4 research outputs found
Strong Influence of Coadsorbate Interaction on CO Desorption Dynamics on Ru(0001) Probed by Ultrafast X-Ray Spectroscopy and \u3cem\u3eAb Initio\u3c/em\u3e Simulations
We show that coadsorbed oxygen atoms have a dramatic influence on the CO desorption dynamics from Ru(0001). In contrast to the precursor-mediated desorption mechanism on Ru(0001), the presence of surface oxygen modifies the electronic structure of Ru atoms such that CO desorption occurs predominantly via the direct pathway. This phenomenon is directly observed in an ultrafast pump-probe experiment using a soft x-ray free-electron laser to monitor the dynamic evolution of the valence electronic structure of the surface species. This is supported with the potential of mean force along the CO desorption path obtained from density-functional theory calculations. Charge density distribution and frozen-orbital analysis suggest that the oxygen-induced reduction of the Pauli repulsion, and consequent increase of the dative interaction between the CO 5Ï and the charged Ru atom, is the electronic origin of the distinct desorption dynamics. Ab initio molecular dynamics simulations of CO desorption from Ru(0001) and oxygen-coadsorbed Ru(0001) provide further insights into the surface bond-breaking process
Symmetry-Resolved CO Desorption and Oxidation Dynamics on O/Ru(0001) Probed at the C K-edge by Ultrafast X-Ray Spectroscopy
We report on carbon monoxide desorption and oxidation induced by 400 nm femtosecond laser excitation on the O/Ru(0001) surface probed by time-resolved x-ray absorption spectroscopy (TR-XAS) at the carbon K-edge. The experiments were performed under constant background pressures of CO (6 Ă 10â8 Torr) and O2 (3 Ă 10â8 Torr). Under these conditions, we detect two transient CO species with narrow 2Ï* peaks, suggesting little 2Ï* interaction with the surface. Based on polarization measurements, we find that these two species have opposing orientations: (1) CO favoring a more perpendicular orientation and (2) CO favoring a more parallel orientation with respect to the surface. We also directly detect gas-phase CO2 using a mass spectrometer and observe weak signatures of bent adsorbed CO2 at slightly higher x-ray energies than the 2Ï* region. These results are compared to previously reported TR-XAS results at the O K-edge, where the CO background pressure was three times lower (2 Ă 10â8 Torr) while maintaining the same O2 pressure. At the lower CO pressure, in the CO 2Ï* region, we observed adsorbed CO and a distribution of OCâO bond lengths close to the CO oxidation transition state, with little indication of gas-like CO. The shift toward âgas-likeâ CO species may be explained by the higher CO exposure, which blocks O adsorption, decreasing O coverage and increasing CO coverage. These effects decrease the CO desorption barrier through dipoleâdipole interaction while simultaneously increasing the CO oxidation barrier
Probing the Transition State Region in Catalytic CO Oxidation on Ru data files
Femtosecond x-ray laser pulses are used to probe the CO oxidation reaction on Ru initiated by an optical laser pulse. On a timescale of a few hundred femtoseconds, the optical laser pulse excites motions of CO and O on the surface allowing the reactants to collide and, with a transient close to a picosecond (ps), new electronic states appear in the O K-edge x-ray absorption spectrum. Density functional theory calculations indicate that these result from changes in the adsorption site and bond-formation between CO and O with a distribution of OCâO bond lengths close to the transition state (TS). After 1 ps 10 % of the CO populate the TS region, which is consistent with predictions based on a quantum oscillator model.https://digitalcommons.chapman.edu/sees_data/1002/thumbnail.jp