Destructive adsorption

Figure 2.7 Illustration of Fe203 catalyst on nanocrystalline CaO for the destructive adsorption of CCI4.

In this chapter, we have discussed the application of metal oxides as catalysts. Metal oxides display a wide range of properties, from metallic to semiconductor to insulator. Because of the compositional variability and more localized electronic structures than metals, the presence of defects (such as comers, kinks, steps, and coordinatively unsaturated sites) play a very important role in oxide surface chemistry and hence in catalysis. As described, the catalytic reactions also depend on the surface crystallographic structure. The catalytic properties of the oxide surfaces can be explained in terms of Lewis acidity and basicity. The electronegative oxygen atoms accumulate electrons and act as Lewis bases while the metal cations act as Lewis acids. The important applications of metal oxides as catalysts are in processes such as selective oxidation, hydrogenation, oxidative dehydrogenation, and dehydrochlorination and destructive adsorption of chlorocarbons. 

Koper, O. Li, Y.-X. Klabunde, K.J. Destructive adsorption of chlorinated hydrocarbons on ultrafme (nanoscale) particles of calcium oxide. Chem. Mater. 1993,5, 500-505. 

Decker, S.R Klabunde, J.S. Khaleel, A. Klabunde, K.J. Catalyzed destructive adsorption of environmental toxins with nanocrystalline metal oxides. Fluoro-, chloro-, bromocarbons, sulfur, and organophosphorus compounds. Environ. Sci. Technol. 2002, 36, 762-768. 

Promotion and deactivation of unsupported and alumina-supported platinum catalysts were studied in the selective oxidation of 1-phenyl-ethanol to acetophenone, as a model reaction. The oxidation was performed with atmospheric air in an aqueous alkaline solution. The oxidation state of the catalyst was followed by measuring the open circuit potential of the slurry during reaction. It is proposed that the primary reason for deactivation is the destructive adsorption of alcohol substrate on the platinum surface at the very beginning of the reaction, leading to irreversibly adsorbed species. Over-oxidation of Pt active sites occurs after a substantial reduction in the number of free sites. Deactivation could be efficiently suppressed by partial blocking of surface platinum atoms with a submonolayer of bismuth promoter. At optimum Bi/Ptj ratio the yield increased from 18 to 99 %. 

Y.-X. Li, K.J. Klabunde, Nano-scale metal oxide particles as chemical reagents. Destructive adsorption of a chemical agent simulant, dimethyl methylphosphonate, on heat-treated magnesium oxide. Langmuir 7, 1388-1393 (1991)... 

Abstract. Destructive adsorption of halocarbons on nanocrystalline oxides has been studied. The effect of nanoparticle size and phase composition on the reaction kinetics is discussed. The reactivity of nanocrystalline oxides has been found to increase after deposition of a permeable carbon coating. The possibility of synthesis of new nanocrystalline halogenated materials using nanoscale oxides as precursors has been demonstrated. 

Nanocrystalline oxides have exceptionally high reactivity with respect to halocarbons. First of all, the temperature of their reaction with halocarbons is much lower than observed for bulk analogs. It is thus possible to carry out destructive adsorption reactions at fairly low temperatures. This is especially important for the development of effective destructive sorbents for neutralization of toxic compounds at mild temperatures. We have also demonstrated that nanocrystalline oxides can be used as precursors for the synthesis of novel halogenated nanocrystalline materials. 

K. J. Klabunde, G. Medine, A. Bedilo, P. Stoimenov, and D. Heroux, Nanocrystalline metal oxides A new family of mesoporous inorganic materials useful for destructive adsorption of environmental toxins, A CSSymp. Ser. 890, 272-276 (2005). 

A number of locations and orientations of Sarin on the regular nanosurface and on the small fragment of MgO were found. In this study it was revealed that Sarin is physisorbed (the nanosurface and hydroxylated small fragment this is undestructive adsorption) or chemisorbed (destructive adsorption) on MgO (see Figure 16-1). The physisorption of GB on the surface of MgO occurs due to the formation of hydrogen bonds and ion-dipole and dipole-dipole interactions between adsorbed GB and the surface. The chemisorption occurs due to the formation of covalent bonds between the molecule and the surface. The adsorption results in the polarization and the electron density redistribution of GB. The adsorption energy obtained at the MP2/6-31G(d) level of theory for the most stable chemisorbed system is... 

As discussed in a previous section, metal oxides represent an important class of materials exhibiting a broad range of properties from insulators to semiconductors and conductors and have found applications as diverse as electronics, cosmetics and catalysts. Metal oxides have been widely used in many valuable heterogeneous catalytic reactions. Typical metal oxide-catalyzed reactions, including alkane oxidation, biodiesel production, methanol adsorption and decomposition, destructive adsorption of chlorocarbons and warfare agents, olefin metathesis and the Claisen-Schmidt condensation will be briefly discussed as examples of metal oxide-catalyzed reactions. 

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