The Acyclic Alkynes Acetylenes

The literature of the vibrational spectra of adsorbed alkynes (acetylene and alkyl-substituted acetylenes) is very much in favor of single-crystal studies, with fewer reported investigations of adsorption on oxide-supported metal catalysts. Fewer studies still have been made of the particulate metals under the more advantageous experimental conditions for spectral interpretation, namely, at low temperatures and on alumina as the support. (The latter has a wide transmittance range down to ca. 1100 cm-1.) A similar number of different single-crystal metal surfaces have been studied for ethyne as for ethene adsorption. We shall review in more detail the low-temperature work which usually leads to HCCH nondissociatively adsorbed surface structures. Only salient features will be discussed for higher temperature ethyne adsorption that often leads to dissociative chemisorption. Many of the latter species are those already identified in Part I from the decomposition of adsorbed ethene. 

In the earlier single-crystal review (4), the low-temperature ethyne spectra were classified in terms of the three types indicated in Table I. Types A and A spectra have in common strong features from uCC modes (as identified with the help of spectra from adsorbed C2H2 and C2D2) in the 1300- to 1050-cm-1 region together with rCH absorptions between 2950 

Spectral Types, Temperatures, and References for the Adsorption of Ethyne (Acetylene) on Single-Crystal Metal Surfaces at Low Temperatures 

In an early paper, Demuth and Ibach (6) suggested that the type A spectrum of ethyne on Ni(lll) may correspond to a surface species involving four metal atoms by adsorption across the central M-M bond of the (111) unit cell of the face-centered cubic (fee) crystal. This involves interactions with the two somewhat different threefold sites on either side of the central M-M bond. This model implies that the plane of the HCCH group is perpendicular to the (111) face so that, as found experimentally, the -yCH 

Clearly, several spectra that exhibit strong type A features, e.g., those of ethyne on Ni(lll) or Pd(100), would merit reinvestigation, with careful attention being paid both to the identification of the specular direction and to the direction of the plane of incidence of the electron beam. If such experiments fail to account for type A/type A variations, the additional-species hypothesis could be tested by remeasuring systems giving type A spectra at markedly lower temperatures, when dissociative adsorption or self-hydrogenation would not be expected, and perhaps by deliberate postadsorption of ethene and/or perdeuterioethene. 

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