Screening of MgO- and CeO2-Based Catalysts for Carbon Dioxide Oxidative Coupling of Methane to C_(2+) Hydrocarbons

摘要

The catalyst screening tests for carbon dioxide oxidative coupling of methane (CO_2-OCM) have been investigated over ternary and binary metal oxide catalysts. The catalysts are prepared by doping MgO- and CeO_2-based solids with oxides from alkali (Li_2O), alkaline earth (CaO), and transition metal groups (WO_3 or MnO). The presence of the peroxide (O_2~(2-)) active sites on the Li_2O_2, revealed by Raman spectroscopy, may be the key factor in the enhanced performance of some of the Li_2O/MgO catalysts. The high reducibility of the CeO_2 catalyst, an important factor in the CO_2-OCM catalyst activity, may be enhanced by the presence of manganese oxide species. The manganese oxide species increases oxygen mobility and oxygen vacancies in the CeO_2 catalyst. Raman and Fourier Transform Infra Red (FT-IR) spectroscopies revealed the presence of lattice vibrations of metal-oxygen bondings and active sites in which the peaks corresponding to the bulk crystalline structures of Li_2O, CaO, WO_3 and MnO are detected. The performance of 5%MnO/15%CaO/CeO_2 catalyst is the most potential among the CeO_2-based catalysts, although lower than the 2%Li_2O/MgO catalyst. The 2%Li_2O/MgO catalyst showed the most promising C_(2+) hydrocarbons selectivity and yield at 98.0% and 5.7%, respectively.