A VIBRATIONAL STUDY OF THE ACTIVATION SEQUENCE OF C-H AND C-C BONDS OF ISOBUTENE AND 1-BUTENE ON MO(110) AND (4X4)-C MO(110) SURFACES/

The thermal decomposition pathways of isobutene and I-butene on both M o(110) and (4 x 4)-C/Mo(110) surfaces have been studied using high-res olution electron energy loss spectroscopy (HREELS) in order to highlig ht the substantially different activities of these two surfaces toward s the cleavage of C-H and C-C bonds. On clean Mo(110), the CH2 group o f isobutene decomposes upon heating to 150 K, producing either a phi/p i-bonded isobutenylidene [(CH3)(2)CCH] species or a 1,1-di-phi/pi-bond ed isobutenyl [(CH3)(2)CC] species. Upon further heating, extensive C- H bond scission occurs to form hydrocarbon fragments which do not cont ain CH3 or CH2 groups, but appear to have largely intact carbon skelet ons. By contrast, isobutene is molecularly adsorbed on the carbide-mod ified surface at 150 K. Further heating produces isobutylidyne [(CH3)( 2)HCC] by 300 K, which subsequently decomposes via C-C bond scission t o generate surface methyl groups. The different activation sequence of the C-H and C-C bonds of isobutene on clean and carbide-modified Mo(1 10) surfaces is also qualitatively confirmed by comparative studies of 1-butene on the two surfaces.