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

Abstract

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-U₁ and Rb-U₁). 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-U₁, the most unstable of the uranyl triperoxide monomers, undergoes ambient direct air capture of CO₂ in the solid-state, converting to Cs₄[U^VIO₂(CO₃)₃], evidenced by single-crystal X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. We have attempted to benchmark the evolution of Cs-U₁ 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^VO₃, where the parent phase has been computationally predicted, but not yet synthesized.

Keywords: CO2 direct air capture; SAXS; X-ray structure; alkalis; peroxide; uranyl.