A FOURIER-TRANSFORM INFRARED STUDY OF THE REPULSIVE INTERACTIONS AND ORIENTATION OF 1-BUTENE AND ISOBUTYLENE ON AG(110)

SURFACE SCIENCE

卷:340

Article

PAWELACREW, J ; MADIX, RJ ; VASQUEZ, N

关键词: ADSORPTION KINETICS;    ALKANES;    ALKENES;    FTIR;    LOW INDEX SINGLE CRYSTAL SURFACES;    SILVER;    VIBRATIONS OF ADSORBED MOLECULES;   

DOI  :  10.1016/0039-6028(95)00666-4

来源: Elsevier

【 摘 要 】

Temperature programmed desorption spectroscopy (TPD) and Fourier transform infrared spectroscopy (FTIR) were used to investigate the origin of the repulsive interactions and the multiple desorption states for 1-butene and isobutylene an Ag(110). The orientation of the alkenes on Ag(110) at various coverages was determined from the observed vibrational modes by use of the dipole selection rule. It was found that isobutylene lies flat on the surface. Unlike isobutylene, only three of the carbons in 1-butene are in a plane parallel to the surface; the methyl group was found to tilt 110 degrees +/- 10 degrees from the surface plane, with the other three carbon atoms lying in a plane parallel to the surface, indicative of a gauche conformer. To verify the mode assignments for 1-butene, 1,1-d(2)-1-butene vibrational spectra were obtained. Both alkenes bond to the surface with the carbon-carbon double bond parallel to the surface plane. The repulsive interactions manifested by the coverage dependence of the activation energy for desorption do not give rise to reorientation of the adsorbate for theta less than or equal to 0.3 ML. Furthermore, the absence of frequency shifts of the vibrational modes in the adsorbed species with increasing coverage indicates that repulsive interactions apparent in desorption are not due to static dipole-dipole interaction. It appears that the forces that lead to the preferred orientation of the double bond in the alkenes leads to an increase in the repulsive pairwise potential between alkenes, leading to the decrease in the activation energy with increasing alkene coverage. The emergence of a second desorption state, beta, at coverage greater than or equal to 0.3 ML and a corresponding abrupt shift in the vibrational frequencies of the =CH2 and CH=CH wagging modes suggests that all adsorbed alkene species reorient at this coverage.

【 授权许可】

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