Adsorption and Oxidation of Ethylene on the Stoichiometric and O-Rich RuO2(110) Surfaces

We investigated the adsorption and oxidation of ethylene (C2H4) on stoichiometric and oxygen-rich RuO2(110) surfaces using temperature-programmed reaction spectroscopy (TPRS) and density functional theory (DFT) calculations. We find that C2H4 binds strongly on the coordinatively unsaturated (cus) Ru sites of RuO2(110), and desorbs in a peak at similar to 315 K during TPRS. According to DFT, C2H4 initially adsorbs in a pi-bonded configuration on atop Ru-cus sites but converts to a more stable di-sigma species of the form C2H4O prior to desorbing or reacting. Our TPRS results show that the stoichiometric RuO2(110) surface exhibits limited reactivity toward C2H4, whereas the O-rich surface is highly active toward promoting the extensive oxidation of C2H4. We find that the absolute yield of reacted C2H4 increases to a maximum with increasing initial coverage of on top O atoms (O-ot) on RuO2(110), and show that this behavior is accurately described by a model that assumes unit reaction probability of C2H4 molecules adsorbed at Rucus-O-ot, surface pairs. Our DFT calculations predict that the C-H bond cleavage of adsorbed C2H4 is energetically prohibitive on stoichiometric RuO2(110) relative to desorption, but that O-br-Ru-cus-O-ot surface ensembles provide facile reaction pathways on O-rich RuO2(110), wherein C-H bond cleavage of adsorbed C2H4 is strongly preferred over desorption.