3.3 CO2 adsorption capacity

and the flake-like structures are formed. Another important reason may be related to the isoelectric point (IEP) of MgAl LDHs. The IEP of MgAl LDHs is around 10, so it can be positively (pH > IEP), neutrally (pH = IEP), or negatively (pH < IEP) charged depending on the relationship between the IEP and the pH. Because it is electronic neutrally on its surface, its growth is inhibited due to the repulsive force

, CO3

2,

between negatively charged TUC-LDH primary particles and Al(OH)4

3.2 TGA analysis

Sorption in 2020s

Figure 5.

132

TGA spectrum of the three different types of synthesized LDHs.

OH anions. Hence, the formation rate of LDHs is so fast that the growth in all directions under such basic conditions. It is well known that the formation process of TUC-LDH includes both nucleation and growth process. In our experiment, the growing of TUC-LDH is based on the mixture solution, which suggests that it supplies a nucleation environment for TUC-LDH growth, then the directions of LDHs growth could be determined. As reported in literature, the deposition of a colloidal suspension of TUC-LDH on substrates, such as glass or silicon, generally leads to the TUC-LDH nanoplatelets having preferred orientation with their c-axis perpendicular to the substrate. The fact that the MgAl-CO3 LDH nanoplatelets are perpendicularly attached to the surface via their edges suggests they are grown onto the substrate via a strong chemical interaction. Wang et al. reported the first synthesize of nanosized spherical MgAl LDHs using IEP method [20].

Calcination is a very important step for activating the MgAl LDHs, because the fresh LDHs do not contain much basic sites so that the CO2 uptake capacity is quite low. Figure 5 shows the TGA spectrum of the three different types of synthesized LDHs. The curves for the LDHs prepared by three methods are all fairly similar in shape. The TGA spectra of TU-LDHs show a weight loss of 13% between 50 and 207°C due to the loss of the physisorbed water. In the second weight loss, 33.2% occurred between 207 and 600°C, which is mainly caused by dehydroxylates and decarbonates of LDHs to a large extent, finally leading to the formation of a mixed oxide with a three-dimensional network [8]. For CC-LDHs, the first weight loss

The CO2 capture capacity of the above-mentioned three types of LDHs, including TU-LDHs, TUC-LDHs and CC-LDHs, were evaluated using isothermal CO2 adsorption tests. In the present work, we are particularly interested in whether the modified methods would influence the final CO2 adsorption capacity or not. The CO2 adsorption capacities of the different types of LDHs are given in Figure 6. All the samples were first calcined at 500°C before each CO2 adsorption test. Then, the thermogravimetric adsorptions of CO2 on the samples were measured at 80, 150 and 200°C using a TGA analyser. It can be found that the CO2 adsorption capacities of TU-LDHs, CC-LDHs and TUC-LDHs at 80°C are 0.30, 0.22 and 0.28 mmol g<sup>1</sup> , respectively. While for the sample at 200°C, the CO2 adsorption capacities of TU-LDHs, CC-LDHs and TUC-LDHs are 0.33, 0.25 and 0.36 mmol g<sup>1</sup> , respectively. The CO2 adsorption capacity of LDHs at 200°C was higher than that of 80 and 150°C. The main reason maybe that a surface phenomenon and chemical interactions are restricted at 80°C due to the higher activation energy [19]. We can see that TUC-LDHs had better adsorption capacity than other two TU-LDHs and CC-LDHs independent of adsorption temperature. The removal of water during calcination process leads to the formation of channels and pores. As the BET surface area of TUC-LDHs is higher than TU-LDHs and CC-LDHs, it could increase more basic sites for CO2 adsorption [7].
