Reetz’ colloid

Reetz et al. have used N-(octyl)4Br-stabilized Pd colloids (typical size, e.g., 3nm) as precursors to form so-called cortex-catalysts, where the active metal forms an extremely fine shell of less than lOnm on the supports (e.g., AI2O3). Within the first 1-4 s, the impregnation of AI2O3 pellets by dispersed nanostructured metal colloids leads to the time-dependent penetration of the support which is complete after 10 s. Cortex catalysts were reported to show a threefold higher activity in olefin hydrogenation than conventionally prepared catalysts of the same metal loading (5% Pd on AI2O3) [388]. 

Wang H, Wingender Ch, Baltmschat H, Lopez M, Reetz MT. 2001b. Methanol oxidation on Pt, PtRu, and colloidal Pt electrocatalysts A DEMS study of product formation. J Electroanal Chem 509 163-169. 

Reetz, M.T. and Maase, M., Redox-controlled size-selective fabrication of nanostructured transition metal colloids, Adv. Mater., 11, 773,1999. 

Reetz, M.T. and Koch, M., Water-soluble colloidal Adams catalyst preparation and use in catalysis,... 

HRTEM images of surfactant-stabilized PtRuOsIr oxide colloids at different magnification, (Reprinted with permission from M. T. Reetz et al.. Journal of Physical Chemistry B, 107, 7414 (2003). Copyright 2007, American Chemical Society.)... 

Because aryl phosphines are not only costly but can also act as aryl sources themselves, giving rise to unwanted byproducts, there has been steady interest in extending ligand-free Heck reactions to aryl bromides and aryl chlorides. Reetz and de Vries recently found that these can be performed with high efficiency using stabilized colloidal Pd catalysts [21]. If the palladium is kept at a low concentration between 0.01 and 0.1 mol%, precipitation of the Pd(0) is avoided and the colloids serve as a reservoir for the catalytically active species (Scheme 5). This economically attractive method has been successfully applied on an industrial scale by DSM [22]. 

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... 

A new method for the size- and morphology-selective preparation of metal colloids using tetraalkylammonium carboxylates of the type NR4 R C02 (R = octyl, R = alkyl, aryl, H) both as the reducing agent and as the stabilizer [Eq. (4)] was reported by Reetz and Maase [42]. 

This very versatile preparation route for nano structured mono- and bimetallic colloids has been further developed by Reetz and his group since 1994 [2e,12a,b]. The overall process of electrochemical synthesis [Eq. (7)] can be divided into six elemental steps (see Figure 6). 

The electrochemical synthesis developed by Reetz and co-workers offers at present the most rational method for control of particle size. Researchers have obtained at will almost monodisperse samples of colloidal Pd and Ni between 1 and 6nm using variable-current densities and suitable adjustment of further essential parameters [12]. For thermal decomposition methods the resulting particle size has been found to depend on the heat source [44f]. Size control has also been reported for the sonochemical decomposition method [45e] and y-radiolysis [48]. 

Beller et al. have shown for the first time that palladium colloids are effective catalysts for the olefination of aryl bromides (Heck reaction). Reetz et al. have studied Suzuki and Heck reactions catalyzed by preformed palladium clusters and palladium/nickel bimetallic clusters and further progress was achieved by Reetz and Lohmert using propylene carbonate stabilized nanostructured palladium clusters as catalysts in Heck reactions. In addition, the use of nanostructured titanium clusters in McMurry-type coupling reactions has been demonstrated by Reetz et... 

In the standard chemical preparation methods, the properties, especially the size and size distribution of the nanoparticles, are defined by the choice of the reaction conditions, reactant concentrations, etc. The use of electrochemical techniques to generate nuclei has the advantage that the supersaturation is determined by the applied potential or current density. Thus, the size of the particles can be controlled by electrochemical instrumentation rather than by changing the experimental conditions. Reetz and Helbig [115] demonstrated how electrochemical methods can be used to produce metal colloids of nanometer size and more importantly how particle size can be controlled in a simple manner by adjusting the current density [159]. First, a sacrificial anode was used as the source of the metal ions, which were then reduced at the cathode. Later, a more general approach was introduced, where metal salts were used as the starting material [160]. The particles were stabilized by alkylammonium or betaine salts. With a suitable choice of surfactants, the electrochemical method can be applied in the preparation of different shapes of particles, e.g., nanorods [161]. 

Later, Reetz and coworkers pointed out the role of Pd colloids in phosphine-free Pd-catalysed Heck reactions, as typically encountered under Jeffery s conditions (Pd(OAc)2, Bu4NBr, K2CO3). Studying the Heck reaction of iodobenzene with ethyl... 

Earher, Reetz and coworkers had seen a similar effect when testing the effect of the additive N,N-dimethyIglycine on the Heck reaction between bromobenzene and styrene [lOj. This additive supposedly stabilises the colloids, leading to good yields when between 0.0009 and 1.5 mol% Pd was used. They noted that only at the very low catalyst concentrations is there no need for the additive and rates are the same as without. 

The radiation method was described by Rogninski and Schalnikoff for the first time and is based on condensation of the metal atoms after collision [154]. Reetz et al. prepared nanoparticles via electrochemical synthesis [155]. Salt reduction was developed by Bonnemann to obtain mono- and bi-metallic nanoparticles in solution [156]. Salt reduction is the most widely practised method for the synthesis of colloidal metal suspensions. Faraday synthesised gold particles by the reduction of HAuCb [157]. 

Reetz et al. reported on catalytically active solvent-stabilized colloids in propylene carbonate, which were prepared electrochemically or by thermal decomposition of [Pd(OAc)2 assisted by ultrasound. The colloidal particles had sizes of 8 to 10 nm, as determined by TEM. After addition of aryl bromide, styrene, and base to the colloid solution, satisfactory conversions were obtained within reaction times of 5-20 h. Isolation of the particles stabilized by propylene carbonate was not possible, however [16]. The same authors also reported Suzuki and Heck reactions with electrochemically prepared Pd or Pd/Ni colloids stabilized by tetraalkylammonium, as well as polyvinylpyrrolidone (PVP)-stabilized palladium colloids prepared by hydrogen reduction (Table 1) [17]. It was assumed that the reaction occurs on the nanopartide surfaces. 

Reetz et al. reported phosphine-free, paUadium-catalyzed Mizoroki-Heck reactions of unactivated aryl halides using paUadium(O) nanoparticles [23, 24]. The thermolytic decomposition of Pd(OAc)2 at 130°C in the presence of ammonium salts resulted in the formation of R4N X -stabilized palladium(O) nanoparticles (Scheme 7.6). The decisive role of palladium colloids was secured by nuclear magnetic resonance (NMR) studies of the oxidative addition step yielding PhPdl and PhPdX3. ... 

At a relatively high reaction temperature (150°C), Reetz et al. also accomplished the Mizoroki-Heck reaction of chlorobenzene with styrene in the presence of Pd(OAc)2 and phosphonium salt PI14PCI [23aj. Again, the high catalytic activity was ascribed to the presence of nanosized palladium colloids, which are beheved to be stabilized by the phosphonium salt... 

Reetz, M. T., Winter, M., Breinbauer, R., Thum-Albrecht, T., Vogel, W. Size-selective electrochemical preparation of surfactant-stabilized Pd-, Ni- and Pt/Pd colloids. Chem-Eur J 2001, 7, 1084-1094. 

Bradley, J.S., Tesche, B., Busser, W Maase, M. and Reetz, M.T. (2000) Surface spectroscopic study of the stabilization mechanism for shape-selectively synthesized nanostructured transition metal colloids. Journal of the American Chemical Society, 122, 4531. 

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