Chemisorption carbonate structures

Juza and Blanke (125) investigated the reaction of carbon and sulfur between 100 and 1000° at various pressures. They thought it unlikely that there was genuine chemical bonding. The phenomenon of sulfur fixation was ascribed to capillary condensation, adsorption, chemisorption, and solution in the carbon structure. 

Nevertheless, C02 is an extremely valuable probe molecule because the infrared spectra of the chemisorbed species respond very sensitively to their environments. Thus, the frequency separation of the typical band pairs of the carbonate structures may be taken as a measure of the local asymmetry at the chemisorption site. The application of 13C-FT-NMR should be extremely valuable for a still more extensive study of the nature of sites by C02 adsorption. Due to the very detailed information on the structure of sites on oxide surfaces that can be obtained by C02 chemisorption studies, this compound should in some cases also be applicable as a specific poison. A very careful study of the type of interaction with the surface, however, has to be undertaken for each particular system before any conclusive interpretation of poisoning experiments becomes meaningful. 

Chemisorption of COj leads to carbonate structures. Carbonate formation is accelerated by preadsorbed water (cf. also [543]). According to Boese et al. [608], the assignment of the corresponding bands is difficult because of band overlap. However, the authors tentatively attributed a band pair found with CO2 on Nai jLiio.y-A at 1660 and 1365 cm to a bidentate structure and a second one at 1588 and 1421 cm to a monodentate structure (cf. [543,641]). From alkali ion-exchanged zeolites, carbonates could be removed by pumping at elevated temperatures, which was, however, not possible in the case of alkaline earth-exchanged zeolites. 

Chemisorption of benzene at 297°C on Ni(110) occurred in a rather different manner. Several patterns, some streaked, were observed, and they followed the same sequence and showed the same behavior as those obtained when acetylene was chemisorbed on this surface (29). These structures have not been fully elucidated, but the streaked patterns suggest (i) that the mobility of adsorbed species along the "furrows of the (110) face is easier than their mobility across them, and (ii) that dissociation of the carbon skeleton of benzene and the formation of other structures occurs. 

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. 

Reaction between carbon monoxide and dihydrogen. The catalysts used were the Pd/Si02 samples described earlier in this paper. The steady-state reaction was first studied at atmospheric pressure in a flow system (Table II). Under the conditions of this work, selectivity was 100% to methane with all catalysts. The site time yield for methanation, STY, is defined as the number of CH molecules produced per second per site where the total number of sites is measured by dihydrogen chemisorption at RT before use, assuming H/Pd = 1. The values of STY increased almost three times as the particle size decreased. The data obtained by Vannice et al. (11,12) are included in Table II and we can see that the methanation reaction on palladium is structure-sensitive. It must also be noted that no increase of STY occurred by adding methanol to the feed stream which indicates that methane did not come from methanol. 

Surface groups consisting of atoms foreign to the structure can be formed on a great variety of substances. It is not intended to discuss all possibilities this would surpass the scope of an article limited in volume. Furthermore, research in this field has but begun surface compounds have been studied only on a selected group of substances. Most of the investigated substances, however, are very important from an industrial viewpoint. Therefore, in this article the chemistry of surface compounds will be described for a few characteristic and well-known examples. Borderline cases, such as the chemisorption of carbon monoxide on metals, will not be considered. 

Ruthenium catalysts, supported on a commercial alumina (surface area 155 m have been prepared using two different precursors RUCI3 and Ru(acac)3 [172,173]. Ultrasound is used during the reduction step performed with hydrazine or formaldehyde at 70 °C. The ultrasonic power (30 W cm ) was chosen to minimise the destructive effects on the support (loss of morphological structure, change of phase). Palladium catalysts have been supported both on alumina and on active carbon [174,175]. Tab. 3.6 lists the dispersion data provided by hydrogen chemisorption measurements of a series of Pd catalysts supported on alumina. is the ratio between the surface atoms accessible to the chemisorbed probe gas (Hj) and the total number of catalytic atoms on the support. An increase in the dispersion value is observed in all the sonicated samples but the effect is more pronounced for low metal loading. 

Concurrent stream of the development of nanomaterials for solid-state hydrogen storage comes from century-old studies of porous materials for absorption of gasses, among them porous carbon phases, better known as activated carbon. Absorption of gases in those materials follows different principles from just discussed absorption in metals. Instead of chemisorption of gas into the crystalline structure of metals, it undergoes physisorption on crystalline surfaces and in the porous structure formed by crystals. The gases have also been known to be phy-sisorbed on fine carbon fibers. 

The role of defects in changing physisorption to chemisorption can not only be limited to disordered carbons and nanocarbons but also discussed in respect to highly ordered nanocarbons, such as fuUerenes and CNTs discussed in the following paragraphs. A view that the nonphysisorbed hydrogen observed in carbon nanostructures would be connected with the formation of structural defects was expressed recently by Zuttel and Orimo [28]. 

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