Palladium dispersion

Fig. 3 Plot of H2 STY at 200 °C versus palladium dispersion for some catalysts reported in the literature. Data taken from ref. 51.

Note 2 Examples of polymer-supported catalysts are (a) a polymer-metal complex that can coordinate reactants, (b) colloidal palladium dispersed in a swollen network polymer that can act as a hydrogenation catalyst. 

A new procedure also carries out hydrogenation without added hydrogen. Triethoxysi-lane and 5 mol% of palladium acetate in a mixture of THF and water yields finely divided palladium dispersed on a polysiloxane matrix with concomitant hydrogen evolution44. Alkenes, present in this reaction mixture, are transformed to the corresponding saturated hydrocarbons in 100% yield at room temperature (equation 1). 

Finally, palladium dispersion was found to decrease asymptotically from 9% (1 wt.% palladium content) to 0.5% (20 wt.% palladium content). The effect of carbon monoxide concentrations exceeding the equilibrium values of the water-gas shift reaction was studied in depth. With increasing time on-stream and deactivation of the catalyst (containing 1% palladium), the carbon monoxide concentration exceeded the equilibrium up to 18-fold. Even higher values were found for lower reaction temperatures. 

Narayanan, S. and Krishna, K. (1999). Structure activity relationship in Pd/hydrotalcite effect of calcination of hydrotalcite on palladium dispersion and phenol hydrogenation. Catal. Today 49, 57. 

Effect of Hydrogen Adsorption on the Magnetic Susceptibility of Palladium Dispersed on Silica Gel... 

Magnetic Susceptibility of Palladium Dispersed on Silica Gel. Figure 1 shows the susceptibility values obtained as increasing amounts of palladium were deposited on the gel surface. As ordinates the values of the relative susceptibility, Xs/x0, are plotted against amounts of palladium on the gel. xs is the measured susceptibility while x0 is that of the bulk metal as given in the literature (4, 5),... 

Palladium nanoparticles vfith a size of a few nanometers supported on carbon are widely used as catalysts, for instance in three-way automotive exhaust catalysts and fuel cells, and can easily be prepared by impregnation of a porous support body with a precursor solution, followed by drying, decomposition of the precursor and, if necessary, reduction. It is well-known that the activity and selectivity of these catalysts for hydrogenation reactions depend on the palladium dispersion for particles sizes in the range 1-10 nm. It is, hence, not surprising that the interaction of Pd with hydrogen, and the infiuence of nanosizing, have been widely studied. 

Palladium dispersed on SiOg, n-AlgOg, SiC -A Og, and Ti 02, was characterized by chemisorption measurements to determine H2, O2. and CO uptakes. Integral, isothermal heats of adsorption were then obtained using a modified differential scanning calorimeter. 

Catalytic tests show (Fig. 3) that nickel and palladium dispersed on carbon supports by this method are active for hydrogenation of ethylene. These tests, performed between 30°C and 80°C after H2 reduction (320°C), allowed to measure apparent activation energies Ea= 53 kJ/mol and Ea = 28 kJ/mol were found for nickel and palladium respectively. These values are in good agreement with those found in the literature [20]. 

The palladium dispersion on the activated charcoal (AC) was somewhat different when compared to that observed on the CNTs catalyst. On the activated charcoal, palladium was present in agglomerate shape instead of individual particles as observed on the CNTs, which led to a less homogeneous dispersion of the metal particles on the support. However, the average particle size estimated fi om TEM was similar to that of the palladium supported on the CNTs, i.e. 5 nm. 

Such a difference in terms of product selectivity was attributed to the complete absence of any acidic sites on the carbon nanotubes sur ce and also to the absence of micropores which could induce re-adsorption and consecutive reaction [16]. The presence of micropores could artificially increase the contact time and as a consequence, modify the hydrogenation pathway. The influence of the support nature on the electronic properties of the metallic phase eould also be put forward to explain these results. Depending on the metal-support interaction, the metal particles could exhibit different exposed faces and as a consequence, significantly modify the chemisorption of the reactant on their surface. According to the interaction between the C=C bond and the laces exposed by the palladium particles, the residence time and the desorption of the intermediate could be different and thus, lead to a different selectivity. The presence of palladium aggregates on the activated charcoal as compared to the individual palladium dispersion on the CNTs could be the illustration of this difference in exposed crystalline feces. 

The BET surface area as well as the palladium metal surface area of the precursor increases by more than two orders of magnitude during the in situ activation. The solid-state reactions occurring in the metallic glass during in situ activation result in a large increase of the BET surface area from 0.02 to 45.5 m2/g. The palladium metal surface area of the as-prepared catalyst determined by CO chemisorption is 6.9m2/g, which corresponds to a palladium dispersion of about 6%. 

The view that the CO stretching absorption just below 2000 cm is due to a linear CO adsorption on edge and corner sites should be verifiable by experiments on samples with different crystallite size. An attempt to do this was carried out by Clarke and co-workers 105). They found that, for palladium dispersed in silica, the effect of increasing the metal in the samples caused a decrease in the high-frequency absorption... 

Palladium dispersed on silica or on other supports (La203, Z1O2, etc.) can also form methanol selectively [127]. Surface-science investigations produced methanol on the Pd(llO) crystal face without the presence of any oxide near atmospheric pressures and at 550 K. The activation energy was 74 kJ/mole. 

Trimerization of acetylene into benzene is known to proceed on a single crystal of palladium and on fine particles of palladium dispersed on a substrate. Among them, Pd (111) surface is the most active for the trimerization because the surface has a site with three fold symmetry at which three acetylene molecules are adequately adsorbed for the trimerization into benzene geometry-controlling reaction. In the trimerization involving a palladium cluster, it is expected that the catalytic activity of the trimerization begins to appear at a critical size as the cluster size increases because a small cluster does not have such an active site with three-fold symmetry but a larger cluster should have. 

Kholer et al. concluded that the activity of the palladium supported catalyst depends on the nature of the oxide support and the palladium dispersion, and in... 

A. Derlinkiewicz, M. Hasik, and M. Kloe, Pd/polyanihne as the catalysts for 2-ethylantra-quinone hydrogenation. The effect of palladium dispersion. Catalysis Lett., 64, 41-47 (2000). 

In the sense of this scenario, palladium dispersions (sols and nanoparticles) are just another group of SRPCs. The assortment of these nano-precatalysts described so far is broad and might be categorized into several distinct groups (1) sols protected by small ions or molecules (2) sols protected by low molecular weight surfactants (3) sols protected by... 

As noble metals seem to be the preferred catalysts for total oxidation we will emphasise these, especially palladium, platinum and gold. In fact, noble metal catalysts, such as platinum and palladium, dispersed on a high area metal oxide, are the commercial catalysts of choice due to their high intrinsic oxidation activity. Table 3.4 presents some advantages and disadvantages of the different catalysts used for the total oxidation of alkanes. Data from the table must be taken with caution, but it can be useful for a quick comparison of the different types of catalysts employed. 

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