Microstructure development of Raney cobalt catalysts

Raney-type catalysts are unsupported catalysts of transition metals such as Ni, Co, Fe and Cu that have been widely used, especially for hydrogenation. Most studies on Raney-type catalysts have focused on Raney nickel or Raney copper and the catalytic properties after leaching. The effects of parent alloys have not been intensively studied. The Al-Co binary system has more intermetallic phases than the Al-Ni, Al-Fe or Al-Cu system, so Raney cobalt may have more possibilities for promising catalysts. The full range of Al-rich binary Al-Co alloys has been investigated. Parent alloys having different composition were produced by induction melting and casting. As-cast and annealed alloys were characterized with optical microscopy, SEM-EDS, and X-ray diffraction. All as-cast alloys showed non-equilibrium structures due to sluggish peritectic reaction and annealed microstructures were consistent with the phase diagram. The AlCo phase was observed in as-cast alloys containing more than 42 wt% Co, which sets the upper limit to select parent alloys suitable for catalysts since this phase is not leachable. Plan-view studies of leached bulk microstructures revealed that in decreasing order, eutectic, Al9Co2, Al13Co4 and Al5Co2 leached slower. Cross-section specimens were prepared by epoxy infiltration, which allows measurement of leaching depth in leached powders for kinetics study, and an in-situ fracture method was developed using the environmental SEM (E-SEM). Fine porous structure observed by FESEM showed Co crystallite with sizes of about 10 and 5 nm, respectively, for the leached as-cast 40 and 41 wt% Co alloys. Leaching kinetics for the as-cast 40 and 41 wt% Co alloys were studied in three independent ways, by measuring total surface area, residual aluminum and leaching depth with time. All these results were consistent with each other and have been correlated to investigate a leaching kinetics model