Hydrogen chemisorption VIII metals

From chemisorption theory we know that adatom adsorption energies wiU decrease in a row of the periodic system of the group VIII metals when the position of the element moves to the right. The rate of hydrogenation of Cads vviU decrease with increasing adsorption energy of Cads and hence wiU decrease in the same order with element position in the periodic system. 

Throughout these studies, no product other than propane was observed. However, subsequent studies by Sinfelt et al. [249—251] using silica-supported Group VIII metals (Co, Ni, Cu, Ru, Os, Rh, Ir, Pd and Pt) have shown that, in addition to hydrogenation, hydrocracking to ethane and methane occurs with cobalt, nickel, ruthenium and osmium, but not with the other metals studied. From the metal surface areas determined by hydrogen and carbon monoxide chemisorption, the specific activities of... 

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. 

The work of Tauster and coworkers (1,2) showed that hydrogen chemisorption is suppressed on group VIII metals supported on a series of oxides after these samples have been reduced at high temperatures. The term strong metal-support interactions (SMSI) was introduced to describe this behavior. A similar suppression in hydrogen chemisorption has since been reported for many other supported metal systems 0-5). However, the use of other chemical probes (4, 5) demonstrated that different mechanisms of metal-support interactions could exist for different types of oxides. Furthermore, even for a so-called SMSI oxide, the degree of interaction could be influenced by many parameters such as crystallite size and reduction temperature. It would thus be desirable to find an approach to systematically compare catalytic behavior of different systems. 

TiO on the surface of a bulk metal can increase the rate of CO hydrogenation and selectivity of products relative to the clean metal surface. In the case of Ni, the effects of surface TiO on CO hydrogenation rate can be rationalized in terms of the perturbation in the relative competition between CO and H2 for chemisorption sites. This explanation cannot be generalized to other Group VIII metals. There is circumstantial evidence that a Ti species may be directly involved in the formation of the site on which reaction occurs on other Group VIII metals, but definitive proof and elaboration of the mechanism await future work. 

The chemisorption of hydrogen is the most widely used method and it has been studied on all the Group VIII metals [3,4,23], Adsorption is routinely dissociative and is applicable around 300 K for all these metals except Fe, on which surface it frequently exhibits activated adsorption [24], especially on very small Fe crystallites [25], thus it is not applicable to this metal. However, it is widely used with the other metals [26], and such a system is represented in Figure 3.3 for a Pt/Al203 catalyst, but other Group VIII metals and different supports, such as SiOi, Ti02, molecular sieves, and carbon, would provide similar examples. Studies of H2 adsorption on... 

Nonsteady behavior of electrochemical systems was observed by Fechner as early as 1828 [ii]. Periodic or chaotic changes of electrode potential under gal-vanostatic or open-circuit conditions and similar variation of current under potentiostatic conditions have been the subject of numerous studies [iii,iv]. The electrochemical systems, for which interesting dynamic behavior has been reported include anodic or open-circuit dissolution of metals [v-vii], electrooxidation of small organic molecules [viii-xiv] or hydrogen, reduction of anions [xv, xvi] etc. [ii]. Much effort regarding the theoretical description and mathematical modeling of these complex phenomena has been made [xvii-xix]. Especially studies that used combined techniques, such as radiotracer (-> tracer methods) ig. 1) [x], electrochemical quartz crystal microbalance (Fig. 2) [vii,xi], probe beam deflection [xiii], surface plasmon resonance [xvi] surface stress [xiv] etc. have contributed considerably to the elucidation of the role of chemisorbed species ( chemisorption), surface reconstruction as well as transport phenomena in the mechanism of oscillations. 

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