A TEST OF ELECTRONEGATIVITY EQUALIZATION DURING FLUORINATED ETHANOL DECOMPOSITION ON PT(331)

In previous studies there has been considerable interest in the use of fluorination to probe the transition state for hydrogen transfer reac tions on metal surfaces. In this paper we note that based on a princip le called ''electronegativity equalization,'' fluorine can speed up re actions under some conditions, and slow the reactions down under other conditions. In this paper, we test this theoretical prediction. The a dsorption and decomposition of monofluoro-, difluoro-and trifluoro-eth anol has been studied on a clean Pt(331) surface and on an oxygen-cove red Pt(331) surface using TPD, LEED and AES. It has been found that on a clean Pt(331) sample, the fluorinated ethanols are considerably les s reactive than unfluorinated ethanol. The formation of ethoxy is prog ressively suppressed as we fluorinate the molecule and the C-C bond sc ission process disappears. We also find that the activation barrier fo r beta-hydride elimination grows as we fluorinate the molecule. Intere stingly, if we switch conditions and use excess oxygen. we see the opp osite effect. In the presence of excess oxygen, ethoxy formation is in stantaneous with the fluorinated ethanols and the activation barrier f or beta-hydride elimination is substantially reduced. We also see evid ence for formation of an acetate ligand at large excesses of oxygen. T hese results show that the role of fluorine is much more complex than people had previously supposed. Fluorine can both activate a molecule under some conditions and deactivate the same molecule under other con ditions. Interestingly, the effects are exactly as expected from the p rinciple of electronegativity equalization. (C) 1998 Elsevier Science B.V.