Stability of Supported Catalysts in the Aqueous Phase

It is mentioned in Section 9.2 that the preparation of bimetallic nanoparticles by surface redox reactions occurs under a controlled atmosphere between a parent monometallic catalyst and the cation of the second metal dissolved in aqueous solution. However, the immersion of the parent catalyst in the solution may induce a modificahon of the metal particle size before the introduction of the ion of the second metal. In an aqueous medium, water molecules could penetrate between the metal-support interface and induce the migration of metal crystallites. Moreover, water and gas, for instance hydrogen, can adsorb and dissociate on metals, leading to a modification of their electronic properties. Consequently, it is important to study the stability of supported monometallic catalysts in an 

The sintering of the parent metal may result from the immersion in the aqueous medium only or from both the immersion in the solution and the nature of the bubbling gas. Table 9.1 shows for example that the metallic dispersion loss of a Pt/silica catalyst is more marked in the presence of hydrogen, compared to nitrogen, whatever the initial pH of the solution. 

These effects are aU the more pronounced as the metal-support interactions of the parent catalyst are weaL In other words, when the metal-support interactions are prevailing, the metallic particles remain stable. Examples are reported in the literature for platinum [70, 72], palladium [38] and rhodium [41], showing that silica leads to a more severe sintering than alumina. Indeed, it appears that the 

In conclusion, the treatment of supported catalysts in an aqueous medium seems therefore unfavorable to the presence of well dispersed nanoparticles. Nevertheless, an in situ reduction of the parent catalyst allows one to prevent or restrict metal sintering, depending on the nature of the support. However, in the absence of any contact of the catalyst with air, many parameters can also influence the metal particle size. Then, for example it is better to avoid especially very acidic (pH 2) or very basic (pH 10) solutions [13, 72]. 

---