(298d) Investigating the Behaviour of Promising Mixed Oxides for Key CO2 Adsorption Technologies

The concentration of CO₂ in the atmosphere has reached record observed levels in 2023 (37.4 Gt), and real time data indicate a continued rise in 2024. This increase is not aligned with the global climate goals set out in the Paris Agreement to achieve a net zero scenario by 2050. Despite this, the total energy-related CO₂ emissions are showing a structural slowdown compared to economy growth rates.[1] The use of clean energy in tandem with CO₂ capture and storage (CCS) are responsible for this positive trend. Therefore, it is crucial to accelerate their deployment.

Gas-solid adsorption is a leading strategy to aid the production of clean fuels coupled with CCS. In this contribution, we investigate the suitability of promising mixed oxides to be used as CO₂ sorbents in key sustainable technologies such as sorption-enhanced H₂ production.[2,3] The materials explored include NaX zeolites, magnesium oxides (MgOs) and hydrotalcites (HTs), which exhibit attractive characteristics to adsorb CO₂ in diverse operating windows and reaction environments required for such processes.[4]

The mixed oxides comprise commercial and as-synthesised adsorbents. The materials are characterised systematically using several physicochemical techniques. The CO₂ adsorption-desorption isotherms of the solids at relevant temperature and pressure ranges (473-773 K, 1-10 bar) are carried out using primarily Thermal Gravimetric Analysis. The stability of the adsorbents is assessed by conducting pressure and temperature-swing cycles (PSA and TSA).

The results of this work show that, under typical conditions for sorption-enhanced H₂ production, the HTs exhibit higher adsorption capacities per unit mass of sorbent at relatively low pressures and per unit surface area over the whole range of pressures analysed, compared to NaX zeolites. It is found that the MgOs assessed outperform these materials in terms of both types of capacity.

Hydrotalcites and also MgOs show adsorption-desorption isotherms with a hysteresis loop indicating a stronger binding energy with CO₂, while the zeolites present a nearly reversible performance. In agreement with this, the NaX zeolites have a very stable performance upon cycling. On the other hand, HTs and MgOs exhibit a marked deactivation, which can be reduced by the use of carbon supports and alkali promoters.[5] In addition, PTSA regeneration appears to be more effective than PSA and TSA alone to mitigate the deactivation observed under the conditions assessed.

The equilibria data of the mixed oxides show good reproducibility and can be successfully fitted to Freundlich and Langmuir models. The values of heat of adsorption calculated for the fresh and cycled samples are useful input data for mathematical models of the sorption units.

References

[1] CO₂ emissions in 2023, International Energy Agency report, 2024.

[2] D. Iruretagoyena, K. Hellgardt, D. Chadwick, Int. J. Hydrog.Energy, 43 (2018), p. 4211.

[3] J. Wong, D. Iruretagoyena, N. Shah, P. Fennell, Proc. R. Soc. A, 479, (2023) p. 20230173.

[4] D. Iruretagoyena, P. Fennell, R. Pini, Chem. Eng. J. Adv., 13, (2023) p. 100437.

[5] R. Menzel, D. Iruretagoyena, et al., Adv. Funct. Mater., 30, (2020) p. 2002788.