Chemisorption and thermal decomposition of ethylene on Pd(110): Electron energy loss spectroscopy, low-energy electron diffraction, and thermal desorption spectroscopy studies

The adsorbed state of ethylene on Pd(110) at 90 K and its thermal decomposition in the temperature region between 90 and 600 K have been studied by the use of high resolution electron energy loss spectroscopy (EELS), low-energy electron diffraction (LEED), and thermal desorption spectroscopy (TDS). At 90 K, ethylene is π bonded to the Pd(110) surface and is adsorbed almost disorderedly. The c(2×2)-C2H4 patches are formed near the saturation coverage (which corresponds to 0.58 C2H4 molecule per surface Pd atom). By heating the C2H4-saturated Pd(110) surface to 260 K, some C2H4 admolecules are desorbed intact and the remaining admolecules rearrange their adsorbed sites to form the c(2×2)-C2H4 structure. At above 300 K, almost all the C2H4 admolecules are dehydrogenated, and the ethynyl (CCH) species, H adatoms and unstable dehydrogenated species [possibly, vinyl (CHCH2) species] are formed; the C2H4 desorption occurs by the recombination of H adatoms and dehydrogenated species. The remaining H adatoms are desorbed as β-H2 at ∼325 K. By heating to 450–520 K, ethynyl is decomposed, and only carbon adatoms remain on Pd(110); the decomposition is accompanied by the H2 desorption. Structural models of chemisorbed C2H4 and CCH on Pd(110) are proposed.