Characterizations of Cobalt Oxide Nanoparticles within Faujasite Zeolites and the Formation of Metallic Cobalt

Abstract

Cobalt oxide (CoO x ) particles within faujasite zeolites have been synthesized by a procedure comprising (i) ion-exchange of cobalt ions into the zeolite, (ii) precipitation of cobalt ions with sodium hydroxide within the supercages of the zeolite, and (iii) calcination. The materials are characterized by XRD, nitrogen sorption, XPS, TEM-EDS, H 2 -TPR, and O 2 -titration. The concentration of sodium hydroxide for precipitation and the temperature for calcination are found to be critical in controlling the locations of the CoO x particles. With appropriate conditions, the CoO x particles formed are located and encapsulated in the supercages of faujasite zeolites. The sizes of the CoO x particles are in the range of 0.7−3 nm with a maximum distribution of 1.3−1.5 nm. These particles exist mainly in the state of CoO. On the other hand, higher calcination temperature or higher concentration of sodium hydroxide may lead to the formation of larger Co 3 O 4 particles located outside the supercages of the faujasite zeolites. The CoO x particles encapsulated in the supercages exhibit a broad reduction peak and can be partly reduced to metallic cobalt at temperatures as low as 573 K, while the cobalt cations exchanged into the zeolites can only be reduced at temperatures higher than 773 K. The metallic cobalt formed by the reduction of cobalt oxide within the supercage exhibits superior catalytic activity in Fischer−Tropsch synthesis.