Fabricating Carbon Foams as CO2 Adsorbent

By Laura ThomsonNov 2 2021

Increasing CO₂ is a global problem and to capture it, various CO₂ adsorbents are needed. This article looks at the design of a porous CO₂ adsorbent reported in Cheng Duan et al's paper published in Materials.

Image Credit: Billion Photos/Shutterstock.com

The increasing amount of carbon dioxide (CO₂) in the atmosphere, a major reason for global climate change, has given rise to CO₂ capture technology development. Numerous porous materials have been used as a CO₂ adsorbent (zeolite, metal-organic frameworks (MOFs), porous organic polymers, and porous carbons). Porous carbons also harbor excellent CO₂ capacities along with exceptional chemical and physical stability and hence have garnered much attention. The high CO₂ capacities of porous carbon relate to the profuse micropore volumes and the heteroatom.

To decrease diffusion pressure in the adsorbent, mesopores or macropores were introduced into the adsorbent. The macroporous structures in polymers are created by removing the internal phase, and an open-cell interconnected porous polymer monolith was obtained.

This article illustrates the use of a concentrated emulsion for developing microporous–macroporou s carbon foams (“ACRFs”), depicted in Scheme 1.

Scheme 1. Preparation route of microporous-macroporous carbon foams (ACRFs). Image Credit: Duan et al., 2021.

The researchers initially prepared macroporous resorcinol-formaldehyde resins (PRFs) with a concentrated emulsion, and later ACRFs were collected by carbonizing PRFs with potassium hydroxide (KOH) as an activator.

Methodology

Reagent

The fabrication requires Resorcinol and Tween 20, formaldehyde 37 wt% aqueous solution, anhydrous sodium carbonate and KOH, toluene, methanol, and ethanol.

Preparation of PRF

Into a 100 mL flask, Resorcinol, formaldehyde aqueous solution, anhydrous sodium carbonate, and deionized water were added and pre-polymerized. After cooling, the resultant solution was mixed with Tween 20, along with adding toluene. A light-pink emulsion forms, which was later transferred into a mold and solidified. PRFs are obtained during this process and the toluene droplets leave unique open-cell macropores simultaneously.

Preparation of ACRF

Carbonization of the PRF monolith was carried out by placing them in a tube furnace and heating, followed by soaking the pre-carbonized PRF in water/methanol solution dissolved with KOH for 3 hours. Upon carbonizing for an hour, ACRF was formed.

Characterization

The researchers analyzed Fourier transform infrared spectroscopy (FT-IR) spectra, X-ray photoelectron spectroscopy spectra (XPS). Macropores in PRFs and ACRFs were observed by scanning electron microscopy. The researchers also measured the N₂ adsorption–desorption isotherms, specific surface areas of the samples, the pore size distributions, CO2 adsorption isotherms, Raman spectra, X-ray diffraction (XRD).

Results and Discussion

Carbonizing Process of ACRFs

KOH when used as an activator during the carbonization process generates more micropores in the resultant carbon. But due to hydrolysis it was difficult to load KOH solution in PRFs and so PRFs were initially carbonized at low temperature, then the resultant macroporous carbon can be impregnated with KOH aqueous solution and further activated at the demanded temperature.

Figure 1 shows the TGA/DTA curves used for determining the pre-carbonizing temperature and activation temperature.

Figure 1. TGA and DTG curves of PRF-50C-85% under nitrogen flow. Image Credit: Duan et al., 2021.

Oxygen in Porous ACRFs

The XPS spectra of ACRF-50C-85%-600, ACRF-50C-85%-700, and ACRF-50C-85%-800 were used to analyze the surface chemistry state of these carbons (Figure 2) and revealed that the ACRFs were oxygen-enriched.

Figure 2. X-ray photoelectron spectroscopy (XPS) spectra of all ACRFs (A) and deconvolution of O 1s peaks of ACRF-50-85%-600 (B), ACRF-50-85%-700 (C) and ACRF-50-85%-800 (D). Image Credit: Duan et al., 2021.

Porosity of ACRFs

Figure 3 depicts the macroporous morphologies of PRFs and ACRFs. In the case of PRFs, SEM of the porous material showed numerous closely-packed cavities (“voids”) interconnected with the adjacent ones by small pores (“windows”). ACRF-50C-80%-800, ACRF-50C-85%-800, and ACRF-40C-85%-800 had macroporous structures of their corresponding PRFs.

Figure 3. Scanning electron microscope (SEM) images of PRFs and corresponding ACRFs. Image Credit: Duan et al., 2021.

Figure 4 shows the N₂ adsorption-desorption isothermals of all the ACRFs and the results showed that there were abundant micropores in ACRFs.

Figure 4. N₂ adsorption-desorption isotherms of ACRFs at 77 K. Image Credit: Duan et al., 2021.

Micropores were dominant in all of the ACRFs and were mostly ultra-micropores (sizes below 0.7 nm) (Figure 5).

Figure 5. Pore size distributions of ACRFs. Image Credit: Duan et al., 2021.

CO₂ Capacity of ACRFs at 1 Bar

Figure 6 shows the CO₂ adsorption isothermals of all the ACRFs. The same is listed in Table 1.

Figure 6. CO₂ capacities of ACRFs at 273 K. Image Credit: Duan et al., 2021.

Table 1. Specific surface areas, pore volume, and CO₂ capacities of ACRFs. Source: Duan et al., 2021.

¹ Calculated BET surface area over the pressure range of 0.01–0.2 P/P₀ values. 
² Calculated by DFT method, cumulative pore volume under 2 nm. 
³ Calculated by DFT method, cumulative pore volume under 0.7 nm. 
⁴ Measured at 273 k, 1 bar.

Figure 7 shows the relationships of CO₂ capacities with porosity. The high adsorption potential of ultra-micropores might be the factor that determines CO₂ capacity at 1 bar.

Figure 7. Relationships between CO₂ capacities and total pore volume (A), micropore volume (B), and ultra-micropore volume (C). Image Credit: Duan et al., 2021.

Effect of Oxygen Content on the CO₂ Capacities of ACRFs

The influence of oxygen content on the CO₂ capacity of ACRFs was analyzed, and the results interpreted showed that an increase of oxygen in ACRFs improves the interaction between carbon surface and the CO₂ molecule. This is important for ACRFs to capture CO₂ at low pressure.

Effect of Macropores on CO₂ Adsorption Kinetics of ACRFs

Figure 8 depicts the CO₂ adsorption kinetics of ACRF-30C-85%-800 and ACRF-40C-85%-800. The analyses also confirmed the accelerating effect of macropores on CO₂ adsorption kinetics.

Figure 8. Comparison of the CO₂ adsorption kinetics of ACRF-40C-85%-800 and ACRF-30C-85%-800. Image Credit: Duan et al., 2021.

CO₂ Adsorbing Properties of ACRF-40C-85%-800

More analyses were carried out on the CO₂ adsorbing behaviors of ACRF-40C-85%-800. The CO₂ adsorption kinetics was carried out as shown in Figure 9.

Figure 9. CO₂ adsorption kinetics of ACRF-40C-85%-800. Image Credit: Duan et al., 2021.

Table 2 shows the kinetics model parameters for CO₂ adsorption.

Table 2. Kinetics model parameters for CO₂ adsorption. Source: Duan et al., 2021.

The kinetics data was also analyzed with the micropore diffusion model. The CO₂ selectivity adsorption over N₂ was measured by the adsorption isotherms of CO₂ and N₂ at 273 K (Figure 10A). The results showed the competitive adsorption mechanism. It showed that they tended to adsorb CO₂ over N₂, and can selectively capture CO₂ over N₂ from a mixed gas feed stream.

Figure 10. CO₂ and N₂ adsorption isothermals of ACRF-40C-85%-800 at 273 K (A); breakthrough curves of CO₂/N₂ of ACRF-40C-85-800 at 298 K (B); CO₂ capacity at 298 K of ACRF-40C-85%-800 for six cycles (C). Image Credit: Duan et al., 2021.

The reproducibility of ACRF-40C-85%-800 revealed the same capacity as the first cycle even after the sixth cycle showing good reproducibility.

Figure 11 shows the CO₂ adsorption heat calculated by Clausius−Clapeyron.

Figure 11. CO₂ adsorption isothermals (A) and CO₂ adsorption heats (B) of ACRF-40C-85%-800. Image Credit: Duan et al., 2021.

Conclusion

This article puts forth a strategy to fabricate monolith-like carbon (ACRFs) with macropores and micropores. The micropores were created by the activation effect of KOH, and the macropores by the macroporous precursor.

The macropores in ACRFs accelerated the CO₂ adsorption rate. ACRFs also showed moderate CO₂ adsorption heat and selective adsorption ability. It is assumed that ACRFs have the potential to be transformed into a CO₂ adsorbent with outstanding comprehensive performance.

Journal Reference:

Duan, C., Zou, W., Zhongjie, Du., Mi, J., Han, J., Zhang, C. (2021) Construction of Oxygen-Rich Carbon Foams for Rapid Carbon Dioxide Capture. Materials, 4(1), p. 173. doi.org/10.3390/ma14010173.

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