Seminar: CO2 splitting by inductively-coupled hybrid plasma catalysis and Boudouard reactions

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.