(638d) Modified Ceria for Low-Temperature Methane Partial Oxidation and Water-Splitting

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 uses oxygen carrier particles, also known as redox catalyst, to partially oxidize 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 can be achieved by cyclic removal of active lattice oxygen in the redox catalyst and replenishment of the same using steam or air. Selecting a redox catalyst particle with high syngas selectivity and facile oxygen transport is very important for CLR. Owing to its capacity to store and release oxygen, ceria has been studied widely as a redox catalyst in a number of thermochemical cycles. The fluorite structure of ceria is stable with fast oxygen diffusion rates. But at low temperatures (<800 °C), syngas selectivity during CLR is low, as the surface oxygen 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 cerium oxide surfaces by platinum group metals such as rhodium. 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. Surface promotion can increase the oxygen removal rate, thereby changing the type of oxygen species present on the surface. This increase in oxygen removal is caused by the enhancement of surface methane activation. In the latter part of the work, the effects of cerium oxide substituted with lanthanum and calcium, coupled with surface promotion are presented. We show increased methane conversion and oxygen release at low temperatures, with the addition of lanthanum to ceria. The reduced oxide is capable of near complete conversion of water to hydrogen.