How process parameters and packing materials tune chemical equilibrium and kinetics in plasma-based CO2 conversion

摘要

Plasma (catalysis) reactors are increasingly being used for gas-based chemical conversions, providing an alternative method of energy delivery to the molecules. In this work we explore whether classical concepts such as equilibrium constants, (overall) rate coefficients, and catalysis exist under plasma conditions. We specifically investigate the existence of a so-called partial chemical equilibrium (PCE), and how process parameters and packing properties influence this equilibrium, as well as the overall apparent rate coefficient, for CO2 splitting in a DBD plasma reactor. The results show that a PCE can be reached, and that the position of the equilibrium, in combination with the rate coefficient, greatly depends on the reactor parameters and operating conditions (i.e., power, pressure, and gap size). A higher power, higher pressure, or smaller gap size enhance both the equilibrium constant and the rate coefficient, although they cannot be independently tuned. Inserting a packing material (non-porous SiO2 and ZrO2 spheres) in the reactor reveals interesting gap/material effects, where the type of material dictates the position of the equilibrium and the rate (inhibition) independently. As a result, no apparent synergistic effect or plasma-catalytic behaviour was observed for the non-porous packing materials studied in this reaction. Within the investigated parameters, equilibrium conversions were obtained between 23 and 71%, while the rate coefficient varied between 0.027 s(-1) and 0.17 s(-1). This method of analysis can provide a more fundamental insight in the overall reaction kinetics of (catalytic) plasma-based gas conversion, in order to be able to distinguish plasma effects from true catalytic enhancement.