Chemisorption state

The chemisorption of a molecule is often a precursor [31] to fiirther reactions such as dissociation (see section A3,9.5.2). that is, the molecule must reside in the precursor state exploring many configurations until finding that leading to a reaction. Where there is more than one distinct chemisorption state, one can act as a precursor to the other [32], The physisorption state can also act as a precursor to chemisorption, as is observed for the 02/Ag(l 10) system [33],... 

There is a very rich literature and a comprehensive book6 on the role of promoters in heterogeneous catalysis. The vast majority of studies refers to the adsorption of promoters and to the effect of promoters on the chemisorptive state of coadsorbed species on well characterized single crystal surfaces. A... 

Increasing catalyst potential and work function leads to a pronounced increase in total oxygen coverage (which approaches unity even at elevated temperatures) and causes the appearance of new chemisorption states. At least two such states are created on Pt/YSZ (Fig. 4.43) A strongly bonded one which, as discussed in Chapter 5, acts as a sacrificial promoter during catalytic oxidations, and a weakly bonded one which is highly reactive and causes the observed dramatic increase in catalytic rate. 

At a Pd(l 11) surface at room temperature, the chemisorption state is disordered when the NO pressure is less than 3 x 10-6 Torr with very noisy STM images due to the high mobility of the adsorbed molecules.14 With increasing pressure (and coverage), the c(4 x 2) state, which is reversible, is locked-in and immobile. The adsorption at lower temperatures (150-200 K), where the coverage exceeds that at room temperature, the c(4 x 2) state coexists with a p(2 x 2) and a c(8 x 2) phase the latter is only present when it coexists with the c(4 x 2) and p(2 x 2) states. 

From the requirement t < 1 in (6.32), comes the chemisorption-state existence condition... 

Fig. 6.5. Chemisorption-state energies for H-Cu/Ni with cs= Cb. After Sulston (1986).

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... 

Enthalpy changes on adsorption and desorption of probe molecules on catalyst surfaces may also be followed by differential thermal analysis (DTA) (67) although this method has been used only sporadically in the past. The experimental techniques have been described by Landau and Molyneux (67) very recently. As an example, Bremer and Steinberg (68) observed three endothermic peaks during the desorption of pyridine from a MgO-Si02 catalyst these peaks were assigned as three different chemisorption states of pyridine. 

The TBMD results demonstrate that the molecular sticking probability of 02/Pt(l 1 1) for the whole energy range can be understood in terms of trapping into the chemisorption states. However, these results can only be obtained and understood if the multidimensionality of the adsorption process is appropriately taken into account. 

Chemisorption state modeling by extended Huckel molecular orbital calculations were made in a collaborative study with R. M. Gavin, and full details of these calculations will be published elsewhere. 

Stereochemical Features of Benzene and Toluene Coordination Chemistry. Benzene forms an ordered chemisorption state on the flat Ni(lll) and Ni(100) surfaces at 20°C with unit cells of (2/3x2/3)R 30° and c(4x4), respectively (13). The symmetry data do not fix the registry of the benzene with respect to the metal atoms nor the orientation of the ring plane to the surface plane. However, basic coordination principles would suggest that the benzene ring plane should be parallel to the surface plane. In Figures 8 and 9, possible registries of the benzene with respect... 

Our studies of olefin and acetylene chemisorption states on platinum surfaces is presently incomplete. Ethylene and acetylene chemisorption on platinum (111) are complicated by the apparent presence of more than one chemisorption state (indicated by thermal desorption studies). When C2HH and C2D1, are chemisorbed on Pt(lll), the small fraction of ethylene thermally desorbed as ethylene comprises nearly a statistical mixture of all possible molecules. Thus we see here reversible C-H (and C-D) bond breaking on this flat platinum surface. In an analogous experiment with C2H2 and C2D2> only a small extent of H-D exchange was observed for the small fraction of acetylene molecules that reversibly desorb from this surface (11). 

Acetonitrile and Methyl Isocyanide (8). Acetonitrile forms an ordered chemisorption state on the fully flat nickel surfaces, a p(2x2) and a c(2x2) on Ni(lll) and Ni(100), respectively. Acetonitrile thermal desorption from these two surfaces was nearly quantitative (a small amount of acetonitrile decomposed at the temperatures characteristic of the reversible thermal desorption from these surfaces). Importantly from an interpretive context, acetonitrile was quantitatively displaced from these two flat low Miller index planes by trimethylphosphine (8). However, the displacement was not quantitative (only 90-95% complete) from the stepped and stepped-kinked surfaces. For the super-stepped (110) surface, chemisorption was nearly irreversible and no acetonitrile could be displaced from this surface by trimethylphosphine. 

Hydrocarbon Fragments - Modeling by Molecular Orbital and Cluster Chemistry. A basic guideline for metal surface coordination chemistry with respect to hydrocarbon or hydrocarbon derivatives may be formulated as follows If the stereochemistry of the chemisorption state allows C-H hydrogen atoms to closely approach surface metal atoms then the chemisorption state should be further stabilized by the formation of a three-center two-electron C-H-metal bond. This effect should be more pronounced the more electron deficient the metal surface. There should be an activation of the C-H bond and the hydrogen atom should become more protonic in character. If the C-H bond is sufficiently weakened by this interaction then C-H bond cleavage should result. 

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