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Chemisorption interaction with simple molecules

The microscopic imderstanding of the chemical reactivity of surfaces is of fundamental interest in chemical physics and important for heterogeneous catalysis. Cluster science provides a new approach for the study of the microscopic mechanisms of smface chemical reactivity [48]. Smfaces of small clusters possess a very rich variation of chemisorption sites and are ideal models for bulk surfaces. Chemical reactivity of many transition-metal clusters has been investigated [49]. Transition-metal clusters are produced using laser vaporization, and the chemical reactivity studies are carried out typically in a flow tube reactor in which the clusters interact with a reactant gas at a given temperature and pressure for a fixed period of time. Reaction products are measured at various pressures or temperatures and reaction rates are derived. It has been found that the reactivity of small transition-metal clusters with simple molecules such as H2 and NH can vary dramatically with cluster size and structure [48, 49, 50, 51 and 52]. [Pg.2393]

H2, N2, or CO dissociates on a surface, we need to take two orbitals of the molecule into account, the highest occupied and the lowest unoccupied molecular orbital (the HOMO and LUMO of the so-called frontier orbital concept). Let us take a simple case to start with the molecule A2 with occupied bonding level a and unoccupied anti-bonding level a. We use jellium as the substrate metal and discuss the chemisorption of A2 in the resonant level model. What happens is that the two levels broaden because of the rather weak interaction with the free electron cloud of the metal. [Pg.311]

The use of CO as a chemical probe of the nature of the molecular interactions with the surface sites of metallic catalysts [6] was the first clear experimental example of the transposition to surface science and in particular to chemisorption of the concepts of coordination chemistry [1, 2, 5], In fact the Chatt-Duncanson model [7] of coordination of CO, olefins, etc. to transition metals appeared to be valid also for the interactions of such probes on metal surfaces. It could not fit with the physical approach to the surface states based on solid state band gap theory [8], which was popular at the end of 1950, but at least it was a simple model for the evidence of a localized process of chemical adsorption of molecules such as olefins, CO, H, olefins, dienes, aromatics, and so on to single metal atoms on the surfaces of metals or metal oxides [5]. [Pg.4]

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. [Pg.351]

The chemisorption is due to an interaction between the states of the H2 molecule and the electrons in the metal. The simple picture is given by the Newns-Anderson model where an adsorbate state is lowered and broadened when interacting with a sea of valence electrons in a metal, as indicated in Figure 4.15. Basically, aU metals have a broad band of sp-electrons that are populated up to the Fermi level. Since these metals can be considered more or less as free electrons their density of states (DOS) is usually assumed to be proportional to v, as indicated in Figure... [Pg.110]

Hydrogen interacts with metals in three principal ways (i) by dissociative chemisorption at the surface (ii) by physical adsorption as molecules at very low temperatures and (iii) by dissolution or occlusion. As we shall see, to these three extreme forms have been added numerous intermediate states of various lifetimes and stabilities, some of which may have importance in catalysis. There is for example clear evidence for a molecular state formed at about 100 K on stepped surfaces saturated with atomic hydrogen (on Ni(510), Pd(510) and (210) ) this is distinct from a molecular precursor stzlt such as that seen with deuterium on Ni( 111) at 100 K. The role of such states will be discussed further below (Sections 3.2.2 and 3.3.3). The small size and electronic simplicity of the hydrogen atom formed by dissociation enable it to bond to metal surfaces in different ways, and simple-minded notions about its forming only a single covalent bond to another atom have to be abandoned. [Pg.94]

In the intrinsic heterogeneous catalytic cycle, the reactants are adsorbed on the catalyst surface at specific locations called active sites, and they are activated by chemical interaction with these sites to form the catalyst-reactant complex, thus rapidly transforming on the active site to adsorbed products which subsequently desorb from these sites allowing them to momentarily return to their original state until other reactant molecules adsorb. The simple hypothesis initiating from Langmuir s work on chemisorption [1, 2] forms the basis of the modern theory used in the interpretation of the kinetics of reactions at the catalyst surface ... [Pg.17]

Another example of the interaction of water with a relatively simple metal oxide surface is provided by the water vapor/a-Al203(0001) system (Figure 7.9(a)). Oxygen Is synchrotron radiation photoemission results indicate that significant dissociative chemisorption of water molecules does not occur below 1 torr p(H20) [149]. However, following exposure of the alumina (0001) surface to water vapor above this threshold p(H20) , a low kinetic energy feature in the Is spectrum grows quickly,... [Pg.482]

Chemisorption is a phenomenon of importance in catalysis which may be treated by MO theory. Experimental studies have been carried out for a variety of systems, but theoretical descriptions of the electronic features of chemisorption beyond simple considerations are in a primitive stage. There are several factors responsible for this state of affairs. One is, of course, the complexity of the substrate system to be modeled, which has forced theorists to work with a small-size representation for the surface, as implied by the surface molecule concept of localized interactions. Although some early work has been done by... [Pg.34]

The electrochemical interface is composed of molecules (solvent, adsorbed molecular species) and ions (ofelectrolyte), which can be partially discharged when chemisorbed, electrons and skeleton ions in the case of metal electrodes, electrons and holes in the case of semiconductor electrodes, mobile conducting and immobile skeleton ions in SEs. Molecules and ions are classical objects but electrons, holes with small effective mass, and protons are quantum objects. Interaction between molecules and surfaces is quantum-mechanical in nature in the case of chemisorption. Thus, microscopic description of the interface requires a combination of quantum and classical methods. One can benefit, however, from simple or more involved phenomenological descriptions of the interface. [Pg.34]

Figure 3.6 The Lennard-Jones curve-crossing model for dissociative chemisorption, left without and right with an energy barrier. The undissociated A2 molecule is physisorbed at the surface. The A atoms are chemisorbed. The energy of the two new metal—A bonds suffices to compensate for the A-A bond energy e and the depth of the physisorption well. Therefore the interaction potential of the undissociated A2 molecule with the surface is asymptotically lower, by the A—A bond energy. But near the surface this potential curve is crossed by the interaction of two A atoms with the surface. The limitation of the two-body point of view is evident in this plot. The A—A bond distance, that is surely a key variable, is not represented in this simple view. More on this topic in Chapter 12. Figure 3.6 The Lennard-Jones curve-crossing model for dissociative chemisorption, left without and right with an energy barrier. The undissociated A2 molecule is physisorbed at the surface. The A atoms are chemisorbed. The energy of the two new metal—A bonds suffices to compensate for the A-A bond energy e and the depth of the physisorption well. Therefore the interaction potential of the undissociated A2 molecule with the surface is asymptotically lower, by the A—A bond energy. But near the surface this potential curve is crossed by the interaction of two A atoms with the surface. The limitation of the two-body point of view is evident in this plot. The A—A bond distance, that is surely a key variable, is not represented in this simple view. More on this topic in Chapter 12.

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See also in sourсe #XX -- [ Pg.147 , Pg.148 ]




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