Carbon monoxide oxidation conclusions

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... 

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. 

A similar plot depicting the effects on hydrocarbon oxidation efficiency of dilution of the catalyst concentration is presented in Figure 9. The conclusion is similar—minor variations at close to uniform distribution are not critical, and the effect is greater for aged catalyst. The effect of maldistribution on aged catalyst was greater for hydrocarbon than for carbon monoxide oxidation (cf. Figure 8). 

The experiments hitherto described dealt with catalytically active electrons and positive holes released by light. They allow only indirect conclusions regarding thermal catalysis. It is felt that direct observations are necessary in the present stage more than ever. Some work along these lines has been mentioned in the Introduction. Other observations on semiconductors of the ferrite type (d) have shown that the carbon monoxide oxidation, a donor reaction, is catalyzed best by inverse spinels, in which ferric ions, situated in octahedral positions, chemisorb carbon monoxide. Zinc ferrite, in which all the occupied octahedral positions carry ferric ions, showed a... 

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. 

One of the conclusions deduced from the thermochemical cycle 2 in Table V, for instance, is that in the course of the catalytic combustion of carbon monoxide at 30°C, the most reactive surface sites of gallium-doped nickel oxide are inhibited by the reaction product, carbon dioxide. This conclusion ought to be verified directly by the calorimetric study of the reaction. Small doses of the stoichiometric reaction mixture (CO + IO2) were therefore introduced successively in the calorimetric cell of a Calvet microcalorimeter containing a freshly prepared sample of gallium-doped... 

However, the analysis of the data was carried out in such a way as to cast doubt on the validity of these conclusions (Vayenas54). Okamoto, Kawamura and Kudo48 went on to use the e.m.f. interpretation from the above work49 to further investigate the mechanism of CO oxidation over platinum by using the cell as a probe of the surface coverage of carbon monoxide. 

Toumay, Murphy, Damon and Van Dolah [41] examined the fumes produced by the detonation of ammonium nitrate-fuel mixtures. They came to the conclusion that carbon monoxide formation is within the allowed limits. However, the concentration of nitric oxide and nitrogen dioxide is much higher than in typical dynamite compositions. On the ground of these experiments they expressed the view that ammonium nitrate-fuel compositions should not be recommended for underground blasting. 

Haber [3] investigated the formation of nitric oxide during the combustion of carbon monoxide and came to the conclusion that charged particles— electrons and ions—have an important catalytic effect on the reaction in the flame. 

The C-cluster of carbon monoxide dehydrogenase is the active site for the oxidation of CO to CO 2. This conclusion is based on rapid kinetic studies in which changes in the spectra of Cluster C undergo changes at rates commensurate with the rate of CO oxidation (Kumar et al., 1993). In addition, cyanide, which is a relatively specific inhibitor of CO oxidation, binds specifically to Cluster C (Anderson et al., 1993). 

The oxidation of carbon monoxide has been studied by both the usual step-response and isotopic experiments and by the TAP system (2/7). The general conclusion is that the fast response of the TAP system did not produce any additional mechanistic information to that obtained from step-response experiments. A number of the points discussed in previous paragraphs are mentioned, and it is suggested that the final pattern of multipulse response experiments be termed a pseudo-steady state. A factor not mentioned is that transient IR experiments are valuable with the step-response method but not compatible with the TAP system. 

Rossi [II] and Khudyakov [12) studied this problem and came to the conclusion that coal can react with COj to yield CO, apatites and potassium salts can bind NOj, molybdenum and some copper ores bind carbon monoxide. Some iron ores can catalyse the oxidation of carbon monoxide to carbon dioxide. Dubnov [6] pointed out that sulphur containing ore may be responsible for the formation of such toxic gases as SO2 and S. 

NiO(250°) contains more metallic nickel than NiO(200°). Magnetic susceptibility measurements have shown that carbon monoxide is adsorbed in part on the metal (33) and infrared absorption spectra have confirmed this result since the intensity of the bands at 2060 cm-i and 1960-1970 cm-1 is greater when carbon monoxide is adsorbed at room temperature on samples of nickel oxide prepared at temperatures higher than 200° and containing therefore more metallic nickel (60). Differences in the adsorption of carbon monoxide on both oxides are not explained entirely, however, by a different metal content in NiO(200°) and NiO(250°). Differences in the surface structures of the oxides are most probably responsible also for the modification of their reactivity toward carbon monoxide. In the surface of NiO(250°), anionic vacancies are formed by the removal of oxygen at 250° and cationic vacancies are created by the migration of nickel atoms to form metal crystallites. Carbon monoxide may be adsorbed in principle on both types of surface vacancies. Adsorption experiments on doped nickel oxides, which are reported in Section VI, B, have shown, however, that anionic vacancies present a very small affinity for carbon monoxide whereas cationic vacancies are very active sites. It appears, therefore, that a modification of the surface defect structure of nickel oxide influences the affinity of the surface for the adsorption of carbon monoxide. The same conclusion has already been proposed in the case of the adsorption of oxygen. 

It is concluded from these calorimetric experiments that adsorption of oxygen at 200° on the surface of NiO(200°) produces adsorbed species which are more reactive than surface anions. This conclusion is in agreement with results presented in an earlier section and it has been shown that these surface species are O (ads) (Section III, A). Carbon monoxide reacts easily at 200° with these species and gaseous carbon dioxide is formed. The quantity of these reactive ions, expressed in cubic centimeters of molecular oxygen per gram of oxide, may be calculated from the calorimetric results in Fig. 35 (Fo, = iFco = 3.6... 

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