Catalytic Decomposition of Methane Over M-Co-Al Catalysts (M = Mg, Ni, Zn, Cu)

The catalytic decomposition of methane over M-Co-Al (M = Mg, Ni, Zn, Cu) was studied. The samples were prepared by co-precipitation and characterized by S-BET, TGA, DTA, TPR and XRD. The carbon produced in the reaction was characterized by SEM and TPO. Activity tests were carried out in a thermobalance between 500 and 750 A degrees C. The results show that the textural properties of the calcined samples did not change significantly with the partial substitution of Co by Mg, Ni, Zn or Cu. On the other hand, there were marked differences in the reduced samples. There was a strong influence on the reducibility of cobalt oxides in the presence of Ni or Cu. Nickel promoted the reduction of Co3O4 at the same temperature as the NiO phase, whereas copper strongly decreased the reduction temperature of both Co3O4 and CoAl2O4 due to a synergistic effect between Cu and Co. The sample containing Cu resulted in low catalytic activity in the whole temperature range because the reduction conditions promoted the formation of a Cu-Co alloy. In the reaction carried out at 700 A degrees C, the observed activity was Co-Al > Mg-Co-Al > Ni-Co-Al. All the samples were deactivated by encapsulation under these conditions due to high rates of carbon deposition. The carbon produced was mainly carbon nanotubes, except for the Cu-Co-Al sample, which produced mostly amorphous carbon.