Catalysts Pd-based

It has been predicted that a Pd skin on a PdsFe core would sit close to the top of the activity/O binding energy volcano curve and thus have significantly higher activity than Pt and Pt/Pd core-shell materials. If these materials could be shown to be stable to long-term PEM conditions, then these could represent viable replacements for Pt. As with other alternative non-Pt catalysts, very few stability studies have been reported. 

---

The2-aminophenethyl alcohols resulting from condensation of orr/ici-nitrotoliienes are good precursors for preparation of indoles. Watanabe and co workers have developed ruthenium-catalyzed dehydrogenadveiV-heterocyclizadon for synthesis of indoles and other hereto cycles from 2-aminophenethyl alcohols or 2-nitrophenylethyl alcohols fEq. 10.52. The oxidadve cycli-zadon of 2-aminophenethyl alcohols are also catalyzed by Pd-based catalysts. ... 

Some evidences of a bifunctionnal mechanism for the reduction of NO on Pd based catalysts. 

The Pd-based catalysts were prepared by wet impregnation of the support with Pd(C5H702)2 dissolved in toluene. After drying at 373 K, the acetylacetonate was decomposed under O2 up to 773 K (0.5 K/min) The catalytic behavior of these solids was compared to that of a reference Pt-Rh/Al203 solid. Some characteristics of the solids are reported in Table 1. 

After 120 s oxygen exposure (500 C), all of the dynamometer-aged Pd-based catalysts gave oxygen up es that could be accounted for by tfie formation of bulk PdO. 

Formation of bulk PdO is the primary oxygen storage mechanism in the dynamometer-aged Pd-based catalysts. 

Very recently Chen and co-workers have applied the previously mentioned Ni-based dimetallic pre-catalyst 14 in the Negishi reaction. Remarkable results were obtained even when unactivated aryl chlorides were chosen as reaction partners providing an alternative to the more expensive Pd-based catalysts. The fact that dinuclear pre-catalyst 14 is more active than its mononuclear analogue 13 indicates a possible cooperative effect between the two metal centres [86] (Scheme 6.23). 

Allyl carbonate esters are also useful hydroxy-protecting groups and are introduced using allyl chloroformate. A number of Pd-based catalysts for allylic deprotection have been developed.209 They are based on a catalytic cycle in which Pd° reacts by oxidative addition and activates the allylic bond to nucleophilic substitution. Various nucleophiles are effective, including dimedone,210 pentane-2,4-dione,211 and amines.212... 

A recent comparative investigation of the NO + CO reaction shows a significant rate enhancement in the formation of N2 on Ce0.98Pda02O2 8 prepared via a combustion synthesis method in comparison with conventional Pd-based catalysts supported on alumina... 

Recent developments showed promising results over platinum and palladium-based catalysts after deposition on reducible materials [ 104-106] (see Figures 10.11 and 10.12). Further improvements might be expected on Pd-based catalysts supported on those... 

Although Pd-based catalyst systems form only the desirable trans-1,4-hexadiene, their industrial value is very doubtful because of the reported poor yield of the C6 diene and the very low activity of the dimerization reactions compared to other catalyst systems. 

The most active Pd based catalysts in the table have been reported by Shishido et al.51 who have employed hydrotalcite precursors, using a solid phase crystallisation (SPC) method. Comparisons made with catalysts prepared via impregnation showed SPC to afford higher activity catalysts, which was ascribed to better dispersion and also easy desorption of CO. The inclusion of a Cr component in these types of catalyst has also been reported to be beneficial.52 In terms of the desorption of CO, isotopic studies performed by McKee53 over ruthenium and platinum catalysts have indicated that the rate determining step of methanol decomposition is the fission of a... 

Platinum-based catalysts are widely used in low-temperature fuel cells, so that up to 40% of the elementary fuel cell cost may come from platinum, making fuel cells expensive. The most electroreactive fuel is, of course, hydrogen, as in an acidic medium. Nickel-based compounds were used as catalysts in order to replace platinum for the electrochemical oxidation of hydrogen [66, 67]. Raney Ni catalysts appeared among the most active non-noble metals for the anode reaction in gas diffusion electrodes. However, the catalytic activity and stability of Raney Ni alone as a base metal for this reaction are limited. Indeed, Kiros and Schwartz [67] carried out durability tests with Ni and Pt-Pd gas diffusion electrodes in 6 M KOH medium and showed increased stability for the Pt-Pd-based catalysts compared with Raney Ni at a constant load of 100 mA cm and at temperatures close to 60 °C. Moreover, higher activity and stability could be achieved by doping Ni-Al alloys with a few percent of transition metals, such as Ti, Cr, Fe and Mo [68-70]. 

Two types of catalysts have been proposed for the CPOM reaction copper and palladium. The catalytic properties of these materials show significant discrepancies with respect to by-product formation and the effect of oxygen partial pressure. The Cu-based catalysts display high selectivity for the CPOM reaction whereas for the Pd-based catalysts CO formation is significant. 

---