Carbon Dioxide Capture by Niobium Polyoxometalate Fragments

Abstract

High oxidation state metal cations have diverse roles in carbon dioxide removal (direct air capture and at-the-source) includ-ing providing basic oxygens for chemisorption reactions, direct binding of carbonate, and low-temperature release of CO2 for regeneration of capture media. Moreover, metal oxide systems and aqueous metal-oxo species are stable in harsh condi-tions. Here we demonstrate the carbon capture ability of niobium polyoxometalates (POMs) as aqueous solutions, specifi-cally [Nb6O19]8-, Nb6. Upon exposure of Nb6 solutions to CO2, Nb6 fragments and binds carbonate, evidenced by crystalliza-tion of Nb-carbonate POMs including [Nb22O53(CO3)16]28-and [Nb10O25(CO3)6]12−. While Rb/Cs+ counter cations yielded crystal structures to understand the chemisorption processes, K+ counter cations enabled higher capture efficiency (based on CO3:Nb ratio), as determined by CHN analysis and thermogravimetry-mass spectrometry of the isolated solids. Sum fre-quency generation (SFG) spectroscopy also showed higher carbon capture efficiency of the K-Nb6 solutions at the air-water interface, while small-angle X-ray scattering (SAXS) provided insight into the role of the alkalis in influencing these process-es. Tetramethylammonium counter cations (TMA), like K+, demonstrated high efficiency of carbonate chemisorption at the interface, but SAXS and Raman of the bulk showed a predominance of a Nb24-POM (HxNb24O72, x~9) that can bind car-bonate only minimally, and any carbonate present is likely free bicarbonate instead of Nb-bound. Control experiments show that all carbonate present at the interface is Nb-bound, and the Nb-carbonate species are stabilized by alkalis, demon-strating a synergistic role of alkalis and high oxidation state metal cations in chemisorption of CO2. Of fundamental im-portance, this study presents rare examples of directing POM speciation with a gas, instead of liquid phase acid or base.