Catalysis PdNPs

A very important concept pioneered in the 1970s is that of catalysis using two different metals such as Au and Pd in the same NP [15]. This idea has been beautifully developed by Toshima s group who used PVP to stabUize core-shell bimetallic Au-PdNPs, i.e. for instance NPs in which the core is Au whereas Pd atoms are located on the shell (Fig. 1.6) [16]. Subsequent to co-reduction, this structure is controlled by the order of reduction potentials of both ions and the coordination abilities of both atoms to PVP. The location of Au in the core and Pd on the shell was demonstrated by EXAFS, and it was shown that such heterobimetallic... 

Au-cored PdNPs are more active in catalysis than simple PVP-stabilized PdNPs. Thus, the Au core enhances the catalytic properties of PdNPs at the PdNP surface [5f,g]. Conversely, design strategies can lead to the opposite core-shell structure (Pd core, Au shell), and specific catalytic properties were obtained for methylacrylate hydrogenation [16]. 

I ) Transition-metal Nanoparticles in Catalysis O cat PdNPs, DCHIN... 

Fig. 1.10 Hydrogenation of 10-(3-propenyl) anthracene catalyzed by PdNPs in water-in-oil microemulsion (much faster than Pd/C catalysis). (Reprinted with permission from Ref [32f] Wai group, Chem. Commun, 2003, 1041).

Ionic liquids (ILs) were introduced in catalysis by Yves Chauvin in the 1990s [33a] and have received considerable attention in this field [33b,cj. Yves Chauvin introduced the imidazolium salts that are the most frequently used ILs in catalysis. ILs are valuable media for catalysis with PdNPs because the substituted imidazolium cation is bulky, favoring the electrosteric stabilization of NPs, as do the t-Bu N salts in Fig. 1.1. The size of the cation (that can eventually be tuned by the choice of the N-alkyl substituents) also has an important influence on the stabilization, size and solubUity of the NPs, these factors playing a role in catalysis. ILs are also non-innocent, however, as they readily produce Pd-N-heterocyclic carbene complexes upon deprotonation of the imidazoHum salt at sufficiently high temperature. Thus, these carbene ligands can be bound to the NP surface or give mononuclear mono- or bis-carbene complexes subsequent to leaching of Pd atoms from the PdNP surface (vide infra) [33]. 

In conclusion, IL are favorable media for the electrostatic stabilization of preformed NPs at room temperature and subsequent catalysis, whereas they give for instance Pd-carbene complexes upon deprotonation of the imidazolium cation, yielding, at high temperature, PdNP catalysts whose mechanism of action is discussed in the ligand-free catalysis section. 

Fig. 1. 12 Sketch of a piece of alumina membrane with metal NP elucidating its application for gas phase catalysis of 1,3-butadiene hydrogenation (PdNPs) and CO oxidation (RuNPs) (Reprinted with permission from Ref [36a], Schmid and Chaudret groups, Z Anorg. Allg. Chem. 2004, 630, 1913).

Fig. 1.13 Principle of the formation of hollow PdNP spheres used for the catalysis of Suzuki reactions in refluxing ethanol with K3CO4 as a base. With 2-iodothiophene and phenylboronic acid (catalyst 3% Pd spheres), at least seven cycles could be achieved with 95-97% yields (Reprinted with permission from Ref [35b], Hyeon group, J. Am. Chem. Soc. 2002, 724, 7642).

The mechanisms of oxide-supported PdNP catalysis are far from being understood. The oxide support has of course a strong influence on the activity. For instance, in the Heck reaction, the activity is dominated by the support according to ... 

Thus the first part of the book (Chapters 1 to 9) deals with NP catalysis, emphasizing the key role of NP supports the second part (Chapters 10 to 12) concerns specific metals (namely Pd, Ru, Ir and Au), and the last part (Chapters 13 to 18) focuses on specific substrates of particular interest for organic chemistry, hydrocarbon reforming and environmental aspects. Among the metals, Pd and Au are the most effective catalysts. Palladium is the most efficient catalyst for carbon-carbon bond formation thus, besides Chapter 10 that is devoted to PdNPs, their catalytic properties also spread over the first part of the book. Gold is the most efficient NP catalyst for a variety of aerobic (thus low-cost) oxidation reactions, and AuNP catalysis is covered in four chapters at the end of the second part and beginning of the third part. Each chapter is introduced in more detail in Chapter 1. 

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