Chemisorption, organic molecules with

Some organic molecules with OH dipoles behave similarly to water molecules when adsorbed on salt surfaces such as CaF2, BaCl2, or NaCl surfaces. On heating they do not desorb as such, but HF or HC1 evaporates and their ions are left behind as an adsorbed layer on the depleted salt surface. Many di- and polyhydroxy anthraquinones, like alizarin (138), behave in this way. The chemisorption reaction is accompanied by characteristic color changes, which could be studied by their absorption spectra, using completely transparent adsorbent layers as obtained by vacuum sublimation of these salts. 

The adsorption and ordering characteristics of the various hydrocarbon molecules on the low Miller index platinum surfaces are discussed in great detail elsewhere. These two surfaces appear to be excellent substrates for ordered chemisorption of hydrocarbons, which permit one to study the surface crystallography of these important organic molecules. The conspicuous absence of C-H and C-C bond breaking during the chemisorption of hydrocarbons below 500 K and at low adsorbate pressures (10 9-10-6 Torr) clearly indicates that these crystal faces are poor catalysts and lack the active sites that can break the important C-C and C-H chemical bonds with near zero activation energy. 

The reactions of ethylene and acetylene with Si( 100)-(2 x 1) were initially described as being [2 + 2] cycloadditions, with the di-configuration predicted by this mechanism believed to provide the dominant reaction products. A variety of alternate reaction products could actually be formed as a result of the chemisorption, with, e.g., the organic molecule spanning silicon atoms in different dimer rows, adhering above a row oriented perpendicular to the silicon dimers, or adhering above a row and parallel to the dimers. As reviewed in Sec. 3, a variety of alternate structures have now indeed been found for chemisorbed acetylene. Hence, while the [2 + 2] cycloaddition mechanism appears apt for ethylene chemisorption, its applicability to similar processes in acetylene is questionable. 

In addition to the chemical steps, which are the only steps involved in stoichiometric or in homogeneous catalysis reactions, heterogeneous catalysis reactions involve also physical steps, i.e. transport (transfer) of organic molecules (and heat) from the reaction mixture to the active sites of the solid catalyst and vice versa.113-151 Another difference deals with the chemical steps, which do not occur in the fluid phase, but for part of them involve both fluid and solid phases (chemisorption and desorption), the other part occurring at the surface of the catalyst.113-151... 

At sufficiently high temperatures, due to not too strong cohesion, the surface Pd atoms may acquire convenient positions to form a bond with reacting hydrocarbon molecule (189). This concept, called extractive chemisorption, was introduced by Burwell et al. (190, 191) as a possible cause of absence of steric hindrance in adsorption and reaction of some complex organic molecules. It was proposed that in chemisorption one or two metal atoms were displaced above the initial planar level, leading to increased bonding to the surface for low-dispersion catalysts. An extension of this concept to the problem of structure sensitivity allows one to explain several cases of the relatively mild (or absent) structure sensitivity in many reactions catalyzed by Pd catalysts. 

Another technologically important reaction is the Fischer-Tropsch synthesis, with iron oxide being one of the components of some catalysts. A detailed understanding of the complex mechanism of this reaction can be obtained by studying the chemisorption of simple molecules on well-characterized surfaces by means of advanced surface-sensitive spectroscopic techniques. A few investigations of the interaction of small molecules (such as CO, CO2, H2O, O2, H2, and NO) (520-522) and organic molecules on iron oxide surfaces (523-527) have been carried out. 

The term adsorption is the complex phenomenon of the interaction of an organic or inorganic molecule with a surface. Physisorption and chemisorption are generally distinguished according to the nature of forces involved. 

The well characterized and stable surface phases observed on the Sn-Pt(l 11) have provided researchers in the chemisorption and catalysis field with a substrate of great interest for studying the properties of bimetallic interfaces. Simple probe gases such as CO have been studied after adsorption on this system [45] as well as a variety of organic molecules such as acetylene [46], cyclohexane and benzene [47, 48], butane and isobutane [49], methanol, ethanol and water [50]. Several surface reactions of the above gases were also studied. 

Simple linear relationships developed between the quantum chemical indexes of organic molecules (their maximum charges and energies of the frontier orbitals) and their empirical donor acceptor parameters as well as the heats of the hydrogen-bonded complexes formation with silica surface OH groups or the chemisorption activation energies on this surface may be used for estimation of the complexes stability and reactivity of organic compounds towards Bronsted sites of other oxide surfaces. The electron donor ability of... 

The H-type isotherm, indicative of very strong adsorbate-adsorbent interaction (i.e., chemisorption), is really an extreme case of the L-type. This isotherm is not often encountered with organic molecules because few of them form strong ionic or covalent bonds with soil colloids. 

The surface of nanocrystalline MgO has been found to interact strongly with polar organic molecules, such as aldehydes, ketones, and alcohols, by a dissociative chemisorption process, which results in the destruction of the organic molecule. This is in contrast to high-surface-area activated-carbon absorbents, which merely absorb the moiety with no resultant reaction. The chemisorption of acetaldehyde on MgO nanocrystals results in... 

The understanding of the relationships between molecular structure of tailored organic molecules, their hierarchical organization in assemblies chemically bound to surfaces and interfaces as well as their fimctionality represent fundamental topics of current interest [104,105]. hi the following we shall focus on so-called chemisorbed, self-assembled monolayers (SAM), which are distinctly different from the physisorbed, hydrogen-bonded adlayers discussed in the previous paragraph. Following a historical development we will use the terminus self-assembled monolayers herein exclusively as molecular assemblies formed by chemisorption of an active surfactant onto a solid surface [106-108]. We will specifically focus on selected results with aromatic SAMs on Au(lll) electrodes at solid-liquid interfaces. 

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