Palladium concentrations

As shown in Fig. 3, bulk Pd-Cu-P glasses also require about 20 at.% P. The palladium concentration needed to form bulk amorphous rods with diameters of 7 mm is restricted to 40 - 60 at.%, which is narrower than that for Pd-Ni-P glasses. During these tests, we noticed that upon replacing part of the Cu by Ni, the glass forming ability of the... 

The effects of tin/palladium ratio, temperatnre, pressnre, and recycling were studied and correlated with catalyst characterization. The catalysts were characterized by chemisorption titrations, in situ X-Ray Diffraction (XRD), and Electron Spectroscopy for Chemical Analysis (ESCA). Chemisorption studies with hydrogen sulfide show lack of adsorption at higher Sn/Pd ratios. Carbon monoxide chemisorption indicates an increase in adsorption with increasing palladium concentration. One form of palladium is transformed to a new phase at 140°C by measurement of in situ variable temperature XRD. ESCA studies of the catalysts show that the presence of tin concentration increases the surface palladium concentration. ESCA data also indicates that recycled catalysts show no palladium sulfide formation at the surface but palladium cyanide is present. 

The rate equation for the reaction scheme in Figure 13.17 shows a zero order dependence in aryl iodide and base, a first order in alkene, and a square root order in the palladium concentration. We can conclude from this that either the complexation of acrylate to palladium or its insertion in the palladium-aryl bond is rate-determining. 

IPPA (Figure 9)] were obtained at 90°C, 150 bar CO, and a palladium concentration of 150 ppm in the aqueous solution with no decomposition of Pd(tppts)3 after the reaction.451 Under these conditions the dehydration product... 

Microwave-heated Heck reactions in water using ultralow palladium catalyst concentrations have been performed by Arvela and Leadbeater [46]. Different catalyst concentrations were investigated using a commercially available 1000 ppm palladium solution as the catalyst source. Impressively, useful Heck arylations were performed with palladium concentrations as low as 500 ppb (Scheme 8). 

Scheme 8 Microwave-promoted Heck reactions using ultralow palladium concentrations...

FIGURE 11 An edge-coated palladium-on-carbon catalyst. A, Photomicrograph of an irregularly shaped particle of wood carbon. B, Palladium map showing the palladium concentrated on or near the surface of the particle. C, Palladium line scan showing the distribution of the palladium at the edge of the carbon. 

Except at very low palladium concentrations on the gel surfaces, the amounts of hydrogen sorbed at 30-mm. pressure by the palladium carrying gels appear to be constant at H/Pd ratio of from 0.70 to 0.75. At the low concentrations of palladium on the gel, the nature of the palladium, deposited on the surface or... 

Figure 3. Maximum hydrogen adsorbed at 30-mm. hydrogen pressure as function of palladium concentration on silica gel surface...

Table 2. The Effect of Palladium Concentration on the Yield of 4-MethylbiphenyP...

Alsenz, H., Zereini, F., Wiseman, C.L.S., Puttmann, W. Analysis of palladium concentrations in airborne particulate matter with reductive co-precipitation. He collision gas, and ID-ICP-Q-MS. Anal. Bioanal. Chem. 395, 1919-1927 (2009)... 

Fig. 5. Activity of HF-treated (Cu,oo-xPdx) Zr67 amorphous alloys in formaldehyde and fromate ion electrooxidation as a function of palladium concentration [O, 0.3M H2CO , 0.3Af HCOONa (I.OAf NaOH, 0.3 V) (reference reversible hydrogen electrode), 303 K]. (Reprinted from Ref. 110 with permission of Elsevier Sequoia, S.A.)...

The palladium concentration in solution (leaching) also depends on the reaction tenperature. This is illustrated for the coupling of bromobenzene and styrene with catalyst 3W (Table 3, entries 5-7.) The most pronounced influence of the... 

Currently, we have not been able to account for all of the lost palladium in the fixed-bed catalyst. For example the palladium concentration in the reaction solution is < 2 ppm which accounts for < 1 % of the lost palladium. The issue of Pd mass balance is under further investigation,... 

The T50 activity of impregnated y-alumina spheres is plotted vs. palladium content in Figure 4. Pure palladium catalysts as well as platinum and palladium co-impregnated catalysts were tested. When palladium concentration was increased above 0.03 wt %, there was no further increase in CO oxidation activity. Furthermore, the activity of catalysts containing both platinum and palladium, with the platinum palladium weight ratio varying tenfold, was virtually unaffected by the presence of platinum. 

The activity for carbon monoxide oxidation was plotted versus varying platinum and palladium concentration at a fixed total metals concen-... 

The rate of formation of by-product 2 is independant of palladium concentration. But on the other hand, the rate of formation of 2 is directly proportional to palladium concentration. This is very important in controlling the selectivity and you have to maintain the activity of the palladium catalyst (avoid poisoning...) in order to obtain good yields... 

From Table 2, it is clear that the variations of the glass transition temperature as a function of the composition for amorphous Pd-Ni-P alloys is small. The weak composition dependence of Tg in Pd-Ni-P alloys is consistent with previously reported results [24,25], where Tg of glassy Pdgo-xNixP2o alloys were found to be between 585 and 602 K when x varies from 0.15 to 0.8. In contrast, Tg for bulk amorphous PdxCugo-xPzo alloys increases gradually as the palladium concentration increases, as illustrated in Figure 6. 

As shown in the phase diagram of Fig. 11.7, if the composition of the Pd-Cu membrane is greater than the pure p boundary and within the a/p phase (at an approximate palladium concentration [in weight percent] of 61%), then a two phase structure can exist and thereby reduce the efficiency of the membrane. For a constant composition, the membrane can transform from the single phase p to the two phase a -I- p structure by merely heating up the membrane and thereby crossing the phase boundary. 

In addition, the Heck reaction of iodobenzene with butyl acrylate was studied in more detail [7]. In all experiments,butyl cinnamate was selectively produced. It was observed that the activity was only dependent on the palladium concentration in the hydrophihc hquid,but neither on the total amount of complex applied nor on the quantity of the liquid used. In recycling experiments, a short induction period occurred in the first runs. This was not observed in the following runs. Furthermore, repeated apphcation of the catalyst resulted in an increased reaction rate. Other investigators observed a decrease of activity during the recycling experiments [5]. The authors explained their results with an accumulation of EtjNHl during consecutive runs. It was assumed that the accumulated ammonium salt had the same promotion effect as tetrabuty-lammonium halide additives in Heck reactions (Jeffery conditions) [8]. 

De Vries and coworkers found another solution based on kinetic considerations of the reactions outUned in Scheme 10.5 The formation and growth of the nanoparticles must be higher order in the palladium concentration, whereas the catalytic cycle is first order or even half order (if the catalyst resting species is dimeric) in palladium. Thus, increasing the substrate catalyst ratio will slow down the former process more than the latter [61]. Indeed this non-obvious solution is the crux to what has been dubbed homeopathic paUadium by Beletskaya [62] and by de Vries [61]. The essence is that the ligand-free paUadium catalyst becomes more active at lower doses. The concept becomes immediately clear upon observation of Fig. 10.11. At 1 mol% of palladium the catalyst almost immediately precipitates as palladium black, leading to low conversions even after prolonged periods. 

Some of the (probably) wrong conclusions in the literature during this period are due to the fact that measurable amounts of palladium are often observed in solution only during the reaction. Due to redeposition, the palladium concentration in solution is very small or not detectable at the end of the reaction. In addition, several experimental tests on the heterogeneity of the reaction were obviously not suitable for the Heck reaction [136]. And, very importantly, most mechanistic investigations were performed with aryl iodides (or activated aryl bromides) only. It has been shown in Sections 10.3 and 10.4 that extremely low (even not detectable) amounts of palladium at the ppm or sub-ppm level are able to convert these substrates easily [58]. 

The reaction of bromobenzene and styrene (points A to C) is complete within a few minutes at 140 °C (without microwave heating ) with the best supported Pd-catalysts (e.g. palladium on carbon, on metal oxides or in zeolites Fig. 10.16). The dissolution of palladium from the support (point A) starts (only ) at reaction temperature (140 C for bromobenzene) and depends on a variety of parameters (substrate, catalyst, solvent, temperature, base, additives, atmosphere). The maximum palladium concentration in solution correlates well with the highest reaction rate (turning point B of conversion curve). Palladium concentration in solution decreases with continued conversion (point C decrease of aryl halide concentration) leading to (nearly) complete re-precipitation (point D) of the originally dissolved palladium (fortunately) onto the support. Increased temperature and/or excess of a reducing agent (e.g. sodium formate) added at the end of the... 

Analysis of Palladium Concentrations in Airborne Particulate Matter with Reductive Co-Precipitation,... 

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