High performance of Mg-La mixed oxides supported Ni catalysts. for dry reforming of methane: the effect of crystal structure

Transcription

1 High performance of Mg-La mixed oxides supported Ni catalysts for dry reforming of methane: the effect of crystal structure Jun Ni, a, c Luwei Chen, b, * Jianyi Lin, b Martin Karl Schreyer b, Zhan Wang b and Sibudjing Kawi a, * 5 a Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore , Singapore. b Institute of Chemical and Engineering Sciences, A*STAR (Agency for Science, Technology and Research), 1 Pesek Road, Jurong Island, Singapore , Singapore. c Institute of Industrial Catalysis, College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou, 14, China. a, * Corresponding author. Tel.: ; fax: chekawis@nus.edu.sg (S. Kawi). b, * Corresponding author. Tel.: ; fax: address: chen_luwei@ices.a-star.edu.sg (L. Chen). 1

2 Abstract A series of Mg-La mixed oxides support with various Mg 2+ /La 3+ mole ratios were prepared via co-precipitation of Mg and La nitrates and then impregnated to form 5 wt.% Ni catalysts. The as-prepared catalysts were evaluated in DRM reaction for 0 h and characterized by means of in situ DRIFTS, XRD, TEM, CO 2 -TPD, XPS, and TGA. It was found that the interaction of suitable amount of MgO with La 2 O 3 stabilized cubic La 2 O 3 species in catalysts, which has high basicity to adsorb CO 2 forming monoclinic La 2 O 2 CO 3 (Ia) species in DRM reaction. The introduction of MgO also created surface oxygen ions (i.e. O ). Both monoclinic La 2 O 2 CO 3 (Ia) and surface oxygen species are able to oxidize and remove deposited carbon, keeping the Ni catalyst at high activity and stability. Low Mg 2+/ La 3+ ratios generated hexagonal La 2 O 3 and La 2 O 2 CO 3 (II) in DRM reaction. The hexagonal La 2 O 2 CO 3 (II) did not play significant role in carbon removal so that the catalysts deactivated fast. Keywords Dry reforming of methane; Mg-La mixed oxides; Ni catalysts; La 2 O 2 CO 3 ; La 2 O 3. 2

3 1 Introduction The catalytic process of CO 2 (dry) reforming of methane (DRM), converting two greenhouse gases CO 2 and CH 4 into synthesis gas (H 2 and CO) for the production of liquid hydrocarbons in the Fischer-Tropsch synthesis [1, 2], is particularly interesting from environmental point of view. In addition, DRM reaction coupled with Fischer-Tropsch synthesis can store the energy applied to drive this endothermic reaction into the final liquid products, providing an opportunity for remote energy storage and utilization [3]. Ni-based catalysts which are among the best catalysts for DRM suffer from fast deactivation caused by serious carbon deposition, which is of high thermodynamic potential [4, 5]. Therefore, the development of highly active and carbon resistant Ni-based catalysts for DRM becomes a major aspect of research in this area. There are essentially two approaches to reduce carbon deposition of Ni-based catalysts: 1) Decrease the Ni catalyst particle size and improve its dispersion on support surfaces [6, 7]; and 2) Tune the catalyst acidity by introduction of a second oxide component [8, 9]. Smaller Ni particle size and better nickel dispersion not only lead to more active sites on the catalyst surface, but also reduce the possibility of carbon deposition, as already demonstrated by many theoretical calculations and experimental results [, 11]. In our previous study by our group on DRM reaction at 700 C over Ni/B 2 O 3 -Al 2 O 3 catalysts the presentation of strong acidic sites on the catalyst was found to favor the dissociation of adsorbed CO 2 and CH 4, resulting in the rapid carbon accumulation and hence catalyst deactivation. In contrast the support of strong basicity can enhance the CO 2 chemisorption on Ni catalyst surface, and favor the removal of carbon deposition via gasification during the reaction [12]. Magnesium oxide has been widely used as support for Ni-based catalysts due to its basicity and the same crystal structure as NiO. The interaction between MgO and NiO may form a non-reducible basic solid-solution, Ni x Mg (1-x) O [13], which not only increases CO 2 adsorption but also can control the Ni particle sizes [14], since only a small amount of NiO can be reduced and the nickel particles formed from the solid solution are often smaller than the critical size for coke formation [14]. In addition, Ni on reduced Ni/MgO catalysts is highly-dispersed and very stable against the particle sintering during the reaction due to the strong metal-support interaction. Thus Ni/MgO catalysts have 3

4 shown long-term stability and low carbon formation in DRM []. However, this strong metal-support interaction also limits the availability of active Ni sites, as a consequence a high metal Ni loading is required (~ wt.% ) to exhibit high activity in DRM [, 16]. Adjusting the interaction between Ni and MgO support would lead to the formation of high active catalysts with low metal loading and high resistance to carbon accumulation during high temperature DRM reaction. Lanthanum oxide (La 2 O 3 ) has also been considered as a promising basic support for nickel based catalysts due to its ability to react with CO 2 to form La 2 O 2 CO 3 in DRM reaction [17, 18]. Verykios [19, ] found these La 2 O 2 CO 3 species participate directly in DRM by decomposing to produce CO and providing oxygen species to react with deposited carbon at the interface of Ni-La 2 O 2 CO 3, restoring the activity of the Ni sites. In such a way, La 2 O 3 supported catalysts can facilitate the dissociation of adsorbed CO 2. Since the rate constant of carbon scavenging is 60 times higher than that of carbon formation during the reaction, the carbon deposition is kinetically inhibited and thus a stable La 2 O 3 supported nickel catalyst results [21]. This unique property of La 2 O 3 also enables La 2 O 3 to be used as promoter for DRM reaction [22]. Although MgO and La 2 O 3 as individual catalyst support has been extensively studied as above mentioned, the use of MgO and La 2 O 3 mixed oxides as the support of Ni catalysts for DRM has not been reported. While interfacing two unlike materials may often give rise to unusual properties resulting from the asymmetry of the structure, it is difficult to predict whether the prepared Mg-La mixed oxides supported Ni catalysts can retain the merits of each support component and have the additional unexpected properties beneficial for DRM reactions. To shed light on these unknowns, in the present study, a series of Mg-La mixed oxides supported Ni catalysts with various Mg 2+ /La 3+ mole ratios has been prepared via co-precipitation of Mg and La nitrates and sequential impregnation of Ni nitrate, keeping the constant metal Ni loading (5 wt.%). The catalytic activity and stability of these catalysts were then evaluated in DRM reaction at 700 ºC. Both reduced and used catalysts have been characterized by means of XRD, in situ DRIFTS, TEM, CO 2 -TPD, XPS, and TGA, with the aim to gain a deep understanding of the influence of the support composition over the catalytic performance of Ni-based catalysts in the long term DRM reaction. 4

5 2 Experimental 2.1 Catalyst Synthesis Mixed Mg-La oxide supports with different Mg 2+ /La 3+ ratios (1, 3, 5,, and ) were prepared by co-precipitation of Mg and La nitrates and then impregnated to form 5 wt.% Ni catalysts. In a typical method of catalyst preparation, desired amount of Mg(NO 3 ) 2 (Sigma-Aldrich, g, 99%, ACS Reagent) and La(NO 3 ) 3 (Aldrich, g, 99.99%) was dissolved in 0 ml of deionised water and heated to 60 ºC under vigorous stirring, respectively. The two precursor solutions were mixed homogeneously and were precipitated using a basic solution of 1M KOH (Sigma-Aldrich, , >85%, ACS Reagent) and 0. M K 2 CO 3 (Sigma-Aldrich, 31,026-3, 99%+, ACS) at a constant ph of in ultrasonic bath. The ultrasonic bath was used to control local nuclei population in order to have a reduction in particle size and an increase in the homogeneity of the mixed Mg and La hydroxides. After completion of the precipitation, the precipitate was collected by centrifuging at 6,000 rpm for min. The collected precipitate was thoroughly washed several times by re-dispersion in de-ionized water followed by re-collection by centrifuging as before. This procedure was repeated until the filtrate became neutral and the EDX analysis revealed the absence of potassium element in catalysts (not shown). The resultant product was oven-dried at 1 ºC for 12 h and calcined at 800 ºC for 4 h. For the catalyst with a pure oxide as support, commercial La 2 O 3 (Aldrich, G, 99%, nanopowder) or MgO (Strem Chemicals , 0G, nanopower) was used without any pretreatment. Five wt.% Ni was then supported on all these supports by wet impregnation method using Ni(NO 3 ) 2 6H 2 O (Merck, ) as precursor. The prepared catalysts which were subsequently dried at 1 C were used without any calcination prior to catalytic reactions. The purpose of using fresh catalysts was to prevent the loss of active Ni phase via formation the of Ni x Mg (1-x) O solid-solution during the calcination process. These catalysts were designated as 5%Ni/La 2 O 3, 5%Ni/xMgO-La 2 O 3 (x = 1, 3, 5,, is the Mg 2+ /La 3+ mole ratio) and 5%Ni/MgO. 2.2 Catalyst Characterizations Powder X-ray diffraction (XRD) 5

6 Powder X-ray diffraction (XRD) patterns were collected with a Bruker D8 Advance X-ray diffraction system equipped with Cu Kα radiation (λ = 0.4 nm). The profiles were collected at a step width of 0.02 in the (2θ) range from to 90, so each XRD pattern was recorded over 24 h that high resolution could be obtained. The reduced catalysts refer to those collected after H 2 -TPR and CO 2 -TPD measurements, while the used catalysts are those have been tested for 0 h, except 5%Ni/MgO-La 2 O 3 and 5%Ni/MgO which have been tested for h Transmission electron microscopy (TEM) Transmission electron microscopy (TEM) analysis over the above mentioned reduced catalysts was performed on a FEI Tecnai G2 TF S-twin microscope Temperature-programmed Desorption of Carbon dioxide (CO 2 -TPD) Temperature-programmed desorption of CO 2 was performed on a reactor equipped with temperature-programmed heating and thermal conductivity detector (Thermo, TPDRO 10). 0 mg of catalyst was subjected to H 2 -activation up to 800 C and cooled to room temperature before CO 2 adsorption. Then pure CO 2 of 50 ml/min was admitted to pass through catalysts at C for min, followed by Ar purge at the same temperature for another min to remove physisorbed CO 2. Temperature programming was then initiated with a heating rate of C /min in 50 ml/min of He until 800 C. TPD profiles were recorded during the whole process X-ray photoelectron spectroscopy (XPS) X-ray photoelectron Spectra were collected on a Thermo ESCA Lab 0 spectrometer using Al Kα = ev as photon source. Measurements were carried out with ev pass energy, 0.1 ev step, and 0.1 s dwelling time. The C 1s peak at ev binding energy was used as a reference for energy corrections. To study the reduced catalysts, samples were reduced ex situ using the standard reduction conditions as H 2 -TPR and transferred into the XPS chamber under Ar protection Thermal Gravimetric Analyses (TGA) 6

7 The amount of carbon deposition on spent catalysts was determined by TGA in a temperature range from room temperature to 900 C with a TG-50-Mettler-Toledo instrument. The measurements were carried out in a flow of air of 0 ml/min at a heating rate of C/min, and the change in weight was monitored continuously In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) The in situ DRIFTS study under DRM condition was carried out in a PerkinElmer Spectrum GX FT-IR Spectrometer with a MIRTGS detector. A powder sample was put into a reaction cell (Harricks HVC-DRP) and reduced in situ at 700 C for min in H 2 flow. After the reduction, a mixture flow of 80 ml/min with CH 4 and CO 2 mole ratio of 1:1 was introduced into the cell at the same temperature. The spectra of the adsorbed species after reaction of 12 min were recorded with a resolution of 4 cm -1 by subtraction of the initial spectrum recorded after the reduction step. 2.3 Catalyst Activity Steady-state experiments were carried out in a quartz tubular fixed-bed continuous flow reactor (i.d. 7mm) with 50 mg of fresh catalyst (particle size less than 56 um) at atmospheric pressure and constant temperature of 700 ºC. The total gas flow rate was kept constant at ml/min with stoichiometric composition (CH 4 /CO 2 = 12.5/12.5). Before reaction, catalysts were activated in situ with H 2 flow (40 ml/min) at 800 ºC for 5 h. After reduction, the system was cooled to 700 ºC in the flow of H 2. The reaction temperature was measured with a thermocouple placed in the reactor bed. The reactants (CH 4 and CO 2 ) and products (CO, C 2 H 4, C 2 H 6 and H 2 ) were analyzed every min by an on-line GC (Agilent 6890D) equipped with two TCD detectors using He and N 2 as carrier gases, respectively. 3 Results 3.1 Catalytic Activity Methane and CO 2 conversions of Ni-based catalysts with various Mg/La mole ratios in DRM as a function of time on stream are shown in Fig. 1 and the corresponding coking 7

8 rates are listed in Table 1. The stability of catalysts increases with the increment of Mg content from the Mg/La ratio of 1:1, 3:1, 5:1 to :1. 5%Ni/MgO-La 2 O 3 has the best catalytic performance in terms of both activity and stability over 0 h of reaction. However, excess amount of Mg caused fast pressure built up in the reactor as such reaction had to be shut down after few hours of the time-on-stream over 5%Ni/MgO-La 2 O 3 and 5%Ni/MgO. Post analysis of the coking amount on these two spent catalysts reveals their coking rates are one to two orders of magnitude higher than others (Table 1), thus the fast pressure built up is most probably due to the large amount of coking. The coking rate has an opposite tendency to the activity and stability of catalysts which follow the order: 5%Ni/MgO-La 2 O 3 > 5%Ni/5MgO-La 2 O 3 5%Ni/3MgO-La 2 O 3 > 5%Ni/MgO-La 2 O 3 5%Ni/La 2 O 3 > 5%Ni/MgO-La 2 O 3 > 5%Ni/MgO. Moreover, the small variation of metal Ni size after reduction and reaction (Table 1) suggests that the strong interaction between metal Ni and MgO and high surface area of supports may be responsible for restraining reduced metal Ni particles from sintering at both high reduction and reaction temperatures. Since the metal Ni sizes of catalysts after reaction are rather similar, one can conclude that the physicochemical properties of support played a significant role in retarding carbon formation. Another interesting point drawing our attention is the CH 4 conversions increase with time on stream during the initial 3- h of reaction over 5%Ni/La 2 O 3 and 5%Ni/xMgO-La 2 O 3 (x=1, 3, 5), then decrease over the rest reaction time of 0 h. This enhancement of activity is a typical feature when using La 2 O 3 supported nickel catalysts in DRM and, was explained by a slow process of establishment of the 'equilibrium' concentration of the La 2 O 2 CO 3 formed by the reaction of La 2 O 3 with CO 2 as well as other surface carbon species on the Ni crystallites [17], since these La 2 O 2 CO 3 species participated in the carbon removal process, maintaining the activity of catalysts [18]. Indeed, the formation of La 2 O 2 CO 3 species are detected on spent catalysts in the XRD study in the following section and the involvement of La 2 O 2 CO 3 in the DRM reaction is evidenced in the in situ DRIFTS studies shown in Section 3.3. The detailed discussion on the correlation of crystalline structure of supports with catalytic performance of catalysts in DRM reaction will be presented in Section 4. Comment [ 雨林木风 1]: This sentence was deleted. Comment [ 雨林木风 2]: A sentence This participation of La 2 O 2 CO 3 species is also reflected by CO 2 conversions which have the similar tendency of CH 4 conversions [12]. was added here. 8

9 3.2 XRD for spent catalysts Because La 2 O 2 CO 3 species has been proposed as an important reaction intermediate during DRM, it is worth to study the crystalline phase of spent catalysts by XRD. The catalysts, after being tested in the DRM reaction for 0 h were quickly quenched to room temperature and transferred to the XRD apparatus. Fig. 2 shows the XRD patterns of spent catalysts with different Mg/La mole ratio. It is found that metallic Ni (PDF: ) and two crystalline structures of lanthanum dioxide carbonate La 2 O 2 CO 3, i.e. monoclinic (PDF: , a: Å, b: Å, c: 4.07 Å) and hexagonal (PDF: , P63/mmc, a=b: Å, c:.9570 Å) are predominant phases in spent 5%Ni/La 2 O 3. These two La 2 O 2 CO 3 crystalline phases also coexist in 5%Ni/MgO-La 2 O 3 and 5%Ni/3MgO-La 2 O 3 catalysts. The 5%Ni/5MgO-La 2 O 3 is found to consist of metallic Ni, hexagonal La 2 O 3 (PDF: ) and La 2 O 2 CO 3 of monoclinic crystalline structure. Hexagonal La 2 O 2 CO 3 contributes insignificantly to the XRD pattern of 5%Ni/5MgO-La 2 O 3. With further increase of MgO content to Mg/La mole ratio :1, well-crystallized monoclinic La 2 O 2 CO 3 can be detected in 5%Ni/MgO-La 2 O 3 while the contribution of the pattern of hexagonal La 2 O 3 is suppressed. On the contrary, in 5%Ni/MgO-La 2 O 3, the peaks ascribed to hexagonal La 2 O 3 are enhanced whereas the intensities of peaks of monoclinic La 2 O 2 CO 3 are greatly reduced. Since La 2 O 2 CO 3 is expected to participate in the carbon removal process and maintain the activity of catalysts [18], and the best 5%Ni/MgO-La 2 O 3 catalyst (Fig.1) has the most intense peaks of monoclinic La 2 O 2 CO 3, we therefore speculate that monoclinic La 2 O 2 CO 3 is more active than hexagonal La 2 O 2 CO 3 in DRM reaction. This speculation is later confirmed in Section 3.3 in situ DRIFTS studies. Comment [ 雨林木风 3]: This sentence was revised as XRD for fresh and spent catalysts 3.3 In situ DRIFTS for DRM reaction In situ DRIFTS spectroscopy was used to study the chemical nature of surface intermediate species formed during DRM reaction at 700 C over the studied catalysts. The spectra of all the catalysts after reduction at 700 C are shown in Fig. 3 (A). Several bands in the range of 18 to 850 cm -1 are due to adsorption of the La 2 O 2 CO 3 species can be seen on those catalysts containing La 2 O 3 (Fig. 3a~f) [23, 24]. The appearance of the bands of La 2 O 2 CO 3 species should be due to the reaction of La 2 O 3 with 9

10 CO 2 during the calcination of catalysts. Although the reduction of catalysts was performed at temperature of 700 C which is the highest temperature of the FTIR cell, the complete removal of carbonaceous species on La 2 O 3 takes place at 750 C based on the H 2 -TPR results (not shown). Therefore the La 2 O 2 CO 3 species which were not detected by XRD for catalysts reduced at 800 o C (Fig. 4) can be seen on the DRIFTS spectra. According to the assignments in literature [23, 24], the bands of La 2 O 3 containing catalysts at 1827, 1750, 53, 1491, 1463, 1400, 84, 854 cm -1 can be attributed to the La 2 O 2 CO 3 species. Among these, the bands at 53, 1463, 84 and 854 cm -1 are related to the carbonate groups positioned between the (LaO) 2+ 2 layers, while the bands at 1827 and 1750 cm -1 correspond to the >C=O group of the La 2 O 2 CO 3 species. The appearance of two bands at 1491 and 1400 cm -1 infers the formation of polydentate carbonates. Further previous detailed studies [-28], as displayed in Table 2, revealed that the bands at 1463, 84, 854 cm -1 can be assigned to the carbonate vibration mode of ν 3, ν 1 and ν 2 in type Ia (monoclinic) La 2 O 2 CO 3 species, respectively. Hence, the existence of type Ia La 2 O 2 CO 3 species on surfaces of La 2 O 3 -containing catalysts is confirmed. Besides these La 2 O 2 CO 3 species, we also observe a broad band centered at 2490 cm -1 which is the result of overtones of several vibrating groups for all La 2 O 3 containing catalysts. As for all the reduced catalysts, in the OH vibration region, the band at 3738 cm -1 may be assigned to basic OH groups and that at 3569 cm -1 to acid OH groups. Fig. 3 (B) shows the in situ DRIFTS spectra of catalysts after exposure to reactant gases CH 4 /CO 2 at 700 C for 12 min. The spectrum of the corresponding reduced catalyst was used as background. It is observed that a number of negative bands corresponding to carbonate groups appear in the frequency region 80~10 cm -1 over La 2 O 3 containing catalysts, suggesting the involvement of these carbonate species in the reaction. The bands at 84 and 855 cm -1 which are ascribed to the characteristic bands of type Ia (monoclinic) La 2 O 2 CO 3 species appear in negative on 5%Ni/La 2 O 3 (Fig. 3(B)(a)) and 5%Ni/MgO-La 2 O 3 (Fig. 3(B)(b)) whereas in positive on other La 2 O 3 containing catalysts, such as 5%Ni/3MgO-La 2 O 3, 5%Ni/5MgO-La 2 O 3, 5%Ni/MgO-La 2 O 3, and 5%Ni/MgO-La 2 O 3. The negative bands for type Ia La 2 O 2 CO 3 species implies the consumption rate was faster than the rate for formation of these species. Considering the fast deactivation of 5%Ni/La 2 O 3 and 5%Ni/MgO-La 2 O 3 in activity measurements (Fig. 1),

11 caused by fast surface carbon deposited on active sites, it seems that the amount of these type Ia (monoclinic) La 2 O 2 CO 3 species was not enough to remove deposited carbon during reaction. Hence, the consumption of these Ia La 2 O 2 CO 3 species was faster than the formation, leading to the bands at 84 and 855 cm -1 negative. On the contrary, the formation of Ia La 2 O 2 CO 3 species was faster than consumption on other La 2 O 3 containing catalysts, rendering the bands at 84 and 855 cm -1 positive. Therefore, these Ia (monoclinic) La 2 O 2 CO 3 species are beneficial in keeping those La 2 O 3 containing catalysts from deactivation by carbon deposition. It is also observable that the OH groups at 3747 cm -1 were involved in DRM reaction for all catalysts, while the OH groups at 3566 cm -1 only participated in reaction on catalysts 5%Ni/MgO-La 2 O 3 and 5%Ni/La 2 O 3. As for 5%Ni/MgO catalyst, a positive broad band ascribed to carbonate groups can be seen in frequencies of cm -1, suggesting these carbonate species were inert species in DRM reaction. Another two sharp bands centered at 16 and 15 cm -1 are due to gas-phase CH 4 [23]. Adsorbed CO 2 at 2350 cm -1 is observable on all the catalysts. 3.4 XRD and TEM for reduced catalysts The XRD patterns of reduced catalysts with different Mg/La mole ratios are shown in Fig. 4. For the 5%Ni/MgO catalyst only characteristic pattern of MgO (PDF: ) can be seen. The absence of metallic Ni peaks should be due to the strong solid interaction between NiO and MgO during reduction. In the solid-solution catalyst, some compounds, such as MgNiO 2, have essentially the same symmetry and unit cell parameters as that for the MgO, therefore they cannot be detected here. The remaining Ni maybe well dispersed and cannot be detected because of low loading and small particle sizes. The presence of La 2 O 3 in 5%Ni/MgO-La 2 O 3 catalyst, greatly reduce the peaks of MgO, instead, hexagonal structure La 2 O 3 (PDF: , Hexagonal, P6 3 /mmc, a=b: Å, c: Å) become obvious. The metallic Ni (PDF: ) peaks are still too weak to be observed. It is worth noting that another crystal phase of La 2 O 3, the cubic La 2 O 3 (PDF: , Cubic, Ia-3, a=b=c: Å), appears the dominant phase on 5%Ni/MgO-La 2 O 3. The intensity of cubic La 2 O 3 peaks then decreases with decreasing Comment [ 雨林木风 4]: This sentence was revised as TEM for reduced catalysts. 11

12 the Mg/La ratio. The cubic La 2 O 3 almost disappears and the hexagonal La 2 O 3 becomes the dominant phase on 5%Ni/La 2 O 3. In comparison with XRD results of the corresponding spent catalysts in Fig. 2, where the presence of monoclinic La 2 O 2 CO 3 has the same trend of cubic La 2 O 3, there is a strong hint that the monoclinic La 2 O 2 CO 3 species is likely associated with cubic La 2 O 3 species and beneficial to catalytic performance of catalysts based on the catalytic activity and in situ DRIFT studies in Section 3.1 and 3.3. This postulation will be validated by means of CO 2 -TPD analysis in Section 3.6 and discussed in Section 4. Representative TEM images of the reduced catalysts, namely 5%Ni/MgO-La 2 O 3 and 5%Ni/MgO-La 2 O 3 are presented in Fig. 5. In the reduced 5%Ni/MgO-La 2 O 3, metallic Ni is covered by hexagonal La 2 O 3 and in close contact with cubic La 2 O 3 (Fig. 5A). With more MgO in the 5%Ni/MgO-La 2 O 3 catalyst (Mg 2+ /La 3+ mole ratio of :1), metallic Ni and NiO can be directly observed in vicinity of MgO (Fig. 5B), which indicates the stronger interaction between Ni/NiO with MgO than with La 2 O 3 [24]. This result is in good agreement with H 2 -TPR studies on catalysts with various Mg/La ratios (not shown), that the reduction temperature of Ni 2 O 3 /NiO shifts to that of 5%Ni/MgO with the increase of MgO content. 3.5 CO 2 -TPD for reduced catalysts CO 2 -TPD is widely used to evaluate the strength and amount of basic sites, such as various oxygen species (O, O 2 and O 2 2 ) [29] and hydroxyl groups which are able to accept protons serving as Brønsted base sites. It was documented [, 31] that hexagonal La 2 O 3 has two CO 2 adsorption sites: 1) at low-coverage on surface basic sites from which CO 2 starts to desorb at temperature about 0 C; 2) at high-coverage on bulk basic sites which adsorb CO 2 to form structural oxycarbonates La 2 (CO 3 ) 3 that decompose above 500 C. However, there is no literature available on the evaluation of basic sites on cubic La 2 O 3. Hence the CO 2 -TPD studies on cubic La 2 O 3 presented in the following probably can be used as a reference for evaluation of basic sites on cubic La 2 O 3. Before the CO 2 -TPD, the catalysts were activated following the same procedure as in catalytic test, so that the real surface status of catalysts can be reproduced. The properties of basic sites (strength and concentration) measured by CO 2 -TPD can thus be corrected to Comment [ 雨林木风 5]: These two paragraphs were moved to Section 3.2. Comment [ 雨林木风 6]: This sentence was revised as This result is in good agreement with H 2 -TPR studies on catalysts with various Mg/La ratios (not shown), that the reduction temperature of Ni 2 O 3 /NiO derived from the decomposition of nickel nitrate shifts to that of 5%Ni/MgO with the increase in MgO content. 12

13 the initial performance of the catalysts in catalytic activity evaluation. Moreover, considering the fact that the loading of Ni is low (5 wt.%) and no consumption of O 2 was observed when TPO analysis was performed on reduced La 2 O 3 (not shown), the contribution of CO signal derived in the process of re-oxidation of catalysts by CO 2 to the overall recorded signals can be negligible. TPD profiles of CO 2 adsorbed on reduced catalysts with various Mg/La mole ratios are shown in Fig. 6. A broad desorption peak at ca. 474 C can be observed after CO 2 chemisorption on reduced 5%Ni/La 2 O 3 at C. Since CO 2 adsorption on NiO is rather weak [32], the 474 C peak corresponds mainly to the surface basic sites of hexagonal La 2 O 3. This peak shifts to lower desorption temperature at 462 C on 5%Ni/MgO-La 2 O 3 and its intensity is also reduced, indicating the number of basic sites on this catalyst were reduced upon the addition of MgO, as evidenced by the decline in peak intensity of hexagonal La 2 O 3 in Fig. 4. This peak continuously decreases and shifts to lower temperature with increasing Mg/La ratio to 3:1 and 5:1, and finally disappears on 5%Ni/MgO-La 2 O 3. Interesting, a shoulder appears at around 550 o C on 5%Ni/MgO-La 2 O 3 and gradually develops into a peak and shifts to 602 o C on 5%Ni/5MgO-La 2 O 3 and eventually shifts to 6 o C and becomes more intense on 5%Ni/MgO-La 2 O 3, The development of this high temperature CO 2 desorption peak indicates the increasing amount of structural oxycarbonate on bulk basic sites with increasing Mg content. This trend is also consistent with the development of cubic La 2 O 3 species from 5%Ni/3MgO-La 2 O 3 to 5%Ni/MgO-La 2 O 3 as shown in XRD analysis (Fig. 4). Therefore, this desorption peak is associated to the adsorption of CO 2 on bulk basic sites on cubic rather than hexagonal La 2 O 3. However, further increase in MgO content in the catalysts with Mg/La ratio up to leads to the disappearance of the high temperature CO 2 desorption peak in region of C (Fig. 6), coincidentally with the almost disappearance of the XRD patterns of the cubic La 2 O 3 on 5%Ni/MgO-La 2 O 3 as shown in Fig. 4. Only a broad peak centered at 400 C can be observed over 5%Ni/MgO-La 2 O 3. Finally, two CO 2 desorption peaks appear at temperatures lower than 0 C on 5%Ni/MgO, indicating very weak basicity of MgO. Considering the decrease of intensity and movement to lower temperature of the CO 2 desorption peaks in medium desorption temperature region ( C) with 13

14 the decrease of hexagonal La 2 O 3 from 5%Ni/La 2 O 3 to 5%Ni/MgO-La 2 O 3, it is reasonable to correlate the CO 2 desorption peaks in this region ( C) to the adsorbed CO 2 on the hexagonal La 2 O 3. In the meantime, it is also reasonable to correlate the CO 2 desorption peaks in the high temperature region of C to the adsorbed CO 2 on the cubic La 2 O 3 since this peak also increases its intensity and desorption temperature with the increase of cubic La 2 O 3 from 5%Ni/La 2 O 3 to 5%Ni/MgO-La 2 O 3, These cubic La 2 O 3 species are more basic than hexagonal La 2 O 3 and their amount increases with increasing MgO content. Since the DRM reaction was carried out at 700 C, the cubic La 2 O 3 species may be responsible for adsorption of CO 2 in CH 4 and CO 2 reactant mixture. In comparison of these CO 2 -TPD results (Fig. 6) with the evolution of La 2 O 2 CO 3 species on the spent catalysts in XRD analysis (Fig. 2), it is revealed that the CO 2 released in medium desorption temperatures ( C) are only observed in catalysts containing both monoclinic and hexagonal La 2 O 2 CO 3 after reaction, such as 5%Ni/La 2 O 3, 5%Ni/MgO-La 2 O 3 and 5%Ni/3MgO-La 2 O 3 catalysts; while those released in high desorption temperatures ( C) are observed in catalysts containing only monoclinic La 2 O 2 CO 3 after reaction, such as 5%Ni/5MgO-La 2 O 3 and 5%Ni/MgO-La 2 O 3. 5%Ni/MgO-La 2 O 3 has the maximum amount of CO 2 desorbed at the highest temperature of 6 C. The high amount of CO 2 adsorbed in 5%Ni/MgO-La 2 O 3 thus accounts for the well-crystallized monoclinic structure of La 2 O 2 CO 3. As discussed previously that the CO 2 desorption peaks in high temperature region of C is correlated to the adsorbed CO 2 on the cubic La 2 O 3, while those appear in medium desorption temperature region of C to the adsorbed CO 2 on the hexagonal La 2 O 3, it is thus concluded that the observed monoclinic La 2 O 2 CO 3 was derived from carbonation of cubic La 2 O 3 and the hexagonal La 2 O 2 CO 3 originated from carbonation of hexagonal La 2 O 3. Supporting of this finding was also provided by Fleming s study that, in CeO 2 -La 2 O 3 mixed oxides, the monoclinic La 2 O 2 CO 3 phase did not decompose to form hexagonal La 2 O 3 as usually observed in lanthanum only materials [33], but rather appeared to form a cubic structure on heating to temperatures of between 700 and 00 C [34]. It was reported that La 2 O 2 CO 3 species exist in three polymorphic crystalline 14

15 modifications: I, Ia, and II. All are layer-type structures built up of slabs of (La 2 O 2+ 2 ) n layers separated by CO 2-3 ions [26]. Type I has square layers and is tetragonal, whereas type Ia is described as a monoclinic distortion of form I [35]. II-La 2 O 2 CO 3 is completely indexed in the hexagonal unit cell [36]. Since the type I La 2 O 2 CO 3 transforms to type II at 552 C via the formation of type Ia as intermediate and decomposes at a faster rate than type II at the same temperature to give La 2 O 3 [26], the tetragonal La 2 O 2 CO 3 species phase normally cannot be detected in XRD after pretreatment at temperature above 552 C. The intermediate type Ia La 2 O 2 CO 3 species can be detected even at temperature as high as 780 C along with the type II La 2 O 2 CO 3 species being the main phase [26]. The hexagonal type II La 2 O 2 CO 3 species decomposes to form hexagonal La 2 O 3 at 9 C, and the reversible carbonation of the hexagonal La 2 O 3 takes place within a few minutes to form type I La 2 O 2 CO 3 at temperature 0 C lower than decomposition temperature [26]. However, the rate of the type I to type II La 2 O 2 CO 3 transformation can be greatly affected by factors, such as the presence of carbon and/or the particle size [26]. It thus seems that the introduction of the second oxide such as CeO 2 or MgO retards this transformation and promotes the distortion of the crystal structure of type I La 2 O 2 CO 3 to type Ia La 2 O 2 CO 3 which then decomposes to form cubic La 2 O 3 as in our case and Fleming s study [34]. 3.5 XPS of reduced catalysts XPS was used to determine the chemical and electronic nature of the catalyst surfaces. Fig. 7a displays the O 1s spectra for the reduced catalysts with various Mg/La mole ratios. The O1s spectra are broad and can be deconvoluted into four peaks which are referred to the lattice oxygen ions (O 2- ) at around ev [37-39], adsorbed oxygen species (O - /O 2- ) at around 5.4 ev [40], hydroxyl and carbonate species (CO 2-3 /OH - ) at around ev and adsorbed water at around ev [41], respectively. The quantification of corresponding proportion of each surface oxygen species was calculated in Table 3. It can be seen from Fig. 7a and Table 3, 5%Ni/MgO-La 2 O 3 has the lowest BE value of lattice oxygen at ev among all the reduced catalysts, showing more electrons on the O 2- ions in the crystal lattice of 5%Ni/MgO-La 2 O 3 than other catalysts with higher

16 MgO contents. This BE value at ev can be assigned to the lattice oxygen within La 2 O 3 [42]. On the other hand, it is also observable the BE value of La 3d 5/2 of this catalyst is located at lower BE value than other catalysts (Fig. 7b), indicating the La species on the surface of 5%Ni/MgO-La 2 O 3 is easy to be reduced [43] and electron-rich. The resultant LaO x (x<3/2) species is electron-rich in nature and tends to donate electrons to their neighboring O species, leading the O species shift to lower binding energy at ev. With the increase of MgO content, the BE value of lattice oxygen first increases to ev on 5%Ni/5MgO-La 2 O 3 then decreases to ev on 5%Ni/MgO-La 2 O 3, while the amount for these lattice oxygen species decreases progressively. This variation of BE value of lattice oxygen can be associated with the replacement of La 2 O 3 with MgO on catalyst surface, as evidenced by the decline in the peak intensities of La 3d in Fig. 7b. This shift of BE value of lattice oxygen may indicate that after reduction the interaction between lattice oxygen with Mg cations in crystal lattice of MgO was weaker than that with La cations in La 2 O 3. Owing to the possible donation of electrons from LaO x (x<3/2) species to O 2- ions in the crystal lattice of 5%Ni/MgO-La 2 O 3, this catalyst has the lowest binding energy. As illustrated in TEM (Fig. 5A) that metallic Ni is in close contact with La 2 O 3 on the surface of the catalyst, the spillover of H activated by metallic Ni onto La 2 O 3 occurred easily and caused the reduction of La 2 O 3 into LaO x (x<3/2) species. In the contrast, La 2 O 3 is located on remote sites from metallic Ni on other catalysts (Fig. 5B), resulting in the reduction difficult. Therefore, there is no significant binding energy shift in Fig. 7b, the contribution of these LaO x (x<3/2) species to the shift of BE values of O 2- ions is insignificant. As a result, 5%Ni/5MgO-La 2 O 3 has the highest binding energy over all the analysed catalysts. Meanwhile, the amount of adsorbed oxygen species first increases then decreases with increasing MgO content, 5%Ni/MgO-La 2 O 3 has the highest amount of adsorbed oxygen species. This maximum amount of adsorbed oxygen species indicates the highest basicity on 5%Ni/MgO-La 2 O 3, which is consistent with the high amount of CO 2 desorbed at high temperature of 6 C in CO 2 -TPD analysis (Fig. 6). 16

17 4 Discussion Mixed MgO-La 2 O 3 supported Ni catalysts with various Mg/La ratios were investigated for DRM at 700 ºC for 0 h. This reaction temperature was chosen in order to produce sufficient amount of carbon deposition during DRM, as predicted by thermodynamic analysis, so that the effect of the support structure and acidity/basicity on carbon suppression can be highlighted. As can be seen in Fig. 1, the activity and stability of catalysts decrease in the order: 5%Ni/MgO-La 2 O 3 > 5%Ni/5MgO-La 2 O 3 5%Ni/3MgO-La 2 O 3 > 5%Ni/MgO-La 2 O 3 5%Ni/La 2 O 3 > 5%Ni/MgO-La 2 O 3 > 5%Ni/MgO. This tendency is well correlated with the corresponding coking rate (Table 1), the amount of cubic La 2 O 3 and monoclinic La 2 O 2 CO 3 (Fig.2 and 4), the strength of basicity (CO 2 -TPD in Fig. 6) and the surface oxygen species (Table 3). Specifically, 5%Ni/MgO-La 2 O 3 having the slowest coking rate, the highest amount of cubic La 2 O 3 of strong basicity and then monoclinic La 2 O 2 CO 3 in DRM reaction, thus exhibits the best performance in terms of catalytic activity and stability throughout the 0 h time-on-stream reaction. This indicates that the support played a significant role in retarding carbon formation, particularly in the kinetic control reaction region. Therefore, the following sections will focus on the discussion on the effect of support. 4.1 Importance of monoclinic La 2 O 2 CO 3 (Ia) The amount of cubic La 2 O 3 species and the basicity increased with Mg/La mole ratios ranging from 0 to in the reduced catalysts was revealed by XRD and CO 2 -TPD analysis (Figs. 4 and 6). These cubic La 2 O 3 species have so strong basicity for CO 2 adsorption, that the adsorbed CO 2 can only be released in high temperature region C (Fig. 6). Since the DRM reaction took place at temperature of 700 C, these cubic La 2 O 3 species may be responsible for adsorption of CO 2 in reactant gases. It was reported that La 2 O 2 CO 3 species favored carbon removal during DRM reaction, maintaining catalysts active by providing oxygen species to react with deposited carbon at the interface of Ni-La 2 O 2 CO 3 [17-]. Our in situ DRIFTS as well as XRD analysis on the spent catalysts have provided the evidence that monoclinic La 2 O 2 CO 3 was formed and participated in DRM the reaction. Among all the catalysts, 5%Ni/MgO-La 2 O 3 has the best catalytic performance and the highest amount of these cubic La 2 O 3 species. 17

18 Hence the high stability of 5%Ni/MgO-La 2 O 3 can be rationalized. 5 Although the spent 5%Ni/MgO-La 2 O 3 shows the presence of monoclinic La 2 O 2 CO 3 species (Fig. 2), they were consumed so rapidly during the DRM reaction that the rate of carbon deposition was faster than its gasification (Fig. 3). Moreover, as indicated in XPS analysis, 5%Ni/MgO-La 2 O 3 possesses active LaO x (x<3/2) species which facilitate the dissociation of CO to produce adsorbed carbon [44]. As a result, the catalyst deactivated fast in DRM reaction (Fig. 1). The same is valid for 5%Ni/La 2 O 3. Further increase of the Mg/La mole ratio to, as in 5%Ni/MgO-La 2 O 3, the amount of hexagonal La 2 O 3 in bulk phase was enriched. The presence of hexagonal La 2 O 3 clearly indicates the lack of momoclinic La 2 O 2 CO 3 species in the bulk phase of 5%Ni/MgO-La 2 O 3, which reduced the amount of surface momoclinic La 2 O 2 CO 3 species involved in DRM reaction. As a consequence, this catalyst also deactivated fast due to a large amount of carbon formation (Fig. 1 and Table 1). Two crystal structures of La 2 O 2 CO 3 (type I and II) were found over Ni/La 2 O 3 during DRM by Zhang et al. [17]. However, whether both types of La 2 O 2 CO 3 species had been involved in reaction was unclear. Our present study clearly demonstrates that monoclinic La 2 O 2 CO 3 (Ia) species are the major contributor for carbon removal and high catalytic stability. The monoclinic La 2 O 2 CO 3 species are originated from carbonation of cubic La 2 O 3 species which possess strong basicity to adsorb CO 2 during DRM reaction. The hexagonal La 2 O 2 CO 3 species, originated from hexagonal La 2 O 3 which can also be detected in XRD analysis, play a minor role in carbon removal in DRM reaction. 4.2 The effect of MgO on stabilizing monoclinic La 2 O 2 CO 3 XRD analysis of reduced catalysts in Fig. 4 shows that the introduction of MgO into La-containing catalysts promotes the formation of cubic La 2 O 3 species and the amount of these cubic species increased with increasing MgO content. However, further rising Mg/La mole ratio to causes the almost complete disappearance of cubic La 2 O 3 species. In general, La 2 O 3 has a double c-axis hcp (dhcp) structure at room temperature [45]. With temperature and pressure variation, La 2 O 3 experiences two structural phase transitions into type-a (hexagonal) and type-c (cubic), which are the characteristic forms at high and low temperatures, respectively [45]. In hexagonal structure, the La 3+ metal 18

19 atoms are surrounded by a 7 coordinate group of O 2- atoms, while the oxygen ions are in an octahedral shape around the metal atom and there is one oxygen ion above one of the octahedral faces [46, 47]. On the other hand, in cubic crystal structure, the La 3+ metal atoms are surrounded by a 6 coordinate group of O 2- ions [48]. The release of one-quarter of the O 2- ions from hexagonal to cubic structure during H 2 activation was probably facilitated by the introduction of Mg 2+ with lower valence than that of La 3+. Density functional calculations [49] have suggested the addition of Mg may weaken the bond strength of oxygen atoms in the surface layer to La 2 O 3, lowering substantially the energy of vacancy formation and making La 2 O 3 easier to form oxygen vacancies. As a consequence, the addition of MgO reduced the amount of lattice oxygen species in catalysts as shown in in XPS analysis (Fig. 7 and Table 3) and promoted the crystal structure transforming from hexagonal to cubic. Once the transformation was completed, the oxygen vacancies were eliminated and could not participate in reactions, such as dissociation of adsorbed CO 2 to CO and oxygen species. On the other hand, the amount of surface adsorbed oxygen species was enhanced with the addition of MgO, which may be related to the reabsorption of lattice oxygen species dissociated from the La 2 O 3 lattice onto catalyst surface with the influence of H 2 [50]. Since the hexagonal structure of La 2 O 3 is thermodynamically stable than cubic structure at high temperature, the presence of cubic La 2 O 3 phase in XRD patterns (Fig. 4) in MgO-containing catalysts indicates that MgO not only facilitated the formation of cubic La 2 O 3 but also prevented them from transformation into hexagonal structure at high temperature. This stabilizing effect of MgO on crystal structure was also reflected by the presence of monoclinic La 2 O 2 CO 3 species in both XRD analysis (Fig. 2) and in situ FTIR study of DRM reaction at 700 C (Fig. 3). 4.3 The effect of MgO on surface oxygen species With the addition of MgO, the amount of oxygen species on catalyst surface varied as illustrated in XPS analysis (Fig. 7 and Table 3). It is found that the amount of nucleophilic lattice oxygen species (i.e. O 2 ) decreases while the amount of electrophilic - adsorbed oxygen species (e.g. O 2, O - and O 2 (ads) ) increases as increasing the MgO content in catalysts. The stability (Fig. 1) and the rate of carbon formation (Table 1) are 19

20 in line with the amount of the electrophilic oxygen species. The most stable catalyst 5%Ni/MgO-La 2 O 3 possesses the most electrophilic oxygen species (e.g. O - 2, O - and O 2 (ads) ) and the slowest carbon formation rate. The lattice oxygen ions (i.e. O 2 ) are considered to be able to initiate a reaction by abstraction of an H + from reactants to form anionic intermediates, while the surface oxygen ions (i.e. O ) are more oxidizing [51] and favor the direct total oxidation of methane to carbon oxides in oxidative coupling of methane reaction [52, 53]. Previous studies by our group [Error! Bookmark not defined.] indicated that these surface lattice oxygen species promoted C-H activation in DRM reaction, resulting in high amount of carbon formation. Therefore, it is understandable that 5%Ni/MgO-La 2 O 3 and 5%Ni/La 2 O 3 which have the highest amount of lattice oxygen species among the other catalysts produced a large amount of carbon in DRM reaction On the contrary, the catalysts with high amount of surface oxygen species electrophilic oxygen species (e.g. O - 2, O - and O 2 (ads) ) can adsorb CO 2 molecules to form bidentate carbonate species, which can then react with the surface carbon species formed during DRM, resulting in higher CO 2 conversion and lower carbon formation [40]. This important role of adsorbed oxygen species was also confirmed by Hwang s study [54] that the surface concentration of adsorbed oxygen species is in parallel correlation with catalytic activity of LaFeO 3 catalysts in soot combustion. As a result, 5%Ni/MgO-La 2 O 3 having the highest amount of these surface adsorbed oxygen species exhibited the best catalytic performance in DRM. 5 Conclusions A series of Mg-La mixed oxides supported Ni catalysts have been prepared, among which 5%Ni/MgO-La 2 O 3 has the best performance towards DRM reaction in terms of both catalytic activity and stability during 0 h time-on-stream reaction. It is identified that two surface La 2 O 2 CO 3 species, i.e. monoclinic and hexagonal La 2 O 2 CO 3 were present on La-containing catalysts but mainly monoclinic La 2 O 2 CO 3 species have participated in DRM. The interaction of suitable amount of MgO with La 2 O 3 stabilized cubic La 2 O 3 species which has strong basicity to adsorb CO 2 and is responsible for the subsequent formation of monoclinic La 2 O 2 CO 3 species in DRM reaction. The

21 introduction of MgO also generated surface adsorbed oxygen ions (i.e. O ) which are able to oxidize deposited carbon. 5%Ni/MgO-La 2 O 3 was found to possess the highest amount of both monoclinic La 2 O 2 CO 3 and surface oxygen species while it also showed the best catalytic activity and stability during DRM reaction of 0 h. The good correlation between the crystal structure and catalytic performance of catalysts thus provides a new approach to design and prepare a good DRM catalyst by controlling the crystalline structure of catalysts. Acknowledgments We gratefully acknowledge the Science and Engineering Research Council of A*STAR (Agency for Science Technology and Research, Singapore) for its financial support of this project (A*STAR Project No and NUS RP No. R ). 21

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25 Captions: 5 Table 1 Physicochemical properties of catalysts with various Mg/La mole ratio and corresponding carbon formation rates during DRM reaction. Table 2 Spectrum frequencies for carbonate modes. Table 3 Quantities of surface oxygen species based on XPS. Comment [ 雨林木风 7]: They were moved to the end of the Captions. Fig. 1 Catalytic activity and stability of Ni-based catalysts with various Mg/La mole ratios in DRM reaction (T = 700 ºC, GHSV =,000 ml/g h, CH 4 /CO 2 = 1/1): (a) CH 4 conversion, (b) CO 2 conversion. Fig. 2 XRD patterns of spent catalysts with different Mg/La mole ratios. ---: Ni 0 ( ); : La 2 O 3 (hexagonal, ); : MgO ( ); : Carbon ( ); : La 2 O 2 CO 3 (Monoclinic, ); : La 2 O 2 CO 3 (Hexagonal ). Fig. 3 In situ DRIFTS spectra of reduced catalysts (A) and, upon exposure to reactant gases after 12 min (B) at 700 C with the spectrum of reduced catalysts as background: (a) 5%Ni/La 2 O 3, (b) 5%Ni/MgO-La 2 O 3, (c) 5%Ni/3MgO-La 2 O 3, (d) 5%Ni/5MgO-La 2 O 3, (e) 5%Ni/MgO-La 2 O 3, (f) 5%Ni/MgO-La 2 O 3, (g) 5%Ni/MgO. Fig. 4 XRD patterns of reduced catalysts with various Mg/La mole ratios. ---: Ni 0 ( ); : La 2 O 3 (hexagonal, ); : La 2 O 3 (cubic, ); : MgO. Fig. 5 TEM images of reduced catalysts: A) 5%Ni/MgO-La 2 O 3 ; B) 5%Ni/MgO-La 2 O 3. Fig. 6 CO 2 -TPD for reduced catalysts with various Mg/La mole ratios. Fig. 7 XPS spectra of reduced catalysts with various Mg/La mole ratios: a) O1s; b) Ni2p+La3d.

26 Table 1 Physicochemical properties of catalysts with various Mg/La mole ratio and corresponding carbon formation rates during DRM reaction. Catalysts Surface Area (m 2 /g) Metal Ni size (nm) c Carbon amount (mol)/ Support a Catalyst b After reduction After reaction total converted CH 4 (mol) d 5%Ni/La 2 O E-04 5%Ni/MgO-La 2 O E-04 5%Ni/3MgO-La 2 O E-04 5%Ni/5MgO-La 2 O E-04 5%Ni/MgO-La 2 O E-04 5%Ni/MgO-La 2 O E-02 5%Ni/MgO E-01 a The support was calcined at 800 ºC for 4 h. 5 b The catalyst was reduced following the procedure for H 2 -TPR but without the thermal treatment. c From TEM. d From TGA. 26

27 Table 2 Spectrum frequencies for carbonate modes. Spectrum Frequencies for Carbonate Modes (cm -1 ) Carbonates ν 1 ν 2 ν 3 ν 4 Ref. 2- Free CO [] La 2 O 2 CO 3 (I, tetragonal) , , ~642 [26] La 2 O 2 CO 3 (Ia, monoclinic) 1129, 88, , 858, , 1467, ~642 [26, 27] La 2 O 2 CO 3 (II, hexagonal) , [26, 28] 5 27

28 Table 3 Quantities of surface oxygen species based on XPS Adsorbed Catalysts Lattice oxygen oxygen Hydroxyl/Carbonate Adsorbed water BE Area BE Area Area BE Area BE (ev) (ev) (%) (ev) (%) (%) (ev) (%) 5%Ni/MgO-La 2 O %Ni/5MgO-La 2 O %Ni/MgO-La 2 O %Ni/MgO-La 2 O

29 Fig. 8 Catalytic activity and stability of Ni-based catalysts with various Mg/La mole ratios in DRM reaction (T = 700 ºC, GHSV =,000 ml/g h, CH 4 /CO 2 = 1/1): (a) CH 4 conversion, (b) CO 2 conversion. 29

30 Fig. 9 XRD patterns of spent catalysts with different Mg/La mole ratios. ---: Ni 0 ( ); : La 2 O 3 (hexagonal, ); : MgO ( ); : Carbon ( ); : La 2 O 2 CO 3 (Monoclinic, ); : La 2 O 2 CO 3 (Hexagonal ).

31 Fig. In situ DRIFTS spectra of reduced catalysts (A) and, upon exposure to reactant gases after 12 min (B) at 700 C with the spectrum of reduced catalysts as background: (a) 5%Ni/La 2 O 3, (b) 5%Ni/MgO-La 2 O 3, (c) 5%Ni/3MgO-La 2 O 3, (d) 5%Ni/5MgO-La 2 O 3, (e) 5%Ni/MgO-La 2 O 3, (f) 5%Ni/MgO-La 2 O 3, (g) 5%Ni/MgO. 31

32 Fig. 11 XRD patterns of reduced catalysts with various Mg/La mole ratios. ---: Ni 0 ( ); : La 2 O 3 (hexagonal, ); : La 2 O 3 (cubic, ); : MgO. 32

33 Hexagonal La 2 O 3 [1 0 1] Ni 0 6.nm Cubic La 2 O 3 [2 2 2] Ni nm nm A NiO [2 00] Ni 0 [1 1 1] 5 nm MgO [2 00] B Fig. 12 TEM images of reduced catalysts: A) 5%Ni/MgO-La 2 O 3 ; B) 5%Ni/MgO-La 2 O 3. 33