Partial oxidation of CH4 and C3H8 over hexaaluminate-type oxides

Hexaaluminate catalysts based on BaMxAl12−xO19−α (M = Ru, Pd, Pt, Ni; x = 0.25, 0.5, 1.0, 1.5) were prepared via the alkoxide method and investigated for partial oxidation of CH4 and C3H8. Ru- and Ni-hexaaluminate catalysts showed excellent activities for CH4 and C3H8 partial oxidation under a high space velocity of 120,000 l kg−1 h−1. Pd-hexaaluminate catalyst displayed catalytic activity comparable to the Ru catalyst only in a low temperature range, whereas Pt-hexaaluminate catalyst showed quite low activity for CH4 partial oxidation over the whole temperature range investigated. The catalytic activity of BaRuAl11O19−α depended weakly on the calcination temperature. We studied the dependence on the composition of BaRuxAl12−xO19−α. CH4 conversion was almost constant over the samples with x = 0.25, 0.5, and 1.0, but the activity was significantly deteriorated in the case of x = 1.5. On the other hand, the activity over the Pd catalysts increased consistently with the x-value and decreased in keeping with the calcination temperature. In the case of the Pd-hexaaluminate catalysts, the amount of carbon monoxide adsorbed on the exposed Pd decreased monotonically as the calcination temperature increased, indicating that most of the Pd species was not incorporated in the hexaaluminate lattice and that sintering of Pd resulted in deterioration of the catalytic activity. The Ru/Al ratio determined by EPMA for the Ru-substituted hexaaluminate catalyst was unchanged between the as-calcined and used catalysts, whereas the ratio of a Ru-supported hexaaluminate catalyst decreased significantly. These results indicate that the strong interaction between Ru and the base oxide in the Ru-substituted hexaaluminate could suppress evaporation of Ru during CH4 partial oxidation reaction.