Chemisorbed intermediates chemisorption

Chemical system, 32 278-283 Chemisorbed intermediates, 38 1-135 see also Oxide electrocatalysts cathodic hydrogen evolution, 38 58-66 chemical identity, 38 16-23 species from dissociative or associative chemisorption, 38 20-23 species from electrochemical discharge steps, 38 16-20... 

While a knowledge of surface mobility is of great interest in physical adsorption, it becomes essential in chemisorption phenomena. For instance in calorimetric work a curve of differential heats of adsorption versus surface coverage will be horizontal if adsorption is localized but shows the customary slope from high to low values of the heat of adsorption if the adsorbed layer is mobile Furthermore if a chemisorbed intermediate takes part in a surface reaction (crystal growth, corrosion, catalysis), it is essential to know whether, after adsorption anywhere on the surface, it can migrate to a locus of reaction (dislocation, etch pit, active center). Yet here again, while Innumerable adsorption data have been scrutinized for their heat values, very few calculations have been made of the entropies of chemisorbed layers. A few can be found in the review of Kemball (4) and in the book of Trapnell (11). 

Related to these matters has been the question whether two-component electrode metals (dual-site model) could lead to an electrocatalyst surface that exhibited catalytic properties better than either of its components. Qualitative ideas about electron spillover between one component and another at microcrystal grain boundaries, or transfer of the chemisorbed intermediate from one site to another, could suggest the possibility of such an effect. However, a quantitative theoretical analysis of this question by Parsons (149), based on his treatment of chemisorption effects at single metals (23) having various AG ,, h values, showed that, for practical applications, almost no... 

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 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). 

Another PES topology for molecular dissociation occurs when an intermediate molecularly chemisorbed state lies parallel to the surface between the physisorption well and the dissociated species as shown in Figure 3.2(b). This molecular state is usually described in terms of a diabatic correlation to a state formed by some charge transfer from the surface to the molecule [16]. In this case, there can be two activation barriers, V] for entry into the molecular chemisorption state of depth Wx and barrier V2 for dissociation of the molecularly chemisorbed state. This PES topology is relevant to the dissociation of some it bonded molecules such as 02 on metals, although this is often an oversimplification since distinct molecularly adsorbed states may exist at different sites on the surface [17]. In some cases, V < 0 so that no separate physisorbed state exists [18]. If multiple molecular chemisorption... 

It should be noted that more complex molecules than CO (e.g., methanol) produce many kinds of intermediates in the course of the catalytic oxidation, and they will chemisorb to form surface states. If the energy of the surface states formed by chemisorption of these intermediates are shallow enough from the delocalized band (conduction band and valence band) edges in the... 

The intermediate case is that the antibonding level of the molecule broadens across the Fermi level, as shown in Figure A.13. It becomes partially filled, and consequently the intramolecular bond is weakened. The relevance for catalysis is that this type of chemisorption weakens (or activates in catalytic language) the A-A bond and, as a result, it may be more reactive than in the gas phase. In molecules such as CO and NO this weakening of the bond is readily observed with infrared spectroscopy. This partial filling of the antibonding orbital of a chemisorbed molecule is often referred to as back donation . 

Chemisorption is vital in catalysis. Transition metals such as Fe, Pd, Pt, Ir, Ni, Co, Cr, Mn, Ti, Hf, Zr, V, Nb, Mo, W, Ru, and Os have the ability to chemisorb simple molecules such as 02, CO, H2, C02, C2H4, and N2 [24,25,27], However, if chemisorption is very strong, the catalytic sites are blocked. Therefore, it is necessary that an intermediate between weak chemisorption, when there is no reaction, and strong chemisorption, when the sites are blocked [24,25], is available. In this sense, the first d-block metals form especially stable surface bonds, while the noble metals form weak bonds. These properties are unfavorable to catalysis. Hence, the best metallic catalysts are in between these two groups [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). 

Burch and Flambard (113) have recently studied the H2 chemisorption capacities and CO/H2 activities of Ni on titania catalysts. They attributed the enhancement of the catalytic activities for the CO/H2 reaction (after activation in H2 at 450°C) to an interfacial metal-support interaction (IFMSI). This interaction is between large particles of Ni and reduced titanium ions the Ti3+ is promoted by hydrogen spillover from Ni to the support, as pictured in Fig. 8. The IFMSI state differs from the SMSI state since hydrogen still chemisorbs in a normal way however, if the activation temperature is raised to 650°C, both the CO/H2 activity and the hydrogen chemisorption are suppressed. They define this condition as a total SMSI state. Between the temperature limits, they assumed a progressive transition from IFMSI to SMSI. Such an intermediate continuous sequence had been... 

From table 2, it appears that the catalyst at 37S°C becomes enriched in nitrogen. Also, coke appears to originate from adsorption of PNAs containing much less nitrogen, and the enrichment is most likely due to chemisorption of intermediate or final products of HDN reactions. This could indicate that not all the nitrogen is associated with the coke, but a major part is chemisorbed on the active NiMoS phase. 

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