RESUMO
CO2 capture and conversion are a key research field for the transition towards an economy only based on renewable energy sources. In this regard, hydride materials are a potential option for CO2 methanation since they can provide hydrogen and act as a catalytic species. In this work, Mg2NiH4 complex hydride is synthesized by in situ monitoring of mechanical milling under a hydrogen atmosphere from a 2MgH2:Ni stoichiometric mixture. Temperature and pressure evolution is monitored, and the material is characterized, during milling in situ, thus providing a good insight into the synthesis process. The cubic polymorph of Mg2NiH4 (S.G. Fm3[combining macron]m) starts to be formed in the early beginning of the mechanical treatment due to the mechanical stress induced by the milling process. Then, after 25 hours of milling, Mg2NiH4 with a monoclinic (S.G. C12/c1) structure appears. The formation of the monoclinic polymorph is most likely related to the stress release that follows the continuous refinement of the material's microstructure. At the end of the milling process, after 60 hours, the as-milled material is composed of 90.8 wt% cubic Mg2NiH4, 5.7 wt% monoclinic Mg2NiH4, and 3.5 wt% remnant Ni. The as-milled Mg2NiH4 shows high reactivity for CO2 conversion into CH4. Under static conditions at 400 °C for 5 hours, the interactions between as-milled Mg2NiH4 and CO2 result in total CO2 consumption and in the formation of the catalytic system Ni-MgNi2-Mg2Ni/MgO. Experimental evidence and thermodynamic equilibrium calculations suggest that the global methanation mechanism takes place through the adsorption of C and the direct solid gasification towards CH4 formation.
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.
