Precursor mechanism

Harris J and Kasemo B 1981 On precursor mechanisms for surface reactions Surf. Sc/. 105 L281... 

Figure 4.18 (a) STM image (39 x 23 nm) 02 molecules at Ag(l 10) at 65 K, illustrating the hot precursor mechanism at a coverage of 0.02. The inset shows an atomic resolution image of the silver surface and the 02 molecules as dark holes. Also shown (b) is a ball model with oxygen molecules (black) and surface silver atoms (white) and second layer silver atoms (grey). (Reproduced from Ref. 32). 

As a complement to these Pt(l 1 1) studies, Nolan et al. investigated 02 dissociation on Pd(l 1 1) [81]. The ability of both surfaces to bind oxygen in similar molecular forms (peroxo-like and superoxo-like) at high coverages, could provide additional support for the sequential precursor mechanism proposed for Pt(l 1 1). EELS and dissociation probability data obtained from this study produced trends similar to that observed for the Pt(l 1 1) experiments [75, 76], supporting the same sequential precursor mechanism described above. 

Data shown in Fig. 6 lead to the following conclusions at temperatures near 0 the desorption rate demonstrates Arrhenius behaviour with the effective activation energy of 50 4 kJ/mol, which is much greater than the value predicted by the simple MP mechanism (25-35 kJ/mol). Also, at temperatures above -40 C, the ice vaporization rate exceeds the desorption rate predicted by the simple MP model. In summary, the mobile precursor mechanism, as formulated by Somorjai and Davy, fails to describe the desorption kinetics of ice at temperatures near its melting point. 

Since increasing the incident kinetic energy, Ej, of the molecule decreases a, Sq will decrease w th increasing kinetic energy in a precursor mechanism. This is one signature of a precursor mechanism often looked for experimentally. ... 

We should point out that more complex one-dimensional PESs are possible with, for example, two different molecularly physisorbed species separated by a barrier from each other and also by a barrier from the chemisorbed state (Grunze et al. 1984 Whitman et al. 1986). In such cases, the bahavior of So for precursor mechanism chemisorption may not be so transparent. 

Results for Sq in the N2/W(100) system are shown in Fig. 9 as a function of not jCos 0j. These data clearly demonstrate that Sq scales with the total energy, which is especially intriguing since there is no indication of a precursor mechanism. Two possible explanations were advanced (Pfnur et al. 

Figure 4.11. Variation of 5 (0)/5 (O) with d for adsorption via precursor mechanism. The precursor parameter K equals k a/k d [36].

We can summarize these effects of moving zones by saying that for the reactant precursor mechanism (either poisoning or coking), the bed is deactivated from front to back, and for the product precursor mechanism deactivated from back to front. The similarity of such behavior to some corresponding reaction schemes involving individual catalyst particles is striking, as will be shown in Chapter 7. 

To simulate this type of operation a mixing-cell sequence is quite useful [J.B. Butt and D.M. Rohan, Chem. Eng. Sci., 23,489 (1968)]. Let us start with the familiar case of first-order kinetics for the main reaction and first-order kinetics for the deactivation, where coke deposition occurs via a reactant precursor mechanism [see scheme (XXVII) in Chapter 3). For the main reaction we may write... 

Reactions involving hot adatoms, such as discussed earlier, clearly represent an intermediate situation between the two limiting cases LH and ER, since the reactants are not in complete thermal equilibrium with the surface (as for LH). Neither does the reaction take place by direct collision from the gas phase (as for ER). Such a kind of intermediate mechanism ("precursor" mechanism) has been proposed [16], whereafter one of the reactants is not fully accommodated with the surface but reacts from a state that is vibrationally excited with respect to motion normal to the surface. 

Planar Intramolecular Charge Transfer Precursor Mechanism... 

Van Ruiten and co-workers [45] studied the drawability and attainable mechanical properties of PA 4,6 yarn using true stress-strain curves. The concept of the molecular network was applied to an analysis of these fibres and yarns and the mechanical properties of yarns with different draw ratios was evaluated in terms of the network draw ratio, which was determined by matching true stress-strain curves. The validity of the molecular network concept for these yarns and its suitability for predicting fibre drawability was assessed. A method for predicting the maximum attainable tenacity of drawn yarns under given drawing conditions from precursor mechanical properties is proposed. 

Figure 6.2. (a) Sticking coefficient as a function of coverage in the case of direct adsorption and a mobile precursor (b) adsorption isotherm of NO on the (111) surface of rhodium showing the effect of the precursor mechanism (from Borg et ai, 1994). 

Harris, J. and B. Kasemo (1981). On precursor mechanisms for surface reactions. Surf. Sci. 105, L281. 

It is difficult to decide which of these principles is the older phylogenetically. Even some unicellular organisms are known to be equipped with acetylcholine and epinephrine. Perhaps both mechanisms developed from one common precursor mechanism. 

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