Studies of Overlayer Vibrational Structure and Identification of Adsorbed Reaction Intermediates Via Electron Energy Loss Spectroscopy

Citation

Anton, Alan Brad (1986) Studies of Overlayer Vibrational Structure and Identification of Adsorbed Reaction Intermediates Via Electron Energy Loss Spectroscopy. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/6yr9-e416. https://resolver.caltech.edu/CaltechETD:etd-04032008-110807

Abstract

Electron energy loss vibrational spectroscopy (EELS) and thermal desorption mass spectrometry (TDMS) have been used to investigate the chemisorption of several molecules on the hexagonally close-packed Ru(001) surface. The adsorption of N₂, O₂, and N₂ with O₂ and CO was investigated to characterize the chemical state of adsorbed molecules, including their interactions with the substrate and with their adsorbed neighbors, through effects manifest in their vibrational spectra. The adsorption of (Ch₃)₂CO and H₂CO and their subsequent thermal decomposition was investigated to identify the structures of reactive and non- reactive adsorbed intermediates, to identify the products of surface reactions and their structures, to identify surface reaction mechanisms, and to correlate reactivity with the structure of adsorbed intermediates.

N₂ binds to the Ru(001) surface at on-top sites with its molecular axis perpendicular to the surface. In contrast to results reported for the isoelectronic molecule CO on the same surface, however, ν(NN) decreases with increasing surface coverage, a result which is explained in terms of increasing population of the 1π_g antibonding orbital of N₂ with increasing surface coverage.

The vibrational spectra of ordered p(2x2) and p(1x2) overlayers of oxygen adatoms on Ru(001) were studied via comparison of experimental EEL spectra to vibrational spectra calculated with a Green's function lattice dynamical technique. The results identify features due to coupling of the overlayers to substrate phonons and illustrate a unique effect of adsorption site symmetry which distinguishes the vibrational spectra of the two overlayers.

EELS and TDMS results used in conjunction to determine the effects of interactions between contrasting adsorbates in mixed adlayers of N₂ with oxygen and N₂ with CO on Ru(001). Preadsorbed oxygen produces a strong chemical effect on subsequently adsorbed N₂, stabilizing σ-donation while destabilizing 1π_g-backdonation in the N₂-surface bond. Preadsorbed N₂ increases the ability of the Ru surface atoms to backdonate electron density into the 2π orbital of subsequently adsorbed CO, producing values of ν(CO) which are lower than are observed under any circumstances for the adsorption of CO on the clean Ru(001) surface.

Adsorption of (Ch₃)₂CO on the clean Ru(001) surface produces η²-bonded molecular acetone which decomposes to CO, carbon and hydrogen upon heating the surface. If the surface is instead precovered with a p(2x2) oxygen overlayer, a significant fraction of the subsequently adsorbed acetone exists in an η¹-bonded configuration which, like η¹ acetone observed on the clean Pt(111) surface, desorbs molecularly upon heating. These results demonstrate in a quantifiable way how the reactivity of the Ru(001) surface can be modified by the presence of a coadsorbed species, and that the change in reactivity can be correlated with the selectivity of the surface toward reactive (η²) and nonreactive (η¹) intermediate bonding configurations.

Adsorption of the chemically similar molecule H₂CO on Ru(001) produces many effects analogous to those observed for (Ch₃)₂CO adsorption: η² bonding is observed on the clean surface and η¹ bonding is observed when the surface is precovered with a p(2x2) oxygen overlayer. The H₂CO is more reactive than (Ch₃)₂CO, however, decomposing at low coverages and low temperature on the clean surface to give hydrogen and CO before any molecular adsorption is observed. At coverages intermediate between total decomposition and near monolayer saturation, where the η²-H₂CO species is observed, partial decomposition to yield an η²-HCO species is observed. The results have important implications for the mechanistic understanding of CO hydrogenation reactions catalyzed under heterogeneous conditions.

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