Temperature-programmed desorption

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With temperature-programmed desorption (TPD), sometimes also called thermal desorption spectroscopy (TDS), the binding energy of adsorbates on the surface can be examined.

procedure

The surface to be examined is covered with the adsorbate in a vacuum chamber , so that the adsorbate molecules or atoms condense on the surface. Various coverings can be achieved here, ranging from sub- monolayers to several monolayers . The surface must be cold enough so that the adsorbate can condense. To do this, the sample holder is cooled with liquid nitrogen or liquid helium.

The surface is then heated at a defined heating rate, typically 2 to 10 K / s . At some point the surface reaches the desorption temperature, i.e. the temperature at which the adsorbates leave the surface because the thermal energy is sufficient to break the bond to the surface. At this temperature the desorption rate increases rapidly and with it 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 as possible to the QMS. Two methods are common: On the one hand, the entire signal of all particles leaving the sample can be detected, and on the other hand, specific masses can be recorded in order to measure the binding energies of a special adsorbate, for example.

Result

The result is a diagram on which the partial pressure (and thus the desorption rate) is plotted as a function of the sample temperature. Several maxima can usually be seen in the diagram. Weaker bound adsorbates leave the surface at lower surface temperatures, more tightly bound ones at higher temperatures.

The maximum at the highest temperature thus corresponds to the binding energy of the adsorbate directly on the surface. At lower temperatures, further maxima appear, which often overlap. These are the binding energies of the 2nd monolayer, which bind the adsorbate atoms on the first layer, as well as the higher monolayers.

In the simplest case, the evaluation of the experimental data takes place with the so-called Redhead formula, which is based on the assumption that both the pre-exponential factor and the binding energy of the kinetic desorption equation are independent of the coverage. This does not necessarily have to be the case; for example, lateral interactions between the adsorbed particles or even just the spatial distribution of the adsorbates can strongly influence the results of TPD experiments. The redhead evaluation method is no longer used today. Evaluation methods such as "complete analysis method" or "leading edge method" are now used. These evaluation methods are compared in an article by AM de Jong and JW Niemantsverdriet.

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

  • TDS spectrum 1 A thermal desorption spectroscopy of NO absorbed on platinum-rhodium (100) single crystal. The unit of the x-directional axis is the temperature in Kelvin, the unit of the y-directional axis is an arbitrary (arbitrary) unit, the intensity from a mass spectrometer measurement .

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

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

Applications

TPD can be used for the following purposes:

  • Determination of the binding energy of the adsorbate on the surface
  • Determination of the quality of the surface (number of steps or defects)
  • Determination of the purity of the surface
  • With the adsorption of two different gases that can react, the catalytic activity of the examined surface for this reaction can be determined. (e.g. adsorption of carbon monoxide and oxygen)

See also

Individual evidence

  1. ↑ P. A. Redhead: Thermal desorption of Gases . In: Vacuum . tape 12 , no. 4 , 1962, pp. 203-211 , doi : 10.1016 / 0042-207X (62) 90978-8 .
  2. ↑ Michael Rieger, Jutta Rogal, Karsten Reuter: Effect of Surface Nanostructure on Temperature Programmed Reaction Spectroscopy . First-Principles Kinetic Monte Carlo Simulations of CO Oxidation at RuO 2 (110). In: Physical Review Letters . tape 100 , no. 016105 , January 2008, doi : 10.1103 / PhysRevLett.100.016105 , arxiv : 0711.2493 .
  3. ↑ D. A. King: Thermal desorption from metal surfaces . A review. In: Surface Science . tape 47 , no. 1 , January 1975, p. 384-402 , doi : 10.1016 / 0039-6028 (75) 90302-7 .
  4. ↑ E. Habenschaden, J. Küppers: Evaluation of flash desorption spectra . In: Surface Science . tape 138 , no. 1 , March 1984, p. L147-L150 , doi : 10.1016 / 0039-6028 (84) 90488-6 .
  5. ↑ A. M. de Jong, J. W. Niemantsverdriet: Thermal desorption analysis . Comparative test of ten commonly applied procedures. In: Surface Science . tape 233 , no. 3 , July 1990, p. 355-365 , doi : 10.1016 / 0039-6028 (90) 90649-S .

Web links

Thermal desorption of large adsorbates