Molecular or dissociative

Step through the sequence of structures depictir dissociation of HCl in thegfls phase. Plot energy (vertic axis) vs. interatomic distance (horizontal axis). How mai energy minima are there Do these structures correspoi to molecular or dissociated HCl ... 

The adsorption of C02 on metal surfaces is rather weak, with the exception of Fe, and no molecular or dissociative adsorption takes place at room temperature on clean metal surfaces. At low temperatures, lower than 180 to 300 K, a chemisorbed COf" species has been observed by UPS6 on Fe(lll) and Ni(110) surfaces, which acts as a precursor for further dissociation to CO and adsorbed atomic oxygen. A further step of CO dissociation takes place on Fe(l 11) above 300 to 390 K. 

Prior to 1970 our understanding of the bonding of diatomic molecules to surfaces, and in many cases the type of adsorption (i.e., molecular or dissociative) was almost entirely dependent on indirect experimental evidence. By this we mean that deductions were made on the basis of data obtained from monitoring the gas phase whether in the context of kinetic studies based on gas uptake or flash desorption, mass spectrometry, or isotopic exchange. The exception was the important information that had accrued from infrared studies of mainly adsorbed carbon monoxide, a molecule that lent itself very well to this approach owing to its comparatively large extinction coefficient. 

In the dark, this reaction is likely to involve ion pairs of the lowest coordination, e.g., MjcOjc, whereas in the presence of UV irradiation other sites are involved, such as 02c. The presence of such ion pairs on the model surface can be seen in Fig. 11. The coordination of the oxygen ion appears to be the dominant feature in determining whether the adsorption is molecular or dissociative. Similar ion sites are probably to be involved in the H2/D2 equilibration reaction, and special configurations such as a triangular array of Ole ions maY play an important role. The increased yield of 02 after adsorption of hydrogen is most likely to arise from electron donation by the —H species, the hydrogen either forms more surface... 

Hydrogen adsorption on MgO can, in principle, be either molecular or dissociative. Dissociative adsorption of hydrogen on high-surface-area MgO has already been reported, and both homolytic and heterolytic pathways have been proposed 12). Homolytic splitting is supposed to operate under UV-irradiation only (117-119) and is not discussed further here. Heterolytic splitting takes place in the dark and at 300 K on coordinatively unsaturated (cus) Mg O surface pairs following the schematic mechanism illustrated in Scheme 2. 

As mentioned earlier, the ultimate accommodation of incident energy occurs when the molecule becomes trapped by the attractive surface potential. The process can occur molecularly or dissociatively, leading to a new chemical species and is discussed in detail in Section 4.3. 

Dissociative adsorption of CO has been found on a variety of transition metal surfaces. Broden el al. (17) and Nieuwenhuys (14) correlated the tendency for CO, N2, and NO to dissociate with the position of the transition metal in the periodic table the tendency for dissociation increases the further to the left the metal appears in the table, and it decreases from 3d to 5d metals. Furthermore, the borderline for dissociative or molecular adsorption moves to the right in the sequence CO, N2, NO to O2, being the same order as the bond strength in the free molecules. There is sufficient evidence for the proposed correlation. For example, W and Mo surfaces dissociate CO easily al room temperature dissociative adsorption has not been reported lor Pi, Ir, and Pd(III) surfaces, and CO dissociation has been reported to occur on Ni, Co, and Ru at elevated temperatures. Ben-zinger (IS) suggested that the state of adsorption (molecular or dissociative)... 

In acid/base, or donor/acceptor, reactions, bonding results form the overlap of filled orbitals on the "donor" and empty orbitals on the "acceptor". Surface cations are generally Lewis acids and act as electron acceptors, while surface O ions are Lewis bases and can donate electrons to acceptor adsorbates. In lower oxides of the transition metals (i.e., in which the cations are in an oxidation state lower than their maximal valency), cations may also be able to donate electrons in an acid/base reaction. Although one talks of donating and accepting electrons in acid/base reactions, the electrons are in no sense free, and there is no actual electron transfer involved. This type of bonding can be either molecular or dissociative. 

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. 

The oxide surface becomes reduced in this process if this were a step in a catalytic reaction, some other O-containing species would have to donate an O atom back to the substrate in order for the reaction to continue. Like the other types of reaction, 0-transfer adsorption can be either molecular or dissociative. 

Tf we consider the generalities of adsorption (the details will be discussed later) of molecules on surfaces, it is important to ponder upon whether adsorption will be molecular or dissociative and the Lennard-Jones type of description of adsorption (Lennard-Jones, 1932) shown in fig. 8 (for the real example of oxygen dissociation on Ag, see Dean and Bowker (1988/89, 1989) Campbell (1985)) makes a good starting point for consideration, the dynamics and kinetics being considered later. 

A key question is whether the diatomic molecule in its interaction with metal surfaces remains molecular or dissociates into carbon and oxygen. Broden et al. (3) predicted, by the perturbation of molecular orbitals for CO adsorbed, that only iron could dissociate CO. However, other metals in Group VIII such as nickel (A) ruthenium (5) and rhodium (6) can dissociate CO. Recently Ichikawa et al.(7) observed that disproportionation of CO to CO2 and carbon occurs on small particles of silica-supported palladium. These results show that carbon deposition phenomena may occur via either dissociation of CO on the metals used or disproportionation of CO to CO and carbon on small platinum particles. Cant and Angove (8) studied the apparent deactivation of Pt/Si02 catalyst for the oxidation of carbon monoxide and they suggested that adsorbed CO forms patches and that oxygen atoms are gradually consumed. 

As previously mentioned, hydrogen (as a elemental component of water) is the only heteroatom we consider in this review. The adorption and subsequent chemical behaviour of moecules on surfaces is certainly one of the most active areas of both experimental and computational surface science studies. For such a small and seemingly simple adorbate, the many types of possible interactions of water on oxide surfaces lead to richly complex chemistry. Water may be physisorbed intact, chemisorbed molecularly or dissociatively. In the last case, surface hydroxyls are formed, with the oxygen constituent of the hydroxyl originating either from the dissociated water molecule or the surface oxygen atoms. The H atom in the hydroxyls may be considered to be acidic (added to or abstracted from oxide anions as tf) or basic (as part of OIT added to or removed from metal cations). For all adsorbed species, molecular or dissociated, water or hydroxyl, there is the capability of strong hydrogen bond formation between other H, OH or O surface species. 

This model consists of sequential steps which depend on the molecular or dissociated adsorption forms and the nature of one or more active sites at the surface. One determines the rates for each step and assumes what the rate limiting step is. 

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... 

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. 

Water may be adsorbed in the molecular or dissociated form on a metallic surface. The oxygen... 

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. 

Therefore, we conclude that there are two critical situations whether the molecules are adsorbed in molecular or dissociative form. The chemisorption is ther-mod5mamically favored. 

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