Thermal desorption spectroscopy

Referred to the temperature - programmed desorption (TPD), and sometimes thermal desorption spectroscopy (TDS), the binding energy of adsorbates can be investigated on the surface.

Expiration

The surface to be examined is in a vacuum chamber with the adsorbate, so that the adsorbate molecules or atoms to condense on the surface. Here, different coverages can be achieved, ranging from submonolayer to several monolayers. The surface has to be cold enough for the adsorbate can condense. For this purpose the sample holder is cooled with liquid nitrogen or liquid helium.

The surface is then heated at a specified heating rate, typically 2 to 10 K / s Eventually, the surface reaches the desorption temperature, ie the temperature at which the adsorbate leave the surface because of the thermal energy is sufficient to break the bond to the surface. At this temperature, the desorption rate increases rapidly and therefore the partial pressure of the adsorbate in the vacuum chamber. The partial pressure is measured continuously with a quadrupole mass spectrometer (QMS). In order to obtain the largest possible signal, the sample is positioned as close to the QMS. There are two methods commonly Firstly, the entire signal all the sample abandoned particles can be detected, on the other hand, specific masses would be included to measure about the binding energies of a specific adsorbate.

Result

The result is a diagram on which the partial pressure (and thus the desorption rate ) is plotted as a function of sample temperature. In the diagram, several maxima are usually recognizable. Weakly bound adsorbates thus leave the surface at lower surface temperatures, more tightly bound at higher.

The peak at the highest temperature thus corresponds to the binding energy of the adsorbate to the surface directly. At lower temperatures, more maxima that overlap frequently appear. These are the binding energies of the second monolayer, which bind to the first layer of adsorbate atoms, and the higher monolayers.

The evaluation of the experimental data is held in the simplest case with the so-called Redhead formula based on the assumption that both the pre-exponential factor and the binding energy of the kinetic Desorptionsgleichung cover are independent. This need not necessarily be the case, for example, lateral interactions between the adsorbed particles or even the spatial distribution of adsorbates can affect the results of TPD experiments strongly. The Redhead evaluation method is used nowadays no longer. Now you nutz evaluation methods as "complete analysis method" or the "leading edge method". These evaluation methods are juxtaposed in an article by AM de JONG and J. W. Niemantsverdriet

Thermal desorption spectroscopy 1 and 2 are typical examples of TDS measurements. Both spectra are models of NO desorbing from single crystals under high vacuum. The desorbing NO is measured with a mass spectrometer and has an atomic mass of 30

TDS spectrum 2 A thermal desorption of NO adsorbed on platinum - rhodium ( 100) single crystal. The spectra of the different NO coverages are combined in a spectrum. The unit of the x - axis direction is the temperature in Kelvin, the unit of the y- axis direction is an arbitrary (arbitrary ) unit, the intensity of a mass spectrometer measurement.

Other evaluation methods for desorption are Thermogravimetric analysis ( TGA), with a thermal imaging camera or a thermal conductivity detector.

Applications

TPD can be used for the following purposes:

• Determination of the binding energy of the adsorbate on the surface of
• Determination of the surface quality ( number of stages or defects)
• Determination of the purity of the surface
• For the adsorption of two different gases which can react, the catalytic activity of the surface under investigation can be determined for this reaction. (for example, adsorption of carbon monoxide and oxygen)