Chemisorption or dissociation

Simple chemisorption or dissociation will only occur if the wave functions involved in the electron transfer have positive overlaps. 

If the chemisorption or dissociated adsorption of N2 is rate determining step mentioned above, isotope exchanges of N2 must be a slow process ... 

If the chemisorption or dissociation of dinitrogen is rate-determining in the ammonia synthesis as has been demonstrated, the isotopic equilibration of dinitrogen... 

In this section we give a simple and qualitative description of chemisorption in terms of molecular orbital theory. It should provide a feeling for why some atoms such as potassium or chlorine acquire positive or negative charge upon adsorption, while other atoms remain more or less neutral. We explain qualitatively why a molecule adsorbs associatively or dissociatively, and we discuss the role of the work function in dissociation. The text is meant to provide some elementary background for the chapters on photoemission, thermal desorption and vibrational spectroscopy. We avoid theoretical formulae and refer for thorough treatments of chemisorption to the literature [2,6-8],... 

Actual electron transfer does occur in oxidation/reduction, or "redox", reactions. In this type of reaction, there is a change in the oxidation state of the adsorbate. A simple example is the chemisorption of an alkali atom, in which it becomes a 1+ ion, transferring its outer electron to empty electron orbitals of the substrate. It is the large electric dipole moment created by this charge transfer process that lowers the work function of surfaces on which alkali atoms are adsorbed (e.g., "cesiation") by up to several eV. This type of bonding is generally strong, and it can also be either molecular or dissociative. 

All aluminum clusters react with CO, but Alg reacts about twice as fast as any other cluster. The Alg cluster is nearly a hundred times more reactive toward CO than Hj. In addition no Al —CO adducts are detected. Studies of CO chemisorption on aluminum surfaces ° indicate that the bond strength is only about 8 kcal/mol, consistent with theoretical calculations.The fact that Al —CO and Cu —CO are not observed as products may be due to unimolecular decomposition in the beam, or dissociation during the photo-ionization/detection process. The answer must await further experiments. 

Thus, when considering the viability of various fuel cell electrocatalysts, it is a requirement that effective chemisorption take place on the electrocatalyst surface. For this reason, there is a large amount of interest in trying to understand the processes involving adsorbed intermediates on electrode surfaces to help in the design of improved electrocatalytic materials. The process of chemisorption on electrode surfaces can be either associative or dissociative. An associative process involves the formation of two single bonds attached to the catalyst surface directly from the c-orbitals from a double... 

Atoms and molecules frequently adsorb on surfaces, where they may decompose and/or react with other adsorbed species. Modern technology is increasingly dependent on surface chemistry which underlies many industrially important processes as well as destructive processes such as corrosion. It is useful to distinguish two types of adsorption physical adsorption, or physisorption, and chemical adsorption, or chemisorption. Physisorption is similar in nature to condensation and involves little chemical interaction with the surface, being associated with van der Waals forces. Chemisorption involves a tme chemical interaction with the surface, with the formation of a chemical bond. Thus, the enthalpy of physisorption is usually of the order of 20 kJ mol while for chemisorption values are in the region of200 kJ mol . A chemisorbed molecule may either remain intact in molecular chemisorption, or fall apart in dissociative chemisorption. In an important recent development, it is now possible to identify individual molecular bonds of adsorbed molecules using STM... 

General rate expressions of the form given in equations and have been experimentally verified for many types of LH reactions. Similar but more complicated rate expressions are easily derived assuming different (non-Langmuir) isotherms, higher-order reaction steps, or dissociative chemisorption of the reactants. In the ER mechanism, surface reaction takes place between a chemisorbed species and a nonchemisorbed species, e.g., Aads + Bg products. The nonchemisorbed species may be physisorbed or weakly held in a molecular precursor state. In this case, the rate expression for the surface reaction becomes... 

Chemical Characterization. Titration methods in aqueous medium are not very informative, because H2O tends to strongly modify surface properties by molecular or dissociative chemisorption (70). Therefore, nonaqueous methods have been proposed, where the solvent (eg, benzene or isooctane) does not (or not strongly) interact with the catalyst surface. Hammett indicators were used to determine the acid strength in terms of the Hammett-Deyrup function Hq which is defined as... 

The dissociation of CO2 depends on the nature of the catalyst support. On an acidic or inert support such as Si02, the CO2 chemisorption and dissociation occurs on a transition metal surface and is dominated by electron transfer, requiring the formation of an anionic C02 precursor [142] ... 

Chemisorption may also proceed by a mechanism involving an electron transfer between the adsorbate and the substrate (oxidation- reduction or redox interaction). It is the case for adsorbates such as O2 or CI2 that are strong electron acceptors. O2 can be molecularly or dissociatively adsorbed, CI2 is dissociatively adsorbed. The redox reactions that involve electronic carriers are expected to occur preferentially on semiconducting or metallic oxides. On wide-bang-gap insulators these reactions are promoted by surface defects such as ion vacancies, which may act as sources or sinks for electrons. 

Adsorption of reactants on the surface of a catalyst represents the first elementary step in a catalytic reaction cycle. Chemisorption and physisorption are two kinds of adsorption, and differ according to the type and strength of bond. In physisorption, the adsorption bond is due to the rather weak van der Waals interactions between permanent or induced dipoles. Chemisorption occurs if a real chemical bond is formed between the substrate and the adsorbate. Molecules may adsorb intact or dissociate on the surface. Catalytic reactions almost always involve the dissociation of at least one of the reacting molecules. In certain highly stable molecules, such as methane or ethane, chemisorption is not possible without the rupture of a C-H bond. Dissociation of molecules on metals leads to predominantly neutral fragments (homolytic bond splitting), whereas on oxides, the dissociation... 

Adsorption comprises physisorption, involving van der Waals interactions, and chemisorption, in which a chemical bond forms between the adsorbate and the substrate. Chemisorption may be molecular or dissociative, depending on temperature, surface structure, and the extent to which the surface is covered by adsorbate species. 

In order to analyze the interaction between a molecule and the catalytic surface that leads to molecular or dissociative chemisorption, it is useful to consider the metal-adsorbate system as a surface molecule. This complex has molecular orbitals, composed of metal orbitals and orbitals of the adsorbing molecule. 

The role of boron in the dissociative chemisorption process (H) may be either to promote adsorption or dissociation (or both). Trigonally coordinated boron is a Lewis acid it can accept the lone pair electrons of nitrogen, promoting NHj adsorption (left-side of scheme H). Low and Ramasubramanian [102] have pointed out that B-O-Si bonds are more easily ruptured than Si-O-Si bonds, thus promoting dissociation (right side of scheme H). The combined effects result in increased nitridation compared to pure silica gels. 

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