DEHYDROGENATION, HYDROGENATION, EXCHANGE AND COMBUSTION REACTIONS OF AROMATIC-HYDROCARBONS ON PD(100)

The reactivity of aromatic hydrocarbons (benzene, toluene and styrene) has been studied on the clean Pd(100), Pd(100)-p(1 x 1)-H(D) and Pd(1 00)-p(2 x 2)-O surfaces using temperature-programmed reaction spectros copy (TPRS) over a temperature range from 125 to 1065 K. On Pd(100) al l three hydrocarbons are adsorbed molecularly at 125 K. Upon heating t he sample, part desorbs molecularly and the remainder dehydrogenates t o C(a) and H(a), the latter evolving associatively as H-2. Dehydrogena tion proceeds to temperatures above 700 K. On Pd(100)-p(1 x 1)-H styre ne can be hydrogenated to ethylbenzene, whereas no hydrogenation occur s for either benzene or toluene. Multiple H-D exchange is observed on Pd(100)-p(1 x 1)-D for benzene, toluene and styrene, and is proposed t o occur through reversible C-H bond scission. The exchange reactivity correlates well with the order of the homolytic C-H bond strengths in the aromatic hydrocarbons. On Pd(100)-p(2 x 2)-O all H atoms in the ar omatic hydrocarbons can react below 500 K to form H2O; further, some C atoms in the molecule react to form CO2 below 500 K. Compared with co mplete dehydrogenation on the clean surface, O(a) activates the C-H bo nd scission by direct reaction with H and/or direct attack of C, which constitutes a direct combustion channel, distinct from the indirect p rocess of dehydrogenation followed by oxidation. Oxidation of styrene is similar to that of buta-1,3-diene, suggesting that reactions occur initially on the vinyl group.