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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. http://resolver.caltech.edu/CaltechETD:etd-04032008-110807

Abstract

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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[...] 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 [...] 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.

[...] 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, [...] decreases with increasing surface coverage, a result which is explained in terms of increasing population of the[...] antibonding orbital of [...] with increasing surface coverage.

The vibrational spectra of ordered p(2x2) and p(lx2) 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 [...] with oxygen and [...] with CO on Ru(001). Preadsorbed oxygen produces a strong chemical effect on subsequently adsorbed [...], stabilizing [...]-donation while destabilizing [...] backdonation in the [...]-surface bond. Preadsorbed [...] increases the ability of the Ru surface atoms to backdonate electron density into the [...] orbital of subsequently adsorbed CO, producing values of [...](CO) which are lower than are observed under any circumstances for the adsorption of CO on the clean Ru(001) surface.

Adsorption of[...] on the clean Ru(001) surface produces [...]-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 [...]- bonded configuration which, like [...] 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 [...] and nonreactive [...] intermediate bonding configurations.

Adsorption of the chemically similar molecule [...] on Ru(001) produces many effects analogous to those observed for [...] adsorption: [...] bonding is observed on the clean surface and [...] bonding is observed when the surface is precovered with a p(2x2) oxygen overlayer. The [...] is more reactive than [...], 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 [...] species is observed, partial decomposition to yield an [...] species is observed. The results have important implications for the mechanistic understanding of CO hydrogenation reactions catalyzed under heterogeneous conditions.

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemical Engineering
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • Weinberg, William Henry
Thesis Committee:
  • Unknown, Unknown
Defense Date:12 July 1985
Record Number:CaltechETD:etd-04032008-110807
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-04032008-110807
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:1257
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:08 Apr 2008
Last Modified:26 Dec 2012 02:36

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