E N V I R O N M E N T , E A R T H A N D P L A N E T A R Y S C I E N C E S
S C I E N T I F I C H I G H L I G H T S
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Real-time monitoring of the structural dynamics of a working CO2 sorbent using X-ray powder diffraction
To reach net-zero emission targets, it is vital to develop effective CO2 sorbents. When promoted with alkali salts, MgO can capture significant amounts of CO2 via carbonation. In-situ, time-resolved X-ray powder diffraction reveals the structural dynamics in NaNO3-promoted MgO and their effect on the rate of CO2 capture.
To combat global warming, it is essential to minimise human-made emissions of carbon dioxide (CO2) into the atmosphere. In this context, CO2 capture technologies can contribute to achieving sustainability goals. However, the implementation of CO2 capture in combustion processes necessitates the development of effective CO2 sorbents. In this context, MgO offers a high theoretical gravimetric CO2 capacity (1.09 g CO2 per g MgO), high abundance and environmental benignity. MgO captures and releases CO2 in the temperature range of 300 550°C via carbonation [1]: MgO+CO2 ↔ MgCO3 ΔH300K = 106.05 kJ mol 1 Despite its high theoretical uptake capacity, the carbonation of bare MgO is kinetically limited and restricted to the MgO surface, resulting in low CO2 uptake. Yet the discovery of promoters, such as molten alkali nitrates, has made MgO-based materials promising for CO2 sorption. The key goals for sorbent development are to increase CO2 sorption rates and improve cyclic stability. To avoid ineffective trial and error approaches, it is crucial to gain an understanding of (i) how promoters facilitate MgO carbonation, (ii) the chemical and structural transformations the sorbents undergo under operational conditions, and (iii) the hitherto uncertain impact of these changes on CO2 capture performance [2].
To unveil which atomic-scale features of MgO-based CO2 sorbents determine their CO2 absorption rate, the structural evolution of a NaNO3-promoted MgO-based sorbent was monitored over several carbonation-regeneration cycles using in-situ synchrotron powder X-ray diffraction (PXRD) at beamline ID31 using a capillary cell reactor. The in-situ, time-resolved (1 s per scan) experiments, in combination with parametric Rietveld refinements, followed the structural evolution of the sorbent over repeated carbonation-regeneration cycles.
The CO2-sorbent showed a deactivation during the second and third cycle, followed by a re-activation during the following cycles, reaching a high CO2 uptake after the seventh cycle (Figure 110a). Comparing the CO2 uptake with the rate constant and the induction period as a function of cycle number, it was concluded that the observed deactivation-reactivation behaviour of the sorbent was due to changes in the kinetics of MgCO3
Fig. 110: CO2 capture performance and structural
descriptors. a) CO2 uptake at t = 85 min, (b) reaction
rate constant (k), (c) Avrami exponent (n), (d) induction
period, (e) surface to volume ratio
