Can plasma be formed in catalyst pores? A modeling investigation

We investigate microdischarge formation inside catalyst pores by a two-dimensional fluid model for various pore sizes in the μm-range and for various applied voltages. Indeed, this is a poorly understood phenomenon in plasma catalysis. The calculations are performed for a dielectric barrier discharge in helium, at atmospheric pressure. The electron and ion densities, electron temperature, electric field and potential, as well as the electron impact ionization and excitation rate and the densities of excited plasma species, are examined for a better understanding of the characteristics of the plasma inside a pore. The results indicate that the pore size and the applied voltage are critical parameters for the formation of a microdischarge inside a pore. At an applied voltage of 20 kV, our calculations reveal that the ionization mainly takes place inside the pore, and the electron density shows a significant increase near and in the pore for pore sizes larger than 200 μm, whereas the effect of the pore on the total ion density is evident even for 10 μm pores. When the pore size is fixed at 30 μm, the presence of the pore has no significant influence on the plasma properties at an applied voltage of 2 kV. Upon increasing the voltage, the ionization process is enhanced due to the strong electric field and high electron temperature, and the ion density shows a remarkable increase near and in the pore for voltages above 10 kV. These results indicate that the plasma species can be formed inside pores of structured catalysts (in the μm range), and they may interact with the catalyst surface, and affect the plasma catalytic process.