Vacuum Thermal Treated Ni-CeO2/SBA-15 Catalyst for CO2 Methanation

Ni-CeO2/SBA-15-V catalyst was prepared by the impregnation method with vacuum thermal treatment and used for CO2 methanation reaction. Compared with Ni-CeO2/SBA-15-air catalyst with thermal treatment in air, the reduced Ni-CeO2/SBA-15-V catalyst with vacuum thermal treatment exhibited higher Ni dispersion and smaller Ni particle size. In CO2 methanation reaction, the Ni-CeO2/SBA-15-V catalyst was more active and selective than the Ni-CeO2/SBA-15-air catalyst. The good activity and selectivity of Ni-CeO2/SBA-15-V catalyst should be due to highly dispersed Ni in contact with small CeO2 particles.


Introduction
With the increasing consumption of fossil fuel, CO 2 released into the atmosphere has resulted in climate change and global warming, and the utilization of CO 2 has received much attention. CO 2 hydrogenation is an effective way to convert CO 2 into fuels and chemicals [1,2]. CO 2 methanation can convert CO 2 and renewable H 2 into methane, which is a promising process for CO 2 conversion and the storage of renewable H 2 [3]. CO 2 methanation is an exothermic reaction, and the low reaction temperature is favorable for the high equilibrium conversion of CO 2 . Therefore, various catalysts have been developed for CO 2 methanation at low temperature. Noble metal-based catalysts, such as Rh [4], Ru [5], and Pd [6], and non-noble Co [7,8] and Ni-based catalysts [9,10] are the most studied CO 2 methanation catalysts. Although noble metal-based catalysts possess good activity for low-temperature CO 2 methanation, Ni-based catalysts are preferred in industrial processes due to the low cost, and they have been applied for CO methanation [11,12] and CO 2 methanation [13]. Ni supported on various metal oxides, such as SiO 2 [13], Al 2 O 3 [14], CeO 2 [15], ZrO 2 [16], and TiO 2 [17], has been reported for CO 2 methanation. Ni supported on SBA-15, which is a kind of mesoporous SiO 2 with high specific surface area, has been reported for CO 2 methanation [18,19]. Adding CeO 2 to a Ni-based catalyst can enhance Ni dispersion and CO 2 adsorption and dissociation [20]. CeO 2 -promoted Ni/SBA-15 catalyst has been used for CO 2 methanation reaction and exhibits excellent catalytic performance [10,19].
However, it is difficult to obtain highly dispersed Ni particles on SBA-15 support by the conventional impregnation method, and Ni particles are mainly located on the SBA-15 outer surface [19]. Various methods, such as two solvents [21], surfactant assistant [22], and ammonia evaporation [23] methods have been used to improve the dispersion of Ni on SBA-15 support. Recently, vacuum thermal treatment was applied to synthesize highly dispersed Cu [24], Pt [25], and Pd [26] particles on mesoporous SBA-15. In the present work, the Ni-CeO 2 /SBA-15 catalyst was prepared by the impregnation method with vacuum thermal treatment and used for CO 2 methanation. The vacuum thermal treated catalyst exhibited high dispersion of Ni and CeO 2 and showed high activity and selectivity for CO 2 methanation reaction.

Synthesis of Catalysts
SBA-15 was prepared by a hydrothermal method according to the literature [27]. First, 16 For comparison, using γ-Al 2 O 3 (241 m 2 /g) as support, a Ni-CeO 2 /Al 2 O 3 catalyst was prepared by impregnation method and calcinated in air as described above.
In these catalysts, the theoretical Ni and CeO 2 content was 10 wt % for both.

Characterization of Catalysts
N 2 isotherms were measured using an Autosorb-iQ analyzer (Quantachrome Instruments, Boynton Beach, FL, USA) at −196 • C, and the pore size distribution was calculated from the desorption branch by Barrett-Joyner-Halenda (BJH) method. X-ray diffraction (XRD) patterns were measured by a DX-2700 X-ray diffractometer (Haoyuan Instrument, Dandong, China) using Cu K α radiation. Transmission electron microscope (TEM) images were measured on a Tecnai G2 F20 microscope (FEI Company, Hillsboro, OR, USA). (XRD). H 2 -temperature programmed reduction (H 2 -TPR) patterns were measured on a TP-5080 adsorption instrument under a 5% H 2 /Ar flow at a rate of 10 • C/min to 900 • C. Before H 2 -TPR measurements, the samples were purged with an argon flow at 400 • C for 5 min. The outgas was monitored by a thermal conductivity detector (TCD) detector. Thermo gravimetric (TG) was carried out using an HCT-1 TG thermal analyzer (Henven Scientific Instrument, Beijing, China). The sample was heated from room temperature to 900 • C with a heating rate of 10 • C/min in air (20 mL/min).

Catalytic Test
The reaction was carried out in a fixed bed quartz reactor (inner diameter (ID) = 8 mm) at atmospheric pressure. The temperature of the reactor was controlled by a thermocouple inserted into the catalyst bed, and the gas flow rates were controlled by mass flow controllers. For a catalytic test, 100 mg of catalyst (60-100 mesh) diluted with 200 mg of inert SiO 2 was used. The Ni-CeO 2 /SBA-15 catalyst was reduced to 50 mL/min of 20% H 2 /Ar stream at 450 • C for 40 min, and. After reduction, the reactor temperature was decreased to 200 • C in 20% H 2 /N 2 . Then the reactant gases (CO 2 :H 2 :Ar = 1:4:5, 100 mL/min) were introduced into the reactor. The reaction was conducted in the temperature range from 200 to 450 • C. The catalyst was kept for 1 h at each reaction temperature before the products were analyzed. The stability tests were performed at 350 and 400 • C. Before stability test, the Ni-CeO 2 /Al 2 O 3 catalyst was reduced at 700 • C for 40 min. The exhaust and feed gas compositions were analyzed on a GC-7900 gas chromatograph (Techcomp, Kwai Chung, China) equipped with a TDX-01 column and a TCD detector. CO 2 conversion, CH 4 selectivity, and carbon balance were calculated using the following formulae: CO 2 conversion = moles of CO 2 in −moles of CO 2 out moles of CO 2 in × 100% CH 4 selectivity = moles of CH 4 out moles of CO 2 in −moles of CO 2 out × 100% Carbon balance = (1 − moles of CH 4 out +moles of CO out +moles of CO 2 out moles of CO 2 in ) × 100% In this work, carbon balance was within ±3% for all the catalytic tests, indicating negligible carbon deposition on the catalysts. Figure 1 shows N 2 adsorption-desorption isotherms and pore size distributions of SBA-15, Ni-CeO 2 /SBA-15-air, and Ni-CeO 2 /SBA-15-V. All samples showed type IV isotherm with a hysteresis loop in the relative pressure 0.65-0.80, confirming the uniform mesoporous structure in these samples. As shown in Figure 1b, the pore size of all samples was 6.5 nm, indicating the addition of Ni and CeO 2 to SBA-15 without changing the mesoporous structures. As listed in Table 1, the Brunauer-Emmett-Teller (BET) surface area and pore volume decreased after the addition of NiO and CeO 2 into SBA-15. In this work, carbon balance was within ±3% for all the catalytic tests, indicating negligible carbon deposition on the catalysts. Figure 1 shows N2 adsorption-desorption isotherms and pore size distributions of SBA-15, Ni-CeO2/SBA-15-air, and Ni-CeO2/SBA-15-V. All samples showed type IV isotherm with a hysteresis loop in the relative pressure 0.65-0.80, confirming the uniform mesoporous structure in these samples. As shown in Figure 1b, the pore size of all samples was 6.5 nm, indicating the addition of Ni and CeO2 to SBA-15 without changing the mesoporous structures. As listed in Table 1, the Brunauer-Emmett-Teller (BET) surface area and pore volume decreased after the addition of NiO and CeO2 into SBA-15.   Figure 2 shows the XRD patterns of the catalysts. The reduced catalysts were reduced in a 20% H2/Ar stream (50 mL/min) for 40 min at 450 °C. As shown in Figure 2a, all the small-angle XRD patterns showed three typical peaks of mesoporous SBA-15 [27]. The result demonstrates that the mesoporous structure of SBA-15 was maintained after loading NiO and CeO2. Figure 2b shows the wide-angle XRD patterns of fresh and reduced catalysts. Compared with Ni-CeO2/SBA-15-air, Ni-CeO2/SBA-15-V exhibited obviously wider reflection peaks of NiO and CeO2, indicating that vacuum treatment can improve the dispersion of NiO and CeO2. After reduction, the reflection peaks of Ni in Ni-CeO2/SBA-15-V were also wider than those in Ni-CeO2/SBA-15-air, confirming that the Ni particle was smaller in the Ni-CeO2/SBA-15-V catalyst after reduction. As listed in Table 2, the crystal sizes of Ni and CeO2 in reduced Ni-CeO2/SBA-15-V were 8.5 and 4.2 nm, which were obviously smaller than   Figure 2 shows the XRD patterns of the catalysts. The reduced catalysts were reduced in a 20% H 2 /Ar stream (50 mL/min) for 40 min at 450 • C. As shown in Figure 2a, all the small-angle XRD patterns showed three typical peaks of mesoporous SBA-15 [27]. The result demonstrates that the mesoporous structure of SBA-15 was maintained after loading NiO and CeO 2 . Figure 2b shows the wide-angle XRD patterns of fresh and reduced catalysts. Compared with Ni-CeO 2 /SBA-15-air, Ni-CeO 2 /SBA-15-V exhibited obviously wider reflection peaks of NiO and CeO 2 , indicating that vacuum treatment can improve the dispersion of NiO and CeO 2 . After reduction, the reflection peaks of Ni in Ni-CeO 2 /SBA-15-V were also wider than those in Ni-CeO 2 /SBA-15-air, confirming that the Ni particle was smaller in the Ni-CeO 2 /SBA-15-V catalyst after reduction. As listed in Table 2, the crystal sizes of Ni and CeO 2 in reduced Ni-CeO 2 /SBA-15-V were 8.5 and 4.2 nm, which were obviously smaller than those in reduced Ni-CeO 2 /SBA-15-air. The XRD results indicate that vacuum thermal treatment can improve Ni and CeO 2 dispersion on SBA-15 support.    Figure 3f. Figure 4 shows H2-TPR profiles of the catalysts. For Ni-CeO2/SBA-15-air, the sharp peak located at around 420 °C was attributed to the reduction of large bulk NiO species with no or very little interaction with SiO2 support [19]. For Ni-CeO2/SBA-15-V, a reduction peak between 300 °C and 600 °C was observed. A reduction peak beyond 500 °C corresponds to the reduction of small NiO particles strongly interacting with SiO2 support [28][29][30][31][32][33]. The result indicates that there is a strong interaction between Ni and SiO2 in Ni-CeO2/SBA-15-V catalyst.     Figure 3f. Figure 4 shows H 2 -TPR profiles of the catalysts. For Ni-CeO 2 /SBA-15-air, the sharp peak located at around 420 • C was attributed to the reduction of large bulk NiO species with no or very little interaction with SiO 2 support [19]. For Ni-CeO 2 /SBA-15-V, a reduction peak between 300 • C and 600 • C was observed. A reduction peak beyond 500 • C corresponds to the reduction of small NiO particles strongly interacting with SiO 2 support [28][29][30][31][32][33]. The result indicates that there is a strong interaction between Ni and SiO 2 in Ni-CeO 2 /SBA-15-V catalyst.

Catalytic Performance
The CO2 conversion and CH4 selectivity of the catalysts for CO2 methanation reaction are shown in Figure 5. Compared with the Ni-CeO2/SBA-15-air catalyst, the Ni-CeO2/SBA-15-V catalyst exhibited obviously higher CO2 conversion and CH4 selectivity. For the Ni-CeO2/SBA-15-V catalyst,

Catalytic Performance
The CO2 conversion and CH4 selectivity of the catalysts for CO2 methanation reaction are shown in Figure 5. Compared with the Ni-CeO2/SBA-15-air catalyst, the Ni-CeO2/SBA-15-V catalyst exhibited obviously higher CO2 conversion and CH4 selectivity. For the Ni-CeO2/SBA-15-V catalyst,

Catalytic Performance
The CO 2 conversion and CH 4 selectivity of the catalysts for CO 2 methanation reaction are shown in Figure 5. Compared with the Ni-CeO 2 /SBA-15-air catalyst, the Ni-CeO 2 /SBA-15-V catalyst exhibited obviously higher CO 2 conversion and CH 4 selectivity. For the Ni-CeO 2 /SBA-15-V catalyst, CO 2 conversion and CH 4 selectivity at 400 • C were 68.8% and 99.0%, respectively. CH 4 selectivity of the Ni-CeO 2 /SBA-15-V catalyst decreased to 97.1% at 450 • C. This is because reverse water gas shift reaction (CO 2 + H 2 → CO + H 2 O) is an endothermic reaction, and high temperatures are more conducive to producing CO and decreasing CH 4 selectivity. The result shows that the Ni-CeO 2 /SBA-15-V catalyst was active and selective for the CO 2 methanation reaction. It should be noted that the Ni-CeO 2 /SBA-15-air catalyst produced a large amount of CO even at low temperature. This may be due to that the large Ni particle in the Ni-CeO 2 /SBA-15-air catalyst was active for both CO 2 methanation and reverse water-gas shift reaction, while small Ni particle in contact with CeO 2 in Ni-CeO 2 /SBA-15-V catalyst was more selective for CO 2 methanation.
CO2 conversion and CH4 selectivity at 400 °C were 68.8% and 99.0%, respectively. CH4 selectivity of the Ni-CeO2/SBA-15-V catalyst decreased to 97.1% at 450 °C. This is because reverse water gas shift reaction (CO2 + H2 → CO + H2O) is an endothermic reaction, and high temperatures are more conducive to producing CO and decreasing CH4 selectivity. The result shows that the Ni-CeO2/SBA-15-V catalyst was active and selective for the CO2 methanation reaction. It should be noted that the Ni-CeO2/SBA-15-air catalyst produced a large amount of CO even at low temperature. This may be due to that the large Ni particle in the Ni-CeO2/SBA-15-air catalyst was active for both CO2 methanation and reverse water-gas shift reaction, while small Ni particle in contact with CeO2 in Ni-CeO2/SBA-15-V catalyst was more selective for CO2 methanation.  Figure 6 shows the stability test of Ni-CeO2/SBA-15-V catalyst at 350 °C and 400 °C. The equilibrium conversions are also shown in Figure 6. The CO2 conversions of Ni-CeO2/SBA-15-V catalyst had a little decrease in initial 10 h on stream, then the catalyst exhibited stable catalytic performance for 50 h at 350 °C and 400 °C. After reaction for 60 h, the CO2 conversion at 350 °C and 400 °C stayed at around 49.5% and 62.5%, respectively. It is worth noticing that, during the stability test at 400 °C, CH4 selectivity achieved over the Ni-CeO2/SBA-15-air catalyst remained almost constant after 10 h on stream approaching values higher than 97%. Among Ni-based methanation catalysts, Ni/Al2O3 catalyst is the most widely investigated and used due to its low costs and availability. Table 3 shows the CO2 conversion and selectivity of reported Ni/Al2O3 catalyst at 250 and 300 °C. Compared with the Ni/Al2O3 catalyst, the CO2 conversion of Ni-CeO2/SBA-15-V was obviously higher.  Figure 6 shows the stability test of Ni-CeO 2 /SBA-15-V catalyst at 350 • C and 400 • C. The equilibrium conversions are also shown in Figure 6. The CO 2 conversions of Ni-CeO 2 /SBA-15-V catalyst had a little decrease in initial 10 h on stream, then the catalyst exhibited stable catalytic performance for 50 h at 350 • C and 400 • C. After reaction for 60 h, the CO 2 conversion at 350 • C and 400 • C stayed at around 49.5% and 62.5%, respectively. It is worth noticing that, during the stability test at 400 • C, CH 4 selectivity achieved over the Ni-CeO 2 /SBA-15-air catalyst remained almost constant after 10 h on stream approaching values higher than 97%. Nanomaterials 2018, 10, x 6 of 10 CO2 conversion and CH4 selectivity at 400 °C were 68.8% and 99.0%, respectively. CH4 selectivity of the Ni-CeO2/SBA-15-V catalyst decreased to 97.1% at 450 °C. This is because reverse water gas shift reaction (CO2 + H2 → CO + H2O) is an endothermic reaction, and high temperatures are more conducive to producing CO and decreasing CH4 selectivity. The result shows that the Ni-CeO2/SBA-15-V catalyst was active and selective for the CO2 methanation reaction. It should be noted that the Ni-CeO2/SBA-15-air catalyst produced a large amount of CO even at low temperature. This may be due to that the large Ni particle in the Ni-CeO2/SBA-15-air catalyst was active for both CO2 methanation and reverse water-gas shift reaction, while small Ni particle in contact with CeO2 in Ni-CeO2/SBA-15-V catalyst was more selective for CO2 methanation.  Figure 6 shows the stability test of Ni-CeO2/SBA-15-V catalyst at 350 °C and 400 °C. The equilibrium conversions are also shown in Figure 6. The CO2 conversions of Ni-CeO2/SBA-15-V catalyst had a little decrease in initial 10 h on stream, then the catalyst exhibited stable catalytic performance for 50 h at 350 °C and 400 °C. After reaction for 60 h, the CO2 conversion at 350 °C and 400 °C stayed at around 49.5% and 62.5%, respectively. It is worth noticing that, during the stability test at 400 °C, CH4 selectivity achieved over the Ni-CeO2/SBA-15-air catalyst remained almost constant after 10 h on stream approaching values higher than 97%. Among Ni-based methanation catalysts, Ni/Al2O3 catalyst is the most widely investigated and used due to its low costs and availability. Table 3 shows the CO2 conversion and selectivity of reported Ni/Al2O3 catalyst at 250 and 300 °C. Compared with the Ni/Al2O3 catalyst, the CO2 conversion of Ni-CeO2/SBA-15-V was obviously higher. Among Ni-based methanation catalysts, Ni/Al 2 O 3 catalyst is the most widely investigated and used due to its low costs and availability. Table 3 shows the CO 2 conversion and selectivity of reported Ni/Al 2 O 3 catalyst at 250 and 300 • C. Compared with the Ni/Al 2 O 3 catalyst, the CO 2 conversion of Ni-CeO 2 /SBA-15-V was obviously higher. For comparison with Ni-CeO 2 /SBA-15-V catalyst, the stability of Ni-CeO 2 /Al 2 O 3 catalyst prepared with a conventional impregnation method was tested at 350 • C. As shown in Figure 7, the CO 2 conversion and CH 4 selectivity of Ni-CeO 2 /Al 2 O 3 catalyst were obviously lower than that of Ni-CeO 2 /SBA-15-V catalyst.  For comparison with Ni-CeO2/SBA-15-V catalyst, the stability of Ni-CeO2/Al2O3 catalyst prepared with a conventional impregnation method was tested at 350 °C. As shown in Figure 7, the CO2 conversion and CH4 selectivity of Ni-CeO2/Al2O3 catalyst were obviously lower than that of Ni-CeO2/SBA-15-V catalyst. Carbon deposition and sintering of metal particle are the main reasons for the catalyst deactivation during CO2 methanation [10,35,36]. The spent Ni-CeO2/SBA-15-V catalyst after stability test at 400 °C was characterized by TG, XRD, and TEM. TG curve was shown in Figure 8a. There was a weight decrease below 100 °C and a weight increase in the temperature region of 180-400 °C. The weight decrease should be due to the desorption of adsorbed water and the weight increase due to the oxidation metal Ni to NiO. There was no obvious weight loss above 100 °C, indicating no obviously carbon deposition was formed on the spent Ni-CeO2/SBA-15-V catalyst. XRD patterns of reduced and spent Ni-CeO2/SBA-15-V catalysts were shown in Figure 8b. Determined by Scherrer's equation from the (111) plane of Ni in XRD patterns, Ni crystal sizes were 8.5 and 9.1 nm for reduced and spent Ni-CeO2/SBA-15-V catalysts, respectively. XRD results indicate that the Ni particle size was slightly increased during the stability test. As shown in Figure 8c, TEM image also showed that no obviously sintering of Ni particle occurred. The TG, XRD, and TEM results indicate the slight decrease of activity and selectivity during stability test was due to the slight increase of Ni particle size. Carbon deposition and sintering of metal particle are the main reasons for the catalyst deactivation during CO 2 methanation [10,35,36]. The spent Ni-CeO 2 /SBA-15-V catalyst after stability test at 400 • C was characterized by TG, XRD, and TEM. TG curve was shown in Figure 8a. There was a weight decrease below 100 • C and a weight increase in the temperature region of 180-400 • C. The weight decrease should be due to the desorption of adsorbed water and the weight increase due to the oxidation metal Ni to NiO. There was no obvious weight loss above 100 • C, indicating no obviously carbon deposition was formed on the spent Ni-CeO 2 /SBA-15-V catalyst. XRD patterns of reduced and spent Ni-CeO 2 /SBA-15-V catalysts were shown in Figure 8b. Determined by Scherrer's equation from the (111) plane of Ni in XRD patterns, Ni crystal sizes were 8.5 and 9.1 nm for reduced and spent Ni-CeO 2 /SBA-15-V catalysts, respectively. XRD results indicate that the Ni particle size was slightly increased during the stability test. As shown in Figure 8c, TEM image also showed that no obviously sintering of Ni particle occurred. The TG, XRD, and TEM results indicate the slight decrease of activity and selectivity during stability test was due to the slight increase of Ni particle size.
The present work clearly demonstrates that the Ni-CeO 2 /SBA-15-V catalyst possessed superior activity and selectivity and exhibited excellent stability during the CO 2 methanation reaction. The XRD and TEM results prove that the vacuum-thermal treated Ni-CeO 2 /SBA-15-V catalyst had higher Ni dispersion and smaller Ni particle size than the Ni-CeO 2 /SBA-15-air catalyst thermal treated in air. Based on the characterization and catalyst performance results, it can be concluded that good activity and selectivity of Ni-CeO 2 /SBA-15-V catalyst can be attributed to the small Ni particle size in contact with CeO 2 . Nanomaterials 2018, 10, x 8 of 10 The present work clearly demonstrates that the Ni-CeO2/SBA-15-V catalyst possessed superior activity and selectivity and exhibited excellent stability during the CO2 methanation reaction. The XRD and TEM results prove that the vacuum-thermal treated Ni-CeO2/SBA-15-V catalyst had higher Ni dispersion and smaller Ni particle size than the Ni-CeO2/SBA-15-air catalyst thermal treated in air. Based on the characterization and catalyst performance results, it can be concluded that good activity and selectivity of Ni-CeO2/SBA-15-V catalyst can be attributed to the small Ni particle size in contact with CeO2.

Conclusions
The effects of vacuum thermal treatment on Ni-CeO2/SBA-15 catalysts were investigated in this paper. Compared with thermal treatment in air, vacuum thermal treatment can improve the dispersion of Ni and CeO2 in the Ni-CeO2/SBA-15 catalyst. The Ni-CeO2/SBA-15-V catalyst with vacuum thermal treatment exhibited better activity and selectivity than the Ni-CeO2/SBA-15-air catalyst thermal treated in air. The excellent catalytic performance of the Ni-CeO2/SBA-15-V catalyst was mainly attributed to higher Ni and CeO2 dispersion.

Conclusions
The effects of vacuum thermal treatment on Ni-CeO 2 /SBA-15 catalysts were investigated in this paper. Compared with thermal treatment in air, vacuum thermal treatment can improve the dispersion of Ni and CeO 2 in the Ni-CeO 2 /SBA-15 catalyst. The Ni-CeO 2 /SBA-15-V catalyst with vacuum thermal treatment exhibited better activity and selectivity than the Ni-CeO 2 /SBA-15-air catalyst thermal treated in air. The excellent catalytic performance of the Ni-CeO 2 /SBA-15-V catalyst was mainly attributed to higher Ni and CeO 2 dispersion.