Rate expressions carbon monoxide oxidation

Figure 10.1 shows the fitting of a rate data set for carbon monoxide oxidation to the expression ... 

One of the inconveniences of TS-PFR methods is that it is difficult, in fact impossible, to compare conversions calculated using the fitted rate expression with raw TS-PFR data. This point was raised previously in connection with the investigation of carbon monoxide oxidation. One can simulate kinetic behaviour using the above equations and parameters to produce the expected isothermal behaviour of the system but not that observed in the experimental results that are obtained from the TS-PFR during temperature ramping. This is unavoidable and results from our lack of knowledge of the axial temperature profile in the experimental set-up. 

These expressions are valid for carbon monoxide and oxygen in the range between 10 and 50 000 ppm. For hydrogen oxidation, the rate equation is valid for mole fractions exceeding 40%. Both RWGS and hydrogen oxidation are inhibited by carbon monoxide and therefore have a negative reaction order. 

To date, numerous model compounds simulating the pollutants in common waste streams have been studied under laboratory-scale conditions by many researchers to determine their reactivities and to understand the reaction mechanisms under supercritical water oxidation conditions. Among them, hydrogen, carbon monoxide, methanol, methylene chloride, phenol, and chlorophenol have been extensively studied, including global rate expressions with reaction orders and activation energies [58-70] (SF Rice, personal communication, 1998). 

According to the International Union of Pure and Applied Chemistry (IUPAC O)) the turnover frequency of a catalytic reac tion is defined as the number of molecules reacting per active site in unit time. The term active sites is applied to those sites for adsorption which are effective sites for a particular heterogeneous catalytic reaction. Because it is often impossible to measure the amount of active sites, some indirect method is needed to express the rate data in terms of turnover frequencies In some cases a realistic measure of the number of active sites may be the number of molecules of some compound that can be adsorbed on the catalyst. This measure is frequently used in the literature of the Fischer-Tropsch synthesis, where the amount of adsorption sites is determined by carbon monoxide adsorption on the reduced catalyst. However, it is questionable whether the number of adsorption sites on the reduced catalyst is really an indication of the number of sites on the catalyst active during the synthesis, because the metallic phase of the Fischer-Tropsch catalysts is often carbided or oxidized during the process. 

All the binary Cu/ZnO catalysts were found highly selective toward methanol without DME, methane, or higher alcohols and hydrocarbons detected in the product by sensitive gas chromatographic methods (59). Several of the composites were also found to be very active when subjected to a standard test with synthesis gas C0/C02/H2 = 24/6/70 at gas hourly space velocity of 5000 hr- pressure 75 atm, and temperature 250°C. The activities, expressed as carbon conversions and yields, are summarized in Table VIII. The end members of the series, pure copper and pure zinc oxide, were inactive under these testing conditions, and maximum activity was obtained for the composition Cu/ZnO = 30/70. The yields per unit weight, per unit area of the catalyst or the individual components, turnover rates per site titratable by irreversible oxygen and by irreversible carbon monoxide, are graphically... 

Fig. 6.5. Temperature dependence of the catalytic activity of supported gold and base metal oxide catalysts in the WGS. Starting reaction gas mixture was 4.88 vol% carbon monoxide in argon water vapour partial pressure 223 Torr SV 4,000 h 1 atm. Au/a-Fe203 (Q) CuO/ZnO/AhOa ( ) a-Fe203 ( ) AU/AI2O3 ( ). Rates are expressed in mol m h x 10 (adapted from [219])...

The reaction rate expression is given for oxidizing carbon monoxide on a catalytic surface. When the inlet mole fraction CO is Wn = 0.02, and the inlet temperature is Tin = 600 K, find the solution to these equations. The parameters are... 

The overall activation energies and frequency factors of overall catalytic rate expressions, and consequently their temperature coefficients, are less easy to constrain. For example, the relatively simple rate expression for the DAM model of the oxidation of carbon monoxide on a platinum on alumina catalyst (see Chapter 11) is thought to be ... 

In the case of the oxidation of carbon monoxide, the mechanistic rate expressions investigated were of the general form ... 

Although these procedures are generally applicable to all chemical reactions, in the following pages we use the simulated behaviour of the catalytic oxidation of carbon monoxide as an example to map and understand the peculiarities of this system. We also search for the best reaction conditions for eliminating carbon monoxide from various streams under certain constraints. The rate expression we will be examining is the same as that previously presented in Chapter 11. 

In the case of the catalytic oxidation of carbon monoxide, an interesting mapping is that of rate of reaction versus conversion at isothermal conditions. The reason is that, as we saw in Chapter 11, certain reaction conditions lead to a maximum in reaction rate at partial conversion. Using the best parameters for the rate expression reported in Chapter 11 we can simulate a variety of reaction conditions and find that at 1 bar total pressure, for a stoichiometric molar feed ratio of CO/O2 = 2, the rate of reaction at various reaction temperatures is predicted to behave in the way shown in Figure 12.1. 

At the present time, the fuels which can be described by this modeling approach include hydrogen, carbon monoxide, methane, methanol, ethane, ethylene, acetylene, propane, and propylene. The reaction mechanism used to describe the oxidation of these fuels has been developed and validated in a series of papers (3-7). The elementary reactions and their rate expressions are summarized in Reference (7) and are not reproduced here due to space limitations. Reverse reaction rates are computed from the forward rates and the appropriate thermodynamic data (8). This mechanism has been shown to describe the oxidation of methane (3,A), methanol (5), ethylene (6), and propane and propylene (7) over wide ranges of experimental conditions. It has also been used to describe the shock tube oxidation of ethane (4,9), and acetylene (10). 

McNeil, M.A., Schack, C.J., and Rinker, R.G. (1989) Methanol synthesis from hydrogen, carbon monoxide and carbon dioxide over a copper oxide/zinc oxide/aliunina catalyst. 11. Development of a phenomenological rate expression. Appl. Catal., 50, 265-285. 

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