Methane production by mechanochemical processing of MgH2-Li2CO3 as sources of H2 and CO2 at room temperature

Abstract

The reutilization of CO2 to produce valuable chemical fuels is an attractive approach to reduce the greenhouse effect and global warming. In this work, the room temperature production of CH4/H2 fuel mixtures via mechanochemical activation of Li2CO3-MgH2 system was explored. The CH4/H2 fuel mixtures were obtained by a free-catalyst reaction, with high selectivity and using Li2CO3 and MgH2 as solid portable sources of CO2 and H2, respectively. The effect of processing parameters on both the evolution of the solid phases and the production of CH4/H2 fuel mixtures was studied using X-ray diffraction, spectroscopic techniques, electron scanning microscopy and gas chromatography. Thermodynamic calculations predict the complete CO2 reduction to CH4, with simultaneous formation of MgO and Li2O. Experimental evidence supports the progressive formation of MgO and the appearance of Li2O. The presence of H2 simultaneously with CH4 in the gas phase was due to kinetic restrictions. Based on experimental results and equilibrium composition calculations, the reaction mechanism was proposed. The kinetic analysis reveals that the rate limiting CH4 production is the nucleation of MgO/Li2O in the surroundings of Li2CO3, while the H2 release is controlled by the reaction interface between MgH2/Mg or MgH2/MgO.