On the role of Ce in CO2 adsorption and activation over lanthanum species

This paper describes the influence of Ce addition on the CO2 adsorption and activation over La2O3. Ce addition is verified to promote the formation of bidentate carbonate on La2O3 and affect the ratio of hexagonal/monoclinic La2O2CO3 on the Ce–La binary oxides.


Introduction
La 2 O 3 , as a common rare earth oxide with strong basicity, is generally used as a support or promotor to facilitate CO 2 adsorption and activation. [1][2][3][4] Typically, CO 2 adsorbs on the surface of La 2 O 3 and subsequently converts La 2 O 3 to La 2 O 2 CO 3 . 5, 6 The formed La 2 O 2 CO 3 species actively reacts with coke thus enhancing catalyst stability. [5][6][7][8] Additionally, La 2 O 2 CO 3 plays a signicant role in the oxidative coupling of methane (OCM), and CH 4 can be activated over the surface of La 2 O 2 CO 3 . 9,10 La 2 O 2 CO 3 has three phases including tetragonal (I type), monoclinic (Ia type), and hexagonal (II type). 11,12 As treatment temperature increases, phase transformation happens. Transformation from I-La 2 O 2 CO 3 to Ia-La 2 O 2 CO 3 takes place readily, namely Ia-La 2 O 2 CO 3 is the monoclinic distortion of I-La 2 O 2 CO 3 . 12,13 Conversion of Ia-La 2 O 2 CO 3 to II-La 2 O 2 CO 3 is a slow process below 600 C. 12 And II-La 2 O 2 CO 3 decomposes to La 2 O 3 at about 750 C. 14 It has been reported that the orientation of carbonate ions is closely related to the crystalline phase of La 2 O 2 CO 3 , 11,15 both of which can affect the catalytic performance. 16,17 It has been found that mixing ZnO with La 2 O 2 CO 3 increases the basicity of the La 2 O 2 CO 3 material. 18 In addition, Metiu et al. reported that dopants can affect CH 4 activation and dissociation on lanthanum oxide and hence improve the OCM performance. 19,20 Generally, ceria possesses good redox properties and has various applications. 21,22 It has been extensively used as an oxygen carrier 23 and is a necessary component of catalysts used in reforming processes, [24][25][26] water-gas shi reaction, 27 CO oxidation, 28 and soot combustion. 29,30 In order to improve the oxygen storage capacity (OSC) and oxygen mobility (OM) of ceria, an appropriate dopant is typically mixed with ceria to enhance the OSC/OM of ceria. [31][32][33] La 3+ , as an aliovalent dopant, has been extensively applied to enhance the OSC/OM of ceria, [34][35][36] during which oxygen vacancies can be formed due to the charge compensation mechanism. 37,38 It should be noted that the reported synergy of Ce-La binary oxide is based on the fact that La addition can largely promote the formation of oxygen vacancy on ceria, [36][37][38] while this paper investigates the inuence of Ce addition on the properties of lanthanum species.
Since the release or uptake of lattice oxygen is closely related to an oxygen/steam atmosphere, herein an oxygen/steam atmosphere is excluded to minimize the involvement of oxygen vacancy. Thus, CO 2 as a so oxidant is selected due to its weak oxidation capacity compared with O 2 or H 2 O molecule. Given that CO 2 adsorbs on the surface of La 2 O 3 and reacts with La 2 O 3 to form La 2 O 2 CO 3 , 5,6 the effect of Ce addition on the properties of La 2 O 2 CO 3 formed under the mild oxidative conditions has been investigated. For the OCM reaction, coke deposition is negligible in an oxidative atmosphere, but it takes place under oxygen lean conditions. 39,40 When O 2 is replaced by CO 2 to rule out the inuence of oxygen vacancy, dry reforming of methane (DRM) mainly occurs, during which CH 4 reacts with CO 2 to form syngas (CO and H 2 ). For the DRM reaction, coke deposition and sintering of metal particles can lead to the deactivation of catalysts. 4,41 Herein, the DRM reaction is used as a probe reaction to examine the coke elimination performance of Ce-La binary oxides.
This paper demonstrates the inuence of Ce addition on the properties of lanthanum species, including the adsorption mode of CO 2 (bidentate carbonate and monodentate carbonate) and the crystalline phase of lanthanum oxycarbonate (hexagonal La 2 O 2 CO 3 and monoclinic La 2 O 2 CO 3 ) formed aer CO 2 and CH 4 adsorption. In situ diffuse reectance infrared Fourier transform spectroscopy (DRIFTS) measurements are applied to investigate surface species on the Ce-La binary oxide during the process of CO 2 /CH 4 adsorption. The physical-chemical properties of the catalysts prior to and aer CO 2 /CH 4 adsorption are investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectra, N 2 -physisorption, transmission electron microscopy (TEM), and H 2 temperatureprogrammed reduction (H 2 -TPR). Periodic density functional theory (DFT) calculations are carried out to estimate CO 2 adsorption energy on Ce-La binary oxides. DRM is selected as the probe reaction to examine the performance of coke elimination, and CO 2 temperature-programmed desorption (CO 2 -TPD) and thermogravimetric analysis (TGA) are applied to examine the basicity of Ce-La binary oxides and properties of the deposited coke during the DRM process.

Results and discussion
Textural properties of Ce-La binary oxide Table 1 sums up the textural properties of the samples with different Ce/La ratios at the moment of "0 min" (Fig. 1, see details in the Experimental section). XRD patterns of a series of Ce-La binary oxides at "0 min" are shown in Fig. 2a and b. Hexagonal lanthanum oxycarbonate (II-La 2 O 2 CO 3 , JCPDF#37-0804) acts as the dominant species for the series of Ce-La binary oxides. When the Ce/La ratio reaches 0.10 and higher, monoclinic lanthanum oxycarbonate (Ia-La 2 O 2 CO 3 , JCPDF#48-1113) can be detected but is minimal. Results show that the peaks of La 2 O 2 CO 3 gradually migrate to a higher degree (Fig. 2b), indicating that lattice parameters decrease correspondingly (Table 1). Since the ionic radius of either Ce 3+ (0.101 nm) or Ce 4+ (0.097 nm) is smaller than that of La 3+ (0.11 nm), 37 the decrease of lattice parameters could be attributed to the fact that Ce-ions are doped into the lattice of La 2 O 2 CO 3 . 42 Furthermore, the surface areas of the series of Ce-La binary oxides were obtained through the N 2 sorption isotherm method. With the increase of Ce/La ratios, surface area gradually decreases, which is related to the decrease of lanthanum composition in the binary oxide and the small surface area of the cerium composition. The types of carbonate, formed upon CO 2 adsorption, have a close relationship with the crystalline phases of La 2 O 2 CO 3 , which will be discussed in the following part.
The valence state of cerium (Fig. 3) and the surface elemental composition (Table 1) were also examined. On the basis of literature reports, [43][44][45][46][47] the Ce 3d region consists of ve doublets. The spin-orbit components with unprimed labels, v and u, are ascribed to the primary Ce 3d 5/2 and Ce 3d 3/2 states while other doublets represent satellite features arising from the Ce 3d 5/2 and Ce 3d 3/2 ionization. 44,46,48 The doublets labeled v 0 /u 0 and v 0 /u 0 are characteristic of Ce 3+ , while the remaining doublets labeled v/u, v 00 /u 00 and v 000 /u 000 are characteristic of Ce 4+ . 44,46 The Ce 3+ surface concentration was calculated via the following equation: 44,48 where c x denotes the concentration of x and I(y) denotes the integral intensity of the specic peak. It should be noted that  the Ce 3+ content of CeO 2 is 21% while other Ce-La binary oxides have much higher Ce 3+ content than CeO 2 (Fig. 3). It is assumed that the enhanced content of Ce 3+ is doped into the lattice of La 2 O 2 CO 3 to maintain the concentration of Ce 3+ , which is consistent with the reduced lattice parameter. As the Ce/La ratio increases from 0.05 to 0.20, the Ce 3+ content gradually decreases (Fig. 3), indicating that the trend to be doped into the lattice of La 2 O 2 CO 3 is close to saturated state and Ce 4+ might exist in the form of CeO 2 distributed on the surface of the binary oxide. Additionally, CeO 2 diffraction peaks are absent even in the enlarged graph of XRD patterns (Fig. 2b), which suggests that CeO 2 particles are uniformly dispersed on the surface of the Ce-La binary oxide. For the series of Ce-La binary oxides (Table 1), bulk Ce/La ratios obtained from ICP-OES were consistent with nominal Ce/La ratios, while surface Ce/La ratios obtained from XPS are higher than bulk Ce/La ratios. The difference in Ce/La ratios between the bulk and surface mainly results from the selected subsequent impregnation method, namely La 2 O 2 CO 3 was rst synthesized and then impregnated with Ce precursor solution so that cerium mainly distributed on the surface of the binary oxide. The morphology and structure of Ce-La binary oxides with different Ce/La ratios were characterized via TEM (Fig. S1 in the ESI †). Particles randomly distribute along the surface of La 2 O 2 CO 3 , and some of them are conrmed as CeO 2 . It can be found that small particles have continuous lattice fringes with the oxide hosts, indicating the formation of the solid solution on the interface between the observed small particles and oxide hosts. 37 Since CeO 2 diffraction peaks are absent in XRD patterns (Fig. 2), UV-vis is applied to measure the indirect band gap of CeO 2 , which reects variation tendency in the size of CeO 2 particles located on the surface of the samples. 49,50 The indirect band gaps of nonoriented polycrystalline CeO 2 and La 2 O 3 are 3.19 eV, 49 and 5.2 eV, 51 respectively. As shown in Fig. S2 in the ESI, † the indirect band gap of CeO 2 migrates to a lower value with the increase of Ce/La ratios, indicating the corresponding increment in the size of CeO 2 particles on the surface of binary oxides. In addition, the variation tendency of the increment remains the same when the Ce/La ratio reaches 0.15 and 0.20. It is reported that the concentration of Ce 3+ increases with the reduction in CeO 2 particle size. 47 Therefore, the obtained results of UV-vis ( Fig. S2 in the ESI †) have a similar variation tendency with the results of XPS (Fig. 3).

In situ DRIFTS measurements
In situ DRIFTS measurements are applied in order to identify surface intermediates present on the Ce-La binary oxide samples from "0 min" to "30 min" (see the denition in the Experimental section), during which coke is formed via CH 4 decomposition and will further react with carbonate formed aer CO 2 adsorption (Fig. 1). In situ DRIFTS spectra are shown in Fig. 4, and corresponding contour graphs are inserted in the x-y planes of Fig. 4 and listed alone in Fig. S3 in the ESI. † The band at 1300 cm À1 corresponds to the C-H bending of gaseous methane, 24,52,53 and it can be found on all samples. The strong band at 1563 cm À1 corresponds to bidentate carbonate with the coexistence of bands at 1300, 1029 and 856 cm À1 , 15,54,55 while the strong band at 1345 cm À1 is ascribed to monodentate carbonate with the coexistence of bands at 1428, 1070 and 856 cm À1 . 15,54,55 In addition, bands at 1748 and 1796 cm À1 are overtones of C]O stretching vibration and CO 3 2À stretching vibration. 15 For the series of La-Ce binary oxides, both bidentate carbonate and monodentate carbonate exist on the surface ( Fig. 4 and S3 in the ESI †), while for CeO 2 , bidentate carbonate is absent ( Fig. 4f and S3f in the ESI †). At "0 min", the intensity of bidentate carbonate on La-Ce binary oxides increases as the Ce/ La ratio increases from 0 to 0.15, reaches the maximum when the Ce/La ratio is 0.15, and nally decreases slightly when the Ce/La ratio reaches 0.20 ( Fig. S3 and S4 in the ESI †).
Simultaneously, the intensity of monodentate carbonate shows a variation tendency, which is opposite to the intensity of bidentate carbonate. Therefore, it is assumed that Ce addition can inuence the CO 2 adsorption mode on the surface of Ce-La binary oxide and promote the transformation from monodentate carbonate to bidentate carbonate.
When CO 2 is adsorbed on La 2 O 3 (30 min prior to "0 min" in Fig. 1), it reacts with La 2 O 3 and then leads to the formation of La 2 O 2 CO 3 . During the in situ DRIFTS measurement (from "0 min" to "30 min" in Fig. 1), CH 4 can be adsorbed on the surfaces of La 2 O 2 CO 3 and dissociates to form coke under non-oxidative conditions, 39,40 while coke will react with La 2 O 2 CO 3 to form La 2 O 3 and CO. 5,8 For the series of Ce-La binary oxides, the intensity of bidentate carbonate keeps decreasing as time on stream goes by, while the intensity of monodentate carbonate remains stable ( Fig. 5 and S3 in the ESI †). Therefore, bidentate carbonate is consumed to react with coke during the introduction of CH 4 . It is concluded that bidentate carbonate is active in the reaction with the coke, while monodentate carbonate is inactive (Fig. 4, S3 and S4 in the ESI †). If we use I B / I M (intensity of $1563 cm À1 /intensity of $1345 cm À1 ) to evaluate the ratio of bidentate carbonate to monodentate carbonate, it is found that when the Ce/La ratio equals 0.15, Ce-La binary oxide possesses the highest ratio of I B /I M (Fig. 5).
Considering that Ce addition can affect the ratio of I B /I M , it is assumed that Ce addition might affect the performance of coke elimination, which will be discussed in the following part.
Inuence on the crystalline phase XRD patterns of the series of Ce-La binary oxides at "30 min" aer reacting with CH 4 are shown in Fig. 2c and d. The CeO 2 phase remains absent indicating that cerium is well dispersed. By comparison of the XRD patterns of samples at "0 min" and "30 min", the intensity of peaks ascribed to II-La 2 O 2 CO 3 decreases signicantly compared with Ia-La 2 O 2 CO 3 . In addition, it is observed that as the Ce/La ratio increases from 0 to 0.15, the decrease in the intensity of II-La 2 O 2 CO 3 peaks becomes much more prominent. The consumption of II-La 2 O 2 CO 3 during the CH 4 adsorption indicates that II-La 2 O 2 CO 3 is more active than Ia-La 2 O 2 CO 3 for coke elimination. For the 0.15Ce-LOC sample, the intensity of II-La 2 O 2 CO 3 is much lower than that of Ia-La 2 O 2 CO 3 so that Ia-La 2 O 2 CO 3 eventually becomes the dominant species on the sample, indicating that Ce addition can inuence the decomposition of II-La 2 O 2 CO 3 caused by the reaction with deposited coke.
Raman spectra of samples at "30 min" are shown in Fig. 6, which are consistent with the XRD patterns. Peaks at 290, 333, 438, 451, 1052, and 1341 cm À1 correspond to Ia-La 2 O 2 CO 3 , while peaks at 355, 385, 740, and 1082 cm À1 are assigned to II-La 2 O 2 CO 3 . 13,56,57 The intensity ratio of peaks at 355 cm À1 / 451 cm À1 can be used as a descriptor to evaluate the dominant surface species. When the Ce/La ratios are 0 and 0.05, the II-La 2 O 2 CO 3 phase is the dominant surface species and Ia-La 2 O 2 CO 3 phase is the subordinate. However, when the Ce/La ratio is higher than 0.10, Ia-La 2 O 2 CO 3 acts as the dominant surface species. For 0.15Ce-LOC, it possesses the lowest intensity ratio of peaks at 355 cm À1 /451 cm À1 , indicating that the ratio of II-La 2 O 2 CO 3 /Ia-La 2 O 2 CO 3 reaches the lowest level. It can be assumed that II-La 2 O 2 CO 3 formed on the 0.15Ce-LOC sample has the fastest decomposition rate. Two additional peaks at 216 and 577 cm À1 are reported to be induced by the dopant, 58 which veries the existence of Ce 3+ in the lattices of the samples. 59,60 DRIFTS results demonstrate that Ce addition is capable of facilitating the formation of bidentate carbonate, which is active for coke elimination. Based on the XRD patterns (Fig. 2) and Raman spectra (Fig. 6), with the decomposition of bidentate carbonate, the ratio of II-La 2 O 2 CO 3 to Ia-La 2 O 2 CO 3 will change correspondingly. Spivey et al. directly ascribed FTIR bands at 1509 cm À1 and 1367 cm À1 to II-La 2 O 2 CO 3 and Ia-La 2 O 2 CO 3 , respectively. 16 They concluded that only II-La 2 O 2 CO 3 acts as a reactive species to eliminate coke while Ia-La 2 O 2 CO 3 merely acts as a spectator species. 16 However, Kawi et al. reported that Ia-La 2 O 2 CO 3 mainly participated in coke elimination rather than II-La 2 O 2 CO 3 . 61 Combined with results of XRD patterns, Raman spectra and in situ DRIFTS spectra, we could conclude that bidentate carbonate is active for coke elimination and closely related to the II-La 2 O 2 CO 3 phase while monodentate carbonate is inactive for coke elimination and closely related to the Ia-La 2 O 2 CO 3 phase.
Catalytic performance of coke elimination H 2 -TPR was carried out to investigate the decomposition behavior of La 2 O 2 CO 3 formed on the series of Ce-La binary oxide under a H 2 atmosphere. The H 2 -TPR proles are shown in Fig. 7. It should be mentioned that the formation of a reduction peak in H 2 -TPR proles is due to the reaction of H 2 and CO 2 released during the decomposition of La 2 O 2 CO 3 . Herein, peaks at 450-500 C correspond to the desorption of chemisorbed CO 2 . 9 With the increase of Ce/La ratios, the desorption temperature of chemisorbed CO 2 increases to higher temperature. It indicates that Ce addition can strengthen the chemisorption of CO 2 on the surface of lanthanum species, while peaks at around 700 C correspond to the decomposition of II-La 2 O 2 CO 3 , 62 since the phase transformation of Ia-La 2 O 2 CO 3 to Fig. 5 The intensity ratios of bidentate/monodentate carbonate in DRIFTS spectra as a function of time on stream. Fig. 6 Raman spectra of the series of Ce-La binary oxide upon CH 4 adsorption for 30 min.
II-La 2 O 2 CO 3 is complete at around 600 C. 12 With the increase of Ce/La ratios, the decomposition temperature of II-La 2 O 2 CO 3 under the H 2 atmosphere decreases to a lower temperature, indicating that Ce addition can promote the decomposition of II-La 2 O 2 CO 3 under the H 2 atmosphere, which is related to the activity in the reaction of II-La 2 O 2 CO 3 and coke.
In order to examine the performance of coke elimination, DRM reaction is used as a probe reaction, and then Ni particles are supported on the series of Ce-La binary oxide to produce coke. Ni loadings are xed at 5 wt% for the series of Ce-La binary oxide to ensure that all catalysts exhibit similar CH 4 conversions under appropriate reaction conditions. CO 2 -TPD was employed to investigate the basicity of the Ni supported samples (Fig. 8). It has been extensively reported that the basicity of the support is benecial to coke elimination. 63 CO 2 desorption will take place when the reaction temperature (600-650 C) is higher than the desorption temperature. Thus, basicity with higher desorption temperature should be investigated. CO 2 -TPD proles show that areas of peaks higher than 700 C increase with the increment of Ce/La ratio, and reach the maximum when the Ce/La ratio is 0.15, and then decrease slightly when the Ce/La ratio is 0.20. For pure CeO 2 as a reference, the area of the peak higher than 700 C is negligible, which indicates that pure CeO 2 itself has much weaker CO 2 adsorption compared with Ce-La binary oxide. According to the DRIFTS spectra (Fig. 4, 5 and S3 in the ESI †), for the 0.15Ce-LOC sample, it has the highest intensity of bidentate carbonate during the CH 4 adsorption. A previous study by Valange et al. has shown that bidentate carbonate has higher stability and hence higher basicity compared with monodentate carbonate. 64 In addition, it has been reported that the decomposition temperature of II-La 2 O 2 CO 3 is higher than 700 C. 13 Therefore, the desorption peaks higher than 700 C can be ascribed to the decomposition of II-La 2 O 2 CO 3 . It should be mentioned that the atmosphere could affect the decomposition of La 2 O 2 CO 3 , 13 hence there is a difference in the decomposition temperature of II-La 2 O 2 CO 3 between CO 2 -TPD and H 2 -TPR ( Fig. 7 and 8). For the 0.15Ce-LOC sample, it has the highest amount of bidentate carbonate aer CO 2 adsorption and the highest basicity above 700 C. It indicates that bidentate carbonate has a close relationship with II-La 2 O 2 CO 3 . Based on these facts, it is assumed that Ce addition can promote the transformation from monodentate carbonate to bidentate carbonate on La 2 O 3 aer CO 2 adsorption, which will promote the formation of II-La 2 O 2 CO 3 . On the other hand, H 2 -TPR (Fig. 7) proles show that Ce addition improves the activity of II-La 2 O 2 CO 3 under the H 2 atmosphere.
DRM activity tests are applied to test the coke elimination performance of Ce-La binary oxide. GHSV has been adjusted to 60 000 mL h À1 g cat À1 to ensure that different catalysts exhibit similar CH 4 conversions. Spivey et al. reported that when the DRM reaction temperature is 550-650 C, the variation   tendency of coke formation is much more severe since coke originates from both CH 4 decomposition and Boudouard reaction. 65 Therefore, the reaction temperature is xed to 650 C to increase the coke formation. As shown in Fig. S5 in the ESI, † all the Ni catalysts have CH 4 conversions at around 44% and exhibit good stability. TGA proles are shown in Fig. 9. The mass loss below 700 C is ascribed to the oxidation of coke and Ni particles. And the mass loss above 700 C is ascribed to the decomposition of La 2 O 2 CO 3 to release CO 2 . At the end of TGA, all samples can be regarded as mixtures of NiO, La 2 O 3 and CeO 2 . According to the Ni loading, specic Ce/La ratio and mass loss obtained by TGA, the content of Ni, CeO 2 , La 2 O 2 CO 3 and La 2 O 3 in the spent catalysts can be estimated ( Table 2). It should be mentioned that the formation of La 2 O 3 on the spent catalysts is due to the spontaneous reaction between II-La 2 O 2 CO 3 and deposited coke. Additionally, since Ia-La 2 O 2 CO 3 is inactive for coke elimination and spontaneously transforms to II-La 2 O 2 CO 3 when the temperature is above 600 C, 12,16 the calculated content of La 2 O 2 CO 3 ( Table 2) is ascribed to the content of Ia-La 2 O 2 CO 3 which transforms to II-La 2 O 2 CO 3 during the programmed temperature process. Therefore, we can use the calculated content of La 2 O 3 and La 2 O 2 CO 3 in Table 2 to estimate the content of II-La 2 O 2 CO 3 and Ia-La 2 O 2 CO 3 , respectively. Based on the results of DRIFTS (Fig. 4, 5 and S3 in the ESI †), bidentate carbonate is active for coke elimination while monodentate carbonate is inactive for coke elimination. When correlating the molar ratio of La 2 O 3 /La 2 O 2 CO 3 in Table 2 with the maximum intensity ratio of bidentate/monodentate carbonate peaks in Fig. 5, a linear relationship is obtained as shown in Fig. 10, which indicates that bidentate carbonate has a close relationship with II-La 2 O 2 CO 3 while monodentate carbonate is closely related to Ia-La 2 O 2 CO 3 .
In addition, it can be found that the amount of coke decreases with the increase of Ce/La ratio. For 5Ni/0.15Ce-LOC, it has the lowest amount of coke and the highest molar ratio of La 2 O 3 /La 2 O 2 CO 3 aer 50 h DRM reaction, indicating that appropriate Ce addition can promote the formation of II-La 2 O 2 CO 3 to react with the deposited coke. According to the DTG proles in Fig. 9, the peak temperature of lamentous coke ($550 C) decreases with the increase of Ce/La ratio which reects the decrease of the graphitic degree of coke, 4 while the peak temperature of II-La 2 O 2 CO 3 (>700 C) exhibits the opposite variation tendency. Based on the above facts, it is concluded that the 0.15Ce-LOC sample shows the best performance for coke elimination.

DFT calculation of CO 2 adsorption energy
DFT calculation was applied to illustrate the inuence of Ce addition on the most stable CO 2 adsorption mode on La 2 O 2 CO 3 (Fig. 11). Ideally, models presuming that Ce atoms distribute along the surface layer were taken into consideration. In this case, a new C-O bond is formed between a CO 2 molecule with a surface O atom on La 2 O 2 CO 3 , which leads to the formation of carbonate. The calculated frequency of the formed C-O bond is 1517 cm À1 , which is close to the characteristic frequency of bidentate carbonate (1560 cm À1 ) collected by in situ DRITFS (Fig. 4). In addition, the calculated CO 2 adsorption energy for pure II-La 2 O 2 CO 3 ((Ce/La) s is equal to 0, the subscript s denotes surface) is À1.39 eV, which is a negative value since it is an exothermic adsorption process.
When the ratio of (Ce/La) s is equal to 1 : 7, there are eight possible sites for a Ce atom to replace a La atom within our selected unit cell (Fig. S6 in the ESI †). DFT calculations predict that the most stable CO 2 adsorption mode takes place when the La atom at site 4 (see the denition in Fig. 11) is replaced by a Ce atom and the calculated CO 2 adsorption energy is À1.50 eV. For samples with higher Ce content, the selected model was based on the most stable structure of samples with lower Ce contents. For example, when the ratio of (Ce/La) s is equal to 1 : 3, one Ce atom is xed at the fourth site on the basis of the obtained calculation (Fig. S7 in the ESI †). In this case, the most stable CO 2 adsorption mode takes place when La atoms located at sites 1 and 4 are replaced by Ce atoms and the calculated adsorption energy is À2.12 eV. Following the same procedure, when the ratio of (Ce/La) s increases to 3 : 5 and 1 : 1, the strongest adsorption energy of CO 2 over Ce-La binary oxides is À2.37 eV (Fig. S8 in the ESI †) and À2.13 eV (Fig. S9 in the ESI †), respectively. Meanwhile, twenty extra models with randomly distributed Ce structures were tested, which did not follow the mentioned procedure. The CO 2 binding over all these randomly generated models is less stable than the ones discussed above ( Table S1 in the ESI †). Additionally, the calculated CO 2 adsorption energy for pure CeO 2 is À1.32 eV, which is weaker than that of La 2 O 2 CO 3 and other Ce-La binary oxides. It indicates that the intensity of CO 2 adsorption on ceria is much weaker than that of La 2 O 2 CO 3 and other Ce-La binary oxides, which is responsible for the absence of bidentate carbonate on CeO 2 as shown in Fig. 4. According to the results of DFT calculation (Fig. 11), with the increase of (Ce/La) s ratios, the CO 2 adsorption energy gradually becomes lower and reaches the lowest value when (Ce/La) s is equal to 3 : 5, and then the CO 2 adsorption energy weakens. The variation tendency of CO 2 adsorption energy with the increase of (Ce/La) s ratios (Fig. 11) is consistent with the variation tendency for the peak intensity of bidentate carbonate on Ce-La binary oxides (Fig. 4). As the CO 2 adsorption energy becomes lower, CO 2 adsorption on binary oxide is strengthened, and the formed carbonate is expected to have better stability. Therefore, Ce addition can affect CO 2 adsorption energy for Ce-La binary oxide and the type of carbonate formed aer CO 2 adsorption. Ce-La binary oxide with the optimal Ce/La ratio exhibits the highest intensity of bidentate carbonate (Fig. 4), and hence has the highest basicity above 700 C (Fig. 8) and shows the best coke elimination performance (Fig. 9).

Conclusion
We have investigated the role of Ce addition in CO 2 adsorption and activation over lanthanum species. Based on results of in situ DRIFTS spectra, DFT calculations and other experimental characterizations, it is concluded that Ce addition can promote the formation of bidentate carbonate on Ce-La binary oxide via tuning CO 2 adsorption energy. With the increase of Ce/La ratio from 0 to 0.20, Ce addition facilitates transformation from monodentate carbonate to bidentate carbonate on Ce-La binary oxides. Bidentate carbonate is veried to be active in the reaction with deposited coke, while monodentate carbonate is inactive for coke elimination. With the consumption of bidentate carbonate, the variation of the intensity ratio of bidentate/ monodentate carbonate can affect the ratio of II-La 2 O 2 CO 3 /Ia-La 2 O 2 CO 3 . Bidentate carbonate is veried to be closely related to II-La 2 O 2 CO 3 and monodentate carbonate has a close relationship with Ia-La 2 O 2 CO 3 . When the Ce/La ratio is 0.15, the corresponding nickel catalyst has the highest intensity ratio of bidentate/monodentate carbonate and the highest ratio of II-La 2 O 2 CO 3 /Ia-La 2 O 2 CO 3 , which exhibits the highest basicity above 700 C and the best performance of coke elimination aer 50 h DRM reaction.

Experimental section
Catalyst preparation The synthesis route of La 2 O 2 CO 3 is described as follows. 2.6 grams of La(NO 3 ) 3 $6H 2 O and 7.2 grams of urea were separately dissolved in de-ionized water. Once dissolved, the two solutions were mixed with constant stirring; the concentrations of La 3+ and urea in the mixture were 0.015 mol L À1 and 0.30 mol L À1 , respectively. Then aqueous ammonia was added into the mixture to adjust the pH to 8.5. A white suspension was obtained aer heating in a water bath at 90 C for 3 h with constant stirring, followed by naturally cooling to room temperature. Aerwards, the white suspension was centrifuged and washed with absolute ethanol three times. La 2 O 2 CO 3 was nally prepared upon drying at 80 C overnight and calcination at 500 C for 2 h.
A series of Ce-La binary oxides were prepared by a wet impregnation method. The stoichiometric Ce/La ratio was chosen as 0, 0.05, 0.10, 0.15, and 0.20, respectively. The prepared La 2 O 2 CO 3 was impregnated with an aqueous solution containing a specied amount of Ce(NO 3 ) 2 $6H 2 O. Upon stirring at 80 C for 3 h, vacuum evaporation was carried out to remove the solvent. Then the sample was dried overnight, and ground and calcined at 600 C for 2 h. The prepared Ce-La binary oxide was marked as "xCe-LOC", where LOC denotes the prepared La 2 O 2 CO 3 and x denotes the specic stoichiometric Ce/La ratio.
A series of Ni catalysts supported on the prepared Ce-La binary oxide were synthesized by a similar procedure to that described above. For the subsequent wet impregnation method the Ni loading was xed at 5 wt% on the basis of reduction conditions. When the sample was impregnated with the Ni precursor and dried overnight, it was ground and calcined at 650 C for 2 h. Aer grinding to 20-40 mesh, the sample was reduced at 650 C under a H 2 atmosphere (H 2 /N 2 ¼ 1 : 3, 40 mL min À1 ) for 60 min. The prepared catalyst was named 5Ni/ xCe-LOC, where LOC denotes the prepared La 2 O 2 CO 3 and x denotes the specic Ce/La ratio.

Characterization
Textural properties of catalysts were measured through nitrogen adsorption-desorption at À196 C using a Micromeritics Tristar 3000 analyzer. All samples were degassed at 300 C for 3 h prior to the tests. The specic surface areas were calculated on the basis of the N 2 isotherms and the Brunauer-Emmett-Teller (BET) method. Combined with the Barrett-Joyner-Halenda (BJH) method and the desorption branches of the N 2 isotherms, cumulative volumes of pores were obtained.
Elemental contents of the prepared catalysts were examined by inductively coupled plasma optical emission spectroscopy H 2 -TPR experiment was applied to analyze the reduction behavior of the catalysts with the aid of a chemisorption apparatus (Micromeritics AutoChem II 2920). 100 mg of the sample was pretreated at 300 C for 1 h with an Ar stream (30 mL min À1 ) to remove moisture and impurities. Aer cooling to 50 C, the system was exposed to a 10 vol% H 2 /Ar stream (30 mL min À1 ) to reduce the sample. Subsequently, the temperature of the system was programmed to rise linearly from 100 C to 900 C with a rate of 10 C min À1 , during which variation of the signal of the thermal conductivity detector (TCD) was recorded.
CO 2 -TPD analysis was applied to investigate the basicity of the catalyst by utilizing the same chemisorption apparatus (Micromeritics AutoChem II 2920). 100 mg of the sample was prereduced at 750 C with a 10 vol% H 2 /Ar stream (50 mL min À1 ) for 30 min to completely decompose existing La 2 O 2 CO 3 on samples before CO 2 adsorption. Aer cooling to 60 C, the system was exposed to a stream of CO 2 gas (50 mL min À1 ) to carry out CO 2 adsorption for 30 min. Next, the system was exposed to a He stream (30 mL min À1 ) and the temperature was programmed to increase to 120 C for the removal of residual CO 2 in the stream. Once the signal of TCD reached a stable state, the temperature of the system was programmed to increase from 120 C to 900 C with a ramping rate of 10 C min À1 and at the same time the system starts to keep record of the TCD signal. An isothermal period lasting for 8 minutes at 900 C was set to ensure that the adsorbed CO 2 was totally desorbed.
A TEM instrument (FEI Tecnai G2 F20) was applied to investigate the morphology and structure of catalysts, and the working voltage was 100 kV. Aer the sample powder was dispersed in absolute ethanol via ultrasonication, the obtained suspension was dripped onto a copper grid-supported transparent carbon foil and dried in air for characterization.
XPS analysis of the catalysts was carried out on a Perkin-Elmer PHI 1600 ESCA system equipped with an Al KR X-ray source (E ¼ 1486.6 eV). Spectra were operated at a pass energy of 187.85 eV. The binding energy (B.E.) scale was measured on the basis of carbon contamination utilizing C 1s peak centered at 285 eV. In addition, core peaks were obtained using a nonlinear Shirley-type background. Besides, quantication of surface elemental composition was carried out according to Scoeld's relative sensitivity factors. 66 Properties of the coke deposited on the spent catalysts were characterized by utilizing a TGA system (STA449F3, NETZSCH Corp.). The TGA experiment was conducted in an air stream (50 mL min À1 ), and the temperature was programmed to rise from room temperature to 900 C with a heating rate of 10 C min À1 . The amount of coke deposition, II-La 2 O 2 CO 3 accumulation and oxidation of Ni particles were calculated according to the mass losses in TGA proles.
A Raman spectrometer (Renishaw inVia Reex) was employed to record Raman spectra under ambient conditions, which was equipped with a 532 nm Ar-ion laser beam as the excitation source. Each sample was examined more than three times at different positions.
UV-visible reectance spectra were collected on a SHIMADZU UV-2550 spectrophotometer using a pressed disc of the sample. Kubelka-Munk transformed diffuse reectance spectra (DRS) of all samples were measured with BaSO 4 powder as a reference.

In situ DRIFTS measurements
In situ DRIFTS experiments were performed on a ThermoFisher Nicolet IS50 spectrometer, which was equipped with a Harrick Scientic diffuse reection accessory and a mercury-cadmiumtelluride (MCT) detector. The scheme of the experimental process is shown in Fig. 1. The Ce-La binary oxide samples were placed in the cell of the DRIFTS apparatus and reduced at 600 C under 30 vol% H 2 /Ar stream (30 mL min À1 ). And then, the gas stream was switched from H 2 /Ar stream to Ar stream (30 mL min À1 ) in order to remove hydrogen in the gas stream. Subsequently, the baseline of DRIFTS was collected continuously until the obtained baseline spectra remained stable. Aerwards, the gas stream was switched to 10 vol% CO 2 /Ar stream (30 mL min À1 ) to carry out CO 2 adsorption for 30 min, aer which the CO 2 adsorption was saturated. The moment at the end of CO 2 adsorption was marked as "0 min". Subsequently, the gas stream was switched from 10 vol% CO 2 /Ar stream to 10 vol% CH 4 /Ar stream (30 mL min À1 ) in order to carry out CH 4 adsorption for 30 min. The moment at the end of CH 4 adsorption was marked as "30 min". In situ DRIFTS measurements were carried out from "0 min" to "30 min". Since DRIFTS spectra remained stable aer "30 min", it could be regarded that the reaction took place completely. Additionally, when it is "0 min" or "30 min", subsequent operations could be skipped and the temperature would decrease from 600 C to room temperature. When the cell was cooled to room temperature, samples could be taken out to carry out ex situ characterizations.

Periodic DFT calculations
Periodic DFT calculations were carried out with the assistance of Vienna ab initio Simulation Package (VASP). 67 The calculations employed the generalized-gradient approximation (GGA) in the form of the Perdew-Burke-Ernzerhof (PBE) exchangecorrelation functional. 68 A Hubbard U value was added to the PBE functional (DFT + U), which is chosen to improve the quality of the DFT calculations in dealing with oxides having narrow f or d bands. 69 The interactions between the atomic cores and electrons were described by the projector augmented wave (PAW) method. 70 The valence wave functions were expanded using plane-wave with a cutoff energy of 400 eV. A 4 Â 2 cell was used for La 2 O 2 CO 3 (110) and CeO 2 (110) surfaces, and a 3 Â 1 Â 1 k-point mesh was used for the Brillouin zone integration. The slab was three layers thick and separated by 15Å of vaccum. The top two layers of the slab were allowed to relax, while the bottom one layer was kept xed. For La, a value of U eff ¼ 7.5 eV was used, which was calculated self-consistently by Metiu et al. 19 U eff ¼ 4.5 eV was employed for Ce, which was reported by Fabris et al. 71 The adsorption energy of adsorbates, E ads , is dened as follows: where E total is the total energy of the system aer adsorption, E gas is the energy of the adsorbate in the gas phase, and E slab is the energy of the clean slab. Thus, a negative value means an exothermic adsorption process.

Coke elimination performance test
Catalytic activity tests were carried out in a quartz xed-bed tubular reactor (F 8 Â 44 mm) under atmospheric pressure. Prior to the test, the catalyst sample (100 mg, 20-40 mesh) was evenly mixed with 1 mL of quartz particles and then the mixture was loaded inside the reactor. Prior to the test, the catalysts were reduced at 650 C under a 25 vol% H 2 /N 2 stream (40 mL min À1 ) for an hour. The ow rate of the feed gas was set at 100 mL min À1 (GHSV ¼ 60 000 mL h À1 g cat À1 , CH 4 : CO 2 : N 2 ¼ 20 : 20 : 60 mL min À1 ) to ensure that the activities of the catalysts are close to each other to simplify the comparison of coke deposition. Here, the mentioned GHSV is based on a total ow. The concentrations of gas species including CH 4 , CO 2 , H 2 , CO, and N 2 were measured online with the assistance of a gas chromatograph (GC2060, Shanghai Ruimin Instrument). Helium was used as the carrier gas. The GC was equipped with a TCD and two columns including a TDX-01 column followed by a 5 A molecular sieve column. Activity test was performed at 650 C for 50 h. Conversions of CH 4 and CO 2 (X CH 4 and X CO 2 ), selectivities to H 2 and CO (S H 2 and S CO ), and the H 2 /CO ratio are dened as follows: S H 2 ¼ F H 2 ;out 2 Â ðF CH 4 ;in À F CH 4 ;out Þ (5) S CO ¼ F CO;out ðF CH 4 ;in À F CH 4 ;out Þ þ ðF CO 2 ;in À F CO 2 ;out Þ (6)

Conflicts of interest
There are no conicts to declare.