CO2 activation by plasma can be a viable route to reduce CO2 to CO, which can be used for further chemistry. The advantage of plasma activation is that it can use intermittent sustainably generated electrical power as energy source. We use inductively coupled Radio Frequency (RF) plasma, because the plasma is readily accessible to diagnostics and energy transfer from grid power to plasma power can be made very efficient.
CO2 conversion in our empty tube reactor at pressures around 100 Pa leads to conversions by the reaction CO2 ® CO + ½ O2 of up to 50%. There is no significant effect of adding an oxidic catalyst like CoO-MgO or NiO-Al2O3 for CO2 conversion. However, there is a clear effect by adding metal meshes or grids into the plasma reactor. O-atom recombination yielding O2 can be the explanation for this effect, because O-atoms can recombine with CO to form CO2. In addition, direct dissociation of vibrationally excited CO2 can explain the enhanced conversion observed. Experiments with a thermal catalytic reactor demonstrate that no dissociation of CO2 is observed at all. Plasma action is required to obtain the catalytic enhancement of the yield. XPS and SEM studies reveal that the surface of the metals does not show any uptake of O or C in the process. The surfaces are damaged in the reaction.
Adding charcoal into the catalyst bed in the RF reactor significantly increases the CO yield. We attribute this to a very efficient reaction of O atoms with C in the catalyst bed. The overall reaction is like the Boudouard reaction: CO2 + C ® 2 CO. However, the Boudouard reaction only runs above 900 K. In the plasma case the C is heated to at most 600 K, so the reaction is induced by the plasma. The effect of C is not catalytic, because it is consumed in the reaction. We carried out prolonged exposure of the C material to the CO2 RF-plasma. After an exposure of about 15 hours, of the initial C only 25% gram was left. Integrating the CO2 flow during the exposure we see that the reaction probability per incident CO2 is about 4%.
We have observed this effect also using high power (15000 W) thermal arcs at atmospheric pressure, where the effect is spectacular. Very high CO yields of more than a factor of 2 higher than the incident flux of CO2 are observed. The energy efficiency of the conversion is high, close to 100%. No O2 is observed in the product stream, which implies that the very expensive separation of the CO and O2 products is not necessary. Thermal arcs allow large scale conversion of CO2. Yields of 100 m3/hr have been observed.