Unified mechanistic understanding on CO2 sorption by Na-doped Li4SiO4: Experimental and DFT study

The addition of molten Na2CO3 has drastically accelerated the extremely slow sorption kinetics of Li4SiO4-based CO2 sorbents in sorption enhanced reforming process. However, the lack of the unified consensus about the doped Na2CO3 on the CO2 capture process, now delays the development of more-effective Li4SiO4 sorbents. Here, sorption tests, sturactural characterzations and periodic density functional theory calculations were combined to develop a better mechanistic understanding CO2 sorption on Na-doped Li4SiO4. The Na-doped Li4SiO4 exhibited a better CO2 capture performance than pure Li4SiO4 even below the melting point of any eutectic mixtures. Structural characterizations revealed that rather than generating physical favorable characteristics, the Na doping substituted at the Li site and created lithium–sodium orthosilicates (Li4-xNaxSiO4). Theoretical calculations agreeing with experimental results further demonstrated that Li4-xNaxSiO4 presented a weaker Na-O bond and generated more active phase (quasi Li2O groups) for larger sorption energies, more charge transfers and stronger CO32– bond formation, in turn providing more reactive sites for enhanced CO2 chemisorption as comparing with pure Li4SiO4. Finally, we propose a new and unified sorption mechanism independent of the traditional "melting layer" explanation.