Acetylene LEED studies

Fig. II. (a) Schematic representation of hydrocarbons adsorbed on the [111] plane of platinum. Intersections of the lines of triangular net denote positions of the centers of platinum atoms. (1) Cyclohexane (2) all-cis conformation of cij-l,3,5-hexatriene (3) transoid conformations of cis- and trans-1,3,5-hexatriene (S4). (b) Adsorption configurations of acetylene and ethylene found most probable according to LEED studies 141).

LEED Studies of Acetylene and Ethylene Adsorption on Rh(lll). Exposing the clean Rh(lll) surface between 230 and 250 K to either C2H2 or C2H4 results in the appearance of sharp half order diffraction spots in the LEED pattern from a (2x2) surface structure. The new diffraction spots from the ordered hydrocarbon structures are sensitive to surface coverage. Although the spots are visible after a 1 L gas exposure, they do not become sharp and intense until 1.5 L and then immediately begin disordering above 1.5 L. 

Complex adsorbates and substrates. The majority of LEED studies have concentrated on monatomic or diatomic adsorbates on low-index surfaces but attention is now being turned to less simple systems. This effort is illustrated by the work of Somorjai and his colleagues who have examined acetylene and ethylene on Pt(l 11), normal paraffins and cyclohexane on Pt(l 11) and Ag(l 11), benzene on Pt(l 11)," ... 

Perhaps the next simplest molecular adsorbates for which quantitative structural information exists are the unsaturated C2 hydrocarbons, notably acetylene (ethyne, HC CH) and ethylene (ethene, H2C=CH2), adsorbed on a number of metal surfaces (especially, Cu, Ni and Pd), and also on Si(100), studied by LEED, SEXAFS, and PhD. In some systems adsorption of ethylene is accompanied by a surface reaction. In particular, on both Pt(lll) [74] and Rh(lll) [75] ethylene is converted to an ethylidyne species, H3C—C—, which bonds to these surfaces through the C atom with the —C axis essentially perpendicular to the surface, in three-fold coordinated hollow sites. In addition, ethylene adsorbed on Ni(l 11) at low temperature dehydrogenates to produce adsorbed acetylene as the surface is warmed towards room temperature this particular system actually provided the first example of the... 

Bonding and Adsorbed Species.—Broden et al. have investigated geometric and electronic effects in the chemisorption and reaction of acetylene, ethylene, and benzene on Ir(lOO) surfaces. Though not a paper on catalysis, the study represents primary information on adsorption processes for molecules which may or may not be involved in catalytic changes. The physical techniques used were LEED, AES, and UPS. 

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