Reduction of CO2 with hydrogen in a non-equilibrium microwave plasma reactor

In the context of converting electricity into value-added chemicals, the reduction of carbon dioxide (CO2) with hydrogen (H2) in a surface-wave-induced microwave plasma discharge, so-called surfatron, was investigated. The effect of different input variables such as gas flow rate, feed gas composition ratio (H2:CO2) and specific energy input (SEI) on the reactor performance, i.e. the CO2 conversion and energy efficiency, was assessed. A maximum CO2 conversion of 85% is obtained when the feed gas mixture ratio (H2:CO2) was equal to 3. Moreover, a trade-off between CO2 conversion and energy efficiency was clearly noticed when varying the supplied microwave power. High SEI resulted in high conversions and low energy efficiencies and vice-versa. Furthermore, the saturation of the carbon monoxide (CO) production was found at high SEI. These results were rationalized by means of a simplified reaction scheme and by optical emission spectroscopy analysis, which showed that the formation of hydrogen (H) and oxygen (O) atoms in the plasma are the dominant channels driving the reaction pathway. We also observed higher electron densities and temperatures at higher H2 content, which may explain the high conversions achieved in the plasma reactor at high H2:CO2 ratios. H2 is then not only capable of acting as a “catalyst” for CO2 decomposition but also modifies the plasma properties, which seems to greatly enhance the potential of chemical reactions and thus the dissociation rates.