Rh-promoted mixed oxides for "low-temperature" methane partial oxidation in the absence of gaseous oxidants

Compared to conventional reforming, chemical looping reforming (CLR), which partially oxidizes methane in the absence of gaseous oxidants such as steam or oxygen, offers a simpler and potentially more efficient route for syngas generation. This is achieved by cyclic removal and replenishment of active lattice oxygen in oxygen carrier particles, a.k.a. redox catalysts. With redox catalysts being at the heart of CLR, their activity and selectivity are crucial for the CLR performance. While many redox catalysts have been developed, their activities toward methane partial oxidation (POx), especially at relatively low temperatures, are often limited due to the high activation energy for the migration and removal of lattice oxygen. Moreover, syngas selectivity is often less than ideal due to the non-selective nature of the surfaces for many oxides. To address these limitations, we investigated the effects of promoting the catalytic activity of oxide surfaces for two redox catalysts: CaMnO₃ and LaCeO₃. Our findings indicate that promoting mixed oxides with a small amount of Rh can lower the onset temperature of methane POx by as much as 300 °C (0.5 wt% Rh loading). Over 93% syngas selectivity and 7.9 mmol of syngas per gram of redox catalyst were obtained for a highly stable, Rh promoted CaMnO₃ at 600 °C, making it a promising redox catalyst for methane POx under a cyclic redox scheme.