The surface structure and thermal decomposition of acetylene on the Rh(111) single crystal surface and the effect of coadsorbed carbon monoxide

Abstract

The surface structure and thermal chemistry of acetylene on the Rh(111) crystal surface have been studied from 30 to 800 K using high resolution electron energy loss spectroscopy (HREELS), low energy electron diffraction (LEED), and thermal desorption spectroscopy (TDS). Adsorbed acetylene on the Rh(111) is disordered at 30 K, but orders into a (2×2) LEED structure at 60 K; this is the lowest ordering temperature that has been reported for chemisorbed acetylene on a metal surface. HREEL spectra of C 2H 2 and C 2D 2 ordered in the (2×2) structure at 77 K on Rh(111) indicated that the carboncarbon bond of chemisorbed acythylene rehybridizes between sp 2 and sp 3. At 270 K, chemisorbed molecular acetylene decomposes predominantly to CCH 2 species. Above 400 K, further decomposition to a mixture of CCH and CH species is observed. When CO and molecular acetylene are coadsorbed at 220 K, we observe a c(4×2) LEED structure rather than the (2×2) LEED structures observed for acetylene or CO adsorbed alone on Rh(111). Also, chemisorbed acetylene, when coadsorbed with CO, decomposes at 270 K to a mixture of CCH 3 (ethylidyne) and CCH (acetylide) species rather than the CCH 2 species.