Monolayer oxide catalysts

Raman spectroscopy has provided information on catalytically active transition metal oxide species (e. g. V, Nb, Cr, Mo, W, and Re) present on the surface of different oxide supports (e.g. alumina, titania, zirconia, niobia, and silica). The structures of the surface metal oxide species were reflected in the terminal M=0 and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports was determined by monitoring the specific surface hydroxyls of the support that were being titrated. The surface coverage of the metal oxide species on the oxide supports could be quantitatively obtained, because at monolayer coverage all the reactive surface hydroxyls were titrated and additional metal oxide resulted in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites in supported metal oxide catalysts has been determined by adsorbing probe mole-... 

Andreaus B, Maillard F, Kocylo J, Savinova ER, Eikerling M. 2006. Kinetic modeling of CO monolayer oxidation on carbon-supported platinum catalyst nanoparticles. J Phys Chem B 110 21028-21040. 

These motivations strengthen the interest for eatalysis towards the development of ordered assemblies of ID nanostruetures for oxide materials, e.g. metal-oxide catalysts in which the 3D macro-structure is constituted by an ordered assembling of regular ID structures with nanometric size. Note that this type of structure is significantly different from that of metal-oxide supported over other metal-oxides, such as monolayer-type V202/Ti02 materials. See also later, when the concept of nanostructured metal-oxide films is defined. 

The surface structure and reactivity of vanadium oxide monolayer catalysts supported on tin oxide were investigated by various physico-chemical characterization techniques. In this study a series of tin oxide supported vanadium oxide catalysts with various vanadia loadings ranging from 0.5 to 6. wt.% have been prepared and were characterized by means of X-ray diffraction, oxygen chemisorption at -78°C, solid state and nuclear magnetic resonance... 

Monolayer coverage of vanadium oxide on tin oxide support was determined by a simple method of low temperature oxygen chemisorption and was supported by solid-state NMR and ESR techniques. These results clearly indicate the completion of a monolayer formation at about 3.2 wt.% V2O5 on tin oxide support (30 m g" surface area). The oxygen uptake capacity of the catalysts directly correlates with their catalytic activity for the partial oxidation of methanol confirming that the sites responsible for oxygen chemisorption and oxidation activity are one and the same. The monolayer catalysts are the best partial oxidation catalysts. 

Promoters. - Many supported vanadia catalysts also possess secondary metal oxides additives that act as promoters (enhance the reaction rate or improve product selectivity). Some of the typical additives that are found in supported metal oxide catalysts are oxides of W, Nb, Si, P, etc. These secondary metal oxide additives are generally not redox sites and usually possess Lewis and Bronsted acidity.50 Similar to the surface vanadia species, these promoters preferentially anchor to the oxide substrate, below monolayer coverage, to form two-dimensional surface metal oxide species. This is schematically shown in Figure 4. 

A very low selectivity (10%) is found by Shchukin et al. [288] for a fully oxidized catalyst in a pulse reactor at 425°C. However, in the absence of oxygen, the selectivity increases to 90% upon reduction of the catalyst to an extent that corresponds to 40% of a monolayer. 

Although not entirely unequivocal, ESCA results appear to be consistent with the monolayer model of the oxidized catalyst. More uncertainty exists for the sulfided catalyst either a MaS2 phase separates from the support or if the monolayer becomes partially sulfided, its interaction with the support is apparently weakened. 

Ratnasamy et al. (27) reported that on reduction, terminal oxygen is lost first, and Fransen et al. (76) found the OH band to reappear. The latter authors speculated that on reduction, breaks in the monolayer appear caused by migration of Mo4+ ions to octahedral sites. An Mo02 phase did not appear and oxidation gave back the same spectra as the freshly oxidized catalyst. 

There are three current theories on sulfiding the CoMo/Al catalyst monolayer, intercalation, and contact synergism. These are schematically depicted in Fig. 4. All start with the assumption of a monolayer model for the oxidized catalyst, differences appearing in the subsequent effect that sulfiding has on the monolayer structure. Space limitations preclude a full description of these models the original references should be consulted for more detail. 

As was stated previously, metal cannot disperse as a monolayer onto catalyst supports. However, oxide precursors of metals can monolayer disperse on supports, and supported metal particles can be prepared from the monolayer-dispersed oxide by reduction. 

Another important consideration in preparing mixed-oxide catalysts is the spontaneous monolayer dispersion of oxides and salts onto surfaces of support substrates on calcination. Both temperature and duration of calcination are important here, as discussed in the reviews by Xie and Tang [63] and by Knozinger and Taglauer [64]. If this dispersion step is inadequate or incomplete, the resulting oxide layer, and any reduced metal surface from it, will not be reproducible from the same catalyst system therefore, one can then have different catalysts prepared at different times and, of course, from one laboratory to another. Spreading and wetting phenomena in preparation of supported catalysts is discussed in Section A.2.2.1.3. 

The historical development of electro-organic chemistry is well documented by several authors, e.g. in refs. 514 and 521-527, and therefore will not be repeated here. Much of the pioneering work in the field was carried out at Pt, i.e. Pt covered with an oxide film of monolayer dimensions in the case of anodic reactions. Electro-organic reactions at Pt have been analyzed in considerable detail by Conway [517] and will not be discussed here. Rather, attention will be focussed on oxide electrocatalysts and metal anodes covered with oxide films of multilayer dimensions, e.g. Ni and Pb. However, before commencing with a discussion of such oxide catalysts, some important factors in electroorganic chemistry will be briefly reviewed. 

The aqueous preparation oT supported niobium oxide catalysts was developed by using niobium oxalate as a precursor. The molecular states oT aqueous niobium oxalate solutions were investigated by Raman spectroscopy as a -function o-f pH. The results show that two kinds o-f niobium ionic species exist in solution and their relative concentrations depend on the solution pH and the oxalic acid concentration. The supported niobium oxide catalysts were prepared by the incipient wetness impregnation technique and characterized by Raman, XRD, XPS, and FTIR as a -function o-f niobium oxide coverage and calcination temperature. The Raman studies reveal that two types o-f sur-face niobium oxide species exist on the alumina support and their relative concentrations depend on niobium oxide coverage. Raman, XRD, XPS, and FTIR results indicate that a monolayer oT sur-face niobium oxide corresponds to 19%... 

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