Epoxides as probes of oxametallacycle chemistry on Rh(111)

Abstract

Temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS) were used to investigate the reactions of ethylene oxide and propylene oxide on the clean Rh(111) surface. This study was performed to determine the reaction pathways and intermediates involved during decomposition of epoxides as a means of understanding the divergence of alcohol and aldehyde decomposition on Rh(111). Ethylene oxide decomposed via C-O scission of the ring. Decarbonylation was observed to have begun by 221 K to release adsorbed carbon monoxide and hydrocarbon fragments. Volatile carbon monoxide and di-hydrogen were the only desorbing products detected. Carbon was also deposited on the Rh(111) surface during ethylene oxide decarbonylation. Similar behavior was observed during propylene oxide decarbonylation. Decarbonylation of the adsorbed species began by 232 K. Again surface carbon was detected as well as volatile carbon monoxide and di-hydrogen. No volatile hydrocarbon products were observed during the decomposition of either epoxide. The reaction pathways and products observed are quite consistent with those observed for ethanol and 1-propanol, but not for the aldehyde isomers of these epoxides. The results of this study support the hypothesis that alcohol decarbonylation occurs via oxametallacycle intermediates on Rh(111), since the chemistry of the epoxides mimics that of the alcohols rather than that of the aldehydes. However, these intermediates were insufficiently stable to permit their isolation and spectroscopic verification by HREELS.