EELS identification of the adsorbed species from acetone adsorption on Pt(111)

Adsorption of acetone on a clean Pt(111) surface at 120 K has been studied by high-resolution EELS and TDS. Two forms of associatively adsorbed acetone have been characterized and their structures determined. The more weakly bound, majority form, desorbs reversibly at 185 K (†Ha≈11.6 kcal mole−1) The EELS spectrum of this species showed a carbonyl stretching band (A1 symmetry) red shifted by 80 cm−1 together with the two B1 skeletal modes viz. vas (CC) and δ(CO), indicating bonding in an end on, through the oxygen, η1(O) configuration, with the Pt-O-C bent most probably in the molecular plane. This form of adsorbed acetone is the02 analog of the labile inorganic acetone complexes which are widely exploited in synthesis reactions. The second more, strongly bound, minority form of acetone, at 210–230 K, partly desorbed molecularly while the remainder decomposed to COa, Ha and an adsorbed hydrocarbon residue with the stoichiometry C2Hx. This form of adsorbed acetone showed no EELS bands due to skeletal modes but instead only bands due to methyl stretching, deformation and rocking modes, indicating side-on bonding in a configuration designated η2(O,C). Its surface concentration was variable, depending on the region of the crystal surface probed by the EELS beam and consequently is associated with accidental, low-coordination, step sites. By inference, the η1(O) form is associated with the well defined close packed regions of the ideal Pt(111) surface.