Thermodynamics of CVD Synthesis of Multiwalled Carbon Nanotubes: A Case Study

Galvanic cells with CaF2 single crystal as electrolyte were utilized to determine the thermodynamic quantities of the solid-state process: δNdNi5 (μm) + MWCNT = {MWCNT−[NdNi5(nm)]δ} where multiwalled carbon nanotubes (MWCNT) react with NdNi5 which is the catalyst of the chemical vapor deposition (CVD) synthesis of MWCNTs through the decomposition of a light hydrocarbon. It was shown elsewhere that the nucleation and growth of a MWCNT occurs by the nanodusting of the micrometer size powder of the unsupported catalyst. The composite {MWCNT−[NdNi5(nm)]δ} is a nanoparticle of catalyst bonded to one of the ends of the MWCNT. The thermodynamic quantities of this process combined with those available for graphite to MWCNTs transformation and for decomposition of a light hydrocarbon, CH4, for instance, provide to obtain a complete picture of the thermodynamics of the CVD synthesis: CH4(g)+δNdNi5(μm) = {MWCNT−[NdNi5(nm)]δ} + 2H2(g). At the standard state, the synthesis becomes feasible at T> 800 K. Between 300 and 1300 K, Δ r H T θ and Δ r S T θ are both positive, changing from 81 to 20 kJ mol−1 and from 87 to 58 J K−1 mol−1, respectively. To derive the thermodynamic quantities of formation of {MWCNT−[NdNi5(nm)]δ}, the formation of NdNi5 was also studied by making use of a suitable CaF2 electrolyte cell. Between 600 and 1000 K, the formation of {MWCNT−[NdNi5(nm)]δ} is always favored while Δ f H T θ and Δ f S T θ are positive but scarcely dependent on T (Δ f H T θ ≈ 7 kJ mol−1 and Δ f S T θ ≈ 12 J K−1 mol−1). A microscopic modeling of the process leading to the formation of {MWCNT−[NdNi5(nm)]δ} has also been presented and discussed.