Plasma catalysis: distinguishing between thermal and chemical effects

The goal of this study is to develop a method to distinguish between plasma chemistry and thermal effects in a Dielectric Barrier Discharge nonequilibrium plasma containing a packed bed of porous particles. Decomposition of CaCO ₃ in Ar plasma is used as a model reaction and CaCO ₃ samples were prepared with different external surface area, via the particle size, as well as with different internal surface area, via pore morphology. Also, the effect of the CO ₂ in gas phase on the formation of products during plasma enhanced decomposition is measured. The internal surface area is not exposed to plasma and relates to thermal effect only, whereas both plasma and thermal effects occur at the external surface area. Decomposition rates were in our case found to be influenced by internal surface changes only and thermal decomposition is concluded to dominate. This is further supported by the slow response in the CO ₂ concentration at a timescale of typically 1 minute upon changes in discharge power. The thermal effect is estimated based on the kinetics of the CaCO ₃ decomposition, resulting in a temperature increase within 80 °C for plasma power from 0 to 6W. In contrast, CO ₂ dissociation to CO and O ₂ is controlled by plasma chemistry as this reaction is thermodynamically impossible without plasma, in agreement with fast response within a few seconds of the CO concentration when changing plasma power. CO forms exclusively via consecutive dissociation of CO ₂ in the gas phase and not directly from CaCO ₃ . In ongoing work, this methodology is used to distinguish between thermal effects and plasma-chemical effects in more reactive plasma, containing, e.g., H ₂ .