Lemaître–Tolman metric

Catalycity is a macroscopic parameter to describe surface catalysis (heterogeneous catalysis). It is usually called a material property, due to its strong dependence on the surface material. This naming is not fully justified, since the catalycity not only dependent on the material but also on its environment.

Catalysis

Gases like Nitrogen, Oxygen or mixtures of the like Air consists at low temperatures of molecules. At elevated temperatures they dissociate, i.e. they break apart into single atoms.

$X_{2}\rightarrow 2X$

Atoms tends to recombine in the gas phase.

$2X\rightarrow X_{2}$

This process can be accelerated with the help of a catalyst. The catalyst provides an alternative reaction path, which is in general faster than the original. Such and reaction path could be:

adsorption:

$X(g)+(s)\rightarrow X(s)$

reaction at the surface:

$2X(s)\rightarrow X_{2}(s)+(s)$

desorption:

$X_{2}(s)\rightarrow X(g)+(s)$

Definition

Catalycity relates the number of reacting atoms to the number of atoms that hit a surface. Therefore, a catalycity of one means that all impinging particles do recombine. $\gamma ={\frac {{\mathcal {M}}_{rec}}{{\mathcal {M}}^{\downarrow }}}$

Usage

The parameter catalycity is mainly used in hypersonics, i.e. the science high speed and high temperature flow. These kind of flows occur for instance in Atmospheric reentry.

Limitations

In mixtures of gases where interactions between the different species occurs, the parameter catalycity is not appropriate.

Determination

The determination of catalycity is usually done in Plasma (physics) wind tunnels like arc-jets or inductively coupled plasma generators.