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

Abstract

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 N2 physisorption) and solid-state 27Al 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...