Rh-doped Ceria: Solar Organics From H2O, CO2 and Sunlight?

AbstractThe depleting supply of fossil fuels and rapidly increasing emissions of anthropogenic greenhouse gases demand sustainable solutions to the unfolding energy and environmental crises. One solution is to store concentrated solar energy in the form of chemical fuels via thermochemical cycles, which produce synthesis gas, a gas mixture of H2 and CO that is the precursor of liquid fuels in the Fischer-Tropsch (FT) processes. To date, research efforts in this field have been devoted exclusively to the improvement of synthesis gas production, and no reports are available in direct generation of organic fuels such as methane and ethanol from H2O and CO2 by solar thermochemical cycles. With the aim to generate higher grade fuels directly from H2O and CO2 via thermochemical processes, we incorporate FT catalysts into the ceria lattice. In this study, we have synthesized rhodium doped ceria by coprecipitation. X-ray powder diffraction (XRD) indicates that the as-synthesized Rh-doped ceria is single-phased. High temperature XRD reveals that the Rh-doped ceria sustains its fluorite structure even at elevated temperatures up to 1400˚C, indicating excellent structural stability highly desired for thermochemical cycles. The formation of oxygen vacancies in ceria due to the substitution of cerium by the lower valent rhodium cations is evidenced by Raman spectra. Rh-doped ceria exhibits an enhanced oxygen storage capacity (OSC) and superior activities in the conversion of H2 and CO2 into methane. These demonstrate great potential of Rh-doped ceria for the production of methane and other chemicals during the reoxidation in a thermochemical cycle, when H2O, instead of H2, is co-fed with CO2. In fact, evidence strongly indicates that the H2 produced from the splitting of H2O is partly consumed when both H2O and CO2 are used for the reoxidation of thermally reduced Rh-doped ceria. Therefore the formation of higher grade fuels seems highly likely