Palladium, pure colloidal

Supported palladium and platinum modified by chiral compounds are largely used as pure heterogeneous hydrogenation catalysts. However, recent studies have been performed starting with catalysts of colloidal nature and particles with dimensions of only a few nanometers. Their development continues to attract substantial interest for three main reasons ... 

Bimetallic colloids containing gold and palladium or platinum have been deposited onto carbon or graphite for use as catalysts for the selective oxidation of organic compounds52 (Section 8.3), in the same way as for pure gold colloids (Section 4.6). 

Physical Varieties of Palladium.—Palladium may be prepared in several different physical states, namely, as the ordinary compact metal, as colloidal metal, as palladium sponge, and finally as palladium black. Of these, the last named is not pure palladium, but an indefinite mixture in a state of very fine division. Each of these varieties has its own peculiar physical characteristics. 

Catalytic Activity.—Palladium is a powerful catalyser in each of its various physical varieties. The colloidal form is most active and its catalytic powers are discussed on page 184. Next comes palladium black (see p. 187), whilst finely divided palladium—as distinct from palladium black, which is not the pure metal-—and compact palladium are also reactive. 

Platinum Catalyst for Reductions (CoU. Vol. i, 452) Directions are given for the preparation of a colloidal platinum (or palladium) catalyst in an anhydrous, alkaline medium. This catalyst is said to be particularly valuable for the reduction of nitriles, oximes, and nitrostyrenes to pure prinaary amines. Skiia and Keil, Ber. 63, 424 (r932). 

An acetylene may be reduced to an olefin by sodium in liquid ammonia, ° by electrolytic reduction at a spongy nickel cathode, or by partial hydrogenation over metal catalysts. Catalysts for the hydrogenation include nickel, ° iron, colloidal palladium, and palladium on barium sulfate or calcium carbonate. Pure trans olefins are obtained from dialkylacetylenes by reduction with sodium in liquid ammonia. The yields ate better than 90%. Catalytic hydrogenation leads to mixtures of cis and trans olefins in which the cis isomers predominate. ° Mono- and di-arylacetylenes have also been reduced. ... 

Recently, novel nanomaterials have become a new frontier for SERS experiments, where different metals are collected together to form, for example, bimetallic particles. Thus, the same nanoparticle could be responsible for both SERS effect and catalytic activity. This is the case of the Ag/Pd colloids synthesized by chemical reduction with sodium borohydride (NaBH4) of silver nitrate (AgNOs) and palladium nitrate (Pd(N03)2), with a 96 4 Ag/Pd molar ratio [11]. The silver nanoparticles provide the SERS enhancement for the ligand molecules, while palladium may induce catalytic reactions. Also, in this case, TEM microscopy provides an important help to characterize these composite materials. In Fig. 20.6 TEM images at different magnifications are reported for bimetallic Ag/Pd particles, in comparison with those constituted by pure silver. While these latter present spheroidal shapes, bimetallic particles show more irregularities, due to palladium clusters in contact with the silver core surface. 

At the opposite of the molecular chemistry described until now, nanoparticles are reminiscent of heterogeneous catalysts. However, these colloid-derived materials have been shown to catalyze efficiently in water coupling reactions which have been previously described in pure homogeneous systems. For instance, poly(N-vi-nyl-2-pyrrolidine)-stabilized palladium nanopartides promote the Suzuki crosscoupling in aqueous media with high yields (see also Section 6.6) [87]. 

Adsorbed carbon monoxide can serve as a useful infrared probe of surface composition in bimetallic colloids if both metals bind CO. This is exemplified in the infrared spectrum of CO on a PVP stabilized colloidal alloy CutgPdjT, [38] Carbon monoxide adsorbs readily onto these PdCu particles (co. 45 A) in dichloro-methane at 25 °C, as shown by the infrared absorption spectrum in Figure 6-28. By comparing this to the IR spectrum of CO on a pure palladium colloid of similar size [34] in Figure 6-27d, it can be clearly seen that CO occupies both palladium and copper sites. Whereas the bands at 2046 cm" and 1936 cm" are in the frequency ranges found for linear and bridged CO on the pure palladium particles, the new band at 2089 cm corresponds to CO on surface copper atoms, thus demonstrating that both metals are present at the surfaces of the particles. 

A pH-responsive palladium catalyst 46, prepared by immobilizing palladium nanoparticles on colloidal core-shell micro-spheres that contain a pH-responsive shell of poly(methylacryUc acid) (PMAA) segments and a coordinative core of poly[2-(acetoxy)ethyl methacrylate] (PAEMA), exhibits good activity in the Suzuki and Heck reactions in a mixture of DMF/water (1 1, v/v) or in pure water [131]. The catalyst is highly dispersed under neutral or basic conditions, behaving like a homogeneous catalyst, but heterogeneous under acidic conditions. As such, it could... 

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