Research areas (interests)

 

Aim of our research in general

- elucidation of fundamental steps of electrocatalytic reactions
- structure and dynamic of the electrical double layer

 

Current Topics

- structure of metal deposits, role of steps
- nanostructuring of electrode surfaces
- organic adsorbates and electrocatalysis
- (ordered bimetallic electrode surfaces)
- structured modification of electrode surfaces by metallic and organic adsorbates (step decoration)
- improved fuel cell catalysis and their action
- electrochemical gas sensors

 

 

 

 

 


The use of single crystalline electrodes is crucial for an atomistic understanding. In many cases we use stepped surfaces because
- they mimic active sites in heterogeneous catalysis
-they allow to prepare ordered bimetallic surfaces by decoration of the steps with a second metal.
The use of nanoparticulate catalysts is crucial for
- an understanding of particle size effects
- obtaining a high surface area to (noble metal) catalyst mass ratio

Click here to enlarge.



Structure of Metal Deposits

 

Metal deposits on a foreign metal substrate play an important role in industry (galvanisation). When the radii of the deposited atoms greatly differ from those of the substrate, the deposit forms 3 dimensional clusters. When the radii coincide, deposition occurs in a “layer by layer” mode. When the rations are close, but not identical, the growth mode seems to depend on the surface orientation.

 


Click here to enlarge.

  

Organic Asorbates and Electrocatalysis

Organic adsorbates are used as modifiers in electrodeposition (galvanic industry) and as corrosion inhibitors. They also occur as intermediates in electrocatalysis.

 

In many cases the organic species can be desorbed either by hydrogenation, displacement by other adsorbing species or by oxidation to CO2. Using differential electrochemical mass spectrometry (DEMS), the product can be quantitatively detected.

  

Well Structured Modification

Well prepared single crystals with a surface plane parallel to a highly indexed miller plane exhibit monoatomic high steps. These step sites differ in their properties from sites within atomically smooth planes. In general they are catalytically more active. As a consequence

- in case of Pt a particular peak for hydrogen adsorption at these sites is visible in cyclic voltammetry
- many foreign metals can be selectively deposited at these sites (leading to a suppression of the corresponding hydrogen adsorption peaks in case of Pt substrates). Thus ordered bimetallic surfaces are obtained.
- many organic materials (and CO) are preferentially adsorbed at these sites.

Fuel cell catalysis and their action

For the low temperature, PEM fuel cell  there are two basic topics related to electrocatalysis which need improvements:

- The large overvoltage for oxygen reduction (at the cathode) decreases the output voltage for any type of fuel cell

- Since hydrogen is difficult to transport (e.g. in vehicles), methanol (or other organic compound) are used as fuel either directly (direct methanol fuel cell) or after conversion to H2 (and CO2) in a reformer. In both cases CO is formed (on the electrode surface or in the reformer) and poisons the electrode surface. It is therefore necessary that the CO be oxidized at a potential as low as possible.

For both anode and cathode one tries to optimize the catalysts by alloying Pt (which in both cases is the best catalysts from all pure metals) with other components (bimetallic and trimetallic catalysts).

The action of a secondary component in a bimetallic surface can be due to the following:

- an electronic (on ligand) effect: the adsorption energy in the vicinity of the second component is influenced

- a geometric effect: Many reactions (e.g. an unwanted poisoning reaction) require a minimal number of adjacent sites of Pt atoms. This number may be reduced by a second component.

- a bifunctional mechanism: An adsorbed species has to react with another one. The second metal component facilitates its adsorption.

 

The action of such co- cataysts is elucidated by model surfaces. In our groups we use step decoration in order to obtain an ordered array of the two components.

 

Example: CO oxidation a Ru modified surface.

Interpretation: On Pt (665)/Ru all adsorbed CO molecules are oxidized at a lower potential than on pure Pt because they can diffuse to the Ru sites where they react with oxygen species adsorbed on Ru. In addition, CO molecules adsorbed in the vicinity of Ru are oxidized even earlier because they are less stable adsorbed due to an electronic affect.




Sensor für aromatische Kohlenwasserstoffe: Wovon hängen Empflindlichkeit und Ansprechzeit ab?
Aromatische Kohlenwasserstoffe lassen sich mit herkömmlichen amperometrischen Sensoren nicht nachweisen, da eine kontinuierliche Oxidation bei konstantem Potential nicht möglich ist. Bei niedrigen Potentialen werden sie aber adsorbiert. Das Adsorbat kann dann bei hohen Po­tentialen durch Oxidation desorbiert werden. Ein Messzyklus umfasst also einen Wechsel des Potentials zwischen Adsorption und Oxidation. Der Signalstrom ergibt sich aus Oxi­dations­ladung und Adsorptionszeit. Das Sensorsteuerungsprogramm stellt die Ad­sorptions­zeit des Sensors selbsttätig auf die jeweilige Analyt­kon­zentration ein. Der lineare Bereich des Sensors erstreckt sich dadurch bis über 200 ppm. Die untere Nachweisgrenze liegt bei 0,1 ppm Toluen.
Die Empfindlichkeit wird bestimmt durch die Diffusionsgeschwindigkeit in der dünnen Elektrolytschicht zwischen Gasphase und der Sensorelektrode. Die Ansprech­zeit wird dagegen bestimmt durch Ad-­ bzw. Absorptionsvorgänge beim Transport in der Gas­phase, insbesondere in der PTFE-­Membran der Gasdiffusionselektrode. Solche Sorptionsvorgänge sind auch für alle anderen Sensortypen für organische Dämpfe im Bereich kleinster Konzentrationen bedeutsam.

Die Arbeiten wurden von der Arbeitsgemeinschaft industrieller Forschungs­vereinigungen „Otto von Guericke“ e.V. im Rahmen des Forschungsvorhabens 12656 N (Elektrochemische Sensoren zum Nachweis organischer Luftschadstoffe durch Adsorption) aus den Mitteln des Bundesministeriums für Wirtschaft und Arbeit (BMWA) gefördert. 

Die Ergebnisse wurden vorgetragen auf dem International Symposium on Instrumentalized Analytical Chemistry and Computer Technology (Incom), Düsseldorf, 29. - 31. März 2005.

Ausführliche Fassung (PDF): Sensor für aromatische Kohlenwasserstoffe: Wovon hängen Empflindlichkeit und Ansprechzeit ab?