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

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.


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 (La 2 O 3 ) has been widely used as a promoter or support in heterogeneous catalysis [11][12][13]. The basicity of La 2 O 3 readily induces the adsorption of CO 2 to form the lanthanum oxycarbonate (La 2 O 2 CO 3 ) phase, which is an important species in the La 2 O 3 -containing catalytic reaction [4,[13][14][15][16]. The crystalline structures of La 2 O 2 CO 3 can be divided into three types of different polymorphs: a tetragonal La 2 O 2 CO 3 (type I), a monoclinic La 2 O 2 CO 3 (type Ia), and a hexagonal La 2 O 2 CO 3 (type II) [16][17][18]. The hexagonal type II La 2 O 2 CO 3 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 La 2 O 2 CO 3 phases affect the interaction between La 2 O 2 CO 3 and ZnO in the La 2 O 2 CO 3 /ZnO composite materials as well as the catalytic behavior of the composite materials on glycerol carbonation with CO 2 [4,21]. Meanwhile, the monoclinic type Ia La 2 O 2 CO 3 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 La 2 O 2 CO 3 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 La 2 O 2 CO 3 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 La 2 O 2 CO 3 phases in the conventional methods and ii) the CO 2 behavior on the La 2 O 2 CO 3 structures, which is an essential step in the CO 2 -involving catalytic reactions, as well as the formation of the different La 2 O 2 CO 3 phases.
In this study, we prepared the nanoparticles with type Ia and type II La 2 O 2 CO 3 crystal structures by conventional wet preparation methods and investigated the formation of different La 2 O 2 CO 3 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 CO 2 behavior on the different La 2 O 2 CO 3 crystal structures was observed by CO 2 -temperature programmed desorption (TPD), thermogravimetric analysis (TGA), and linear sweeping voltammetry (LSV) of CO 2 electrochemical reduction. The superior CO 2 behavior of the hexagonal La 2 O 2 CO 3 phase to the monoclinic phase was additionally explained by the crystalline structures of both La 2 O 2 CO 3 phases, which was predicted by the density functional theory (DFT) calculation.

Materials
A total of 1.00 g of La(NO 3 ) 3 ·6H 2 O 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 La 2 O 2 CO 3 samples synthesized by the hydrothermal method were designated as HL-yh (y = 6, 12, or 24), where y represents the hydrothermal treatment time.

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 CO 2 -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 CO 2 gas (10 vol.% CO 2 /He) was fed into the reactor with a flow rate of 30 mL/min at 50 • C for CO 2 adsorption before conducting the CO 2 -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 CO 2 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 cm 2 ). The working electrode was tested 20 times at a scan rate of 20 mV/s. The electrolyte was 0.1 M NaHCO 3 saturated with CO 2 . Before each experiment, high-purity CO 2 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 La 2 O 2 CO 3 , 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 La 2 O 2 CO 3 , respectively [25]. Figure 1 shows the XRD patterns of La 2 O 2 CO 3 nanoparticle materials prepared at each reaction time. The two types of La 2 O 2 CO 3 phases are primarily detected in the PL samples: the monoclinic type Ia and hexagonal type II La 2 O 2 CO 3 phases. For 6 h of precipitation (PL-6h), the characteristic XRD peaks in the hexagonal La 2 O 2 CO 3 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 La 2 O 2 CO 3 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 La 2 O 2 CO 3 phase during the initial precipitation time, followed by the transformation into the monoclinic La 2 O 2 CO 3 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 La 2 O 2 CO 3 phase, regardless of the reaction time during the hydrothermal preparation, which demonstrates that there is no change in the La 2 O 2 CO 3 phase during the preparation process ( Figure 1(Ba-Bc)).

Synthesis of Monoclinic and Hexagonal La 2 O 2 CO 3 Nanoparticles
The FT-IR spectra of the PL and HL samples also confirm the formation of each La 2 O 2 CO 3 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 La 2 O 2 CO 3 phases, the bands at 745, 855, 1066, and 1518 cm −1 are interpreted as CO 3 2− vibrations related to the La 2 O 2 CO 3 phase [4,6,21,27]. The three-fold splitting bands at approximately 845 cm −1 (υ 2 ) and a strong band at 1367 cm −1 (υ 3 ) are assigned to the unique carbonate vibrational mode for the monoclinic type Ia La 2 O 2 CO 3 phase. The FT-IR spectra in Figure 2b,c of only the PL-12h and PL-24h samples show the characteristic bands (υ 2 and υ 3 ) of type-Ia La 2 O 2 CO 3 , whereas the FT-IR spectra of the other samples show the typical bands of the La 2 O 2 CO 3 phase, which further confirms that the formation of the type Ia and II La 2 O 2 CO 3 phases depends on the preparation conditions. In the precipitation method, the monoclinic type Ia La 2 O 2 CO 3 phase is mainly formed when the precipitation time is longer than 12 h, whereas the hydrothermal method produces only the hexagonal type II La 2 O 2 CO 3 phase. This is consistent with the XRD results in this study.
Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 12 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 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 Moreover, TEM measurements also provide additional evidence for the existence of the monoclinic and hexagonal La 2 O 2 CO 3 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 La 2 O 2 CO 3 phase is detected in the PL-12h sample, whereas the (260) plane of the hexagonal type II La 2 O 2 CO 3 phase is observed in the HL-12h sample. Similarly, the PL-6h sample shows the (004) plane of the type II La 2 O 2 CO 3 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) [1,6,12,21,26-28], 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  To further understand the formation mechanism of the monoclinic and hexagonal La 2 O 2 CO 3 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 Nanomaterials 2020, 10, 2061 6 of 13 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)CO 3 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)CO 3 . First, the strong bands at 1438 and 1491 cm −1 shown in Figure 4(Bd) can be assigned to the bending vibrations of CO 3 2− , 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)CO 3 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 CO 3 2− 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)CO 3 through the reaction with CO 2 . In the hydrothermal method, the crystallization of La(OH) 3 goes further, which results in the high crystallinity of La(OH) 3 .
Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 12 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)CO3, 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 CO3 2− , 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 CO3 2− 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 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., CO 2 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)CO 3 phase by the reaction with CO 2 at a time longer than 12 h, even though the 6-h precipitation produces only a weakly crystallized La(OH) 3 . Under the continuous CO 2 supply condition, there is a transformation from La(OH) 3 into La(OH)CO 3 . 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)CO 3 phase is finally converted into the monoclinic type Ia La 2 O 2 CO 3 phase in the precipitation, while La(OH) 3 is transformed into the hexagonal type II structure in the hydrothermal method. The sufficient supply of CO 2 into the aqueous solution produces the La(OH)CO 3 that can be changed into the monoclinic La 2 O 2 CO 3 phase.

CO 2 Behavior on La 2 O 2 CO 3 Nanoparticles
To investigate the CO 2 behavior on each La 2 O 2 CO 3 phase, TGA, CO 2 -TPD and CV of CO 2 electrochemical reduction for PL-12h (monoclinic type Ia La 2 O 2 CO 3 phase) and HL-12h (hexagonal type II La 2 O 2 CO 3 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 CO 2 gases that leave from the La 2 O 2 CO 3 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 CO 2 peak, which appears during the decomposition of La 2 O 2 CO 3 above 600 • C, can be assigned to CO 2 gases leaving from the bulk structure of the La 2 O 2 CO 3 phases, which is then transformed into the La 2 O 3 phase. The CO 2 decomposition from the bulk structure of the hexagonal La 2 O 2 CO 3 phase occurs at approximately 800 • C, which is higher than the temperature of CO 2 production during the decomposition of the bulk structure of the monoclinic La 2 O 2 CO 3 phase. This result shows that the thermal stability of the hexagonal La 2 O 2 CO 3 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 CO 2 gas that is adsorbed on the surface of the La 2 O 2 CO 3 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 CO 2 is adsorbed onto the surfaces of the La 2 O 2 CO 3 phase than that released from the bulk structure. Furthermore, based on each peak's intensity, shown in Figure 5A, the hexagonal type II La 2 O 2 CO 3 phase contains more CO 2 on the surface than that on the monoclinic type Ia phase. Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 12 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.

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 To better understand the CO 2 adsorption ability on the surface of each La 2 O 2 CO 3 phase, the CO 2 -TPD profiles of PL-12h and HL-12h were acquired. Before conducting the CO 2 -TPD experiments, both samples were thermally treated at 600 • C for 1 h in He gas, and then CO 2 was introduced into the reactor at 50 • C to perform the CO 2 adsorption. Therefore, CO 2 can be assumed Nanomaterials 2020, 10, 2061 8 of 13 to adsorb on the surface of La 2 O 2 CO 3 phases and then desorb from the adsorption surface sites, which demonstrates the CO 2 adsorption behavior on the monoclinic and hexagonal La 2 O 2 CO 3 phases. In Figure 5B, the CO 2 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 CO 2 adsorption modes on each basic site have been studied by a combination of FT-IR spectroscopy and CO 2 -TPD measurements [31,32]. Manoilova et al. [30] investigated the CO 2 adsorption onto La 2 O 3 by IR spectroscopy, TPD, and DFT calculations. The DFT calculation for the CO 2 adsorption on La 2 O 3 predicted that CO 2 gas adsorbed on the surface in the form of polydentate and monodentate species as a starting structure, and then La 2 O 3 made a stable connection with polydentate and asymmetric CO 2 adsorptions at the saturated coverage. The CO 2 desorption peak at approximately 290 • C in the CO 2 -TPD profile of LaOCl was assigned to the decomposition of coupled bridged CO 2 adsorbate species [31]. On the basis of the results from the FT-IR and CO 2 -TPD measurements of Mg-Al basic oxides, Di Cosimo et al. [32] suggested that the three types of CO 2 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 CO 2 species adsorbed on weak, medium and strong basic sites, respectively. Figure 5B and Table 1 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 La 2 O 2 CO 3 phase provides more CO 2 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 La 2 O 2 CO 3 phases ( Figure 6). The lattice constant of La 2 O 2 CO 3 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 La 2 O 2 CO 3 nanoparticles in both monoclinic and hexagonal structures, similar to the available experimental data from the literature, which are shown in Table 2 [34,35]. On the basis of the DFT calculation, we can optimize the hexagonal type II and monoclinic type Ia La 2 O 2 CO 3 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 La 2 O 2 CO 3 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 La 2 O 2 CO 3 .
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.      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/cm 2 at −1.26 V vs. Ag/AgCl ,whereas a maximum current density of −17.97 mA/cm 2 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.  Figure 8 shows LSV curves ranging from 0 to −0.6 V vs. Ag/AgCl for PL-12h and HL-12h in CO 2 -saturated 0.1 M NaHCO 3 electrolyte. HL-12h exhibits a maximum total current density of −25.2 mA/cm 2 at −1.26 V vs. Ag/AgCl ,whereas a maximum current density of −17.97 mA/cm 2 for PL-12h is achieved at −1.438 V vs. Ag/AgCl. In addition, HL-12h shows a more positive onset potential toward CO 2 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 CO 2 electrochemical reduction in HL-12h compared to that of PL-12h. CO2-saturated 0.1 M NaHCO3 electrolyte. HL-12h exhibits a maximum total current density of −25.2 mA/cm 2 at −1.26 V vs. Ag/AgCl ,whereas a maximum current density of −17.97 mA/cm 2 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 CH 4 , C 2 H 4, C 2 H 6 and H 2 . 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. C 2 H 4 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 La 2 O 2 CO 3 samples . This indicates that La 2 O 2 CO 3 catalysts are efficient for C-C coupling rather than desorption to form CO gas, since CO is an intermediate for CO 2 transformation to ethene during CO 2 reduction [36]. The superior electrocatalytic activity of HL-12h to PL-12h would result from the better CO 2 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 La 2 O 2 CO 3 of HL-12h leads to the better CO 2 adsorption ability [38]. Nanomaterials 2020, 10, x FOR PEER REVIEW 10 of 12

CO 2 Electrochemical Reduction
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, C2H4, 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].

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

Conclusions
In this study, La 2 O 2 CO 3 nanoparticles with hexagonal and monoclinic phases were prepared by different preparation methods, and the CO 2 behavior on each crystalline structure was investigated by CO 2 -TPD, TGA measurements, and CO 2 electrochemical reduction. The hydrothermal method produced the hexagonal type II La 2 O 2 CO 3 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)CO 3 phase by the reaction with CO 2 supplied from air in the precipitation method. The hexagonal La 2 O 2 CO 3 phase showed a higher CO 2 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 La 2 O 2 CO 3 phase of HL-12h had a higher current density and a more positive onset potential than the monoclinic La 2 O 2 CO 3 of PL-12h in CO 2 electrochemical reduction.
Author Contributions: H.Y. performed the experiments and wrote the paper; K.J., S.G.K. and Y.M. contributed data analysis; E.W.S. supervised the work and polished the paper. All authors have read and agreed to the published version of the manuscript.