Chemisorption of Hydrogen on Carbons

Sherman and Eyring (14) from theoretical considerations proposed that in case of the chemisorption of hydrogen on carbon the activation energy... 

Attempts have been made by Eyring and Sherman (29) and by Okamoto, Horiuti, and Hirota (30) to evaluate the activation energy for the chemisorption of hydrogen on carbon or nickel on the assumption that the surface atoms behave as isolated atoms. The calculated values, although too high, vary markedly with the spacing of surface atoms. Quantitatively, however, we must at present rely upon experimental data. The absolute rates of chemisorption will be calculated here using the observed temperature coefficient. 

EXAFS (Extended X-Ray Absorption Fine Structure). Characterization of the surface of metal nanoparticles had been limited to chemical methods, e.g., chemisorption of hydrogen and carbon monoxide. In 1970s, the situation was surprisingly changed due to the advances in x-ray absorption spectroscopy, especially extended x-ray absorption fine structure (EXAFS), Advances in this method have been achieved with the use of synchrotron radiation, which runs effectively at Tsukuba (Japan), Grenoble (France), etc. Now it is one of the most valuable tools to get structural information on bimetallic nanoparticles. 

Figure 5.12 Adsorption energy and surface coverage, (a) Physical adsorption of nitrogen on rutile at 85 K71. (b) Chemisorption of hydrogen on tungsten169, (c) Physical adsorption of krypton on graphitised carbon black166. (See Figure 5.6) (By courtesy of (a) Science Progress, (b) Discussions of the Faraday Society and (c) The Canadian Journal of Chemistry)...

The characterization of zeolite-entrapped metal clusters by chemisorption of hydrogen or carbon monoxide provides valuable information, requiring, however, careful interpretation. When metal particles in zeolites approach monoatomic dispersion, the ratio of chemisorbed hydrogen to metal (H/M) fails to provide reliable information on metal dispersion. A first convincing example that the H/M ratio can actually decrease with increasing metal dispersion has been provided by determining the H/M ratio and the EXAFS profiles of reduced Pd/NaY and Pd/HY. It was found that the H/M ratio in Pd/HY is significantly smaller than that in Pd/NaY, particularly for small Pd particles when reduction was performed at temperatures below 500°C... 

Maclver and Tobin (56) have reported that there is a weak and nearly reversible chemisorption of hydrogen on a-Cr203 at —195° which is suppressed almost completely by chemisorption of carbon monoxide. The amount of this adsorption at 300 torr is about equal to that of carbon monoxide at —78°. Adsorption of hydrogen on a-Cr203 has also been studied by Weller and Voltz (8). 

Selective chemisorptions of hydrogen and carbon monoxide have also been used to determine the surface area of other Group VIII metals, especially by Yates et al. (6). Development of methods applicable to other metals is only a matter of ingenuity and from now on every investigation of catalysis on supported metals must include a determination of the surface area of the metal. 

Several catalyst samples of tungsten carbide and W,Mo mixed carbides with different Mo/W atom ratios, have been prepared to test their ability to remove carbon monoxide, nitric oxide and propane from a synthetic exhaust gas simulating automobile emissions. Surface characterization of the catalysts has been performed by X-ray photoelectron spectroscopy (XPS) and selective chemisorption of hydrogen and carbon monoxide. Tungsten carbide exhibits good activity for CO and NO conversion, compared to a standard three-way catalyst based on Pt and Rh. However, this W carbide is ineffective in the oxidation of propane. The Mo,W mixed carbides are markedly different having only a very low activity. 

Studies of chemisorption of hydrogen, water, carbon monoxide, and carbon dioxide alone and in sequence on a Cu-Cr-Zn low temperature methanol synthesis catalyst show that the catalyst surface contains two different types of active sites. Hydrogen and water are chemisorbed in competition on one type, carbon monoxide and carbon dioxide on the other. The results for Cu-Zn-Al catalysts follow the same pattern. 

FIGURE 6.6 Hydrogen spillover includes dissociative chemisorption of hydrogen on metal nanoparticles, and subsequent migration of hydrogen atoms onto adjacent surfaces of a sorbent (e.g., single-wall carbon nanotube) via spillover and surface diffusion (Ref. [58]). 

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. 

S.P. Chan, G. Chen, X.G. Gong, Z.F. Liu, Chemisorption of hydrogen molecules on carbon nanotubes under high pressure. Phys. Rev. Lett. 87 (2001) 20. 

The most common techniques utilize the chemisorption of hydrogen, oxygen, or carbon monoxide on the surface of the sample. The volume of a gas produced in a catalytic reaction may also be used to calculate catalytically active surface area. 

When ethylene is adsorbed on bare nickel at 35° C. or on either bare or hydrogen-covered nickel at 150° C., the intensity of the C—H bands, shown as A of Fig. 3, is small compared with those of the associated chemisorbed ethylene shown in Fig. 2. When the species represented by A is treated with H2 at 35° C., the band intensities increased as is shown in B of Fig. 3. This behavior shows that A is due to a dissociatively chemisorbed ethylene in which the number of hydrogens per carbon is low (7). The species obtained by dissociative chemisorption will be referred to as a surface complex. It is doubtful whether the surface complex has a specific stoichiometric composition. Rather it appears that the carbon-hydrogen ratio will depend on the severity of the dehydrogenation conditions. In some cases it appears that a surface carbide, which has no hydrogens, is obtained. Even in this case the carbons appear to be easily rehydrogenated to adsorbed alkyl groups. 

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