Fig. 5. F–Zn(ii) catalytic carbon dioxide hydration and sequestration.

a, Schematic representation of F–Zn(ii) catalytic CO₂ hydration and sequestration on reaction with CaCl₂ to produce solid CaCO₃. b, UV–visible absorption spectral changes of the pH indicator phenol red (5 × 10⁻⁵ M) in a mixture containing CO₂-treated deionized water in Tris buffer (pH 8) with (+) and without (−) the F–Zn(ii) (3.7 × 10⁻⁴ M) catalyst. The black arrows show the changes in the peaks following addition of F-Zn(ii). Inset: images of the corresponding solutions without (left) and with (right) F–Zn(ii). c, The time-dependent absorbance at 560 nm and the corresponding pH profile of phenol red in CO₂-treated deionized water, Tris buffer (pH 8) in the absence (−) and presence (+) of F–Zn(ii). d, Stepwise drop in absorbance at 560 nm and respective pH by sequential addition of CO₂-treated deionized water (10 μl, red arrows) into the Tris buffer (25 mM, pH 8) containing F–Zn(ii). e, Scanning electron micrograph of the solid precipitate after sequestration of CO₂-treated F–Zn(ii) aqueous solution with CaCl₂. f, Energy dispersive X-ray spectrum of the precipitate showing calcium (Ca), along with carbon (C), oxygen (O) and zinc (Zn) elements.