Characterization of Reaction Intermediates and Adsorbate Interactions on the Ru(001) Surface

Citation

Hills, Malina Manon (1987) Characterization of Reaction Intermediates and Adsorbate Interactions on the Ru(001) Surface. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/7afx-0m86. https://resolver.caltech.edu/CaltechETD:etd-03032008-141343

Abstract

The adsorption and reaction of ethylene and methylamine on the Ru(001) surface have been investigated in order to characterize reaction intermediates and adsorbate interactions. Experiments were conducted under ultrahigh vacuum conditions using electron energy loss spectroscopy and thermal desorption mass spectrometry. Molecularly chemisorbed ethylene is di-σ-bonded and the carbon atoms of the ethylene are rehybridized to sp³. Upon annealing to room temperature, ethylene forms ethylidyne (CCH₃) and acetylide (CCH) via unstable η²-CHCH₂ and η²-CCH₂ intermediates. These species decompose below 400 K to carbon adatoms and methylidyne (CH) with concurrent evolution of hydrogen.

The interactions of ethylene and its decomposition intermediates with hydrogen and also with carbon monoxide have been explored. Hydrogen and carbon monoxide reduce the adsorption and decomposition of ethylene by blocking adsites. Neither coadsorbed hydrogen nor carbon monoxide alter the identities of the decomposition intermediates, although the selectivity of the reaction is altered. Thus on the clean, hydrogen-precovered and carbon monoxide-precovered Ru(001) surfaces, ethylene tends to decompose via sp³-hybridized, η²-bonded intermediates.

Oxygen overlayers perturb the electronic structure of the Ru(001) surface, changing both the bonding of coadsorbates and the decomposition intermediates. For example, ethylene adsorbed molecularly on the Ru(001)-p(2x2)O surface is sp²-hybridized and π-bonded. This ethylene dehydrogenates to ethylidyne which forms vinylidene (CCH₂). The vinylidene is coordinated to the surface via one carbon atom with π-electron donation from the carbon-carbon double bond to the surface occurring. Thus the presence of oxygen changes the nature of the molecularly adsorbed ethylene and the decomposition intermediates, favoring sp²-hybridized, η¹-bonded adspecies. This result holds also for adspecies such as acetylene, acetone and formaldehyde and is discussed using the Dewar-Chatt-Duncanson model.

Methylamine adsorbed on the Ru(001) surface is coordinated through the lone pair of electrons on the nitrogen atom. This species dehydrogenates to a CH₂NH₂ intermediate at 300 K, that dehydrogenates to CHNH₂ upon annealing to 350 K. The lone pair of electrons, previously located on the nitrogen, are delocalized over the ruthenium-carbon and carbon-nitrogen bonds of CHNH₂. This species decomposes below 400 K via two competing pathways, forming carbon adatoms and ammonia or dehydrogenating to side-on bonded C=N with concurrent evolution of hydrogen.

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