Factors influencing the decomposition of CO2 in AC fan-type plasma reactors:: Frequency, waveform, and concentration effects

The decomposition of CO2 in fan-type ac glow discharge reactors coated with platinum or rhodium was studied as a function of concentration of CO2 in the feed, frequency, and waveform shape. The progress of the reaction was monitored with an ion-quadrupole mass spectrometer coupled with a partial pressure analyzer, CO was the main carbonaceous product with selectivities >80%. Mixtures of 2,5, 10, and 20% CO2 in He were studied and the conversion is observed to increase with decreasing CO2 concentration, although the power consumed by the reactor remains relatively constant. The reaction efficiency for the reaction (proportional to yield) is observed to increase with increasing CO2 concentration. The frequency has little effect on the conversion of CO2 in the plasma, but the plasma power consumption is observed to decrease as the frequency is increased at constant applied voltage, resulting in an increase in reaction efficiency with increasing frequency. The conversion of CO2 increases with increasing input voltage in the range 411-2050 V root mean square (rms), then levels off up to 10.91 kV rms, The plasma power shows the same trend as conversion, whereas the efficiency and excitation temperature of the plasma are observed to decrease up to 2050 V rms and then level off. The effects of sine, square, and triangular waveforms were examined and found to yield similar conversions, plasma powers, and efficiencies. The order for conversion and power is square > sine > triangular and the order for efficiency is triangular > sine approximate to square. CO2 conversion is maximized for a square waveform and low concentrations of CO2 at input voltages near 2 kV rms, Conversion is independent of the ac frequency. Conversely, maximization of reaction efficiency occurs at low input voltages (<2 kV rms) and high concentrations of CO2 with a triangular waveform at frequencies in the kiloHertz range. Optimization of conditions has resulted in an efficiency of 11.4% for CO2 dissociation to CO and O-2. (C) 1999 Academic Press.