The effect of adsorbed oxygen for selectivity of acetophenone (AP) hydrogenation on Pd/SiO2 catalyst at 298 K has been studied by means of gas phase acetophenone hydrogenation, infrared (IR) spectra, and temperature-programmed desorption. Acetophenone hydrogenation on reduced Pd/SiO2 catalyst reveals a typical series reaction in which phenylethanol (PE) is the intermediate for ethylbenzene (EB) formation. The selectivity of the reaction is towards phenylethanol at low temperature. The oxidized Pd/SiO2 catalyst exhibits very different catalytic selectivity with reduced catalyst. The selectivity of ethylbenzene can be significantly boosted to over 90%, even if the reaction approaches zero conversion, suggesting that phenylethanol needs not be an intermediate for production of ethylbenzene from acetophenone. The formation of ethylbenzene and phenylethanol on oxidized Pd may be controlled by a parallel reaction pathway. The numbers of adsorbed oxygen on Pd surface strongly dominate the rate of EB formation. The bulk Pd oxide cannot be reduced by hydrogen at 298 K, so the oxygen atoms in Pd bulk act a poison for AP hydrogenation, leading to deactivation of oxidized Pd catalyst. The adsorbed oxygen on Pd surface plays the important role that can activate the C-H bond of CH3 group in acetophenone, leading to the formation of a new intermediate (perhaps acetophenone enolate). This intermediate is the key species that will be further hydrogenated to ethylbenzene. (C) 2003 Elsevier B.V. All rights reserved.