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Revisado por pares
Fischer−Tropsch Synthesis on Anchored Co/Nb 2 O 5 /Al 2 O 3 Catalysts: The Nature of the Surface and the Effect on Chain Growth
2006; American Chemical Society; Volume: 110; Issue: 18 Linguagem: Inglês
10.1021/jp060175g
ISSN1520-6106
AutoresFabiana Magalhães Teixeira Mendes, Carlos A. Perez, Fábio B. Noronha, Carlos Daniel Davi de Souza, Deborah Vargas César, Hans‐Joachim Freund, Martín Schmal,
Tópico(s)Catalytic Processes in Materials Science
ResumoA series of Co/x%Nb2O5/Al2O3 catalysts were prepared by anchoring niobia on an Al2O3 support at different niobia concentrations. Characterization of the structure and nature of surface active sites was attempted in order to correlate the CO hydrogenation activity of these systems with those of the Co/Al2O3 and Co/Nb2O5 catalysts. The effect of the reduction temperature on the CO hydrogenation activity and selectivity was studied, showing that interaction of cobalt and niobia surface species favored the selectivity for hydrocarbon chain growth. However, this effect is less pronounced on the niobia-promoted Co/Al2O3 compared to Co/Nb2O5 catalysts. X-ray photoelectron spectroscopy (XPS) and diffuse reflectance spectroscopy (DRS) results on Co/x%Nb2O5/Al2O3 showed prevailing amounts of Co2+ and Co3+ after calcination and reduction at 573 K, while, after reduction at 773 K, besides metallic cobalt, the Co2+ species still remains in contact with alumina, even for higher niobia loading. It seems that during this process formation and destruction of new interfaces involving Co0−NbOx sites takes place. Results suggest that Co0, Co0−Co2+, and Co0−NbOx are the active sites at the surface. The relative abundance of Co2+ species affects greatly the performance of the catalysts. DRIFTS and selectivity results suggest that these sites might be responsible for the reaction chain growth and therefore for the drastic change in the selectivity of CH4 and C5+ hydrocarbons mainly on the Co/Nb2O5 catalyst. DRIFTS results on Co/Nb2O5/Al2O3 showed the formation of CC and CH3 besides CHxO species. With increasing reduction temperature, the CC species disappear while CH3 fragments increased markedly, suggesting the formation of increasing amounts of hydrocarbons with higher chain length.
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