Effect of rare earth oxides on the formation of gaseous products from low rank coal pyrolysis

Rare earth elements and their compounds can be used as catalysts or promoters for pyrolysis of organic matter including those in coal, but relevant investigations, especially regarding the role of rare earth materials in coal pyrolysis are very scarce. This study focuses on the effect of cerium oxide (CeO2) and lanthanum oxide (La2O3) on the formation of gaseous products from low-rank coal pyrolysis by means of gas chromatography (GC). The mechanisms of two kinds of rare earth oxides (REOs) involved in the coal pyrolysis are also discussed tentatively. Results show that CeO2 can promote the formation of CO2 (400 degrees C-600 degrees C), CO (550 degrees C-650 degrees C), CH4 (300 degrees C-500 degrees C), C2H6 and C3H8 (350 degrees C-750 degrees C), and it is believed to catalyze the bond breaking reactions of the aliphatic side chain, carboxyl group and phenolic hydroxyl group, or the bridge bonds between aromatic nuclei. The catalytic effect of CeO2 on the breaking of bridge bonds is likely to be completed by the movement of oxygen vacancies (i.e., oxygen storage and release ability of CeO2) in the water vapor environment. In addition, the results also reveal that it is La2O(CO3)(2), not La2O3, playing a role in coal pyrolysis, which can promote the formation of CO2 (500 degrees C-750 degrees C), H-2 and unsaturated hydrocarbons (350 degrees C-500 degrees C), and could be considered to be a catalyst for dehydrogenation at low-medium temperature. The mechanism of CO2 formation catalyzed by La2O(CO3)(2) is probably related to the decomposition of oxygen-containing ligands formed by La(III) and carboxyl group at high temperature.