Tetraperoxotitanates for High Capacity Direct Air Capture of Carbon Dioxide

Materials chemists play a strategic role towards achieving ambitious global climate goals, including removing legacy CO2 via direct air capture (DAC). Innovating diverse DAC materials will enable their effective use in varying conditions, and bring forth a better understanding of CO2 capture mechanisms. In our current contribution, we have synthesized a family of homoleptic alkali tetraperoxotitanate materials (generally formulated A4Ti(O2)4, A=Li, Na, K) and studied their DAC reactivity. Synthesis was achieved with inexpensive reagents, and >90% yields. We present the first single-crystal X-ray structures (five total) of A4Ti(O2)4 compounds, along with supplemental bulk characterization and computation. We compare their DAC behavior in simple ambient benchtop experiments, determining CO2 uptake by combustion analysis of post-capture materials. The K-analogue exhibited the most rapid and high capacity DAC, 8.17 mmol CO2/gram sorbent, translating to nearly 3 moles CO2 per mole Ti, and reaching near maximum capacity in under 10 days. The Li and Na analogues both exhibit delayed reactivity, but also with high DAC capacity (respectively 6.66 and 8.18 mmol CO2/gram sorbent). Characterization of the DAC products via scanning electron microscopy shows phase separation of alkali-rich and Ti-rich regions in core-shell morphologies for the Na and Li analogues, and this is discussed with respect to the role of the titanium vs the alkali in DAC. On the other hand, no phase separation was observed for the K-analogue. In situ monitoring detailed the early-stage CO2 capture behavior of the K-analogue, and it reaches ~50% of maximum capacity within one hour. The differentiating behavior of the K-analogue is attributed to its unique composition, containing four H2O2 lattice molecules in addition to the four O2- peroxide anions bonded to TiIV. While H2O2 (aq) alone does not exhibit CO2 chemisorption, the basic environment of the A4Ti(O2)4 lattice activates its rapid DAC, inspiring future exploration of peroxosolvate materials for DAC.