Understanding heterogeneous catalysis: Why is Ruthenium a so much more efficient catalyst than conventional exhaust converting materials?

Animated gif showing how a gas-phase CO molecule adsorbes,
interacts with an O neighbor, and finally desorbs as a CO₂ molecule (C in green, O in red, the catalyst in grey). Click here to see snapshots of the transition state.

Today's car exhaust converters rely on the high catalytic power of the transition metals Pt, Pd or Rh for a reduction of the CO content in the emission. Unfortunately, the performance of these materials is substantially reduced under highly oxidizing conditions (as e.g. in diesel engines), presumably through oxide formation. Ru, on the other hand, does not seem to suffer from this drawback displaying remarkable conversion rates (CO + 1/2 O₂ -> CO₂) even with high oxygen concentrations at the surface. Not withstanding,  the fact that during this process also the volatile and toxic RuO₄ may be formed impedes the practical use of Ru in open catalytic reactors. Nevertheless, this anomalous behavior compared to all other transition metals (e.g. the efficiency enhancement by a factor of about 50) makes Ru an interesting candidate for fundamental research aiming to understand the ongoing catalytic reactions on a microscopic level. Corresponding Surface Science experiments are typically performed in Ultra High Vacuum (UHV) hoping that the conclusions drawn may be extrapolated towards the high pressure regime of working catalysts. Yet, until recently Ru seemed to contradict this general rule exhibiting extremely poor catalytic activity under UHV conditions, thus preventing a further elucidation of the high pressure results. This discrepancy between real world and laboratory (the so-called "pressure gap phenomenon") could now be overcome with a theoretical study employing first-principles calculations.  It could be shown that the high conversion rates may only be obtained in presence of more than one adsorbed oxygen  monolayer, the formation of which was kinetically hindered in the previous UHV experiments. With this result, which has in the meanwhile been confirmed by recent experiments using higher partial O pressure, which indicate that surface oxides form which in fact lead to the enhanced performance, the door has been opened to systematically investigate the involved catalytic reactions under controlled conditions.