Atomic intermediates, chemisorbed

If other gases than the reactants and products of the catalytic reaction are present and if these foreign atoms are chemisorbed much more weakly than the intermediates of the catalytic reaction, the presence of the foreign gas will be of no consequence for the kinetic of the catalytic reaction. In this case the foreign gas behaves as an inert. 

CHx coverage and to C as the most abundant carbon-containing reactive intermediate. Chemisorbed carbon is then removed by steam or C02 as a coreactant. These elementary steps are consistent also with kinetic and isotopic measurements on other noble metal-based catalysts such as Pt and Ir studied by them. When exposed metal atoms are the abundant surface species, only the rate constant for the activation of... 

We shall assume that the surface of the catalyst contains chemisorbed atomic oxygen and that it is these chemisorbed oxygen atoms that act, when in the ion-radical state, as adsorption centers for CO molecules. In this case, during the adsorption of CO molecules, surface ion radicals C02-are formed as intermediate compounds, which, after being preliminarily neutralized, are desorbed in the form of C02 molecules. 

Intermediate cases in which the antibonding chemisorption orbital is broadened across the Fermi level can also arise (Fig. A. 14b). In such cases the antibonding orbital is only partially filled and the atom A will be chemisorbed, though with a weaker chemisorption bond than in Fig. A. 14a. 

The system of H chemisorbed on Cu/Ni is examined both with and without surface segregation. In the case of cs = Cb (i.e., no surface segregation), the curve of AE vs q, is shown in Fig. 6.3(a), and is seen to have a monotonic behaviour, which is almost linear for intermediate values of c, . In the dilute limits (cfe close to 0 or 1), AE is closer to the value for the corresponding pure system than a purely linear relationship would produce, which suggests that the effect of any minority atoms, even near the surface, is cancelled by the averaging process used in the CPA. 

If the foreign gas is chemisorbed much more strongly than the intermediates, the surface will to a large extent become covered by adsorbed foreign atoms. In this case the foreign gas behaves as a poison and the consequences for the kinetics will mainly be a large decrease in the reaction rate. 

The chemisorption case is exemplified by oxygen and sulfur on metals, the physisorption case by krypton and xenon on metals and graphite. Intermediate cases do exist for example, undissociated CO on metals is not physisorbed but chemisorbed and nevertheless it seems in many cases to be able to produce close-packed hexagonal overlayers. Also, some metal surfaces [for example, Pt(lOO), Ir(lOO), Au(100)] reconstruct into different lattices, exhibiting the effect of adsorbate-adsorbate interactions (here the adsorbate is just another metal atom of the same species as in the substrate). 

How much simpler things would be if these were monatomic gases and there was no need for all the juggling between intermediate species to dispose of unused molecular fragments We saw in our discussion of LEED that molecules of this sort are chemisorbed at metal surfaces in the dissociated state. The combination of chemisorbed hydrogen and oxygen atoms to form water clearly follows a different mechanism than in the gas phase. The fact that the reaction occurs rapidly in the presence of platinum and not at all when the reactants are mixed without the metal shows that the activation energy has been lowered tremendously by this modification. 

Figure 6 The rotation mechanism occurs via a allylic intermediate. a) The hydrogen attached to C3 can either bond to a surface Pt atom or recombine with a chemisorbed D atom to form gaseous HD.

Transition metals are active as catalysts because of their capacity to chemisorb atoms, given that the main role of transition metals as catalysts is to atomize gaseous molecules, such as H2, 02, N2, and CO, thereby providing atoms to other reactants and reaction intermediates [27],... 

Metals frequently used as catalysts are Fe, Ru, Pt, Pd, Ni, Ag, Cu, W, Mn, and Cr and some of their alloys and intermetallic compounds, such as Pt-Ir, Pt-Re, and Pt-Sn [5], These metals are applied as catalysts because of their ability to chemisorb atoms, given an important function of these metals is to atomize molecules, such as H2, 02, N2, and CO, and supply the produced atoms to other reactants and reaction intermediates [3], The heat of chemisorption in transition metals increases from right to left in the periodic table. Consequently, since the catalytic activity of metallic catalysts is connected with their ability to chemisorb atoms, the catalytic activity should increase from right to left [4], A Balandin volcano plot (see Figure 2.7) [3] indicates apeak of maximum catalytic activity for metals located in the middle of the periodic table. This effect occurs because of the action of two competing effects. On the one hand, the increase of the catalytic activity with the heat of chemisorption, and on the other the increase of the time of residence of a molecule on the surface because of the increase of the adsorption energy, decrease the catalytic activity since the desorption of these molecules is necessary to liberate the active sites and continue the catalytic process. As a result of the action of both effects, the catalytic activity has a peak (see Figure 2.7). 

Polanyi represents the reaction thus suppose that there are four valencies proceeding from four atoms of the solid surface, and the reacting atoms Ay By CtD are attracted, perhaps chemisorbed, to these four atoms of the surface. We have a surface intermediate compound involving all four atoms, in which the spacings may differ widely from those of the unstrained forms of AB and CD. With suitable disposition of the atoms and the forces adsorbing them, the reaction may proceed very easily, particularly if the end products AC and BD are not strongly adsorbed on the surface, and so evaporate as soon as they are formed. 

Electrochemical reduction of specific oriented chemisorbed intermediates is also very selective [77-79]. For example, reduction of chemisorbed thioph-enols (and mercaptans) result in selective scission of the carbon-sulfur bond to yield an unadsorbed hydrocarbon and an adsorbed sulfur atom. 

Here, K is the free form of the active center, and Ki and K2 are catalytic intermediates (the chemisorbed forms of atomic oxygen and carbon monoxide). 

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