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X-ray Absorption Spectroscopic Investigation of Partially Reduced Cobalt Species in Co-MCM-41 Catalysts during Synthesis of Single-Wall Carbon Nanotubes

Domenii publicaţii > Chimie + Tipuri publicaţii > Articol în revistã ştiinţificã

Autori: D. Ciuparu, P. Haider, M. Fernández-García, Y. Chen, S. Lim, G.L. Haller, L. Pfefferle

Editorial: Journal of Physical Chemistry B, 109(34), p.16332-39, 2005.

Rezumat:

Chemometric tools were employed to analyze the in-situ dynamic X-ray absorption spectroscopy data to probe the state of Co-MCM-41 catalysts during reduction in pure hydrogen and under single-wall carbon nanotube synthesis reaction conditions. The use of the progressive correlation analysis established the sequence in which changes in the spectral features near the Co K edge occurred, and the evolving factor analysis provided evidence for the formation of an intermediate Co1+ ionic species during reduction of the Co-MCM-41 catalyst in pure hydrogen up to 720 C. This intermediate species preserves the tetrahedral environment in the silica framework and is resistant to complete reduction to the metal in H2. While the Co2+ species is resistant to reduction in pure CO, the intermediate Co1+ species is more reactive in CO most likely forming cobalt carbonyl-like compounds with high mobility in the MCM-41. These mobile species are the precursors of the metallic clusters growing carbon nanotubes. Controlling the rates of each step of this two-stage reduction process is key to controlling the size of the metallic Co clusters formed in Co-MCM-41 catalysts.

Cuvinte cheie: X-ray absorption spectroscopy, Co reduction, MCM-41

URL: http://dx.doi.org/10.1021/jp052243p