AVS2011 Session SS2-MoM: Surface Chemical Dynamics

New possibilities for the study of chemical reactions on surfaces using X-ray free-electron lasers (Linac Coherent Light Source, or LCLS, at SLAC National Accelerator Laboratory) will be presented. We induced the hot electron mediated excitation of CO on Ru(0001) with synchronized excitation by a femtosecond optical laser pulse. We have followed the ultrafast evolution of the bond distortions, weakening and breaking, using x-ray emission spectroscopy resonantly tuned to the oxygen core level with ultrashort x-ray pulses delivered from LCLS. We can directly study the time evolution of the molecular orbitals in an atom-specific way on a subpicosecond timescale. Different adsorption configurations explored have been characterized by comparing the measured time dependent energy shifts of the molecular orbitals in the valence band with theoretical results.

This research was carried out on the SXR Instrument on the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory. The SXR Instrument is funded by a consortium whose membership include the LCLS, Stanford University through the Stanford Institute for Materials Energy Sciences (SIMES), Lawrence Berkeley National Laboratory (LBNL), University of Hamburg through the BMBF priority program FSP 301, and the Center for Free Electron Laser Science (CFEL). The LCLS is funded by the U.S. Department of Energy's Office of Basic Energy Sciences.

In order to understand alcohol conversion on gold, we have explored the mechanism of the oxidation of benzyl alcohol (C₆H₅-CH₂OH) on a Au(111) surface - a test reaction commonly used to determine the efficiency and selectivity of novel gold-based catalysts. Using adsorbed atomic oxygen as the active oxidizing species, we find that a high selectivity towards the conversion to aldehyde (benzaldehyde, C₆H₅-CHO) is achieved at low oxygen coverages. However, in excess oxygen the selectivity decreases dramatically, and benzaldehyde is readily transformed to adsorbed benzoate, which is eliminated as benzoic acid (C₆H₅-COOH) and CO₂. In addition, we show that the production of the ester produced by self-coupling (benzyl benzoate) does not require the separate presence of acid, as might be expected from acid-alcohol condensation reactions. These surface transformations are extremely facile and exhibit small kinetic barriers of the rate-limiting steps. Lastly, by comparing the mechanism of benzyl alcohol oxidation to that of toluene oxidation, we extract information of a model hydrocarbon-to-alcohol reaction (toluene to benzyl alcohol, C₆H₅-CH₃ to C₆H₅-CH₂OH). Our results suggest that due to the relatively large kinetic requirements for toluene conversion, the alcohol cannot be selectively produced on a gold surface; indeed, preliminary TPRS data indicates that the gold-mediated conversion of toluene yields benzoic acid and combustion gases.

The development of molecular switches on the single molecule level is a major challenge on the path towards incorporating molecules as building units into nanoelectronic circuits. With a scanning tunneling microscope (STM) it is possible to induced chemical reactions on a single molecule basis by electrons tunneling inelastically from the STM tip into a molecule. The method is based on high-resolution imaging at low-temperature (5K) that allows us to identify different groups within the molecule. Chemical reactions are induced by injecting selectively electrons into specific parts of the molecule. The success of the manipulation is visualized in the recorded tunneling current during the manipulation and in STM images taken afterwards. We investigate isomerization of individual molecules adsorbed on metal surfaces. For chlorobenzene and azobenzene derivatives, the effects of different substitutional groups and different substrates are explored.

Methane dissociation on transition metal surfaces is the rate-limiting step in the steam reforming reaction, which is the principal route for converting CH₄ to H₂. Understanding the dynamics of energy flow during this process has both fundamental and practical impact. Experiments that measure the reactivity of methane prepared in select vibrational states reveal how specific nuclear motions promote methane dissociation. State-resolved measurements for methane reactivity on Ni and Pt surfaces show that molecules prepared in vibrationally excited states are more reactive than those without laser excitation, and the efficacies of vibrational energy in different excited states can differ significantly.

Here, we present state-resolved reactivity measurements of the ν₂+ν₄ bending combination vibration of CH₄ on Ni(111) as a function of translational energy. This state is a member of the pentad of vibrational states that can play an important role in the thermal activation of methane. We are able to quantify the state-resolved reactivity of methane in this particular vibrational state over a wide range of translational energies. We compare the efficacy for ν₂+ν₄ bend with that of the ν₃ stretching and 3ν₄ bending states to gain insight into the ability of ν₂ excitation to promote dissociative chemisorption. The result permits a detailed comparison of the role of stretch and bend excitation of methane dissociative chemisorption on Ni(111).

Spatial distributions of N₂ and N₂O desorbing from Pd(211), Pd(S)-[3(111)×(100)], in temperature programmed desorption (TPD) were measured after the N₂O dosing on the surface at 60 K. About one-third of N₂O molecules on the surface are found to desorb as N₂ by the decomposition with sharp concentration to 25° (θ = -25°) (100) side from the normal direction of (211) plane at 108 K in the parallel plane to [211] and [-111] directions at N₂O coverage of 1.0 ML. This indicates that N₂ desorbs without thermal equilibration by N₂O decomposition on Pd(211). From the preferential desorption direction (θ= -25°), the adsorption structure of N₂O just before the desorption is assigned as the bridge-structure at step-site, where O and terminal-N bonded with Pd at ontop (100) and 3fold hollow(111), respectively along the [-111] directions. On the other hand, the spatial distribution of desorbing N₂O obeyed cosine distribution directing the normal direction to the (211) plane, suggesting the existence of pre-desorption state on the surface.