W-Fe-MgO and Mo-Fe-MgO catalysts were prepared by the "citric acid" method. CH₄ (60 mL/min) was decomposed on these catalysts in a small fluidized bed reactor, at 1273 K and normal pressure with Ar (200 mL/min) as carrier gas. It was found that adding small amount of water into the feed gas significantly affected the SWCNT content in the products. Using dry feed gas, the products of CH₄ decomposition on the catalysts were mainly amorphous carbon and multi-wall carbon nanotubes (MWCNT). Whereas, when the feed gas contained small amount of water (partial pressure 0.67 to 2.0 kPa), the products of CH₄ catalytic decomposition were mainly bundles of single-wall carbon nanotubes (1～3 nm diameter for single tube), and the carbon yield was 40～52 mg/100 mg catalyst. But, when the partial pressure of water increased to 2.7 kPa, no carbon product can be obtained. By XRD, MS analysis and comparison of experimental results, the following conclusion can be deduced: The metal oxides and carbides in Mo-Fe-MgO and W-Fe-MgO catalysts are the active phases for CH₄ decomposition to amorphous carbon; the Mo-Fe alloys are the active phases for producing SWCNT. With dry feed gas, CH₄ decomposition to amorphous carbon on metal oxides and carbides were dominant and the produced SWCNT on Mo-Fe alloys was relatively small, so products are mainly amorphous carbon and MWCNT with very little SWCNT. Adding a little water (partial pressure 0.67～2.0 kPa) into the feed gas, the decomposition of CH₄ to amorphous carbon on oxides and carbides were hindered, so the main products are SWCNT. But adding more water (partial pressure higher or equal to 2.7 kPa) would keep the catalysts all at oxide state with no alloy, so SWCNT could not be formed and no other carbon product either.

Key words: SWCNT, H₂O, oxide, Mo-Fe alloy