Catalytically active nanoparticles

An example of a consecutive use of block copolymer micelles as endo- and exo-templates is the preparation of mesoporous silica with embedded Pd-nanoparticles [113]. As a first step Pd-nanoparticles are prepared in the micellar core (see Sect. 4.1). In a lyotropic phase of these micelles they are further employed as exotemplates for the preparation of mesoporous silica (see Sect. 5). After removal of the block copolymer by calcination, nanoparticles within the open mesopore structure are obtained (Fig. 20). This represents a promising way to incorporate catalytically active nanoparticles into mesoporous oxides as stable catalyst supports. 

We have reviewed the family of dealloyed Pt-based nanoparticle electrocatalysts for the electroreduction of oxygen at PEMFC cathodes, which were synthesized by selective dissolution of less-noble atoms from Pt alloy nanoparticle precursors. The dealloyed PtCua catalyst showed a promising improvement factor of 4-6 times on the Pt-mass ORR activity compared to a state-of-the-art Pt catalyst. The highly active dealloyed Pt catalysts can be implemented inside a realistic MEA of PEMFCs, where an in situ voltammetric dealloying procedure was used to constructed catalytically active nanoparticles. The core-shell structural character of the dealloyed nanoparticles was cmifirmed by advanced STEM and elemental line profile analysis. The lattice-contracted transition-metal-rich core resulted in a compressive lattice strain in the Pt-rich shell, which, in turn, favorably modified the chemisorption energies and resulted in improved ORR kinetics. 

The catalytic lifetime was studied by reusing the aqueous phase for three successive hydrogenation runs of toluene, anisole and cresol. Similar turnover activities were observed during the successive runs. These results show the good stability of the catalytically active iridium suspension as previously described with rhodium nanoparticles. 

Finally, Jessop and coworkers describe an organometalhc approach to prepare in situ rhodium nanoparticles [78]. The stabilizing agent is the surfactant tetrabutylammonium hydrogen sulfate. The hydrogenation of anisole, phenol, p-xylene and ethylbenzoate is performed under biphasic aqueous/supercritical ethane medium at 36 °C and 10 bar H2. The catalytic system is poorly characterized. The authors report the influence of the solubility of the substrates on the catalytic activity, p-xylene was selectively converted to czs-l,4-dimethylcyclohexane (53% versus 26% trans) and 100 TTO are obtained in 62 h for the complete hydrogenation of phenol, which is very soluble in water. 

Endo et al. [96] prepared AuPt, AuPd, and PtPd bimetallic nanoparticles with 2 nm in particle size in order to investigate catalytic activity for reduction of p-nitrophenol in water. The binary features of the nanoparticles were characterized by UV-Vis spectroscopic measurements. 

In 1989, we developed colloidal dispersions of Pt-core/ Pd-shell bimetallic nanoparticles by simultaneous reduction of Pd and Pt ions in the presence of poly(A-vinyl-2-pyrrolidone) (PVP) [15]. These bimetallic nanoparticles display much higher catalytic activity than the corresponding monometallic nanoparticles, especially at particular molecular ratios of both elements. In the series of the Pt/Pd bimetallic nanoparticles, the particle size was almost constant despite composition and all the bimetallic nanoparticles had a core/shell structure. In other words, all the Pd atoms were located on the surface of the nanoparticles. The high catalytic activity is achieved at the position of 80% Pd and 20% Pt. At this position, the Pd/Pt bimetallic nanoparticles have a complete core/shell structure. Thus, one atomic layer of the bimetallic nanoparticles is composed of only Pd atoms and the core is completely composed of Pt atoms. In this particular particle, all Pd atoms, located on the surface, can provide catalytic sites which are directly affected by Pt core in an electronic way. The catalytic activity can be normalized by the amount of substance, i.e., to the amount of metals (Pd + Pt). If it is normalized by the number of surface Pd atoms, then the catalytic activity is constant around 50-90% of Pd, as shown in Figure 13. 

Figure 13. Normalized catalytic activity (in mmol H2 per mmol surface Pd per s) as a function of metal composition of PVP-stabilized Pd/Pt bimetallic nanoparticles. The normalization was determined by the number of Pd atoms on the surface of the nanoparticle, assuming that Pd atoms exist selectively on the surface. (Reprinted from Ref. [48], 1993, with permission from Royal Society of Chemistry.)...

This means that the improvement of catalytic activity of Pd nanoparticles by involving the Pt core is completely attributed to the electronic effect of the core Pt upon shell Pd. Such clear conclusion can be obtained in this bimetallic system only because the Pt-core/Pd-shell structure can be precisely analyzed by EXAFS and Pd atoms are catalytically active while Pt atoms are inactive. 

The bimetallic nanoparticles were generally more active than the corresponding monometallic nanoparticles. The highest catalytic activity was observed for Au/Rh and/or... 

The measurement of catalytic activity of PdPt bimetallic nanoparticles over methane combustion showed that the difference in activity with increasing and decreasing reaction temperatures disappeared probably due to the synergestic effect of the formation of the PdPt bimetallic nanoparticles [176]. 

These results on catalytic activity of bimetallic nanoparticles are summarized in Table 2. 

Molecular-dynamics simulations also showed that spherical gold clusters is stable in the form of FCC crystal structure in a size range of = 13-555 [191]. This is more likely a key factor in developing extremely high catalytic activity on reducible Ti02 as a support material. Thus, it controls the electronic structure of Au nanoparticles (e.g. band gap and BE shift of Au 4f7/2 band) and thereby the catalytic activity. 

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