Tetraperoxotitanates for High Capacity Direct Air Capture of Carbon Dioxide

Abstract

Materials chemists play a strategic role towards meeting ambitious global climate goals of net-zero CO2 emissions by 2050. It is important to develop a suite of direct air capture (DAC) materials that can effectively remove legacy CO2, bringing the atmospheric concentration closer to pre-industrial levels, and halt the current steady rise. A diversity of DAC materials will improve flexible functionality in varying ambient conditions, and bring forth a better understanding of CO2 chemisorption and physisorption mechanisms. In our current contribution to this mission, we have developed a general synthesis for A4Ti(O2)4 (A=Li, Na, K), alkali tetraperoxotitanates, and present their crystal structures and DAC reactivity in both ambient year-long experiments and enriched CO2 environment studies. Characterization of DAC products by CHN analysis, thermogravimetry-mass spectrometry (TGA MS), Fourier transform infrared spectrometry (FTIR), Raman spectrometry, powder X-ray diffraction (PXRD), and scanning electron microscopy-energy dispersive X-ray spectrometry (SEM EDX) all point toward a mechanism of Ti-centered CO2 chemisorption for the K-analogue, and hydrolysis/autodegradation for the Li and Na analogues. K4Ti(O2)4 exhibits remarkable DAC capacity of 8.73 mmol CO2/gram sorbent, reaching nearly maximum capacity in 20 days. This translates to nearly three CO2 molecules chemisorbed, per Ti-center. Na4Ti(O2)4 and Li4Ti(O2)4 are much slower, but also with high capacity (respectively 6.74 and 8.33 mmol CO2/gram sorbent). Characterization of the Na4Ti(O2)4 DAC products via SEM EDX elemental mapping and imaging shows phase separation of Na-rich and Ti-rich phases in a core-shell morphology, where the Ti-rich core is largely passivated by the Na-rich coating. This both sheds light on its poorer and delayed performance, and also points toward the superior performance of TiIV in CO2 chemisorption by DAC, in comparison to its alkali counter cations.