Carbon monoxide oxidation catalyst

Table 3 Deactivation of carbon monoxide oxidation catalyst at 523K by HMDS ...

Since 1975 catalysts have been fitted to vehicles to control emissions, initially of hydrocarbons and carbon monoxide (oxidation catalysts), and latterly also of oxides of nitrogen (three way catalysts). This contribution will demonstrate the ability of precious metal based catalysts not only to control carbon monoxide, hydrocarbons and nitrogen oxides but also the polynuclear aromatic fraction from both gasoline and diesel fuelled vehicles. The data will include that from both fresh and aged catalyst systems and also those exposed to leaded gasoline. 

Many commercial heterogeneous catalysts are not impregnated in a uniform fashion. For example, various precious-metal catalysts consist of an exterior active shell and an inert core in order to enhance the effectiveness factor. Several automobile-muffler catalysts have a carbon-monoxide-oxidation catalyst in one shell and an NOx-reducing catalyst in another shell. Our understanding of the reaction-diffusion interaction facilitated this rational design of the optimal profile of catalyst-activity distribution and shape. It would be of both practical importance and academic interest to develop a rational procedure for enhancing the performance of metallocenes by their nonuniform impregnation on the support. 

Yu et al. [104] also have found a positive effeet of lanthanum for carbon monoxide oxidation catalysts (Au/Ti02). They noted that lanthanum improved the activity, a fact that was assigned to the presence of lanthamun promoting the reactivity of CO adsorbed on gold sites and to the formation of a second active phase on the surface. It was observed that lanthanum increased the specific surface area and restrained the growth of titania partieles. 

Chromium Oxide-Based Catalysts. Chromium oxide-based catalysts were originally developed by Phillips Petroleum Company for the manufacture of HDPE resins subsequendy, they have been modified for ethylene—a-olefin copolymerisation reactions (10). These catalysts use a mixed sihca—titania support containing from 2 to 20 wt % of Ti. After the deposition of chromium species onto the support, the catalyst is first oxidised by an oxygen—air mixture and then reduced at increased temperatures with carbon monoxide. The catalyst systems used for ethylene copolymerisation consist of sohd catalysts and co-catalysts, ie, triaLkylboron or trialkyl aluminum compounds. Ethylene—a-olefin copolymers produced with these catalysts have very broad molecular weight distributions, characterised by M.Jin the 12—35 and MER in the 80—200 range. 

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). 

Catalysts in an oxidized state showed high activity in the oxidation of carbon monoxide [nickel catalysts (146) ] and hydrogen [molybdenum catalysts (146a)]. 

Zheng X-C, Wu S-Fl, Wang S-P, Wang S-R, Zheng S-M, Huang W-P (2005) The preparation and catalytic behavior of copper-cerium oxide catalysts for low-temperature carbon monoxide oxidation. Appl Catal A 283(l-2) 217-223... 

IX yer, S. M. Transient infrared studies of carbon monoxide oxidation on a supported platinum catalyst. M. S. Thesis, University of Connecticut, 1980. 

Sconox A catalytic process for oxidizing oxides of nitrogen and carbon monoxide. The catalyst is in the form of a ceramic honeycomb coated with platinum and containing potassium carbonate. The platinum oxidizes the carbon monoxide to carbon dioxide, and the potassium carbonate absorbs the NO,. Developed in 1995 by Sunlaw Energy Corporation, CA, and Advanced Catalytic Systems, TN. 

Figure 1.6 Representative TEM image (a) and particle size distribution (b) obtained for a Au/Ti02 catalyst prepared by grafting of a [Au6(PPh3)6](BF4)2 complex onto Ti02 particles followed by appropriate reduction and oxidation treatments [42], The gold particles exhibit approximately spherical shapes and an average particle size of 4.7 nm.The measured Au particle sizes could be well correlated with the activity of the catalyst for carbon monoxide oxidation and acetylene hydrogenation. (Reproduced with permission from Springer.)...

Based on the experimental data and some speculations on detailed elementary steps taking place over the catalyst, one can propose the dynamic model. The model discriminates between adsorption of carbon monoxide on catalyst inert sites as well as on oxidized and reduced catalyst active sites. Apart from that, the diffusion of the subsurface species in the catalyst and the reoxidation of reduced catalyst sites by subsurface lattice oxygen species is considered in the model. The model allows us to calculate activation energies of all elementary steps considered, as well as the bulk... 

Carbon monoxide oxidation is a relatively simple reaction, and generally its structurally insensitive nature makes it an ideal model of heterogeneous catalytic reactions. Each of the important mechanistic steps of this reaction, such as reactant adsorption and desorption, surface reaction, and desorption of products, has been studied extensively using modem surface-science techniques.17 The structure insensitivity of this reaction is illustrated in Figure 10.4. Here, carbon dioxide turnover frequencies over Rh(l 11) and Rh(100) surfaces are compared with supported Rh catalysts.3 As with CO hydrogenation on nickel, it is readily apparent that, not only does the choice of surface plane matters, but also the size of the active species.18-21 Studies of this system also indicated that, under the reaction conditions of Figure 10.4, the rhodium surface was covered with CO. This means that the reaction is limited by the desorption of carbon monoxide and the adsorption of oxygen. 

Lanthanum cobaltate catalysts carbon monoxide oxidation, kinetics, 36 281-283... 

S. Ozkara, and A. E. Aksoylu, Selective low temperamre carbon monoxide oxidation in H2-rich gas streams over activated carbon supported catalysts, Appl. Catal. 251(1), 75—83 (2003). 

Carbon monoxide oxidation, ethane dehydrogenation, ethane hydrogenolysis, ethene hydrogenation. Pt, Mg, Zn catalysts placed either in the pores of the membrane or at the entrance of the membrane pores. 

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... 

Carbon monoxide oxidation. SEP was used to investigate a La(Sr)MnC>3.5 catalyst during carbon monoxide oxidation at 500 and 550°C.3S This material was chosen because of its phase stability throughout the conditions employed. Changes in composition and temperature do not result in phase changes, rather they modify the value of 5. 

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. 

VI. Carbon Monoxide Oxidation on Modified Nickel Oxide Catalysts.. 68... 

The method outlined above in the case of zinc oxide will now be applied to the carbon monoxide oxidation on nickel oxide catalysts modified in both ways. If it is assumed, as before, that semiconductivity trends in the bulk and in the surface layer are qualitatively the same, a correlation between semiconduetivity and catalysis will be established if cationic impurities of valences lower and higher than 2 are found to affect the catalytic rate in opposite directions. 

Such a proof of the carbon monoxide oxidation was first given by one of us (28). It is very important to remark that the catalysts containing impurities were prepared by firing together in air at 600°C. for 3 hr. a mechanical mixture of the required components in adequate proportions. As pointed out by Fensham (54), this is much too low a temperature to ensure homogeneous solid solutions. Consequently, when a catalyst is described as NiO + 0.1% Li2O, there is no assurance that this nominal composition is realized at all either in the surface layer or in the bulk of the sample. As will be shown, this reservation is quite essential. 

If further work confirms our explanations which connect catalytic inversion with the inversion of physical properties of the modified nickel oxide catalysts, the correlation between semiconductivity and oxidation catalysis found in the Princeton work and in Schwab s studies will appear quite convincing. To sum up, the activation energy of the carbon monoxide oxidation has been found to decrease with increasing semiconductivity on both sides of the inversion point of physical properties of nickel oxide catalysts. 

Goodman, M. G., Kenney, C. N., Morton, W., Cutlip, M. B. and Mukesh, D., 1982, Transient studies of carbon monoxide oxidation over platinum catalyst. Surf. ScL 120, L453-460. 

The success of the correlation of catalytic behavior with bulk Mossbauer parameters by Skalkina et al. is also reflected in the work of Tops0e and Boudart (96). As discussed earlier, these authors found a decrease in the isomer shift of the octahedral iron ions in a lead-promoted Cr-Fe304 carbon monoxide shift catalyst, indicative of an increased covalency of these ions. Schwab et al. (203) have proposed a correlation of the activity for CO oxidation by ferrites with the octahedral ions in these materials, and the electron transfer required for this catalytic process may be facilitated by an increased covalency of the metal ions (204). In view of these suggestions, the lead-promoted catalyst is expected to possess a higher catalytic activity for the CO shift reaction than an unpromoted catalyst, as evidenced by the Mossbauer parameters of these two samples. This has in fact been shown experimentally to be the case (96). For the reverse CO shift reaction over supported europium (176), the success of the correlation between catalytic activity and the Mossbauer parameters (in this case the reducibility) has already been noted in Section III, A, 4. 

While the photoenhancement of the hydrogen deuterium exchange at MgO has been found to be due solely to an increase in the rate constant, investigations of the photocatalyzed carbon monoxide oxidation with ZnO, NiO and Co304 as catalysts have shown that the photoenhancement in these cases is due to drastic changes in the apparent activation energy 82-88)... 

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

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... 

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