Production of pure hydrogen by cyclic decomposition of methane and oxidative elimination of carbon nanofibers on supported-Ni-based catalysts

Theoretically, the catalytic decomposition of methane (CH4 --> C + 2H(2)) followed by the oxidations of the deposited carbon with CO2 and O-2 ((1/2)C + (1/2)CO2 --> CO, (1/2)C + (1/2)O-2 --> (1/2)CO2) give just an over-all reaction CH4 + (1/2)O-2 --> 2H(2) + CO (DeltaH(1073) = -20 U/mol), requiring no energy input with zero CO2-emission. The catalytic performances of Ni/SiO2, Ni/TiO2, Ni/Al2O3 and Pd-NiNO2 for the repeated decomposition of methane and oxidation of the formed carbon nanofibers with O-2 have been studied in detail. Ni/TiO2, Ni/Al2O3 and Pd-Ni(1:3)/SiO2 were found to be promising catalysts because the catalytic activities completely lost during precedent decomposition of methane could be recovered repeatedly by the oxidation of the formed carbon nanofibers with O-2. All the carbons fibers that formed on supported-Ni catalysts were oxidized at >753 K. Temperatures higher than 773 K were needed for complete oxidative removal of carbon fibers on Pd-Ni/SiO2. The apparent activation energies for the oxidation of the carbon fibers did not differ much (150-170 kJ/mol) for all the catalysts. The activation energies for the carbon fibers after Ni-elimination treatment gave similar values. These results indicated that the catalysts did not contribute to the oxidation of the carbon nanotibers. (C) 2004 Elsevier B.V. All rights reserved.