Carbon monoxide oxidation— reaction rate

A similar oxidation-reduction mechanism in the carbon monoxide oxidation reaction on oxide catalysts has been proposed by Benton (71), Bray (72), Frazer (73), and Schwab (74). In this reaction also, Mooi and Selwood (57) found that a decrease in the percentage of iron oxide, manganese oxide or copper oxide on the alumina support first increased the rate, and then at lower percentages decreased the rate, of carbon monoxide oxidation, indicating that valence stabilization is again operative in these cases. 

Reaction Rate Oscillations During the Carbon Monoxide Oxidation Reaction Over Pt/y-Al203 Catalysts An IR-Transmission Spectroscopy Study... 

Figure 3. Global rate constant vs. 1/ t for carbon monoxide oxidation reaction (Eq. 19).

Oxidation. Carbon monoxide can be oxidized without a catalyst or at a controlled rate with a catalyst (eq. 4) (26). Carbon monoxide oxidation proceeds explosively if the gases are mixed stoichiometticaHy and then ignited. Surface burning will continue at temperatures above 1173 K, but the reaction is slow below 923 K without a catalyst. HopcaUte, a mixture of manganese and copper oxides, catalyzes carbon monoxide oxidation at room temperature it was used in gas masks during World War I to destroy low levels of carbon monoxide. Catalysts prepared from platinum and palladium are particularly effective for carbon monoxide oxidation at 323 K and at space velocities of 50 to 10, 000 h . Such catalysts are used in catalytic converters on automobiles (27) (see Exhaust CONTHOL, automotive). 

Figure 8. Rate of carbon monoxide oxidation on calcined Pt cube monolayer as a function of temperature [27]. The square root of the SFG intensity as a function of time was fit with a first-order decay function to determine the rate of CO oxidation. Inset is an Arrhenius plot for the determination of the apparent activation energy by both SFG and gas chromatography. Reaction conditions were preadsorbed and 76 Torr O2 (flowing). (Reprinted from Ref. [27], 2006, with permission from American Chemical Society.)...

Potentiometric techniques have been used to study autonomous reaction rate oscillations over catalysts and carbon monoxide oxidation on platinum has received a considerable amount of attention43,48,58 Possible explanations for reaction rate oscillations over platinum for carbon monoxide oxidation include, (i) strong dependence of activation energy or heat of adsorption on coverage, (ii) surface temperature oscillations, (iii) shift between multiple steady states due to adsorption or desorption of inert species, (iv) periodic oxidation or reduction of the surface. The work of Sales, Turner and Maple has indicated that the most... 

If it is assumed that the mobile oxygen differs from the extralattice oxygen by the absence of an additional electron supplied by the solid, it is quite likely that modifications of the electronic levels of nickel oxide by impurities will not affect substantially the low-temperature rate of carbon monoxide oxidation. Indeed, the rate depends on surface diffusion with subsequent reaction of the adsorbed partners if our scheme is correct. On the contrary such modifications might affect the rate of the high-terapera-ture process insofar as it depends on the availability and heat of adsorption of the extralattice oxygen. As will be seen later, this prediction is correct. 

In combustion systems it is generally desirable to minimize the concentration of intermediates, since it is important to obtain complete oxidation of the fuel. Figure 13.5 shows modeling predictions for oxidation of methane in a batch reactor maintained at constant temperature and pressure. After an induction time the rate of CH4 consumption increases as a radical pool develops. The formaldehyde intermediate builds up at reaction times below 100 ms, but then reaches a pseudo-steady state, where CH2O formed is rapidly oxidized further to CO. Carbon monoxide oxidation is slow as long as CH4 is still present in the reaction system once CH4 is depleted, CO (and the remaining CH2O) is rapidly oxidized to CO2. 

Carbon Monoxide Oxidation. Analysis of the carbon monoxide oxidation in the boundary layer of a char particle shows the possibility for the existence of multiple steady states (54-58). The importance of these at AFBC conditions is uncertain. From the theory one can also calculate that CO will bum near the surface of a particle for large particles but will react outside the boundary layer for small particles, in qualitative agreement with experimental observations. Quantitative agreement with theory would not be expected, since the theoretical calculations, are based on the use of global kinetics for CO oxidation. Hydroxyl radicals are the principal oxidant for carbon monoxide and it can be shown (73) that their concentration is lowered by radical recombination on surfaces within a fluidized bed. It is therefore expected that the CO oxidation rates in the dense phase of fluidized beds will be suppressed to levels considerably below those in the bubble phase. This expectation is supported by studies of combustion of propane in fluidized beds, where it was observed that ignition and combustion took place primarily in the bubble phase (74). More attention needs to be given to the effect of bed solids on gas phase reactions occuring in fluidized reactors. 

For highly exothermic reactions, such as carbon monoxide oxidation, it may be hard to keep the catalyst isothermal, and if the rate of heat dissipation no longer keeps pace with heat generation it becomes hotter than the temperature shown by the sensing device the rate will then escalate... 

There is little information of the chemisorption of water on gold surfaces, although its presence has a marked acceleratory effect on the rate of carbon monoxide oxidation over Au/Si02 132 since the support is not expected to be involved in the reaction, it was considered that it might help the adsorption of oxygen on gold panicles. Water occupies oxygen vacancies on... 

After heating to 623 K in helium, which also effected reduction according to XAFS, treatment with sodium cyanide solution removed the Au°, and left 10% of gold as cationic Au111.54 The specific rate of carbon monoxide oxidation was constant, irrespective of the treatment with sodium cyanide, as were activation energy and orders of reaction. It appeared that the Au111 was reduced under reaction conditions, and was not an active species. 

The oxidation of carbon compounds is treated only very briefly and avoids the reactions of the carbon monoxide oxidation. Although kinetic studies of the phosphorus oxidation have not until now yielded any rate coeflScients, we have nonetheless included a survey of the work done because it is obvious that once a few rate coelficients have been unequivocally determined, the relationships discussed will then yield many other quantitative results. 

Catalytic properties in the reactions of carbon monoxide oxidation (all oxides) and butene oxidative dehydrogenation (iron oxides) were studied using a microreactor with the vibrofluidized bed of catalysts and pulse/flow kinetic installation [4], Catalytic activities were characterized by the reaction rate W (molec. COWs) in differential conditions and first-order rate constant K (dm butene (STP) /m -s-atm), respectively. 

Mechanisms of the Carbon Monoxide Oxidation and Nitric Oxide Reduction Reactions over Single Crystal and Supported Rhodium Catalysts High Pressure Rates Explained using Ultrahigh Vacuum Surface Science", G.B. Fischer, Se H. Oh, J.E. Carpenter, C.L. DiMaggio, S.J. Schmieg,... 

Classical analysis has demonstrated that a given quantity of active material should be deposited over the thinnest layer possible in order to minimize diffusion limitations in the porous support. This conclusion may be invalid for automotive catalysis. Carbon monoxide oxidation over platinum exhibits negative order kinetics so that a drop in CO concentration toward the interior of a porous layer can increase the reaction rate and increase the effectiveness factor to above one. The relative advantage of a thin catalytic layer is further reduced when one considers its greater vulnerability to attrition and to the deposition of poisons. 

It is evident from examples like these that the investigation of electron transfer in catalysis is dependent on the availability of test reactions of well-known acceptor or donor type. Lately, it has become clear that sometimes the same reaction can exert both functions, depending on the conditions. Thus, the carbon monoxide oxidation is a donor reaction on most p-conducting catalysts, like nickel oxide 13) when the chemisorption of carbon monoxide governs the reaction rate. However, on zinc oxide, the chemisorption of the acceptor oxygen is rate-determining. 

In order to substantiate this measure of chromia area, the rates of carbon monoxide oxidation over the various catalysts were measured. It was found that the alumina portion of the surface could be rendered inactive by selective poisoning with water and, under these conditions, the reaction was catalyzed exclusively by the ehromia surface. Since the activation energy was independent of the chromium content, it was reasonable to expect a linear variation of specific activity (i.e., activity per unit total surface area) with the fraction 0 (Table I) of the total surface contributed by the chromia phase. In Fig. 3 the specific rate is... 

The current knowledge of the CO + O2 reaction mechanism makes possible to state rather justified theoretical models giving insight to the features of spatio-temporal dynamics of reaction on the platinum surface. Carbon monoxide oxidation over Pt(lOO) single crystal has been studied comprehensively. It was shown that under certain conditions (partial pressures of reactants and temperature), the adsorbate coverages and the reaction rate undergo self-oscillations attended by the spatio-temporal pattern of COads and Oads formation on the surface [1,2,94]. The observed phenomena are associated with the reversible adsorbate-induced surface phase transition hex 1 X 1. The platinum state in unreconstructed 1x1 phase is catalytically active due to the ease of oxygen molecules dissociation S (02) 0.3—0.4 S (02) 10. The CO... 

Quantitative information about the activity of various catalysts is obt ned by rate me lsurements in a recirculation system. The results indicate that the observed syner sm between the noble metal and tin(IV)oxide is due to spillover of oxygen. The comparison of reaction rates, measured with catalysts mainly differing in the sorption capacity relative to oxygen, shows that the rate determining step of the carbon monoxide oxidation should be the migration of adsorbed oxygen. 

In the present study we have aimed at the preparation and examination of catalysts with different oxygen sorption capacities. For that purpose the characteristics of the tin(IV)oxide had to be altered and thus, firstly, a more general investigation of the modification of tin(IV)oxide powder by an adequate temperature treatment was performed. Subsequently, reaction rates for the carbon monoxide oxidation were determined with catalysts of different oxygen sorption capacities in order to obtain additional information concerning the synergism between the noble metal and tin(IV)oxide. 

Carbon monoxide oxidation on Pt catalysts is an example of a reaction of practical importance that can lead to multiplicity of steady states when the resistance to diffusion leads to significant concentration gradients. Typical for multiple steady states is a sudden jump from a relatively low rate of reaction or conversion to relatively high values upon an increase of the catalyst temperature. Upon decreasing the temperature, the jump back to the low reaction rate occurs... 

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