Ferric oxide supported particles

In addition to titania, alumina and ceria supports (Table 4.4), the DPU method has been applied to other supports. Gold particles supported on ferric oxide were quite small (3-7 nm) after thermal treatment is static air at 623 K.80 Another study reported the use of the DPU method to deposit gold onto other supports (MgO, CaO, SrC>2 and BaO).81 After calcination at 673 K, gold particles of moderate size were obtained on magnesia (8 nm) and on calcia (6nm). In all cases, all the gold in solution was deposited on the support. 

The success of Haruta s early work lay in his choice of preparation method and support. Gold particles of the necessary small size were first obtained by coprecipitation (COPPT) and later by deposition-precipitation (DP) (see Sections 4.2.2 and 4.2.3) classical impregnation with HAuCLj does not work. The choice of support is also critical transition metal oxides such as ferric oxide and titania work well, whereas the more commonly used supports, such as silica and alumina, do not work well or only less efficiently. This strongly suggests that the support is in some manner involved in the reaction. 

The dependence of the isomer shift (41) and recoil-free fraction (9, 42-48) on particle size has also been suggested, but such relationships may be somewhat tenuous. It is clear that caution must be exercised in the use of methods hitherto described in the interpretation and correlation of microcrystallite size. Recent work has suggested (12) that ferric oxide may react with the support when calcined at high temperatures, e.g. for 2 hr at 500°C. The presence and contribution... 

Such a reaction of Fe(CO)5 (at 293-363 K, PVP) without ultrasonic radiation proceeds very slowly and only after few days there, a material is formed with very low Fe content (2%, the isolated particles 2-5 nm in size). It is of interest that the sonochemical decomposition of Fe(CO)5 does not proceed in the presence of PVP if THF is used as the solvent, but the reaction is very effective when anisole is used as the solvent and PFO is used as the polymer matrix [93]. A black product formed contains up to 10% (in mass) of the spheric particles of nonoxidized Fe (mainly y-Fe, with little content of a-Fe) with 1-12 nm in size (the mean diameter is 3nm, as shown in Figure 3.7). It is likely that the big particles present the flocks of little ones ( 2-2.5nm). The sonochemical synthesis allows us to produce the functionalized amorphous nanoparticles of ferric oxide with 5-16 nm in diameter [94]. The ultrasonic irradiation in the PFO presence allows us to also produce the stabilized nanoparticles of copper, gold, and so on. In the literature the findings are not about the bimetallic particle formation in the ultrasonic fields by carbonyl metal reduction in the polymer matrices presence (as, for example, in the case of the carbon-supported Pt-Ru from PtRu5C(CO)i6 reduced clusters [95]). 

With the development of nanoscaled catalysts, several recent studies have pointed out the great interest of recording Mossbauer spectra below 4.2 K up to 0.055 K (100,203,204) it allowed the identification at the surface of various support such as mesoporous silica or Zr02, the formation of nanometric iron oxide clusters this identification was not possible in classical low temperature studies conducted above 4.2K, which concluded to the presence of larger particles. The analysis by Mossbauer spectroscopy performed at lower temperature enabled to show that these larger particles were agglomerates of nanometric iron clusters and allowed to reach another level of resolution of ferric particles structures (204). 

All of the hydrothermally treated samples were brown in colour indicating the presence of some iron oxide/hydroxide species. The Mossbauer spectra at RT and 78 K of these samples were all quite similar and indicated that essentially all of the iron in these samples was in the ferric state. The spectra consisted of a symmetric doublet with IS of 0.33 mm s indicating iron in octahedral coordination while the QS of 0.84 mm s is close to that for a-Fe203 particles supported on silica [33]. 

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