X-ray photo of a variable catalytic converter
New techniques to clarify a common chemical reaction: burning carbon monoxide in a car exhaust catalytic converter. Richard van Rijn defended his PhD on this subject on 8 May.
For many years now, every car that runs on petrol has been fitted with a catalytic converter. This device converts toxic carbon monoxide into carbon dioxide (at the same time rendering petrol and nitrous oxide molecules harmless). Yet it is still unclear how this process actually works, according to Richard van Rijn (1983), whose dissertation investigates this chemical reaction using X-rays. To put it crudely: the exhaust gases in a catalytic converter pass through particles of the precious medal palladium. The carbon monoxide molecules and oxygen atoms combine at the surface of the palladium to produce a chemical reaction.
The details are far from clear: the exact influence of temperature, pressure, and, for instance the roughness of the metallic surface, cannot be predicted with any accuracy. Trial and error plays a major role in the development of catalytic converters for such chemical processes. Until recently basic research was limited to low gas pressure because the measuring technique used did not work well with more realistic, higher gas pressures. Van Rijn terms this the ‘pressure gap’. He has been able to bridge this gap with a new kind of trial reactor suitable for realistic gas pressures, as well as for measurements on the surface of the catalytic converter using surface X-ray diffraction.
This measuring technique was carried out using the powerful X-ray clusters of the European Synchrotron Radiation Facility (ESRF) in Grenoble, where Van Rijn worked for three and a half years. ‘With such an X-ray cluster there is little space for a reactor, so it was a matter of smart design.’ But the result was there: the measurements offered a surprising insight into the mysterious oscillations that were sometimes exhibited by the palladium catalytic converter. The rapidity with which carbon monoxide was converted changed every hundred seconds from high to low and back again.
Van Rijn shows why: a palladium catalytic converter begins life as a pure metallic surface, and during the chemical reaction gradually becomes covered with oxygen atoms, or palladium oxide. As Van Rijn puts it: ‘Every expert “knows” that an oxide catalyses less effectively than the metal, but our research shows that it actually does so more effectively.’ The X-ray measurements also show that the oxide layer becomes thicker and rougher, until it becomes chemically unstable, and within a few seconds is transformed back into pure palladium. At first this layer is very rough, but the atoms disseminate easily, so that the surface gradually becomes smoother again, until the entire cycle begins again a hundred seconds later.
This step in our fundamental understanding was published in NatureChemistry. ‘I think we're getting closer to a custom-built catalytic converter,’ says Van Rijn. A spin-off company of the research group is already marketing his reactor to other researchers. For the time being, the trial-and-error method rather than theoretical chemistry will dominate the development of catalytic converters. As Van Rijn puts it: ‘At present it is still a bit like a chemist telling a cook how to fry a beefsteak.’
Richard van Rijn, MSc
A Structural View of Pd Model Catalysts; High-Pressure Surface X-Ray Diffraction
8 May 2012, 13.45 hrs
Academy Building, Rapenburg 73
Supervisor: Profesor Joost Frenken
(9 May 2012)
- Richard van Rijn
- Leiden Institute of Physics, Interface Physics Group
- Fundamentals of Science research profile area