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

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.