COPRECIPITATED NI-ALUMINA AND NI-CU-ALUMINA CATALYSTS OF METHANE DECOMPOSITION AND CARBON DEPOSITION .2. EVOLUTION OF THE CATALYSTS IN REACTION

Using coprecipitated Ni-alumina and Ni-Cu-alumina catalysts, up to 250 g/g(cat) of filamentous carbon was produced by methane decomposition at 823 K and a methane pressure of 100 kPa. The process evolved three general stages. In the induction period carbon was dissolved into the Ni particles as evidenced by in situ X-ray powder diffraction measurem ents. This promoted the formation of large metal particles pear-shaped in Ni catalysts and quasi-octahedral in Ni-Cu ones. The steady-state growth of the filaments occurred on the second stage. Finally, the cat alyst was deactivated producing porous granules composed of interlaced long carbon filaments. In deactivated Ni catalysts the nickel was fou nd in metal state, while in Ni-Cu samples about half of the nickel was atomically dispersed in carbon as revealed by extended X-ray absorpti on fine structure spectroscopy. The deactivation of the catalysts was suggested to include the fragmentation of the metal particles as well as an atomic erosion in Ni-Cu samples. In addition, in both catalysts the filaments growth could also be limited when the close-packed struc tures of porous carbon were achieved.