Non-activated dissociation

There are a great many examples of non-activated dissociation at surfaces. H2 dissociation on Pd(100) and the other single crystal faces of Pd are perhaps the best studied examples, both experimentally and theoretically. Part of the interest in these systems is the importance of Pd as a hydrogenation catalyst. The other major technological impetus is that bulk Pd readily absorbs large amounts of H, so is a prototype for metal-hydride hydrogen storage systems. 

In reality, the various mechanisms dominant for dissociation of H2/Pd at low Ei form a continuum. Steering implies only modest molecular energy transfer (En —Ej, ) prior to dissociation, dynamic trapping implies considerable molecular energy transfer (En — Ej, ) so that dissociation is indirect and a precursor-mediated process implies not only an indirect interaction from (En — Ej,E ), but also thermalization with the lattice prior to dissociation [317]. 

The 02 dissociation on Pt(lll) is another of the well-studied paradigms in the dynamics of surface chemistry, both because of the richness of its dynamics and because Pt is an important oxidation catalyst, e.g., in automotive exhausts. 

Spectroscopic measurements of 02 adsorption at very low Ts (and low En) demonstrate that only the physisorbed state is populated under this condition, and this converts thermally to chemisorbed 02 at Ts = 35 [30,325]. It was therefore proposed that the dissociation of 02 at low E is due to sequential precursors the physisorbed state acts as a precursor to molecular chemisorption and the molecularly chemisorbed state is a precursor to dissociation [30]. This can be written as the following kinetic equation  

STM experiments are in general agreement with the picture above. STM directly observes the two different molecular precursor states at low Ts and observes their dissociation by thermal annealing, photochemistry and viatunneling electrons [153,326]. The molecularly adsorbed states and dissociated show a strong Ts dependent clustering at low Ts, and this is evidence for a very mobile physisorbed precursor since the molecularly chemisorbed states are not significantly mobile at these Ts [326,327]. 

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DFT calculations [342,343] demonstrate that the only stable molecularly adsorbed N2 state is for an atop vertically oriented molecule with W = 1.1 eV, although a shallow metastable well parallel to the surface also is predicted. Classical calculations on a 6D DFT PES observe dynamic trapping and non-activated dissociation throughout the energy range studied (Et < 1 eV) (Figure 3.36(c)), although the... 

Later the barrier puzzle was resolved in a close collaboration between experiment and electronic structure calculations. It turned out that it is not sufficient to just consider the H2 dissociation on clean Si(l 0 0). Instead it was realized that it is very important to take into account the exact surface structure and surface coverage in the determination of the adsorption/desorption barriers [64, 72]. At surface imperfections such as steps the reactivity of a surface can be extremely modified. It was found experimentally on vicinal Si(l 0 0) surfaces that the sticking coefficient at steps is up to six orders of magnitude higher than on the flat terraces [71]. This finding was supported by DFT studies which showed that non-activated dissociation of H2 on the so-called rebonded DB steps on Si(l 0 0) is possible [71, 78], while on the flat Si(l 0 0) terraces the dissociative adsorption is hindered by a barrier of 0.4 eV [37],... 

Figure 26.11 displays two examples of dissociative chemisorption, which depend on the shapes of the PESs. In one case one has a non-activated dissociative... 

Dissociative chemisorption of a diatomic molecule can also happen through the dissociation in a gas phase and a creation of two gas phase atoms these two atomic species can be then adsorbed on the surface (this way is almost always non-activated). If the curves describing molecular and atomic adsorption intersect at or below the zero potential energy line, then the precursor physisorbed molecule can experience non-activated dissociation, followed by chemisorption (Fig. 4. la). In contrast, if the energetic for these two pathways are such that the intersection occurs above the zero eneigy plane, then chemisorption wiU be activated with activation energy, Ead, as indicated in Fig. 4. lb. 

As mentioned in the discussion of the reaction mechanism for this transformation, the active species is a dicoordinate Pd(0) complex, and it is unclear whether an associative or a dissociative process is operative for oxidative addition. In this context, different NHC complexes containing only one carbene ligand have been tested in the Mizoroki-Heck reaction. The most successful are those prepared by Beller, which were able to perform the Mizoroki-Heck reaction of non-activated aryl chlorides with moderate to good yields in ionic liquids (Scheme 6.13). The same compounds have also been applied to the Mizoroki-Heck reaction of aryldiazonium... 

The stoichiometric Pt3Co(lll) surface has one-quarter of the Pt atoms replaced with Co atoms. Bulk Co is very reactive toward oxygen, on which O2 dissociation is probably non-activated. The Co atoms on the alloy surface are diluted by Pt, but... 

The experimental evidence, first based on spectroscopic studies of coadsorption and later by STM, indicated that there was a good case to be made for transient oxygen states being able to open up a non-activated route for the oxidation of ammonia at Cu(110) and Cu(lll) surfaces. The theory group at the Technische Universiteit Eindhoven considered5 the energies associated with various elementary steps in ammonia oxidation using density functional calculations with a Cu(8,3) cluster as a computational model of the Cu(lll) surface. At a Cu(lll) surface, the barrier for activation is + 344 k.I mol 1, which is insurmountable copper has a nearly full d-band, which makes it difficult for it to accept electrons or to carry out N-H activation. Four steps were considered as possible pathways for the initial activation (dissociation) of ammonia (Table 5.1). 

For molecular dissociation, the qualitative behavior of S(Et, 0 , Ts) depends greatly on the mechanism of dissociation, i.e., whether it is direct or precursor-mediated. When the dissociation is direct, absolute values of S are generally only weakly dependent on Ts. On the other hand, 5 is a very strong function of Ts for precursor-mediated dissociation (eq. (2.7)), decreasing nearly exponentially with Ts when EcEd. Generally, the entrance to the precursor state is non-activated and S decreases with Ej due to a... 

For non-activated haloalkanes, a dissociative electron capture7,15,28 has been proposed mainly based on theoretical calculations and experimental findings in the gas phase, solid matrices and electrochemistry in polar solvents. The ET to the halide is accompanied by bond fragmentation and therefore radical anion intermediates are not formed. However, this is not necessarily the case for bridgehead alkyl halides. For these compounds, in which... 

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