RESUMO
Carbon capture and storage using regenerable sorbents are an effective approach to reduce CO2 emissions from stationary sources. In this work, lithium orthosilicate (Li4SiO4) was studied as a carbon dioxide sorbent. For a deeper understanding of the synthesis and carbonation mechanism of Li4SiO4, an in situ synchrotron radiation powder X-ray diffraction technique was used. The Li4SiO4 powders were synthesized by a combination of ball milling of a Li2CO3 and SiO2 mixture followed by a thermal treatment process at low temperature. In situ studies showed that formation of Li4SiO4 from the as-milled 2Li2CO3-SiO2 mixture involves decomposition of Li2CO3 by reaction with SiO2via Li2SiO3 as an intermediate compound. No evidence of Li2Si2O5 formation was obtained, in spite of thermodynamic predictions. The CO2 capture by Li4SiO4 was evaluated dynamically over a wide temperature range, reaching a maximum weight increase of 34 wt% and good cyclability after about 10 cycles. By thermogravimetric and microstructural analyses in combination with ex situ and in situ measurements, a two step carbonation mechanism and its influence on the final CO2 capture was clearly elucidated. Under dynamical conditions up to 700 °C, the lower number of Li2CO3 nuclei initially formed retards the double shell formation and the nucleation and growth of the Li2CO3 particles remains the controlling step up to higher CO2 capture capacity. Isothermal carbonation at 700 °C favours the formation of a higher number of Li2CO3 nuclei that creates a thin carbonate shell. The CO2 diffusion through this shell is the limiting step from the beginning and further carbonation is hindered as the reaction progresses.
RESUMO
Lithium fast-ion conductors have shown positive effects on the hydrogen storage properties of the Li-Mg-N-H system. In the present work, Li4(NH2)3BH4 doped Mg(NH2)2-2LiH was formed by milling the 2LiNH2-MgH2-0.2LiBH4 composite and posterior annealing under hydrogen pressure to reduce the kinetic barrier of the Li-Mg-N-H system. The effect of repetitive dehydrogenation/rehydrogenation cycles on the kinetic and thermodynamic performance was evaluated. The dehydrogenation rate in the doped composite was twice that in the un-doped sample at 200 °C, while hydrogenation was 20 times faster. The activation energy decreases by 9% due to the presence of Li4(NH2)3BH4 compared to the un-doped composite, evidencing its catalytic role. The presence of Li4(NH2)3BH4 in the composite stabilized the hydrogen storage capacity after successive sorption cycles. Thermodynamic studies revealed a variation in the pressure composition isotherm curves between the first dehydrogenation cycle and the subsequent. The Li4(NH2)3BH4 doped composite showed a sloped plateau region at higher equilibrium pressure in regard to the flat plateau of the un-doped composite. Detailed structural investigations revealed the effective influence of Li4(NH2)3BH4 in different reactions: the irreversible dehydrogenation in the presence of MgH2 and the reversible hydrogen release when it reacts with Li2Mg2(NH)3. The role of Li4(NH2)3BH4 in improving the dehydrogenation kinetics is associated with the weakening of the N-H bond and the mobile small ion mass transfer enhancement.