The chemisorption of acetylene and ethylene on Rh(111): A low energy electron diffraction (LEED), high resolution electron energy loss (ELS), and thermal desorption mass spectrometry (TDS) study

Abstract

We report the results of a detailed investigation of the chemisorption and reactivity of acetylene (C₂H₂) and ethylene (C₂H₄) on the Rh(111) single crystal surface. Below 270 K ELS measurements indicate that acetylene chemisorbs on Rh(111) with its C–C bond oriented parallel to the surface forming an approximately sp² hybridized species. LEED investigations show that both C₂H₂ and C₂H₄ form metastable (2×2) surface structures on Rh(111) below 270 K. An irreversible order–order transformation occurs between 270 and 300 K to a stable c(4×2) hydrocarbon overlayer. The stable species formed from both molecules are identical. Hydrogen addition to chemisorbed acetylene is necessary to complete this conversion. The geometry of the adsorbed ethylene species does not change during this transformation although the overlayer structure does. This stable hydrocarbon species is identical to the hydrocarbon species formed from the chemisorption of either C₂H₄ or C₂H₄ and hydrogen on Pt(111) above 300 K. The addition of H₂ to chemisorbed C₂D₂ or C₂D₄ results in H–D exchange, but no change in the adsorbate geometry is detected by ELS. Decomposition of these molecules occurs on the Rh(111) surface above ∼420 K.