THz-Pulse-Induced Selective Catalytic CO Oxidation on Ru

We demonstrate the use of intense, quasi-half-cycle THz pulses, with an associated electric field component comparable to intramolecular electric fields, to direct the reaction coordinate of a chemical reaction by stimulating the nuclear motions of the reactants. Using a strong electric field from a THz pulse generated via coherent transition radiation from an ultrashort electron bunch, we present evidence that CO oxidation on Ru(0001) is selectively induced, while not promoting the thermally induced CO desorption process. The reaction is initiated by the motion of the O atoms on the surface driven by the electric field component of the THz pulse, rather than thermal heating of the surface.

Figure 1

CO temperature programmed desorption traces of $\mathrm{CO}/\mathrm{Ru}(0001)$ before and after THz radiation. No CO desorption was induced by THz radiation.

Figure 2

CO temperature programmed desorption traces of $\mathrm{CO}/\mathrm{O}/\mathrm{Ru}(0001)$: (a) before THz irradiation; (b) after THz irradiation; (c) after THz irradiation and redosed with a CO to form a saturated CO layer. The ratio between (a) and (b) measures the decrease in CO coverage after THz irradiation. In (c), the desorption peak for redosed CO appears at the high-temperature side, 380–430 K, which is assigned to the desorption of CO from O depleted regions. No CO desorption was found above 450 K.

Figure 3

Schematic of the molecular orbitals of O and CO bonding to the Ru surface. The antibonding Ru─O oribtals and the Ru─CO nonbonding orbital are crossing the Fermi level. These orbitals will be further populated as conduction electrons are polarized towards the surface. The Ru─O bond will be weakened due to the partial filling of the antibonding orbital while the Ru─CO bond will be unaffected due to the nonbonding character of the orbital.

Figure 4

Strong electric field stimulated surface chemistry. In the presence of the THz field, (a) conduction electrons are polarized towards the surface due to the intense electric field and an electron occupation of the Ru─O antibonding orbital elongates the Ru─O bond. In the absence of the THz field, (b) the electron transfers back to the surface, leaving vibrationally excited the Ru─O bond leading to CO oxidation to produce ${\mathrm{CO}}_2$.