LOW-ENERGY ELECTRON-DIFFRACTION STUDIES OF ADSORPTION OF UNSATURATED HYDROCARBONS AND CARBON MONOXIDE ON PLATINUM (111) AND (100) SINGLE-CRYSTAL SURFACES

The interaction of simple aliphatic hydrocarbons and carbon monoxide with Pt(111) and Pt(100) surfaces, at gas pressures ≤1X10-7 torr, has been studied using low-energy electron diffraction (LEED), mass spectrometry, flash desorption, and work-function (φ) measurements. The Pt(111) substrate was characterized by a (1×1) diffraction pattern while a (5×1) surface structure was visible on Pt(lOO). In contrast to the saturated hydrocarbons, all the other gases used (CO, C2H2, C 2H4, C3H6, 1,3-butadiene and the isomeric butenes) were readily chemisorbed on both substrates. Relatively large decreases in φ resulted from the chemisorption of the unsaturated hydrocarbons while a small increase in φ was measured due to the adsorption of CO. A (2×2) surface structure was produced on the (111) face by all of the unsaturated hydrocarbons studied, with the exception of isobutylene which exhibited a larger unit mesh. Although C2H2 and C 2H4 produced a C(2×2) structure on the (100) face of platinum, the adsorption of the other hydrocarbons was disordered. The necessity of close packing of the olefins appeared to be responsible for the presence of ordered structures on the Pt(111) surface and for their absence on Pt(100). The results of these structural studies indicate that the carbon atoms are sp2 hybridized in the adsorbed state. The chemisorption of CO resulted in the appearance of a C(4× 2) structure on both (111) and (100) surfaces. This structure was transformed by electron-beam desorption into a (21/2 × 51/2) .R45̈ structure on the Pt(l00) surface. Three bonding states of CO were apparent on this substrate whilst only one was detected on Pt(111). A simple model can rationalize these differences.