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Catalysis
Atmospheric pressure non-thermal plasma can be used to promote chemical reactions. Collisions between hot temperature electrons and cold gas molecules can lead to dissociation reactions and the subsequent formation of radicals. This kind of discharges exhibits reacting properties that are usually seen in high temperature discharge systems. Non-thermal plasma is also used in conjunction with a catalyst to further enchance the chemical conversion of reactants or to alter the products chemical composition.

Among the different application fields, there are:


 * Ozone production at a commercial level


 * Pullution abatment, both solid (PM, VOC) and gaseous (SOx, NOx)


 * CO2 conversion in fuels (methanol, syngas) or value added chemicals


 * Methanol syntesis


 * Liquid fuels syntesis from lighter hydrocarbons (e.g. methane)


 * Hydrogen production via hydrocarbons reforming

Configurations
The coupling between the two different mechanisms can be done in two different ways:


 * Two-stage configuration, also called Post-Plasma Catalysis (PPC)
 * One-stage configuration, also called In-plasma catalisys (IPC) or Plasma Enchanced Catalysis (PEC)

In the fisrt case the catalytic reactor is placed after the plasma chamber. This means that only the long-lived species can reach the catalyst surface and react, while short-lived radicals, ions and excited speceis decay in the first part of the reactor. As an example, the oxygen ground state atom O(3P) has a lifetime of about 14 μs in a dry air atmospheric pressure plasma. This means that only a small region of the catalyst is in contact with active radicals. In a such two-stage set-up, the main role of the plasma is to alter the gas composition fed to the catalytic reactor. In a PEC system, synergistic effects are greater since short-lived excited species are formed near the catalyst surface. The way the catalyst is inserted in the PEC reactor influence the overall performace. It can be placed inside the reactor in different ways:


 * in powder form (Packed bed)
 * deposited on foams
 * deposited on structured material (honeycomb)
 * coating of the reactor walls

Packed bed plasma-catalytic reactor are commonly used for fundamental studies and a scale-up to industruial applications is difficult since the pressure drop increase with the flow rate.

Plasma-Catalysis Interactions
In a PEC system, the way the catalyst is positioned in relation to the plasma can affect the process in different ways. The catalyst can positively influence the plasma and viceversa resulting in an output that cannot be obtained using each process individually. The synergy that is exstablished is ascribed to different cross effects :


 * Plasma effects on catalyst:
 * Change in the physiochemical properties. Plasma change the adsorbtion/desorption equilibrium on the catalyst surface leading to higher adsorption capabilities. An interpretation to this phenomenon is not yet clear.
 * Higher catalyst surface area. A catalyst exposed to a discharge can give rise to the formation of nanoparticles. The higher surface/volume ratio leads to better catalyst performanaces.
 * Higher adsorbtion probability.
 * Change in the catalyst oxydation state. Some metallic catalyst (e.g. Ni, Fe) are more active in their metallic form. The presence of a plasma discharge can induce a reduction of the catalyst metal oxides, improving the catalytic activity.
 * Reduced coke formation. When dealing with hydrocarbons, coke formation leads to a progressive deactivation of the catalyst. The reduced coke formation in presence of plasma reduces the poisoning/deactivation rate and thus extending the life of a catalyst.
 * Presence of new gas phase species. In a plasma discharge a wide range of new speceis is produced allowing the catalyst to be exposed to them. Ions, vibrationally and rotationally excited species do not affect the catalyst since they loose the charge and the additional energy they posses when they reach a solid surface. Radicals, instead, show high sticking coefficients for chemisorption, incresing the catalytic activity.


 * Catalyst effects on plasma:
 * Local electric field enhancement. This aspcect is mainly related to a packed-bed PEC configuration. The presence of a packing material inside an electric field generates local field enhancements due to the presence of asperities, solid material surface inhomogeneities, prensence of pores and other physical aspects. This phenomenon is related to surface charge accumulation on the packing material surface and it is present even if a packed-bed is used without a catalyst. Despite this is a physical aspect, it also affects the chemistry since it alters the electron energy distribution in proximity of the asperities.
 * Discharges formation inside pores. This aspect is strictly related to the previous one. Small void spaces inside a packing material affect the electric field strength. The enhancement can also lead to a change in the discharge characteristics, which can be different from the discharge condition of the bulk region (i.e. far from the solid material). The high intensity of the electric field can also lead to the production of different species that are not observed in the bulk.
 * Change in the discharge type. Inserting a dielectric material in a discharge region leads to a shifting in the discharge type. From a filamentary regime a mixed filamentary/surface discharge is estlablished. Ions, excited species and radicals are formed in a wider region if a surface discharge regime is present.

Catalyst efects on plasma are mostly releated to the presence of a dielectric material inside the discharge region and do not necessarly require the presence of a catalyst.