Chemisorption mixed metals

Figure 1.4 Proposed steps in the chemisorption of OH on/in Pt, starting with arrays of OH groups over the uppermost metal atom layer, increasing the coordination number of the adsorbed OH by place exchange, and next generating a mixed, metal/oxygen overlayer while further oxidizing to form O atoms. From Conway et al. [1990].

Bonding modifiers are employed to weaken or strengthen the chemisorption bonds of reactants and products. Strong electron donors (such as potassium) or electron acceptors (such as chlorine) that are coadsorbed on the catalyst surface are often used for this purpose. Alloying may create new active sites (mixed metal sites) that can greatly modify activity and selectivity. New catalytically active sites can also be created at the interface between the metal and the high-surface-area oxide support. In this circumstance the catalyst exhibits the so-called strong metal-support interaction (SMSI). Titanium oxide frequently shows this effect when used as a support for catalysis by transition metals. Often the sites created at the oxide-metal interface are much more active than the sites on the transition metal. 

Oxygen-containing compounds such as alcohols also undergo dissociative chemisorption, an example being the adsorption of gaseous methanol on molybdenum oxide catalysts (Eq. 5-28). Such metal oxides, and in particular mixed metal oxides, act as redox catalysts, as we shall see in Section 5.3.3. 

An unexpected result was the progressive apparent dechlorination of SiOTiCla. We have verified that this phenomenon was not related to the presence or absence of TiCU either physically adsorbed or in the gas phase. We could also observe the growth of the same IR bands between 1000 and 600 cm using a self-supporting disc. Therefore, the dechlorination of TiCU on silica and the eventual incorporation of Ti as a random mixed metal surface oxide is probably entropy driven. Although the initial chemisorption follows reaction (3) and (4), further dechlorination probably results in the formation of SiCl surface species. The vibrations of this near 7(X) cm would be impossible to detect with a thin film given the low extinction coefficient [15], and in any case, they would be masked by the much stronger SiOTi vibrations. Finally, the results have implications for mixed oxide catalysts which are prepared by chemical vapor deposition. Structural models which are based on the notion that only reactions like those depicted in schemes (3) and (4) occur are probably not valid. 

F1 NMR of chemisorbed hydrogen can also be used for the study of alloys. For example, in mixed Pt-Pd nanoparticles in NaY zeolite comparaison of the results of hydrogen chemisorption and F1 NMR with the formation energy of the alloy indicates that the alloy with platinum concentration of 40% has the most stable metal-metal bonds. The highest stability of the particles and a lowest reactivity of the metal surface are due to a strong alloying effect. 

An asterisk ( ) here represents an adsorption site. There are several observations which provide strong support for the association mechanism.266 First, Hf is observed from some metals such as Au, which is known not to show dissociative chemisorption of H2 at low temperatures. Second, for some metals such as Ir, even though chemisorption of H2 is dissociative, few Hj ions are observed. Third, when a H2-D2 mixed gas is used, atomic exchanges always occur regardless of whether hydro-... 

A database of molecularly adsorbed species on various surfaces is also included (see Table 4.3). In all cases, the chemisorption energies have been calculated on stepped surfaces using density functional theory (see [56] for details). The metals have been modeled by slabs with at least three close-packed layers. The bcc metals are modeled by the bcc(210) surface and the fee and hep metals have been modeled by the fcc(211) surface. A small discrepancy between the adsorption on the hep metals in the fcc(211) structure is thus expected when the results are compared to the adsorption energies on the correct stepped hep structure instead. When mixing... 

As early as in 1937, Nyrop (17) suggested that electron transfer may occur during chemisorption. Dowden (18) clarified the situation by classifying the possible reactions with respect to the type of bond (ionic, covalent, or mixed) and the type of adsorbent (metal, semiconductor, or insulator). He attempted to indicate some probable criteria to be used in the choice of the best adsorbent for use with a given adsorbate. 

One may speculate about the causes of strong chemisorption of some ions and the weak chemisorption of others. The four metals, with strongly chemisorbed ions on GaAs, (Ru, Pb, Ir, Rh) have several stable oxidation states and thus radii, some of which approach those of the lattice components. The orbitals of these metals and ions also have substantial mixed sp ( 60%) and d( 40%) character, which makes varying orbital hybridizations and thus a range of orbitals of different directionality possible.41 In some cases, orbitals binding at two or more surface sites can be envisaged. 

Solidification with cement generally is accomplished with a Portland cement and other additives. The quantity of cement can be varied according to the amount of moisture in the waste. Heavy metal cations in the waste form insoluble carbonates and hydroxides at the high pH of the mixture. The surface of the hardened mass can be coated with asphalt or other material to reduce leaching of hazardous components. If the waste is mixed with anhydrous cement and water there is the possibility of ions incorporation in the cement structure during the hydrolysis process. Heavy metal ions could bind with the cement by the process of chemisorption, precipitation, surface adsorption,... 

Beckler, R. K. and M. G. White, Polynuclear Metal Complexes as Model Mixed Oxide Catalysts Selective Chemisorption of NH3 and NO , J. Catal, 109, pp. 25-36 (1988) Beckler, R. K. and M. G. White, Polynuclear Metal Complexes as Model Mixed Oxide Catalysts Isomerization Activity , J. Catal, 110, pp. 364-374 (1988). Coulier, L., V. G. Kishan, J. A. R. van Veen, and J W. Niemantsverdriet, Surface science models for CoMo Hydrodesulfurization Catalysts the Influence of the support on hydrodesulfurization acidity , J. Vac Scl Technol A. 19, Issue 4, 1 July/August 2001, pp 1510-5. 

There is no single interpretation to explain the effects of particle size, alloying, and metal-support interaction on the chemisorption and catalytic properties of supported metal particles. Depending on the particle size, the nature of co-metal and support, and the nature of the reaction, the change of chemisorption and catalytic properties can be interpreted in terms of geometric features, electronic modifications, and/or mixed sites. This is due to the formation of various adsorbed species and intermediates. Moreover, in many cases, the promotion of catalytic properties will be directly related to the method of catalyst preparation, which affects the architecture of the active site, with respect to chemical and electronic states of components and topology. 

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