THE CHEMISTRY OF BIBENZYL ON PT(111) AS PROBED WITH BPTDS

The dissociation of 1,2-diphenylethane (bibenzyl) on Pt(111) has been studied by using TDS and bismuth postdosing thermal desorption mass spectroscopy (BPTDS). In spite of this molecule's very weak central C-C bond, it did not dissociate prior to extensive dehydrogenation. Adsorbed bibenzyl dissociates on Pt(111) in a 3 step process: (1) Between 280 and 380 K adsorbed bibenzyl (C6H5-CH2CH2-C6H5,a) partially dissociates and partially dehydrogenates to produce adsorbed stilbene (C6H5-CHCH-C6H5,a) and H-a. (2) At similar to 500 K, this adsorbed stilbene dissociates to eliminate six hydrogen atoms as H-2 gas (E(a) congruent to 110 kJ/mol). At this point, six ''C2Ha'' species are proposed to be produced, which may be polymeric in nature and of slightly different C:H ratio (C14H6). (3) Between 560 and 800 K, the remaining six hydrogens, assumed to be bonded in these species, are evolved to leave graphitic carbon (E(a) congruent to 123-178 kJ/mol). No evidence was seen for a benzyl (C6H5-CH2,a) species at any stage of the dissociation process, although BPTDS experiments using coadsorbed D-a would have identified this species if it were present.