Dehydrogenation, hydrogenation, exchange and combustion reactions of aromatic hydrocarbons on Pd(100)

Abstract

The reactivity of aromatic hydrocarbons (benzene, toluene and styrene) has been studied on the clean Pd(100), Pd(100)-p(1 × 1)-H(D) and Pd(100)-p(2 × 2)-O surfaces using temperature-programmed reaction spectroscopy (TPRS) over a temperature range from 125 to 1065 K. On Pd(100) all three hydrocarbons are adsorbed molecularly at 125 K. Upon heating the sample, part desorbs molecularly and the remainder dehydrogenates to C(a) and H(a), the latter evolving associatively as H₂. Dehydrogenation proceeds to temperatures above 700 K. On Pd(100)-p(1 × 1)-H styrene can be hydrogenated to ethylbenzene, whereas no hydrogenation occurs for either benzene or toluene. Multiple H–D exchange is observed on Pd(100)-p(1 × 1)-D for benzene, toluene and styrene, and is proposed to 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 × 2)-O all H atoms in the aromatic 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 complete dehydrogenation on the clean surface, O(a) activates the C—H bond scission by direct reaction with H and/or direct attack of C, which constitutes a direct combustion channel, distinct from the indirect process 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.