Inhibition of C-H and C-O bond activation by surface oxygen: stabilization of surface phenoxide in the reaction of phenol on oxygen-precovered Mo(110)

The presence of surface oxygen on Mo(110) is shown to increase the kinetic stability of surface phenoxide formed after saturation phenol exposure with respect to decomposition. Temperature-programmed reaction and x-ray photoelectron spectroscopies have been used to study the reactions of phenol (C{sub 6}H{sub 5}OH) on oxygen-precovered Mo(110) ({theta}{sub 0} = 0.33). At saturation coverage, surface phenoxide is formed from cleavage of the phenol O-H bond at temperatures below 400 K and is stable on the surface to 650 K, where it undergoes disproportionation to gaseous phenol (E{sub d} = 42 kcal/mol) and the decomposition products gaseous water, gaseous dihydrogen, and surface carbon. A trace amount of benzene is also produced from decomposition of surface phenoxide. The hydrogen released by O-H bond cleavage in phenol at low temperature reacts with oxygen in the oxide overlayer to form gaseous water at temperatures below 400 K. Intact molecular phenol is also desorbed at temperatures below 350 K. The atomic oxygen overlayer dramatically stabilizes C-O and C-H bonds in the surface phenoxide species, which are cleaved at {approx} 370 K on the clean Mo(110) surface. The stability of surface phenoxide on the oxygen-precovered surface is dependent on the coverage of phenol-derived surface species, with decomposition occurring at lower temperature for lower phenol exposures. No gaseous phenol is reformed from phenoxide for low phenol exposures.