Chemisorption heterolytic

Parravano and Boudart (1S6) have depicted chemisorption of hydrogen on zinc oxide, as occurring through heterolytic splitting on a pair of adjacent zinc-oxygen sites, followed by proton-transfer, e.g.,... 

The presence of solution can dramatically affect dissociative chemisorption. In the vapor phase, most metal-catalyzed reactions are homolyticlike, whereby the intermediates that form are stabilized by interactions with the surface. Protic solvents, on the other hand, can more effectively stabilize charge-separated states and therefore aid in heterolytic activation routes. Heterolytic paths can lead to the formation of surface anions and cations that migrate into solution. This is directly relevant to methanol oxidation over PtRu in the methanol fuel cell. The metal-catalyzed route in the vapor phase would involve the dissociation of methanol into methoxy or hydroxy methyl and hydrogen surface intermediates. Subsequent dehydrogenation eventually leads to formation of CO and hydrogen. In the presence of an aqueous media, however, methanol will more likely decompose heterolytically into hydroxy methyl (—1) and intermediates. 

Summary Types of heterolytic reactions with the participation of the silica surface sites are examined. Chemisorption regularities of the organosilicon compounds, cotaining trimethylsilyl group (trimethylhalo- and trimethylpseudohalosilanes), in reaction with surface silanols of fumed silica are analyzed. Peculiarities of surface reactions with the participation of some chlorides and oxochlorides are discussed Some examples of the addition reactions are considered. 

In Sections III-V, we speculated about the nature of active sites on chromia and the relations of such speculations to chemisorption and heterogeneous catalytic reactions. In particular, we suggested that many types of active sites would involve coordinatively unsaturated surface (cus) ions of Cr3+ and 0 - and that the following types of chemisorption might occur at such sites simple coordinative adsorption at Cr3+(cus), adsorption of generalized acids at 02-(cus), heterolytic dissociative adsorption at pair sites of Cr +(cus) and 02-(cus), and reductive adsorption. In addition, we considered the possibility of ligand displacement adsorption which does not depend upon (cus) ions. 

In conclusion, the combined experimental and theoretical study of methanol adsorbed on MgO films with different defect densities allows for a better identification of the surface sites responsible for the MgO reactivity. On the inert terrace sites only physisorption is observed. Molecular chemisorption, activation, and heterolytic dissociation occur on irregular sites. The low-coordinated Mg-O pairs of ions located at edges and steps can lead to strongly activated and even dissociated methanol molecules. Adsorption of CHsO" and H+ fragments seems to be preferred over dissociation into and OH ... 

A.B. Anderson, Z.Y. Al-Saigh W.K. Hall (1988). J. Phys. Chem, 92, 803-809. Hydrogen on M0S2. Theory of its heterolytic and homolytic chemisorption. 

SCHEME 49 Possible heterolytic chemisorption of H2 by Cr(ll)/silica to make the monoattached species thought to produce a-olefins in situ during polymerization. 

If the experimental conditions prior to recording the UV diffuse reflectance spectra are such that exclusively the saturation limited irreversible heterolytic H2 chemisorption (I) may occur, the intensity loss of the absorption band at 270 nm is small (Fig.6d) as compared to the pretreatment in terms of UV induced homolytic H2 splitting (Figs. 6b and c). This shows that the majority of the 3-coordinated O anions which exhibit a uniform UV excitation at 270 nm, are not able to split H2 heterolytically in the dark. For this purpose the specific site must — as discussed above — fulfill an additional requirement, namely, of being constituent of a low coordinated anion vacancy. 

Surface-trapped electrons can be formed at the surface of alkali-earth oxides by different methods, among these methods the UV irradiation of the solid in the presence of hydrogen has been found the most reliable and reproducible and leads to the formation of a particular type of surface colour centre named Fj (H) centre . The mechanism leading to the formation of these centres implies the heterolytic chemisorption of hydrogen at the surface of activated MgO and the formation of and H ions stabilised onto a couple of low-coordinated O -Mg ions. Upon UV irradiation a fraction of the hydride H ions are ionised and the released electrons stabilised into suitable surface anion vacancies close to the OH formed by reaction of H with surface O ions. At the end of this process the sample develops a blue colour and exhibits an EPR signal with g values slightly lower than the free spin value... 

Anodier example of dissociative chemisorption is the heterolytic cleavage of hydrogen on metal oxide surfaces. The reaction of hydrogen wifli a zinc oxide surface produces a zinc-hydride bond and a proton boimd to an oxygen center (Eq. 5-25) [T3 9]. 

The chemisorption of simple gases on semiconductors can be relatively simply understood in terms of the chemical reaction of the adsorbate with the catalyst Reducing gases like hydrogen and CO are strongly and irreversibly adsorbed. On heating, only water and carbon dioxide are detectable. On adsorption, H2 mainly undergoes heterolytic dissociation (Eq. 5-47) ... 

Between processes of adsorption and chemisorption there is a close relationship, since in the process of evolution of adsorbed species one can observe formation of transient complexes that are products of reactions proceeding by the Langmuir-Hinshelwood mechanism. According to the accepted classification of reactions in a surface layer of SiOi they are subdivided into two main classes, namely reactions with substitution of structural hydroxyl protons (SeI processes) and reactions with substitution of OH groups at silicon atoms (SnI processes). Reactions of heterolytic decomposition of siloxane bonds proceeding by the AdN3 mechanism form a separate class. 

Several literature reports confirm, in fact, that zeolite catalysts are able to adsorb NO2 in the form of nitrates. NO2 chemisorption can be described by the following two-step mechanism, schematically representing disproportionation and heterolytic chemisorption of NO2 to form surface nitrites and nitrates, step (9.1), followed by NO2 oxidizing the nitrites to nitrates, step (9.2) ... 

The relative ease with which hydrogen chemisorbs on the surface of a metal oxide surface mainly depends on the chemical nature of the oxide and on the O-vacancies. Thus, hydrogen adsorbs dissociatively on a perfect titanium oxide surface [10,11]. The energetically most favorable mode for the adsorption of atomic hydrogen is the adsorption on the outermost O atom, accompanied by the reduction of a Ti atom. In this mode, protons are formally adsorbed while an equivalent amount of Ti(IV) atoms are reduced to Ti(III). Theoretical calculations have demonstrated that H adsorption is less favorable on a defective surface than on a perfect surface. However, the best adsorption mode for the atomic chemisorption on a defective surface is heterolytic adsorption, which involves two different adsorption sites one H+/0= and one H on the surface. This adsorption mode is best on irreducible oxides such as MgO however, it is less favorable than adsorption on the perfect Ti02 surface [10]. The heat of atomic adsorption in all cases is very weak and dissociation onto the surface is unlikely. The molecular adsorption (physisorption), thus, remains the most stable system. 

Chemisorption [114] on an oxide surface differs significantly from that on metals. One of the main reasons for this difference is the ionic character of the solid, which favors acid-base or donor-acceptor reactions. Lewis sites are localized on the cations and basic sites on the anions. An example of this type of interaction is given by CO2, which reacts with basic to give a surface carbonate COj . Similarly, a donor molecule such as H2O or NH3 can be molecularly adsorbed via its lone-pair electrons, which react with an acidic (cation) site. An alternative to the molecular adsorption is that resulting from the heterolytic dissociation of the molecule. It may occur by abstraction of H atom transferred to a basic site, producing a hydroxyl group. 

In general, the ammoxidation reaction of methyl aromatics and/or hetero aro-maties runs via redox mechanism as proposed by Mars and van Krevelen [12]. Most of the catalysts used so far eontain transition metal oxides with easily ehanging valence states (e.g., V, Mo, etc.). Essential steps of the reaction mechanism are (i) chemisorption of the methyl aromatic or hetero aromatic reactant on the catalyst surface followed by H-abstraetion (i.e., C-H bond disassociation) to form a benzylic intermediate, (ii) insertion of nitrogen into a surface bonded partially oxidized intermediate and (iii) desorption of the formed nitrile and iv) reoxidation of the catalyst by gas-phase oxygen. Literature survey [114, 115] revealed that the H-abstraction oeeurs via C-H bond dissociation in three different possible ways, such as (i) heterolytic with the abstraction of hydrogen atom in an anionic form followed by carbocation. 

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