Carbon Dioxide Capture by Niobium Polyoxometalate Fragmentation

High oxidation state metal cations in the form of oxides, oxoanions, or oxoperoxoanions have diverse roles in carbon dioxide removal (direct air capture and point source). Features include providing basic oxygens for chemisorption reactions, direct binding of carbonate, and catalyzing low-temperature CO<INF>2</INF> release to regenerate capture media. Moreover, metal oxides and aqueous metal-oxo species are stable in harsh, point-source conditions. Here, we demonstrate aqueous niobium polyoxometalate (POM) carbon capture ability, specifically [Nb<INF>6</INF>O<INF>19</INF>]8-, Nb<INF>6</INF>. Upon exposure of aqueous Nb<INF>6</INF> to CO<INF>2</INF>, Nb<INF>6</INF> fragments and binds chemisorbed carbonate, evidenced by crystallization of Nb-carbonate POMs including [Nb<INF>22</INF>O<INF>53</INF>(CO<INF>3</INF>)<INF>16</INF>]<SUP>28-</SUP>and [Nb<INF>10</INF>O<INF>25</INF>(CO<INF>3</INF>)<INF>6</INF>]12-. While Rb/Cs+ counter cations yield crystal structures to understand the chemisorption processes, K+ counter cations enable higher capture efficiency (based on CO<INF>3</INF>/Nb ratio), determined by CHN analysis and thermogravimetry-mass spectrometry of the isolated solids. Sum frequency generation spectroscopy also showed higher carbon capture efficiency of the K-Nb<INF>6</INF> solutions at the air-water interface, while small-angle X-ray scattering (SAXS) provided insights into the role of the alkalis in influencing these processes. Tetramethylammonium counter cations, like K+, demonstrate high efficiency of carbonate chemisorption at the interface, but SAXS and Raman of the bulk showed a predominance of a Nb<INF>24</INF>-POM (H<INF>x</INF>Nb<INF>24</INF>O<INF>72</INF>, x similar to 9) that does not bind carbonate. Control experiments show that carbonate detected at the interface is Nb-bound, and the Nb-carbonate species are stabilized by alkalis, demonstrating their supporting role in aqueous Nb-POM CO<INF>2</INF> chemisorption. Of fundamental importance, this study presents rare examples of directing POM speciation with a gas, instead of liquid phase acid or base.