La-doped silica porous composites were synthesized following the procedure described in our previous report [7]. Anhydrous ethanol and tetraethoxysilane (TEOS) were mixed under stirring while maintaining the mixture at 0˚C to prevent ethanol evaporation. La(NO₃)₃∙6H₂O was added to achieve Si:La molar ratios of 2:1, 4:1, and 8:1. After complete dissolution, hydrogen peroxide was added, and the mixture was stirred for 2 h. The resulting gel was dried at 60˚C for one week, ground into powder, and stored in a desiccator.

Thermal analysis (TG-DTA) was conducted to determine the calcination temperature, which was set to 600˚C. Samples were heated at 2˚C/min to 600˚C, held for 3 h, and cooled at 5˚C/min. Figure 1 shows the synthesis flow chart and sample appearance.

2.2.1. Structural Analysis

XRD measurements were performed using a Rigaku RINT2500 diffractometer (Cu Kα radiation, 3 kW, 50 mA, 2θ = 20–60°). Microstructural observations were conducted using a JEOL JEM-4000FX TEM and a Hitachi S-4500 SEM.

2.2.2. CO₂ Adsorption Measurements

To measure CO₂ adsorption capacity, CO₂ adsorption experiments were conducted on samples of pure silica and La-doped silica with different compositions using a fully automated gas adsorption analyzer (specific surface area and pore size distribution analyzer; AUTOSORB), based on the volumetric method.

Figure 1. Flow chart of the production process of La-doped Silica specimen by a precursor solution method and sample views.

First, CO₂ was adsorbed onto the SiO₂-La composite sample by passing it through the sample under pressure. After holding the sample at this pressure for a set period, the CO₂ was desorbed by reducing the pressure, and the process was then repeated by reapplying pressure. For a sample under a single set of experimental conditions, this cycle was repeated several times; the amounts of gas adsorbed and desorbed during this process were measured, and the amount of gas adsorbed on the sample surface was determined based on these changes. The adsorption measurement is performed by opening a solenoid valve to introduce carbon dioxide from a manifold containing carbon dioxide at a specified pressure into the cell containing the sample. The amount of adsorption can be calculated from the difference between the amount of carbon dioxide introduced into the system and the amount of gas remaining in the system after adsorption.

Figure 2 shows a flowchart of the volumetric adsorption measurement apparatus.

Figure 2. Flowchart for measuring carbon dioxide (CO₂) adsorption capacity using a AUTOSORB.

For two experimental temperatures—100˚C and 400˚C—we performed the same procedure on 20 to 25 samples at each temperature, varying the experimental pressure from 0 kPa to 110 kPa to obtain data on the pressure dependence of the CO₂ gas volume.

In AUTOSORB volumetric measurements, the amount of gas removed (i.e., the amount adsorbed) can be calculated using the equation of state based on the “pressure change” before and after adsorption and the “known instrument volume” when gas is introduced from the manifold into the sample cell. In this way, the amount of CO₂ that can be measured by repeatedly performing desorption and adsorption under adsorption equilibrium pressure can be plotted at each pressure value at a constant temperature based on the actual material’s adsorption system, generally yielding the adsorption isotherm shown in Figure 3.

Figure 3. Carbon dioxide adsorption isotherms.

Line (a) shows the initial adsorption isotherm, while Line (b) shows the adsorption isotherm obtained after performing an adsorption measurement, followed by a vacuum evacuation, and then conducting another adsorption measurement. The difference between these two adsorption isotherms (Vi) corresponds to the amount of carbon dioxide that is irreversibly adsorbed—that is, the amount of chemically adsorbed carbon dioxide—which cannot be desorbed at the measurement temperature due to strong adsorption forces.

On the other hand, the Vr shown in (b) corresponds to physical adsorption of carbon dioxide that can be reversibly adsorbed and desorbed in response to changes in carbon dioxide pressure. This analytical method has also been used in previous studies on CO₂ adsorption [8]. Possible sites for this adsorption include extremely small pores that can only be reached by gas diffusion, the interface between the support and the metal, or surface lattice defects.

Figure 4 shows the XRD patterns of samples with different La contents. The Si:La = 2:1 sample exhibited eight diffraction peaks corresponding to La₂O₃, indicating significant crystallization. The Si:La = 4:1 sample showed two La₂O₃ peaks ((002) and (011)), while the Si:La = 8:1 sample exhibited no distinct peaks, suggesting that La₂O₃ crystallization was suppressed and the amorphous silica structure was largely retained.

Figure 4. XRD pattern for the La-dope Silica specimens. (a) Si:La = 2:1; (b) Si:La = 4:1; (c) Si:La = 8:1.

Although we did not perform SEM or TEM observations on all of the obtained samples, we did conduct microscopic observations on multiple specimens of each type. As a representative example, Figure 5 shows SEM and TEM images of a sample with a Si:La composition ratio of 4:1.

SEM (Figure 5(a)) revealed no obvious porous structure, although bright contrast regions were attributed to dispersed La₂O₃ particles. TEM analysis (Figure 5(b)) confirmed lattice fringes corresponding to La₂O₃ (100) and (012) planes.

Figure 5. (a) SEM image and the electron diffraction pattern of La-dope Silica specimen; (b) TEM image and the electron diffraction pattern of La-dope Silica specimen (Si:La = 4: 1).

Figure 6 summarizes the CO₂ adsorption capacities. Pure silica exhibited negligible chemical adsorption, whereas La-doped samples showed measurable irreversible adsorption. However, in all samples, the proportion of chemical adsorption remained small, in dicating that physical adsorption dominated the overall CO₂ uptake.

Reversible adsorption showed strong temperature dependence. At 100˚C, lower La concentrations (Si:La = 8:1 and 4:1) yielded higher adsorption capacities. At elevated temperatures, adsorption decreased, except for the Si:La = 2:1 sample, which showed an anomalous increase at 400˚C.

The adsorption capacity varied significantly with temperature, and a temperature de pendence was confirmed. At the low temperature of 100°C, the higher the La doping level, the greater the reversible adsorption capacity; however, as the temperature increased, the CO₂ adsorption capacity tended to decrease.

The above results are summarized in Table 1.