The CO O2 reaction

Johnson B R and Scott S K 1990 Period doubling and chaos during the oscillatory ignition of the CO + O2 reaction J. Chem. Soc. Faraday Trans. 86 3701-5... 

J. Mai, W. von Niessen. The CO -(- O2 reaction on metal surfaces. Simulation and mean-field theory The influence of diffusion. J Chem Phys 95 3685-3692, 1990. 

Figure 2.1S. The rate of CO2 formation in the CO + O2 reaction on Pt(l 110) oscillates synchronously with the surface reconstruction, from (1x1) to (1x2), shown in Fig. 2.14. [Adapted from M. Eiswirth, P. Moeller, K. Wetzl, R. Imbihl, and G. Ertl, /. Chem. Phys. 90 (1989) 510.1...

Carbon monoxide adsorbed on sufficiently small palladium particles disproportionates to surface carbon and carbon dioxide. This does not occur on large particles. The CO-O2 reaction is shown to be structure-insensitive provided the metal surface available for the reaction is estimated correctly. 

Thus, although CO disproportionation is structure-sensitive, the CO-O2 reaction appears to be structure-insensitive at both 445 K and 518 K, provided we define correctly the number of Pd sites available for reaction at both temperatures. It should also be noted that the turnover rate at 445 K is the same on metal particles supported on 1012 a-A O, 0001 a-A O, and... 

Fig. 4.51. Steady-state rates of CO2 production for the CO + O2 reaction over the (111), (100), (410) and (210) surface of a Pt-0.25-Rh-0.75 single crystal (from ref. 88).

Dissociation is often a crucial step in a catalytic reaction chain. However this is not always the case. Metals active for the O2 + H2 reaction are the noble metals of group 8-10, especially Pt, a metal with a low O-metal bond strength. The same metals are active for the CO + O2 reaction. The most active metals do not easily break the C-O bond and do not form oxides under reaction conditions. Both reactions are Langmuir-Hinshelwood reactions between adsorbed O and adsorbed H or CO. Desorption of products of the reaction intermediates here is reaction rate controlling. 

The gas phase homogeneous catalysis of the CO + O2 reaction in shock waves by addition of chromium, iron and nickel carbonyls has been described by Izod et al. [507], and Matsuda [508, 509]. It will not be discussed further here. 

The reader will discover in what follows that most of the models were developed for the CO/O2 reaction on noble metals. This is due to the fact that these have been the most intensely investigated systems and the fact that sometimes contradictory results have been published, thus requiring adaptation and refinement of accepted models in order to explain new experimental findings. Further, other reactions are more complex and less understood than CO/O2, thus researchers have characterized the oscillations in these systems, but often did not attempt to construct detailed models. On the other hand, mechanisms for oscillations in other systems are sometimes almost unanimously accepted, and this leaves no incentive for the development of competing models. 

Species B represents a less reactive species that acts as a buffer. The formation and removal of this buffer is slow compared to the main reaction given by Eq. (14). Further application of this model to the CO/O2 system will be discussed in Section IV,B. Chang and Aluko were able to predict the bifurcation and oscillatory behavior of the CO oxidation reaction and exploited the oscillatory data to obtain refined estimates for the reaction parameters of the CO/O2 reaction. 

Morton and Goodman determined whether oscillations could be predicted for various values of m, n, o, and p. Their analyses showed that even for the simple Langmuir-Hinshelwood mechanism of the CO/O2 reaction, oscillations are in principle possible if the reactor mass balance is taken into account. However, the amplitudes of the resultant numerically simulated oscillations were very small. 

The use of IR spectroscopy and solid electrolyte potentiometry has provided additional experimental evidence for these types of models. Lind-strom and Tsotsis (93,120) report an absorption band around 2120 cm for the CO/O2 reaction on Pt in addition to the typically observed Pt-CO band around 2070 cm". This new band was assigned to CO molecules adsorbed on an oxidized Pt surface. The intensity of this band oscillated counterphase to the reaction rate, but with a much lower amplitude than the 2070-cm" ... 

The role of NO during the CO/NO reaction is analogous to that played by oxygen in the CO/O2 reaction discussed above. Different sticking coefficients have been measured on the two forms of the (100) surface for NO as well 319). A model similar to that of Imbihl et al. (52) for CO/O2 has been proposed by the same group for NO/CO on Pt(lOO) (144). The hex 1 X 1 phase transition was modeled in the same manner, and oscillations and multiple steady states similar to experiments were predicted. As discussed in Section IV,B,1, this model could also display oscillations at lower temperatures, at which the phase transition was not involved. This general model of the hex 1 x l phase transition may also be applicable to the recently discovered oscillations in the NO/H2 reaction on Pt(lOO) (2/5). 

A third Pt surface on which oscillations of the CO/O2 reaction have been observed is the (210) plane (75,78). In this case, a surface-phase transition was again proposed as an explanation for the oscillations. The (210) surface was observed to facet into (310) and (110) orientations during an induction period, after which oscillations began to occur on the (110) facets (78). An alternative model for the oscillatory behavior has been proposed by Ehsasi et al. (76). 

Another mechanism also belongs to this class of models, although in this case there is no phase transition in the first layer of the catalyst, but rather in the adsorbate. Wicke and Bocker (125,126) proposed a model in which a CO-induced transition of the 2 x 2 oxygen structure to the more reactive Vs X Vs// 30° structure is the key step in an oscillation cycle of the CO/O2 reaction on Pd. This was concluded from IR spectra, wherein the characteristic bands for the two different adsorbate structures were identified in the respective phases of the oscillation cycle. Similar results... 

At the root of these behaviors is the phenomenon of synchronization of a macroscopic oscillating system. This topic has been discussed occasionally in the literature but has rarely been explicitly treated. In principle there are several stages or hierarchical levels on which oscillations can occur (1) the single-crystal plane, (2) the catalytically active metal crystallite, (3) the catalyst pellet, (4) arrangements of several pellets in one layer of a flow reactor or a CSTR, and finally (5) the catalytic packed-bed reactor (327). On each of these levels, different types of oscillations may exist, but to become observable on the next level oscillations on the respective sublevels must be synchronized. For example, if oscillations of the CO/O2 reaction on a Pt(lOO) face of a Pt crystallite supported on a pelletized support material in a packed-bed reactor occur, the reaction on the (100) facet as a whole must oscillate in synchrony, other (100) facets of the crystallite have to synchronize, other crystallites in the pellet must couple to the first crystallite, and, finally, all pellets in one layer of the bed must display oscillations in synchrony. If the synchronization on one of these levels fails, different oscillators will superimpose and their effects will cancel. One would then only observe a possible increased level of noise in the measured conversion. On the other hand, if synchronization occurs independently over several regions of a system, then it might exhibit apparently chaotic behavior caused by incomplete coupling. 

Fig, 21. The influence of coupling strength on oscillations of three Pt/AUOs pellets during the CO/O2 reaction. Top, no coupling middle, weak coupling bottom, strong coupling. (From Ref. 103.)... 

Large differences were found by Siera et al. (45) in the steady-state reaction rates of both reactions catalyzed by four Pto.25-Rho.75 alloy surfaces in the low-pressure range. An example is shown in Fig. 25 for the NO CO reaction and in Fig. 9 for the CO + O2 reaction. 

The apparatus used for these experiments was dedicated to the study of the CO+O2 reaction which was carried out under transient conditions at any given frequency close to I Hz. This system consists in two parts the gas manifold and the mass spectrometer. The global experimental setup is schematized on Fig. 7.8. 

In well-stirred flow reactors, the CO + O2 reaction supports five different modes of response [63,64] steady slow reaction, steady glow, oscil-... 

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