Perfomances of different additives on NiO/[gamma]-Al^sub 2^O3 catalyst in CO methanation

Abstract

In this work, NiO/γ-Al₂O3 catalyst with different additives prepared by excessive dipping method, was investigated in CO methanation in an attempt to improve the reaction activity and enhance the anti-coking property. The influences of additives, such as Zr, Co, Ce, Zn and La, on catalysts in performance of CO methanation were studied in a fixed-bed reactor. The catalysts were characterized by XRD, H2-TPR, H2-TPD and CO-TPD. The results showed that the addition of the additives could promote the dispersion of nickle species on support and decrease the crystallite size of Nickel species. The reduction temperatures of catalysts were all reduced except the one with Mg. The catalytic results showed that additives improved the reaction activity of CO methanation. Especially, the addition of La gave the best catalytic performance of 100 % CO conversion and 99.61 % CH4 selectivity, respectively. The space-time yield of CH4 was achieved as high as 2134.5 g kg ^-1 h^-1.

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Perfomances of different additives on NiO/c-Al2O3 catalyst in CO methanation

Zhong He1 Xiaolong Wang1 Rong Liu1 Shiwang Gao1 Tiancun Xiao1

Received: 27 January 2016 / Accepted: 24 May 2016 / Published online: 14 June 2016 The Author(s) 2016. This article is published with open access at Springerlink.com

Abstract In this work, NiO/c-Al2O3 catalyst with different additives prepared by excessive dipping method, was investigated in CO methanation in an attempt to improve the reaction activity and enhance the anti-coking property. The inuences of additives, such as Zr, Co, Ce, Zn and La, on catalysts in performance of CO methanation were studied in a xed-bed reactor. The catalysts were characterized by XRD, H2-TPR, H2-TPD and CO-TPD. The results showed that the addition of the additives could promote the dispersion of nickle species on support and decrease the crystallite size of Nickel species. The reduction temperatures of catalysts were all reduced except the one with Mg. The catalytic results showed that additives improved the reaction activity of CO methanation. Especially, the addition of La gave the best catalytic performance of 100 % CO conversion and 99.61 % CH4 selectivity, respectively. The spacetime yield of CH4 was achieved as high as 2134.5 g kg-1 h-1.

Keywords CO methanation Nickel-based catalysts

Additives Fixed-bed reactor

Introduction

Methanation is one of the core technologies of coal to synthetic natural gas (SNG) process [1]. This technology requires catalyst with high activity and high stability due to

the high exothermic of the methanation reaction [24]. Currently, commercial methanation technology was mastered by Lurgi (LURGI), Johnson Matthey (DAVY) and Topsoe (TOPSE) [5]. All of these technologies are performed in adiabatic xed-bed reactor. Ni-based catalyst is the most widely investigated system due to its high efciency and low cost. NiO/c-Al2O3 catalyst has received renewed interests for the methanation of carbon monoxide.

It is known that the main reasons for Ni-based catalyst deactivation are the carbon deposition and sintering in methanation reaction process [68]. The addition of rare earth oxide can change the interaction between NiO and the support, as well as the pH of the catalyst surface to improve the Ni dispersion. They are benecial for the catalyst activity, stability and resistance to carbon deposition [9]. The suitable content of MgO could increase the monolayer capacity of NiO on the surface of the supports. And the reduction of the nickel grain was become smaller due to the introduction of Mg additives. But MgO would enhance the chemical interaction between the NiO and the supports, resulting in a new formation of NiOAl2O3MgO catalyst, which was not easy to be reduced [10]. Jun et al. [11]

developed a NiCo bimetallic aerogel catalyst prepared by a solgel method for the production of SNG via CO methanation. The catalyst exhibited higher conversion of CO, higher yield and selectivity of CH4. Moreover, the

NiCo catalyst showed no deactivation in 100 h reaction. This good stability was attributed to the superior property. However, the aerogel NiCo catalyst showed poor uidization through the nanoparticle agglomerates. Guido et al. developed nickel doped with cobalt catalyst for hydrogen production by ethanol steam reforming. The structure characteristic of NiCoZnAl catalyst was studied [12]. The calcination of the layered precursors performed a high surface area of mixed oxides. It was a

& Tiancun Xiao [email protected]

1 Huaneng Clean Energy Research Institute, China Huaneng Group, Future Science and Technology City, Changping District, Beijing 102209, China

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well distributed mixture of a rock salt phase (NiO), a wurtzite phase (ZnO) and a spinel phase. But application of catalyst with Zn and Co in the process of methane had not been reported. In the last decade, many efforts have been made to improve the catalytic performance of the Ni/Al2O3

catalyst. The addition of small amount of second metal, such as Mg, Co, Zr, Ce, La and Zn has been attempted to enhance the stability and catalytic activity of the methanation of carbon monoxide [13].

Although the Ni/c-Al2O3 catalysts have been reported in some methanation reaction of H2-rich syngas, there are few works reported on methanation catalysts promoted by different additives. Especially, the optimized amount of structural promoter, electronic promoter and lattice defects promoter in methanation reaction have not been reported. In this work, the effect of different additives metal oxides on the interactions of NiO and support, and carbon deposition resistance over such catalysts has been studied. The selection of different additives for Ni/c-Al2O3 catalysts with low Ni contents for syngas methanation was conducted. A series of NixMyA1 (M = Mg, Co, Zr, Ce, La, Zn) catalysts were prepared using excessive dipping method and characterized using XRD, H2-TPR, H2-TPD and CO-TPD. The inuences of different additives on the performances of NixMyAl catalysts were investigated.

Experiment

Catalyst preparation

The catalysts were prepared with excessive dipping method. First, Ni(NO3)2 6H2O were mixed with

Mg(NO3)2, Co(NO3)2 6H2O, La(NO3)3 6H2O, Ce(NO3)3-

6H2O, Zr(NO3)4 5H2O and Zn(NO3)2 6H2O, respectively.

Then, the Al2O3 (surface area 246 m2/g, Shandong Aluminum Co. Ltd) were impregnated in the different solution for 24 h at room temperature, dried for 12 h at 120 C then calcined for 4 h at 450 C to give the oxide catalyst. The catalysts were noted as NixMyAl (M = Mg, Co, Zr, Ce, La, Zn) catalyst. The catalyst composes of 12 wt% of NiO and 4 wt% MzOw (M = Mg, Co, Zr, Ce, La, Zn).

Catalyst evaluation

Initially, the catalyst was loaded into a pressurized xedbed reactor and reduced with H2/N2 mixture gas at the ratio of 1:4 at 550 C for 4 h. Then, the reactor was cooled to 200 C. The pressure of the system was slowly increased with nitrogen gas. The feed gas was switched into the reactor gradually to the desired pressure. The activity of methanation catalyst was tested at 400 C with the reaction

pressure of 1 MPa. The ow ratio of the H2/CO was 3:1 at the space velocity of 12,000 h-1.

Catalyst characterization

The crystalline structure of the catalyst was determined with X-ray diffraction (XRD) recorded on a powder diffractometer (Bruker Advance D8, 40 kV, 40 mA) using a Cu Ka radiation source in the range of 1085. H2-TPR was carried out in a quartz-tube xed-bed micro reactor system. The sample (20 mg) was pretreated with Ar at 300 C for 1 h and then cooling to 50 C. Afterwards, the owing gas was switched to 10 vol% H2/Ar and heated to 800 C at a ramping rate of 10 C/min. H2-TPD and CO

TPD experiments were performed with similar procedure. The sample was pretreated in Ar ow (50 ml/min) at 300 C for 1 h, then cooling to 50 C and kept at this temperature for 40 min in 10 vol% H2/Ar or 10 vol% CO/

Ar. Afterward, the sample was swept with He for 60 min. Finally, the desorption step was performed from 50 to 800 C at a heating rate of 10 C/min with He ow of 50 ml/min. The desorbed products were all monitored using TCD.

Results and discussion

The inuences of additives on the catalytic activity of the nickel-based catalyst for CO methanation

The catalytic results of the catalyst for CO methanation are shown in Table 1. The loading amount of additives and NiO were 4 and 12 % in weight, respectively. The results showed that the addition of La, Zn, Ce, Co and Zr into Ni/ c-Al2O3 catalyst improved the catalytic performance of CO methanation. The most effective catalyst was the one with

La promoter. The CO conversion was 100 %, and CH4 selectivity was 99.6 % at CH4 spacetime yield of 2134.5 g/kg h. However, the Mg promoter decreased the catalytic activity. In summary, it was showed that they would change the chemical environment and the state, which would ultimately affect the catalytic activity for CO methanation.

X-ray diffraction (XRD) characterization

The XRD characterization of the catalyst was shown in Fig. 1. The diffraction peaks at the 2h peaks of 37.2, 43.2

were the characteristic peaks of NiO. Compared with Ni12Al catalyst, the introduction of Zr, Co, Ce, Zn and La additives decreases the intensity of diffraction peak of NiO.

The diffraction peak tends to become broader due to high

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Appl Petrochem Res (2016) 6:235241 237

Table 1 The inuences of additives on the catalytic activity of the nickel-based catalyst for CO methanation

Con-CO (%) Sel-CH4 (%)/standard deviations STY-CH4 (g kg-1 h-1)/standard deviations

Ni12La4Al 100 99.61/0.28 2134.5/13.19 Ni12Zn4Al 100 98.64/0.29 2113.7/13.28 Ni12Ce4Al 100 96.28/0.26 2063.1/14.04 Ni12Co4Al 100 94.83/0.24 2032.1/13.22 Ni12Zr4Al 100 94.72/0.27 2029.7/12.88 Ni12Mg4Al 100 92.74/0.28 1987.3/12.76 Ni12Al 100 95.10/0.29 2037.9/13.24

Reaction conditions: CO:H2 = 1:3, 12,000 h-1, 1 MPa, 400 C

10 20 30 40 50 60 70 80

R

Fig. 1 The XRD patterns of nickel-based catalysts with different additives star MgNiO2 square NiO inverted triangle CeO2 triangle c-

Al2O3

dispersion of NiO. The dispersion of NiO decreased in the order La [ Co [ Ce [[ Zr [ Zn. It was reported that the rare earth metal of La can effectively improves the dispersion of NiO on the surface of the support and the particle size of NiO crystal descended after calcinations [14, 15]. The peaks of Ni12Mg4Al sample at the 2h peaks of 37.1, 43.1, 62.6 are due to MgNiO2. It showed that the MgNiO2 formation was difcult to be reduced due to the strong interaction between MgO and NiO. The peaks of Ni12Ce4Al at the 2h of 28.5, 33, 47.4 and 56.3 belong to CeO2, The Ce3? was oxidized to Ce4? after Ce was introduced into the c-Al2O3, The lling of Ce4? in Al2O3 crystal lattice vacancies made the original lattice distorted because of different valence of Ce4? and Al3?. Therefore, Ce4? mainly located on the surface of the crystal phase with a lower energy state or precipitate to form crystal

[16, 17]. The characteristic diffraction peaks of Zr, Co, Zn and La oxides were not observed in XRD spectra. This indicated that Zr, Co, La and Zn were well dispersed on the surface of the support [18]. It suggests that the introduction of additives could improve the dispersion of NiO on the surface of c-Al2O3.

The XRD characterization of the reduced catalyst for CO methanation as shown in Fig. 2. The 2h peaks of 44.4,51.8, 76.3 were the characteristic diffraction peaks of nickel. The characteristic diffraction peaks of nickel were decreased compared with the catalyst Ni12Al, when Zr, Co,

Ce, Zn, La, Mg additives is introduced. Considering the particle size data of nickel in Table 2, the catalyst of nickel particle size had been decreased. The Ni12Ce4Al and

Ni12La4Al catalysts showed the weakest peak intensity of nickel. This indicated that the addition of La and Ce additives in the process of catalyst preparation could effectively reduce the particle size of nickel. The 2h peaks

10 20 30 40 50 60 70 80

R

Fig. 2 The XRD patterns of different additives of reduced nickel-based catalysts. star MgNiO2, diamond Ni triangle c-Al2O3

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Table 2 Analytic results of the reduced Ni crystallites

Catalyst 2h (o) FWHM (o) Average diameter (nm)

Ni12Al 44.41 0.72 15.08 Ni12Mg4Al 44.47 1.06 10.34 Ni12Zr4Al 44.39 1.02 10.76 Ni12Co4Al 44.37 0.88 12.48 Ni12Ce4Al 44.38 0.15 7.22 Ni12Zn4Al 44.41 1.13 9.70 Ni12La4Al 44.54 1.23 8.91

200 300 400 500 600 700 800

T/oC

Fig. 3 H2-TPR proles of nickel-based catalysts

of 37.1, 43.1, 62.6 belong to MgNiO2 formation of characteristic diffraction peaks. It indicated that the MgNiO2 formation could not be reduced completely in the reduction condition of 550 C [19]. Therefore the quantity of nickel in the surface of active center could be reduced and the activity of CO methanation was not enhanced. In addition, all the catalysts do not have characteristic diffraction peaks of oxide of Zr, Co, Ce, Zn, La, maybe due to the formation of amorphous phases. Another reason for this is probably that Zr, Co, Ce, Zn, La metal or oxide grain is too small to be detected by XRD. In summary, the introduction of additives changed the chemical environment of the catalyst surface as well as the particle size of the nickel [20].

H2-TPR characterization

The H2-TPR characterization of the catalyst for CO methanation was shown in Fig. 3. The Gauss tting analysis of H2-TPR spectrum was given in Table 3. It was observed that H2 consumption started at about 300 C for all the Ni-based catalysts. The reducible NiO species are usually classied to three types, including low temperature peak a (200350 C), middle temperature peak b (300600 C), high temperature peak c1 (500700 C) [21]. Each peak corresponded to different states of NiO species. The peaks located in the temperature region (200350 C) were assigned to a-type NiO species, which was attributed to free nickel oxides possessing a weak interaction with alumina support. The middle temperature peaks (300600 C) represent b-type NiO species, which has a stronger interaction with alumina than the a-type NiO. It could be further classied into b1-type (300500 C) and b2-type (400600 C). The former peak located at 300500 C was attributed to the more reducible NiO in Ni-rich mixed oxide phase. While the latter at 400600 C was attributed to the less reducible one in Al-rich phase. The high temperature peaks (500700 C) were assigned to c1-type NiO species, which was stable nickel

aluminum phase with the spinel structure. The new high temperature reduction peak c2-type at 680 C was assigned to the reduction peak of MgNiO2, when the Mg promoter was employed. This showed that addition of Mg promoter increased the reduction temperature of the catalyst. The reduction peak of NiO at the low temperature region shifted to lower temperature direction, and the peak area was increased. The high temperature reduction peak also shifted to low temperature direction, and the area of reduction peak was reduced when the promoter of Ce, Zr, La was introduced. It was suggested that the introduction of Ce, Zr, La make the NiO easier to be reduced, while inhibiting the strong interaction between NiO and c-Al2O3

[22]. The low temperature reduction peaks of Ni-based catalysts with Ce promoter and CeO2 were overlapped.

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Appl Petrochem Res (2016) 6:235241 239

Table 3 Gaussian tting analysis of H2-TPR patterns

Catalyst Reduction temperature/C Relative content/%

a b1 b2 c1 c2 a b1 b2 c1 c2

Ni12Al 288.76 406.74 500.42 590.68 / 3.51 27.41 47.09 21.99 / Ni12Mg4Al / 408.49 488.53 567.86 684.22 / 5.97 27.61 40.44 25.98 Ni12Zr4Al / 424.08 495.31 589.96 / / 30.47 51.67 17.85 / Ni12Co4Al 266.91 361.21 456.79 545.86 / 11.88 28.93 31.43 27.75 / Ni12Ce4Al 252.10 399.80 483.78 569.24 / 3.95 37.71 31.96 26.38 / Ni12Zn4Al 298.77 432.84 494.28 571.37 / 3.88 20.66 38.03 37.43 / Ni12La4Al / 404.48 496.43 601.18 / / 28.33 55.59 16.07 /

Combined with the catalyst hydrogen consumption data in Table 4, it could be noticed that the catalyst consumption of La promoter was larger compared with Ce, Zr additives. The number of active center nickel was also the largest in all the reduced catalyst. The Co promoter was introduced to greatly decrease the reduction temperature of the catalyst. The reduced Co was also an important catalytic active center in the process of methanation [23]. The reduction peak shifted to the lower temperature, and the peak area decreased after the introduction of the Zn promoter. This shows that Zn has an inhibitory effect on the Ni-based catalyst [24].

CO-TPD characterization

The CO-TPD characterization results of the catalysts are showed in Fig. 4. All of the catalysts had a low temperature desorption peak near 100 C, which is attributed to the desorption of CO absorbed on nickel on the surface of the catalyst. The low temperature peak of CO desorption of the catalyst with promoter shifted to a even lower temperature compared to the results of Ni12Al catalyst. This showed that nickel surface was reduced when the additives were introduced. At the same time, there were three other chemical desorption peaks named a, b and c, ranging from 400 to 600 C, corresponding to three different states of the

Ni species [25]. CO desorption temperature of the catalyst is related to desorption activation energy [26]. The adsorbed component with lower activation energy was easily desorbed from the catalyst, which leads to the decrease of desorption temperature. The adsorption component with high desorption activation energy was more conducive to the adsorption on the catalyst, which leads to the increase of desorption temperature [27]. The c-desorption peak disappeared when Mg promoter was added. The a-desorption peak was attributed to NiO of loading on the surface of the support. The b-desorption peak was attributed to the MgNiO2 formation. CO desorption peak area was relatively small due to a large number of MgNiO2 formation, which is difcult to be reduced. The CO desorption peak area of catalyst Ni12Co4Al was slightly increased. This indicated that the introduction of Co promoter improved

Table 4 H2-consumption analysis of nickel-based catalysts

Catalyst H2-consumption/mmol g-1

Ni12Al 1.64

Ni12Mg4Al 1.36

Ni12Zr4Al 1.63

Ni12Co4Al 2.20

Ni12Ce4Al 1.66

Ni12Zn4Al 1.84

Ni12La4Al 2.03

0 100 200 300 400 500 600 700 800

T/ oC

Fig. 4 CO-TPD proles of nickel-based catalysts

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the dispersion of Ni species on the catalyst surface and enhanced the adsorption of CO. The c-desorption peak disappeared in Ni12Zr4Al, but a-desorption peak area and b-desorption peak area were greatly increased. Thus, the introduction of Zr promoter improved the dispersion of Ni to a great extent and enhanced the adsorption of CO. The b-desorption peak and the c-desorption peak of CO desorption were disappeared on Ni12Ce4Al and Ni12La4Al

catalyst. The a-desorption peak area was increased. Therefore, the introduction of rare earth elements Ce, La can improve the dispersion of nickel on the surface, as well as reduce the CO desorption peak temperature and the desorption activation energy, which was more benecial for the adsorption of CO form the activated site.

H2-TPD characterization

The H2-TPD proles of the supported Ni-based catalysts usually contain two or more desorption peaks. The number of desorption peaks reveals the types of catalyst surface active sites. The H2-TPD results of the catalyst for CO methanation are given in Fig. 5. All the H2-TPD proles of the catalysts showed a low temperature desorption peak (nearby 150 C) and a high temperature desorption peak (nearby 400 C). The low temperature peak was attributed to desorption of hydrogen adsorbed on the metal surface, and the high temperature peak was attributed to desorption of the hydrogen spillover hydrogen adsorbed on the surface of the oxide [28].

The Ni12Al catalyst H2-TPD proles have desorption peak (90 C) and a satellite peak (150 C) at low temperature. This can be attributed to the weak and strong adsorption of hydrogen on the nickel surface. The desorption peak at high temperature (400 C) was attributed to the desorption peak of spillover hydrogen [29]. The low temperature desorption peak disappeared when the Mg promoter was introduced. This showed that the amount of adsorbed hydrogen on the surface of nickel metal was very low. It is likely that the MgNiO2 formation occurred during the calcination process. It was hard to be reduced, and directly affected the quantity of active center of nickel. The two desorption peaks (400 and 460 C) with larger area at high temperature were also showed. This indicated that there was a large number of spillover hydrogen [30]. It was possible that a small amount of MgNiO2 formation was reduced for high dispersion of nickel with strong ability to activate hydrogen.

There were a desorption peak (100 C) and a satellite peak (130 C) in the low temperature range, when the Zr, Co, Ce, Zn and La promoter were introduced into the catalyst. These two desorption peaks could be attributed to weak adsorption and strong adsorption of hydrogen on the surface of nickel. The high temperature desorption

peak (400 C) shifted to the low temperature direction. The area of high temperature desorption peak was signicantly increased when the catalyst was added with La, Co and Zr. This indicated that the spillover hydrogen and the quantity of activate hydrogen on the catalyst surface were increased. The Co has also an important catalytic activity in FischerTropsch synthesis, but the ability to stick to its metallic cobalt is weak in harsh environment. There is a strong interaction between Ni and Zr, which makes it easy to form NiZrO2 and reduces the dispersion of nickel. The introduction of La promoter not only increased the number of spillover hydrogen on the catalyst surface, but also reduced the activation temperature of the spillover hydrogen.

Conclusion

To develop a high reaction activity and anti-coking coal to natural gas CO methanation catalyst, La, Zn, Ce, Co, Zr and Mg additives were introduced to the Ni/c-A12O3 dur

ing catalyst preparation. The rare earth metal La, Zr and Ce can effectively improve the dispersion of NiO on the surface of the support. This makes the nickel grains become easier reduced and gives higher catalytic activity. The La promoter introduced into the catalyst could effectively increase the amount of easily reduced NiO and provide more active centers. It was benecial to generate active carbon species, which increased the number of spillover hydrogen on the catalyst surface and reduced the activation energy of hydrogen in CO methanation reaction.

0 100 200 300 400 500 600 700 800

T/ oC

Fig. 5 H2-TPD proles of nickel-based catalysts

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Acknowledgments We would like to thank Beijing Municipal Science and Technology Commission for the support of the project. Professor Shuren Hao is also appreciated for his constructive advice.

Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/

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