Two-Dimensional Layered Double Hydroxide Derived from Vermiculite Waste Water Supported Highly Dispersed Ni Nanoparticles for CO Methanation

Expanded multilayered vermiculite (VMT) was successfully used as catalyst support and Ni/VMT synthesized by microwave irradiation assisted synthesis (MIAS) exhibited excellent performance in our previous work. We also developed a two-dimensional porous SiO2 nanomesh (2D VMT-SiO2) by mixed-acid etching of VMT. Compared with three-dimensional (3D) MCM-41, 2D VMT-SiO2 as a catalyst support provided a superior position for implantation of NiO species and the as-obtained catalyst exhibited excellent performance. In this paper, we successfully synthesized a layered double hydroxide (LDH) using the spent liquor after mixed-acid etching of VMT, which mainly contained Mg2+ and Al3+. The as-calcined layered double oxide (LDO) was used as a catalyst support for CO methanation. Compared with Ni/MgAl-LDO, Ni/VMT-LDO had smaller active component particles; therefore, in this study, it exhibited excellent catalytic performance over the whole temperature range of 250–500 ◦C. Ni/VMT-LDO achieved the best activity with 87.88% CO conversion, 89.97% CH4 selectivity, and 12.47 × 10−2·s−1 turn over frequency (TOF) at 400 ◦C under a gas hourly space velocity of 20,000 mL/g/h. This study demonstrated that VMT-LDO as a catalyst support provided an efficient way to develop high-performance catalysts for synthetic natural gas (SNG) from syngas.


Introduction
Since carbon oxide methanation was discovered by Sabatier and Senderens in 1902, it has been well developed and widely used due to its capability as a chemical storage for excess H 2 generated from renewable energy and a solution for greenhouse gas recycling [1][2][3].Synthetic natural gas (SNG) via methanation can result in the clean utilization of coal and biomass, and this can prove to be an ideal method to solve the problem of the lack of natural gas in China [4,5].In addition, methanation of carbon monoxide can also guarantee energy supplies, an important advantage owing to the increasing energy demand that accompanies the rapid development of the world economy.
Al 2 O 3 has been one of the most commonly used catalyst supports due to its large specific surface area and moderate active component-support interaction [6].The performance of an Al 2 O 3 -based catalyst is quite sensitive to the preparation method; catalysts prepared using solution combustion synthesis and the single-step sol-gel method can reduce the catalysts' disadvantages, such as sintering of Ni particles, facile carbon deposition, and severe sulfur poisoning at high temperature [7][8][9][10][11].MgO as a promoter can also improve resistance to carbon deposition and minimize Ni particle sintering [12].The Ni/MgO catalyst has a strong interaction between Ni and MgO, and MgO can depress the aggregation of Ni metal particles, leading to higher thermal stability and better performance [13,14].
2D materials have excellent electronic properties and exceptional mechanical robustness, and are favorable to be used as catalyst supports [15].As a kind of 2D material, layered double hydroxide (LDH) with the advantages of unique structure, composition diversity, high stability, easy preparation, and low cost has shown great potential in the design and synthesis of novel supported catalysts.LDH has been widely employed in the synthesis of Ni-based catalysts with a high degree of dispersion and stability by utilization of anchoring effects to enhance the immobilization of the active phase [16,17].Ni-Al LDHs are found to be promising catalyst precursors for CO 2 methanation reaction, and Ni/Al 2 O 3 -LDO can decrease the CO 2 methanation reaction temperature to 260 • C due to its high dispersion of Ni, and Fe(III) can further decrease the Ni particle size, resulting a higher catalytic performance of 58.2% methane yield at 220 • C [7,18].MgAl-LDH has been used as a catalyst support in oxidation reactions [19,20], and the as-calcined samples, named layered double oxide (LDO) or mixed metal oxides (MMO) are also used as supports due to their large specific surface area and the strong interaction between active components and the support [16].Li et al. [21] found that the NiMgAl-LDO catalyst showed excellent CO methanation performance in high-temperature region: 400-650 • C, the transmission electron microscopy (TEM) and X-ray diffraction (XRD) results indicated that upon calcination of the NiMgAl-LDH precursor, the NiO phase gains a strong interaction with MgO and/or Al 2 O 3 to form a Ni x Mg 1−x O solid solution, leading to high-temperature stability and high Ni dispersion.Fan et al. [12] used Mg-Al oxide-supported Ni catalysts which not only have excellent thermal stability at high temperature, but also showed catalytic activity at low temperature of 250 • C with a Ni loading of 40 wt %, which is ascribed to the large specific surface area and strong metal-support interaction.Buelens et al. [22] developed a "super-dry" CH 4 reforming through Le Chatelier's principle reaction using Ni/MgAl 2 O 4 as a CH 4 -reforming catalyst.In this reaction Fe 2 O 3 /MgAl 2 O 4 was used as a solid oxygen carrier and oxidized CH 4 into CO 2 and H 2 O, while the Fe 2 O 3 reduced to Fe; CaO/Al 2 O 3 as a CO 2 sorbent formed CaCO 3 , then the CaCO 3 decomposed into CaO and CO 2 , and CO 2 reduced to CO by Fe through a redox reaction.Compared with conventional dry reforming, "super-dry" CH 4 reforming can result in higher CO production, and showed both practical and economic benefits.
We have developed a type of 2D porous SiO 2 nanomesh from a mixed-acid etching of 2D expanded multilayered vermiculite (EML-VMT).In this paper we synthesized a 2D VMT-LDH using the spent liquor, which contains Mg 2+ , Al 3+ , etc., with a Mg/Al molar ratio of 1.5:1 [23,24].The spent liquor also contains a small amount of Fe 2+ and Ca 2+ .Fe as the best promoter, can combine with Ni to form enhanced methanation catalysts, and Ca can improve the dispersion of nickel [25][26][27].The as-calcined 2D VMT-LDO was used as a catalyst support for CO methanation and Ni/VMT-LDO exhibited better catalytic performance than Ni/MgAl-LDO in the entire temperature range.This VMT-LDH catalyst support realized recycling utilization of the spent liquors, effectively preventing the metal ions in the spent liquors pollute the environment at the same time.

XRD, SEM, and TEM Characterization of LDHS and LDOS
From Figure 1a, MgAl-LDH and VMT-LDH presented the characteristic diffraction peaks of a well-crystallized LDH phase (JCPDS NO. 22-0452) at 2θ of 11.71 • , 34.33 • , 61.34 • , and 62.68 • , corresponding to the reflections of planes (001), (100) , (110), and (111), respectively.The diffraction peaks indicated that the LDH was successfully synthesized by using the spent liquor after mixed dilute aqua regia solution etching of vermiculite.After calcining at 550 • C for 5 h in a muffle furnace, the as-obtained MgAl-LDO and VMT-LDO (Figure 1b) showed the characteristic diffraction peaks of MgO (JCPDS NO. 65-0476) at 2θ of 36.94 • , 42.92 • , and 62.30 • , corresponding to crystal lattice indices of (111), (200), and (220), respectively.No peaks indicative of Al 2 O 3 species was detected in either MgAl-LDO or VMT-LDO, these observations suggested that Al 2 O 3 was in an amorphous state or that Al 3+ ions became incorporated into MgO to form spinel phases, such as MgAl 2 O 4 [28][29][30].The SEM and TEM images of the as-obtained LDOs are displayed in Figure 2. The plate-like structure of VMT-LDO can be obviously seen in the TEM images.The stratified structure as perfect as MgAl-LDO, which is more evidence that VMT-LDH was successfully synthesized.The Brunauer-Emmett-Teller (BET) surface of MgAl-LDO and VMT-LDO were 177.3 m 2 /g and 72.4 m 2 /g respectively.

XRD,SEM, and TEM Characterization of LDHS and LDOS
From Figure 1a, MgAl-LDH and VMT-LDH presented the characteristic diffraction peaks of a well-crystallized LDH phase (JCPDS NO. 22-0452) at 2θ of 11.71°, 34.33°, 61.34°, and 62.68°, corresponding to the reflections of planes (001), (100) , (110), and (111), respectively.The diffraction peaks indicated that the LDH was successfully synthesized by using the spent liquor after mixed dilute aqua regia solution etching of vermiculite.After calcining at 550 °C for 5 h in a muffle furnace, the as-obtained MgAl-LDO and VMT-LDO (Figure 1b) showed the characteristic diffraction peaks of MgO (JCPDS NO. 65-0476) at 2θ of 36.94°,42.92°, and 62.30°, corresponding to crystal lattice indices of (111), (200), and (220), respectively.No peaks indicative of Al2O3 species was detected in either MgAl-LDO or VMT-LDO, these observations suggested that Al2O3 was in an amorphous state or that Al 3+ ions became incorporated into MgO to form spinel phases, such as MgAl2O4 [28][29][30].The SEM and TEM images of the as-obtained LDOs are displayed in Figure 2. The plate-like structure of VMT-LDO can be obviously seen in the TEM images.The stratified structure as perfect as MgAl-LDO, which is more evidence that VMT-LDH was successfully synthesized.The Brunauer-Emmett-Teller (BET) surface of MgAl-LDO and VMT-LDO were 177.3 m 2 /g and 72.4 m 2 /g respectively.

XRD,SEM, and TEM Characterization of LDHS and LDOS
From Figure 1a, MgAl-LDH and VMT-LDH presented the characteristic diffraction peaks of a well-crystallized LDH phase (JCPDS NO. 22-0452) at 2θ of 11.71°, 34.33°, 61.34°, and 62.68°, corresponding to the reflections of planes (001), ( 100) , (110), and (111), respectively.The diffraction peaks indicated that the LDH was successfully synthesized by using the spent liquor after mixed dilute aqua regia solution etching of vermiculite.After calcining at 550 °C for 5 h in a muffle furnace, the as-obtained MgAl-LDO and VMT-LDO (Figure 1b) showed the characteristic diffraction peaks of MgO (JCPDS NO. 65-0476) at 2θ of 36.94°,42.92°, and 62.30°, corresponding to crystal lattice indices of ( 111), (200), and (220), respectively.No peaks indicative of Al2O3 species was detected in either MgAl-LDO or VMT-LDO, these observations suggested that Al2O3 was in an amorphous state or that Al 3+ ions became incorporated into MgO to form spinel phases, such as MgAl2O4 [28][29][30].The SEM and TEM images of the as-obtained LDOs are displayed in Figure 2. The plate-like structure of VMT-LDO can be obviously seen in the TEM images.The stratified structure as perfect as MgAl-LDO, which is more evidence that VMT-LDH was successfully synthesized.The Brunauer-Emmett-Teller (BET) surface of MgAl-LDO and VMT-LDO were 177.3 m 2 /g and 72.4 m 2 /g respectively.

TPR Characterization of Catalysts
The surface state and the reduction behavior of the catalysts NiO/VMT-LDO and NiO/MgAl-LDO were analyzed by H2-TPR measurement (Figure 3b).There were three reduction peaks of NiO species observed in the H2-TPR profile, which could be classified into three types: free nickel oxides possessing a weak interaction with the support (350-500 °C, α-type); NiO species that have a stronger interaction with the support (520-700 °C, β-type); the reduction peaks of MgNiO2 solid solution (730-850 °C,γ-type) [10,21].The first two reduction temperatures of NiO/VMT-LDO were centered at 338 °C and 491 °C, respectively.While the first two peaks' reduction temperatures of NiO/MgAl-LDO

TPR Characterization of Catalysts
The surface state and the reduction behavior of the catalysts NiO/VMT-LDO and NiO/MgAl-LDO were analyzed by H 2 -TPR measurement (Figure 3b).There were three reduction peaks of NiO species observed in the H 2 -TPR profile, which could be classified into three types: free nickel oxides possessing a weak interaction with the support (350-500 • C, α-type); NiO species that have a stronger interaction with the support (520-700 • C, β-type); the reduction peaks of MgNiO 2 solid solution (730-850 • C, γ-type) [10,21].The first two reduction temperatures of NiO/VMT-LDO were centered at 338 • C and 491 • C, respectively.While the first two peaks' reduction temperatures of NiO/MgAl-LDO were centered at 284 • C and 410 • C, respectively.The reduction temperatures of NiO/VMT-LDO were slightly higher than those of NiO/MgAl-LDO, suggesting that the interaction between NiO particles and VMT-LDO was stronger than that in MgAl-LDO.However, all of the reduction temperatures were below 500 • C, which meant that they belong to the α-type and could be reduced to generate active sites during the pretreatment before reaction.MgNiO 2 solid solution could not be reduced in pretreatment or during the reaction due to the strong interaction between Ni and MgO, resulting in a higher thermal stability [13].

Catalytic Performance of Catalysts
The CO methanation performance of the catalysts were carried out in the temperature range 250 • C-500 • C, as shown in Figure 4a.Ni/MgAl-LDO had shown no activity below 350 • C, and when the reaction temperature reached 400 • C, the catalyst Ni/MgAl-LDO showed 79.5% CO conversion, and the catalytic performance had shown no decline at the higher temperature.For Ni/VMT-LDO, the catalyst not only showed low-temperature activity at 250 • C, but also exhibited better catalytic performance than Ni/MgAl-LDO over the whole temperature range, reaching a maximum CO conversion of 87.9% at 400 • C, additionally.The higher catalytic performance was no doubt due to the smaller and higher dispersion of active components particle size.The trend of CH 4 selectivity (Figure 4b were centered at 284 °C and 410 °C, respectively.The reduction temperatures of NiO/VMT-LDO were slightly higher than those of NiO/MgAl-LDO, suggesting that the interaction between NiO particles and VMT-LDO was stronger than that in MgAl-LDO.However, all of the reduction temperatures were below 500 °C, which meant that they belong to the α-type and could be reduced to generate active sites during the pretreatment before reaction.MgNiO2 solid solution could not be reduced in pretreatment or during the reaction due to the strong interaction between Ni and MgO, resulting in a higher thermal stability [13].

Catalytic Performance of Catalysts
The CO methanation performance of the catalysts were carried out in the temperature range 250 °C-500 °C, as shown in Figure 4a.Ni/MgAl-LDO had shown no activity below 350 °C, and when the reaction temperature reached 400 °C, the catalyst Ni/MgAl-LDO showed 79.5% CO conversion, and the catalytic performance had shown no decline at the higher temperature.For Ni/VMT-LDO, the catalyst not only showed low-temperature activity at 250 °C, but also exhibited better catalytic performance than Ni/MgAl-LDO over the whole temperature range, reaching a maximum CO conversion of 87.9% at 400 °C, additionally.The higher catalytic performance was no doubt due to the smaller and higher dispersion of active components particle size.The trend of CH4 selectivity (Figure 4b) for Ni/MgAl-LDO was in good agreement with that of CO conversion for Ni/MgAl-LDO in the temperature region: 250 °C-400 °C, and declined with the temperature, which resulted from the carbon deposition and the agglomeration of the active Ni component.When it came to the catalyst Ni/VMT-LDO, which showed an interesting curve of CH4 selectivity, the CH4 selectivity of Ni/VMT-LDO declined at 350 °C compared with 300 °C.Both samples obtained a maximum CH4 selectivity at 400 °C.See Table 2 for a comparison of catalytic performance in different works.
It can be clearly seen in Figure 4c that Ni/VMT-LDO exhibited a higher turnover frequency (TOF) than Ni/MgAl-LDO over the whole temperature range.Ni/VMT-LDO reached a maximum TOF of 12.47× 10 −2 •s −1 at 400 °C, while for Ni/MgAl-LDO, a TOF value of 11.29× 10 −2 •s −1 was achieved at the same temperature.The highest TOF value of Ni/MgAl-LDO catalyst was 11.57 × 10 −2 •s −1 at 500 °C.Ni/VMT-LDO also achieved a higher CH4 yield in the whole temperature range compared with the catalyst Ni/MgAl-LDO (Figure 4d), the highest CH4 yield was obtained at 400 °C for both samples.When the reaction temperature was higher than 400 °C, the CH4 yield decreased with the temperature increased.However, Ni/MgAl-LDO and Ni/VMT-LDO still have ≥ 70% CH4 yield at 500 °C.All of these results indicate that the catalytic performance of Ni/VMT-LDO catalyst was much better than that of Ni/MgAl-LDO catalyst.

TEM and XRD Characterization of the Used Catalysts
After reaction, as shown in Figure 5

2.5.TEM and XRD Characterization of the Used Catalysts
After reaction, as shown in Figure 5, both catalysts Ni/MgAl-LDO (used) and Ni/VMT-LDO (used) exhibited no obvious agglomeration, and especially for catalyst Ni/VMT-LDO (used) there were no obvious Ni particles to be seen, the absence of Ni particles in the catalysts being due to the smaller Ni particles and the formation of Ni-Mg solid solutions [21].XRD patterns of the used catalysts are shown in Figure 6.For Ni/MgAl-LDO (used) and Ni/VMT-LDO (used), both samples showed the diffraction peaks of metal Ni (JCPDS NO. 65-0380) at 2θ of 44.35°, 51.67°, and 76.09°.Diffraction peaks of MgNiO2 (JCPDS NO. 24-0712) could also be seen at 2θ of 37.10°, 43.10°, and 62.59°, which was another argument in favor of the formation of Ni-Mg solid solutions.The MgNiO2 solid solution inhibited the agglomeration of Ni nanoparticles, which maintained high CO conversion and thermal stability at high temperatures.However, for Ni/MgAl-LDO (used), the diffraction peaks of spinel (MgAl2O4) could be observed at 2θ of 19.03°, 31.27°,36.85°,44.83°, 55.66°, 59.37°, and 65.24°, suggesting spinel (MgAl2O4) formation during the reaction.Spinel could promote catalytic performance, leading to enhanced reactivity with increasing temperature [12,31].

2.5.TEM and XRD Characterization of the Used Catalysts
After reaction, as shown in Figure 5, both catalysts Ni/MgAl-LDO (used) and Ni/VMT-LDO (used) exhibited no obvious agglomeration, and especially for catalyst Ni/VMT-LDO (used) there were no obvious Ni particles to be seen, the absence of Ni particles in the catalysts being due to the smaller Ni particles and the formation of Ni-Mg solid solutions [21].XRD patterns of the used catalysts are shown in Figure 6.For Ni/MgAl-LDO (used) and Ni/VMT-LDO (used), both samples showed the diffraction peaks of metal Ni (JCPDS NO. 65-0380) at 2θ of 44.35°, 51.67°, and 76.09°.Diffraction peaks of MgNiO2 (JCPDS NO. 24-0712) could also be seen at 2θ of 37.10°, 43.10°, and 62.59°, which was another argument in favor of the formation of Ni-Mg solid solutions.The MgNiO2 solid solution inhibited the agglomeration of Ni nanoparticles, which maintained high CO conversion and thermal stability at high temperatures.However, for Ni/MgAl-LDO (used), the diffraction peaks of spinel (MgAl2O4) could be observed at 2θ of 19.03°, 31.27°,36.85°,44.83°, 55.66°, 59.37°, and 65.24°, suggesting spinel (MgAl2O4) formation during the reaction.Spinel could promote catalytic performance, leading to enhanced reactivity with increasing temperature [12,31].

Catalyst Preparation
The procedure to prepare expanded multilayered vermiculite (EML-VMT) followed the protocol as described in our previous work [24].The schematic of the preparation process of VMT-LDH is displayed in Figure 7, vermiculite was expanded by H 2 O 2 to obtain the EML-VMT, then the EML-VMT was etched with acid to remove SiO 2 .Subsequently the spent liquor was used to prepare LDH.The LDO was obtained by calcination of LDH, and then the catalyst was prepared through the impregnation method.
Specific steps to prepare the LDH using the spent liquor are as follows: a 200 mL portion of 6 M mixed acid solution (hydrochloric acid and nitric acid with volume ratio of 3:1) was placed into round-bottom flask equipped with a reflux condenser.Ten grams of EML-VMT was added and boiled in the flask at 80 • C for 4 h with constant stirring.The obtained samples were filtered.The filtrate, which mainly contained Mg 2+ and Al 3+ , was collected and neutralized by adding 4 M NaOH until precipitates started to appear in the solution.Then, the filtrate solution was added dropwise to Na 2 CO 3 solution (100 mL) containing 0.04 mol Na 2 CO 3 , the pH value of the mixture was kept constant at 10 by addition of 4 M NaOH and then the solution was aged at room temperature for 4 h with continuous stirring.Afterwards, the mixture was filtered and washed with deionized water until pH = 7 and then dispersed with acetone for 2 h.The sample was filtered and washed with acetone again and then left to dry at 60 • C in an oven.The as-obtained support was denoted as VMT-LDH.In comparison, the MgAl-LDH with an Mg/Al molar ratio of 1.5:1 was also synthesized.Then VMT-LDH and MgAl-LDH were calcined at 550 • C for 4 h, and the as-obtained layered double oxides named VMT-LDO and MgAl-LDO.

Catalyst Preparation
The procedure to prepare expanded multilayered vermiculite (EML-VMT) followed the protocol as described in our previous work [24].The schematic of the preparation process of VMT-LDH is displayed in Figure 7, vermiculite was expanded by H2O2 to obtain the EML-VMT, then the EML-VMT was etched with acid to remove SiO2.Subsequently the spent liquor was used to prepare LDH.The LDO was obtained by calcination of LDH, and then the catalyst was prepared through the impregnation method.
Specific steps to prepare the LDH using the spent liquor are as follows: a 200 mL portion of 6 M mixed acid solution (hydrochloric acid and nitric acid with volume ratio of 3:1) was placed into round-bottom flask equipped with a reflux condenser.Ten grams of EML-VMT was added and boiled in the flask at 80 °C for 4 h with constant stirring.The obtained samples were filtered.The filtrate, which mainly contained Mg 2+ and Al 3+ , was collected and neutralized by adding 4 M NaOH until precipitates started to appear in the solution.Then, the filtrate solution was added dropwise to Na2CO3 solution (100 mL) containing 0.04 mol Na2CO3, the pH value of the mixture was kept constant at 10 by addition of 4 M NaOH and then the solution was aged at room temperature for 4 h with continuous stirring.Afterwards, the mixture was filtered and washed with deionized water until pH = 7 and then dispersed with acetone for 2 h.The sample was filtered and washed with acetone again and then left to dry at 60 °C in an oven.The as-obtained support was denoted as VMT-LDH.In comparison, the MgAl-LDH with an Mg/Al molar ratio of 1.5:1 was also synthesized.Then VMT-LDH and MgAl-LDH were calcined at 550 °C for 4 h, and the as-obtained layered double oxides named VMT-LDO and MgAl-LDO.The catalyst was prepared using an impregnation method; the support (VMT-LDO or MgAl-LDO) was impregnated with an aqueous solution which contains certain quantities of Ni(NO3)2•6H2O to achieve 10 wt % Ni loading, with vigorous stirring under the conditions of 80 °C for 12 h, and then evaporating the liquid.The as-prepared precursor was subsequently dried at 110 °C for 12 h and further calcined at 550 °C for 4 h in air at the heating rate of 10 °C/min.The obtained catalysts were denoted as NiO/VMT-LDO and NiO/MgAl-LDO.

Characterization of Samples
Powder X-ray diffraction (XRD) patterns were carried out on a Bruker D8 Advance X-ray diffractometer (Bruker Biosciences Corporation, Billerica, MA, USA) operated at 40 kV and 40 mA with Cu Kα radiation (λ = 0.15406 nm).The Brunauer-Emmett-Teller (BET) specific surface area and Barrett-Joyner-Halenda (BJH) pore structure of the catalysts were evaluated using a Micromeritics ASAP 2020 BET apparatus (Micromeritics Instrument Ltd., Norcross, GA, USA).The Micromeritics TPx System (Micromeritic ASAP 2720, Micromeritics Instrument Ltd, Norcross, GA, USA) was also employed to analysis the H2 temperature programmed reduction (H2-TPR) properties of catalysts in a temperature range from room temperature to 900 °C, with a gas (10 vol % H2/Ar) flow rate of 40 mL/min and a heating rate of 10 °C/min.Scanning electron microscopy (SEM) (Hitachi S-4300, Hitachi

Characterization of Samples
Powder X-ray diffraction (XRD) patterns were carried out on a Bruker D8 Advance X-ray diffractometer (Bruker Biosciences Corporation, Billerica, MA, USA) operated at 40 kV and 40 mA with Cu Kα radiation (λ = 0.15406 nm).The Brunauer-Emmett-Teller (BET) specific surface area and Barrett-Joyner-Halenda (BJH) pore structure of the catalysts were evaluated using a Micromeritics ASAP 2020 BET apparatus (Micromeritics Instrument Ltd., Norcross, GA, USA).The Micromeritics TPx System (Micromeritic ASAP 2720, Micromeritics Instrument Ltd., Norcross, GA, USA) was also employed to analysis the H 2 temperature programmed reduction (H 2 -TPR) properties of catalysts in a temperature range from room temperature to 900 • C, with a gas (10 vol % H 2 /Ar) flow rate of 40 mL/min and a heating rate of 10 • C/min.Scanning electron microscopy (SEM) (Hitachi S-4300, Hitachi Limited, Japan) and transmission electron microscopy (TEM) (TecnaiG2 F20, FEI Instrument Ltd., Hillsboro, OR, USA) were employed to observe the microscopic of the catalysts.

Catalytic Performance Evaluation
The evaluation of CO methanation performance of the catalysts were performed in a fixed bed reactor at 1.5 Mpa, with a stainless steel tubular microreactor.Prior to the feed syngas H 2 /CO with a volume ratio of 3:1 was introduced to the reactor, the catalyst (0.2 g) was activated in 28 mL/min H 2 at 500 • C for 2 h.Then the feed syngas introduced to the reactor with a gas flow rate of 65 mL/min and a weight space velocity of 20,000 mL/g/h, and the catalytic performance of the catalysts were evaluated in the temperature region of 250-500 • C. The outlet gases were analyzed by gas chromatography (GC-2014C, SHIMADZU) (GC-2014C, Shimadzu, Kyoto, Japan) online.

Conclusions
In this work, layered double hydroxide (LDH) was successfully synthesized, and the as-calcined layered double oxide (LDO) was used as a support for CO methanation.The as-obtained catalyst Ni/VMT-LDO had smaller Ni nanoparticles compared with catalyst Ni/MgAl-LDO, which was ascribed to Fe and Ca modification improved the dispersion of nickel, which resulted in a low temperature activity.The stronger metal-support interaction between Ni and VMT-LDO maintained the higher catalytic performance than the catalyst Ni/MgAl-LDO at higher temperature, so it was no doubt that the catalyst Ni/VMT-LDO exhibited excellent catalytic performance and thermal stability over the whole temperature range.Compared with Ni/MgAl-LDO, Ni/VMT-LDO achieved a better catalytic performance at 400 • C, including a 87.88% CO conversion, 89.97% CH 4 selectivity, 12.47 × 10 −2 •s −1 turn over frequency (TOF) and 79.1% CH 4 yield.This study demonstrated that VMT-LDO was a superior catalyst support to develop a catalyst for synthesis of SNG from syngas, and the VMT-LDO which contains Fe and Ca can also be used as catalyst support in other catalytic systems, such as super-dry reformation of methane.
) for Ni/MgAl-LDO was in good agreement with that of CO conversion for Ni/MgAl-LDO in the temperature region: 250 • C-400 • C, and declined with the temperature, which resulted from the carbon deposition and the agglomeration of the active Ni component.When it came to the catalyst Ni/VMT-LDO, which showed an interesting curve of CH 4 selectivity, the CH 4 selectivity of Ni/VMT-LDO declined at 350 • C compared with 300 • C. Both samples obtained a maximum CH 4 selectivity at 400 • C. See Table2for a comparison of catalytic performance in different works.It can be clearly seen in Figure4cthat Ni/VMT-LDO exhibited a higher turnover frequency (TOF) than Ni/MgAl-LDO over the whole temperature range.Ni/VMT-LDO reached a maximum TOF of 12.47× 10 −2 •s −1 at 400 • C, while for Ni/MgAl-LDO, a TOF value of 11.29× 10 −2 •s −1 was achieved at the same temperature.The highest TOF value of Ni/MgAl-LDO catalyst was 11.57 × 10 −2 •s −1 at 500 • C. Ni/VMT-LDO also achieved a higher CH 4 yield in the whole temperature range compared with the catalyst Ni/MgAl-LDO (Figure4d), the highest CH 4 yield was obtained at 400 • C for both samples.When the reaction temperature was higher than 400 • C, the CH 4 yield decreased with the temperature increased.However, Ni/MgAl-LDO and Ni/VMT-LDO still have ≥ 70% CH 4 yield at 500 • C. All of these results indicate that the catalytic performance of Ni/VMT-LDO catalyst was much better than that of Ni/MgAl-LDO catalyst.Catalysts 2017, 7, 79 5 of 10
, both catalysts Ni/MgAl-LDO (used) and Ni/VMT-LDO (used) exhibited no obvious agglomeration, and especially for catalyst Ni/VMT-LDO (used) there were no obvious Ni particles to be seen, the absence of Ni particles in the catalysts being due to the smaller Ni particles and the formation of Ni-Mg solid solutions [21].XRD patterns of the used catalysts are shown in Figure 6.For Ni/MgAl-LDO (used) and Ni/VMT-LDO (used), both samples showed the diffraction peaks of metal Ni (JCPDS NO. 65-0380) at 2θ of 44.35 • , 51.67 • , and 76.09 • .Diffraction peaks of MgNiO 2 (JCPDS NO. 24-0712) could also be seen at 2θ of 37.10 • , 43.10 • , and 62.59 • , which was another argument in favor of the formation of Ni-Mg solid solutions.The MgNiO 2 solid solution inhibited agglomeration of Ni nanoparticles, which maintained high CO conversion and thermal stability at high temperatures.However, for Ni/MgAl-LDO (used), the diffraction peaks of spinel (MgAl 2 O 4 ) could be observed at 2θ of 19.03 • , 31.27 • , 36.85 • , 44.83 • , 55.66 • , 59.37 • , and 65.24 • , suggesting spinel (MgAl 2 O 4 ) formation during the reaction.Spinel could promote catalytic performance, leading to enhanced reactivity with increasing temperature [12,31].Catalysts 2017, 7, 79 6 of 10

Figure 7 .
Figure 7. Schematic of the preparation process of VMT-LDH and NiO/VMT-LDH.

Figure 7 .
Figure 7. Schematic of the preparation process of VMT-LDH and NiO/VMT-LDH.

Table 1 .
Structural physical properties of the supports and catalysts.

Table 1 .
Structural physical properties of the supports and catalysts.

a Pore Volume (cm 3 •g −1 ) b Pore Size (nm) b
a Obtained from the Brunauer-Emmett-Teller (BET) method.b Obtained from the Barrett-Joyner-Halenda (BJH) desorption average pore volume and pore diameter.

Table 2 .
The comparison of catalytic performance in different works.

Table 2 .
The comparison of catalytic performance in different works.

Table 2 .
The comparison of catalytic performance in different works.