Catalysis of Organic..

156 Raney® Metastable PrecursorsIntroductionRaney® catalyst is a very interesting material for metallurgists (1-5). The externalappearance of a precursor particle is not significantly changed by leaching. Anexample is shown in Fig.1 for Raney Cu. However, the crystallographic structurewas changed from tetragonal Al 2 Cu to face centered cubic (fcc) Cu due to(a) リーチング before 前 leaching急 冷 粉 末100μm(b) after leaching例 アルミ- 銀 合 金 A l 80 Ag 20Figure 1. Shape retention during leaching (Al 2 Cu precursor)composition change. It means that the structure change was not caused by thedeposition of a new phase from leaching solution. Cu atoms may be retained onoriginal lattice sites at intermediate stages of leaching. Many vacancy sites may beintroduced by removal of Al atoms during leaching as shown in Fig.2(a)-(b). Therearrangement of remained Cu atoms can be easily due to the vacancies Fig.2 (c).This process is viewed as a sort of phase transformation.Figure 2. Crystallographic structure changes during leachingFigure 3 shows an equilibrium phase diagram of Al-Cu system (6). This is thebasis for the phase transformation. The skeletal fcc Cu was finally formed from theAl 2 Cu ( θ ) phase by leaching at a constant temperature. The process of the