In-situ Raman Spectroscopic Study of La₂O₃-BaO Catalysts

For a series of La-Ba-Ca-（O,CO₃） catalysts under methane-oxidative-coupling conditions at 973K,the Ca-free sample was characterized by Raman bands due to surface carbonates of BaCO₃ and （LaO）₂（O,CO₃） phases,and a weak and broad band around 810cm^-1 ,probably due to O₂^2-;the Ca-containing samples showed Raman bands of surface carbonates associated with （Ba,Ca）CO₃ phase and minor amount of BaCO₃ phase （but not with lanthania phase）,and active-oxygen bands at 1135cm^-1 （w） and 810cm^-1 （w,br） attributed to O₂^- and O₂^2- transient species,respectively.At 973～1053K,the activities of all the samples were found to increase with increasing OCM temperature and with increasing calcium contents.The C₂-selectivities increased with increasing temperature for all the samples,and increased with increasing calcium contents at 973K,but decreased slightly with increasing calcium contents at 1053K.These results strongly indicate that O₂^- might be an active oxgen-species reacting with CH 4 with higher C₂-selectivity and higher activation energy,becoming sufficiently active only at higher temperature.