Chemisorption of hydrogen

Figure Bl.26.1. Sorption isothemi for chemisorption of hydrogen on palladium film at 273 K (Stephens S J 19597. Phys. Chem.. 63 188-94).

Madhavan P and Whitten J L 1982 Theoretical studies of the chemisorption of hydrogen on copper J. Chem. Phys. 77 2673-83... 

Studies of the static and frequency response chemisorption of hydrogen on Rh catalysts supported on SlOo and TIO7 have shown that ... 

In the foregoing it has been discus.sed how a metal can dissociate H2. Fig. 3.6 explains the principle of catalysis with an example of the hydrogenation of ethylene, for which dissociative chemisorption of hydrogen is an elementary step in the catalytic cycle. The adsorption of alkenes, on the other hand, is non-dissociative. 

Oudar and co-workers studied the dissociative chemisorption of hydrogen sulfide at Cu(110) surfaces between 1968 and 1971.3,14 As in the case of Ni(110) described below, a series of structures were identified, which in order of increasing sulfur coverage were described as c(2 x 2), p(5 x 2) and p(3 x 2). In contrast to nickel, the formation of the latter phase is kinetically very slow from the decomposition of H2S and could only be produced at high temperatures and pressures. The c(2 x 2) and p(5 x 2) structures were confirmed by LEED,15 17 but the p(3 x 2) phase has not been observed by H2S adsorption since Oudar and colleagues work. 

Fig. 2. (a) Magnetization-volume isotherms for the chemisorption of hydrogen and of benzene on kieselguhr-supported nickel at 150° C (16). (b) Average number of bonds formed by benzene adsorbed on nickel-silica as a function of temperature (17). From J. Amer. Chem. Soc. 79, 4637 (1957) 83, 1033 (1961). Copyright by the American Chemical Society. Reprinted by permission of copyright owner. 

The conclusions from this work were (i) that the mechanism that operates is of wide applicability, (ii) that exchange proceeds by either the dissociative chemisorption of benzene or by the dissociation of benzene which has previously been associatively chemisorbed, and (iii) that M values of about 2 indicate that further dissociation of surface-area measurements. Surface areas of metal films determined by the chemisorption of hydrogen, oxygen, carbon monoxide, or by physical adsorption of krypton or of xenon concur... 

The surface molecule model has been used to study chemisorption of hydrogen 47) and nitrogen 48) on tungsten (100). The parameters used in these calculations are collected in Table IV. Preliminary calculations on the diatomic molecules WH and WW showed that inclusion of tungsten 5 p orbitals is essential to produce a minimum in the energy/ distance curves. However, the repulsion due to inner electrons could be calculated by the empirical relationship ... 

The surface areas of the iridium and palladium catalysts were determined by chemisorption of hydrogen and carbon monoxide, respectively, the monolayer volume being determined from an adsorption isotherm taken at 20°C. 

Alloy films are commonly sintered during preparation by deposition on substrates heated to, say, 400°C or by subsequent annealing at such temperatures, and, consequently, rather small surface areas have to be measured, perhaps in vessels of substantial volume. Krypton adsorption at liquid nitrogen temperature was used with induction-evaporated Cu-Ni, Fe-Ni, and Pd-Ni films, and BET surface areas of 1000-2000 cm2 were recorded (48), after correction for bare glass. The total area of Cu-Ni films was measured by the physical adsorption of xenon at — 196°C (70) in addition, the chemisorption of hydrogen on the same samples enabled the quantity a to be determined where... 

Data of the chemisorption of hydrogen on small copper particles were obtained by Ward (Proc Roy Soc London A133 506, 1031) in the units P, Torr, and V in cc/g STP. The plots show the Langmuir to be a better fit than the Freundlich. 

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 adsorption (chemisorption) of hydrogen on clean metal surfaces is almost always accompanied by absorption of hydrogen into the interior of the structure. This absorption is a slow activated process and has in the past been mistaken for activated adsorption of hydrogen on the surface. 

The heats of chemisorption of hydrogen on nickel and iron are nearly identical and decrease from about 30,000 calories for the sparsely covered surface to about 18,000 calories for the completely covered surface. The heat of chemisorption on tungsten decreases from 45,000 calories to about 13,000 calories as a function of surface coverage (Roberts, also Beeck and coworkers). The lower values of Frankenburg for higher... 

The activation energy for the chemisorption of hydrogen on all metals studied is extremely low. 

On nickel, chemisorption of hydrogen is still taking place at 20°K. 

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. 

Chemisorption of a saturated molecule generally involves its dissociation and consequent bonding between the fragments and the surface. For the chemisorption of hydrogen on a metal this process can be represented by the familiar Lennard-Jones (71) diagram depicted in Fig. 6. Most adsorption processes are exothermic, and hence the activation energy for desorption (Ed) is greater than that for adsorption (Ea) thus, desorption is likely... 

Fio. 7. Dependence of the heat of chemisorption of hydrogen (Q) and the catalytic activity for ethylene hydrogenation (K) on the percent d-character of the metallic bonding [after Beeck, O., Discussions Faraday Soc. 8, 126 (1950)). 

Other important measurements on the chemisorption of hydrogen include those of Rideal and Trapnell (90-92) on tungsten films and those of Schuit and his coworkers (93, 94a, b) on silica-supported metal catalysts. In each case the heat of adsorption was observed to decrease to very low values as complete coverage was approached. 

Fio. 10. Changes in electrical resistance during the chemisorption of hydrogen on nickel films. [Suhrmann, R., in Chemisorption (W. E. Garner, ed.), p. 106. Academic Press, New York, 1967.]... 

Typical magnetization-volume and pressure-volume isotherms obtained by this method are shown in Fig. 12. The magnetization-volume isotherm is linear i.e., each hydrogen molecule adsorbed causes the same decrease in magnetization. In the interpretation of these results, and those for other gases to which this method has been applied, it has been assumed on the basis of reasonable, but not conclusive, evidence that the slope of this isotherm corresponds to the entry of one electron into the nickel d-band for each chemisorhed H atom. From Fig. 12 it is evident that chemisorption of hydrogen is not complete at one atmosphere, and recent measurements (Vaska and Selwood, 102) indicate that it continues to much higher pressures. 

Fig. 13. Variation of the differential heat of chemisorption of hydrogen on nickel-kieselguhr calculated from magnetic measurements. Relative magnetization (

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