Desorption

Desorption (from the Latin de-sorbere ; sorbere : (to) suck up) describes the process in which atoms or molecules leave the surface of a solid (reverse process: adsorption ) or from a liquid in which they were dissolved into pass the gas phase (reverse process: absorption ). In the latter case, procedurally speaking, stripping is also used. Desorption thus generally represents the reverse process of sorption .

In order to be able to desorb, the particle must possess or be supplied with sufficient energy to overcome the binding energy .

Classification

According to the origin of the energy required for this and depending on the case at hand, there are

• thermal desorption (temperature swing )
• optical desorption (through absorption of photons )
• acoustic desorption by ultrasound
• electrical desorption
• ion-stimulated desorption (by bombardment with fast ions )
• Pressure desorption ( pressure swing desorption )
• technical desorption (different desorption systems for example with fixed bed, moving bed , fluidized bed )
• Displacement desorption (use of a more easily adsorbable substance; technically mostly water vapor)

Desorption rate

Thermal desorption rate

The desorption rate of the particles from the surface depends initially on the number n of adsorbed or absorbed particles / molecules, the temperature T and the desorption energy (also binding energy ): ${\displaystyle E_{\mathrm {D} }}$

${\displaystyle R_{\mathrm {D} }=n^{m}k_{\mathrm {D} }^{0}\exp \left({\frac {-E_{\mathrm {D} }}{k_{\mathrm {B} }T}}\right)}$

where an oscillation frequency, Boltzmann constant and m is the order of desorption. ${\displaystyle k_{\mathrm {D} }^{0}}$ ${\displaystyle k_{\mathrm {B} }}$

The order of desorption depends on the specific mechanism of desorption:

• ${\displaystyle m=0}$: Desorption from a multilayer
• ${\displaystyle m=1}$: Desorption of atoms
• ${\displaystyle m=2}$: Desorption of two-atom molecules
• Etc.

Ion-stimulated desorption rate

The observed desorption rates of the ion-stimulated desorption range from 1 (vertical incidence, low energies of the incident ions) to approx. 25,000 (grazing incidence, energies in the range of a few MeV / u of the incident ions). In general, the rate of ion-stimulated desorption is a function of:

${\displaystyle R_{\mathrm {D} }=f\left({\frac {\Delta E}{\Delta x}}\right)^{2}\max \left({\frac {d}{\cos(\theta )}},\lambda \right)}$,

where is the penetration depth of the ions into the material, the electronic energy loss per penetration depth, the angle of incidence and a constant that is initially determined empirically. The speed of desorption and the energy required for it is a decisive factor in whether the combination of adsorption and desorption is an option as a technical process. ${\displaystyle \lambda }$ ${\displaystyle \Delta E/\Delta x}$ ${\displaystyle \theta }$ ${\displaystyle d}$

application

In epitaxy , desorption is used to maintain a certain concentration in the gas phase.

In research, ion-stimulated desorption is used to clean surfaces of adsorbates that could not be cleaned by means of thermal desorption.

When cleaning process and exhaust gases, the desorption mechanism is used to regenerate a loaded adsorbent . This is usually done by introducing heat or reducing the pressure .

Web links

• Boldly going where no mass spectrometer has gone before - use of the ion-stimulated desorption in combination with a mass spectrometer (English)

Individual evidence

1. ↑ VDI 3674: 2013-04 Exhaust gas cleaning through adsorption; Process gas and waste gas cleaning (Waste gas cleaning by adsorption; Process gas and waste gas cleaning). Beuth Verlag, Berlin. P. 15.