Catalyst on the support

Because XPS is a surface sensitive technique, it recognizes how well particles are dispersed over a support. Figure 4.9 schematically shows two catalysts with the same quantity of supported particles but with different dispersions. When the particles are small, almost all atoms are at the surface, and the support is largely covered. In this case, XPS measures a high intensity Ip from the particles, but a relatively low intensity Is for the support. Consequently, the ratio Ip/Is is high. For poorly dispersed particles, Ip/Is is low. Thus, the XPS intensity ratio Ip/Is reflects the dispersion of a catalyst on the support. Several models have been reported that derive particle dispersions from XPS intensity ratios, frequently with success. Hence, XPS offers an alternative determination of dispersion for catalysts that are not accessible to investigation by the usual techniques used for particle size determination, such as electron microscopy and hydrogen chemisorption. 

Various surface thermodynamic considerations relevant to supported metal catalysts are presented. They include the thermodynamics of (1) spreading of the active catalyst on the support, (2) crystallite vs. film stability, (3) thin planar patches, (4) the phase separation on the substrate, and (5) the rupture of thin films. These thermodynamic considerations explain a number of phenomena observed during experiments with model catalysts. 

The Rh catalysts on the supports mentioned previously have been further characterized and their performance in the CO2 reforming of methane has been measured 178,179). The best catalysts in terms of both activity and stability (50 h) are those supported on YSZ, Si02, and AI2O3. FTIR spectra obtained during the first 30 min on stream for the Rh/A Os catalyst at 500°C are shown in Fig. 26. The formate at 1590 cm accumulates but is... 

In the presence of catalyst on the support, evidently being a platinum complex with diallylamine fixed on the porous glass, the reaction did not proceed at 70 - 80 °C, but at 90- 100 °C ethylene was actively absorbed and the ethyltriethoxysilane content in the mixture amounted to 94.3%. When the synthesis was repeated on the same catalyst, the reaction proceeded more slowly, but the triethoxysilane was completely consumed. 

Fig. 5 XRD patterns of the support (a), CLD catalysts on the support dried at 393 K (b) and 573 K (c), and IMP catalysts (d). The amount of added vanadium was equivalent to monolayer coverage. Thick arrows, anatase dotted arrows, rutile and vertical dotted lines, V2O5.

Investigating the influence of the loading of the hydrophilic liquid phase, it was observed that maximum activity was obtained with a pore filling of 10%. That amount of hydrophihc phase corresponded to a theoretical film thickness of 16 A. Molecular modelHng of the Pd-TPPMS complex revealed that the average diameter of the complex was 11 A, the largest diameter being 15 A. Thus, a monolayer of catalyst on the support was assumed. 

Several approaches to immobilize transition metal catalysts on polymer supports have been reported in the literature. The most representative ones are (1) immobilization of ligands as well as ligand-metal complexes by covalent and/or coordination bonds (2) adsorption of catalysts on the supports (3) formation of ionic pairs between, for example, the surface of the support bearing anionic functional groups and cationic metal species and (4) entrapment of catalysts... 

Polymerization of ligand monomers is a useful tool for preparing polymer-supported ligands. The cross-linked polystyrene bound ferrocenyl bisphos-phine ligand 21 was prepared by the copolymerization of styrene, divinylben-zene,and l,r-bis(diphenylphosphino)-2-vinylferrocene (20) (Scheme 8) [46]. The loading density of the catalyst on the support was readily controlled by the ratio of the monomers used. 

This example illustratively shows that inorganic materials are well suited for continuous flow processes in column-like reactors. Thus, covalently immobilized NH-benzyl-(li, 2S)-(-)-norephedrine 10 on silica inside a column was doped with ruthenium. This setup was used to carry out continuous asymmetric transfer hydrogenation reactions (Scheme 10) [38]. Remarkably,no catalyst deactivation occurred over a period of one week, which the authors ascribed to the successful site isolation of the catalyst on the support. 

Structural parameter derivation from curve-fitting analysis of EXAFS spectra of the supported PdQ2 catalyst and the attached Pd catalyst, respectively, established the Pd-Q distances in the catalysts to be almost the same, but with the latter having a higher palladium coordination number close to four (Table 5.6-2). This confirmed the quite uniform preparation of the attached catalyst on the support with mainly [PdCU] " species present. 

Very recently a critical review of the work on heat-treated macrocylic complexes on carbon was published It showed that the results for gas diffusion electrodes obtained can hardly be compared with each other. The activity and pyrolysis behaviour of carbon-supported transition metal chelates is determined by various factors such as the chelate, the texture of the carbon, the dispersion of the catalyst on the support, the kinetics of the pyrolysis reactions, etc. The processes which occur during the metal loading on the carbon and the pyrolysis were discussed. The authors developed a new carbon modified rotating ring disc electrode which can be used for quick comparisons of both the activity and selectivity of carbon-supported catalysts... 

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