High-Temperature Stability of Cobalt Grafted on Low-Loading Incorporated Mo−MCM-41 Catalyst for Synthesis of Single-Walled Carbon Nanotubes

Abstract

Cobalt metal is a good catalyst for CO disproportionation to single-walled carbon nanotubes (SWNT) when particle size is stabilized on the nanometer scale. With reaction temperatures in the range of 973−1123 K, stabilization of nanometer Co particles against sintering requires a chemical bonding and/or occlusion of the Co particles. Catalysts with Co-grafted Mo-incorporated MCM-41 with different Mo loadings (<1 wt %) have been synthesized. Incorporation of Mo cations in the silica framework of MCM-41 provides different species as grafting sites. These grafting sites affect the strength of the metal−support interaction that controls sintering of active Co metal. The stability of Co at high temperature has been investigated by TPR and in situ/ex-situ X-ray absorption spectroscopy. The reduced Co metal particle size, after hydrogen prereduction, is independent of Mo loading. However, the Co particle size after SWNT synthesis is greatly affected by Mo anchoring and varied with loading. The Co particle size of low-Mo-loading samples after SWNT synthesis is little changed (from 0.88 nm after prereduction to 0.85 nm after SWNT synthesis), but the Co particle size difference of high-Mo-loading samples is gradually increased as the Mo loading is increased.