Stepwise Dissociation of Thermally Activated Phenol on Pt(111)

The thermal reactions of phenol on Pt(111), including reaction intermediates, have been studied between 125 and 1100 K by using phenol- h 6, phenol- 2,4,6-d 3, phenol- 2,3,4,5,6-d 5, and phenol- d 6 with temperature-programmed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS), and X-ray photoelectron spectroscopy (XPS). Phenol adsorbs molecularly at 125 K, with the ring plane parallel to the surface. During heating, the O−H bond breaks below 200 K. The O−H bond dissociation product, phenoxy, has a quinoidal structure with η 5 -π-adsorption geometry via the C2 through C6 atoms with the C1 atom tilted away from the ring plane. Bonds in the phenoxy species rearrange toward oxocyclohexadienyl upon heating. The reactivity of phenoxy depends on the initial phenol coverage. Below 0.5 monolayer (ML), phenoxy fully decomposes to CO(g), H 2 (g), and C(a). Above 0.7 ML coverages, it follows two different pathways: (1) 85 ± 7% forms CO(g), H 2 (g), C 2 (a), and C 3 H 3 (a) at 490 K, the latter dehydrogenating to H 2 (g) and C(a) above 550 K, and (2) 15 ± 7% forms C 6 H 6 (g) and O(a) between 380 and 530 K. TPD and HREELS of isotopically labeled phenol indicate selective C2−H and C3−H dissociation within phenoxy. The common vibrational spectra and desorption features above 550 K during TPD of phenol, iodobenzene, and benzene strongly suggest a common intermediate species, C 3 H 3 . After annealing to 1100 K, only carbidic C remains.