https://doi.org/10.3390/su14084750
“TiO(OH)2 is a fundamentally different metal oxide nanoparticle that was recently studied for the regeneration process of solvents in an absorption based process for CO2 capture [24,38,110]. The past studies proposed a catalytic behavior instead of a heat and mass transfer enhancement. Past studies focused more on the synthesizing and characterization of the nanoparticle as well as its cyclic performances. Table 6 depicts the summary regarding the utilization of TiO(OH)2 in past studies.”
Table 6. Summary of past studies using TiO(OH)2 nanoparticles in the regeneration of solvent.
| Solvent | Concentration | Temperature °C |
Remarks | Ref. |
|---|---|---|---|---|
| Sodium Carbonate Na2CO3 | – | 40–70 |
|
[38] |
| MEA | 1–3 wt% | 88 |
|
[24] |
| K2CO3 | 0.010 vol% 0.014 vol % (For Cu/TiO(OH)2 |
100 |
|
[110] |
“In 2017, Yao et al. [38] studied the employment of nanoporous TiO(OH)2 on the regeneration of aqueous Na2CO3. At 65 °C, the quantity of CO2 desorbed is approximately 800% more than the case without the presence of TiO(OH)2. The study concluded on the increase in desorption rate upon the increase in stirring rate from 400 to 550 ppm, increase in quantity of TiO(OH)2 added and increase in temperature (although effects gradually decreased with time). The specific surface area of the fresh and cycled nanoparticles experienced a 14% drop at 807.4 m2/g and 693.1 m2/g. The quantity of the CO2 desorption (mmol) was also reported to be very similar, even after five absorption–desorption cycles, which demonstrates the particles’ ability to be recycled.
In 2018, Lai et al. [24] studied the employment of TiO(OH)2 in MEA solvent, where a drastic increase in the rate of CO2 desorption (up 4500%) was seen. The regeneration was conducted at a temperature below 100 °C at 20 wt% MEA solvent. The same team reported the utilization of TiO(OH)2 in MDEA solvent, where the nanoparticle exhibited a stronger catalytic effect. The catalytic mechanism for TiO(OH)2-catalyzed MEA was proposed. The highest rate achieved with the employment of TiO(OH)2 was 0.204 mmol/s at only 792 s, whereas at the same time, the MEA solvent without the nanoparticle only achieved a rate of 0.0162 mmol/s. The study compared 50 absorption–desorption cycles, and it was reported that there was no obvious decrease in the amount of CO2 absorbed and desorbed. The catalytic mechanism was also confirmed by observing the weak peak intensity of HCO3−, using RAMAN spectroscopy of the TiO(OH)2 utilized solvent.
Another study on TiO(OH)2 nanoparticle for enhancing solvent regeneration was by Liu et al. [110]. In their study, a nanostructured Cu/TiO(OH)2 was prepared to enhance the desorption process of K2CO3 solvent solution. In the study, 0–0.014 vol% nanoparticle was employed and an increase in the desorption capacity was observed by 45%. The CO2 desorption rate increased as the volume fraction of the nanoparticle increased but its peak performance was observed at 0.014 vol%. A further increase in the volume fraction caused the nanoparticle to agglomerate, which, in turn, affected the thermal conductivity of the nanoparticles. Its cyclic ability for 10 cycles also demonstrated great stability, where no significant changes was observed in terms of its absorption and desorption capacity at a specific time frame. The study also proved its stability using XRD, where no new crystalline phases were formed in an unwanted reaction. According to the toxicological classification, it was reported to be slightly toxic and relatively harmless to the environment.”