Research Article | Open Access
Wen Yang, Yanyan Feng, Wei Chu, "Promotion Effect of CaO Modification on Mesoporous Al2O3-Supported Ni Catalysts for CO2 Methanation", International Journal of Chemical Engineering, vol. 2016, Article ID 2041821, 7 pages, 2016. https://doi.org/10.1155/2016/2041821
Promotion Effect of CaO Modification on Mesoporous Al2O3-Supported Ni Catalysts for CO2 Methanation
The catalysts Ni/Al2O3 and CaO modified Ni/Al2O3 were prepared by impregnation method and applied for methanation of CO2. The catalysts were characterized by N2 adsorption/desorption, temperature-programmed reduction of H2 (H2-TPR), X-ray diffraction (XRD), and temperature-programmed desorption of CO2 and H2 (CO2-TPD and H2-TPD) techniques, respectively. TPR and XRD results indicated that CaO can effectively restrain the growth of NiO nanoparticles, improve the dispersion of NiO, and weaken the interaction between NiO and Al2O3. CO2-TPD and H2-TPD results suggested that CaO can change the environment surrounding of CO2 and H2 adsorption and thus the reactants on the Ni atoms can be activated more easily. The modified Ni/Al2O3 showed better catalytic activity than pure Ni/Al2O3. Ni/CaO-Al2O3 showed high CO2 conversion especially at low temperatures compared to Ni/Al2O3, and the selectivity to CH4 was very close to 1. The high CO2 conversion over Ni/CaO-Al2O3 was mainly caused by the surface coverage by CO2-derived species on CaO-Al2O3 surface.
In present years, various technological options have been considered to reduce the amount of carbon dioxide emitted in the atmosphere by combustion of fossil fuels [1–7]. In case hydrogen is available or can be produced by renewable energy such as solar energy in order to achieve a low-carbon society , the hydrogenation of captured CO2 is an interesting option as a CO2 Capture and Storage (CCS) technology . The methane produced can be injected in chemical and petrochemical industries as natural gas use. Thus, the possibility to transform H2 and CO2 into CH4 becomes a real alternative in an environmental and ecological point of view [7–9].
CO2 methanation, also called the Sabatier reaction , has been known for a long time and a great number of metal catalysts have been conducted in the reaction [3, 4, 6–8, 11–14] such as Rh [4, 15–18], Ru [19, 20], Ni [1, 4, 21–26] supported on various oxide solids like Al2O3 [1, 2, 4, 15, 22, 27], SiO2 [1, 4, 28], ZrO2 [1, 21–24], or TiO2 . Evidences have shown that CO2 adsorption and dissociation are the first steps in the mechanism of CO2 methanation, and CO2 dissociation into CO is sometimes considered as a poisoning effect on hydrogen adsorption [4, 29]. Moreover, the increase of catalytic activity was also related to the enhancement of the hydrogen adsorption capacity. Therefore, in order to increase catalytic activity of the methanation, it is necessary to enhance hydrogen supply at the surface of the catalyst [15, 18]. It was suggested that nickel supported catalysts could supply active catalysts in CO2 methanation to increase the reaction performances , and hence this was the principal objective of this work.
The Ni-based catalyst has been extensively investigated under widely varying experimental conditions, and Ni/Al2O3 catalysts showed moderate catalytic activity in methanation of carbon dioxide with hydrogen . For Ni-based catalysts, the promoters, such as MgO, CeO2, and La2O3, have played important roles in the activity for CO2 methanation. The basic properties, special electronic structure, and strong interaction with Ni could enhance the CO2 adsorption and dissociation and consequently improve the activity of the catalyst for CO2 methanation. In our previous study , NiCe/CNTs and NiCe/Al2O3 catalysts prepared by impregnation method were found to be highly active and stable for CO2 methanation. Therefore, it is desired to develop an effectively promoted Ni-based catalyst which exhibits high activity in methanation of carbon dioxide, namely, to combine a well-known catalyst Ni/Al2O3 with CaO in CO2 adsorption and dissociation. The aim of this work was to investigate the effects of CaO-Al2O3 as the support and the content of CaO on the catalytic performance of CO2 methanation over Ni/CaO-Al2O3 catalysts and to optimize the reaction temperature.
2.1. Preparation of CaO-Al2O3 Supports and Ni-Based Catalysts
The CaO-Al2O3 composite support (, 10, 20, 30%) was prepared by the sol-gel method. Ca(NO3)2·4H2O, Al(NO3)3·9H2O, and citric acid with a stoichiometric ratio were dissolved in deionized water and stirred and evaporated at 70°C until the solution became viscous colloids. Sequently the support was dried in an oven at 120°C for 24 h and placed in the muffle furnace calcined at 600°C in air for 3 h [2, 22, 31, 32].
The Ni-based catalysts were prepared by impregnating the supports (100Al2O3, 10CaO-90Al2O3, 20CaO-80Al2O3, and 30CaO-70Al2O3) with an aqueous solution of Ni(NO3)2·6H2O [2, 9, 22]. The Ni loading was fixed at 15 wt.% in the samples. The impregnated granules were dried at 120°C for 12 h and then calcined at 500°C in air for 3 h. The prepared catalysts were denoted as 15Ni/100Al2O3, 15Ni/10CaO90Al2O3, 15Ni/20CaO80Al2O3, and 15Ni/30CaO70Al2O3, respectively.
2.2. Characterization of Catalysts
Temperature-programmed reduction of H2 (H2-TPR) was performed in a fixed bed reactor to observe the catalyst reducibility [2, 31, 32]. 100 mg sample was loaded in the middle of the reactor tube. The temperature of the reactor was raised from 100°C to 800°C at a heating rate of 10°C/min under 5% H2/95% N2 with a flow rate of 30 mL/min. The H2 consumption was analyzed online by a SC-200 gas chromatograph with a thermal conductivity detector (TCD).
Experiments of temperature-programmed desorption of CO2 (CO2-TPD) were conducted employing an automated gas sorption analyser TP-5080 . The samples were pretreated at 500°C (5°C/min) under N2 flow (30 mL/min) for 1 h and reduced at the same temperature under H2 flow (30 mL/min) for 1 h. After that, the materials were cooled and exposed to CO2 (30 mL/min) for 1 h at 50°C. CO2-TPD measurements were carried out up to 750°C with the heating rate of 10°C/min under N2 flow (30 mL/min).
2.3. Catalytic Performance
The catalytic performance of the catalyst was conducted under atmospheric pressure in a fixed bed reactor with an interior diameter of 6 mm. 100 mg of catalyst was pretreated at 500°C in 30 mL/min H2 flow for 1 h and then cooled down to room temperature in N2. Next, a mixture of H2 and CO2 (molar ratio = 4.0) was switched to the reactor. The catalyst was then heated to the reaction temperature at a rate of 5°C/min. The composition of the outlet gases was analyzed online by a GC-1690 model gas chromatograph with a TDX-01 column and a thermal conductivity detector (TCD). The CO2 conversion () and CH4 yield () were estimated by the following:
3. Results and Discussion
3.1. Textural Properties of the Catalysts
The textural properties of the catalysts were characterized by N2 adsorption/desorption analysis, and the N2 adsorption/desorption isotherms and pore size distributions obtained by the BJH equation were shown in Figure 1. The pristine Ni/Al2O3 catalyst had a large pore size (average pore diameter) of 3.60 nm. After modification, the pore sizes of the catalysts changed significantly. The increase of pore size was attributed to deposition of nanoparticles in the pores of the Al2O3, which formed new porosity and extra surface area. As seen from Figure 1(b), the pristine catalyst exhibited the pore size distribution centered at about 3.60 nm. After the modification, the small pores below 20 nm were formed by the self-organization of nanoparticles inside the large pores of Al2O3. The formation of the pore structure by CaO modification would improve the dispersion of Ni species.
Specific surface area, pore volume, and pore diameter of the Ni/CaO-Al2O3 catalysts with different CaO contents calcined at 500°C were presented in Table 1. It could be seen that the addition of CaO resulted in slight increases in specific surface area and pore volume of the catalysts. Among the samples, 15Ni/10CaO90Al2O3 exhibited the largest specific surface area of 9.59 m2/g. This could be mainly attributed to the blocking of part of mesopores of the catalysts and the change in NiO and Al2O3 structures through their interactions with CaO. Furthermore, the pore diameters of the catalysts had changed.
|Specific surface area evaluated using the Brunauer-Emmett-Teller (BET) method. Pore volume calculated from the volume of nitrogen held at / = 0.98~0.99. BJH desorption average pore diameter.|
3.2. Crystal Phase Analysis of the Catalysts
The XRD patterns of the catalysts calcined at 500°C were displayed in Figure 2, from which the diffraction peaks at 37.2°, 43.3°, 62.9°, 75.4°, and 79.4° corresponding to NiO species were observed. In addition, no clear characteristic diffraction peaks of CaO or CaCO3 phases can be detected, indicating that CaO was highly dispersed on the surface of Al2O3 or formed micromorphology grain which was below the detection limit of XRD. Furthermore, it was found that the peak intensities of the NiO phase became weaker in the modified catalysts than that in the pure catalyst, probably due to the superior dispersion of the NiO species induced by CaO addition.
The XRD patterns of the catalysts reduced at 500°C were displayed in Figure 3. There was a new phase assigned to metallic Ni crystallites corresponding to the diffraction peaks at 44.5°, 51.8°, and 76.4°, and the diffraction peaks corresponding to NiO phase were not observed, as well as the diffraction peaks of CaO or CaCO3, indicating that Ni2+ ions in the NiO phase were fully reduced to metallic Ni and Ca2+ ions in the CaO phase were not reduced.
3.3. Temperature-Programmed Reduction and Reducibility of the Catalysts
H2-TPR measurements were performed to investigate the reducibility of the catalysts and to examine the interaction between nickel species and the supports. Figure 4 showed H2-TPR profiles of the catalysts with different CaO contents calcined at 500°C for 3 h. It was clear that all the samples showed a H2 consumption peak in the range of 350°C~600°C corresponding to the reduction of NiO species. For the catalyst 15Ni/100Al2O3, main reduction peak was located at 435°C. After CaO modification, the reduction peak was shifted to lower temperatures, which was attributed to the reduction of NiO species that interacted weakly with the CaO-Al2O3 supports and the improvement of the catalyst reducibility. This behavior indicated that the incorporation of CaO into Al2O3 supports promoted the reduction of NiO species. However, it was found that, relative to that of 15Ni/20CaO80Al2O3, the reduction temperature peak of 15Ni/30CaO70Al2O3 shifted slightly to higher temperature, which was resulted from the high coverage of CaO on the surface of active sites. This meant that CaO content had an optimal range in the Ni/Al2O3 catalyst to affect its catalytic performance.
3.4. Temperature-Programmed Desorption of the Catalysts
The H2-TPD profiles of the catalysts were presented in Figure 5. The sample 15Ni/100Al2O3 exhibited one peak located at 50~400°C. With CaO modification, the catalysts 15Ni/10CaO90Al2O3 and 15Ni/20CaO80Al2O3 showed two peaks, of which the low-temperature peak corresponded to the physical adsorption of H2 weakly adsorbed on the metal surface, and the high-temperature peak located at 250~550°C could be originated from chemisorbed H2. Compared to 15Ni/10CaO90Al2O3 and 15Ni/20CaO80Al2O3, the sample 15Ni/30CaO70Al2O3 only owning the low-temperature peak showed a great difference.
Temperature-programmed desorption of CO2 (CO2-TPD) was performed to determine the basicity of the tested catalysts, and the results were shown in Figure 6. For these catalysts, the peak at about 120°C was assigned to the weak basicity related to CO2 weakly adsorbed on the support surface. For the CaO modified catalysts 15Ni/10CaO90Al2O3 and 15Ni/20CaO80Al2O3, there were two low-temperature desorption peaks centered at 120°C and 350°C, respectively. Furthermore, there was a high-temperature desorption peak of all the samples corresponding to the formation of or , leading to the strong basicity, and the catalytic performance was correlated with the strength of basicity of the catalyst.
3.5. Catalytic Performance
The effect of the reaction temperature on the CO2 conversion and CH4 selectivity, as well as CH4 yield, was studied and the result was shown in Figure 7. The unmodified catalyst 15Ni/100Al2O3 was almost not active at 240°C; with the reaction temperature increasing up to 360°C, its catalytic activity was slightly improved; after that, its CO2 conversion was increased from 10.3% at 360°C to 56.6% at 450°C; and with the reaction temperature further increasing, the CO2 conversion was declined to 52.1% at 510°C. However, with modification, the catalysts became more active at low temperatures (<450°C). It was seen that the addition of CaO content had an appropriate amount in the Ni/Al2O3 catalyst for the CO2 methanation performance. For the modified catalysts, the activities possessed a maximum at the CaO content of 20 wt.% and then decreased with increasing CaO content to 30 wt.%. Namely, the 15Ni/20CaO80Al2O3 catalyst presented the best catalytic performance over the investigated temperature range, and the CO2 conversion ranged from 6.1% at 240°C to 66.6% at 450°C. This result indicated that the superior dispersion of the NiO species and more available CO2 adsorption sites induced by CaO addition finally promoted the catalytic performance of CO2 methanation for the modified catalysts.
As shown in , the 12Ni5Ce/Al2O3 catalyst displayed better catalytic activity with CH4 yield of 62.4% than the catalyst 12Ni/Al2O3 (47.0% CH4 yield) at 350°C. As shown in , Ni15La/Al2O3 displayed the CH4 yield of 87.5%, and the pristine catalyst Ni/Al2O3 possessed 80.0% CH4 yield at 320°C. In addition, as shown in , the 12Ni5Ca/CNTs catalyst displayed 85.0% CH4 yield at 350°C, but the unmodified catalyst 12Ni/CNTs revealed CH4 yield of 71.9%. However, in this work, with CaO modification, the sample 15Ni/20CaO80Al2O3 exhibited the CH4 yield of 63.2% at 450°C, while the catalyst 15Ni/100Al2O3 presented the CH4 yield of 53.1%. Hence, compared with the above literatures (displayed in Figure 8), CaO as the promoter, owing to its relative low price and high activity, has more advantages than adding CeO2 and La2O3.
Figure 9 showed the CO2 conversion and CH4 yield versus the reaction time on stream in the CO2 methanation at 450°C, and it was seen that the catalytic performance of 15Ni/20CaO80Al2O3 was better than that of the 15Ni/100Al2O3 catalyst. For the catalyst 15Ni/100Al2O3, it was observed that CO2 conversion decreased from 56.6% at 40 min to 36.7% at 360 min, while the catalyst 15Ni/20CaO80Al2O3 exhibited better stability under the selected operating conditions than the pristine catalyst, decreasing from 66.6% at 40 min to 53.7% at 360 min. From the XRD analysis of the reduced catalyst before and after reaction shown in , it was found that the Ni particles became bigger and the sintering of nickel occurred during the reaction, which could be probably due to the weakened interaction between Ni species and the support; that is, as the CO2 methanation proceeded, the catalyst with bigger Ni particles was deactivated. Hence, with CaO modification, the catalysts could apparently possess higher activity and better stability than the unmodified catalyst.
The CO2 methanation has been studied over Ni/CaO-Al2O3 catalysts prepared by impregnating CaO-Al2O3 composite support with an aqueous solution of nickel nitrate. The presence of CaO was found to be beneficial for improving the catalytic activity and exhibited excellent activity for CO2 methanation in a low-temperature range. The catalyst activity strongly depended on the addition amount of CaO for the CaO-Al2O3 support. A suitable CaO content could cause a significantly effect on the interaction between Ni and Al2O3 support, leading to an excellent catalytic performance. The results showed that, among the studied catalysts, the catalyst 15Ni/20CaO80Al2O3 showed optimal catalytic performance (highest CO2 conversion and CH4 yield) under the tested reaction conditions, which was mainly attributed to the fact that the highly dispersed CaO inhibited the incorporation of nickel species into the lattice of Al2O3.
The authors declare that they have no competing interests.
This work was supported by the National 973 Program of Ministry of Sciences and Technologies of China (2011CB201202).
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