Catalysis of Organic..

158 Raney® Metastable Precursorsfew decades, some researchers have intended to produce new Raney catalysts byadding a third element (9). In many cases, a part of the additional element wasdissolved in a precursor. However, if the amount of the additive exceeded a limit,most of the additive was not dissolved in the precursor and the excess additive oftencrystallized or precipitated as a secondary phase. The secondary phase that isenriched with the third element often remains after leaching. In this case, the thirdelement in the secondary phase may not contribute to the skeletal structure formation.The research studies on development of new catalyst by additives have been limited.If the solubility of the third element into the precursor can be expanded, the abovelimitation may be decreased. By applying rapid solidification to prepare theprecursor, the solubility may often be increased significantly. The other well-knownprocess for increasing the solubility is mechanical alloying (MA).Figure Fig.4 4. Schematic illustration of of Rotating-Water-Atomization Process Process Unit UnitOne ordinary rapid solidification process, the single roller method, forms thinribbon of about 20μm in thickness (10). On the other hand, we have developed anew method called the “Rotating-Water-Atomization (RWA) Process” for particulatespecimens (7). A schematic illustration of the process is shown in Fig.4. In thisprocess, the molten jet was injected to the rotating water layer through a nozzle. Themolten jet impinges on the rotating water surface layer and is broken into finedroplets. The fine droplets are immediately cooled by water. Thus fine and rapidlysolidified particles can be easily obtained. The cooling rate was estimated about 10 4K/s -10 5 K/s and the mean particle size was about 200μm depending on the holesize of the nozzle and the rotating water layer velocity (7).In most Al-containing alloys, the shape of the particles was tear-drop like due tothe tight surface oxide film. The typical shape was shown in Fig.1. The effect ofrapid solidification on microstructures is shown in Fig.5 for Al 2 Cu (precursor forRaney Cu) with a small amount of Pd (11). In the case of slowly solidified(conventional) precursor, most of the added Pd was solidified as a secondary Pd richphase shown by white dendritic structure in Fig.5 (a). On the other hand, no suchsecondary phase was observed in a rapidly solidified precursor as shown in Fig.5 (b).