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trend is consistent with a Pt-C stretching mode assignment.This assignment is also anticipated on the basis of the surface dipole selection rule which states that only vibrations which belong to totally symmetric representations can be observed.12 In C,, symmetry, the CO and Pt-CO stretches transform as the totally symmetric Z representation while the Pt-C-0 bend transforms as a II representation. For combination modes we must consider the products of representations for the two fundamentals involved. The product 2x2 is again totally symmetric while the product Z X I I is not. Thus the Pt-C-O bending mode is dipole forbidden both as a fundamental and in combination with the C-O stretch.For comparison with our data, EELS studies of CO on a flat Pt( 11 1) and a stepped Pt[(611)X(11 l)] surface showed terminal and bridging CO stretches at 21 10 and 1870 cm-'.13 Loss features at 390 and 480 cm-' were respectively assigned to Pt-C stretches for bridging and terminal CO. The 390-cm-' loss showed coverage-dependent frequency shifts. A very weak loss at 2580 cm-' was assigned to a combination loss involving CO and Pt-CO stretching modes. The frequencies of the terminal Pt-CO and CO stretching modes we find for Pt/TiO, differ from those measured by Barb and Ibach on the flat and stepped Pt surfaces. Hydrogen chemisorption measurements on a sample of Pt/TiO, reduced a t 520 K indicate that the Pt particles are large, ca. 65 A, so bulklike Pt behavior is expected.16 The discrepancies may indicate that other crystal planes are preferentially exposed on the supported Pt particles. Blyholder and Sheets studied CO adsorbed onto Pt particles suspended in an oil film on salt wind o w~. '~ Their values for the CO stretching (2045 and 1815 cm-')and the Pt-C stretching (480 cm-I) modes were also different from those measured here. It may be that components of the hydrocarbon also chemically interact with the Pt particles. It is interesting to compare the effects of varying C O coverage on the frequencies of the T-CO and proposed Pt-CO bands. When the CO-saturated sample shown in Figure 1 is evacuated for 30 min at 523 K and then cooled to room temperature, we find the T-CO band intensity decreases by 20% and its position red shifts by 12 cm-l to 2071 cm-I. At the same time the H F band increases in frequency by 11 cm-' to 2494 cm-' and the calculated position of the Pt-CO fundamental increases in frequency by 21 cm-' to 423 cm-I. Analogous coverage-dependent frequency behavior has been found for the T-CO fundamental on supported and unsupported Pt. On a Pt foil the T-CO band appears near 2065 cm-' at low CO coverage and increases in frequency to 2100 cm-' as C O coverage approaches ~aturati0n.I~ This frequency shift is in large part ascribed to increasing intermolecular dipole cou-~1 i n g . l~ At very low coverages, donation of Pt d-electron density to empty 2?r* antibonding molecular orbitals on C O (Le. ?r backbonding) is thought to play a major role in defining the CO stretching frequency. On Pt/Ti02 the T-CO frequency drops wit...

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