Metal oxide supported catalysts

Nanoparticle Geometry at Oxide-supported Metal Catalysts... 

D.C. Meier, X. Lai, and D.W. Goodman, Surface chemistry of model oxide-supported metal catalysts An overview of gold on Titania, in Surface Chemistry and Catalysis, eds. A.F. Carley et al. Kluwer, New York, 2002, pp. 147-189. 

O2 adsorption, 28 38 surface modility, 28 34 surface structure, 28 30, 31 oxidation of CO on, 28 65 oxide-supported metal catalysts, 41 10, 11 -phosphine catalysts achiral, 25 83-85 recovery, 32 354-356, 367-369 selectivity, 30 348 platinum, 30 355 -silica catalysts... 

The presence of oxygen enhances the catalyst stability. Breen et al. [187] investigated SRE over a range of oxide-supported metal catalysts. They concluded that the support plays an important role in the reaction. In fact, they observed that alumina-supported catalysts are very active at low temperatures for dehydration of ethanol to ethylene, which at higher temperatures (550 °C) is converted into H2, CO and CO2 as major products and CH4 as a minor product. The activities of the metal decrease in the order of Rh > Pd > Ni PS Pt. Ceria/zirconia-supported catalysts are more active and exhibit 100% conversion of ethanol at high space velocity and high temperature (650 °C). 

The literature of the vibrational spectra of adsorbed alkynes (acetylene and alkyl-substituted acetylenes) is very much in favor of single-crystal studies, with fewer reported investigations of adsorption on oxide-supported metal catalysts. Fewer studies still have been made of the particulate metals under the more advantageous experimental conditions for spectral interpretation, namely, at low temperatures and on alumina as the support. (The latter has a wide transmittance range down to ca. 1100 cm-1.) A similar number of different single-crystal metal surfaces have been studied for ethyne as for ethene adsorption. We shall review in more detail the low-temperature work which usually leads to HCCH nondissociatively adsorbed surface structures. Only salient features will be discussed for higher temperature ethyne adsorption that often leads to dissociative chemisorption. Many of the latter species are those already identified in Part I from the decomposition of adsorbed ethene. 

In a few cases, adsorption on particulate nickel has been studied other than in the form of the conventional oxide-supported metal catalysts. Nash and De Sieno (73) exploded nickel wires in a rare-gas atmosphere to give Ni particles of ca. 20-nm diameter. Results were reported (but not illustrated) characterizing adsorption of ethyne they are similar to those found by Eischens and Pliskin. 

In 1954 R. P. Eischens, W. A. Pliskin, and S. A. Francis (5) of the Texaco Research Center in New York published the first infrared spectra of chemisorbed species, namely of carbon monoxide adsorbed on the silica-supported finely divided metal catalysts of Ni, Pd, Pt, and Cu. Also, in 1956, Pliskin and Eischens (5) were the first to obtain spectra of the hydrocarbons ethylene (ethene), acetylene (ethyne), and propene adsorbed on an oxide-supported metal catalyst, Ni/Si02. Eischens and his colleagues followed this up with further studies of chemisorbed zj-alkenes and their surface-hydrogenation products on Ni/Si02 (7). 

Since these early days, many infrared studies of hydrocarbons adsorbed on oxide-supported metal catalysts have been carried out. Although much success has ultimately been achieved in identifying the chemisorbed species present, progress has tended to be slow for a number of reasons ... 

The performance of a catalyst is well known to be sensitive to its preparation procedure. For this reason, ideally an oxide-supported metal catalyst should be subjected to a number of characterization procedures. These may include measurements of the metal loading within the overall catalyst (usually expressed in wt%), the degree of metal dispersion (the proportion of metal atoms in the particle surfaces), the mean value and the distribution of metal particle diameters, and qualitative assessments of morphology including the particle shapes and evidence for crystallinity. These properties in turn can depend on experimental variables used in the preparation, such as the choice and amounts of originating metal salts, prereduction, calcination or oxygen treatments, and the temperature and duration of hydrogen reduction procedures. 

Characterization Data for Oxide-Supported Metal Catalysts Employed by Research Groups Studying the Adsorption of Hydrocarbons by Infrared (IR) or Raman (Ra) Spectroscopy... 

VI. Experimental Vibrational Spectroscopic Results on Oxide-Supported Metal Catalysts Classified by Alkene Adsorbate... 

The summary of the single-crystal results is followed by the presentation and illustration (with permission from the authors and publishers) of significant results from the literature on finely divided metals, mostly infrared spectra from adsorption on oxide-supported metal catalysts. Particular emphasis is given to results obtained at room temperature or below, where the structures present are likely to be the best-defined. 

In this article (Part I) we have comprehensively reviewed the structural implications of the vibrational spectroscopic results from the adsorption of ethene and the higher alkenes on different metal surfaces. Alkenes were chosen for first review because the spectra of their adsorbed species have been investigated in most detail. It was to be expected that principles elucidated during their analysis would be applicable elsewhere. The emphasis has been on an exploration of the structures of the temperature-dependent chemisorbed species on different metal surfaces. Particular attention has been directed to the spectra obtained on finely divided (oxide-supported) metal catalysts as these have not been the subject of review for a long time. An opportunity has, however, also been taken to update an earlier review of the single-crystal results from adsorbed hydrocarbons by one of us (N.S.) (7 7). Similar reviews of the fewer spectra from other families of adsorbed hydrocarbons, i.e., the alkynes, the alkanes (acyclic and cyclic), and aromatic hydrocarbons, will be presented in Part II. 

Wang, H-F, Kaden, WE, Dowler, R, Sterrer, M, and Freund, H-J. Model oxide-supported metal catalysts - comparison of ultrahigh vacuum and solution based preparation of Pd nanoparticles on a single-crystalline oxide substrate. Phys Chem Chem Phys. 2012 14 11525-11533. 

One of the most active current research areas in metal oxide surface science is oxide-solid interfaces. Work to date falls into three broad categories. The earliest work was the growth of metal films on single-crystal oxide substrates. One of the motivations there was to produce model systems to study the basic properties of oxide-supported metal catalysts. Quite a few groups have been involved in that work, and several papers have reviewed parts of the field [35-37]. An example of this approach is presented in Chap. 9 here. 

CHEMICAL AND NANOSTRUCTURAL ASPECTS OF THE PREPARATION AND CHARACTERISATION OF CERIA AND CERIA-BASED MIXED OXIDE-SUPPORTED METAL CATALYSTS... 

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