(433f) Effect of Sorbent Incorporation to a ZnZrO2 Catalyst in Its CO2 Capture Capacity and Catalytic Activity

Recently process intensification through reactive carbon capture (RCC) using catalytic sorbents (CS) or dual function materials (DFM) has been explored as a potential process in the transition to a CO₂ circular economy.¹ So far, most of the CS reported in literature for CH₄, syngas and MeOH synthesis consist of metal-based supported catalysts that are impregnated with an alkali/alkaline salt or physical contact (mixing or stacking) of the catalyst with an alkali-promoted MgO-based sorbent. Wirner et al. studied RCC to MeOH using stacked beds of NaO/Al₂O₃ adsorbent and Cu/ZnO/Al₂O₃ catalyst and reported a MeOH productivity of 12 µmol/g.² On the contrary, Jeong-Potter et al. reported higher productivity (59 µmol/g) when using a 5 wt% K₂O impregnated Cu/Zn/Al₂O₃ CS.¹ However, it is not clear how the different configurations impact the two processes occurring during RCC: CO₂ adsorption and catalytic hydrogenation of CO₂. Thus, in this work we evaluated the effect of sorbent incorporation into a ZnZrO₂ catalyst on the CO₂ uptake and CO₂ hydrogenation catalytic activity of the material. Two different configurations were evaluated, (i) a physical mixture with a Mg₃Al hydrotalcite derived oxide (Mg₃AlO) with a weight ratio of (6:4) and (ii) sorbent and catalyst impregnation with 10 wt.% of NaNO₃.

Figure 1a presents TGA uptake curves for the different materials. It can be observed that ZnZrO₂ without sorbent modification presents the lowest CO₂ uptake (0.05 mmol/g). The physical mixture of ZnZrO₂ with Mg₃AlO sorbent in a ratio of 6:4 results in an increased CO₂ uptake of 0.15 mmol/g. The highest CO₂ uptake capacity was obtained for the 10% NaNO₃ impregnated materials. Figure 1b presents CO₂ conversion vs. MeOH selectivity at 150 psi and 300 °C for all the materials. The physical mixture of ZnZrO₂ with Mg₃AlO did not significantly affect the catalytic activity. On the contrary, the impregnation with NaNO₃ decreased the CO₂ conversion and MeOH selectivity, especially when done directly on the catalyst domain. This could be related to the titration of the O defects in ZrO₂, which have been reported to be the active sites for CO₂ activation.³ This negative impact was reduced when the impregnation was done on the Mg₃AlO domain. CO₂-TPD experiments revealed that ZnZrO₂ basic sites are mostly weak and moderate with desorption peaks between 100-200 °C, which are associated to the catalyst high activity. On the other hand, Mg₃AlO has mostly strong CO₂ adsorption (desorption at 500 °C). Impregnation with NaNO₃ resulted in an increase in all basic sites. Thus, while the physical mixture of ZnZrO₂ with Mg₃AlO maintains the high selectivity of the catalyst with a moderate increase in total CO₂ uptake, it is possible that strongly bound CO₂ species are not reactive during RCC. On the other hand, NaNO₃ impregnation enhanced the total CO₂ uptake, along with weak and moderate basic site, and the poisoning effect on catalytic activity can be moderated by depositing NaNO₃ on the Mg₃Al sorbent. This understanding of CS configuration in the separated process provides insights for future rational design of CS that require high CO₂ uptakes and catalytic activity.

References

1 Jeong-Potter, C. et al. EES Catalysis 2 (2024)

2 Wirner, L. C. et al. Chemical Engineering Journal 470 (2023).

3 Araújo, T. P. et al. Advanced Energy Materials 13 (2023).