CO chemistry on Ru (001): thermal vs. electronic excitation

As an example of the type of ultrafast chemical phenomena which occur on a catalytic sur face, and which lend themselves to pump-probe investigations at the Swiss- FEL, results are presented from the oxidation of carbonmonoxide on ruthenium [8]. This is a model reaction for catalytic processes of great practical interest, e.g. for the automotive industry.

Consider a Ru (001) sur face with co-adsorbed CO and atomic oxygen (see Fig. II.9); the goal is the catalytic oxidation of CO to less toxic CO2. According to conventional wisdom, sur face catalysis is initiated by energy transfer in thermal equilibrium from the substrate phonons to the chemical adsorbates. However, in the present instance, heating of the substrate simply causes desorption of CO. Upon photoexcitation with a visible laser, energy is transferred to near-sur face conduction electrons. Time-resolved experiments of molecular yields [9] have established that the energy then flows from the hot electrons to the adsorbates in two different ways: via excitation of the substrate phonons or by direct transfer of hot electrons to the adsorbates. The result of model calculations for the time-dependent temperatures of the electron and phonon heat reservoirs is shown in Figure II.10.

Energy transfer to the absorbates via phonons is relatively slow compared to the direct action of hot electrons. This can be understood in terms of the wave-packet diagram in Figure II.11. Since the phonon reservoir can only accept energy from the hot electrons in units of the small energy ℏωphonon, many units must accumulate in order to initiate a catalytic reaction. Alternatively, a hot electron can directly hop to an adsorbate, forming an excited, antibonding ionic species, which leads to rapid wave-packet motion away from the sur face.

The resulting picture of CO and O catalytic photochemistry on Ru (001) is thus as follows (see Fig. II.12): The conventional “slow” channel of energy transfer (left) proceeds via the stepwise excitation of Ru phonons, resulting in CO desorption, before the oxidation reaction is activated. However, if photoexcited hot electrons are present, a “fast” reaction channel is opened up (right), whereby adsorbed oxygen becomes activated, combines with CO and desorbs as CO2<\sub>. The experimental proof of this hot electron channel is provided by exciting the sur face with two separate laser pulses. When the pulse separation is sufficiently long that the electron reservoir can cool down in between, only the phonon channel is active. If the pulses are applied in rapid succession, however, the electrons can heat up sufficiently to open the fast hot electron channel.

These ultrafast investigations of catalytic CO oxidation were per formed using fs laser pulses. The elemental specificity, sensitivity to electronic configuration and wavelength-matching to atomic dimensions provided in addition by the SwissFEL open a multitude of fur ther experimental possibilities. The identification of an ultrafast photosensitive hot-electron channel for the initation of catalytic reactions is very promising for ultrafast pumpprobe studies of heterogeneous catalysis with the SwissFEL.