MODIFICATION OF THE BENZENE-METAL INTERACTION BY C AND H MONOLAYERS AT PT(111) SURFACES

The interaction of benzene with clean and C (H) monolayers covered Pt( 111) surfaces was studied with vibrational (HREELS) and thermal desorp tion (TDS) spectroscopies at 125 K. At clean Pt, first, second and mul tilayer benzene adsorption was found to exhibit characteristic feature s in TD and HREEL spectra, Benzene in the first (chemisorbed) layer pa rtially decomposes, partially desorbs molecularly around 400 K. Benzen e in the second and multilayers desorbs intact around 163 and 150 K, r espectively. HREEL spectra indicate that chemisorbed benzene lies flat at the surface, while it is tilted in multilayers. Adsorption of C or H monolayers prior to benzene adsorption removes the chemisorbed stat e. HREEL spectra suggest that benzene lies flat at Pt(111)/C and Pt(11 1)/H and from the frequency of the prominent out-of-plane CH bending m ode it is inferred that the metal gets effectively shielded. Monolayer s of benzene at sandwiched C and H layers behave like second layer ben zene at clean Pt. Thus, only molecular desorption is observed in that case. The effect of H could be achieved either by predosing hydrogen t o clean Pt or by exposing H atoms to benzene covered Pt surfaces.