A study of the formation, purification and application as a SWNT growth catalyst of the nanocluster [HxPMo12O40⊂H4Mo72Fe30(O2CMe)15O254(H2O)98]

Abstract

The synthetic conditions for the isolation of the iron–molybdenum nanocluster FeMoC [H_xPMo₁₂O₄₀⊂H₄Mo₇₂Fe₃₀(O₂CMe)₁₅O₂₅₄(H₂O)₉₈], along with its application as a catalyst precursor for VLS growth of SWNTs have been studied. As-prepared FeMoC is contaminated with the Keplerate cage [H₄Mo₇₂Fe₃₀(O₂CMe)₁₅O₂₅₄(H₂O)₉₈] without the Keggin [H_xPMo₁₂O₄₀]ⁿ⁻ template, however, isolation of pure FeMoC may be accomplished by Soxhlet extraction with EtOH. The resulting EtOH solvate is consistent with the replacement of the water ligands coordinated to Fe being substituted by EtOH. FeMoC-EtOH has been characterized by IR, UV-vis spectroscopy, MS, XPS and ³¹P NMR. The solid-state ³¹P NMR spectrum for FeMoC-EtOH (δ −5.3 ppm) suggests little effect of the paramagnetic Fe³⁺ centers in the Keplerate cage on the Keggin ion's phosphorous. The high chemical shift anisotropy, and calculated T₁ (35 ms) and T₂ (8 ms) values are consistent with a weak magnetic interaction between the Keggin ion's phosphorus symmetrically located within the Keplerate cage. Increasing the FeCl₂ concentration and decreasing the pH of the reaction mixture optimizes the yield of FeMoC. The solubility and stability of FeMoC in H₂O and MeOH–H₂O is investigated. The TGA of FeMoC-EtOH under air, Ar and H₂ (in combination with XPS) shows that upon thermolysis the resulting Fe : Mo ratio is highly dependent on the reaction atmosphere: thermolysis in air results in significant loss of volatile molybdenum components. Pure FeMoC-EtOH is found to be essentially inactive as a pre-catalyst for the VLS growth of single-walled carbon nanotubes (SWNTs) irrespective of the substrate or reaction conditions. However, reaction of FeMoC with pyrazine (pyz) results in the formation of aggregates that are found to be active catalysts for the growth of SWNTs. Activation of FeMoC may also be accomplished by the addition of excess iron. The observation of prior work's reported growth of SWNTs from FeMoC is discussed with respect to these results.