Nanoparticles, catalytic activity

Studies of bimetallic nanoparticles received great attention from both scientific and technological communities because of most of the nanoparticle catalytic activity depends on their structural aspects [1]. Among the various structural aspects it is of most important to control the homogeneity, dispersion and alloying extent as they have profound influence on the surface properties which affect to catalytic activity and stability of the bimetallic nanoparticles. There-... 

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. 

The catalytic activity in the CO oxidation should significantly increase due to the gold/oxide interface around the perimeter of nanoparticles [147], therefore, we assumed that this is true regardless of the sequence of gold or FeO c deposition provided that iron oxide is amorphous. When iron oxide is deposited onto Au/Si02/ Si(l 0 0) we call it inverse interface . 

We infer the importance of the FeO c/Au interface which occurs for films as well as nanoparticles. However, since we observe FeO c/Au nanoparticles/Si02/Si(l 0 0) to be the most active of all samples we advance the hypothesis of the occurrence of a strong electronic effect at the FeOJ nanoparticle interface coupling through the FeO layer thus producing the high catalytic activity. Au promotion was also experienced in the catalytic activity of Ti02 overlayers [207]. 

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