Palladium Sol

Fig. 1. Reduction of silver ions by hydroquinone 1, no catalyst added 2, qui-none added 3, silver sol added 4, gold sol added 5, palladium sol added 6, silver sulfide sol added. Gelatin (1 %) was present as stabilizer.

By using as fondamental methodology the formation of metallic colloidal solutions, we set up two different procedures a) formation of gold sol followed by further formation of palladium sol in the same aqueous medium and then immobilised on carbon b) formation of gold sol, immobilisation on carbon, and subsequent formation of palladium sol, in the presence of Au/C. In this report we will name (Pd Au)/C (1) the material obtained from the first procedure and Pd Au/C (2) the one from the second. The total metal loading was l%wt and Au/Pd molar ratio 6/4. 

As early as 1997, Antonietti and coworkers [167] showed that palladium nanoparticles supported by polystyrene-polyvinylpyrrolidone block copolymer effect the Mizoroki-Heck reaction while retaining a monodisperse size distribution throughout. Very recently, a similar effect was demonstrated for A-vinylimidazole-A-vinylcaprolactam copolymer, in the presence of which a monodisperse palladium sol was generated directly in the reacting system [168]. 

Scenario 4. Nanoparticles are formed during the reaction initiated by soluble palladium precatalysts. Our experience in phosphine-free (mostly aqueous) systems led us at an early stage to hypothesize that these protocols go hand in hand with formation of palladium sols [4, 83]. Since 2000, the observation of palladium sols in Mizoroki-Heck reactions has become ubiquitous. So, if one were to decide to compile a list of references that mention the formation of sols, that list would practically coincide with a list of references which deal with the development of new phosphine-free catalytic systems. In fact, the idea that it is the dispersed palladium metal which catalyses (or, more correctly, serves as precatalyst) the Mizoroki-Heck reaction should be traced back to the seminal pubUcations by Heck in 1972 [2], who clearly attiibuted catalytic activity in a phosphine-free catalytic system to palladium metal formed by reduction of Pd(OAc)2 by the alkene or the amine. This idea fell into oblivion until its rediscovery in the course of a general interest in nanoscale systems in the late 1990s. 

Since then, statements that palladium nanoparticles or sols are true catalysts in a given catalytic system have become not infrequent. Indeed, in many cases it is clearly seen that, after the addition of precatalyst, a brownish-grey colour of palladium sol appears immediately and hansmission electron microscope measurements reveal the nanoparticles of metal. From the discussion of various preformed sols in Mizoroki-Heck reactions (Scenarios 1-3), it becomes evident that palladium nanoparticles display rather hmited activity with respect to scope, TONs and TOF values. On the other hand, in the discussion of various SRPCs, we see many examples of high activity and quite extended scope of substrates reaching to aryl chlorides. 

The experimental conditions were that 10 cc. palladium sol, 2 gms. nitrobenzol, and 10 cc. alcohol were put into a small flask with a condenser and hydrogen conducted in continuously at 70 degrees. In a similar manner the activation of platinum was found to be 6700 to 37,000, that of iridium 2000 to 4000, silver and osmium very small, while gold and copper are not affected by hydrogen. 

Kostic et al. recently reported the use of various palladium(II) aqua complexes as catalysts for the hydration of nitriles.456 crossrefil. 34 Reactivity of coordination These complexes, some of which are shown in Figure 36, also catalyze hydrolytic cleavage of peptides, decomposition of urea to carbon dioxide and ammonia, and alcoholysis of urea to ammonia and various carbamate esters.420-424, 427,429,456,457 Qggj-jy palladium(II) aqua complexes are versatile catalysts for hydrolytic reactions. Their catalytic properties arise from the presence of labile water or other solvent ligands which can be displaced by a substrate. In many cases the coordinated substrate becomes activated toward nucleophilic additions of water/hydroxide or alcohols. New palladium(II) complexes cis-[Pd(dtod)Cl2] and c - Pd(dtod)(sol)2]2+ contain the bidentate ligand 3,6-dithiaoctane-l,8-diol (dtod) and unidentate ligands, chloride anions, or the solvent (sol) molecules. The latter complex is an efficient catalyst for the hydration and methanolysis of nitriles, reactions shown in Equation (3) 435... 

Palladium catalyzed reaction of aryl halides and olefins provide a useful synthetic method for C-C bond formation reaction [171, 172], The commonly used catalyst is palladium acetate, although other palladium complexes have also been used. A sol-vent-free Heck reaction has been conducted in excellent yields using a household MW oven and palladium acetate as catalyst and triethylamine as base (Scheme 6.51) [173], A comparative study revealed that the longer reaction times and deployment of high pressures, typical of classical heating method, are avoided using this MW procedure. 

A new class of heterogeneous catalyst has emerged from the incorporation of mono- and bimetallic nanocolloids in the mesopores of MCM-41 or via the entrapment of pro-prepared colloidal metal in sol-gel materials [170-172], Noble metal nanoparticles containing Mex-MCM-41 were synthesized using surfactant stabilized palladium, iridium, and rhodium nanoparticles in the synthesis gel. The materials were characterized by a number of physical methods, showed that the nanoparticles were present inside the pores of MCM-41. They were found to be active catalysts in the hydrogenation of cyclic olefins such as cyclohexene, cyclooctene, cyclododecene, and... 

Reetz, M.T. and Dugal, M., Entrapment of Nanostructured Palladium Clusters in Hydrophobic Sol-Gel Materials, Catal. Lett., 58, 207, 1999. 

In the following section, we describe the case of adsorption of a Sn complex onto a palladium oxide suspension. In an alkaline medium (a basic PdO hydrosol), chlorides in the SnCL complex are substituted in the coordination sphere of tin(IV) by hydroxo anions, which are in excess, yielding the stannate Sn(OH)g complex. The Sn Mossbauer spectroscopy spectrum of a bimetallic sol (frozen in liquid nitrogen) is compared with a true stannic solution. At the same tin concentration, it shows the changes in the Sn environment due to adsorption onto the PdO surface (Fig. 13.27). The isomer shift S is found to be close to zero for the stannate solution and increases when contacted with the PdO suspension, indicating a modification of the coordination sphere of tin. The increase in 5 can be correlated to an increase in the core level electronic density of tin. The quadrupole splitting A, is related to a modification of the symmetry of the close environment of tin, due to adsorption of Sn(OH)g complexes onto the PdO colloidal nanoparticles. 

The Wacker process (Eq. 1) was developed nearly 50 years ago [1-3] and represents one of the most successful examples of homogeneous catalysis in industry [4-9]. This palladium-catalyzed method for the oxidation of ethylene to acetaldehyde in aqueous solution employs a copper cocatalyst to facilitate aerobic oxidation of Pd° (Scheme 1). Despite the success of this process, certain features of the reaction have Umited the development of related aerobic oxidation reactions. Many organic molecules are only sparingly sol-... 

Ceramic and semiconductor thin films have been prepared by a number of methods including chemical vapor deposition (CVD), spray-coating, and sol-gel techniques. In the present work, the sol-gel method was chosen to prepare uniform, thin films of titanium oxides on palladium Titanium oxide was chosen because of its versatility as a support material and also because the sol-gel synthesis of titania films has been clearly described by Takahashi and co-workers (22). The procedure utilized herein follows the work of Takahashi, but is modified to take advantage of the hydrogen permeability of the palladium substrate. Our objective was to develop a reliable procedure for the fabrication of thin titania films on palladium, and then to evaluate the performance of the resulting metalloceramic membranes for hydrogen transport and ethylene hydrogenation for comparison to the pure palladium membrane results. 

Preparation of Thin Tilms of TiOx on Palladium Foils. Following the procedure reported by Takahashi, an isopropanol based sol of titanium tetra-isopropoxide (TnP) was prepared (10 mL of TUP in 48.4 mL of 2-propanol), to which 6.7 mL of diethanolamine was added to inhibit the formation of oxides. This mixture was stirred for two hours, at which time gel formation was initiated by the addition of 28.1 mL of a dilute water solution (1.42 mL water in 30 mL of 2-propanol). The resulting molar ratio of water to TUP was 2 1, as was recommended by Takahashi and co-workers (22). The palladium substrates were dipped vertically into this solution and then clamped on edge while draining to allow the excess to drip off and to prepare as even a coating as possible. The membranes were then air-dried in a vertical position at 100°C for one hour, followed by a post-treatment with hydrogen (see Figure 2). 

Figure 2. Preparation of titania films on palladium by sol-gel synthesis and thermal treatment.

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