The Role of Alkalis in Orchestrating Uranyl‐Peroxide Reactivity Leading to Direct Air Capture of Carbon Dioxide

Spectator ions have known and emerging roles in aqueous metal‐cation chemistry, respectively directing solubility, speciation, and reactivity. Here, we isolate and structurally characterize the last two metastable members of the alkali uranyl triperoxide series, the Rb + and Cs + salts (Cs‐ U 1 and Rb‐ U 1 ). We document their rapid solution polymerization via small‐angle X‐ray scattering, which is compared to the more stable Li + , Na + and K + analogues. To understand the role of the alkalis, we also quantify alkali‐hydroxide promoted peroxide deprotonation and decomposition, which generally exhibits increasing reactivity with increasing alkali size. Cs‐ U 1 , the most unstable of the uranyl triperoxide monomers, undergoes ambient direct air capture of CO 2 in the solid‐state, converting to Cs 4 [U VI O 2 (CO 3 ) 3 ], evidenced by single‐crystal X‐ray diffraction, transmission electron microscopy, and Raman spectroscopy. We have attempted to benchmark the evolution of Cs‐ U 1 to uranyl tricarbonate, which involves a transient, unstable hygroscopic solid that contains predominantly pentavalent uranium, quantified by X‐ray photoelectron spectroscopy. Powder X‐ray diffraction suggests this intermediate state contains a hydrous derivative of CsU V O 3 , where the parent phase has been computationally predicted, but not yet synthesized.