Unsupported Palladium

The explanation given above recalls earlier speculations presented by Ledoux et al. (22, 23). The authors assigned a different role to various surface centers on a palladium film in the reactions of isomerization, hydrogenation, and deuterium exchange of butenes. The authors defined 

In the case of chemisorption of acetylene on Pd(lll), high-resolution electron energy loss spectroscopy (HREELS) data suggest that ethylidyne coexists with vinylidene ( =C=CH2) at this surface (31). In their ultra- 

It looks that in using these rather meager data on catalytic hydrogenation on well-defined surfaces of unsupported Pd for probing more complex Pd surfaces (as we have in supported catalysts), one has to be very cautious and not forget the possible effect of Pd hydride formation. The /3-PdH may be more easily formed in massive Pd specimens (36 but see also 37, 38), whereas for highly dispersed Pd catalysts we can deal with the absence of the hydride phase (see Section IV,A). 

To the best of our knowledge, there are no published data on the reactions of alkanes with H2 over palladium single crystals. Deuterium exchange reactions with various alkanes over palladium films were extensively investigated by Kemball et al. (39-42). Because those results were already discussed in several excellent review articles (43-46), we do not intend to dwell upon analyzing all the data at greater length. We only specify here the basic characteristics of Pd films exhibited in D2/alkane reactions  

Palladium is a very active metal (cf. other Group VIII metals) in the D2 exchange of all hydrocarbons when the so-called a/3 process [step (c) 

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Zhou WP, Lewera A, Larsen R, Masel RI, Bagus PS, Wieckowski A. 2006. Size effects in electronic and catalytic properties of unsupported palladium nanoparticles in electrooxidation of formic acid. J Phys Chem B 110 13393-13398. 

Palladium, 26 107, 30 18-19, 32 73-74,37 45-47 see also Supported palladium Unsupported palladium on active charcoal, 20 128-131... 

Having in consideration the problem of chemical probing of Pd/support systems, it is relevant to know the catalytic properties of unsupported palladium. Three types of reactions will be considered in this article ... 

Vannice (232) measured turnover numbers for methanation on a variety of well-characterized palladium catalysts. The supported catalysts were all more active than unsupported palladium (Pd black) and PdHY was intermediate between Pd/Si02 and Pd/Al203 in specific activity (see Table VII). As is evident from the data there is no obvious correlation betwen particle size and turnover number. It was therefore suggested that the enhanced activity of various supported catalysts was due to a metal-support interaction. Figureas et al. (105) also found good evidence for a support effect during benzene hydrogenation studies. In this case the palladium zeolite... 

Unsupported palladium catalysts are often unstable in use at elevated temperatures and/or high hydrogen pressures. Yada et al. obtained a highly stable palladium black by precipitating palladium hydroxide from palladium chloride solution with an aqueous sodium aluminate instead of sodium hydroxide.177 The palladium catalyst obtained by reduction of the palladium hydroxide prepared with a 1M sodium aluminate solution at pH 10, in the same way as described above, kept a high surface area of 139 m2 g 1 even after the reduction at 200°C in a flow of hydrogen, compared to only 2.0 m2 g 1 with the catalyst prepared using a sodium hydroxide solution as precipitant. The thermally stable catalyst was found to contain 13 ppm of sodium and 1400 ppm (0.14%) of aluminum. 

Complexes with Unsupported Palladium-Metal Bonds... 

Supported Palladium. - For catalysts prepared by impregnation with PdQ2 on silica, titania, and alumina supports and reduced (673 K), the width of the Pd 3c 5/2 lines in the photoelectron spectra was greater than the value for unsupported palladium and decreased in the order... 

W. Kitdiing, Z, Rappoport, S. Winstein, and W. G. Young,v4m. Soc., 88,2054 (1966) Palladium catalysts, see also Lindlar catalyst. Engelhard Industries supplies 5% palladium catalyst on C, Al, AljOa, BaCOa, BaS04, or CaCOj also pelleted, granular catalysts, and unsupported palladium oxide and palladium black. MacKay supplies 10% Pd-C catalyst Mozingo describes the preparation of four catalysts ... 

Further examples that deal with the preparation and catalytic activity of unsupported palladium nanoparticles in microemulsion are reported by Spiro de Jesus... 

In the past, there was no need for small-scale (< 1 MW) distributed hydrogen production from hydrocarbons where such reactors would likely be first employed. In any case, unsupported palladium membranes have required too much expensive metal to be economically feasible for all but special purposes. The potential availability of cost effective, high hydrogen permeance, palladium membranes supported on porous, stainless steel, tubular substrates combined with the advent of stable, high volumetric activity WGS catalysts is the driving force behind the system modeling... 

In type 1 and type 2 Mizoroki-Heck reactions, high temperature is required when SRPCs are involved as palladium sources. In type 2 reactions, the need to speed up the oxidative addition step is more important when the palladium catalyst has no particular activating ligands. In this case, a lower rate of oxidative addition would result in the build-up of unsupported palladium(O) with concomitant deactivation. 

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