Surfactant-Assisted Preparation of Nanorod La2O3 Catalysts for Oxidative Coupling of Methane

It is generally agreed upon that oxidative coupling of methane is structure-sensitive over the La2O3 catalysts and that morphology such as nanowire or nanorod exhibited the highest activity. Therefore, three crystalline La2O3 catalysts were synthesized by the hydrothermal method without (denoted as La-NS) or with polyvinylpyrrolidone (PVP) (denoted as La-PVP) or cetyltrimethylammonium bromide (denoted as La-CTAB) as the surfactant. The HRTEM results suggested (110) and (101) planes were both exposed in La-PVP, while only the (101) facet in La-NS and the (002) facet in La-CTAB were exposed. CO2-TPD characterization showed that La-PVP exhibited a larger capacity of CO2 adsorption and more amounts of moderate basic sites than La-NS and La-CTAB. Then, the formation of the chemisorbed oxygen species O2– was facilitated. In situ DRIFT spectra revealed that surface O2– species are the activated oxygen species which would convert methane to a methyl radical and then couple to C2+ hydrocarbons during the OCM reaction over the nanorod La2O3 catalysts. Therefore, the La-PVP catalyst outperformed its peers in both activity and selectivity.