CO₂ capture from industry sectors or directly from the atmosphere is drawing much attention on a global scale because of the drastic changes in the climate and ecosystem which pose a potential threat to human health and life on Earth. In the past decades, CO₂ capture technology relied on classical liquid amine scrubbing. Due to its high energy consumption and corrosive property, CO₂ capture using solid materials has recently come under the spotlight. A variety of porous solid materials were reported such as zeolites and metal-organic frameworks. However, amine-functionalized porous materials outperform all others in terms of CO₂ adsorption capacity and regeneration efficiency. This review provides a brief overview of CO₂ capture by various amines and mechanistic aspects for newcomers entering into this field. This review also covers a state-of-the-art regeneration method, visible/UV light-triggered CO₂ desorption at room temperature. In the last section, the current issues and future perspectives are summarized.

The doping of Au/TiO₂ with FeO_x is shown to result in a strong enhancement of its photocatalytic activity in the degradation of the azo dye Orange II. In order to examine the source of this enhancement, Au-FeO_x/TiO₂ nanocomposites containing different molar ratios of Au:Fe were synthesized, and X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), and high-resolution transmission electron microscope (HRTEM) analyses indicated that the TiO₂-supported Au nanoparticles were partially covered with an amorphous layer of FeO_x species, in which the iron was present as Fe²⁺ and Fe³⁺. The metal-semiconductor system, i.e., Au/TiO₂, showed only a moderate degradation rate, whereas doping with FeO_x strongly enhanced the degradation activity. The bandgap energy decreased gradually from Au/TiO₂ (3.13 eV) to the catalyst with the highest FeO_x loading Au-FeO_x (1:2)/TiO₂ (2.23 eV), and this decrease was accompanied by a steady increase in the degradation activity of the catalysts. XPS analyses revealed that compared to Au/TiO₂, on Au-FeO_x/TiO₂ a much higher population density of chemisorbed and/or dissociated oxygen species was generated, which together with the decreased bandgap resulted in the highest photocatalytic activity observed with Au-FeO_x (1:2)/TiO₂. The processes occurring during reaction on the catalyst surface and in the bulk liquid phase were investigated using operando attenuated total reflection IR spectroscopy (ATR-IR) combined with modulation excitation spectroscopy (MES), which showed that the doping of Au/TiO₂ with FeO_x weakens the interaction of the dye with the catalyst surface and strongly enhances the cleavage of the azo bond.

Lowering the regeneration temperature for solid CO₂-capture materials is one of the critical tasks for economizing CO₂-capturing processes. Based on reported pKa values and nucleophilicity, we compared two different polyethylenimines (PEIs): branched PEI (BPEI) and linear PEI (LPEI). LPEI outperformed BPEI in terms of adsorption and desorption properties. Because LPEI is a solid below 73–75 °C, even a high loading amount of LPEI can effectively adsorb CO₂ without diffusive barriers. Temperature-programmed desorption (TPD) demonstrated that the desorption peak top dropped to 50.8 °C for LPEI, compared to 78.0 °C for BPEI. We also revisited the classical adsorption model of CO₂ on secondary amines by using in situ modulation excitation IR spectroscopy, and proposed a new adsorption configuration, R1(R2)-NCOOH. Even though LPEI is more expensive than BPEI, considering the long-term operation of a CO₂-capturing system, the low regeneration temperature makes LPEI attractive for industrial applications.

Amines supported on porous solid materials have a high CO₂ adsorption capacity and low regeneration temperature. However, the high amine load on such substrates and the substrate itself may lead to substantial pressure drop across the reactor. Herein, we compare the CO₂ adsorption capacity and pressure drop of fumed silica powder to 3D-printed monolithic fumed silica structures, both functionalized by polyethylenimine (PEI), and find a drastically reduced pressure drop for 3D-printed substrates (0.01 bar vs. 0.76 bar) in the sorption bed with equal CO₂ adsorption capacity. Furthermore, the effect of 3D-printing nozzle diameter and PEI loading on the adsorption capacity are investigated and the highest capacities (2.0 mmol/g at 25 °C with 5000 ppm CO₂) are achieved with 0.4 mm nozzle size and 34 wt% PEI loading. These high capacities are achieved since the 3D printing and subsequent sintering (700 °C) of monolithic samples does not compromise the surface area of the fumed silica. Finally, the comparison between 3D-printed monoliths and extruded granulate of varying diameter reveals that the ordered channel system of 3D-printed structures is superior to randomly oriented granulate in terms of CO₂ adsorption capacity.