Molecular pathways in the cyclotrimerisation of acetylene on Pd(111): Vibrational spectra of the C4H4 intermediate and its thermal decomposition products

HREELS and XPS have been used to study a hydrocarbon surface species, C4H4, which can be isolated on the Pd(111) surface at 150 K or 300 K by dissociative chemisorption of C4H4Cl4 (cis-3,4-dichlorocyclobutene). The C4H4 hydrocarbon is deduced to be a metallacycle and is a key intermediate in the formation of benzene from acetylene on Pd(111) and can also lead to ethylene formation at higher temperatures: it transforms to another species between 250 and 350 K, possibly vinylidene. Above 400 K, CH is the only hydrocarbon surface species detectable. HREEL spectra of the three hydrocarbon species are discussed in detail and compared to IR spectra of model compounds.