The role of molybdenum in Fe–Mo–Al 2O 3 catalyst for synthesis of multiwalled carbon nanotubes from butadiene-1,3

Abstract

MoO 3–Fe 2O 3–Al 2O 3 catalysts were prepared by co-precipitation of mixed hydroxides from soluble salts. It was found that the 55%Fe 2O 3–Al 2O 3 aluminum–iron catalyst calcined in air at 500–700 °C consisted of a solid solution based on hematite where a part of iron atoms was substituted for aluminum. The modification of the aluminum–iron catalyst with molybdenum results in the formation of a solid solution based on hematite where a part of iron atoms is substituted for aluminum and molybdenum ions. At 700 °C the MoO 3–Fe 2O 3–Al 2O 3 catalysts are reduced under the action of the feed. The dependence of the multiwalled carbon nanotube (MWNT) yield over MoO 3–Fe 2O 3–Al 2O 3 catalysts on the molybdenum concentration has a maximum. Small concentrations of MoO 3 (up to 6.5 wt.%) added to the aluminum–iron catalyst increase the dispersity of the active metal particles and alter their properties due to the formation of a Fe–Mo alloy. Its formation leads to the decrease of the nanotube growth rate and makes the catalyst more stable. The overall yield of the carbon nanotubes increases. Further increase of the molybdenum concentration leads to the decrease of the nanotube yield because the catalytically active Fe–Mo particles are enriched too much with molybdenum. In addition, excess molybdenum forms molybdenum carbide α-Mo 2C, in which part of the molybdenum atoms are substituted for iron atoms, and which is inactive in the MWNT growth.