Fritz-Haber-Institut der Max-Planck-Gesellschaft  

Inorganic Chemistry – Electronic Structure Group
  

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Electronic Structure:
Axel Knop-Gericke
 
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Beamline
 
Methods:
  High-pressure XPS / XAS

Obviously, the understanding of the interaction of a catalysts surface with the reactants plays a key role in a detailed description of catalytic processes. However, a spectroscopic characterisation of the reacting surface under ambient condition is challenging. The FHI in collaboration with the group of M. Salmeron (Berkley) developed a set-up suitable to record photoelectron spectra in the presence of a reacting gas.

Read more about this method (PDF – 50 kB)   ... ››
Characterization of the atomic and mesoscopic surface structure:    
Low-Energy Electron Diffraction (LEED)   Ion Scattering Spectroscopy (ISS)    

LEED pattern (60eV) of ~1ML FeO(111)
on Pt(111)
 
ISS spectrum of a-Fe2O3(0001)
   
Characterization of the composition and electronic surface structure:    
X-ray Photoelectron Spectroscopy (XPS)   Ultraviolett Photoelectron Spectroscopy (UPS)   Near Edge X-ray Absorption Fine Structure (NEXAFS)

Fe 2p XP spectra after deposition of a
thick potassium layer on Fe3O4(111)
at 200K and annealing to the indicated
temperatures
 
UP spectra of H2O on Fe3O4(111)
 
NEXAFS spectra of ethylbenzene layers
on iron oxide films measured under grazing
and normal incidence
Thermal desorption spectroscopy:    
Thermal Desorption Spectroscopy (TDS)        

Thermal desorption spectra of H2O on FeO(111)
       
Reactor studies:    
Electro-chemical in situ cell   Gas-Chromatographical Mass Spectrometry    

 
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Gas chormatogram and mass spectra for
iron oxide model catalyst films
   
Additional methods:    
Phase diagram calculations with EquiTherm        

Calculated temperature-pressure phase diagram for the system Fe-O2
       
Used Theoretical Methods:    
Read more about the used theoretical methods (PDF – 50 kB)   ... ››
Calorimetry:

Motivation
Measure differential heat of adsorption of probe molecules, e.g. to determine number and strength of acid sites.

Equipment
We are using SETARAM microcalorimeters of the Calvet-type, specifically MS80 and HT1000. The supporting stainless steel triangle is vibrationally disconnected from floor and room. The calorimeter cells are connected to a home-built vacuum & gas dosing system with calibrated volume, so that during the stepwise adsorption the adsorbed amount can be determined through barometry/volumetry.
Location: F 3.16 Phone: 4403
 
Equipment
Sample Preparation and Activation
Samples (typically 0,5 – 1g powder) are pressed and then filled into the cell. Activation is performed outside the calorimeters in a separate vacuum unit where temperatures up to 773 K can be applied. The sample stays in vacuum while the cell is transferred into the calorimeter.
 

Sample Preparation and Activation
 
Measurement of Heat Evolution
 
Data Analysis
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Measurement of Heat Evolution
The only way for heat transfer from the calorimetric cell to the surrounding reservoir is through a thermopile with more than 400 thermocouples in series. The thermopile also generates a signal which allows determination of the heat flow. The heat evolved is measured in comparison to a second cell, the reference cell.

Data Analysis
Raw data show the rising equilibrium pressure in the cell and the thermosignal for each adsorption step. The experiment finally yields two plots: the adsorption isotherm, and the differential heat of adsorption vs. the coverage.
   
Reproducibility of Adsorption Isotherms and Heats of Adsorption
   
     
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Application
Adsorption microcalorimetry is a direct method to determine number, strength and energy distribution of adsorption sites on a catalyst. It allows calculation of the differential heats evolving when known amounts of gas probe molecules are adsorbed on the catalyst surface. The released heat is related to the energy of the bonds formed between the adsorbed species and the adsorbent and hence to the nature of the bonds and to the chemical reactivity of the surface.

Read more about this method (PDF – 1.4 MB)   ... ››

Microcalorimetry tutorial (PDF – 6.0 MB)   ... ››
   
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