CO-O2 system

Surprisingly, the origin of the eomplex oseillations and ehaos in the CO + O2 system (where traee quantities of Ft-eontaming speeies have a major infliienoe on the reaetion and, eonsequently, many of the reaetions of the Fl2 + O2 system predominate) are far from established to date. 

Hegedus, L. L., Chang, C. C., McEwen, D. J. and Sloan, E. M., 1980, Response of catalyst surface concentrations to forced concentration oscillations in the gas phase the NO, CO, O2 system over a-alumina. Ind. Engng Chem. Fundam. 19, 367-373. 

Fig. 2. Bifurcation diagrams describing the behavior of the CO/O2 system as CO pressure is varied, (a) CO2 effluent concentration (which is proportional to the reaction rate) as a function of CO inlet concentration at four different temperatures in an atmospheric reactor over a pulverized Pt/silica/alumina catalyst. Oscillation existence regions are indicated by vertical hatching (from Ref. 98). (b) Work function maxima and minima plotted as a function of CO pressure at 540 K on Pt(l 10). The periodicity of the oscillations (as indicated above the curve) is seen to increase as CO pressure is decreased. (From Ref. 231.)...

Additionally, some of the oxidation/reduction models developed so far can be considered buffer-step models. The most extensive theoretical treatment of such a model was performed by Aluko and Chang (273,280,281), who investigated under nonisothermal conditions the model first proposed by Sales et al. (272). The original isothermal model proposed nominally for the CO/O2 system is able to predict oscillations and can be considered a surface reaction model. The model, described in the general terms of Chang and Aluko, consists of... 

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. 

With the COS-O2 system it is possible that the SO is formed in an electronically excited state via (44) and may undergo reaction with O2 more efficiently than S0( 2 ). In this system, attack on the substrate would also be favorable even with ground-state SO, since... 

There is conflict over the results of adsorption of O2 on Pt a recent paper points to saturation coverage at 0 = 0.5 on stepped Pt and on Pt(lll). In the CO + O2 system a form of unreactive O has been identified on Pt. In the CO + O2 reaction on Pd a transient form of CO was identified. The role of steps in the CO + O2 reaction has been established as important on Pt where several stepped surfaces were prepared on a Pt single crystal cylinder. 

Figure A3.14.6. P T ignition limit diagram for CO + O2 system in a flow reactor showing location of ignition limits and regions of simple and complex (shaded area) oscillations. 

The effects of including the triple excitations in coupled cluster linear response theory for evaluating the dynamic polarizabilities have been assessed for a set of closed-shell (Ne, HF, N2, CO) and open-shell (CN, CO, O2) systems, in view of exploring a new accuracy regime for molecular properties. The main conclusions include that i) for systems with little or no static correlation, CC3 is nearly identical to CCSDT, ii) CC3 and PS(T) [pole shifted technique where the CCSD-LR poles are corrected by adding a noniterative correction due to the triples] methods perform better than CCSD but their relative accuracy is not determined yet, iii) differences between CCSD and CC3 results as well as the errors with respect to CCSDT drop when the basis set is increased, and iv) ROHF-based CC-LR approaches should be favored over their UHF counterparts while the dilfer-ences between the ROHF and UHF appear as an appropriate criterion for determining whether higher-order UHF-based CC calculations can be used. 

Many simple systems that could be expected to form ideal Hquid mixtures are reasonably predicted by extending pure-species adsorption equiUbrium data to a multicomponent equation. The potential theory has been extended to binary mixtures of several hydrocarbons on activated carbon by assuming an ideal mixture (99) and to hydrocarbons on activated carbon and carbon molecular sieves, and to O2 and N2 on 5A and lOX zeoHtes (100). Mixture isotherms predicted by lAST agree with experimental data for methane + ethane and for ethylene + CO2 on activated carbon, and for CO + O2 and for propane + propylene on siUca gel (36). A statistical thermodynamic model has been successfully appHed to equiUbrium isotherms of several nonpolar species on 5A zeoHte, to predict multicomponent sorption equiUbria from the Henry constants for the pure components (26). A set of equations that incorporate surface heterogeneity into the lAST model provides a means for predicting multicomponent equiUbria, but the agreement is only good up to 50% surface saturation (9). 

For the reduction of NO with propene, the catalyst potential dependence of the apparent activation energies does not show a step change and is much less pronounced than it is for the CO+O2 and NO+CO systems. There is persuasive evidence [20] that the step change is associated with a surface phase transition - the formation or disruption of islands of CO. It is reasonable to assume that this phenomenon cannot occur in the NO+propene case, since there is no reason to expect that large amounts of chemisorbed CO can be present under any conditions. That there should be a difference in this respect between CO+O2/CO+NO on the one hand, and NO+propene on the other hand, is therefore understandable however, the chemical complexity of the adsorbed layer in the NO+-propene precludes any detailed analysis of the Ea(VwR> effect. 

The equations are written for the specific reaction of NH3 formation as a fully general approach would be unwieldy. The modification of the approach to other reactions is not trivial but could be done following the outline below. Another application of the formalism to a very complex reaction system (CO + O2 on a Pt/Sn disordered catalyst) is demonstrated, as well as the generality of the stochastic ansatz [28],... 

The essential point is that the initiation steps provide H atoms that react with the oxygen in the system to begin the chain branching propagating sequence that nourishes the radical reservoir of OH, 0, and H that is, the reaction sequences for the complete H2-O2 system must be included in any high-temperature hydrocarbon mechanism. Similarly, when CO forms, its reaction mechanism must be included as well. 

In 2001, Mdstern reported on the oxidative alkenylation of arenes with olefins using a RU/O2/CO catalyst system (Eq. 9.12) [26], but details of the reaction mechanism have not been elucidated. Very recently, Gunnoe reported ethylation and propylation of ben-... 

Nearly all workers detect the first and second limits of explosion, and the third limit has been detected in the presence of a trace of moisture or hydrogen. However, it is noticeable that the positions of these limits may vary considerably from one group of researchers to another indeed, in the case of a dry CO/O2 mixture, the limits sometimes differ from one similar vessel to another, despite identical experimental technique. It emerges that the surface can play a complex and confusing role in the combustion of CO, and that the system is very sensitive to traces of hydrogenous impurities which can make experimental work both arduous and tedious. 

Practically, one of the most important reactions of CO is its exothermic oxidation to CO, from which a very large proportion of the world s useful energy is derived. It has been known for many years that dry CO/O2 flames are difficult to ignite and have lower burning velocities than similar flames derived from moist CO [354]. In this particular, the explosion and flame systems behave analogously. 

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