Dispersed Surface Species

In bulk catalysts prepared by the methods described previously, the active sites on the surface are formed by termination of the crystal or the amporphous solid. In contrast, supported catalysts consist of catalytically active species dispersed over a 

In the case of higher loadings, impregnation is used. In this technique, the pores of the support are filled with a solution that contains a precursor of the active species, 

The activation of catalysts has been studied frequently, but mainly empirically and less systematically. Very often, in particular in academic research, the activation is performed under less defined conditions with respect to temperature control and gas-phase environment. Instead of parameter variation based on trial and error, the application of powerful standard techniques, like thermal analysis or temperature-programmed reaction, and the complementary use of in situ spectroscopic methods will contribute to a deeper understanding of activation processes. 

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Highly dispersed surface species, with the limiting form of single-site active centers, play a primary role in a number of catalytic materials because of their peculiar features in terms of activity and selectivity. Both oxo-species and transition metal ions supported on oxides (or in zeotype materials) belonging to these types... 

The glass phase furnished an active surface for the possible deposition of dispersed plutonium species. 

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... 

Figure 4.3 Different configurations of surface hydroxyl groups produce different, atomically dispersed surface bound metal species.

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]. 

Professors Alex Bell and Enrique Iglesia of the University of California, Berkeley have used UV-Vis DRIFTS and Raman spectroscopy to elucidate the role of many catalytic systems ranging from mixed metal oxides to precious metal formulations for applications ranging from dehydrogenation of hydrocarbons to oxidation of alkanes to the role of exposed species on dispersed surfaces. ... 

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. 

No discussion has been devoted to the recent use of Fourier transform spectrometers rather than dispersion instruments. The ease with which the spectral data can be manipulated and background subtracted make the FT methods particularly useful for studies of surface species, particularly during catalytic reaction. Recently there has been a surge of interest in the coupling of computer subtraction techniques to conventional grating instruments. For many IR surface studies, where only limited frequency range is required, this... 

Time Resolution. Time-resolved studies of surface species are of considerable interest in the field of catalysis since they offer a means for investigating the kinetics of adsorption and surface reaction and for distinguishing between species active and inactive in catalysis (32, 33, 34). Dispersive spectrometers can be used for this purpose (33, 35) but are restricted to the observation of either a single frequency or a narrow range of frequencies, unless the dynamics of the observed phenomenon are very slow compared to the time required for the acquisition of a spectrum. FT spectroscopy allows these limitations to be surmounted and opens up the possibility of recording complete spectra very rapidly. 

The potential of IR ellipsometric spectroscopy (IRES) for investigating surface processes and reactions relevant to gas-solid heterogeneous catalysis is examined, both for single crystal and model dispersed catalytic systems. With it, structural and chemical changes can be followed over a wide range of temperature and gas pressure, allowing one to thermally stabilize intermediates for investigation, and study surface species under conditions close to those in practical catalytic reactions. 

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