In Situ XRD and Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy Unravel the Deactivation Mechanism of CaO-Based, Ca3Al2O6-Stabilized CO2 Sorbents

CaO is an effective high temperature CO2 sorbent that, however, suffers from a loss of its CO2 absorption capacity upon cycling due to sintering. The cyclic CO2 uptake of CaO-based sorbents is improved by Ca3Al2O6 as a structural stabilizer. Nonetheless, the initially rather stable CO2 uptake of Ca3Al2O6-stabilized CaO yet starts to decay after around 10 cycles of CO2 capture and sorbent regeneration, albeit at a significantly reduced rate compared to the unmodified reference material. Here, we show by a combined use of in situ XRD together with textural and morphological characterization techniques (SEM, STEM, and N-2 physisorption) and solid-state Al-27 NMR (in particular dynamic nuclear polarization surface enhanced NMR spectroscopy, DNP SENS) how microscopic changes trigger the sudden onset of deactivation of Ca3Al2O6-stabilized CaO. After a certain number of CO2 capture and regeneration cycles (approximately 10), Ca3Al2O6 transformed into Ca12Al14O33, followed by Al2O3 segregation and enrichment at the surface in the form of small nanoparticles. Al2O3 in such a form is not able to stabilize effectively the initially highly porous structure against thermal sintering, leading in turn to a reduced CO2 uptake.