Tetraperoxotitanates for High-Capacity Direct Air Capture of Carbon Dioxide

Abstract

Materials chemists play a strategic role in 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 improve our 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, or 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 postcapture materials. The K analogue exhibited the most rapid and high-capacity DAC, 8.17 mmol CO₂/g sorbent, translating to nearly 3 mol CO₂ per mole Ti and reaching near maximum capacity in under 10 days. The Li and Na analogues exhibit delayed reactivity along with high DAC capacity (6.66 and 8.18 mmol of CO₂/g sorbent, respectively). 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; this is discussed with respect to the role of titanium vs 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, reaching ∼50% of maximum capacity within 1 h. 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 Ti^IV. While H₂O₂ (aq) alone does not exhibit CO₂ chemisorption, the basic environment of the A₄Ti(O₂)₄ lattice activates its rapid DAC, inspiring the future exploration of peroxosolvate materials for DAC.