Chemisorption dioxide

Carbon dioxide cannot be recommended for routine determinations of specific surface on the other hand, it should be particularly suitable for the study of the polarity of surfaces in systems where chemisorption can be excluded from consideration. 

Many molecules undergo partial oxidation on adsorption and many alkanes and alkenes are believed to yield an adsorbed CHO group on adsorption (Petrii, 1968). These processes usually lead to the complete oxidation of the organic molecule to carbon dioxide and few workers have attempted to halt the reaction at an intermediate stage. Hence, although there are undoubtedly possibilities for using dissociative chemisorption for synthetic reactions, this chapter will not consider these processes further. 

Carbon dioxide chemisorptions were carried out on a pulse-flow microreactor system with on-line gas chromatography using a thermal conductivity detector. The catalyst (0.4 g) was heated in flowing helium (40 cm3min ) to 723 K at 10 Kmin"1. The samples were held at this temperature for 2 hours before being cooled to room temperature and maintained in a helium flow. Pulses of gas (—1.53 x 10"5 moles) were introduced to the carrier gas from the sample loop. After passage through the catalyst bed the total contents of the pulse were analysed by GC and mass spectroscopy (ESS MS). 

Whereas determination of chemisorption isotherms, e.g., of hydrogen on metals, is a means for calculating the size of the metallic surface area, our results clearly demonstrate that IR studies on the adsorption of nitrogen and carbon monoxide can give valuable information about the structure of the metal surface. The adsorption of nitrogen enables us to determine the number of B5 sites per unit of metal surface area, not only on nickel, but also on palladium, platinum, and iridium. Once the number of B5 sites is known, it is possible to look for other phenomena that require the presence of these sites. One has already been found, viz, the dissociative chemisorption of carbon dioxide on nickel. 

The adsorption of oxygen on diamond was studied by Barrer (156). Essentially no chemisorption was observed at —78°. From 0 to 144° oxygen was chemisorbed, but no carbon oxides were liberated. Some carbon dioxide was formed as well from 244 to 370° by interaction of oxygen and diamond surface not covered with surface oxides. Surfaee oxide formation was observed at low pressures. The coefficient of friction of diamond increases considerably after heating in a high vacuum. The measurements by Bowden and Hanwell (157) showed a decrease in the friction on access of oxygen, even at very low pressures. 

Adsorption of carbon dioxide or oxygen on the praseodymium samples was carried out in the pressure range of 1-40 Pa to evaluate the number of chemisorption sites on the samples. Praseodymium oxide irreversibly adsorbed 9.5 x 10" mol g of carbon dioxide. The amount of oxygen irreversibly adsorbed on the sample was 15.2 x 10" mol g Carbon dioxide or oxygen was not adsorbed on the samples containing chlorine, i.e., praseodymium chloride and praseodymium oxychloride prepared from the chloride by heating under oxygen flow at 750°C for 1 h. 

To reduce to the simple rate Equation (2), it is necessary that some of the steps in the general scheme occur at a negligible or an extremely fast rate. It has been observed (37) that carbon monoxide gas is an immediate product of the chemisorption of carbon dioxide on carbon and that the adsorption of carbon dioxide is not reversible to give immediate desorption of carbon dioxide. Therefore, it may be assumed that the lives of CCCOs) and C(CO)x are short. Consequently, the general expressions can be simplified to... 

Mechanisms A and B both state that carbon monoxide retards the gasification of carbon by carbon dioxide by decreasing the fraction of the surface which is covered by oxygen atoms under steady state conditions. In mechanism A, fli is decreased by the chemisorption of carbon monoxide by a fraction of the active sites. In mechanism B, 61 is decreased by the reaction of a portion of the chemisorbed oxygen with gaseous carbon monoxide to produce gaseous carbon dioxide. Reif (57) shows that only one of these reactions can control retardation at one time. 

Ketones and nitriles are rather soft bases their coordination onto electron-deficient sites on oxides is, therefore, relatively weak. One may, however, expect an improved specificity of chemisorption due to their softness. Unfortunately, however, these substances very easily undergo chemical transformations at oxide surfaces. Thus, carboxylate structures are formed on adsorption of acetone on alumina (194, 245-247), titanium dioxide (194), and magnesium oxide (219, 248, 249). Besides, acetone is also coordinated onto Lewis acid sites. A surface enolate species has been suggested as an intermediate of the carboxylate formation (248, 249). However, hexafluoroacetone also leads to the formation of trifluoroacetate ions (219). The attack of a basic surface OH ion may, therefore, be envisaged as an alternative or competing reaction path ... 

Carbon dioxide fulfills some of the relevant criteria and contradicts others. Evidently, although C02 exhibits acidic properties, the adsorbed amounts cannot be taken as a measure of surface basicity strong chemisorption of C02 occurs through interaction with acid-base pair sites preferentially. Thus, specific poisoning of basic sites by C02 chemisorption is not possible. Furthermore, a... 

The adsorption of formic acid and acetic acid leads to the formation of car-boxylate groups on aluminas (194, 295-299), titanium dioxides, (134, 135b, 176, 194, 300, 301), chromium oxide (134, 302, 303), zinc oxide (298, 304-306), and magnesium oxide (299, 304, 306). The corresponding dissociative chemisorption step most probably takes place on acid-base pair sites of the type... 

Extensive information concerning distribution of the promoters, penetration below the promoters of adsorbed atoms, and chemical behavior of the promoters was obtained by Brunauer and Emmett (25,26). They used chemisorption of carbon monoxide, carbon dioxide, nitrogen, hydrogen, and oxygen, individually and successively measuring the influence of one type of chemisorption upon another type. It was concluded that CO and C02 were chemisorbed as molecules, H2 and N2 as atoms, and 02 probably as ions. C02 is chemisorbed on the alkali molecules located at the surface, whereas H2, N 2, CO, and 02 are chemisorbed on the iron atoms. From the effect of presorbed CO upon the chemisorption of C02 and vice versa it was concluded that the promoters are concentrated on the surface and are distributed so effectively that most surface iron atoms are near to a promoter atom. Strong indication... 

Adjacent oxygen site for carbon dioxide chemisorption... 

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