Electron pair donation surfaces

During the collision of two polymer grains areas with a size of a few square micrometers come into contact. In the contact zone an electron transfer can take place as described in [2] (Fig. 2a). In case of the collision of two chemically different polymer particles one of them preferably interacts as electron pair donator and the other species as electron pair acceptor. In this way laterally expanded charge domains can be formed (Fig. 2b). The low electrical conductivity of the polymer bulk and surface prevents a rapid charge dissipation, hence the formed domain structure seems to be permanently stable. 

Fig. 2 Model concept of the contact charging of polymer grains, a Contact between an electron pair donator domain (empty dots) of the particle 1 and an electron pair acceptor domain (grey dots) of the particle 2. Charge transfers (e-) are taking place. After separation the two particles (b), a positively charged ( ) and a negative charged ( ) domain, remain on the particles surface...

The formaiton of anionic species indicates the existence of an electron or electron pair donating sites on the surface. The surface acts as a Lewis base toward CO. 

Clay minerals behave like Bronsted acids, donating protons, or as Lewis acids (Sect. 6.3), accepting electron pairs. Catalytic reactions on clay surfaces involve surface Bronsted and Lewis acidity and the hydrolysis of organic molecules, which is affected by the type of clay and the clay-saturating cation involved in the reaction. Dissociation of water molecules coordinated to surface, clay-bound cations contributes to the formation active protons, which is expressed as a Bronsted acidity. This process is affected by the clay hydration status, the polarizing power of the surface bond, and structural cations on mineral colloids (Mortland 1970, 1986). On the other hand, ions such as A1 and Fe, which are exposed at the edge of mineral clay coUoids, induce the formation of Lewis acidity (McBride 1994). 

The surface basicity of a solid catalyst can be defined in a way analogous to that applied to conventional bases. Thus, a surface Lewis base site is one that is able to donate an electron pair to an adsorbed molecule. If we take the definition of surface basicity in a more general way, it could be said that the active surface corresponds to sites with relatively high local electron densities. This general definition will include not only Lewis basicity but also single electron donor sites. We emphasize that the literature of heterogeneous catalysis often reports that both single-electron and electron-pair donor sites exist on basic catalysts. 

Catalytic hydrodesulfurization (HDS) is a very important industrial process that involves removal of sulfur from crude oils by high-temperature ( 400°C) treatment with hydrogen over Co- or Ni-promoted Mo or W catalysts supported on alumina. In an attempt to determine the mechanism of this process, many transition metal complexes of thiophene, a sulfur-containing heterocycle that is particularly difficult to desulfurize, have been prepared and their reactivities studied in order to compare their behavior with those of the free thiophenes that give H2S and C4 hydrocarbons under HDS conditions (88ACR387). Thiophene can conceivably bind to the catalyst surface by either cr-donation via a sulfur electron pair or through a variety of -coordination modes involving the aromatic system... 

The abiotic hydrolysis of triazines in soil environments is catalyzed by acidic sites on the surfaces of both organic and inorganic soil constituents. The surfaces of soil constituents have both Lewis acid sites (which accept electron pairs) and Bronsted acid sites (which donate protons). However, triazines are not competitive with water and OH groups for complexation with Lewis acid sites, so in soil environments hydrolysis is catalyzed primarily by Bronsted acid sites. Four types of Bronsted acid sites are found on soil surfaces (Mortland, 1970) ... 

In addition to the intrinsic states, tha-e are also Lewis sites and adsorbed electroactive species. Lewis sites arise from the acid/base properties of the surface. A Lewis acid site is a site attractive to electron pairs from an adsorbing molecule and a Lewis basic site is able to donate electron pairs to an adsorbing molecule. For example, a Lewis acid site M or a basic site N may adsorb a OH" ion or a from water, respectively ... 

Superbasic Surface Centres with Ionic Character. - As mentioned earlier reactions between alkali metal atoms and surface acceptor centres such as hydroxyl groups or holes near cationic vacancies lead to the creation of centres of higher basicity. In the first of these examples the reason is the replacement of a hydrogen atom by a more electropositive element, such as an alkali metal atom in the second example it is the result of introduction of an electron from the alkali metal to the hole trapped on the O " anion, the vacancy being filled by a univalent cation. It should be noted that both of the surface configurations so formed can cause strong one-electron or two-electron donor activity. Closer physico-chemical examination has shown that these centres tend to be electron pair rather than one-electron donating in character. They mostly occur on surfaces on which alkali-metal vapours have reacted with oxides heated at the lower temperatures, e.g., MgO calcined... 

Interactions between the precious metal and support influence the performance of the catalyst. Beil (1987) has defined metal-support interaction as depending on contact between the metal particle and the support which can be a dissolution of the dispersed metal in the lattice. The interaction could also depend on the formation of a mixed metal oxide, or the decoration of the metal particle surface with oxidic moieties derived from the support. It is possible that in this study, the differences in catalytic performance of the same active material supported on different washcoats can be attributed to any of these phenomena. Another explanation could be that the support materials exhibit different acid-base properties. According to the Bronsted and Lewis definitions, a solid acid shows a tendency to donate a proton or to accept an electron pair, whereas a solid base tends to accept a proton or to donate an electron pair. The tendency of an oxide to become positively or negatively charged is thus a function of its composition, which is affected by the preparation method and the precursors used. Refer to the section Catalyst characterization for further discussion on the influence of support material on catalyst performance. To thoroughly examine the influence of the support... 

When dioxygen is chemisorbed on a transition-metal surface, an electron pair is donated from a filled 2n orbital of O2 to a vacant da orbital on the metal, followed by back-donation from a filled djr (or dpjr) metal orbital to an empty 2tt orbital on dioxygen. As a result, the 0=0 bond is weakened considerably.For example, at 100 K, chemisorption of O2 on most metals is molecular, but the vibrational frequency of the 0-0 stretch mode (vqo) is drastically redshifted, 630 cm (78.1meV) on Ag(110), 870cm (108 meV) on Pt(lll), and 850cm (105 meV) on Cu(lll), relative to that in the gas phase 1580cm (196 meV). At room temperature, O2 is dissociatively chemisorbed as oxygen atoms on the same metals. 

There are various acidic and basic sites on surfaces. Coordinatively unsaturated cations act as electron pair acceptors and are called Lewis acid sites. The Bronsted add sites on catalyst surfaces are hydroxyl groups able to donate protons. The basic sites are, in most cases, oxygen anions, including oxygen from hydroxyl groups. 

Bronsted and Lewis acids and bases are frequently encountered in the chemistry of solid surfaces, but in some instances these terms are used in a slightly different context from their original proposals. From the point of view of the Bronsted definition, a solid is acidic if it is able to donate (or at least transfer partially) a proton to a basic probe molecule to which the proton bonds (or with which it, at least, becomes partially associated). According to the Lewis definition, a solid is acidic if it is able to accept an electron pear from a basic molecule and also form a coor dinative bond with it. The transfer of an electron pair in the meaning of the Lewis definition should be clearly distinguished from surface redox processes in which electron transfer (ofone or two electrons) occurs without coordination. 

Reaction 9 represents the reaction between a carbonyl compound, perhaps part of a polymer chain, and a cation on a substrate surface this is the kind of reaction that might occur at some interfaces in adhesive bonds. The oxygen atom of the carbonyl group donates a non-bonding electron pair, this time into a vacant orbital in the metal ion, so, again it is a Lewis acid-base reaction. Lewis acids are also known as electrophiles, Lewis bases as nucleophiles. 

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