Natural lavas as catalysts for efficient production of carbon nanotubes and nanofibers.

Nanocarbons, especially carbon nanotubes and nanofibers CNTs/CNFs), are key materials in nanoscience and nanotechnology, but their application on a large scale (for instance in catalysis) is limited by their difficult production. Current processes including preparation of the support, normally silica or alumina, and impregnation with catalytically active metal for hydrocarbon decomposition, are not suitable for mass production. Herein we report on the fabrication of CNTs/CNFs on Mount Etna lavas used both as support and as catalyst, the first step for industrial production without preparation of support and its wet-chemical treatment. Mount Etna is the highest and most active volcano in Europe. The eruptions in 2002 and 2003 were the most explosive eruptions in recent years. On November 12, 2002, the local newspaper “La Sicilia” estimated the volume of emitted lava to be about 10–11 million cubic meters, while the volume of tephra exceeded 20 million cubic meters. The Mount Etna lavas are slightly evolved forms of an alkaline series (trachybasalts and alkali basalts) exhibiting a porphyritic texture with abundant phenocrysts of plagiocalase and fewer clinopyroxene and olivines.[1] The main components are Si (SiO2 48 wt%), Al, Mn, Mg, Ca, Na, and Fe (Fe2O3).[1] The total amount of Fe2O3, as high as 11 wt%, is distributed among silicate phases and FeTi oxides.[1] The presence of iron oxide particles in Etna lava makes it an ideal material for growing and immobilizing nanocarbons. Metal particles such as Fe are necessary for the catalytic growth of CNTs/CNFs.[2] They catalyze the decomposition of the carbon source and control the growth kinetics of the tubes/fibers.[3] The Mount Etna lava used in the present work was collected on the north flank of Etna on November 23, 2003 (Figure 1a). Examination by scanning electron microscopy reveals macropores and mesopores in the silicate matrix (see Figure 1b). The pores contain metal oxide particles ranging from 10 to 100 μm in diameter, giving the bright contrast in Figure 1b. The lava stone was broken into small pieces with a hammer and ground in a mortar. The crushed lava powder (200 mg, the size of the small pieces is less than 0.5 mm) was put into a vertical quartz reactor and reduced with H2 at a flow rate of 50 mLmin at a temperature of 700 °C for 2 h. After reduction, a mixture of ethylene and H2 (total flow rate was 200 mLmin, 50 vol% C2H4) was introduced into the reactor. The temperature of the lava powder was kept at 700 °C in the flow gas at atmospheric pressure for 2 h. The color of the lava powder changed to black after the catalytic chemical vapor deposition process (CVD). The iron oxides are reduced to metallic iron by H2 and catalyze the decomposition of ethylene for growing CNTs/CNFs. The scanning electron micrograph in Figure 1c shows the CNTs/CNFs immobilized on the surface of Etna lava. The transmission electron micrograph in Figure 1d indicates a carbon nanotube with an inner wall diameter of less than 10 nm. Representative Raman spectra of the lava powder, recorded before and after the catalytic CVD process, are shown in Figure 1e. The