Nanocatalyst

TEM experiments show nanoparticles with an average diameter of 5.7 nm similar to the initial diameter consequently, Finke and coworkers concluded that deactivation at 41% of conversion was due to a surface deactivation of their nanocatalyst. 

Scheme 7 Partial hydrogenation of benzene by Ru(0) nanocatalysts in room temperature imidazolium ionic liquid...

Two soluble nanocatalysts have been investigated in partial hydrogenation. The results obtained by Einke or Dupont s catalysts are unsatisfactory but prove that nanoparticles are a potential catalyst for this reaction, hi summary, partial hydrogenation of benzene and its derivatives is still a challenge but will be the focus of future research. 

Ab initio methods allow the nature of active sites to be elucidated and the influence of supports or solvents on the catalytic kinetics to be predicted. Neurock and coworkers have successfully coupled theory with atomic-scale simulations and have tracked the molecular transformations that occur over different surfaces to assess their catalytic activity and selectivity [95-98]. Relevant examples are the Pt-catalyzed NO decomposition and methanol oxidation. In case of NO decomposition, density functional theory calculations and kinetic Monte Carlo simulations substantially helped to optimize the composition of the nanocatalyst by alloying Pt with Au and creating a specific structure of the PtgAu7 particles. In catalytic methanol decomposition the elementary pathways were identified... 

In case of fuel cell cathodes, theoretical considerations were directed towards optimizing catalysts for O2 reduction [103]. This has led to the synthesis of Pt3Co/C nanocatalyst systems and preliminary results again indicate perfect agreement between the calculations and the wet electrochemical results obtained with metal nanoparticles of the composition which theory had recommended [106]. 

The catalytic applications of Moiseev s giant cationic palladium clusters have extensively been reviewed by Finke et al. [167], In a recent review chapter we have outlined the potential of surfactant-stabilized nanocolloids in the different fields of catalysis [53]. Our three-step precursor concept for the manufacture of heterogeneous egg-shell - nanocatalysts catalysts based on surfactant-stabilized organosols or hydrosols was developed in the 1990s [173-177] and has been fully elaborated in recent time as a standard procedure for the manufacture of egg-shell - nanometal catalysts, namely for the preparation of high-performance fuel cell catalysts. For details consult the following Refs. [53,181,387]. 

Controlled decomposition of organometallics leads to nanocatalysts which exhibit a very clean and truly zerovalent metallic surface (cf. [339]). As indicated in Figure 15 in Section 3.10, the organoaluminium shell of the one-shell Pti3 cluster (size 0.75+0.1 nm) obtained via the decomposition of [(COD)Pt(CH3)2] in the presence of excess trioctylaluminium [352] is transferred to an AI2O3... 

Finally, two major industrial applications of nanocatalysts should be mentioned that are currently in the transition from basic research to industrial scale-up. Headwaters NanoKinetix and Degussa have developed and patented [412-A15] a direct synthesis method for the... 

Figure 18. TEM electron micrograph of the supported Pd/Pt nanocatalyst (ca. 4nm) for the direct production of H2O2. (Printed with permission of B. Zhou, Headwaters, Inc.)...

Further, encapsulation in the template prevents the nanoparticles from agglomeration, maintaining the nanocatalyst performance for long term which is crucial for commercial application. 

It is anticipated that the new, environmentally friendly technology designed by Headwaters for H2O2 production will soon replace the current anthraquinone process because of the high activity, selectivity, and durability of the novel nanocatalyst. 

This whole process will be again illustrated step by step in the following section with reference to the case study of nanocatalysts. 

Electron Holography applied to Size-Controlled Nanocatalysts... 

It should be mentioned here that Sn sites are not considered to be the solitary source for OHad, which could be adsorbed on Pt sites owing to the influence of adjunct Sn atoms [Stamenkovic et al., 2005], The promotional effect of Sn was later confirmed on a PtSn/C nanocatalyst [Arenz et al., 2005], which exhibits similar behavior that was assigned primarily to the formation of reactive OH species at much lower potential than on pure Pt catalysts. Based on these findings, the bifunctional effect was unambiguously confirmed for Pt-Sn surfaces, where Sn sites serve as a source of oxygenated species that boost CO oxidation at low potentials and allow these surfaces to be employed as CO-tolerant catalysts. 

Figure 9.9 Examples of single-step hydrogenations using Cu4Rui2C2 nanocatalysts.

Tsang, S.C., Caps, V., Paraskevas, I., Chadwick, D. and Thompsett, D. (2004) Magnetically separable, carbon-supported nanocatalysts for the manufacture of fine chemicals. Angewandte Chemie International Edition, 43 (42), 5645-5649. 

Shylesh, S., Schunemann, V. and Thiel, W.R. (2010) Magnetically separable nanocatalysts bridges between homogeneous and heterogeneous... 

Wu, J.C.S and Chen, C.H. (2004) A visible-light response vanadium-doped titania nanocatalyst by sol-gel method. 

For this purpose, all three catalyst supports were initially synthesized by a chemical vapor deposition (CVD) process and thereafter, using a wet impregnation method, loaded with cobalt as the active component for FTS. The as-synthesized Co/nanocatalysts were then characterized by applying electron microscopic analysis as well as temperature-programmed desorption, chemi- and physisorption measurements, thermogravimetric analysis, and inductively coupled plasma... 

ICP) measurements. The catalytic performance of the nanocatalysts was finally tested in the Fischer-Tropsch synthesis carried out in a fixed bed reactor. The obtained results were compared with literature data of commercially used Fischer-Tropsch catalysts. 

A direct comparison of the productivities of the Co/nanomaterials and a typical Co catalyst23 (promoted Co/Ru-alumina catalyst) is presented in Table 2.3. Bearing in mind that the nanocatalysts are unpromoted systems and that only a simple wetness impregnation technique was employed for catalyst production, the obtained activities are quite promising, especially in the case of the Co/MW catalyst. 

In materials chemistry, nanoparticles of noble metals are an original family of compounds. Well-defined in terms of their size, structure and composition, zero-valent transition-metal colloids provide considerable current interest in a variety of applications. Here, the main interest is their application in catalysis. Zerovalent nanocatalysts can be generated in various media (aqueous, organic, or mixture) from two strategic approaches according to the nature of the precursor, namely (i) mild chemical reduction of transition-metal salt solutions and (ii) metal atom... 

Electrosteric stabilization can be also obtained from the couple ammonium (Bu4N+)/polyoxoanion (INWnNb C>62)- The significant steric repulsion of the bulky Bu4N+ countercation, when associated with the highly charged polyoxo-anion (coulombic repulsion), provides efficient electrosterical stability towards agglomeration in solution of the resultant nanocatalysts [2, 5, 6]. 

Alkene hydrogenation is a common field of catalytic application for metal nanoparticles. Various approaches have been utilized to obtain stable and active nanocatalysts in hydrogenation reactions. The main approaches are described in the following sections, and are classified according to the stabilizing mode retained for the nanoparticles. 

Recently, Liew et al. reported the use of chitosan-stabilized Pt and Pd colloidal particles as catalysts for olefin hydrogenation [51]. The nanocatalysts with a diameter ca. 2 nm were produced from PdCl2 and K2PtCl4 upon reduction with sodium borohydride in the presence of chitosan, a commercial biopolymer, under various molar ratios. These colloids were used for the hydrogenation of oct-1-ene and cyclooctene in methanol at atmospheric pressure and 30 °C. The catalytic activities in term of turnover frequency (TOF mol. product mol. metal-1 h-1)... 

The team of Crooks is involved in the synthesis and the use of dendrimers and, more particularly, poly(amidoamine) dendrimers (PAMAM), for the preparation of dendrimer-encapsulated mono- or bimetallic nanoparticles of various metals (Pt, Pd, Cu, Au, Ag, Ni, etc.) [55, 56]. The dendrimers were used as nanocatalysts for the hydrogenation of allyl alcohol and N-isopropylacrylamide or other alkenes under different reaction conditions (water, organic solvents, biphasic fluorous/or-ganic solvents or supercritical COz). The hydrogenation reaction rate is dependent on dendrimer generation, as higher-generation dendrimers are more sterically... 

Rhee and coworkers published the synthesis of bimetallic Pt-Pd nanoparticles [57] or Pd-Rh nanoparticles [58] within dendrimers as nanoreactors. These nanocatalysts showed a promising catalytic activity in the partial hydrogenation of 1,3-cyclooctadiene. The reaction was carried out in an ethanol/water mixture at 20 °C under dihydrogen at atmospheric pressure. The dendrimer-encapsulated nanoclusters could be reused, without significant loss of activity. 

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