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 desorption (TDS) spectroscopies at 125 K. At clean Pt, first, second and multilayer benzene adsorption was found to exhibit characteristic features in TD and HREEL spectra, Benzene in the first (chemisorbed) layer partially decomposes, partially desorbs molecularly around 400 K. Benzene in the second and multilayers desorbs intact around 163 and 150 K, respectively. 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 state. HREEL spectra suggest that benzene lies flat at Pt(111)/C and Pt(111)/H and from the frequency of the prominent out-of-plane CH bending mode it is inferred that the metal gets effectively shielded. Monolayers of benzene at sandwiched C and H layers behave like second layer benzene at clean Pt. Thus, only molecular desorption is observed in that case. The effect of H could be achieved either by predosing hydrogen to clean Pt or by exposing H atoms to benzene covered Pt surfaces.