Tetraperoxotitanates\nfor High-Capacity Direct Air\nCapture of Carbon Dioxide

Materials chemists play a strategic role in achieving\nambitious\nglobal climate goals, including removing legacy CO<sub>2</sub> via\ndirect air capture (DAC). Innovating diverse DAC materials will enable\ntheir effective use in varying conditions and improve our understanding\nof CO<sub>2</sub> capture mechanisms. In our current contribution,\nwe have synthesized a family of homoleptic alkali tetraperoxotitanate\nmaterials (generally formulated A<sub>4</sub>Ti(O<sub>2</sub>)<sub>4</sub>, A = Li, Na, or K) and studied their DAC reactivity. Synthesis\nwas achieved with inexpensive reagents and >90% yields. We present\nthe first single-crystal X-ray structures (five total) of A<sub>4</sub>Ti(O<sub>2</sub>)<sub>4</sub> compounds along with supplemental bulk\ncharacterization and computation. We compare their DAC behavior in\nsimple ambient benchtop experiments, determining CO<sub>2</sub> uptake\nby combustion analysis of postcapture materials. The K analogue exhibited\nthe most rapid and high-capacity DAC, 8.17 mmol CO<sub>2</sub>/g sorbent,\ntranslating to nearly 3 mol CO<sub>2</sub> per mole Ti and reaching\nnear maximum capacity in under 10 days. The Li and Na analogues exhibit\ndelayed reactivity along with high DAC capacity (6.66 and 8.18 mmol\nof CO<sub>2</sub>/g sorbent, respectively). Characterization of the\nDAC products via scanning electron microscopy shows phase separation\nof alkali-rich and Ti-rich regions in core–shell morphologies\nfor the Na and Li analogues; this is discussed with respect to the\nrole of titanium vs alkali in DAC. On the other hand, no phase separation\nwas observed for the K analogue. In situ monitoring detailed the early-stage\nCO<sub>2</sub> capture behavior of the K analogue, reaching ∼50%\nof maximum capacity within 1 h. The differentiating behavior of the\nK analogue is attributed to its unique composition, containing four\nH<sub>2</sub>O<sub>2</sub> lattice molecules in addition to the four\nO<sub>2</sub><sup>–</sup> peroxide anions bonded to Ti<sup>IV</sup>. While H<sub>2</sub>O<sub>2</sub> (aq) alone does not exhibit\nCO<sub>2</sub> chemisorption, the basic environment of the A<sub>4</sub>Ti(O<sub>2</sub>)<sub>4</sub> lattice activates its rapid DAC, inspiring\nthe future exploration of peroxosolvate materials for DAC.