**4. The synergy of plasma and catalyst**

Synergism in plasma catalysis in the single-stage reactor is not fully understood due to the complex interaction between the various plasma-catalyst interaction processes [45–49]. Kim et al. [27] discussed the criteria for interaction between nonthermal plasma and the porous catalyst. The chemical species in nonthermal plasma are highly reactive; the lifetime is very short, e.g., O (1 D), 10 ns; O (3P), 50 μs; and OH, 100 μs: their one-dimensional diffusion length is limited from 0.7 to 65 μm. Plasma generated species within diffusion length from the external surface of pellets would contribute to the plasma-induced reaction pathways. The interdependence of plasma and catalyst can be discussed as two aspects: catalyst affects plasma, and plasma affects catalyst.
