Background
Plasma-catalysis
Non-thermal plasma
A plasma is a (partially) ionized gas, consisting of neutral species (molecules, radicals, excited species), ions, photons and electrons. In a non-thermal plasma (NTP), or nonequilibrium plasma, the electron temperature is much greater than the temperature of the heavy species (ions and neutrals), and thus the radicals and excited species are formed at temperatures closer to ambient. This non-thermal distribution of energy offers a potential avenue to overcoming both the kinetic and thermodynamic limitations on chemical transformations of reactants into desired products. This energy, appropriately directed, can in principle drive endothermic, equilibrium-limited reactions at conditions at which equilibrium conversions are small. Similarly, the energy can accelerate reaction pathways that are kinetically slow at prevailing conditions. The highly energetic electrons in an NTP produce (rotationally, vibrationally and electronically) excited species, ions and radicals through inelastic collisions with feedstock molecules, yielding a plethora of new species and states that are inaccessible at the bulk thermal temperature..
Plasma-catalysis
Because NTPs can contain a diverse mix of highly reactive species, they are difficult to operate in such a way as to produce single products in high yield and at high selectivity. Integration of plasma and catalysts together promises to combine the advantages of the two, to effect transformations that are currently difficult or impossible to achieve.
There is, however, a paradigm shift, often overlooked, in the concept of catalyst operations. In conventional heterogeneous catalysis, reactants chemisorb and follow some surface-mediated reaction paths that ultimately determine the types and rates of products formed. On the other hand, the
NTP provides external activation, and thus the issue is different: how can the catalyst interact with these highly reactive (or energetic) species without simply quenching them and how can it provide a selective path of transformation? Thus, the conceptual mechanism of operation and control of yield
and selectivity is different for thermal catalysts compared to catalysts that operate on plasma-activated reactive species.
The relevance for using renewable energy sources
A great advantage of plasmas is that they allow conversion of the reactants using renewable energy (RE) sources rather than thermal energy, as in conventional catalysis. Thus, plasmas can drive chemical processes with RE, including chemical energy storage of RE, enabling a new lowcarbon
technology for chemical production and a new solution to store/transport RE. Note that microwaves or electrical heating can also be applied to use RE in chemical processes, but up to now, limited examples exist of these solutions.
Plasmas are thus another element in the portfolio of nonconventional catalysis technologies being developed, which also includes photo- and electro-catalysis. In the latter processes, however, the reaction is limited to the surface of the electrodes, and thus a 2D-like catalytic process occurs, and mass/charge transport often limits the process. In plasma catalysis, on the other hand, the full reactor volume can in principle be used, as in thermal catalysis (3D-like processes). Thus, the potential productivity is larger. In addition, electro-catalysis often relies on scarce elements like noble metals. There are also differences in the typology of reactions possible, due to the different mechanisms of conversion between plasma catalysis
and photo-/electro-catalytic processes. Therefore, they are complementary, rather than competing, technologies to drive chemical processes using RE.
Typologies of approach for plasma-catalysis
Two types of plasma catalysis can be distinguished, based on whether the catalyst is placed inside the plasma or not. The first type is called ‘in-plasma catalysis’ (IPC), or one-stage or single-stage catalysis, while the second type is called ‘postplasma catalysis’ (PPC), or two-stage catalysis. IPC can only be applied in NTP devices, such as DBDs (Dielectric barrier discharge), that operate at low enough temperature (300–1000 K) for the catalyst to be inserted in the plasma region. On the other hand, so-called warm plasmas, such as (atmospheric-pressure) microwave and gliding arc discharges, typically operate at temperatures that are too high (several 1000 K) to directly insert the catalysts (unless specific fluidized-bed configurations were applied). In this case, PPC is more appropriate, and thus, only long-lived species that can escape from the plasma will interact with the catalyst, while in the case of IPC, short-lived reactive plasma species (radicals, ions, photons, electronic and vibrationally excited species) can also interact with the catalyst, giving additional possible pathways for the chemical conversions.