Stoichiometric hydrogenation of ethene on Rh(111); mechanism, importance of weakly adsorbed ethene, and relationship to homogeneous catalysis

Abstract

The hydrogenation of ethene is an important reaction in heterogeneous catalysis and, despite its apparent simplicity, many aspects of the reaction mechanism remain unclear. By contrast, the mechanism using homogeneous catalysts such as Wilkinson's catalyst [(RhCl(PPh₃)₃] is thought to be well understood. To allow a comparison between the homogeneous and heterogeneous reactions we have studied ethene/hydrogen interactions on the (111) plane of rhodium in the temperature range 160–500 K. Under UHV conditions no catalytic reaction was detected. However, we have been able to observe stoichiometric hydrogenation and exchange in the chemisorbed layer. A mixed adlayer of either ethene/deuterium (or perdeuteroethene and hydrogen) was formed at ca. 160 K, and allowed to warm up. From previous spectroscopic studies, ethene is adsorbed at 165 K as partially rehybridised,π bonded species with a C-C bond order of ca. 1.5, similar to ethene in Zeise's salt. At 190–210 K we observe coincident desorption of undeuterated ethene — the major species — together with much smaller quantities of deuterated ethane and partially deuterated ethenes. The influence of both hydrogen and ethene pre-coverage has been studied as has the relative extent of hydrogenation and exchange. The ethane formation results parallel those reported by other authors on Pd(110) and Pt(111) and Pt(110). We propose that on all three metals both hydrogenation and exchange follow the same pathway, with a common intermediate for exchange and hydrogenation. This isa weakly held, π bonded species formed during the desorption process, which can be convertedreversibly into an adsorbed ethyl species. A detailed comparison indicates that the mechanism of heterogeneous hydrogenation closely parallels that in the homogeneous phase.