Lasers Jump-Start Catalytic Research
BERLIN — How do you study chemical processes that last only an average of 100 femtoseconds? And how do you ensure that you are observing just the effects of that particular activity?
Researchers at the Fritz Haber Institute of the Max Planck Society solved both of these questions with an amplified, 800-nm Ti:sapphire laser. In the Aug. 13 issue of Science, physical chemist Mischa Bonn reported how he and his colleagues deciphered the mechanism behind the catalytic chemical reactions that take place on the surface of transition metals such as ruthenium. Catalysts speed up the rate of a chemical reaction without being altered themselves; these reactions typically occur in places such as an automobile's catalytic converter, where poisonous carbon monoxide molecules combine with oxygen to form nontoxic carbon dioxide.
Left illustration -- In simple heating of the ruthenium substrate, no oxidation of CO molecules takes place. Instead, CO molecules simply leave the surface. Right illustration -- When excited by the pulse of a laser beam, the surface metal electrons heat up to several thousand Kelvin above the metal's melting point, but this energy ultimately leads to the formation of CO2.
Until now, the intermediate steps of this chemical conversion -- in which a series of short-lived chemical complexes called transition states are formed -- were largely unknown. Transition states come and go so quickly that it is difficult to observe them by physical means.
To complicate matters, the team sought to discover whether surface metal electrons or phonons (quanta caused by thermal vibrations in a metal's crystal lattice structure) initiate this nearly instantaneous reaction. Heating the substrate by conventional means would not work, as the temperatures of metal electrons and phonons rise at an equal rate, eliminating any possible reaction between oxygen and carbon monoxide. Instead, the strongly bound oxygen molecules stay in place, while the carbon monoxide molecules simply leave the metal substrate altogether.
With the aid of a laser made by Quantronix GmbH of Darmstadt, Germany, Bonn's team selectively studied surface metal electrons by means of a novel chemical pathway. The researchers switched on the reaction with a short 4.5-mJ pulse. Since the metal electrons are quicker to absorb the laser's energy, this threw the electrons and phonons out of equilibrium for about 1 ps, during which time the team could determine the source of energy for the reaction and how long it takes -- key issues in surface chemical physics. To observe the subsequent evolution of transition states over time, they took a series of "snapshots" of the process with pulses of the laser.
The team found that electrons at the surface play an important and underestimated role in surface reactivity. The laser energy was initially absorbed as heat by the surface metal atoms, then transferred to the oxygen-metal bond, which in turn became so weakened that an oxidation reaction with the neighboring carbon monoxide molecule became possible. Carbon dioxide was formed and left the metal surface.
With this improved understanding of surface chemical physics, chemists can design better and more efficient catalysts, Bonn said.
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