(699e) Promoted Mixed Oxides for “Low-Temperature” Methane Partial Oxidation in Absences of Gaseous Oxidants

Reforming of methane to syngas is an important but energy intensive operation in the synthesis of chemical and liquid fuels from natural gas. Steam reforming does not give an ideal H₂ to CO ratio for liquid fuel or chemical synthesis and conventional methane partial oxidation (POx) approaches require costly air separation. Chemical looping reforming (CLR), which partially oxidizes methane into H₂ and CO in the absence of steam or gaseous 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 also known as redox catalysts. Selecting a redox catalyst particle with high selectivity and facile oxygen transport is very important for CLR. Several redox catalysts have been reported in literature, but their activities toward methane POx, at low temperatures (<800 °C), are often limited due to the high activation energy for the migration and removal of lattice oxygen. Moreover, syngas selectivity is often low, as metal oxide surface species that catalytically activate methane at these temperatures tend to be non-selective. To address these limitations, we investigated the effects of promoting catalytic activity of oxide surfaces for a number of mixed metal oxide based redox catalysts. Our findings indicate that surface promotion can lower the onset temperature of methane POx by as much as 300 °C while achieving >90% syngas selectivity.

While the highly dynamic nature of the system makes mechanistic studies challenging, understanding the mechanism is important to aid rational catalyst design. We report pulse reaction and isotope studies indicating that the selectivity of the redox catalysts is related to the relative rates of lattice oxygen (O^2-) conduction to the surface and the surface oxygen removal by the gas-solids reactions. This is particularly the case at the early stage of the reaction. Surface promotion can increase the oxygen removal rate from the surface, thereby changing the type oxygen species present on the surface. This increase in oxygen removal is caused by the enhancement of surface methane activation. It is also shown that the rate of bulk lattice oxygen diffusion to the surface can be notably affected by surface promotion of the redox catalysts. This results from the change in driving force for O^2- conduction.