Dictionary

Working in a multi-disciplinary project can be very challenging regarding the fact that even while speaking English everyone uses a “different language” due to their different backgrounds. Of course the same applies for the PIONEER project where people from plasma physics, catalysis and chemical engineering work together.

To make discussion as smooth as possible we came up with the idea of a dictionary to make sure everyone is speaking the same language. The different topics are marked by symbols as indicated in the legend below.

To get of a full explanation of the respective term click on it to expand it!

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℗   Plasma Physics
©   Catalysis
⊂∈ Chem. Engineering


The term is often applied to those sites for adsorption which are the effective sites for a particular heterogeneous catalytic reaction. The terms active site and active centre are often used as synonyms, but active centre may also be used to describe an ensemble of sites at which a catalytic reaction takes place. [IUPAC Gold book]


Certain materials used as catalysts or supports consist of spheroids smaller than 10 nm in diameter, cemented into larger entities. A primary particle should be defined as the smallest discrete identifiable entity and the method of identification should be mentioned (e.g. transmission electron microscopy, scanning electron microscopy). An assemblage of such primary particles exhibiting an identifiable collective behaviour (e.g. chemical nature of the aggregated primary particles, texture of the aggregate, resistance to mechanical separation upon grinding) constitutes an aggregate. When describing the aggregates the criterion of identification should be mentioned. Strongly bonded aggregates are called agglomerates. [IUPAC Gold book]


The Boltzmann equation rules the space and time variations of the distribution function of a certain species. In low-temperature plasmas it is the key equation describing the properties of the electrons: (∂/∂t + v*∇x + (F/m)*∇v)f(x,v,t) = (∂f/∂t)col. In its formal derivation, the left hand side describes the evolution of the electrons under the effect of external forces, according to Newton’s laws of motion. The right hand side describes particle interactions, i.e. the effects of “collisions”. French mathematician Cédric Villani, Fields medal 2010, considers the Boltzmann equation as “the most beautiful equation in the world.” [At PIONEER we can only endorse this opinion.].


A substance that increases the rate of a reaction without modifying the overall standard Gibbs energy change in the reaction; the process is called catalysis. The catalyst is both a reactant and product of the reaction. The words catalyst and catalysis should not be used when the added substance reduces the rate of reaction (see inhibitor).

Acronym/Symbol: fA with fA in %
mol fraction of a transformed limiting reactant: fA = (FA0FA)/(FA0) × 100% with FA0 the molar flowrate of A (limiting reactant) at the reactor inlet and FA the molar flowrate of A at the reactor outlet.

Acronym/Symbol: σ with σ in m2
The cross section is an effective interaction area presented by a target particle to an incoming beam, that gives the probability for a scattering event to occur.


The presence of 3D order on the level of atomic dimensions. Crystallinity may be detected by diffraction techniques, heat-of-fusion measurements, etc. The amount of disorder within the crystalline region is not incompatible with this concept. [IUPAC Gold book]

Acronym/Symbol: λD with λD in m
A pertubation given by the electric field of a charged particle in a plasma gets shielded by surounding particles. The Debye length gives meaning to the words “quasi-neutral” in the definition of the plasma state. Therefore, particles outside a sphere around this particular particle with a radius corresponding to a few Debye lengths λD = (ε0kTT/n0e2)1/2 do not experience the electric field anymore. Here, ε0 is the dielectric constant, kT the Boltzmann constant, n0 the plasma densty and e the elementary charge. It is an example of the collective behaviour of a plasma. gets shielded by surounding particles. Therefore, particles outside a sphere around this particular particle with a radius corresponding to the Debye length do not experience the electric field anymore.

Acronym/Symbol: DBD
A reactor design usually at atmospheric pressure, in which the transition to an arc is prevented by a dielectric on at least one electrode. Without further adjustments it is a filamentary discharge.


A discharge describes the transport of current through a fuid.

Acronym/Symbol: EEDF
The Electron Energy Distribution Function (EEDF) f(e) allows gives the fraction of electrons f(e)de in an energy intervall de, even in non-equilibrium conditions. [In thermal equilibrium the EEDF is the Maxwell-Boltzmann distribution.] The electron energy distribution function contains the detailed microscopic information about the behaviour of the electrons. Its different averages allow the calculation of all observable macroscopic properties. This process makes the bridge between the microscopic world of atoms and electrons and the macroscopic world of tangible objects… and technological applications! culate the electron density f(E)dE in an energy intervall dE even out of the thermal equilibrium. By this, crucial key parameters of the plasma can be derived.

Acronym/Symbol: GHSV with GHSV in h-1
Ratio between volumetric flow of reactants (volume × time-1) and volume of the catalyst (flow reactors).

Acronym/Symbol: GAD
An intermediate between a thermal and a non thermal plasma at atmospheric pressure. Starting as an arc at the shortest gap length between the electrodes the discharge moves upwards and the gap length increases. Due to heat loss there occurs a transition to a non equilibrium state with hot electrons and cold ions.


Catalysis can be classified as homogeneous catalysis, in which only one phase is involved, and heterogeneous catalysis, in which the reaction occurs at or near an interface between phases. Catalysis brought about by one of the products of a reaction is called autocatalysis. Catalysis brought about by a group on a reactant molecule itself is called intramolecular catalysis. The term catalysis is also often used when the substance is consumed in the reaction (for example: base-catalysed hydrolysis of esters). Strictly, such a substance should be called an activator. [IUPAC Gold book]


The energy to accelerate electrons is supplied by a microwave electromagnetic field (MW). Therefore no electrodes are needed. The plasma density is quite high.

Acronym/Symbol: NTP
“Non-thermal” means that not all the species obey one unique thermal distribution. Most commonly, it refers to the situation in which both electrons and heavy particles obey their own thermal distributions, where electrons are much hotter than the heavy particles. In case the heavy particles are close to room temperature, it is often emphasized as “cold plasma.” Other aspect of non-thermal may refer to non-Maxwellian features in the electron energy distribution function. These may arise when the thermalization timescale is longer than heating and/or reaction rates. This can lead, for instance, to significant amounts of hot electrons (determined by the local or global electric field) or to a deplected high energy tail of the electron energy distribution function. neutrals wherein the electrons have high temperature, while the heavy particles are close to room temperature.


A primary particle should defined as the smallest discrete identifiable entity [IUPAC 1991]


The Paschen curve describes the breakdown voltage in dependency of the product of pressure and gap length between the electrodes.


A RF voltage is applied to a pair of electrodes. In the oscillating field electrons get accelerated, while the ions are too heavy to follow it. Because the current is mainly displacement current, insulating surfaces can be treated.

Acronym/Symbol: τ with τ in h
1/GHSV or 1/WHSV

Acronym/Symbol: Si with Si in %
mol fraction of the limiting reactant that is transformed into a given product (B or C): SB = FB/(FA0 – FA) × 100%, SC = FC/(FA0FA) × 100% with SB and SC the selectivity towards B and C products respectively and FB and FC the molar flowrate of B and C at the reactor outlet. Notice that the sum of all selectivity must be equal to 1 (or 100%).


Quasi-neutrality ensures that the electrostatic potential is zero at the plasma centre. Close to a boundary, the light and fast electrons can leave the plasma quickly. The ions are a lot heavier and cannot leave the plasma that fast. Thus, the walls acquire a negative potential. Because of Debye shielding, the potential variation occurs in a layer of thickness of a few Debye lengths, called the sheath.


A breakdown mechanism. The transition from simple townsend avalanches towards streamer occurs more easily when the multiplication of charge (meaning when the effective ionisation coefficient) is very efficient and varying strongly with the field. This is very often the case at high pressure, but nevetheless streamers can be ignited at any pressure. A lot of charged particles are produced and seperated in the applied electric field. The resulting dipole exhibits an electric field with a comparable strength to the applied one. Therefore, the ionisation is carried by the electric field, which the charged particles create themselves. charged particles are produced and seperated in the applied electric field. The resulting dipole exhibits an electric field with a comparable strength to the applied one. Therefore, the ionisation is carried by the electric field, which the charged particles create themselves.

Acronym/Symbol: t with t in h
Time of reaction

Acronym/Symbol: TOF with TOF in s-1
Commonly called the turnover number, N, and defined, as in enzyme catalysis, as molecules reacting per active site in unit time.[IUPAC Gold book]. Number of revolutions of the catalytic cycle per unit time. TOFs of commonly used heterogeneous catalysts are commonly on the order of one per second. The life of the catalyst can be defined as the number of turnovers observed before the catalyst ceases to operate at an acceptable rate. Clearly, this number must be larger than unity, otherwise the substance used is not a catalyst but a reagent. Catalyst life can either be short, as in catalytic cracking of oil, or very long, corresponding to as many as 109 turnovers in ammonia synthesis. [Principles of Heterogeneous JA Dumesic, GW Huber, M Boudart]

Acronym/Symbol: WHSV with WHSV in h-1
Ratio between mass flow of reactants (mass. Time-1) and mass of the catalyst (flow reactors).

Acronym/Symbol: Yi with Yi in %
mol fraction of initial limiting reactant that is transformed in a given product: YB = FB/(FA0) × 100%, YC = FC/(FA0) × 100% Notice that the yield of a given product (B or C) can be estimated by multiplying the conversion of A (limiting reactant) and the corresponding selectivity (SB or SC).