Bonding and thermal decomposition of propylene, propadiene, and methylacetylene on the Rh(111) single-crystal surface

The reactions of propylene, propadiene, and methylacetylene with Rh(111) crystal faces in ultrahigh vacuum (UHV) from 80 to 800 K have been studied by low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS), and thermal desorption spectroscopy (TDS). All three hydrocarbons adsorb intact at 80 K. At this temperature, methylacetylene forms a p(2×2) LEED pattern while propylene and propadiene adsorb as disordered monolayers. Above 200 K, propylene and propadiene also form p(2×2) LEED patterns which are stable up to 270 K. The propylene fragment in this temperature range is propylidyne (CCH₂CH₃). By room temperature all the C₃ hydrocarbons have decomposed to ethylidyne (CCH₃) and polymerized C_xH fragments. Surface hydrogen is instrumental in the C-C bond breaking, and partial deuteriation studies were performed to determine which C-C bonds break first. The ethylidyne species are stable up to 400 K, above which they decompose to C_xH fragments. The bonding and the thermal decomposition pathways for propylene on Rh(111) are compared to Pt(111) surface chemistry.