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Copy of A TPD study of formic acid and oxygen
Transcript of Copy of A TPD study of formic acid and oxygen
Programmed Desorption TPD is a method of measuring the approximate heat of adsorption by using kinetic information obtained by rapidly heating the surface to above the temperature When molecules adsorb onto a surface the surface energy is minimized, due to the fact a chemical bond is formed with the surface.
The binding energy therefore varies with the combination of the adsorbate and the surface.
Heating the surface causes energy to be transferred to the surface allowing species to desorb. The temperature at which this occurs is called the desorption temperature.
The temperature of the peak maximum can therefore be used to determine the binding energy of the bound surface. In order to determine kinetic parameters for desorbing species the change in peak temperature as a function of oxygen coverage can be studied.
Activation energy of desorption can also be determined via TPD.
- The mass spectrum signal is directly related to the rate of desorption, which can be used to give an Arrhenius plot where the gradient is equal to the activation energy of desorption/the ideal gas constant.
Single peaks correspond to single activation energies. Using the computer programme the following results were obtained. For an oxygen exposed surface 4 peaks are seen as shown below HCOOH + O --> HCOO + OH
HCOOH + OH --> HCOO + H2O
2HCOO --> CO2+H20
H + OH --> H2O
H+O --> OH
H+H --> H2 Formic acid adsorbed onto the surface only, desorbs at a higher temperature than the second layer of formic acid with 1st order kinetics This graph indicates the desorption occurs with zero order kinetics Order of desorption is first.
There is a constant peak temperature with increaing coverage The Basics at which the adsorbed layer desorbs.
- A sample is heated and mass spectrometry can be used to measure
the partial pressures of the atoms or
molecules evolved from the surface
- TPD techniques are often used to determine kinetic and thermodynamic parameters for desorption processes. References Investigating the adsorption and decomposition of formic acid on clean and oxygen dosed surfaces, using a TPD simulator computer programme. XPS DATA Xp data of formic acid adsorption, on a partially oxidised surface. Calculated coverages from the peak areas are given on the figure. For a clean surface a peak corresponding to a m/e of 46 was found. This corresponds to formic acid. Peaks with m/e values of 46, 2, 18 and 44 correspond to HCOOH, H2, Water and CO2 respectively Clean Surface Rate of H2 desorption with increasing oxygen exposure Rate of desorption of H2O with increasing oxygen exposure Background Knowledge Any Questions? Conclusion Activation energy Order of desorption is first.
There is a constant peak temperature with increasing coverage Order of desorption is first.
There is a constant peak temperature with increasing coverage The second layer of formic acid desorbs at a lower temperature than the single layer, with 0th order kinetics Activation energy can be calculated using the complete analysis method. - Formic acid was seen to desorb from the clean surface
- Formic acid, hydrogen, water and carbon dioxide were found to desorb from the oxygen dosed surface.
- Formic acid is bound weakly on the clean surface
- Formic acid was shown to desorb with 0th order kinetics whilst the remaining species were seen to desorb with 1st order kinetics.
- Ea = -32465.338 Jmol-1 Sven L.M. Schroeder and Michael Gottfried, Temperature-Programmed Desorption (TPD), June2002 Atkins, Physical Chemistry, OUP, 9th ed. Pg 896 Surfaces, Gary Attard, Colin Barnes, OUP, 1998 Phil Davies