(760c) Composite Mixed Ionic-Electronic Conducting Materials for Low-Temperature Thermochemical CO2 Splitting and Syngas Generation

Reforming of methane to syngas and CO₂ splitting are energy intensive operations in the synthesis of chemical and liquid fuels from natural gas and CO₂. 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. Then the CO will be produced from CO₂ during the oxidation process. This can be achieved by cyclic removal of active lattice oxygen and replenishment using CO₂ 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 this cyclic redox. Owing to its capacity to store and release oxygen, mixed oxides, and partial perovskites have been studied widely as the redox materials. But at low temperatures (<750 °C), syngas selectivity during methane partial oxidation is low, as the metal oxide surface species that catalytically activate methane at these temperatures tend to be non-selective.

To address these limitations, we report a thermochemical cyclic redox scheme which uses a redox-active oxide to split CO₂ for CO production followed with methane partial oxidation to produce syngas with 2:1 H₂:CO molar ratio. To achieve the effective conversion of both methane and CO₂ at low temperatures, we investigated composite mixed ionic-electronic conductive (MIEC) materials. Specifically, two compatible yet structurally distinct MIEC oxides are prepared with different ratios and elemental compositions. It was determined that the lattice oxygen (O^2-) transport and donation of the composite materials were enhanced at relatively low temperatures. Moreover, the reactions were catalyzed by active sites dynamically formed on the surface of the oxide materials. The as-prepared and cycled materials were tested by XRD, in-situ XRD, TEM, and EDX. Experimental studies demonstrated that up to 85% methane conversion, 95% CO selectivity in partial oxidation of methane, and 92% CO₂ to CO conversion at 700^oC without using platinum group metals. The redox stability was tested for 50 cycles, and the results showed the conversion and selectivity remained at a high level as the first cycle. Furthermore, the performance of the redox materials at different temperatures of 600 ^oC, 650 ^oC, 700 ^oC, 750 ^oC, 800 ^oC was tested. The effects of the compositions of the composite materials were systematically investigated.