Different degrees of weak metal-support interaction in Fe-(Ni)/Al <inf>2</inf>O<inf>3</inf> catalyst governing activity and selectivity in carbon nanotubes' production using ethylene

Abstract

Two Fe-based alumina supported catalysts differing in active phase nature, alumina support origin and methods of synthesis were compared by achieved carbon nanotube (CNT) activity and selectivity in Catalytic Carbon Vapor Deposition reaction using ethylene as a carbon feedstock. Bimetallic Fe-Ni catalyst having 2.5 wt.% of each metals, performed three times an order of magnitude better in terms of CNTs' yield than the monometallic Fe-based catalyst. Besides, very high selectivity was reached in the case of the preferred catalyst, giving structurally uniformed, multiwalled CNTs with diameters in the range 10-30 nm. The concept of week metal-support interaction (MSI) governing the particular Fe-based catalysts' behavior was proven by the observed "tip-growth" mechanism of CNTs. However, a various degrees of achieved WMSI in both catalysts, dictated by the addition of the second metal (Ni) and either predefined textural characteristics of the commercial alumina support or the same for the in-house synthesized alumina, is substantial for their different single behavior. Thus, WMSI in the case of calcined Fe/Al2O 3 sample during its reduction and/or use resulted in metal particles' mobility and coalescence, being finally materialized as big particles of size above the nano-range. Those particles resulted in very poor activity and selectivity in terms of CNTs' growth and in addition undergone deactivation by graphene layer deposition. In contrast, somewhat modified interaction has been brought in the case of Fe-Ni/Al2O3 catalyst sample after calcinations, keeping optimally sized metal particles stable in the following steps of its reduction/use. In addition, observed encapsulation of metallic particles by growing CNTs, occurring by different magnitudes in examined catalysts, can be understood as directly proportional to the degree of their MSI achieved. © 2011 Elsevier B.V.