Activation of Pd

Fig. 7. Changes of the coefficient of recombination, y, of H atoms on the surface of Pd-Au alloy foil catalysts at room temperature. O, Initial values of log y, final values representing catalytic activity of Pd and its alloys containing absorbed hydrogen. Broken line denotes the alloy Pd40Au60 which represents the upper limit of gold content in Pd-Au alloys closing the region of Pd-Au hydride formation. After Dickens et al. (86).

Fig. 9. Decrease of the catalytic activity of palladium on pumice with time. A— catalytic activity of Pd in initial measurements at 30°C B—catalytic activity of Pd at 30°C after mercury vapor is frozen out C—catalytic activity of Pd at 118°C after removing mercury vapor. (r0)i and (r0) are the initial reaction rates for the first and nth reactions (mm Hg/min). After Mann and Lien (41)-...

Both NO and N20 reduction on Pd/YSZ64-66 exhibit electrophilic NEMCA behavior with negative current or potential application. Within the temperature range of the studies64 66 (600-750K) the catalytic activity of Pd for the reduction of NO or N20 by CO was enhanced up to 300% and 200%, respectively, while the rate increase of NO reduction was typically more than 700 times larger than the rate of O2 removal from the catalyst via negative current application. 

Supports effects do not drastically modify Pd this is shown by XPS and by IR spectroscopy of adsorbed CO Nevertheless, the catalytic performances of the materials have been significantly improved with the supports containing zirconia. For instance, the activity of Pd/Al203-Ba0-Zr02 nearly reaches the activity of the Pt-Rh/Al203 reference. 

This means that the improvement of catalytic activity of Pd nanoparticles by involving the Pt core is completely attributed to the electronic effect of the core Pt upon shell Pd. Such clear conclusion can be obtained in this bimetallic system only because the Pt-core/Pd-shell structure can be precisely analyzed by EXAFS and Pd atoms are catalytically active while Pt atoms are inactive. 

Except Ru (not usable in TWC because of the volatility of its oxide [68]), the most active metal is the rhodium. This has been largely confirmed by further studies so that Rh may be considered as a key-component of TWC for NO reduction [69,70], As far as Pd is concerned, it seems that the active site is composed of Pd"+ —Pd° pairs, which may explain the higher activity of Pd in N0+C0+02 mixture (T5( 200°C) [71]. A detailed kinetic study by Pande and Bell on Rh catalysts has evidenced a significant support effect [72], The kinetic data were represented by a conventional power law expression ... 

Substrate reactivity was as expected (Arl > ArBr ArCl). In contrast to the Suzuki cross-coupling, however, Cu and Ru clusters were not active in the Heck reactions, and the activity of Cu/Pd clusters was lower than that of pure Pd clusters. Note the higher activity of Pd clusters prepared in situ (row F) compared to pre-prepared clusters (rows B and G). This increased activity tallies with our findings for Suzuki cross-coupling (7). After reaction, palladium black was observed in all the vials in rows B and G, but not in row F. 

The enantioselective hydrogenation of a,p-unsaturated acids or esters, using 5wt% Pt/Al203 or Pd/Al203 commercial catalysts doped with cinchonidine (CD), was deeply investigated to evidence the specific activity of Pd or Pt and the role of the reaction parameters and solvent polarity. Finally, the steric and electronic effects of different substituent groups were also studied. 

The results from the batch reactor and the fixed-bed reactor agree in terms of activity and selectivity for the different zeolites. The activity of Pd/H-MCM-22 is higher than that of Pd/H-ZSM-5, however, the selectivity to ethyltoluenes is much higher for Pd/H-ZSM-5. The yield of the desired ethyltoluene products is also higher with the ZSM-5 catalyst since it is less active for toluene disproportionation. However, with regard to the achievable yields and selectivities of the desired ethyltoluenes, the batch reactor is clearly inferior to the fixed-bed reactor. The reason might be the excess of toluene in the batch reactor. 

Scheme 2 General routes to the preparation and activation of Pd(II) polymerization catalysts...

Kohnle, Slaugh, and Nakamaye reported that carbon dioxide greatly affects the catalytic activity of Pd(PPh3)4 and Pt(PPh3 )4 in the reaction of butadiene to give 1,3,7-octatriene. Surprisingly, in the absence of carbon dioxide, vinylcyclohexene was formed (35). Musco and Silvani found... 

Rose and Lepper found that phosphites, especially o-substituted phenyl phosphites, enhance the catalytic activity of Pd(acac)2 or Pd(OAc)2 and affect the ratio of 47 to 48 (52). When Pd(OAc)2 and PPh3 (1 1) were used at 50°C for 1.5 hours, the yield was 20% and the ratio of 47 to 48 was... 

Table 9.6 Hydrogenation activity of Pd-apo-ferritin nanoparticles in water. (Reprinted with permission of Wiley [71].)...

The push-spectator stabilization system enables one to employ various alkyl groups with different types of steric environment, which differentiate amino(alkyl) carbenes dramatically from the NHCs as ligands. Taking advantage of their steric and electronic properties, Bertrand et al. nicely demonstrated the utility of CAACs as ligands in the palladium catalyzed a-arylation of ketones. Depending on the nature of the aryl chloride used, dramatic differences were observed in the catalytic activity of Pd-complexes with CAACs featuring different types of steric environment [36]. 

In the case of ethanol, Pd-based electrocatalysts seem to be slightly superior to Pt-based catalysts for electro-oxidation in alkaline medium [87], whereas methanol oxidation is less activated. Shen and Xu studied the activity of Pd/C promoted with nanocrystalline oxide electrocatalysts (Ce02, C03O4, Mn304 and nickel oxides) in the electro-oxidation of methanol, ethanol, glycerol and EG in alkaline media [88]. They found that such electrocatalysts were superior to Pt-based electrocatalysts in terms of activity and poison tolerance, particularly a Pd-NiO/C electrocatalyst, which led to a negative shift of the onset potential ofthe oxidation of ethanol by ca 300 mV compared... 

The active component for olefin oxidation is Pd2+, while Cu2+ acts as a promoter for the reoxidation of Pd. The sequence of ion exchange of Pd + and Cu2+ on the faujasite zeolite influences the catalytic performance. Best results seem to be obtained when Pd + is introduced in the second step of the ion exchange as it will then be located mainly at the more easily accessible cation sites II and/or III [23], The amount of exchanged Pd + determines the catalytic activity of Pd +Cu +Y, provided that Cu2+ is present in sufficient amounts to assure fast regeneration of Pd2+. A Pd/Cu atomic ratio of four is required here. Increasing acidity in Pd +Cu +NaY results in a decrease of both the activity and selectivity in the olefin oxidation [26]. 

Table II. Hydroisomerization of n-Pentane Influence of Silica-Alumina Molar Ratio of Activity of Pd-H-Mordenite Catalysts...

Cinneide and Clarke (770) have studied the activity of Pd-Au films for the deuteration and exchange of benzene and the hydrogenation of p-xylene. The authors report that the activity for the exchange reaction between benzene and deuterium persists to the palladium-lean compositions, which is in agreement with results obtained by Honex et al. (Ill) in a study of the exchange of toluene over alloys of the same kind. The rates are much reduced (by 102 to 103) compared to those found with palladium-rich films. 

Pumice-supported Pd showed similar selectivity (100% up to complete conversion) which is due to the very low hydrogenation rate of the intermediate cyclooctene284. The activity was invariant up to 35% dispersion, then decreased slowly, which is explained by the increasing electron density of Pd brought about by Na+ and K+ ions present in the support. The decrease in both activity and selectivity is anticipated when Pd is alloyed with Pt, a nonselective metal285. The substantial decrease in activity of Pd-Pt-pumice catalysts (TOF values are 90 s 1 for Pd-pumice and 12 s 1 for Pd-Pt-pumice) was attributed to the reduced amount of Pdr units necessary for the surface reaction and the very strong Pt-diene bond. 

In this respect it should be said that even open faces such as the (210) and the stepped (001) surface do not dissociatively adsorb CO at 25-125°C (97). This suggests that unsupported Pd is a rather poor methanation catalyst. Under 1 atm total pressure in a CO + H2 mixture, the Pd black catalyst (210-nm crystallites) produces methane but, here again, the activity level is about two times lower than that of Pd/Si02 catalysts (4.6-nm Pd particle size), and about two orders of magnitude less active than Pd/ A1203 catalysts (4.8-nm Pd particle size) (98). It therefore seems that the effect of dispersion here is not pronounced with respect to the support effect. Silica, as an inert support, does not influence the activity of Pd to the same extent as does more the acidic alumina. 

Finally, it should be mentioned that the lack of kinetic data for singlecrystal surfaces of Pd encouraged Hicks and Bell to calculate specific activities of Pd(l 11) and Pd(100) planes from the results obtained on Pd/ Si02 and Pd/La203 catalysts (99). The distribution of these planes was inferred from the IR spectra of adsorbed CO, based on the relative intensities of the B, and B2 (bridging) bands (100). In the case of the Pd/La203 catalyst, the methanol turnover frequency depends on the crystallographic orientation of the metal surface for Pd(100) planes, turnover frequency is 18 x 10-3 s l for Pd(lll) planes, 6.5 x 10-3 s-1. However, as Hicks... 

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