Metal oxide-adsorbate interactions molecular adsorption

The basic experimental studies of the interactions between organophosphorus compounds and metal oxide surfaces have been carried out intensively during the last several years. Metal oxides, such as MgO, AI2O3, FeO, CaO, Ti02 a-Fe203, ZnO, and WO3, are currently under consideration as destructive adsorbents for the decontamination of chemical warfare agents [46, 47], For example, several studies have addressed adsorption of dimethyl methylphosphonate (DMMP) (a widely used model compound for the simulation of interactions of phosphate esters with a surface) on the surface of these metal oxides [48-60], In most of these works, the authors have observed that, at first, DMMP is adsorbed molecularly via hydrogen... 

Methane is the main constituent of natural gas. Adsorption of methane at ambient temperature has been studied with a special relevance to methane storage. However, methane is a spherical molecule and the intermolecular interaction is quite weak. Furthermore, the bulk critical temperature is 191 K. It is quite difficult to adsorb methane sufficiently at ambient conditions. The adsorption conditions of CH4 by activated carbon have been studied with molecular simulations[59,60]. The chemisorption-assisted micropore filling concept was applied to adsorption of supercritical CH4. As the intermolecular interaction of CH4 is quite weak compared with that of NO, CH4 adsorption must be examined at high pressure region. As basic metal oxides such as MgO, CaO, AI2O3, NiO, and Cr203... 

Adsorption complexes of methane at MgO are interesting because they relate to the conversion of methane to ethylene and methanol. In particular, oxidative coupling of methane on metal-oxide catalysts attracted great attention [119]. Usage of methane as a probe to identify and characterize adsorption sites of different acid strength on oxide catalysts is another important aspect. Because CH4 is not easily captured by surfaces of metal oxides, the nature of the interaction of methane with surface sites was little understood until recently. A FTIR spectroscopy investigation of methane on MgO at 173 K revealed adsorbed molecular species preferentially bound at Lewis basic sites CH4 adsorption on a Lewis acid-base pair has also been proposed [120]. 

The adsorption of water on most metal surfaces is typically rather weak and controlled by a balance between the strength of the metal-water bond and the water waterl interactions. Molecular water adsorbs on metal and metal oxide substrates through the donation and back-donation of electrons between the frontier molecular orbitals of water and the states of the metal near the Fermi level. 

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 this chapter, recent results are discussed In which the adsorption of nitric oxide and its Interaction with co-adsorbed carbon monoxide, hydrogen, and Its own dissociation products on the hexagonally close-packed (001) surface of Ru have been characterized using EELS (13,14, 15). The data are interpreted In terms of a site-dependent model for adsorption of molecular NO at 150 K. Competition between co-adsorbed species can be observed directly, and this supports and clarifies the models of adsorption site geometries proposed for the individual adsorbates. Dissociation of one of the molecular states of NO occurs preferentially at temperatures above 150 K, with a coverage-dependent activation barrier. The data are discussed in terms of their relevance to heterogeneous catalytic reduction of NO, and in terms of their relationship to the metal-nitrosyl chemistry of metallic complexes. 

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