Carbon monoxide, adsorption oxidation

Carbon corrosion, 300 Carbon monoxide adsorption, 248,250,255, 325-327, 347, 386-391,528-532 Carbon monoxide oxidation... 

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. 

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. 

Now possibilities of the MC simulation allow to consider complex surface processes that include various stages with adsorption and desorption, surface reaction and diffusion, surface reconstruction, and new phase formation, etc. Such investigations become today as natural analysis of the experimental studying. The following papers [282-285] can be referred to as corresponding examples. Authors consider the application of the lattice models to the analysis of oscillatory and autowave processes in the reaction of carbon monoxide oxidation over platinum and palladium surfaces, the turbulent and stripes wave patterns caused by limited COads diffusion during CO oxidation over Pd(110) surface, catalytic processes over supported nanoparticles as well as crystallization during catalytic processes. 

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... 

Fig. 13. The dependence of the carbon monoxide saturation adsorption (total) and irreversible adsorption (irreversible) on the Cu/ZnO ratio in the binary copper-zinc oxide catalysts...

Kemball kept an active interest in physical adsorption for several years, but a defining change came in 1946 with the award of a fellowship from the Commonwealth Fund of New York for research and travel in the United States. Influenced by Eric Rideal s long-standing friendship with Hugh S. Taylor, who had known Rideal since World War I when they worked together in London on the catalysis of carbon monoxide oxidation, Charles chose to spend his fellowship year with Professor Taylor at Princeton University. Here he was introduced to experimental work on catalysis and. 

There is a strong difference between the voltammetric profiles of the carbon monoxide oxidative desorption on a pure platinum single crystal in the presence of an ad-metal species and the same in the absence of an ad-metal species. The carbon monoxide adsorption and oxidation are also different depending on the type of the deposited metal as shown in Figure 11.4. 

Generally, Cu based catalysts are more active than Cu-Cr based catalysts. This observation is in agreement with previous reports about carbon monoxide oxidation on Cu/AbOs catalysts(lO). From the results of oxygen chemisorption it can be seen that reaction sites are more dispersed on Cu catalysts as compared to Cu-Cr catalysts. Dispersity towards CO chemisorption is relatively very low for all the catalysts and therefore no definite conclusion could be arrived at from CO chemisorption data In addition, poor affinity of the catalysts for carbon monoxide in chemisorption experiments indicates low level of carbon monoxide adsorption during reaction... 

Quite a number of years ago we carried out some experiments on films evaporated from Alloy 99 wire. We observed that films evaporated from this wire exhibited quite different properties than those evaporated from Nickel A or Hoskins 651. The films prepared from Alloy 99 showed (211) preferential orientation and were highly resistant to sintering as measured by the hydrogen adsorption. The activity of these films for carbon monoxide oxidation was 15 times greater than the activity of films prepared from the other nickel wires. For the disproportionation of carbon monoxide, 50-60% more CO2 was found than was theoretically possible. When the wire was heated in hydrogen a large decrease in hydrogen pressure with subsequent water formation was observed. 

Fig. 4 Energy distribution function, (p(e t) (cmol/kJ/mol/), against the dimensionless product of the lateral interaction energy (P) and the local isotherm (0)P0, for carbon monoxide adsorption over a bimetalhc Pto.25-Rho.75 silica supported catalyst, at 698 K. Source From Gas chromatographic kinetic study of carbon monoxide oxidation over platinum-rhodium catalysts, in J. Chromatogr.

Rate measurements on these catalysts imply the following conclusions The synergism between the noble metal and the oxidic component is due to spillover of oxygen. The oxygen adsorption sites are neither exclusively located on platinum nor on the three phase boundary. The rate determining step of the carbon monoxide oxidation should be the migration of adsorbed oxygen. 

There has been a general updating of the material in all the chapters the treatment of films at the liquid-air and liquid-solid interfaces has been expanded, particularly in the area of contemporary techniques and that of macromolecular films. The scanning microscopies (tunneling and atomic force) now contribute more prominently. The topic of heterogeneous catalysis has been expanded to include the well-studied case of oxidation of carbon monoxide on metals, and there is now more emphasis on the flexible surface, that is, the restructuring of surfaces when adsorption occurs. New calculational methods are discussed. 

Very recently, considerable effort has been devoted to the simulation of the oscillatory behavior which has been observed experimentally in various surface reactions. So far, the most studied reaction is the catalytic oxidation of carbon monoxide, where it is well known that oscillations are coupled to reversible reconstructions of the surface via structure-sensitive sticking coefficients of the reactants. A careful evaluation of the simulation results is necessary in order to ensure that oscillations remain in the thermodynamic limit. The roles of surface diffusion of the reactants versus direct adsorption from the gas phase, at the onset of selforganization and synchronized behavior, is a topic which merits further investigation. 

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