FT-IR SPECTRA OF ETHYLENE MOLECULARLY ADSORBED ON METAL-OXIDES

The FT-IR spectra of ethylene molecularly adsorbed on some oxidic surf aces at low temperature are presented and discussed both in terms of vibrational frequencies and of band intensities. A hydrogen-bonding interaction is observed between ethylene and surfaces having active hydroxy groups, like amorphous silica and HY zeolite. Strong adsorption is observed on some transition metal oxide surfaces (TiO2, ZrO2, Fe2O3, ZnO) where interaction is thought to occur on coordinatively unsaturated cationic centers. In both these cases the bands that are only Raman active in the ghas-phase molecule, become activated, some bands (namely CH stretchings and C = C stretching) are shifted towards lower frequencies while some others (CH2 out-of-plane bendings) are shifted towards higher frequencies. Band intensities reveal a strong increase on the protonic charge on the ethylene hydrogens, evidencing that electron flows from the ethylene molecule to the surface sites, the C2H4 moiety retaining its planar structure. On rock-salt structure metal oxides (MgO, CaO and NiO) the interaction is weaker, and is thought to occur on surface oxide anions. When incompletely reduced centers like Cu(I) are exposed, interaction dominated by backbonding from the surface to pi*-type antibonding orbitals of ethylene occurs, like on typical ethylene-metal homogeneous complexes.