The Organometallic Chemistry of the Ru(001) Surface

Citation

Parmeter, John Ethan (1988) The Organometallic Chemistry of the Ru(001) Surface. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/xv82-qr39. https://resolver.caltech.edu/CaltechTHESIS:03182013-115728432

Abstract

The organometallic chemistry of the hexagonally close-packed Ru(001) surface has been studied using electron energy loss spectroscopy and thermal desorption mass spectrometry. The molecules that have been studied are acetylene, formamide and ammonia. The chemistry of acetylene and formamide has also been investigated in the presence of coadsorbed hydrogen and oxygen adatoms.

Acetylene is adsorbed molecularly on Ru(001) below approximately 230 K, with rehybridization of the molecule to nearly sp³ occurring. The principal decomposition products at higher temperatures are ethylidyne (CCH₃) and acetylide (CCH) between 230 and 350 K, and methylidyne (CH) and surface carbon at higher temperatures. Some methylidyne is stable to approximately 700 K. The preadsorption of hydrogen does not alter the decomposition products of acetylene, but reduces the saturation coverage and also leads to the formation of a small amount of ethylene (via an η²-CHCH₂ species) which desorbs molecularly near 175 K. Preadsorbed oxygen also reduces the saturation coverage of acetylene but has virtually no effect on the nature of the molecularly chemisorbed acetylene. It does, however, lead to the formation of an sp²-hybridized vinylidene (CCH₂) species in the decomposition of acetylene, in addition to the decomposition products that are formed on the clean surface. There is no molecular desorption of chemisorbed acetylene from clean Ru(001), hydrogen-presaturated Ru(001), or oxygen-presaturated Ru(001).

The adsorption and decomposition of formamide has been studied on clean Ru(001), hydrogen-presaturated Ru(001), and Ru(001)-p(1x2)-O (oxygen adatom coverage = 0.5). On clean Ru(001), the adsorption of low coverages of formamide at 80 K results in CH bond cleavage and rehybridization of the carbonyl double bond to produce an η²(C,O)-NH₂CO species. This species is stable to approximately 250 K at which point it decomposes to yield a mixture of coadsorbed carbon monoxide, ammonia, an NH species and hydrogen adatoms. The decomposition of NH to hydrogen and nitrogen adatoms occurs between 350 and 400 K, and the thermal desorption products are NH₃ (-315 K), H₂ (-420 K), CO (-480 K) and N₂ (-770 K). At higher formamide coverages, some formamide is adsorbed molecularly at 80 K, leading both to molecular desorption and to the formation of a new surface intermediate between 300 and 375 K that is identified tentatively as η¹(N)-NCHO. On Ru(001)-p(1x2)-O and hydrogen-presaturated Ru(001), formamide adsorbs molecularly at 80 K in an η¹(O)-NH₂CHO configuration. On the oxygen-precovered surface, the molecularly adsorbed formamide undergoes competing desorption and decomposition, resulting in the formation of an η²(N,O)-NHCHO species (analogous to a bidentate formate) at approximately 265 K. This species decomposes near 420 K with the evolution of CO and H₂ into the gas phase. On the hydrogen precovered surface, the Η¹(O)-NH₂CHO converts below 200 K to η²(C,O)-NH₂CHO and η²(C,O)-NH₂CO, with some molecular desorption occurring also at high coverage. The η²(C,O)-bonded species decompose in a manner similar to the decomposition of η²(C,O)-NH₂CO on the clean surface, although the formation of ammonia is not detected.

Ammonia adsorbs reversibly on Ru(001) at 80 K, with negligible dissociation occurring as the surface is annealed The EEL spectra of ammonia on Ru(001) are very similar to those of ammonia on other metal surfaces. Off-specular EEL spectra of chemisorbed ammonia allow the ν(Ru-NH₃) and ρ(NH₃) vibrational loss features to be resolved near 340 and 625 cm⁻¹), respectively. The intense δ_g(NH₃) loss feature shifts downward in frequency with increasing ammonia coverage, from approximately 1160 cm⁻¹ in the low coverage limit to 1070 cm⁻¹ at saturation. In coordination compounds of ammonia, the frequency of this mode shifts downward with decreasing charge on the metal atom, and its downshift on Ru(001) can be correlated with the large work function decrease that the surface has previously been shown to undergo when ammonia is adsorbed. The EELS data are consistent with ammonia adsorption in on-top sites. Second-layer and multilayer ammonia on Ru(001) have also been characterized vibrationally, and the results are similar to those obtained for other metal surfaces.

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