Chemisorption layer

Before describing the high temperature diffusion experiments, some properties of the chemisorption layer will be discussed. Changes, including evaporation, are inappreciable below 800°K for fully or partially covered surfaces. At this temperature and above, irreversible changes occur in the... 

The experiments discussed above were all carried out with total pressures below 10-4 Torr. However, Hori and Schmidt (187) have also reported non-stationary state experiments for total pressures of approximately 1 Torr in which the temperature of a Pt wire immersed in a CO—02 mixture was suddenly increased to a new value within a second. The rate of C02 production relaxed to a steady-state value characteristic of the higher temperature with three different characteristic relaxation times that are temperature dependent and vary between 3 and 100 seconds between 600 and 1500 K. The extremely long relaxation time compared with the inverse gas phase collision rate rule out an explanation based on changes within the chemisorption layer since this would require unreasonably small sticking coefficients or reaction probabilities of less than 10-6. The authors attribute the relaxation times to characteristic changes of surface multilayers composed of Pt, CO, and O. The effects are due to phases that are only formed at high pressures and, therefore, cannot be compared to the other experiments described here. 

As the chemisorption technique is very convenient, this layer is widely used for optical and optoelectronic devices. Among a number of chemisorption layer techniques, the use of compounds with carboxyl functional group is most prevalent for preparation of the chemisorption layer of probe molecules on the surface of anodic oxidized aluminum. As the probe molecules are arranged on the solid surface directly by using this technique, the chemisorption layer may possess a lower diffusion barrier for oxygen. Thus, highly sensitive devices for PSP can be accomplished by using a chemisorption layer. In this section, the fluorescence probes for PSP based on the chemisorption layer are introduced. 

Porphyrins with carboxylate, as shown in Fig. 13, are also widely used as fluorescence probes for PSP based on the chemisorption layer. This section de-... 

The superscripts (R), (H) and (cr) characterize the boundary layer, the neutral interior of the semiconductor, and the chemisorption layer respectively. If we denote by n the number of defects per cubic centimeter, then we may write for the boundary layer of the n or p-type oxide (Fig. 6) ... 

We and others countered these arguments by invoking effects of mutual displacement of exchange partners in the chemisorption layer (75,76), the likelihood of the allyl radical formation from propene (77) and the proof for the occurrence of exchange reaction between deuturated ethylene and butene-1 (78). 

The method using selective chemisorption to determine the surface area of the active component is that a kind of stylet-molecule is adsorbed on the surface atoms of the active component, which can fully react with the atoms of active component and form a single chemisorption layer. The scattering state or surface area of the active component can be determined and calculated by the quantity of chemisorption. This method is often applied in the case of metal used as active component. At an early stage, H2 was often used as adsorbate to measure the surface area for the highly dispered metal such as Pt, Pd, Os, Fe etc. at about 200° C. Later, H2 was used to titrate to atoms of Pt, Ni, etc. on the surface under low temperatures and low pressures. It is clear that, for the measurement of different metals, one needs to choose the right stylet-molecule as adsorbate and conditions. 

The optical effect of a chemisorption layer cannot be expected to be as simple as the case of physisorption. Bootsma and Meyer found that the magnitude of the ellipsometric signals from reactive gases adsorbed on silicon and germanium surfaces did not agree with their model calculation, which assumed a simple layer of adsorbed gas molecules, and were sometimes even of opposite sign. 

Fujiwara, Y Amao, Y. Controlling the oxygen-sensitivity of 1-pyrenedecanoic acid chemisorption layer onto anodic oxidized aluminum plate. Bull. Chem. Soc. Jpn. 2002, 75,2697-2698. 

The adsorption of nonelectrolytes at the solid-solution interface may be viewed in terms of two somewhat different physical pictures. In the first, the adsorption is confined to a monolayer next to the surface, with the implication that succeeding layers are virtually normal bulk solution. The picture is similar to that for the chemisorption of gases (see Chapter XVIII) and arises under the assumption that solute-solid interactions decay very rapidly with distance. Unlike the chemisorption of gases, however, the heat of adsorption from solution is usually small it is more comparable with heats of solution than with chemical bond energies. 

Restructuring of a surface may occur as a phase change with a transition temperature as with the Si(OOl) surface [23]. It may occur on chemisorption, as in the case of oxygen atoms on a stepped Cu surface [24]. The reverse effect may occur The surface layer for a Pt(lOO) face is not that of a terminal (100) plane but is reconstructed to hexagonal symmetry. On CO adsorption, the reconstruction is lifted, as shown in Fig. XVI-8. 

The matter of surface mobility has come up at several points in the preceding material. The subject has been a source of confusion—see Ref. 112. Actually, two kinds of concepts seem to have been invoked. The first is that invoked in the discussion of physical adsorption, which has to do with whether the adsorbate can move on the surface so freely that its state is essentially that of a two-dimensional nonideal gas. For an adsorbate to be mobile in this sense, surface barriers must be small compared to kT. This type of mobile adsorbed layer seems unlikely to be involved in chemisorption. 

The saturation coverage during chemisorption on a clean transition-metal surface is controlled by the fonnation of a chemical bond at a specific site [5] and not necessarily by the area of the molecule. In addition, in this case, the heat of chemisorption of the first monolayer is substantially higher than for the second and subsequent layers where adsorption is via weaker van der Waals interactions. Chemisorption is often usefLil for measuring the area of a specific component of a multi-component surface, for example, the area of small metal particles adsorbed onto a high-surface-area support [6], but not for measuring the total area of the sample. Surface areas measured using this method are specific to the molecule that chemisorbs on the surface. Carbon monoxide titration is therefore often used to define the number of sites available on a supported metal catalyst. In order to measure the total surface area, adsorbates must be selected that interact relatively weakly with the substrate so that the area occupied by each adsorbent is dominated by intennolecular interactions and the area occupied by each molecule is approximately defined by van der Waals radii. This... 

A second class of monolayers based on van der Waal s interactions within the monolayer and chemisorption (in contrast with physisorption in the case of LB films) on a soHd substrate are self-assembled monolayers (SAMs). SAMs are well-ordered layers, one molecule thick, that form spontaneously by the reaction of molecules, typically substituted-alkyl chains, with the surface of soHd materials (193—195). A wide variety of SAM-based supramolecular stmctures have been generated and used as functional components of materials systems in a wide range of technological appHcations ranging from nanoHthography (196,197) to chemical sensing (198—201). 

Only the surface layers of the catalyst soHd ate generaHy thought to participate in the reaction (125,133). This implies that while the bulk of the catalyst may have an oxidation state of 4+ under reactor conditions, the oxidation state of the surface vanadium may be very different. It has been postulated that both V" " and V " oxidation states exist on the surface of the catalyst, the latter arising from oxygen chemisorption (133). Phosphoms enrichment is also observed at the surface of the catalyst (125,126). The exact role of this excess surface phosphoms is not weH understood, but it may play a role in active site isolation and consequently, the oxidation state of the surface vanadium. 

Adsorption of dispersants at the soHd—Hquid interface from solution is normally measured by changes in the concentration of the dispersant after adsorption has occurred, and plotted as an adsorption isotherm. A classification system of adsorption isotherms has been developed to identify the mechanisms that may be operating, such as monolayer vs multilayer adsorption, and chemisorption vs physical adsorption (8). For moderate to high mol wt polymeric dispersants, the low energy (equiUbrium) configurations of the adsorbed layer are typically about 3—30 nm thick. Normally, the adsorption is monolayer, since the thickness of the first layer significantly reduces attraction for a second layer, unless the polymer is very low mol wt or adsorbs by being nearly immiscible with the solvent. 

Physisorption occurs when, as a result of energy differences and/or electrical attractive forces (weak van der Waals forces), the adsorbate molecules become physically fastened to the adsorbent molecules. This type of adsorption is multilayered that is, each molecular layer forms on top of the previous layer with the number of layers being proportional to the contaminant concentration. More molecular layers form with higher concentrations of contaminant in solution. When a chemical compound is produced by the reaction between the adsorbed molecule and the adsorbent, chemisorption occurs. Unlike physisorption, this process is one molecule thick and irreversible... 

Further chemisorption of atomic oxygen into a second layer and/or further physical adsorption of Oj. 

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