Catalysts dispersed surface species

This study, in conjunction with that discussed in 12.2.1.2, show that when using aqueous electrolytes or Nafion saturated with H20, the induction of NEMCA on finely dispersed noble metal catalysts is rather straightforward. The role of the electronically conducting porous C support is only to conduct electrons and to support the finely dispersed catalyst. The promoting species can reach the active catalyst via the electrolyte or via the aqueous film without having to migrate on the surface of the support, as is the case when using ceramic solid electrolytes. 

In this paper we report (i) the catalytic activity for SCR of VOx/Zr02 samples prepared by various methods (adsorption from aqueous metavanadate solutions at different pH values, dry impregnation, and adsorption from VO(acetylacetonate)2 in toluene), (ii) sample characterization (nuclearity, dispersion and oxidation state) by means of XPS, ESR and FTIR and (iii) the nature and reactivity of the surface species observed in the presence of the reactant mixture. Catalytic results are here reported in full. Characterization data relevant to the discussion of the catalytic activity will be given, whereas details on the catalysts preparation and... 

In order to combine the catalytic activity of highly dispersed metal species and that of zeolites, metals can be deposited in the pores and on the external surface of zeolite particles. In this way, a catalyst is formed with both a metal functionality, e.g., redox or hydrogenation activity, and an acidic function. The metals can be deposited by different methods. Impregnation of a zeolite with a metal... 

The evolution of the initial surface species in several oxides was studied early on [72, 73] the first studies already indicated that catalysts derived from iron carbonyls can be more than one order of magnitude more dispersed than catalysts prepared by conventional techniques using salts of Fe as precursors [72]. 

Analysis of structure-activity relationships shows that various species characterized by different reactivities exist on the surface of vanadium oxide-based catalysts.339 The redox cycle between V5+ and V4+ is generally accepted to play a key role in the reaction mechanism, although opposite relationships between activity and selectivity, and reducibility were established. More recent studies with zirconia-supported vanadium oxide catalysts showed that vanadium is present in the form of isolated vanadyl species or oligomeric vanadates depending on the loading.345,346 The maximum catalytic activity was observed for catalysts with vanadia content of 3-5 mol% for which highly dispersed polyvanadate species are dominant. 

In contrast, comparable rates were determined over platinum of low dispersion suggesting that isomerization occurs without alkene formation.161 The carbene-alkyl species (21) formed with the involvement of terminal carbon atoms is a probable surface intermediate in this selective mechanism. Highly dispersed platinum catalysts are active in nonselective isomerization in which the precursor species is the 22 dicarbene allowing ring closure between methyl and methylene groups. On iridium a pure selective mechanism is operative,162 which requires a dicarbyne surface species (23). 

The use of dimethyldichlorosilane as a coupling agent for the grafting of VOx structures on the MCM-48 surface, produces a material that is simultaneously hydrophobic (inmiscible with water) and very active (all V-centers are accessible, even for water molecules and the catalytic activity for methanol oxidation has increased). The VOx surface species are grafted by the Molecular Designed Dispersion of VO(acac)2 on the silylated surface, followed by a calcination in air at 450°C. These hydrophobic MCM-48 supported VOx catalysts are stable up to 500°C and show a dramatic reduction in the leaching of the V-centers in aqueous media. Also the structural stability has improved enormously. The crystallinity of the materials does not decrease significantly, even not when the samples are subjected to a hydrothermal treatment at 160°C and 6.1 atm. pressure. 

Spectrum 10F by Binet et al. (60) is of particular interest as it is obtained on an exceptionally highly dispersed (51%) catalyst of low loading (0.45% Pd) at 295 K. It is seen that, although once again all three surface species are present, in this case the n complex gives the dominant spectrum. 

The surface properties of importance for adsorbents, catalysts, adherent surfaces, and corrodable surfaces are those properties which control interactions with adsorbable species. These interactions always involve dispersion force interactions and may or may not involve specific interactions. The ability of a surface to interact with another material can be determined at present best by observing its interactions with test materials, and these observations are never done in high vacuum and generally involve wet chemical techniques. 

Even if the tin does not actually form a species corresponding to a surface tin aluminate compound, the oxidized tin is widely dispersed, the anion associated with it must be an oxide anion, or possibly a chloride ion if the catalyst still contains this element, and the oxidized tin interacts strongly with the alumina support. Thus, even if the tin is not present in a specific tin aluminate compound containing either Sn2+ or Sn4+, the actual structure will resemble a tin aluminate widely dispersed over the surface. Thus, we only consider two states for tin a surface species for the oxidized tin that is or resembles a tin aluminate and Sn° that is essentially all present as PtSn =1 1 alloy. Thus,... 

The question as to what is the active site of Cu-based catalysts in MSR is still unclear and debated in the literature. Similar to the methanol synthesis reaction, either metallic Cu° sites, oxidized Cu+ sites dispersed on the oxide component or at the Cu-oxide interface, or a combination of both kinds of sites are thought to contribute to the active ensembles at the Cu surface. Furthermore, the oxidic surface of the refractory component may take part in the catalytic reaction and provide adsorption sites for the oxygenate-bonded species [126], whereas hydrogen is probably adsorbed at the metallic Cu surface. Similar to methanol synthesis, factors intrinsic to the Cu phase also contribute to the MSR activity in addition to SACu- There are two major views discussed in the literature relating these intrinsic factors either to the variable oxidation state of Cu, in particular to the in situ adjustment of the Cu°/Cu+ ratio at the catalyst s surface [102, 107, 127 132], or to the defect structure and varying... 

Supported metal oxide catalysts consist of dispersed surface metal oxide species, the catalytic active sites, which are supported on high-surface-area oxides [1-3]. The... 

---