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

The addition of molten Na 2 CO 3 can drastically accelerated the extremely slow sorption kinetics of Li 4 SiO 4 -based CO 2 sorbents in the sorption enhanced reforming process. However, the lack of a unified consensus about the effect of doped Na 2 CO 3 on the CO 2 capture process, now delays the development of more-effective Li 4 SiO 4 sorbents. Here, sorption tests, sturactural characterzations and periodic density functional theory calculations were combined to develop a better mechanistic understanding CO 2 sorption on Na-doped Li 4 SiO 4 . The Na-doped Li 4 SiO 4 exhibited a better CO 2 capture performance than pure Li 4 SiO 4 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 (Li 4-x Na x SiO 4 ). Theoretical calculations agreeing with experimental results further demonstrated that Li 4-x Na x SiO 4 presented a weaker Na-O bond and generated more active phases (quasi Li 2 O groups) for larger sorption energies, more charge transfers and stronger CO 3 2– bond formation, in turn providing more reactive sites for enhanced CO 2 chemisorption as comparing with pure Li 4 SiO 4 . Finally, we propose a new and unified sorption mechanism independent of the traditional “melting layer” explanation.