Promotion Effect of CaO Modification on Mesoporous Al 2 O 3-Supported Ni Catalysts for CO 2 Methanation

The catalysts Ni/Al 2 O 3 and CaO modified Ni/Al 2 O 3 were prepared by impregnation method and applied for methanation of CO 2 . The catalysts were characterized by N 2 adsorption/desorption, temperature-programmed reduction of H 2 (H 2 -TPR), X-ray diffraction (XRD), and temperature-programmed desorption of CO 2 and H 2 (CO 2 -TPD and H 2 -TPD) techniques, respectively. TPR and XRD results indicated that CaO can effectively restrain the growth of NiO nanoparticles, improve the dispersion of NiO, andweaken the interaction betweenNiOandAl 2 O 3 . CO 2 -TPDandH 2 -TPD results suggested thatCaOcan change the environment surrounding of CO 2 andH 2 adsorption and thus the reactants on the Ni atoms can be activatedmore easily.Themodified Ni/Al 2 O 3 showed better catalytic activity than pure Ni/Al 2 O 3 . Ni/CaO-Al 2 O 3 showed high CO 2 conversion especially at low temperatures compared to Ni/Al 2 O 3 , and the selectivity to CH 4 was very close to 1. The high CO 2 conversion over Ni/CaO-Al 2 O 3 was mainly caused by the surface coverage by CO 2 -derived species on CaO-Al 2 O 3 surface.


Introduction
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][2][3][4][5][6][7].In case hydrogen is available or can be produced by renewable energy such as solar energy in order to achieve a lowcarbon society [4], the hydrogenation of captured CO 2 is an interesting option as a CO 2 Capture and Storage (CCS) technology [4].The methane produced can be injected in chemical and petrochemical industries as natural gas use.Thus, the possibility to transform H 2 and CO 2 into CH 4 becomes a real alternative in an environmental and ecological point of view [7][8][9].
The Ni-based catalyst has been extensively investigated under widely varying experimental conditions, and Ni/Al 2 O 3 catalysts showed moderate catalytic activity in methanation of carbon dioxide with hydrogen [8].For Ni-based catalysts, the promoters, such as MgO, CeO 2 , and La 2 O 3 , have played important roles in the activity for CO 2 methanation.The basic properties, special electronic structure, and strong interaction with Ni could enhance the CO 2 adsorption and dissociation and consequently improve the activity of the catalyst for CO 2 methanation.In our previous study [2], 2 International Journal of Chemical Engineering NiCe/CNTs and NiCe/Al 2 O 3 catalysts prepared by impregnation method were found to be highly active and stable for CO 2 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/Al 2 O 3 with CaO in CO 2 adsorption and dissociation.The aim of this work was to investigate the effects of CaO-Al 2 O 3 as the support and the content of CaO on the catalytic performance of CO 2 methanation over Ni/CaO-Al 2 O 3 catalysts and to optimize the reaction temperature.

Characterization of Catalysts
. N 2 adsorption/desorption was carried out using a Quantachrome Nova 1000e apparatus at 77 K. Before measurement, the samples were degassed at 120 ∘ C for 3 h [2,9].
Temperature-programmed reduction of H 2 (H 2 -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% H 2 /95% N 2 with a flow rate of 30 mL/min.The H 2 consumption was analyzed online by a SC-200 gas chromatograph with a thermal conductivity detector (TCD).
Experiments of temperature-programmed desorption of CO 2 (CO 2 -TPD) were conducted employing an automated gas sorption analyser TP-5080 [22].The samples were pretreated at 500 ∘ C (5 ∘ C/min) under N 2 flow (30 mL/min) for 1 h and reduced at the same temperature under H 2 flow (30 mL/min) for 1 h.After that, the materials were cooled and exposed to CO 2 (30 mL/min) for 1 h at 50 ∘ C. CO 2 -TPD measurements were carried out up to 750 ∘ C with the heating rate of 10 ∘ C/min under N 2 flow (30 mL/min).

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 H 2 flow for 1 h and then cooled down to room temperature in N 2 .Next, a mixture of H 2 and CO 2 (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 CO 2 conversion ( CO 2 ) and CH 4 yield ( CH 4 ) were estimated by the following: ) .
(1)  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/10CaO90Al 2 O 3 exhibited the largest specific surface area of 9.59 m 2 /g.This could be mainly attributed to the blocking of part of mesopores of the catalysts and the change in NiO and Al 2 O 3 structures through their interactions with CaO.Furthermore, the pore diameters of the catalysts had changed.

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 a Specific surface area evaluated using the Brunauer-Emmett-Teller (BET) method.b Pore volume calculated from the volume of nitrogen held at / 0 = 0.98∼ 0.99.c BJH desorption average pore diameter.
and 79.4 ∘ corresponding to NiO species were observed.In addition, no clear characteristic diffraction peaks of CaO or CaCO 3 phases can be detected, indicating that CaO was highly dispersed on the surface of Al 2 O 3 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 CaCO 3 , indicating that Ni 2+ ions in the NiO phase were fully reduced to metallic Ni and Ca 2+ ions in the CaO phase were not reduced.

Temperature-Programmed Reduction and Reducibility of the
Catalysts.H 2 -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 H 2 -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 H 2 consumption peak in the range of 350 ∘ C∼600 ∘ C corresponding to the reduction of NiO species.For the catalyst 15Ni/100Al 2 O 3 , 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-Al 2 O 3 supports and the improvement of the catalyst reducibility.This behavior indicated that the incorporation of CaO into Al 2 O 3 supports promoted the reduction of NiO species.However, it was found that, relative to that of 15Ni/20CaO80Al 2 O 3 , the reduction temperature peak of 15Ni/30CaO70Al 2 O 3 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/Al 2 O 3 catalyst to affect its catalytic performance.

Temperature-Programmed Desorption of the Catalysts.
The H 2 -TPD profiles of the catalysts were presented in Figure 5.The sample 15Ni/100Al   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 CO 2 weakly adsorbed on the support surface.For the CaO modified catalysts 15Ni/10CaO90Al 2 O 3 and 15Ni/20CaO80Al 2 O 3 , 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 HCO 3 − or CO 3 2− , leading to the strong basicity, and the catalytic performance was correlated with the strength of basicity of the catalyst.

Catalytic Performance.
The effect of the reaction temperature on the CO 2 conversion and CH 4 selectivity, as well as CH 4 yield, was studied and the result was shown in Figure 7.The unmodified catalyst 15Ni/100Al 2 O 3 was Intensity (a.u.) 100 0 200 300 400 500 600 700 800 almost not active at 240 ∘ C; with the reaction temperature increasing up to 360 ∘ C, its catalytic activity was slightly improved; after that, its CO 2 conversion was increased from 10.3% at 360 ∘ C to 56.6% at 450 ∘ C; and with the reaction temperature further increasing, the CO 2 conversion was declined to 52.1% at 510 ∘ C.However, with modification, the catalysts became more active at low temperatures (<450  owing to its relative low price and high activity, has more advantages than adding CeO 2 and La 2 O 3 .
Figure 9 showed the CO 2 conversion and CH 4 yield versus the reaction time on stream in the CO 2 methanation at 450 ∘ C, and it was seen that the catalytic performance of 15Ni/20CaO80Al 2 O 3 was better than that of the 15Ni/100Al 2 O 3 catalyst.For the catalyst 15Ni/100Al 2 O 3 , it was observed that CO 2 conversion decreased from 56.6% at 40 min to 36.7% at 360 min, while the catalyst 15Ni/20CaO80Al 2 O 3 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 [2], 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 CO 2 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.

Conclusions
The CO 2 methanation has been studied over Ni/CaO-Al 2 O 3 catalysts prepared by impregnating CaO-Al 2 O 3 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 CO 2 methanation in a low-temperature range.The catalyst activity strongly depended on the addition amount of CaO for the CaO-Al 2 O 3 support.A suitable CaO content could cause a significantly effect on the interaction between Ni and Al 2 O 3 support, leading to an excellent catalytic performance.The results showed that, among the studied catalysts, the catalyst 15Ni/20CaO80Al 2 O 3 showed optimal catalytic performance (highest CO 2 conversion and CH 4 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 Al 2 O 3 .

3. 1 .
Textural Properties of the Catalysts.The textural properties of the catalysts were characterized by N 2 adsorption/desorption analysis, and the N 2 adsorption/desorption isotherms and pore size distributions obtained by the BJH equation were shown in Figure1.The pristine Ni/Al 2 O 3 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 Al 2 O 3 , which formed new porosity and extra surface area.As seen from Figure1(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 selforganization of nanoparticles inside the large pores of Al 2 O 3 .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-Al 2 O 3 catalysts with different CaO contents calcined at 500 ∘ C were presented in Table

Figure 4 :
Figure 4: H 2 -TPR profiles of the catalysts with different CaO contents calcined at 500 ∘ C for 3 h.

Figure 6 :Figure 7 :
Figure 6: CO 2 -TPD profiles of the catalysts with different CaO contents.

Figure 8 :Figure 9 :
Figure 8: Comparison of the results in this work with the previous literature.

Table 1 :
Textural properties of Ni/CaO-Al 2 O 3 catalysts with different CaO contents.
2 O 3 exhibited one peak located at 50∼400 ∘ C. With CaO modification, the catalysts 15Ni/10CaO90Al 2 O 3 and 15Ni/20CaO80Al 2 O 3 showed two peaks, of which the low-temperature peak corresponded to the physical adsorption of H 2 weakly adsorbed on the metal surface, and the high-temperature peak located at 250∼550 ∘ C could be originated from chemisorbed H 2 .Compared to 15Ni/10CaO90Al 2 O 3 and 15Ni/20CaO80Al 2 O 3 , the sample 15Ni/30CaO70Al 2 O 3 only owning the low-temperature peak showed a great difference.Temperature-programmed desorption of CO 2 (CO 2 -TPD) was performed to determine the basicity of the tested [34].It was seen that the addition of CaO content had an appropriate amount in the Ni/Al 2 O 3 catalyst for the CO 2 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/20CaO80Al 2 O 3 catalyst presented the best catalytic performance over the investigated temperature range, and the CO 2 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 CO 2 adsorption sites induced by CaO addition finally promoted the catalytic performance of CO 2 methanation for the modified catalysts.As shown in[2], the 12Ni5Ce/Al 2 O 3 catalyst displayed better catalytic activity with CH 4 yield of 62.4% than the catalyst 12Ni/Al 2 O 3 (47.0%CH 4 yield) at 350 ∘ C. As shown in[33], Ni15La/Al 2 O 3 displayed the CH 4 yield of 87.5%, and the pristine catalyst Ni/Al 2 O 3 possessed 80.0% CH 4 yield at 320 ∘ C. In addition, as shown in[34], the 12Ni5Ca/CNTs catalyst displayed 85.0% CH 4 yield at 350 ∘ C, but the unmodified catalyst 12Ni/CNTs revealed CH 4 yield of 71.9%.However, in this work, with CaO modification, the sample 15Ni/20CaO80Al 2 O 3 exhibited the CH 4 yield of 63.2% at 450 ∘ C, while the catalyst 15Ni/100Al 2 O 3 presented the CH 4 yield of 53.1%.Hence, compared with the above literatures (displayed in Figure8), CaO as the promoter,