Fuels synthesis: Not such a vicious cycle

Energy Environ. Sci. http://doi.org/bj7n (2016)

Thermochemical cycling processes can be used to make useful products such as H₂ and CO from water and carbon dioxide, respectively. These processes require high temperatures, which can be generated by solar thermal concentrators, providing a pathway to convert solar energy into fuels. The approach exploits the fact that redox active materials — such as ceria (CeO₂) — can be partially reduced when heated to high temperatures in inert gas, releasing oxygen. When water or carbon dioxide is subsequently introduced at lower temperature, the ceria is re-oxidized, with concomitant production of H₂ or CO. Syngas (a mixture of CO + H₂) can then be converted into hydrocarbon fuels through other processes. Now, Ivo Alxneit and colleagues in Switzerland show that water and carbon dioxide can be directly converted into methane by thermochemical cycling in the presence of ceria doped with rhodium, with no additional processes.

The researchers cycled the Rh-doped ceria between 1,400 °C and 500 °C over 50 times to reduce and oxidize the ceria sequentially, finding that methane is only produced when water and carbon dioxide are flowed simultaneously. Crucially, the metallic rhodium nanoparticles — the catalytic phase likely responsible for the methane selectivity — are relatively stable to sintering even at the extreme temperatures required. Conversely, nickel-doped ceria, while active for methane production, is not stable due to sublimation and sintering of the metallic nanoparticles.