Mobile precursor

After some time in the mobile precursor state, the atom finds a free site and forms a true chemical bond with the surface ... 

Clearly, the sticking coefficient for the direct adsorption process is small since a considerable amount of entropy is lost when the molecule is frozen in on an adsorption site. In fact, adsorption of most molecules occurs via a mobile precursor state. Nevertheless, direct adsorption does occur, but it is usually coupled with the activated dissociation of a highly stable molecule. An example is the dissociative adsorption of CH4, with sticking coefScients of the order 10 -10 . In this case the sticking coefficient not only contains the partition functions but also an exponential... 

Frequently, adsorption proceeds via a mobile precursor, in which the adsorbate diffuses over the surface in a physisorbed state before finding a free site. In such cases the rate of adsorption and the sticking coefficient are constant until a relatively high coverage is reached, after which the sticking probability declines rapidly. If the precursor resides only on empty surface sites it is called an intrinsic precursor, while if it exits on already occupied sites it is called extrinsic. Here we simply note such effects, without further discussion. 

Equation (12) also contains a pre-exponential factor. In Section 3.8.4 we treated desorption kinetics in terms of transition state theory (Figure 3.14 summarizes the situations we may encounter). If the transition state of a desorbing molecule resembles the chemisorbed state, we expect pre-exponential factors on the order of ek T/h = 10 s . However, if the molecule is adsorbed in an immobilized state but desorbs via a mobile precursor, the pre-exponential factors may be two to three orders of magnitude higher than the standard value of 10 s . ... 

Using threshold ionization mass spectrometry and in situ ellipsometry, Schroder and Bauer [555] have shown that the Si2H4 radical may well be the species responsible for deposition, rather than SiH3 as in PECVD. This larger and less mobile precursor is thought to be the cause of the observed differences in the deposition conditions required in HWCVD and PECVD to obtain device quality material. 

The oceanic burden in December 2004 shows the contamination of the ocean after 50 years of PFOA emissions (Figure 3.14). Highest PFOA burden is located in the northern Atlantic, Mediterranean, and the Arctic ocean. Contaminations of the Atlantic, Mediterranean and Pacific can be related to the vicinity to the oceanic source. PFOA in remote regions, however, such as in the Arctic must have been transported via atmosphere or ocean. MPI-MTCM does not simulate degradation of PFOA from volatile, highly mobile precursor substances, that contribute to the ocean burden in the Arctic by deposition. Then annual dry and wet deposition rates of PFOA in the model are small compared to the mass emitted directly to the ocean. This implies that the burden in the Arctic is results mainly from oceanic long-range transport. 

The exponent key is called the exposure number of the ensemble Q. k is the number of potential poison sites within the passivation range, which may each deactivate one or more of the metal atoms of the ensemble (1. The weight factor, qi, is the probability that the S-ensemble has an exposion number, k. It was shown for ensembles containing three or more sites that this model predicted ensemble concentrations corresponding to those calculated by Monte-CarTo simulation of chemisorption via a mobile precursor. As this is the most common chemisorption mechanism, the model is expected to be of rather general applicahility. 

The Kislink factor, K, which takes values between 0 and 1, gives the degree of mobility of the precursor, with lower values associated with a highly mobile precursor. Values for the three CO/Pt systems are all around 0.4, indicating fairly mobile molecnlar extrinsic precursors. 

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. 

On the other hand, the excimer emission because it is 80% non-correlated with monomer trap emission and because it is effectively quenched in the copolymers even at low temperatures, must largely arise from a mobile precursor. The activation energy for hopping of this precursor is implied to be <10 cm l. This is not unreasonably low(12,17), and indeed, the zero-point energy of the phenyl chromo-phore could in principle allow completely activationless hopping (tunneling) at reasonable rates. Determination of the true situation will require measurements at still lower temperatures, which are now in progress. We note that the polystyrene emission spectrum at 4.2K reported in (Id) indicates a monomer/excitner intensity ratio nearly the same as our 20K spectra. 

The systems discussed above are, in many ways, ideal in that adsorption is very site-specific or limited to the surface layer. Many systems are known to absorb as well as adsorb. This effect is sometimes reflected in sticking probability versus coverage profiles. These may show an increase in s because of a sudden freeing of surface sites due to the absorption process. One example is 02 on A1 111] [434]. However, the adsorbed species may form at all coverages and the s versus N profiles look like those of a typical mobile precursor-trapping model. Fromm [435] has proposed a model to fit this absorption—adsorption mechanism. His... 

If the s versus N profile for a mobile precursor is typically initially independent of coverage, then a typical profile for the case of activated adsorption follows an exponential decay and s can be written [273, 453]... 

If we consider first the case of an incident As4 beam, the essential features of the model proposed by Foxon and Joyce [343] are illustrated in Fig. 43. As4 is adsorbed into a mobile precursor state and in the absence of a surface Ga population has a zero sticking coefficient but a... 

Perfluoro chemicals are present in the environment and have been detected all over the globe [6-14], whereby fluorotelomer alcohols may act as highly mobile precursors for perfluorinated carboxylic acids [8]. PFC are present in drinking water resources, where they probably persist for a long time due to their high environmental stability. Thus drinking water suppliers have to deal with the possibility of elevated PFC concentrations in their raw water and thereby need to consider treatment strategies as barriers for PFC. 

The idea behind this method is the ion exchange of two different metal ions in the faujasite cages whidi is then fcrilowed by reduction. Random collisions of the metal atoms in the supercages will then presumably lead to the formation of bimetallic dusters. It has been postulated that l etallic clusters are favored if the precursor of the less redudble metal is motnle in the zeolite cages. For instance, if Pt is the noUe (easily reduced) component of a bimetallic duster, it will be reduced first, and the subsequent reduction of the other mobile precursor in contact with a Pt would then be catalyzed by the Pt. Hence, the temperature required to reduce the less redudble element is markedly lower than that required for its reduction in the absence of Pt. 

The surface electronic structure of Mg (0001) was analysed by angle resolved photoemission (Karlsson et al, 1982), Two sharp peaks due to surface states were identified. No experimental studies on H adsorption on single crystalline Mg are available. Self consistent calculations of the potential energy surface for a molecule on Mg (0001) were performed by N rskov et al, (1981), They reveal (Fig, 11) an activation barrier for adsorption into a mobile precursor state and an activation barrier for dissociation which depends strongly on the adsorption site geometry. 

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). 

This is the observed behavior when molecules adsorb directly. However, frequently the rate of adsorption decreases less steeply than predicted by (6.27), which is caused by the trapping of molecules in a mobile precursor state (Figure 6.2). The... 

The observed rate of adsorption of CO does not quite have the expected Langmuir form where the rate of adsorption is proportional to the fraction of empty sites. This is because CO is initially trapped in a mobile precursor site. Quantitative agreement with the experimental oscillations requires using (Sco/ cosat) with a power higher than unity, say between 3 and 4. 

Oxides of heavier elements, such as Mb, Hf, Ti, Zr, and Ta are very stable in their highest oxidation state. The mechanism for rare-earth inhibition seems to originate from the alkaline precipitation of protective oxide films at active cathodes. However, soluble and mobile precursors of these oxides remain difficult to stabilize in aqueous solution with the slight exception of Ce, which is the only lanthanide element that exhibits a tetra-valent oxidation state that is stable as a complex in aqueous... 

The sticking coefficient is close to unity and remains constant up to 60% of a monolayer on Fe(l 00) [383] and up to 40% of a monolayer on Fe(l 11) [437] indicating the existence of a mobile precursor for the adsorption. The sticking coefficient decreases with temperature. For the catalyst the adsorption kinetics is first order [402]. 

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