H2 production from CH4 decomposition: Regeneration capability and performance of nickel and rhodium oxide catalysts

Nickel–lanthanum (LaNiO3) and nickel–rhodium–lanthanum (LaNi0.95Rh0.05O3) perovskite-type oxide precursors were synthesized by different methodologies (co-precipitation, sol–gel and impregnation). They were reduced in an H2 atmosphere to produce nickel and rhodium nanoparticles on the La2O3 substrate. All samples were tested in the catalytic decomposition of CH4. Methane decomposed into carbon and H2 at reaction temperatures as low as 450 °C—no other reaction products were observed. Conversions were in the range of 14–28%, and LaNi0.95Rh0.05O3 synthesized by co-precipitation was the most active catalyst. All catalysts maintained sustained activity even after massive carbon deposition indicating that these deposits are of the nanotube-type, as confirmed by transmission electron microscopy (TEM). The reaction seems to occur in a way that a nickel or rhodium crystal face is always clean enough to expose sufficient active sites to make the catalytic process continue. The samples were subjected to a reduction–oxidation–reduction cycle and in situ analyses confirmed the stability of the perovskite structure. All diffraction patterns showed a phase change around 400 °C, due to reduction of LaNiO3 to an intermediate La2Ni2O5 structure. When the reduction temperatures reach 600 °C, this structure collapses through the formation of Ni0 crystallites deposited on the La2O3. Under oxidative conditions, the perovskite system is recomposed with nickel re-entering the LaNiO3 framework structure accounting for the regenerative capability of these solids.