Chemisorption and hydrogenation

Reversible chemisorption and hydrogen-deuterium exchange on zinc oxide have been observed (Taylor et al., 136,137 , Harrison and McDowell,... 

Table 5.2 Comparison of particle sizes (nm) estimated by oxygen chemisorption and hydrogen titration of adsorbed oxygen with those obtained by physical techniques.

The interlamellar transformation of 1-octene involves the chemisorption and hydrogenation of reactant molecules on interlayer Pd atoms as active sites. Reactants can be supplied via diflusion from the bulk phase if the product molecules leave the interlamellar space. However, octane molecules formed tend to interact with the hydrophobic alkyl chains of HD AM, which results in the displacement of the solvent and prevents the entrance of further reactant molecules. If so, transport phenomena become predominant and the reaction slows down. Meanwhile, the accumulation of octane in the interlamellar space decreases the basal spacing (dL = 1.78 nm in pure octane where no swelling occurs at all), and thereby makes the internal Pd sites less accessible. 

A. M. Efstathiou, T. Chafik, D. Bianchi, and C.O. Bennett, CO Chemisorption and Hydrogenation of Surface Carbon Species Formed after CO/He Reaction on Rh/MgO A Transient Kinetic Study Using FTIR and Mass Spectroscopy, J. of Catalysis 141,24 1994). 

We consider first some experimental observations. In general, the initial heats of adsorption on metals tend to follow a common pattern, similar for such common adsorbates as hydrogen, nitrogen, ammonia, carbon monoxide, and ethylene. The usual order of decreasing Q values is Ta > W > Cr > Fe > Ni > Rh > Cu > Au a traditional illustration may be found in Refs. 81, 84, and 165. It appears, first, that transition metals are the most active ones in chemisorption and, second, that the activity correlates with the percent of d character in the metallic bond. What appears to be involved is the ability of a metal to use d orbitals in forming an adsorption bond. An old but still illustrative example is shown in Fig. XVIII-17, for the case of ethylene hydrogenation. 

Roohefort A, Andzelm J, Russo N and Salahub D R 1990 Chemisorption and diffusion of atomlo hydrogen In and on oluster models of Pd, Rh, and bimetalllo PdSn, RhSn, and RhZn oatalysts J. Am. Chem. Soo. 112 8239-47... 

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

F1 NMR of chemisorbed hydrogen can also be used for the study of alloys. For example, in mixed Pt-Pd nanoparticles in NaY zeolite comparaison of the results of hydrogen chemisorption and F1 NMR with the formation energy of the alloy indicates that the alloy with platinum concentration of 40% has the most stable metal-metal bonds. The highest stability of the particles and a lowest reactivity of the metal surface are due to a strong alloying effect. 

As this field is very wide, we will discuss first the gases that can be used to study metal dispersion by selective chemisorption, and then some specific examples of their application. The choice of gases, is, of course, restricted to those that will strongly chemisorb on the metal, but will not physically adsorb on the support. Prior to determining the chemisorption isotherm, the metal must be reduced in flowing hydrogen details are given elsewhere. The isotherm measurement is identical to that used in physical adsorption. 

Because XPS is a surface sensitive technique, it recognizes how well particles are dispersed over a support. Figure 4.9 schematically shows two catalysts with the same quantity of supported particles but with different dispersions. When the particles are small, almost all atoms are at the surface, and the support is largely covered. In this case, XPS measures a high intensity Ip from the particles, but a relatively low intensity Is for the support. Consequently, the ratio Ip/Is is high. For poorly dispersed particles, Ip/Is is low. Thus, the XPS intensity ratio Ip/Is reflects the dispersion of a catalyst on the support. Several models have been reported that derive particle dispersions from XPS intensity ratios, frequently with success. Hence, XPS offers an alternative determination of dispersion for catalysts that are not accessible to investigation by the usual techniques used for particle size determination, such as electron microscopy and hydrogen chemisorption. 

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

Ruthenium-copper and osmium-copper clusters (21) are of particular interest because the components are immiscible in the bulk (32). Studies of the chemisorption and catalytic properties of the clusters suggested a structure in which the copper was present on the surface of the ruthenium or osmium (23,24). The clusters were dispersed on a silica carrier (21). They were prepared by wetting the silica with an aqueous solution of ruthenium and copper, or osmium and copper, salts. After a drying step, the metal salts on the silica were reduced to form the bimetallic clusters. The reduction was accomplished by heating the material in a stream of hydrogen. 

In a series of studies of carefully prepared catalysts of Pt on silica gel (7,10-12) we have shown that the Pt particles are equi-axed, (and de-finitely not cuboidal as is often assumed) that the size (or percent metal exposed) agrees with results from hydrogen chemisorption, and that the particles are free of microstrain faults or twins, except when the average size is similar to the pore size of the support. In this latter case, the particles are elongated, and there is microstrain, probably due to differ-... 

The type catalyst, Pt/Ti02 reduced at 300 C behaves much like Pt/Si02, but reduction at 500°C largely eliminates its capacity for the chemisorption of H2 and hydrogenation while inducing activity for the hydrogenation of CO. Ti suboxide formed by reduction encapsulates the Pt particles. 

Let us now use the sequence of elementary steps to explain the activity loss for some of the catalysts The combination of hydrogen chemisorption and catalytic measurements indicate that blocking of Pt by coke rather than sintering causes the severe deactivation observed in the case of Pt/y-AljOj The loss in hydrogen chemisorption capacity of the catalysts after use (Table 2) is attributed mainly to carbon formed by methane decomposition on Pt and impeding further access. Since this coke on Pt is a reactive intermediate, Pt/Zr02 continues to maintain its stable activity with time on stream. 

When the metal nanoparticles are inserted into zeolite supercages, the size of the metal particles is confined according to the size of the supercage. However, after reduction of the precursor metal ions in a stream of hydrogen, the protons replacing the metal ions in the cation exchange position also interfere with the metal particles, influencing thereby their chemisorption and catalytic properties. 

Titrations of carbon monoxide and hydrogen sulfide up to 800 torr were performed at 30°C each volumetric titration was composed of two adsorption isotherms the first isotherm was a combination of chemisorption and physisorption. 

Beeck at Shell Laboratories in Emeryville, USA, had in 1940 studied chemisorption and catalysis at polycrystalline and gas-induced (110) oriented porous nickel films with ethene hydrogenation found to be 10 times more active than at polycrystalline surfaces. It was one of the first experiments to establish the existence of structural specificity of metal surfaces in catalysis. Eley suggested that good agreement with experiment could be obtained for heats of chemisorption on metals by assuming that the bonds are covalent and that Pauling s equation is applicable to the process 2M + H2 -> 2M-H. 

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. 

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