Interface Physics

 

ReactorSTM, -AFM and -SXRD

 

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 stephanopoulos

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.