Photothermally triggered ▫$CO_2$▫ regeneration from monolithic silica-iron oxide sorbents

Elevated atmospheric CO2 disrupts the Earth’s carbon cycle, creating an urgent need for sustainable management strategies. Carbon Capture, Utilisation, and Storage (CCUS) technology can be seen as a potential solution but remains underutilised due to the high energy demand, particularly in desorption process. Conventional thermal and pressure swing methods require bulk heating or depressurisation of the sorbent chamber, leading to excessive energy consumption (1). Localised photothermal (PT) heating, with light that matches PT active material’s absorption maxima, can present a low-energy alternative for desorption (2). We prepared PT-active SiO2-Fe3O4 hybrid monoliths, where silica acts as the CO2 sorbent and Fe3O4 nanoparticles (IONPs) as the PT-active component. Silica monoliths were coated with different layer thickness of IONP, which was confirmed by SEM-EDXS and EDXS line profile analysis. N2 isotherms (BET analysis) showed that all the samples are hierarchically porous and that the hybrid monoliths samples display increased surface area compared to pristine silica. A similar trend is also observed in CO2 uptake where the CO2 capacity of the hybrid monoliths ≈1.77 mmol/g at 0°C and ≈0.96 mmol/g at 25°C. Additionally, thermally programmed desorption showed that for total CO2 regeneration, ≈100 °C needs to be reached. To evaluate the PT properties of the hybrid monoliths, PT experiments were conducted using different laser sources, i.e., 808nm (peak-off) and 447nm (peak-on). Surface temperatures were monitored by HD-IR camera and core temperatures by optical-fibre thermocouple. (Figure 1). We observed that illumination using the peak-on light leads to a higher surface temperature, compared to when using peak-off light. However, no notable difference in core temperatures was observed between peak-off and peak-on light conditions. This observation confirms a better energy efficiency when using peak-on light than peak-off at equal light powers. We also observed a non-linearity in the surface temperature with increasing layer in the case of peak-on illumination. This non-linearity revealed us that the PT effect in the case of monoliths is not only dependent on the layer thickness and light power but also on the concentration of PT nanoparticles in the layer and the penetration depth of the light. Moreover, further evidence for the effectiveness of peak-on illumination was provided by in-situ PT-triggered desorption, which achieved a 20% higher CO2 release during regeneration. This highlights the potential of peak targeted PT desorption approach for low-energy CCUS.