Mixed metal catalysts chemisorption

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. 

Strength (FLS) empirical approach are discussed in Section 3 as methods for determining the molecular structures of metal-oxide species from their Raman spectra. The state-of-the-art in Raman instrumentation as well as new instrumental developments are discussed in Section 4. Sampling techniques typically employed in Raman spectroscopy experiments, ambient as well as in situ, are reviewed in Section S. The application of Raman spectroscopy to problems in heterogeneous catalysis (bulk mixed-oxide catalysts, supported metal-oxide catalysts, zeolites, and chemisorption studies) is discussed in depth in Section 6 by selecting a few recent examples from the literature. The future potential of Raman spectroscopy in heterogeneous catalysis is discussed in the fmal section. 

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. 

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. 

Adsorption measurements with different supports or catalysts were carried out by using a mixed solution of cinchonidine and substrate 1 (4 mmol/1 for each) in solvent B. After stirring at 298 K for 1 h, the amount of each compound adsorbed was determined from the decrease in the concentration of the solution, The concentrations were monitored by HPLC. The mean crystallite sizes of Pd in the reduced catalysts were obtained from XRD line broadening. The total surface areas (Sbet) and the areas of Pd metal surface (Spa) were determined using the nitrogen adsorption at 77 K and by the CO chemisorption at 323 K, respectively. 

Supported metals are used extensively in heterogeneous catalysis. In the present investigation platinum is loaded onto titania and titania-alumina supports to study the SMSI effects in detail. The catalysts were characterized by X-ray Diffraction(XRD), Stepwise Temperature Programmed Reduction (STPR) and chemisorption measurements. All the samples exhibit eharacteristic behaviour showing SMSI effect after HTR, though there is only moderate interaction in the mixed oxide sample. From STPR studies, the reducibility of platinum and the support in supported platinum systems is shown to depend on the extent of the interaction at the interface. 

Transition metal carbides (mainly of W and Mo) have been shown to be effective catalysts in some chemical reactions that are usually catalyzed by noble metals such as Pt and Pd (ref.1). Their remarkable physical properties added to lower cost and better availability could make them good candidates for substitute materials to noble metals in automobile exhaust catalysis. Hence, for this purpose, we have prepared several catalysts of tungsten carbide and W,Mo mixed carbides supported on y alumina with different Mo/W atom ratios. The surface composition has been studied by XPS while the quantitative determination of catalytic sites has been obtained by selective chemisorption of hydrogen and of carbon monoxide. The catalytic performances of these catalysts have been evaluated in the simultaneous conversion of carbon monoxide, nitric oxide and propane from a synthetic exhaust gas. 

In this section, recent progress in the chemical characterization of conventional cerium-containing oxide-supported gold powder catalysts will be briefly discussed. We will focus our attention on Hg and CO chemisorption studies. There are several reasons justifying this choice. As discussed in Bernal et Hg and CO are by far the most commonly used probe molecules in the characterization of noble metals supported on ceria and closely related mixed oxides. Also very importantly, ceria-based gold catalysts are known to be highly active materials for CO oxida-... 

In accordance with the considerations above, it is obvious that an in-depth analysis of CO adsorption for cerium-based oxide-supported gold catalysts represents a very challenging issue, which requires first, the identification of the nature of the main CO forms occurring on the surface of both the metal phase and the support, and second, the quantitative establishment of the contribution of each of these forms to the total amount of adsorbed CO. As will be discussed below in some detail, these goals have been successfully achieved in a series of recently published studies in which the chemisorption of CO on catalysts consisting of gold nanoparticles supported on ceria-zirconia mixed oxides was investigated. 

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