Thermogravimetric studies of carbon nanofiber formation from methane at low temperature over Ni-based skeletal catalysts and the effect of substrate pre-carburization

Using thermogravimetry (TG) under conditions that minimize inhibition by the hydrogen produced, the intrinsic catalytic rates of skeletal Ni, pure and alloyed with solute metals Fe, Co, or Cu, were evaluated in methane decomposition to carbon nanofibers. In "standard" tests, i.e., after pre-reduction in H-2 and exposure to CH4 directly at 450 degrees C, several catalysts reached stable activities exceeding 4 mg C/mg cat./h, comparable with literature values obtained at 500 degrees C or above. TG evidence is presented for partial bulk carburization of Ni in CH4 below 350 degrees C, which leads to substantially increased coking rates. TEM evidence supports the view that carburization promotes catalyst particle disintegration, thereby inducing faster and more stable nanofiber growth. Irregularities in alloy response to carburization are interpreted in terms of the stability of the respective mixed-metal carbides. TEM also shows that alloying changes the metal nanocrystallite shape (habit), with consequences for the carbon nanofiber structure. Evidence for the easy dissociation of CH4 is corroborated by direct catalyst activation in the absence of H2. Reduction begins in pure hydrocarbon around 300 degrees C and leads to coking activities at 450 degrees C comparable to those for samples pre-reduced in H-2- Skeletal metal catalysts offer distinct advantages in low-temperature natural gas conversion. (C) 2007 Elsevier Ltd. All rights reserved