Hexagonal and Monoclinic Phases of La₂O₂CO₃ Nanoparticles and Their Phase-Related CO₂ Behavior

Abstract

In this study, we prepared hexagonal and monoclinic phases of La₂O₂CO₃ nanoparticles by different wet preparation methods and investigated their phase-related CO₂ behavior through field-emission scanning microscopy, high-resolution transmission electron microscopy, Fourier transform infrared, thermogravimetric analysis, CO₂-temperature programmed desorption, and linear sweeping voltammetry of CO₂ electrochemical reduction. The monoclinic La₂O₂CO₃ phase was synthesized by a conventional precipitation method via La(OH)CO₃ when the precipitation time was longer than 12 h. In contrast, the hydrothermal method produced only the hexagonal La₂O₂CO₃ phase, irrespective of the hydrothermal reaction time. The La(OH)₃ phase was determined to be the initial phase in both preparation methods. During the precipitation, the La(OH)₃ phase was transformed into La(OH)CO₃ owing to the continuous supply of CO₂ from air whereas the hydrothermal method of a closed system crystallized only the La(OH)₃ phase. Based on the CO₂-temperature programmed desorption and thermogravimetric analysis, the hexagonal La₂O₂CO₃ nanoparticles (HL-12h) showed a higher surface CO₂ adsorption and thermal stability than those of the monoclinic La₂O₂CO₃ (PL-12h). The crystalline structures of both La₂O₂CO₃ phases predicted by the density functional theory calculation explained the difference in the CO₂ behavior on each phase. Consequently, HL-12h showed a higher current density and a more positive onset potential than PL-12h in CO₂ electrochemical reduction.

1. Introduction

Recently, the synthesis of nanomaterials with controllable morphologies and phases has attracted considerable attention in the fields of materials science and inorganic chemistry because the physicochemical and structural properties of the nanomaterials strongly correlate with the types of crystal structures as well as the morphologies of nanoparticles [1,2,3,4,5,6]. The unique properties of nanomaterials can be properly tuned by controlling various factors, which results in potential applications of nanomaterials in catalysis, biological labeling, sensing, and optics [1,7,8,9]. Among the methods for synthesizing nanomaterials, wet chemical processes have been considered as the most effective and convenient approaches for the controllable phases of ceramic materials [10].

Lanthana (La₂O₃) has been widely used as a promoter or support in heterogeneous catalysis [11,12,13]. The basicity of La₂O₃ readily induces the adsorption of CO₂ to form the lanthanum oxycarbonate (La₂O₂CO₃) phase, which is an important species in the La₂O₃-containing catalytic reaction [4,13,14,15,16]. The crystalline structures of La₂O₂CO₃ can be divided into three types of different polymorphs: a tetragonal La₂O₂CO₃ (type I), a monoclinic La₂O₂CO₃ (type Ia), and a hexagonal La₂O₂CO₃ (type II) [16,17,18]. The hexagonal type II La₂O₂CO₃ has a higher chemical stability to water and carbon dioxide than the monoclinic type Ia [4,19,20]. In addition, the different crystalline structures of the La₂O₂CO₃ phases affect the interaction between La₂O₂CO₃ and ZnO in the La₂O₂CO₃/ZnO composite materials as well as the catalytic behavior of the composite materials on glycerol carbonation with CO₂ [4,21]. Meanwhile, the monoclinic type Ia La₂O₂CO₃ phase closely resembles the crystalline structure of lanthanum (La) oxysalts (e.g., oxysilicates, oxyhalides, and oxysulfates), whereas the hexagonal type II one is similar to A-type La sesquioxides. Thus, the type Ia La₂O₂CO₃ phase has been readily prepared by the thermal decomposition of La compounds (e.g., oxalates and acetates); however, it is difficult to prepare type II La₂O₂CO₃ in a single phase by the conventional wet preparation methods [20]. Accordingly, it is necessary to investigate i) the preparation conditions used to form type Ia and type II La₂O₂CO₃ phases in the conventional methods and ii) the CO₂ behavior on the La₂O₂CO₃ structures, which is an essential step in the CO₂-involving catalytic reactions, as well as the formation of the different La₂O₂CO₃ phases.

In this study, we prepared the nanoparticles with type Ia and type II La₂O₂CO₃ crystal structures by conventional wet preparation methods and investigated the formation of different La₂O₂CO₃ phases with Fourier transform infrared (FT-IR), X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and high-resolution transmission electron microscopy (HR-TEM). Furthermore, the CO₂ behavior on the different La₂O₂CO₃ crystal structures was observed by CO₂-temperature programmed desorption (TPD), thermogravimetric analysis (TGA), and linear sweeping voltammetry (LSV) of CO₂ electrochemical reduction. The superior CO₂ behavior of the hexagonal La₂O₂CO₃ phase to the monoclinic phase was additionally explained by the crystalline structures of both La₂O₂CO₃ phases, which was predicted by the density functional theory (DFT) calculation.

2. Materials and Methods

2.1. Materials

A total of 1.00 g of La(NO₃)₃·6H₂O was added to 50.0 mL of deionized water, and the resultant solution was vigorously stirred to ensure complete dissolution. The pH of the solution was adjusted to 12 with a 10 wt% NaOH solution, which yielded a white precipitate after the mixture was stirred for approximately 10 min. The sample was continuously stirred for another 6, 12, or 24 h, and the obtained product was centrifuged. The separated precipitate was washed with distilled water and ethanol and then dried at 80 °C for 12 h, followed by the calcination step at 500 °C for 2 h. Depending on the precipitation time, the solid samples prepared by the precipitation method were denoted as PL-xh (x = 6, 12, or 24), where x represents the precipitation time.

For the hydrothermal method, the procedure was almost the same as that in the precipitation method, except using an autoclave for the hydrothermal treatment. The pH-adjusted solution containing the La precursor was transferred to an autoclave (200 mL), heated to 160 °C, and maintained at this temperature for 6, 12, or 24 h. The obtained product was centrifuged, and the remained steps were also the same as those in the precipitation method. The La₂O₂CO₃ samples synthesized by the hydrothermal method were designated as HL-yh (y = 6, 12, or 24), where y represents the hydrothermal treatment time.

2.2. Characterizations

The morphologies of the samples were observed by a field-emission scanning electron microscope (JEOL, JSM-600F, Tokyo, Japan) instrument equipped with an energy-dispersive spectrometer. HR-TEM images were obtained using a JEOL JEM-2100F instrument (JEOL Ltd., Tokyo, Japan). The samples were prepared by suspending and grinding in an ethanol solution whose drops were placed on a carbon-film-coated copper grid. XRD patterns were measured at room temperature on a Rigaku D/MAX-2200 powder X-ray diffractometer (Rigaku Corporation, Tokyo, Japan) using a Cu Kα radiation source (λ = 0.15418 nm). The X-ray tube was operated at 35 kV and 20 mA, and the 2θ angle was scanned from 10° to 90° (with a step of 0.02°) at a speed of 2°/min. The FT-IR spectra of the samples were collected for the KBr powder-pressed pellets on a Nicolet 380 FT-IR spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA) under ambient conditions.

The CO₂-TPD experiments were conducted in a quartz flow reactor. The calcined samples were preheated from room temperature up to 600 °C (with a ramping rate of 15 °C/min) for 1 h under He flow (100 mL/min). The CO₂ gas (10 vol.% CO₂/He) was fed into the reactor with a flow rate of 30 mL/min at 50 °C for CO₂ adsorption before conducting the CO₂-TPD measurements. Finally, the temperature was increased from 50 to 600 °C at the ramping rate of 1.5 °C/min in He flow (30 mL/min). The weight loss in the samples was determined by a thermogravimetric analyzer (TA Instruments Q50, New Castle, DE, USA). A total of 20 mg of the samples was charged into the sample pan and heated to 1000 °C at a rate of 5 °C/min in air flow. The CO₂ electrochemical reduction was carried out via the LSV measurement with an Ag/AgCl electrode as a reference electrode and Pt wire as a counter electrode. The working electrode was prepared by dispersing 10 mg of the samples in a mixture of 2 mL of alcohol and 100 μL of 5% Nafion and then pipetting 10 μL of suspension on the GCE (0.07065 cm²). The working electrode was tested 20 times at a scan rate of 20 mV/s. The electrolyte was 0.1 M NaHCO₃ saturated with CO₂. Before each experiment, high-purity CO₂ gas was bubbled at a flow rate of 30 mL/min for 30 min to remove all oxygen from the electrolyte. The gases in the measurement were analyzed by a GC instrument.

Using the Vienna Ab initio Simulation Package (VASP) [22,23], DFT calculations were conducted along with the GGA–PBE (Perdew–Burke–Ernzerhof) functional [24]. The cutoff energy of 600 eV was chosen in our calculations. The criteria of convergence of energies and forces for geometry optimization were 10⁻⁴ eV and 10⁻² eV/Å, respectively. For the calculation of disordered hexagonal La₂O₂CO₃, the lowest energy configuration among the other randomly selected 50 structures was used. The Monkhorst-Pack k-point meshes of 3 × 5 × 2 and 9 × 9 × 3 were used for the geometry optimization of monoclinic and hexagonal phase of La₂O₂CO₃, respectively [25].

3. Results and Discussion

3.1. Synthesis of Monoclinic and Hexagonal La₂O₂CO₃ Nanoparticles

Figure 1 shows the XRD patterns of La₂O₂CO₃ nanoparticle materials prepared at each reaction time. The two types of La₂O₂CO₃ phases are primarily detected in the PL samples: the monoclinic type Ia and hexagonal type II La₂O₂CO₃ phases. For 6 h of precipitation (PL-6h), the characteristic XRD peaks in the hexagonal La₂O₂CO₃ crystal phase are clearly observed at 2θ = 25.7, 30.2, 47.2, and 56.6° (JCPDS 37-0804) (Figure 1(Aa)) [1,4,20,21,26,27]. However, when the precipitation time is increased to 12 and 24 h, the characteristic XRD peaks in the monoclinic La₂O₂CO₃ phase clearly appear at 2θ = 22.8, 29.3, 31.0, 39.9, and 44.4° with a C12/c1 space group (JCPDS 48-1113) (Figure 1(Ab,Ac)), which indicates the prevalence of the hexagonal La₂O₂CO₃ phase during the initial precipitation time, followed by the transformation into the monoclinic La₂O₂CO₃ phase after 12 h of precipitation. In contrast, the HL samples show the XRD patterns that contain the characteristic peaks in only the hexagonal type II La₂O₂CO₃ phase, regardless of the reaction time during the hydrothermal preparation, which demonstrates that there is no change in the La₂O₂CO₃ phase during the preparation process (Figure 1(Ba–Bc)).

The FT-IR spectra of the PL and HL samples also confirm the formation of each La₂O₂CO₃ crystal phase depending on the preparation methods, as shown in Figure 2. According to the assignments of typical FT-IR bands for carbonates in the La₂O₂CO₃ phases, the bands at 745, 855, 1066, and 1518 cm⁻¹ are interpreted as CO₃²⁻ vibrations related to the La₂O₂CO₃ phase [4,6,21,27]. The three-fold splitting bands at approximately 845 cm⁻¹ (υ₂) and a strong band at 1367 cm⁻¹ (υ₃) are assigned to the unique carbonate vibrational mode for the monoclinic type Ia La₂O₂CO₃ phase. The FT-IR spectra in Figure 2b,c of only the PL-12h and PL-24h samples show the characteristic bands (υ₂ and υ₃) of type-Ia La₂O₂CO₃, whereas the FT-IR spectra of the other samples show the typical bands of the La₂O₂CO₃ phase, which further confirms that the formation of the type Ia and II La₂O₂CO₃ phases depends on the preparation conditions. In the precipitation method, the monoclinic type Ia La₂O₂CO₃ phase is mainly formed when the precipitation time is longer than 12 h, whereas the hydrothermal method produces only the hexagonal type II La₂O₂CO₃ phase. This is consistent with the XRD results in this study.

Moreover, TEM measurements also provide additional evidence for the existence of the monoclinic and hexagonal La₂O₂CO₃ phases in the samples. Figure 3a–c shows the TEM images and fast Fourier transform patterns of PL-6h, PL-12h, and HL-12h. The (207) plane of the monoclinic type Ia La₂O₂CO₃ phase is detected in the PL-12h sample, whereas the (260) plane of the hexagonal type II La₂O₂CO₃ phase is observed in the HL-12h sample. Similarly, the PL-6h sample shows the (004) plane of the type II La₂O₂CO₃ phase, which is in good agreement with the XRD and FT-IR data. However, the morphological structures of PL-12h, HL-12h, and PL-6h samples are similar, as shown by the FE-SEM images; the aggregates of nanoparticles have different sizes: smaller than 10 nm for PL-12h, 10–30 nm for HL-12h, and 30–60 nm for PL-6h (Figure 3d–f).

To further understand the formation mechanism of the monoclinic and hexagonal La₂O₂CO₃ phases, uncalcined samples after precipitation or hydrothermal treatment were investigated. The XRD and FT-IR measurements indicate that different chemical products are also produced depending on the preparation conditions (Figure 4). The XRD peaks in Figure 4(Aa–Ac), shown as circles, are indexed to the pure hexagonal phase of La(OH)₃ with a P63/m(176) space group (JCPDS 36-1481) [1,6,12,21,26,27,28], which clearly shows that the initial La(OH)₃ phase remains unchanged in the hydrothermal method. With an increase in the preparation time during the hydrothermal method, the crystallinity of the La(OH)₃ structure becomes stronger with sharper XRD characteristic peaks. Meanwhile, in the precipitation method, the La(OH)₃ phase is produced with a very low crystallinity for PL-6h (weak and broad characteristic XRD peaks in Figure 4(Ac)). However, when the precipitation time is increased up to 12 h, the characteristic XRD peaks assigned to the orthorhombic La(OH)CO₃ structure (JCPDS 49-0981) appear with the disappearance of the XRD peaks in the La(OH)₃ structure (Figure 4(Ad)) [9,29]. Therefore, in the precipitation method, the dominant phase evolves from La(OH)₃ to La(OH)CO_3, with an increase in the precipitation time. However, the initial La(OH)₃ phase in the hydrothermal method is more crystallized during the hydrothermal treatment.

The FT-IR spectra of the uncalcined samples are monitored to confirm the existence of La(OH)₃ and La(OH)CO₃. First, the strong bands at 1438 and 1491 cm⁻¹ shown in Figure 4(Bd) can be assigned to the bending vibrations of CO₃²⁻, which confirms the presence of carbonate species in the intermediate [9]. A band at 3616 cm⁻¹ and a broad band at 3410 cm⁻¹ represent the O–H stretching mode in La–OH [6,9,27]. The bands at 850 and 1052 cm⁻¹ correspond to the vibrational modes of carbon-related bonds, such as CH and CO, which remain before the calcination step. Thus, the FT-IR spectrum in Figure 4(Bd) clearly confirms the existence of La(OH)CO₃ as an intermediate in the PL-12h sample, which is consistent with the XRD data shown in Figure 4A. For La(OH)₃, the characteristic FT-IR bands for the O–H stretching and bending modes in La–OH are clearly observed at 3616, 3410, and 1640 cm⁻¹, as shown in Figure 4(Ba–Bc) [6,9,27]. Other bands at approximately 2800–3000, 850, and 1052 cm⁻¹ can also be assigned to the vibrational modes of carbon-related bonds. Interestingly, for the samples in the precipitation method, the characteristic IR bands for CO₃²⁻ at approximately 1350–1500 cm⁻¹ become sharp and strong with an increase in the reaction time (Figure 4(Bc,Bd)), whereas the characteristic IR band for OH at 3616 cm⁻¹ is strongly intensified during the hydrothermal method (Figure 4(Ba,Bb)). Therefore, the precipitation method induces the transformation from La(OH)₃ into La(OH)CO₃ through the reaction with CO₂. In the hydrothermal method, the crystallization of La(OH)₃ goes further, which results in the high crystallinity of La(OH)₃.

A critical difference between the two preparations is an open or closed reaction system, which is related to the supply of carbonate sources. For either the precipitation or hydrothermal method, the La precursor in the aqueous solution is dissociated into La cations and is then readily crystallized into the La(OH)₃ phase, because the initial pH conditions are strongly basic (i.e., pH = 12). In the hydrothermal method, a Teflon-lined autoclave reactor is used as a closed reaction system. Because it is a closed system, there is no further transformation of the La intermediate, which only results in the strong crystallization of the La(OH)₃ phase for the HL-12h and HL-24h samples. However, in the precipitation method, the precipitation is carried out in an open beaker; thus, the carbonate source (i.e., CO₂ from the air) can be continuously dissolved into the aqueous solution. Therefore, the initial phase, La(OH)₃, can be converted into the La(OH)CO₃ phase by the reaction with CO₂ at a time longer than 12 h, even though the 6-h precipitation produces only a weakly crystallized La(OH)₃. Under the continuous CO₂ supply condition, there is a transformation from La(OH)₃ into La(OH)CO₃. In the literature, it was reported that La(OH)₃ changed into an La carbonate when it was exposed to air [6,27,28]. More importantly, the La(OH)CO₃ phase is finally converted into the monoclinic type Ia La₂O₂CO₃ phase in the precipitation, while La(OH)₃ is transformed into the hexagonal type II structure in the hydrothermal method. The sufficient supply of CO₂ into the aqueous solution produces the La(OH)CO₃ that can be changed into the monoclinic La₂O₂CO₃ phase.

3.2. CO₂ Behavior on La₂O₂CO₃ Nanoparticles

To investigate the CO₂ behavior on each La₂O₂CO₃ phase, TGA, CO₂-TPD and CV of CO₂ electrochemical reduction for PL-12h (monoclinic type Ia La₂O₂CO₃ phase) and HL-12h (hexagonal type II La₂O₂CO₃ phase) were conducted in this study. Figure 5A shows the derivative TGA (DTGA) profiles of PL-12h and HL-12h, where the decomposition peaks correspond to CO₂ gases that leave from the La₂O₂CO₃ phases. The weight loss due to the thermal decomposition occurs at 326 °C and in the temperature range of 770–800 °C. According to previous studies [4,30], the CO₂ peak, which appears during the decomposition of La₂O₂CO₃ above 600 °C, can be assigned to CO₂ gases leaving from the bulk structure of the La₂O₂CO₃ phases, which is then transformed into the La₂O₃ phase. The CO₂ decomposition from the bulk structure of the hexagonal La₂O₂CO₃ phase occurs at approximately 800 °C, which is higher than the temperature of CO₂ production during the decomposition of the bulk structure of the monoclinic La₂O₂CO₃ phase. This result shows that the thermal stability of the hexagonal La₂O₂CO₃ phase is higher than that of the monoclinic phase [21]. The weight loss at approximately 326 °C is assumed to be due to the release of CO₂ gas that is adsorbed on the surface of the La₂O₂CO₃ phase. The decomposition peak at approximately 326 °C has a much smaller intensity than that at 650 °C, which indicates that a much lower amount of CO₂ is adsorbed onto the surfaces of the La₂O₂CO₃ phase than that released from the bulk structure. Furthermore, based on each peak’s intensity, shown in Figure 5A, the hexagonal type II La₂O₂CO₃ phase contains more CO₂ on the surface than that on the monoclinic type Ia phase.

To better understand the CO₂ adsorption ability on the surface of each La₂O₂CO₃ phase, the CO₂-TPD profiles of PL-12h and HL-12h were acquired. Before conducting the CO₂-TPD experiments, both samples were thermally treated at 600 °C for 1 h in He gas, and then CO₂ was introduced into the reactor at 50 °C to perform the CO₂ adsorption. Therefore, CO₂ can be assumed to adsorb on the surface of La₂O₂CO₃ phases and then desorb from the adsorption surface sites, which demonstrates the CO₂ adsorption behavior on the monoclinic and hexagonal La₂O₂CO₃ phases. In Figure 5B, the CO₂ desorption peaks can be approximately categorized into three types. The peak at approximately 100 °C is related to a weak basic site, and the peaks in the range of 200–400 °C correspond to medium and strong basic sites [2,12,31,32]. The CO₂ adsorption modes on each basic site have been studied by a combination of FT-IR spectroscopy and CO₂-TPD measurements [31,32]. Manoilova et al. [30] investigated the CO₂ adsorption onto La₂O₃ by IR spectroscopy, TPD, and DFT calculations. The DFT calculation for the CO₂ adsorption on La₂O₃ predicted that CO₂ gas adsorbed on the surface in the form of polydentate and monodentate species as a starting structure, and then La₂O₃ made a stable connection with polydentate and asymmetric CO₂ adsorptions at the saturated coverage. The CO₂ desorption peak at approximately 290 °C in the CO₂-TPD profile of LaOCl was assigned to the decomposition of coupled bridged CO₂ adsorbate species [31]. On the basis of the results from the FT-IR and CO₂-TPD measurements of Mg–Al basic oxides, Di Cosimo et al. [32] suggested that the three types of CO₂ adsorption modes (e.g., bicarbonate, bidentate carbonate, and unidentate carbonate) were low-strength, medium-strength, and high-strength basic sites, respectively. It was determined that bidentate and unidentate carbonates remained on the surface at approximately 300 °C; only unidentate carbonate was detected at 350 °C [32]. Therefore, in this study, the peak at 110 °C, peaks at approximately 240 °C, and shoulders at approximately 310 °C can be assigned to the desorption of CO₂ species adsorbed on weak, medium and strong basic sites, respectively. Figure 5B and

Table 1. The peak intensities quantified in the CO₂-TPD patterns.

Samples	Temperature at Maximum (°C)	Quantity (cm3/g STP)
PL-12h	119	31.7
	306	3.38
HL-12h	109	24.6
	241	29.0

shows that the HL-12h sample has a higher combined intensity of medium and strong basic sites than PL-12h, which suggests that the hexagonal type II La₂O₂CO₃ phase provides more CO₂ adsorption sites on the surface. This observation is in good agreement with the TGA results shown in Figure 5A.

A DFT calculation was performed to optimize the bulk structures of both La₂O₂CO₃ phases (Figure 6). The lattice constant of La₂O₂CO₃ in the disordered hexagonal structure was predicted by considering the ratio (c/a) of lattice parameters (a and c) of the hexagonal structure [33]. Our DFT calculated lattice constants of La₂O₂CO₃ nanoparticles in both monoclinic and hexagonal structures, similar to the available experimental data from the literature, which are shown in

Table 2. Lattice constants of La₂O₂CO₃ in monoclinic and hexagonal structures. The experimental lattice data of monoclinic [34] and hexagonal [35] structures are available from the literature.

La2O2CO3	DFT Calculated Data	Experimental Data [33,34]
Monoclinic	a = 12.286 Å	a = 12.239 Å
	b = 7.097 Å	b = 7.067 Å
	c = 16.531 Å	c = 16.465 Å
	β = 75.677	β = 75.690
Hexagonal	a = 4.100 Å	a = 4.076 Å
	b = 4.100 Å	b = 4.076 Å
	c = 16.053 Å	c = 16.465 Å
	γ = 120	γ = 120

[34,35]. On the basis of the DFT calculation, we can optimize the hexagonal type II and monoclinic type Ia La₂O₂CO₃ nanopartilces, as shown in Figure 7. From the optimized structure of each phase, the La atom is determined to have seven and eight oxygen atoms as nearest neighbors in monoclinic and hexagonal structures, respectively. The eight coordination numbers of the La atom in the hexagonal type II La₂O₂CO₃ nanoparticles can produce stronger bonding with carbonate species, which results in the higher stability of the hexagonal type II structure compared to that of the monoclinic type Ia La₂O₂CO₃.

3.3. CO₂ Electrochemical Reduction

Figure 8 shows LSV curves ranging from 0 to −0.6 V vs. Ag/AgCl for PL-12h and HL-12h in CO₂-saturated 0.1 M NaHCO₃ electrolyte. HL-12h exhibits a maximum total current density of −25.2 mA/cm² at −1.26 V vs. Ag/AgCl ,whereas a maximum current density of −17.97 mA/cm² for PL-12h is achieved at −1.438 V vs. Ag/AgCl. In addition, HL-12h shows a more positive onset potential toward CO₂ electrochemical reduction than PL-12h in Figure 8. Both the higher current density and more positive onset potential apparently indicate a higher activity toward the CO₂ electrochemical reduction in HL-12h compared to that of PL-12h.

The chronoamperometry (CA) experiments were performed at different potentials for each 10 min, and gaseous products were determined by GC. For both PL-12h and HL-12h, the main gaseous products are CH₄, C₂H_4, C₂H₆ and H₂. Figure 9 shows the Faraday efficiency (FE) of carbon-containing products for PL-12h and HL-12h, resulting in a much higher FE for HL-12h than those for PL-12h. C₂H₄ is a dominant carbonaceous product at lower potential. A maximum of the ethene FE (9.4%) for HL-12h is achieved at −0.6 V (vs Ag/AgCl), while that for PL-12h is lower than 5%. Interestingly, CO gas was not detected in the potential range, even for the two La₂O₂CO₃ samples_. This indicates that La₂O₂CO₃ catalysts are efficient for C-C coupling rather than desorption to form CO gas, since CO is an intermediate for CO₂ transformation to ethene during CO₂ reduction [36]. The superior electrocatalytic activity of HL-12h to PL-12h would result from the better CO₂ adsorption ability which can optimize the first step involving electron and proton transfer to form a *COOH intermediate, which is then converted to other carbonaceous products [37]. The higher electronegativity of hexagonal La₂O₂CO₃ of HL-12h leads to the better CO₂ adsorption ability [38].

4. Conclusions

In this study, La₂O₂CO₃ nanoparticles with hexagonal and monoclinic phases were prepared by different preparation methods, and the CO₂ behavior on each crystalline structure was investigated by CO₂-TPD, TGA measurements, and CO₂ electrochemical reduction. The hydrothermal method produced the hexagonal type II La₂O₂CO₃ phase, whereas the monoclinic type Ia phase was synthesized by the precipitation method (PL-12h and PL-24h). The initial La(OH)₃ phase was transformed into the La(OH)CO₃ phase by the reaction with CO₂ supplied from air in the precipitation method. The hexagonal La₂O₂CO₃ phase showed a higher CO₂ adsorption ability on the surface and a higher stability in the bulk structure than the monoclinic phase, owing to the differences in optimized crystalline structures predicted by the DFT calculation. Consequently, the hexagonal La₂O₂CO₃ phase of HL-12h had a higher current density and a more positive onset potential than the monoclinic La₂O₂CO₃ of PL-12h in CO₂ electrochemical reduction.