ReactorSPM and ReactorSXRD
Click here to
check out our results on catalytic reaction oscillations and the role of steps
in catalysis (Nature Chemistry)
ReactorSPM
The Interface Physics Group has developed two classes of instruments for the
investigation of catalytic reactions on model catalysts under realistic or
semi-realistic conditions. The ReactorSTM and ReactorAFM are two scanning probe
microscopes that allow us to prepare and characterize single-crystal surfaces
under ultrahigh vacuum conditions, after which they are transferred to a tiny
reactor cell in which they are simultaneously exposed to a high-pressure gas
flow at high temperatures and imaged with the tip of an STM or AFM. More
information about this ReactorSPm approach can be found by clickling
here.
ReactorSXRD
Similarly, we have developed a
ReactorSXRD setup, which is a dedicated combination of sample preparation in UHV
with Surface X-Ray Diffraction for surface structure determination under
high-pressure, high-temperature gas-flow conditions. This work is carried out in
collaboration with the group of Dr. Roberto Felici at
beamline ID03 of the European Synctrotron Radiation Facility (ESRF)
in Grenoble. More information about this ReactorSXRD activity can be found by
clicking here.
Why do this?
There is a good reason for going to the conditions of high pressures and
high temperatures with surface structure sensitive techniques, such as scanning
probe microscopy and surface X-ray diffraction. Most techniques that have been
developed for the investigation of the surfaces of materials with high, e.g.
atomic, precision work best under ultrahigh vacuum conditions. This is why much
of our present-day fundamental knowledge about catalytic processes at surfaces
has been obtained from experiments at very low pressures, for example up between
10-9 and 10-5 mbar. However, it may well be that between
these low pressures of traditional surface science and the much higher pressures
of 10 mbar - 100 bar of industrial catalysis processes surfaces change their
structure and composition. In addition to changes in the equilibrium structure
of the catalyst structure and composition, the catalytic process may even keep
the surface permanently out of equilibrium, leading to a dynamic rather than a
thermodynamics state for the surface. The only way to find out whether or not
this is so is to investigate catalytic systems under conditions that are as
close as possible to those that are used in practice. Although we may recognize
overall trends in the behavior, the investigation of (model) catalysts under
realistic conditions needs to be carried out on a case-by-case basis.
A striking example of the strong
interplay between the surface structure of a catalyst and the actual catalytic
reaction process is shown by the picture below, which shows three scanning
electron microscopy (SEM) images of a Pt particle that serves as the catalyst
for the oxidation of carbon monoxide, CO, at elevated temperatures and pressures.
The image on the left shows the structure before reaction, while the two images
on the right, taken after the reaction, show that the catalyst surface has been
altered strongly by having been in intimate conact with the reactants, O2
and CO, and the product, CO2.
|
 |
|
Pt catalyst particle
before (left panel) and after exposure to elevated pressures of CO and
O2 (middle and right panel). The effect of the catalytic conditions on
the surface morphology is dramatic. Reproduced from:
Flytzani-Stephanopoulos et al., Journal of Catalysis, 49
(1), 1977.
A
recent result
We have used the combination of ReactorSTM and ReactorSXRD
measurements to reveal the presence of spontaneous reaction oscillations
in the catalytic oxidation of CO at atmospheric pressures. The
measurements show that the driving force for the oscillations is the
periodic build-up and decay of roughness of the catalyst surface that
makes the surface switch back and forth between a metallic state and an
oxidic structure. These results have appeared in
Nature Chemistry. More about the high-pressure reaction
oscillations can be found by clicking
here. |
