Hydrogen chemisorption, water

The host oxide lattice, moreover, is able to be reduced by spiltover hydrogen, producing water. Spillover induces lower temperatures of reduction for vanadium, uranium, chromium, cobalt, cadmium, and tin oxides (127), among others. The reduction may involve bulk transformation or it may be confined to the surface. The most studied example of this phenomena involves Ti02 and the resultant changes in sorption capabilities of the surface (SMSI), as discussed above. SMSI seems to be an extreme example of the change in chemisorptive properties with reduction and subsequent occulta-tion of the supported metal. 

The precious metals have the high turnover frequencies required for CO oxidation in the presence of hydrogen and water, in particular the chemisorption of both CO and 02. An optimum range of 02/C0 ratio is required in order to obtain the proper balance of adsorbed CO and adsorbed oxygen on adjacent sites. However, not all pure supported precious metals can achieve selectivity that is required for Prox. One good example of this is palladium because of its strong affinity for hydrogen chemisorption.16... 

Increased amounts of palladium on the charcoal surface increased the rate of hydrogen chemisorption, but exerted little effect on the equilibrium amounts of hydrogen chemisorbed. Extensive evacuation of the chemisorbed catalyst at elevated temperatures gave little hydrogen desorbed, but large amounts of water, methane, propylene, oxides of carbon, and so on. 

As discussed previously (2), the contribution to 0 from hydrogen chemisorption must be negligible. By means of Equations (2) and (6), values for the equilibrium constant, Ki, have been determined for palladium black and palladium-silver alloys of different compositions at various temperatures and are collected in Fig. 2. For every sample, data were obtained at two different water-vapor pressures. The lines drawn through the experimental points were taken as straight lines, whose slopes were used to com-... 

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. 

A reaction mechanism is suggested which involves dissociative chemisorption of hydrogen and water in competition on one type of active sites and chemisorption of carbon dioxide on the other type. Chemisorption of carbon dioxide is so strong that it prevents chemisorption of carbon monoxide. Chemisorbed carbon dioxide and hydrogen are in equilibrium on the surface. Reverse shift takes place by dissociation of the reaction product into carbon monoxide and a chemisorbed hydroxyl-species. The shift reaction is taking place by reaction between carbon monoxide from the gas phase and hydroxyl-species on the surface. Methanol is formed by step-wise hydrogenation of chemisorbed carbon dioxide. 

W. J. Lo, Y. W. Chung, and G. A. Somoijai, Electron spectroscopy studies of the chemisorption of oxygen, hydrogen and water on the titanium dioxide (100) surfaces with varied stoichiometry evidence for the photogeneration of titanium (3+) and for its importance in chemisorption. Surf. Sci. 77 199 (1978). 

To summarize, the use of heavy water as a deuterium source has provided a wealth of experimental information. Evidence for the associative ir-adsorption of benzene [species (I) J is secure (2). Evidence for hydrogen exchange in the benzene ring by an abstraction-addition mechanism is less well established, partly because of uncertainties that surround the mode of chemisorption and reaction of water at metal surfaces. Nevertheless, it would be wrong to deny that Scheme 6 is consistent with a large body of experimental work. 

Infrared studies show that when water is adsorbed on the surface, the background intensity in the hydroxyl region increases new bands may appear but hydrogen-bonding effects make such conclusions uncertain. If such a catalyst is then exposed to hydrogen (or deuterium), no bands due to adsorbed hydrogen (or deuterium) are observed. Thus, adsorption of water apparently occurs on the active sites and blocks out type I chemisorption. 

---