective complexes Ni(bipy)(alkene). The unstable species CoH(LL)2, where LL = bipy or phen, undergo substitution reactions with, for example, carbon monoxide, in which one LL ligand is replaced. One LL can also be replaced by alkyl halides here the reaction is oxidative elimination rather than simple substitution. ... 

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