Langmuir 1985, 1 , 162-166
162
Adsorption of Hexafluoroacetone on Pt( 11 1): Is Bonding End-on ( q l ) or Side-on (q2)? Neil R. Avery CSIRO Division of Materials Science, University of Melbourne, Parkville, Victoria 3052, Australia Received July 13, 1984 The adsorption and bonding configuration of hexafluoroacetone, (CF3),C0, on a Pt(111)surface have been studied by high-resolution electron energy loss and thermal-desorption spectroscopies. Adsorption was entirely reversible and weak, with the majority (95%)monolayer desorbing with first-order kinetics near 135 K 35 kJ mol-'). Minority (5%) monolayer desorbed at temperatures up to 400 K. Multilayer desorption occurred near 105 K (AH,, 35 f 4 kJ mol-'). Distinguishable vibrational spectra from thick and thin multilayers were associated with the second layer (first multilayer) condensing with the skeletal plane approximately parallel to the surface. For the majority monolayer, the slightly red-shifted (20 cm-') carbonyl stretch, activity of the in-plane (A, and B,) and screening of the out-of-plane(B,)modes established that, like (CH3),C0, (CF3),C0 was bonded to the Pt(ll1) surface in a monohapto (q') configuration with the Pt-C=O bond bent in the skeletal plane in a manner well-known in ketone molecular complexes. In view of the previously reported q2-(CHJ2C0configuration on the geometrically related, but more electropositive, Ru(001) surface, the present result shows that fluorine substitution in this ketone does not induce q2 bonding, as in many molecular complexes. Instead, the inherent Lewis acidity of the Pt(ll1) surface prevails, and (CF3),C0 forms a weak donor bond with the surface. N
N
1. Introduction
Recent thermal-desorption spectroscopy (TDS) and high-resolution electron energy loss spectroscopy (EELS) studies of the adsorption of a typical ketone, acetone, on the geometrically related Pt(lll)'-3and R U ( O O ~surfaces )~~~ have revealed contrasting bonding configurations which provide an insight into the electronic structure of these surfaces. On the well-defined regions of the Pt(ll1) surface, bonding is weak and reversible (AHa= 48 kJ mol-') and occurs in an end-on manner through the oxygen atom as shown in I (X = H). In this monohapto ($1 configu-
cx3 I
/c - cx3 :0
, i, I
0 --
cx3 c p cx3
\ I 777777
I1
ration the local molecular CZusymmetry of the acetone skeleton is preserved although on a more extended basis should be regarded as being lowered to Cs with the mirror plane perpendicular to the surface. From the activity of the in-plane modes and the screening of the out-of-plane mode it was concluded that the Pt-O=C bond was bent in the molecular plane leading to an inequivalence of the two methyl groups. Additionally, adsorption was characterized by an 80-cm-l red shift of the carbonyl stretch as expected for a bonding configuration of this kind. Here, the Pt(ll1) surface may be regarded as acting as a Lewis acid, accepting a nonbonding electron pair from the oxygen atom into empty acceptor states near the Fermi level. Weak, donor-bonding configurations of this kind are well-known in inorganic molecular complexes, where X-ray structure determinations have shown the same bending of the M-O=C bond (135-153') in, or close to, the acetone molecular plane. Similarly, the infrared data show a red (1) Avery, N. R. Surf. Sei. 1983, 125, 771-786. (2) Avery, N. R.; Anton, A. B.; Toby, B. H.; Weinberg, W. H. J. Electron Spectrosc. Relat. Phenom. 1983, 29, 233-237. (3) Avery, N. R.; Weinberg, W. H.; Anton, A. B.; Toby, B. H.Phys. Rev. Lett. 1953, 51, 682-685. ~
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shift of the carbonyl stretch (
