PSI team demonstrates direct hydrocarbon fuel production from water and CO2 by solar-driven thermochemical cycles
Solar-driven thermochemical cycles offer a direct means of storing solar energy in the chemical bonds of energy-rich molecules. By utilizing a redox material such as ceria (CeO2) as a reactive medium, STCs can produce hydrogen and carbon monoxide—i.e., syngas—from water and CO2. The syngas can subsequently be upgraded to hydrocarbon fuels by the Fischer-Tropsch process.
Now, a team from the Paul Scherrer Institute (PSI) in Switzerland has demonstrated the direct production of hydrocarbon fuel—specifically methane—from water and CO2 by incorporating a catalytic process into STCs. A paper on their work is published in the RSC journal Energy & Environmental Science.
|Schematic illustration of direct hydrocarbon (CxHyOz) formation from water and carbon dioxide during the reoxidation of reduced ceria doped with a catalyst (Cat.). Lin et al. Click to enlarge.|
Most research efforts in designing redox materials for STCs are devoted to improving syngas production. This can be achieved by doping ceria with heterocations, or by fabricating porous structures to facilitate mass/heat transfer and improve the redox kinetics. A very attractive route is the direct production of hydrocarbon fuels from water and carbon dioxide by realistic STCs. This new concept, if high selectivity for hydrocarbons is achieved, inherently bypasses a second stage conversion, such as methanation or Fischer-Tropsch processes. This could potentially make the solar fuel production chain much more economical. In addition, storage and transportation of syngas would not be required.
… With the aim to produce hydrocarbon fuels directly from water and carbon dioxide, we propose a strategy of incorporating a catalytic process into STCs by adding a catalyst to ceria. This study shows that direct methane production in STCs is possible. The primary role of the catalyst is to drive the formation of hydrocarbon molecules. The formation of these hydrocarbon fuels can be either from the conversion of the syngas generated by water and carbon dioxide splitting, or directly from water and carbon dioxide without the intermediate formation of syngas, or from a combination of both.—Lin et al.
In the study, the team worked with nickel-doped ceria and rhodium-doped ceria. Both materials, after being reduced by hydrogen at 600 ˚C, are active in producing methane during their reoxidation by water and carbon dioxide at 500 ˚C.
The researchers then further evaluated both materials in realistic thermochemical cycles, in which they were activated by thermal reduction.
Generally, they found that the nickel-doped ceria exhibited poor dynamic redox capacity; they further concluded that nickel-doped ceria is not chemically stable at the extreme temperatures required for realistic STCs.
However, with rhodium on ceria, they were able to achieve direct and sustained production of methane from water and CO2. The material exhibited methane, hydrogen and carbon monoxide formation activity during 58 cycles with the activation of the material carried out at 1400 ˚C.
Encouragingly, the material exhibits steady increase of activity for methane production when activated at 1500 ˚C. X-ray diffraction reveals the presence of metallic rhodium in the materials after cycling at 1400|500 ˚C and after additional cycling at 1500|500 ˚C, indicating metallic rhodium as the active catalyst for methane formation. This proof-of-principle study leaves significant room for improvement and may stimulate a new research area of solar thermochemical fuel production. Future research efforts shall be directed towards improving the product selectivity to methane and potentially to other hydrocarbons, preferably liquid hydrocarbons like oxygenates.—Lin et al.
Fangjian Lin, Matthäus Rothensteiner, Ivo Alxneit, Jeroen A. van Bokhoven and Alexander Wokaun (2016) “First demonstration of direct hydrocarbon fuel production from water and carbon dioxide by solar-driven thermochemical cycles using rhodium-ceria” Energy Environ. Sci. doi: 10.1039/C6EE00862C