A Study of CO2 Methanation over Ni-Based Catalysts Supported by CNTs with Various Textural Characteristics

This work studied the influence of textural characteristics of CNTs on catalytic performance of Ni/CNTs for CO 2 methanation. The CNTs supports were prepared by chemical vapor deposition method using Ni/MgO catalysts, and acetonitrile and ethanol were used as carbon sources, respectively. The Ni/CNTs catalysts were prepared via impregnation method and characterized by X-ray diffraction (XRD), N 2 adsorption/desorption, and temperature-programmed reduction (H 2 -TPR) techniques. The results indicated that the textural characteristics of CNTs supports significantly impacted on the catalytic performance of Ni/CNTs. The catalyst Ni/CNTs-E (CNTs using ethanol as carbon source) had good reducibility, high specific surface area, and moderate defects, resulting in higher CO 2 conversion and CH 4 yield, followed by Ni/CNTs-C (commercial CNTs) and Ni/CNTs-A (CNTs using acetonitrile as carbon source). Based on Arrhenius formula, activation energies of the catalysts were calculated and were found decreased for Ni/CNTs-A and Ni/CNTs-E.


Introduction
CO 2 emissions increased dramatically due to human activities, including power generation, transportation, deforestation, and fossil fuels [1][2][3].The CO 2 recycling has become an important topic in recent years, and a lot of researches have been conducted [4][5][6].As CO 2 is one of the main contributors to greenhouse effect and hence to climate change, there is a growing interest in its use as a feedstock in chemical processes [3,7], wherein CO 2 capture and storage (CCS) and CO 2 chemical conversion [8] have attracted wide interests.It is important to convert CO 2 to fuels or raw materials which must be also easily transportable.CH 4 is suitable for this because it benefits from the existing infrastructure for transport and storage of natural gas [9][10][11].
Carbon nanotubes (CNTs), as a novel nano-carbon catalyst support, have been widely used due to their interesting mechanical and electronic properties and stability [21][22][23].The enhanced activity of CNTs systems has been attributed to an electronic interaction between the metal and the support, as well as the overflow effect of hydrogen [24,25].The effect of CNTs with various textural characteristics and defects has not been studied in detail for CO 2 methanation.It has been shown that in the case of catalysts supported by CNTs with high specific surface area, highly dispersed metal of small particles was easily formed and more active than large metal ensembles.For catalysts supported by CNTs with more defects, it was advantageous to promote the metal loading and the dispersion of active components.

International Journal of Chemical Engineering
The present work aimed to clarify some aspects of the activity of nickel based catalysts supported by CNTs with various textural characteristics toward CO 2 methanation.The objective was to study the effect of textural characteristics on the catalysts and their catalytic activity for CO 2 methanation at different temperatures.The synthesized catalysts were characterized by X-ray diffraction (XRD), N 2 adsorption/desorption, and temperature-programmed reduction (H 2 -TPR) techniques, respectively.

Experimental
2.1.Catalysts Preparation.The CNTs supports were prepared by chemical vapor deposition (CVD) method using Ni/MgO catalysts, and acetonitrile and ethanol were used as carbon sources, respectively.The Ni/MgO catalyst was prepared by citrate complex method.Ni(NO 3 ) 3 ⋅6H 2 O and Mg(NO 3 ) 2 ⋅6H 2 O were mixed in a citric acid aqueous solution, stirred and gelated at 70 ∘ C.After drying at 120 ∘ C overnight, the dried gel was calcined at 500 ∘ C for 3 h in air to get the Ni/MgO catalyst.The loading of Ni on the catalyst was kept at 50 wt.%.To grow CNTs, 100 mg of catalyst powder was dispersed on boat and was then placed in a horizontal quartz reactor.Firstly, the catalyst powder was reduced in situ in H 2 at 550 ∘ C for 1 h.Subsequently, the sample was heated to 650 ∘ C in flowing N 2 and then bubbled the acetonitrile and ethanol for 1 h.After 1 h of reaction, the obtained CNTs were cooled to room temperature in a N 2 atmosphere.The CNTs were further purified in 100 mL of concentrated HCl at room temperature for 12 h.After purification, the CNTs were filtered and washed with deionized water until the pH of the filtrate was around 7.0.Finally, the product was dried at 100 ∘ C overnight and labeled as "CNTs-A" and "CNTs-E," where A and E represented the as-prepared CNTs using acetonitrile and ethanol as carbon source, respectively.For comparison, the commercial CNTs purchased from the Chengdu Organic Chemical Limited Company in China were applied and named as "CNTs-C." The catalysts Ni/CNTs-A, Ni/CNTs-E, and Ni/CNTs-C were prepared via impregnation method.An aqueous solution of Ni(NO 3 ) 2 was prepared by dissolving 1.19 g Ni(NO 3 ) 2 ⋅6H 2 O into 4.0 mL H 2 O. Next, 1.76 g of CNTs sample was impregnated with the Ni(NO 3 ) 2 solution and stirred at room temperature for 2 h and then evaporated to dryness at an 80 ∘ C water bath.The impregnated Ni/CNTs (Ni 12 wt.%)were dried at 110 ∘ C for 12 h and calcined in N 2 at 450 ∘ C for 3 h.The catalyst Ni/AC (using activated carbon as the support) was prepared with aqueous solution of Ni(NO 3 ) 2 by the same method.

Catalytic Activity Evaluation.
The catalytic activity measurement was conducted under atmospheric pressure in a fixed-bed reactor with an internal diameter of 6 mm. 100 mg of catalyst was loaded into the reactor and reduced with H 2 at 450 ∘ C for 1 h prior to catalytic evaluation.The CO 2 methanation reaction was performed at various reaction temperatures using a mixture of H 2 and CO 2 at molar ratio of 4. Effluent gases from the reactor were analyzed online by a SC-200 model gas chromatograph with a TDX-01 column and a thermal conductivity detector (TCD).The length and temperature of column were 2.0 m and 80 ∘ C, while the detector temperature was 60 ∘ C, and the retention times of H 2 , CO, CH 4 , and CO 2 were 0.9 min, 2.5 min, 6.0 min, and 10.5 min, respectively.
2.3.Characterization.N 2 adsorption/desorption isotherms were measured at 77 K with a Quantachrome Nova 1000e apparatus using an automated gas sorption system.Prior to the measurement, the sample was degassed at 120 ∘ C for 3 h.The specific surface areas ( BET ) of the catalysts were determined using the BET equation, and the total pore volume (  ) was calculated from the volume of nitrogen held at / 0 = 0.98∼0.99.Pore size distributions (PSDs) were estimated using the DFT (Density Functional Theory) equation.
The weight change of the sample was determined with a thermogravimetric analyzer (TGA Q500).In this protocol, 10 mg sample in the TGA was kept at room temperature for about 30 min, and then it was heated up to 900 ∘ C with a heating rate of 10 ∘ C/min in the air.
Raman spectroscopy measurements of the samples were obtained with a Raman microspectrometer (LabRAM HR) that used an argon laser as the excitation ( = 532 nm).The spectra were recorded in the wave number range of 400-4000 cm −1 with typical exposure times of 2 s.During the analysis, at least four tests were performed on different sections of each individual sample.
Phase analysis was performed using power X-ray diffraction (XRD) technique.The X-ray diffraction patterns were carried on X-ray diffraction apparatus (D/MAX-2500, Rigaku Company) with Cu K (50 KV, 100 mA) radiation.
Temperature-programmed reduction (H 2 -TPR) experiments were carried out in a fixed-bed reactor.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).

Results and Discussion
3.1.Characterization of CNTs. Figure 1(a) showed the N 2 adsorption/desorption isotherms obtained on CNTs samples.These isotherms were type III, that is, the obviously increasing uptakes at the high relative pressure values.It was obvious that the isotherms of samples dramatically increased on reaching a relative pressure of unity.The isotherms exhibited remarkable hysteresis loops at higher relative pressure values (/ 0 > 0.4).Moreover, the adsorption volume of sample CNTs-E was the highest among the three samples, while the adsorption volume of CNTs-A was the lowest.The pore size distribution (PSD) computed with the DFT method was displayed in Figure 1(b).In order to make the graph full scale, the maximum value on the -axis was 35 nm.The CNTs containing larger pores were produced.The pore diameter range presented in Figure 1 Table 1 presented the textural parameters for the samples.The variation of carbon sources led to differences in the  BET , total pore volume, and average pore diameter (  ).Among the samples, the CNTs-E displayed the highest  BET and lowest   , indicating that the porosity of CNTs-E was developed.Compared with others, the CNTs-A sample had the lowest  BET , being 65.98% and 46.48% of the CNTs-C and CNTs-E, respectively.
Thermogravimetric analysis (TGA) has been widely used to analyze types of carbon materials.Typically, those losses were attributed to the removal of amorphous carbon (300∼ 400 ∘ C), CNTs (400∼700 ∘ C), and massive graphite carbon (> 700 ∘ C). Figure 2(a) showed the weight loss of the purified products during heating in air.Compared with CNTs-A and CNTs-E, the CNTs-C possessed most of the weight loss (97.3%) at 400∼700 ∘ C, indicating high purity of the CNTs.The weight loss was 90.0%, 91.8%, and 97.3% for CNTs-A, CNTs-E, and CNTs-C, respectively.It was observed that the temperature of initial weight loss was 440 ∘ C for CNTs-A, 478 ∘ C for CNTs-E, and 618 ∘ C for CNTs-C, suggesting that the grown CNTs-C were more highly graphitic with higher initial weight loss temperature.
Raman spectra of the three samples, shown in Figure 2(b), presented a prominent -band (defects) at 1350 cm −1 and a less-intense -band at 1580 cm −1 , attributed to a perfect graphite structure [10,26,27].It was clear that the band intensity was higher than the -band intensity and CNTs-C had a better developed -band, as well as the band.The increase of the -band relative intensity indicated an improvement of the crystallite orientation, while the increasing -band relative intensity suggested more defects and an increasing amount of disordered structure of the sample.The pertinent Raman parameter,   /  , which has already been mentioned to account for the degree of structural order, as displayed in Figure 2(b), confirmed the structure development of the samples (  /  values of CNTs-A, CNTs-E, and CNTs-C were 1.09, 1.27, and 1.62, resp.).

Characterization of Catalysts.
The N 2 adsorption/desorption isotherms and pore size distributions were presented in Figure 3. Table 2 summarized the textural parameters for the catalysts.As shown in Figure 3 other catalysts, the specific surface area of Ni/AC catalyst was decreased significantly due to the existence of Ni.This would result in a low catalytic activity for the CO 2 methanation using Ni/AC as the catalyst.
The XRD analysis was carried out to investigate the crystal structure of the catalyst.As shown in Figure 4, it revealed that the diffraction peak at 26.2 ∘ in the catalysts can be well indexed as the (002) reflection of graphite, and the graphite peak of CNTs-supported catalysts was much stronger and narrower than that of the AC-supported catalyst, indicating the crystallinity of CNTs-supported catalysts was high.Metallic Ni species existed in all the reduced samples, with diffraction peaks 44.5 ∘ , 51.8 ∘ , and 76.5 ∘ .According to the crystal plane Ni (111), the particle size of Ni in the  catalysts was calculated by Scherrer equation.It was found that the particle size of metallic Ni was 8.9, 9.8, 10.3, and 11.3 nm for Ni/AC, Ni/CNTs-A, Ni/CNTs-E, and Ni/CNTs-C, respectively.Due to the largest specific surface area, ACsupported catalyst was favorable for the decrease of the particle size of Ni.Temperature-programmed reduction of H 2 (H 2 -TPR) is usually employed for estimating reducibility of the catalyst, and the results were displayed in Figure 5. Compared with Ni/AC, the interaction was weaker between CNTs and nickel species for Ni/CNTs series.The low reduction temperatures can be attributed to the overflow effect of hydrogen, which activated hydrogen molecules, thereby promoting the reduction reaction of hydrogen and nickel species at low temperatures.As seen from Figure 5, CNTs-E was to promote the reduction of the nickel species, proved by a significant increase in low temperature reduction.

Catalytic Performance.
The activity of the catalysts was tested by measuring the conversion of CO 2 and the yield of CH 4 as a function of reaction temperature on stream.As shown in Figure 6, except for the catalyst Ni/CNTs-A, the CNTs-supported catalysts displayed better catalytic activity than the AC-supported catalyst, which can be attributed to the high H 2 storage capacity of CNTs, thus promoting the hydrogenation of CO 2 .Compared with Ni/AC, Ni/CNTs-E catalyst had higher catalytic activity, but, with the reaction temperature increasing to 500 ∘ C, Ni/CNTs-E had lower catalytic activity than that of Ni/AC.On the whole, the catalytic activity decreased in the following order: Ni/CNTs-E > Ni/CNTs-C > Ni/AC > Ni/CNTs-A.
It revealed that the catalytic activity was improved not only by the specific surface area of the supports, for the specific surface area of CNTs-E, CNTs-C, CNTs-A, and AC was 254.1 m 2 /g, 179.0 m 2 /g, 118.1 m 2 /g, and 927.5 m 2 /g, respectively, but also by the overflow effect of hydrogen for CNTs supports.It was observed that the defects over the various CNTs samples have shown no obvious effects on the catalytic activity.For the CNTs supports, CNTs-A had lower defects than CNTs-E and CNTs-C.The catalyst Ni/CNTs-A, with lower defects and specific surface area, showed the lowest catalytic activity among the CNTs-supported samples.Herein, for Ni/CNTs-A, the catalytic activity was mainly lowered by the low specific surface area.The CNTs-E and CNTs-C had similar textural properties, but CNTs-C possessed more defects in comparison to CNTs-E.Unexpectedly, the sample Ni/CNTs-C did not exhibit a higher catalytic activity.Hence, compared with the difference in pore structure, the defects played a minor role in catalytic performance of Ni/CNTs for CO 2 methanation.Figure 7(a) displayed the catalytic stability of the catalyst Ni/CNTs-E.With the reaction proceeding, the catalytic activity decreased slightly, which could be due to sintering of active component.Therefore, it is very important to find the catalyst additives which can improve the stability of the catalyst with low price.

Conclusions
The nano Ni-based catalysts supported by CNTs with various textural characteristics were synthetized by impregnation method for CO 2 methanation performance.The prepared CNTs were characterized by N 2 adsorption/desorption, TGA, and Raman spectroscopy, respectively, in order to investigate the textural characteristics of the CNTs.The catalysts were determined by X-ray diffraction (XRD), N 2 adsorption/desorption, and temperature-programmed reduction (H 2 -TPR) techniques.The results showed that the textural characteristics of CNTs supports significantly impacted on the catalytic performance of Ni/CNTs, and the catalyst Ni/CNTs-E with good reducibility, large specific surface area, and moderate defects resulted in higher CO 2 conversion and CH 4 yield.

Figure 1 :
Figure 1: N 2 adsorption/desorption isotherms (77 K) (a) and pore size distributions (b) obtained applying the DFT equation corresponding to the CNTs samples.

Figure 4 :
Figure 4: XRD profiles for the reduced catalysts.

Figure 5 :Figure 6 :
Figure 5: H 2 -TPR profiles of the catalysts prepared with different supports.

Figure 7 :
Figure 7: CO 2 conversion and CH 4 yield versus reaction time at 350 ∘ C (a) and logarithm of CO 2 conversion versus the reciprocal of temperature (b) on the catalysts.
Figure 7(b) showed the relationship between logarithm of CO 2 conversion and reciprocal of temperature.Based on Arrhenius formula, activation energies of the catalysts were calculated from the slope of the fitted lines.It was observed that the activation energy decreased for Ni/CNTs-A and Ni/CNTs-E.The activation energies of the catalysts were 29.23, 36.61,52.54, and 50.77KJ/mol for Ni/CNTs-A, Ni/CNTs-E, Ni/CNTs-C, and Ni/AC, respectively.

Table 1 :
Structural characteristics of the studied samples.

Table 2 :
Structural characteristics of the catalysts.