Chemical reactions at surfaces: CO oxidation
- Coadsorption of CO and O
- CO oxidation: surface reaction
- Laser-induced CO2 formation
Coadsorption of CO and O on Ru(0001)
Coadsorption systems are of high importance in the understanding of heterogeneous catalysis since the various reaction fragments adsorb on the surface prior to reaction, and the presence and nature of neighboring species can affect the physical and chemical properties, and hence reaction, markedly.
Studies for the (mCO+nO)/Ru(0001) system show that for the systems that form in nature where CO occupies the top site, with increasing oxygen coverage, the adsorption energy of CO can remain practically unchanged or even exhibit a slight increase. We attribute the increase to an O-induced lateral weakening of Ru-Ru bonds of non-O-bonded surface Ru atoms. Thus, these non-O-bonded Ru atoms can form stronger bonds to an on-top CO adsorbate. In contrast, a more expected behavior of a notable decrease in CO adsorption energy with increasing O coverage is observed only if the O atoms bond to the same Ru atoms as CO as, for example, is the case when CO occupies hollow sites. Furthermore, for some of the structures, we find that there is a manifestation of small activation energy barriers for CO adsorption well above the surface. [C. Stampfl and M. Scheffler, Phys. Rev. B 65, 155417 (2002).]
CO oxidation: surface reaction
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).
4 snapshots showing how an adsorbed CO molecule (step 1) interacts with its O neighbor, finally (step 4) desorbing as a CO2 molecule (C in green, O in red, the catalyst in grey).
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 RuO 4 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.
Laser-induced CO2 formation
Oxidation of Carbon Monoxide by Laser Beams: Investigations by Finite Temperature DFT
Increasing the electronic temperature by laser pulses leads to a weakening of the O-Ru bond which allows reaction to CO2 take place by lowering the energy activation barrier.
O-Ru bond weakening due to partially occupying a previously unoccupied antibonding orbital through laser-induced increase in the electronic temperature. [M. Bonn, S. Funk, Ch. Hess, D.N. Denzler, C. Stampfl, M. Scheffler, M. Wolf, and G. Ertl, Science 285, 1042 (1999)]