Palladium catalysts determination

The dispersion values for the palladium catalyst determined by titration at temperatures of reduction at 573 and 773 K are close to the values obtained by H2chemisorption. The difference is about 15 % and can be attributed to experimental error. 

Scholten J J and van Montfoort A 1962 The determination of the free-metal surface area of palladium catalysts J. Catal. 1 85-92... 

The kinetics of hydrogenation of phenol has already been studied in the liquid phase on Raney nickel (18). Cyclohexanone was proved to be the reaction intermediate, and the kinetics of single reactions were determined, however, by a somewhat simplified method. The description of the kinetics of the hydrogenation of phenol in gaseous phase on a supported palladium catalyst (62) was obtained by simultaneously solving a set of rate equations for the complicated reaction schemes containing six to seven constants. The same catalyst was used for a kinetic study also in the liquid phase (62a). 

The authors stated at the beginning of their work that to understand the mechanism of the reactions studied required an unambiguous determination of the influence of the hydrogen pressure on the rate of conversion or equilibration reactions. This could be possible only when dealing with a palladium catalyst incapable of absorbing hydrogen, i.e. with the palladium samples previously fully transformed into the /3-hydride phase, in which the H/Pd ratiq would be constant, almost independent of the hydrogen pressure. Then, for example, at room temperature under p = 1 atm, H/Pd = 0.68 when under p = 10 atm, H/Pd = 0.70 and under p = 1000 atm, H/Pd = 0.80 only. 

These spectra show even fewer features than those of the palladium catalysts. Absorption takes place almost exclusively in the region 2000-2100 cm-1. There are some weak bands below 2000 cm-1, but our experimental method did not allow us to determine their frequencies with reasonable accuracy. It is clear that also with the iridium catalysts the particle size has an effect on the spectra. The spectrum of Ir-8 shows only one intense band at 2048 cm-1, whereas the other two have additional bands at higher frequencies. There is also a marked dependence of the intensity of the 2048 cm-1 band on the CO pressure, especially in the case of Ir-37 and Ir-100. We shall not try to interpret the CO spectra of the iridium samples, as we consider the data available insufficient for the purpose. 

The surface areas of the iridium and palladium catalysts were determined by chemisorption of hydrogen and carbon monoxide, respectively, the monolayer volume being determined from an adsorption isotherm taken at 20°C. 

Fig. 5.15b shows a thin-film continuous flow reactor used by Bruno et al. (1991) for determining the dissolution rate of U02 under reducing conditions. A known weight of U02(s) was enclosed into the reactor between two membrane filters (0.22 jum). The reducing conditions of the feed solution were obtained by bubbling H2(g) in the presence of a palladium catalyst. The dissolution rates determined using continu-... 

As in the case of palladium catalysts, formyl species can be assumed to be transformed either into hydrocarbons (on Rh ) or into oxygenates (on Rh ). The rates of these transformations can be calculated from the activities of two catalysts having different selectivities and their formyl concentrations. Once the rates are known, an expected concentration can be evaluated from the activity of a given catalyst. The calculated results are in good agreement with the experimentally determined formyl concentration (Table II). 

The high MIBK selectivity over the palladium catalysts suggested that the hydrogenation of MO was facile and that the overall rate-determining step may lie in the aldol part of the sequence rather than with the hydrogenation. If this was the case the case then we may have expected to have similar activation energies for the formation of MO and MIBK. In a previous study (II) an activation energy of 23+4 kj.mof was calculated for the formation of MO from... 

To determine the mechanism of the above reaction, a modified palladium catalyst, namely Me2Pd(DMPE) 346 (DMPE = l,2-bis(dimethylphosphino)ethane), was reacted with stannylborane 345 and it was observed that the catalysis is triggered by the oxidative insertion of the B-Sn bond across the palladium atom, which was evident by the formation of SnMe4 as well as 5-methyldiazaborolidine 349 along with the expected palladium insertion product 347 (Equation 17) <1996OM5450>. 

Catalytic isomerization of 3,4-dichlorobutene catalyzed by Pd nanoparticles of Pd-PPX film was studied at 100°C [91], The ratio of trans- to cis-1, 4-dichlorobutene for the reaction in this system with low concentration of Pd nanoparticles is 10, and coincides with the ratio obtained for the reaction with the usual palladium catalyst. But the selectivity of the reaction decreases with increasing of Pd concentration the yield of trans-l, 4-dichlorobutene decreases while the yield of cA-1,4-dichlorobutene remains constant. This result shows that the change in the catalytic properties of the composite is determined by interactions between nanoparticles rather than by the size effects. At catalytic reaction catalyzed by Pd-PPX films, where the volume content of Pd nanoparticles is close to percolation threshold, the trans-to-cis ratio for produced isomers of 1,4-dichlorobutene is 2.9 that is close to equilibrium value of this ratio. 

The hydrogenation of a-methylstyrene was investigated to demonstrate the performance of a packed-bed microreactor with a palladium catalyst supported on activated carbon [324]. The microreactor was operated at 50 °C, and conversions from 20 to 100% were measured. It was determined that the reaction is first order for hydrogen and zero order for a-methylstyrene. Initial reaction rates were close to 0.01 mol/min per reaction channel and were achieved without additional activation of the catalyst. This is in agreement with the literature data on intrinsic kinetics. 

Palladium catalysts have been shown to be selective in this hydrogenation. Hanika et al. studied the hydrogenation of 1,5-COD with 0.56% Pd-y-Al203 and two Lindlar catalysts, Pd-CaC03 (Farmakon, 5% Pd) and Pd-CaC03 (Engelhard), as catalysts in heptane at 27°C and 1 atm H2.73 The kinetic constants have been determined accord-... 

The experiments were run with a view to determining how four experimental variables influence the yield in the catalytic hydrogenation of furan over a palladium catalyst. The variables were x2, the amount of palladium catalyst x2, the hydrogen pressure x3, the reaction temperature and x4, the stirring rate. The factorial design, 24, shown in Table 2 was used to fit a second-order intraction model to the yields obtained ... 

Probably the most widdy applicable conditions developed for palladium catalysts utilize silyl enol ethers.In one instance,an excellent yield of enone was ob ned using 0.5 equiv. each of palla-dium(II) acetate and p-benzoquinone in acetonitrile. The method has the advantage that the position of the double bond is determine by the geometry of the precursor silyl enol ether (Scheme 26). Palla-... 

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