Reduction colloidal palladium

Palladium catalysts have been prepared by fusion of palladium chloride in sodium nitrate to give palladium oxide by reduction of palladium salts by alkaline formaldehyde or sodium formate, by hydrazine and by the reduction of palladium salts with hydrogen.The metal has been prepared in the form of palladium black, and in colloidal form in water containing a protective material, as well as upon supports. The supports commonly used are asbestos, barium carbonate, ... 

Dextro-dihydroverbenol melts at 58° C. and boils at 218° C. it yields an acetic ester, the odour of which recalls that of bornyl acetate. Dextro-dihydroverbenone is produced by the oxidation of the above alcohol by means of chromic acid, or by the reduction of verbenone by means of hydrogen in presence of colloidal palladium. It boils at 222° C. (Djj 0-9685 [a]o + 52-1 9° 1-47535 molecular refraction 44 45) and gives... 

By reduction of the corresponding acetylene with hydrogen and colloidal palladium. Bourguel, Bull. soc. chim. 41, 1475 (1927). 

Michaelis, M. and Henglein, A., Reduction of palladium (II) in aqueous solution stabilization and reactions of an intermediate cluster and palladium colloid formation, J. Phys. Chem., 96, 4719, 1992. 

There are two main uncertainties associated with this general mechanism. First, there are a number of C-C coupling reactions where there is no direct evidence for the reduction of the Pd(II) precatalyst into a zero-valent palladium species. Second, like the hydrosilylation system, a number of these reactions may involve colloidal palladium. Also, the general catalytic cycle needs to be substantially modified to rationalize the successful use of 7.63 as a precatalyst. 

Although the selectivity of palladium catalysts in the hydrogenation of 1,5-COD is thus very high, the results also indicate that the hydrogenation of COE to cyclooctane (COA) does not cease after the maximum yield of COE has been attained. Hirai et al. studied the hydrogenation of 1,5-COD over a colloidal palladium catalyst, prepared by reduction of palladium(II) chloride in the presence of poly(iV-vinyl-2-pyrrolidone) in refluxing methanol with addition of sodium hydroxide, in methanol at 30°C and 1 atm H2, and obtained a mixture consisting of 0.4% 1,5-COD, 0.3% 1,4-COD, 97.8%... 

Colloidal nickelous hydroxide is reduced to colloidal nickel by hydrogen in the presence of colloidal palladium as catalyst. The freshly precipitated hydroxide may also be reduced in a similar manner. It is advantageous to add sodium protalbinate to the mixture before reduction in order to increase the stability of the hydrosols produced.4... 

Palladium Hydrosol or Colloidal Palladium is readily prepared by the reduction of the chloride with acrolein5 or with hydrazine hydrate, in either case in the presence of an extract of Iceland moss 6 or in contact with sodium lysalbinate or protalbinate,7gum acacia,8 or with lanolin9 in a precisely similar manner to platinum,10 the function of the organic additions, which are protective colloids, being to increase the stability of the colloidal phase. 

Levene and LaForge,1 in 1912, made the interesting observation that the nucleoside uridine I, which is obtained by hydrolysis of nucleic acid, is reduced practically quantitatively to dihydrouridine II by PaaTs2 method of catalytic reduction, namely, by means of colloidal palladium and hydrogen. This transformation involves an addition of hydrogen at the double bond joining positions 4 and 5 in the uracil nucleus and the change is represented as follows ... 

The process which we have applied successfully for the reduction of uracil is based on the method of reduction developed by Skita.8 This investigator used both colloidal palladium and platinum in his work and utilized as a support for his colloidal metal gum arabic. He showed also that his colloidal metals were active in both acid and alkaline solutions. 

In addition, other forms of Pd(0) stabilized in less conventional ways should be cited. Catalytically active palladium colloids are obtained by reduction of palladium acetate in DMSO, or in the presence of a number of polymers. " Some of the latter are easy to recycle and avoid the leaching of Pd during the catalytic runs. 

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. ... 

The preparation of palladous oxide-palladium black and its use as a catalyst in the reduction of organic compounds have been studied by Shriner and Adams. Palladium black and colloidal palladium have been widely used as hydrogenation catalysts. ... 

A variety of compounds has been reduced employing colloidal palladium catalysts supported by synthetic polymers. Table I summarizes the results of various reductions (13). 

When nitrobenzene (23) is reduced with colloidal palladium, the rate is found to be independent of the pH of the solution (22,5). When this reduction is conducted with colloidal rhodium, the rate is found to be maximal in the alkaline range, median in the acid range, and minimal in neutral solution. Further, when substituted nitrobenzene derivatives are reduced with these catalysts, it is found that the rate of reduction with rhodium is dependent both on the nature and position of the substituent group. With palladium, however, the rate of reduction is virtually independent of these factors. In all cases, the reduction was found to be first order with respect to the catalyst and zero order with respect to the substrate. 

The first studies that intentionally used colloidal nanocatalysts were reported independently by Beller et al. [50] and Reetz et al. [51] using chemical reduction and electrochemical techniques, respectively, to synthesize colloidal palladium nanoparticles for the Heck reaction. Both Beller and Reetz concluded that the solution-phase catalysis occurred on the surface of the nanoparticle, without confirming that a homogeneous catalytic pathway was nonexistent. Le Bars et al. [52] demonstrated an inverse relationship between the size of Pd nanoparticles and the TOF (normalized to the total number of surface atoms) for the Heck reaction (Fig. 18.4a). After normalizing the rate to the density of defect sites (for each nanoparticle size) (Fig. 18.4b), the TOF for all particle sizes was identical. Colloidal PVP-capped palladium nanoparticles synthesized by ethanol reduction are effective catalysts for Suzuki cross-coupling reactions in aqueous solution [53]. The El-Sayed group reported that the initial rate of reaction increased linearly with the concentration of Pd nanoparticles [53] and the catalytic activity was inversely proportional to the... 

Reduction of /3-chlorocodide in dilute acetic acid with a colloidal palladium catalyst gives mainly dihydrodesoxycodeine-D [ix] but also a considerable amount of tetrahydrodesoxycodeine [vm] [30, 34]. The former is presumably formed by initial 1 4-reduction of the system... 

An a-dihydrodesoxycodeine , amorphous, alkali-insoluble, and poorly characterized, was reported by Freund [4] to result from the reduction of a-chlorocodide using a colloidal palladium catalyst. A reinvestigation of this reduction, however, showed that under the conditions prescribed by Freund the product consists of 95 per cent. dihydrodesoxycodeine-D with palladized barium sulphate the results are substantially the same, but an amorphous product is obtained in 40 per cent, yield if the amount of catalyst used is large, and this can bo increased to 96-100 per cent, using palladized calcium carbonate and 100 per cent, using platinum oxide as catalyst [29]. The product appears to bo bis- 6 0 -dihydrodosoxycodeino-D [xlvj] and is probably formed... 

Mild catalytic reduction of thebaine hydrochloride in aqueous or dilute acetic acid solution using a platinum [1] or colloidal palladium catalyst [2-6] results in saturation of the 8 14-double bond and production of dihydrothebaine [i]. The yield is never high, as considerable amounts of dihydrothebainone [n] and tetrahydrothebaine [hi] are produced simultaneously, not by further reduction of dihydrothebaine, which is stable under these conditions. 

Catalytic hydrogenation of dihydrothebaine methine [ix] with colloidal palladium in dilute acetic acid causes saturation of the 9 10 double bond, scission of the cyclic ether, and hydrolysis of the enol ether group giving dihydrothebainone dihydromethine [xvm] [3], and this is the most satisfactory method of preparing the latter. The dihydromethine [x] may be reduced to [xvm] under the same conditions [3]. [xvm] can also be prepared by hydrolysis of dihydrothebaine dihydromethine [x] to dihydrocodeinone dihydromethine [xix] (also accessible by the chromic acid oxidation of a-tetrahydrocodeimethine [xx] [3]) followed by aluminium amalgam reduction [3]. 

Apart from poisoning by adsorbing impurities, the working electrode potential can also contribute to suppress electrocatalytic activity. Platinum metals, for instance, passivate or form surface oxygen and oxide layers above 1 V (Section IV,D), which inhibit Oj reduction (779,257,252) and oxidation of carbonaceous reactants (7, 78, 253, 254) however, decomposition of hydrogen peroxide on platinum is accelerated by oxygen layers (255). Some electrocatalysts may corrode or dissolve, especially in acidic electrolytes, while reactants may contribute to dissolution. Thus, ethylene oxidation on palladium to acetaldehyde proceeds via a Pd-ethylene complex, which releases colloidal palladium in solution (28, 29). Equivalent to this is the surface roughening and the loss of Pt in gas phase ammonia oxidation (256, 257). 

Palladium chloride Reductive opening of the piperidine ring Colloidal palladium as catalyst s. 4, 64 PdClz c... 

Biffis et al. employed soluble microgels to stabilize colloidal palladium particles [27, 28]. These cross-linked polyacrylates, characterized by gel permeation chromatography, contained amino groups, which coordinate palladium ions. Reduction results in colloids with particle sizes of 3 to 9 nm. Particle sizes can be influenced by the degree of crosslinking of the microgel. Heck reactions of activated aryl bromides proceed with high activities (Table 1). 

Roopan, SM., Bharathi, A., Kumar, R., Khanna, V.G., Prabhakam, A., 2012a AcaricidaL insecticidal, and larvicidal efficacy of aqueous extract of Annona squamosa L peel as biomaterial for the reduction of palladium salts into nanoparticles. Colloid. Surf. B 92,209—212. 

Similarly irradiation of Pd(NH3)4Q2 in aqueous uo-propanol results in the reduction of the palladium complex to colloidal palladium by both solvated electrons and 1-hydroxy-1-methylethyl radicals (CH3)2COH (formed by reaction of iyo-propanol with the initial radiolysis products H and OH). [102] Since mixtures of acetone and iso-propanol also yield (CH3)2COH under both radiolytic and photolytic conditions, they have been used in many studies on the radical chemistry of metal colloids. In many of these studies, the techniques of flash photolysis and pulse radiolysis were used to detect short lived intermediates, measure the kinetics, and determine the mechanisms of metal cation reduction, nucleation, and colloidal cluster formation. 

Some Pd-Pt bimetallic colloids protected by polymers [162-165] have been prepared by the coreduction of the metal salts with aqueous alcohols. Similar bimetallic sols in nonpolar organic solvents were obtained by N2H4 or NaBH4 reduction of palladium and platinum salt mixtures after their extraction into the organic phase (cyclohexane or chloroform) with such surfactants as trioctylphos-phine oxide or distearyldimethylammonium chloride. [166]... 

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