Synergy between dielectric barrier discharge plasma and calcium oxide for reverse water gas shift

This study reports on synergy between Dielectric Barrier Discharge plasma and calcium oxide as a catalyst for the Reverse Water Gas Shift (RWGS). Any effect of the presence of the catalyst on the contribution of plasma chemistry, i.e. chemical conversion in the plasma, is minimized by using a fixed bed of α-Al 2 O 3 particles as a blank experiment and adding a small amount of calcium oxide, with similar dielectric constant, particle size and shape. This approach results in constant plasma power and ensures also that the residence time and specific energy input remain unchanged. Furthermore, synergy is determined based on reaction rates at fixed conditions, i.e. concentrations and temperature, based on kinetic equations derived from integral experiments, describing both thermal operation and plasma operation and both in absence and presence of calcium oxide. This approach has not been applied so far to study plasma-catalysis synergy. The experimental results in thermal and plasma operation are well described with kinetic equations based on power rate laws. Synergy is observed at the lower operational temperatures (640 °C) with a rate-enhancement factor of 1.7, steadily decreasing with increasing temperature until disappearing at 750 °C. The concentrations of CO 2 , H 2 and H 2 O have no significant influence. Synergy is attributed to a new reaction pathway involving interaction of plasma activated intermediates with the CaO surface, with a relatively low apparent activation barrier of 40 kJ mol −1 . Much higher activation barriers are observed for both thermal-catalytic RWGS on CaO (140 kJ mol −1 ) as well as for plasma operation with Al 2 O 3 only (90 kJ mol −1 ). We suggest that reaction of surface CaCO 3 with plasma generated H radicals is the rate determining step, in contrast to plasma chemistry where CO 2 cannot be activated with H radicals.