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Al ratio of NiMgAl mixed oxide catalyst derived from hydrotalcite for carbon dioxide reforming of methane
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Effect of Mg/Al ratio of NiMgAl mixed oxide catalyst derived from hydrotalcite for carbon dioxide reforming of methane Yongjun Zhu, Shaohua Zhang, Bingbing Chen, Zhaoshun Zhang, Chuan Shi ∗ State Key Laboratory of Fine Chemicals, School of Chemistry, Dalian University of Technology, Dalian 116024, China
a r t i c l e
i n f o
Article history: Received 15 May 2015 Received in revised form 27 July 2015 Accepted 28 July 2015 Available online xxx Keywords: Carbon dioxide Methane Dry reforming Hydrotalcite Mg/Al ratio
a b s t r a c t How to stabilize Ni during the carbon dioxide reforming of methane (DRM) reaction is crucial for practical use of Ni catalysts for the DRM reaction. It is revealed in the present paper that there are two points that play key role in stabilization of Ni: one is formation of hydrotalcites precursors, and the other is formation of MgNiO2 . Therefore, the NiMgAl mixed oxides with higher Mg/Al ratio exhibits better catalytic activity and coke resistance, and the NiMgAl with Mg/Al ratio of 1 gives the best activity and stability for the DRM reaction. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Dry reforming of methane (DRM) provides a means of disposing and recycling two important greenhouse gases of CH4 and CO2 and a route to produce H2 /CO synthetic gas [1–3]. Theoretically, this process has a unity molar ratio of H2 /CO, which is suitable for future synthesis of hydrocarbons or oxygenated hydrocarbons [4,5]. The reaction described above has a highly endothermic nature for hydrogen production because of the reaction occurrence requiring high temperature ca. 800 ◦ C [2,6]. Ni was popularly used as the catalyst for the DRM reaction because of its higher activity and lower price. But Ni deactivated easily due to coke deposition (stemming from the decomposition of CH4 ) and metal sintering [7–9]. Therefore, great efforts have been devoted to developing carbon resistant Ni catalysts. It was found that coke formation is a structure-sensitive process, and depended on the surface Ni species, particle size and electron density [6–9]. A lot of promotors to Ni catalysts and many kinds of supports were tried in order to eliminate the coke deposition. Among the reported promotors, alkali and alkaline earth metals were helpful for improvement of coke resistance ability of Ni, while they decreased the reforming activity of the catalysts [10]. Ni/MgO possessed good coke resistance ability due to formation of solid
∗ Corresponding author. E-mail address: [email protected] (C. Shi).
solution Ni–Mg–O, but the activity of Ni/MgO was low compared with Ni/Al2 O3 under the same reaction conditions. Recently, it was also found that the preparation method and structure of support exert an important influence on the catalytic activity of Ni in the DRM reaction. By using precursors containing homogeneously distributed metal in the structure resulted in the formation of highly dispersed and stable metal particles on the surface. This method has been named as “solid phase crystallization (SPC)” [11]. This method was successfully applied to the preparation of Ni supported catalysts for the partial oxidation and CO2 reforming of CH4 to syngas by many perovskite-type metal oxides as the precursors [11]. Hydrotalcite like compounds (HTs) are layered double hydroxides (LDHs) and a classic of synthetic two-dimensional (2D) nanostructured anionic clays consisting of positively charged layers with charge-balancing anions between them. In the procedure used, the metal cations were dispersed in LDHs [12]. Since Ni2+ ion could be randomly distributed in the layered structure and probably insulated by Mg and Al ions, it was believed that Ni aggregation will be minimized in HTs-derived catalyst [7]. Tsyganok et al. [13] have compared the preparing method of Ni-based catalyst derived from HTs precursor. The corresponding results showed that the NiMgAl catalyst prepared by co-precipitation exhibited superior catalytic behavior than the one prepared using other conventional impregnation method. Additionally, many researchers have modified Ni-based catalyst from HTs using Ce, La, Ge, Pr and other metallic elements, and have obtained some results [14–17].
http://dx.doi.org/10.1016/j.cattod.2015.07.037 0920-5861/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Zhu, et al., Effect of Mg/Al ratio of NiMgAl mixed oxide catalyst derived from hydrotalcite for carbon dioxide reforming of methane, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.07.037
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2
Table 1 Component, BET surface area and pore volume of the NiMgAl sample. Sample
Nia (%)
Mg/Al ratio
BET surface areab (m2 /g)
Pore volumeb (cm3 /g)
NiMg5 Al1 NiMg3 Al1 NiMg1 Al1 NiMg1 Al3 NiMg1 Al5
7.6 7.7 8.4 7.8 7.8
5/1 3/1 1/1 1/3 1/5
176 180 190 147 101
0.33 0.34 0.39 0.27 0.19
a
The elemental composition was determined by ICP. Specific surface area and pore volume calculated by applying the multi-point BET equation in the linear range. b
In most pervious works, the results were obtained using catalysts with a fixed Mg/Al ratio. While it is worth mentioning that several groups have realized that the Mg/Al ratio has great influence on the catalytic properties. As reported by Shen et al, the effect of M2+ /M3+ on the basicity of catalysts was addressed [18]. By using CSCRM (a combined H2 O and CO2 reforming of CH4 ) as the probe reaction, it was reported that the catalyst with Mg/Al of 0.5 exhibited the highest activity [19]. In the present study, a series of NiMgAl catalysts with various Mg/Al ratios derived from LDHs were designed, characterized and tested for the DRM reaction. Parameters, like Mg/Al ratio and LDHs structure were both taken into account to clarify the influence of them on the catalytic activity and stability for the DRM reaction. The influence of Mg/Al ratio on the chemical states of nickel and therefore on the catalytic activities for the DRM reaction were clarified. Mg-rich sample (Mg/Al = 3 or 5) exhibited higher activity and better coke resistance. Such properties were ascribed to the formation of MgNiO2 , in which nickel was better stabilized than in the form of NiO. 2. Experimental 2.1. Catalyst preparation A series of NiMgAl samples derived from HTs were prepared by the co-precipitation method [12]. In order to examine the effect of the atomic ratio of Mg/Al for catalyst activity, it was varied from 5/1 to 1/5 in NiMgAl series sample. In briefly, pre-determined amounts of Al(NO3 )3 ·9H2 O, Mg(NO3 )2 ·6H2 O, Ni(NO3 )2 ·6H2 O were gradually dissolved in distilled and deionized water (solution I). A pre-determined amount of Na2 CO3 was also gradually dissolved in deionized water (solution II). Thereafter, the two solutions were simultaneously dropwise added into a tank with vigorous stirring at 63 ◦ C, maintaining the pH of the corresponding slurry at approximately 10.5 which was adjusted by adding dropwise of 1 M NaOH aqueous solution. The slurry was vigorous stirred at 63 ◦ C for 24 h and then aged for 24 h at 63 ◦ C, after which the product was washed by heated deionized water (55–65 ◦ C) until the hydroxide ion concentration was minimum. Subsequently, the resulting precipitate was dried at 110 ◦ C for 12 h, and calcined in the static air at 500 ◦ C for 10 h. Finally, the NiMgAl mixed oxides were obtained. For convenience, the obtained NiMgAl mixed-oxide catalysts derived from HTs were designated as NiMgx Aly , where x and y represent the nominal Mg molar amount and Al molar amount, respectively. The detailed list of the NiMgAl mixed-oxide samples is presented in Table 1. 2.2. Characterization techniques The actual Ni content in the catalyst was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Optima 2000DV, USA).
The specific surface area of the sample was determined by the Brunauer–Emmett–Teller (BET) method with nitrogen adsorption at −196 ◦ C in a Quantachrome NOVA station A instruments. X-ray diffraction patterns of powdered sample was recorded using a Shimadzu D3 diffractometer with a Cu-K˛ irradiation ˚ at voltage 30 kV and current 30 mA. Two source ( = 1.54056 A) types of scan were used to record the XRD spectra, hereby designated as normal scan (2 of 8◦ /min, step size 2 = 0.02◦ ) and slow scan (48–55◦ , 2 of 0.24◦ /min, step size 2 of 0.01◦ ). Hydrogen temperature programmed reduction (H2 -TPR) measurements were performed on a Micromeritics AutoChem II 2920 chemisorption analyzer. 50 mg samples were loaded and pretreated under Ar at 500 ◦ C for 1 h to remove adsorbed CO2 and H2 O. After cooling to 30 ◦ C and introducing the reducing gas (5% H2 /Ar) at a flow rate of 50 ml/min, the temperature was ramped to 900 ◦ C at 10 ◦ C/min. Transmission electron microscopy (TEM) images of the catalysts were obtained on a JEM-2000 microscope transmission electron microscope (JEOL, Japan). Carbon dioxide-temperature programmed oxidation (CO2 -TPO) analysis was performed to measure the coke deposition over the used catalyst. With the sample placed in a quartz tubular reactor, CO2 -TPO was carried out by introducing 1% CO2 /Ar (with a total flow rate of 100 ml/min) into the system while the sample temperature was raised from RT to a desired temperature at a rate of 10 ◦ C/min. The sample was firstly treated by Ar for 1 h at 300 ◦ C to remove somewhat moisture and other adsorbed gases on the surface of sample. The signal intensity of CO (m/z = 28) and CO2 (m/z = 44) were monitored and recorded using an online Mass spectrometer (OmnistarTM GSD 301). 2.3. Activity measurements Catalytic tests were performed in a quartz continuous flow fixed-bed micro reactor. In each experiment, 0.045 g catalyst was packed in the reactor (secured with quartz wool) with a thermocouple inserted into the center of the catalyst bed. Before the reaction, the catalyst was activated with a hydrogen flow of 50 ml/min at 800 ◦ C for 10 min. Then, a CH4 and CO2 mixture of 1:1 molar ratio was introduced into the catalyst bed (F/W = 80,000 ml/g h). The compositions of the effluent gas were analyzed on-line by two online gas chromatographs (GC112A and Shimadzu GC14A). The chromatographs were equipped with TCD using a Molecular Sieve 13X column and a Molecular Sieve 5A column respectively for complete separation and analysis of the gaseous components. Systematic error was much less than the random experimental error shown less than 3%. The conversions of CH4 (XCH4 ) and CO2 (XCO2 ) were calculated as follows: XCH4 =
Cin − Cout × 100% Cin
(1)
XCO2 =
Cin − Cout × 100% Cin
(2)
3. Results and discussion 3.1. Physical and chemical properties of the NiMgAl series samples The physical and chemical properties of the as-synthesized NiMgAl series catalysts were summarized in Table 1. The ICP results show that the actual Ni content in the NiMgAl series samples is in line with the nominal content. The BET surface areas and the pore volume of the NiMgAl series sample in terms of the Mg/Al ratios are 176, 180, 190, 147, 101 m2 /g and 0.33, 0.34, 0.39, 0.27, 0.19 cm3 /g, respectively, in which the order of the Mg/Al ratios is as followed
Please cite this article in press as: Y. Zhu, et al., Effect of Mg/Al ratio of NiMgAl mixed oxide catalyst derived from hydrotalcite for carbon dioxide reforming of methane, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.07.037
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ο • ο
ο
•
••
NC-NiMg1Al3
NC-NiMg1Al1
NC-NiMg3Al1 ∗
∗
10
20
30
NC-NiMg5Al1
40 50 60 2 theta (degree)
70
80
90
600oC
NiMg1Al1
NiMg1Al3
NiMg1Al5
200
400
600
800 o
Temperature ( C)
(B)
Fig. 2. H2 -TPR profiles of NiMgAl series samples.
Δ-MgO ⊗−NiAl2O4 ∇-MgNiO2 -NiO
⊗
Δ ∇
⊗
Δ ∇
Intensity (cps)
4000
NiMg5Al1
NiMg3Al1
NC-NiMg1Al5
Intensity (cps)
•
0.03
ο ο
ο-Al(OH)3 ∗−Mg5(CO3)4(OH)2·3H2O
ο •−hydrotalcite
Response (mV)
5000
(A)
3
⊗
NiMg1Al5 NiMg1Al3 NiMg1Al1 NiMg3Al1
NiMg5Al1
10
20
30 40 50 60 2 theta (degree)
70
80
Fig. 1. XRD patterns of NiMgAl series samples (A) non-calcined and (B) calcined at 500 ◦ C for 10 h. Where NC denotes the sample is not calcined at 500 ◦ C for 10 h but just post drying at 110 ◦ C.
5/1, 3/1, 1/1, 1/3 and 1/5. The NiMg1Al sample has the biggest BET surface area and the maximum pore volume. 3.2. X-ray powder diffraction characterization X-ray powder diffraction patterns of the as-synthesized NiMgAl series sample derived from HTs with various Mg/Al ratios and fixed Ni content are shown in Fig. 1. As can be seen in Fig. 1A, the XRD profiles of the as-synthesized sample without calcining (prefix as NC) clearly show three intense and sharp diffraction peaks at 2 of 11.27◦ (0 0 3), 22.78◦ (0 0 6) and 34.46◦ (0 0 9), which are attributed to the hydrotalcite characteristic peaks. In spite of the diffraction peaks at 2 of 18.7◦ (0 0 1), 40.5◦ (1 1 1), 20.3◦ (1 0 0) and 53.0◦ (1 1 2) which could be ascribed to Al(OH)3 were observed in Al-rich samples of NC-NiMg1 Al3 and NC-NiMg1 Al5 , and the diffraction peaks at 2 of 15.3◦ (0 1 1) and 30.8◦ (3 1 0) assigned to Mg5 (CO3 )4 (OH)2 ·3H2 O phase could be observed over the Mg-rich NC-NiMg5 Al1 sample. Furthermore, the intensity of the diffraction peaks due to hydrotalcite is much lower over Al-rich samples than over Mg-rich samples, indicating that higher Mg/Al ratios (from 1/1 to 5/1) facilitates the formation of hydrotalcite. On the other hand, the results also suggest that the NC-NiMgAl series samples with various
Mg/Al ratios and fixed Ni content have a typical layered structure with 3R symmetry that consists in a rhombohedric arrangement [12,13,17,20–22]. Meanwhile, compared to the XRD pattern of the NC-NiMg1 Al1 sample, the XRD diffraction positions at 2 of 11.27◦ (0 0 3), 22.78◦ (0 0 6) and 34.46◦ (0 0 9) of the NC-NiMg5 Al1 and NCNiMg3 Al1 samples slightly systematic shift to lower 2 values due to the expansion of the lattice spacing [23,24], corresponding to the appearance of new crystal phase such as Mg5 (CO3 )4 (OH)2 ·3H2 O phase. Fig. 1B shows the diffraction patterns of the NiMgAl series sample after calcining at 500 ◦ C for 10 h. For Mg-rich samples, such as NiMg5 Al1 , NiMg3 Al1 and NiMg1Al1, reflections corresponding to MgO and/or MgNiO2 (2 = 43.1◦ , 62.5◦ and 37.2◦ ) were observed. However, due to the overlapping of these three diffraction peaks, it is difficult to discriminate them exactly from MgO or MgNiO2 [25–28]. While based on the following H2 -TPR characterizations, it could be deduced that nickel should be mostly in the form of MgNiO2 due to the absence of the reduction peak ascribed to NiO. For Al-rich samples, namely NiMg1 Al3 and NiMg1 Al5 , three characteristic peaks at 2 = 37.1◦ , 44.8◦ and 65.5◦ were observed, which could be ascribed to NiAl2 O4 spinal phase and/or NiO crystal phase. Nevertheless, the intensity of the peak at 2 of 44.8◦ was stronger than that of 2 of 37.1◦ in well consistence with the reflections of NiO phase, therefore the existence of NiO may clearly be confirmed in these samples. However, the characteristic peak at 2 of 65.5◦ could stem from NiAl2 O4 spinal phase, and this sufficiently indicates the coexistence of NiAl2 O4 spinal crystal phase and NiO crystal phases in these samples. 3.3. Reducibility of NiMgAl series sample H2 -temperature programmed reduction (H2 -TPR) experiments were performed to obtain information about the NiMgAl series sample under reductive atmosphere, the results were shown in Fig. 2. The reductive degree of Ni2+ species in these samples were calculated according to the amount of H2 consumption, and the results were summarized in Table 2. As can be seen in Fig. 2, one broad reduction peak with a maximum temperature between 600 and 850 ◦ C is clearly observed in all NiMgAl series samples, assigned to the reduction of the Ni2+ species from mixed oxide, such as MgNiO2 and NiAl2 O4 , in spite of the differences in the maximum temperature. This reduction temperature is higher than that required for pure NiO and is a typical feature of catalysts derived from hydrotalcites. Specifically, the NiMg1Al5 sample showed one
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Fig. 3. TEM images and particle size histograms of (A) NiMg5 Al1 , (B) NiMg1 Al1 and (C) NiMg1 Al5 . These samples were pretreated at 800 ◦ C for 30 min under H2 atmosphere.
strong reduction peak centered at 460 ◦ C, which could be ascribed to the reduction of NiO [10,29–31]. In addition, based on the XRD measurements (Fig. 1B), the diffraction peak due to NiO could be clearly observed over NiMg1 Al5 sample. Therefore, it is reasonable to ascribe the reduction peaks at lower temperatures to the reduction of bulk NiO [10,25,29,30]. The reductive degree of Ni2+ specie was calculated as a function of H2 consumption during TPR process [18,32], and the corresponding results are presented in Table 2. For the NiMg5 Al1 , NiMg3 Al1 and NiMg1 Al3 samples, the reductive degree of Ni2+ specie are quite similar, which are only approximately 40%. As mentioned above, nickel is mostly in the form of MgNiO2 in the Mg-rich samples. Correlated this with the reductive behaviors, it is indicated that nickel
in the form of MgNiO2 is very difficult to reduce, only ca. 40% of it could be reduced at 850 ◦ C. However, with increase the content of Al, the reductive degree increased apparently. It reached as high as ca. 96% for NiMg1 Al5 sample. As indicated by XRD measurements (Fig. 1B), nickel is in the form of NiO over NiMg1 Al5 , which was easily reduced at lower temperature around 400 ◦ C. This accounts for the highest reductive degree of NiMg1 Al5 sample. However, the lower reductive degree over other samples indicates that H2 reduction of nickel in form of MgNiO2 and NiAl2 O4 needs even higher temperature than 850 ◦ C [32–35]. Fig. 3 shows the TEM images of the reduced NiMg5 Al1 , NiMg1 Al1 and NiMg1 Al5 catalysts. The more TEM pictures are shown in Fig. S1. The dark spots should be ascribed to the Ni nanoparticles dispersed
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Table 2 H2 -TPR properties over NiMgAl series sample. Degree of reductiona (%) [18,32]
NiMg5 Al1 NiMg3 Al1 NiMg1 Al1 NiMg1 Al3 NiMg1 Al5
22.7 22.6 28.3 22.3 54.6
39.8 39.8 49.6 39.1 95.8
Degree of reduction =
Measured H2 consumption during H2 −TPR Calculated H2 consumption
× 100%
on the supports [13]. For the NiMg5 Al1 sample, the Ni particles dispersed in a wide size range from 6 nm to 17 nm, and the size of 10–11 nm shows the highest frequency (the particle size was obtained from the TEM measurement of 100 Ni-NPs). In the case of NiMg1 Al1 sample, it can be observed the dispersion of the Ni particles is more uniform. Moreover, the Ni particle size in range of 6–9 nm presents more than 50% of the whole distribution, showing that a smaller average particle size was obtained over NiMg1 Al1 . In contrast, TEM images of the sample with the lowest Mg/Al ratio showed the presence of aggregated Ni nanoparticles, with a size distribution from 5 nm to 30 nm. This result is consistent with XRD measurements. Supplementary Fig. S1 can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2015.07.037 3.4. DRM activity of the NiMgAl series sample Fig. 4 shows CH4 and CO2 conversions at 800 ◦ C over the NiMgAl series samples. In all case, the catalytic conversion of CH4 is lower than that of CO2 , indicating that CO2 is consumed in parallel with the reverse water gas shift reaction (RWGS) [13]. It is clear that the catalytic activity relies on the Mg/Al ratios of the sample. For instance, the optimum NiMg1 Al1 sample shows the highest conversion of CH4 (83%) and CO2 (90%), and the conversions remain stable across the test period. As indicated by XRD measurements, the NiMgAl sample with Mg/Al of 1 exhibit the highest diffraction peaks ascribed to hydrotalcite, suggesting that Mg/Al ratio of 1 is the most optimum ratio for the formation of hydrotalcite, which is in accordance with the most of previous reports [13]. Correlating this with the activity results, it is clear that hydrotalcite as precursor for the mixed oxide is crucial for achieving the good activity for the DRM reaction. Nevertheless, for the other samples, it is obvious that the higher Mg/Al ratios provide a better activity. The activity follows the order of NiMg5 Al1 > NiMg3 Al1 > NiMg1 Al3 > NiMg1 Al5 . The NiMgAl sample with the highest Mg/Al ratio of 5/1 gives the highest conversion, and the NiMgAl sample with the lowest Mg/Al ratio of 1/5 has the lowest activity. The results indicate that magnesium oxide also plays a key role for the DRM reaction. In order to further examine the stability of the NiMgAl series sample in the DRM reaction, the NiMg5 Al1 , NiMg1 Al1 and NiMg1 Al5 samples were chosen to test their stability at 800 ◦ C for 30 h and the results are shown in Fig. 5. Fig. 5A shows that the conversion of CH4 of the NiMg5 Al1 and NiMg1 Al1 samples are substantially maintained constant whereas that of the NiMg1 Al5 sample has a little decrease in the long investigation process from 70% to 67%. Meanwhile, from Fig. 5B, the conversions of CO2 of these samples are close analogy with the trend of the corresponding conversion of the CH4 . 3.5. X-ray powder diffraction characterization of the spent NiMgAl series catalysts Fig. 6 shows the XRD patterns of the spent NiMgAl series samples after the DRM reaction at 800 ◦ C for 6 h. It is clear that the diffraction
90
80
70 NiMg5Al1 NiMg1Al5 NiMg1Al1
60
NiMg1Al3 NiMg3Al1
50 1
2
3
4
5
Time on stream (h)
(B) 100 90 Conversion of CO 2 (%)
H2 consumption (cm3 g−1 cat )
Conversion of CH4 (%)
(A) 100
Sample
a
5
80
70 NiMg5Al1 NiMg1Al5 NiMg1Al1
60
NiMg1Al3 NiMg3Al1
50 1
2
3
4
5
Time on stream (h) Fig. 4. Conversion of CH4 (A) and conversion of CO2 (B) over NiMgAl series catalysts. Conditions: CH4 /CO2 = 1/1, WHSV = 80,000 ml/h gcat , 800 ◦ C and ambient pressure.
patterns are very similar with those of the calcined fresh samples. But there is a new diffraction peak observed over the spent samples, it is at 2 of 51.6◦ , which is ascribed to the diffraction peak of metallic nickel (2 0 0) [36,37]. As can be seen in the inset of Fig. 6, although Ni0 could be observed over all the spent samples, no matter for the Mg-rich or Al-rich samples, especially over the spent NiMg1 Al5 sample, the characteristic peak intensity of Ni0 is the strongest. This suggests that though nickel oxide could be reduced to metallic nickel during the DRM reaction, nickel oxide in the NiMg1 Al5 sample is the most unstable one, and easily aggregated to large particles. Additionally, this is in agreement with the characterizations for the fresh samples that there is segregated NiO particles observed over the NiMg1 Al5 sample as proved by XRD measurement and H2 -TPR results, which is easily to reduce and aggregate to large particles during the DRM reaction. But over other samples, most of nickel oxide is stabilized in form of the NiMgO2 or NiAl2 O4 , which is more difficult to reduce as indicated by H2 -TPR. 3.6. The coke deposition behavior over the spent NiMgAl series catalysts CO2 -temperature programmed oxidization (CO2 -TPO) is usually employed to evaluate the amounts and types of coke deposition on sample surface [38,39], and the results are shown in Fig. 7. It is clear that the CO2 -TPO profiles of the spent NiMg5 Al1 and the spent NiMg1 All samples exhibit only two peaks at ca. 600 and 800 ◦ C, indicating that there are two types coke formation in the spent Mg-rich
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6
100 1.4E-08
(A)
m/z=28
60
m/z=44 Intensity
Conversion of CH4 (%)
80
S-NiMg5Al1
m/z=44
40
NiMg5Al1 NiMg1Al1 NiMg1Al5
20
5
10
15
20
25
m/z=28 m/z=44
0 0
S-NiMg1Al1
m/z=28
30
S-NiMg1Al5
Time on stream (h)
200
400
600
800 o
(B)
Temperature ( C)
100
Fig. 7. CO2 -TPO profiles of the spent NiMgAl series samples in DRM reaction for 30 h. Reaction condition: 1% CO2 /Ar. “S” denotes spent sample in DRM reaction.
Conversion of CO2 (%)
80 Table 3 Relative amount of carbon deposition on the spent sample.
60
NiMg5Al1 NiMg1Al1 NiMg1Al5
20
0
5
10
15
20
25
30
Time on stream (h) Fig. 5. Conversion of CH4 (A) and conversion of CO2 (B) as a function of time on stream over NiMgAl series samples. Conditions: CH4 /CO2 = 1/1, WHSV = 80,000 ml/h gcat , 800 ◦ C and ambient pressure.
Ni(200)
200
♦-Ni Δ-MgO ∇-MgNiO2 ⊗−NiAl2O4
Intensity (cps)
4000
Peak temperature (◦ C)
NiMg5 Al1 NiMg1 Al1 NiMg1 Al5
600 600 600
Total amount of carbon deposition (mg/gcat h)
40
0
Intensity (cps)
Spent sample
Δ ∇
S-NiMg1Al5
⊗
⊗
S-NiMg1Al3
NiMg1Al5 NiMg1Al1 NiMg5Al1
48 49 50 51 52 53 54 55 2 Theta (degree)
♦
Δ∇
⊗
700
850 800 850
3.3 10.6 18.2
samples of NiMg5 Al1 and NiMg1 Al1 . According to literatures [39], the first one appears around 600 ◦ C, which is defined as ␣-carbon; and the other appearing around 800 ◦ C is designated -carbon. The ␣-carbon might be ascribed to the readily activated carbonaceous deposits, whereas the -carbon is probably less reactive toward CO2 than ␣-carbon. The ␣-carbon species should be formed preferentially during the initial stages of the reforming process, mainly via CH4 decomposition [38–40]. In contrast, the -carbon should be attributed to the relatively inert carbon [39–41], which is more stable and therefore is oxidized at higher temperatures. While over the spent Al-rich sample of NiMg1 Al5 , there is a very sharp CO generation observed at 700 ◦ C, which has been assigned to ␥-carbon and might be a kind of a metastable fiber carbon [42]. By integrating the area of the CO2 consumption attributed to oxidation of the deposited carbon, the results are summarized in Table 3. One can clearly see that the total amount of coke on the spent Al-rich NiMg1 Al5 sample is six times higher than that of Mg-rich sample of NiMg5 Al1 . The results indicate that the Mg-rich sample results in decrease of carbon deposition. 4. Discussion
S-NiMg1Al1 S-NiMg3Al1 S-NiMg5Al1 10
20
30
40
50
60
70
80
2 Theta (degree) Fig. 6. XRD patterns of the spent NiMgAl series sample which were performed at DRM conditions for 6 h. Conditions: WHSV = 80,000 ml/h gcat , CO2 /CH4 = 1, 800 ◦ C.
It is well recognized that the nickel particles grow in size as a function of time on stream and then larger crystallites of nickel promote coke formation causing catalyst deactivation. Consequently, it is still a challenge that the current commercial industrial nickelbased catalyst is modified by various means to maintain superior catalytic behavior at higher space velocity and lower operating temperature [6–9]. Nevertheless, hydrotalcites (HTs) is extensively applied in the field of industrial catalysis due to unique physical and chemical properties [12]. The structure of HTs can be accommodated wide variations in the Mg2+ /Al3+ molar ratio, the type of interlayer anions, and different 2+ and 3+ metallic cations. More
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recently, the containing Ni2+ catalyst derived from HTs used in DRM reaction is a research topic. Since Ni2+ ions are able to be randomly distributed in the layered structure and somewhat Ni2+ ions are probable able to be insulated by Mg and Al ions, it is believed that nickel aggregation will be minimized in HTs-derived catalyst. In the present paper, a series of NiMgAl mixed oxides with Mg/Al ratios from 5/1 to 1/5 derived from HTs were characterized and tested for the DRM reaction. The NiMg1 Al1 sample with Mg/Al ratio of 1/1 presents the most perfect hydrotalcite crystalline phase [12]. For Mg-rich samples (Mg/Al of 5/1 and 3/1), there is detected of Mg5 (CO3 )4 (OH)2 ·4H2 O, while for Al-rich samples (Mg/Al of 1/3 and 1/5), there is Al(OH)3 formed. After calcination, all these precursors changed into mixed oxides. It is found that nickel is mostly stabilized as MgNiO2 for Mg-rich samples, while there are coexistence of NiO and NiAl2 O4 and MgNiO2 for the Al-rich samples. As indicated by H2 -TPR studies, nickel in MgNiO2 is very difficult to reduce as evidenced by its higher reduction temperature and lower reductive degree. But for the Al-rich sample, especially for the NiMg1 Al5 sample, most of nickel oxide was reduced at ca. 400 ◦ C, which is in accordance with the reduction temperature for aggregated NiO [35,43,44]. Both XRD and H2 -TPR results indicated that nickel is in form of aggregated NiO over the NiMg1 Al5 sample, but it is mostly stabilized in form of NiAl2 O4 and MgNiO2 over the other samples [32–35,45–47]. Correlation the characterization results with activity measurement, it becomes easier to understand the performances of the catalysts. First of all, hydrotalcite as precursor is especially important to achieve an excellent catalytic performance, as evidenced by the highest activity of NiMg1 Al1 for the DRM reaction with the best perfect HTs phase formation, actually, it is in agreement with elsewhere results [13]. Next, it is found that higher Mg/Al ratio leads to higher activity. As we known, MgO is base oxide, and this should be helpful for CO2 activation. In addition, as shown by H2 -TPR, nickel is stabilized in form of MgNiO2 in Mg-rich samples, which is very difficult to reduce. Thus, this should be the reason for its higher activity as well as stability. In contrast, the most Al-rich sample of NiMg1 Al5 exhibits the lowest activity and the lowest stability. Characterizations for the spent catalyst show that there is apparent of metallic nickel only over the most Al-rich sample. Correlated the characterization for the fresh sample, it is clear that aggregated NiO was reduced to metallic Ni during the DRM process, and then metallic nickel sintered into large particle size which is not favorable for the DRM reaction but very active for coke deposition. Actually, this has been verified by CO2 -TPO results in well agreement with literature [39].
5. Conclusions A series of NiMgAl mixed oxide samples with Mg/Al ratios from 5/1 to 1/5 derived from LDHs were tested for the DRM reaction. There are two factors which influence the activity: the first is the formation of hydrotalcite precursor, and the second is higher Mg/Al ratios. Actually, these two factors both related to stabilize nickel during the DRM reaction performed at high temperature. It is also found that nickel could be better stabilized in form of MgNiO2 over Mg-rich samples. Better nickel dispersion even after the DRM reaction renders the NiMg1 Al1 catalyst as both good activity and stability for the DRM reaction.
Acknowledgements We thank the National Natural Science Foundation of China for funding (Nos. 21073024 and 21373037).
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Effect of Mg/Al ratio of NiMgAl mixed oxide catalyst derived from hydrotalcite for carbon dioxide reforming of methane Yongjun Zhu, Shaohua Zhang, Bingbing Chen, Zhaoshun Zhang, Chuan Shi ∗ State Key Laboratory of Fine Chemicals, School of Chemistry, Dalian University of Technology, Dalian 116024, China
a r t i c l e
i n f o
Article history: Received 15 May 2015 Received in revised form 27 July 2015 Accepted 28 July 2015 Available online xxx Keywords: Carbon dioxide Methane Dry reforming Hydrotalcite Mg/Al ratio
a b s t r a c t How to stabilize Ni during the carbon dioxide reforming of methane (DRM) reaction is crucial for practical use of Ni catalysts for the DRM reaction. It is revealed in the present paper that there are two points that play key role in stabilization of Ni: one is formation of hydrotalcites precursors, and the other is formation of MgNiO2 . Therefore, the NiMgAl mixed oxides with higher Mg/Al ratio exhibits better catalytic activity and coke resistance, and the NiMgAl with Mg/Al ratio of 1 gives the best activity and stability for the DRM reaction. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Dry reforming of methane (DRM) provides a means of disposing and recycling two important greenhouse gases of CH4 and CO2 and a route to produce H2 /CO synthetic gas [1–3]. Theoretically, this process has a unity molar ratio of H2 /CO, which is suitable for future synthesis of hydrocarbons or oxygenated hydrocarbons [4,5]. The reaction described above has a highly endothermic nature for hydrogen production because of the reaction occurrence requiring high temperature ca. 800 ◦ C [2,6]. Ni was popularly used as the catalyst for the DRM reaction because of its higher activity and lower price. But Ni deactivated easily due to coke deposition (stemming from the decomposition of CH4 ) and metal sintering [7–9]. Therefore, great efforts have been devoted to developing carbon resistant Ni catalysts. It was found that coke formation is a structure-sensitive process, and depended on the surface Ni species, particle size and electron density [6–9]. A lot of promotors to Ni catalysts and many kinds of supports were tried in order to eliminate the coke deposition. Among the reported promotors, alkali and alkaline earth metals were helpful for improvement of coke resistance ability of Ni, while they decreased the reforming activity of the catalysts [10]. Ni/MgO possessed good coke resistance ability due to formation of solid
∗ Corresponding author. E-mail address: [email protected] (C. Shi).
solution Ni–Mg–O, but the activity of Ni/MgO was low compared with Ni/Al2 O3 under the same reaction conditions. Recently, it was also found that the preparation method and structure of support exert an important influence on the catalytic activity of Ni in the DRM reaction. By using precursors containing homogeneously distributed metal in the structure resulted in the formation of highly dispersed and stable metal particles on the surface. This method has been named as “solid phase crystallization (SPC)” [11]. This method was successfully applied to the preparation of Ni supported catalysts for the partial oxidation and CO2 reforming of CH4 to syngas by many perovskite-type metal oxides as the precursors [11]. Hydrotalcite like compounds (HTs) are layered double hydroxides (LDHs) and a classic of synthetic two-dimensional (2D) nanostructured anionic clays consisting of positively charged layers with charge-balancing anions between them. In the procedure used, the metal cations were dispersed in LDHs [12]. Since Ni2+ ion could be randomly distributed in the layered structure and probably insulated by Mg and Al ions, it was believed that Ni aggregation will be minimized in HTs-derived catalyst [7]. Tsyganok et al. [13] have compared the preparing method of Ni-based catalyst derived from HTs precursor. The corresponding results showed that the NiMgAl catalyst prepared by co-precipitation exhibited superior catalytic behavior than the one prepared using other conventional impregnation method. Additionally, many researchers have modified Ni-based catalyst from HTs using Ce, La, Ge, Pr and other metallic elements, and have obtained some results [14–17].
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2
Table 1 Component, BET surface area and pore volume of the NiMgAl sample. Sample
Nia (%)
Mg/Al ratio
BET surface areab (m2 /g)
Pore volumeb (cm3 /g)
NiMg5 Al1 NiMg3 Al1 NiMg1 Al1 NiMg1 Al3 NiMg1 Al5
7.6 7.7 8.4 7.8 7.8
5/1 3/1 1/1 1/3 1/5
176 180 190 147 101
0.33 0.34 0.39 0.27 0.19
a
The elemental composition was determined by ICP. Specific surface area and pore volume calculated by applying the multi-point BET equation in the linear range. b
In most pervious works, the results were obtained using catalysts with a fixed Mg/Al ratio. While it is worth mentioning that several groups have realized that the Mg/Al ratio has great influence on the catalytic properties. As reported by Shen et al, the effect of M2+ /M3+ on the basicity of catalysts was addressed [18]. By using CSCRM (a combined H2 O and CO2 reforming of CH4 ) as the probe reaction, it was reported that the catalyst with Mg/Al of 0.5 exhibited the highest activity [19]. In the present study, a series of NiMgAl catalysts with various Mg/Al ratios derived from LDHs were designed, characterized and tested for the DRM reaction. Parameters, like Mg/Al ratio and LDHs structure were both taken into account to clarify the influence of them on the catalytic activity and stability for the DRM reaction. The influence of Mg/Al ratio on the chemical states of nickel and therefore on the catalytic activities for the DRM reaction were clarified. Mg-rich sample (Mg/Al = 3 or 5) exhibited higher activity and better coke resistance. Such properties were ascribed to the formation of MgNiO2 , in which nickel was better stabilized than in the form of NiO. 2. Experimental 2.1. Catalyst preparation A series of NiMgAl samples derived from HTs were prepared by the co-precipitation method [12]. In order to examine the effect of the atomic ratio of Mg/Al for catalyst activity, it was varied from 5/1 to 1/5 in NiMgAl series sample. In briefly, pre-determined amounts of Al(NO3 )3 ·9H2 O, Mg(NO3 )2 ·6H2 O, Ni(NO3 )2 ·6H2 O were gradually dissolved in distilled and deionized water (solution I). A pre-determined amount of Na2 CO3 was also gradually dissolved in deionized water (solution II). Thereafter, the two solutions were simultaneously dropwise added into a tank with vigorous stirring at 63 ◦ C, maintaining the pH of the corresponding slurry at approximately 10.5 which was adjusted by adding dropwise of 1 M NaOH aqueous solution. The slurry was vigorous stirred at 63 ◦ C for 24 h and then aged for 24 h at 63 ◦ C, after which the product was washed by heated deionized water (55–65 ◦ C) until the hydroxide ion concentration was minimum. Subsequently, the resulting precipitate was dried at 110 ◦ C for 12 h, and calcined in the static air at 500 ◦ C for 10 h. Finally, the NiMgAl mixed oxides were obtained. For convenience, the obtained NiMgAl mixed-oxide catalysts derived from HTs were designated as NiMgx Aly , where x and y represent the nominal Mg molar amount and Al molar amount, respectively. The detailed list of the NiMgAl mixed-oxide samples is presented in Table 1. 2.2. Characterization techniques The actual Ni content in the catalyst was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Optima 2000DV, USA).
The specific surface area of the sample was determined by the Brunauer–Emmett–Teller (BET) method with nitrogen adsorption at −196 ◦ C in a Quantachrome NOVA station A instruments. X-ray diffraction patterns of powdered sample was recorded using a Shimadzu D3 diffractometer with a Cu-K˛ irradiation ˚ at voltage 30 kV and current 30 mA. Two source ( = 1.54056 A) types of scan were used to record the XRD spectra, hereby designated as normal scan (2 of 8◦ /min, step size 2 = 0.02◦ ) and slow scan (48–55◦ , 2 of 0.24◦ /min, step size 2 of 0.01◦ ). Hydrogen temperature programmed reduction (H2 -TPR) measurements were performed on a Micromeritics AutoChem II 2920 chemisorption analyzer. 50 mg samples were loaded and pretreated under Ar at 500 ◦ C for 1 h to remove adsorbed CO2 and H2 O. After cooling to 30 ◦ C and introducing the reducing gas (5% H2 /Ar) at a flow rate of 50 ml/min, the temperature was ramped to 900 ◦ C at 10 ◦ C/min. Transmission electron microscopy (TEM) images of the catalysts were obtained on a JEM-2000 microscope transmission electron microscope (JEOL, Japan). Carbon dioxide-temperature programmed oxidation (CO2 -TPO) analysis was performed to measure the coke deposition over the used catalyst. With the sample placed in a quartz tubular reactor, CO2 -TPO was carried out by introducing 1% CO2 /Ar (with a total flow rate of 100 ml/min) into the system while the sample temperature was raised from RT to a desired temperature at a rate of 10 ◦ C/min. The sample was firstly treated by Ar for 1 h at 300 ◦ C to remove somewhat moisture and other adsorbed gases on the surface of sample. The signal intensity of CO (m/z = 28) and CO2 (m/z = 44) were monitored and recorded using an online Mass spectrometer (OmnistarTM GSD 301). 2.3. Activity measurements Catalytic tests were performed in a quartz continuous flow fixed-bed micro reactor. In each experiment, 0.045 g catalyst was packed in the reactor (secured with quartz wool) with a thermocouple inserted into the center of the catalyst bed. Before the reaction, the catalyst was activated with a hydrogen flow of 50 ml/min at 800 ◦ C for 10 min. Then, a CH4 and CO2 mixture of 1:1 molar ratio was introduced into the catalyst bed (F/W = 80,000 ml/g h). The compositions of the effluent gas were analyzed on-line by two online gas chromatographs (GC112A and Shimadzu GC14A). The chromatographs were equipped with TCD using a Molecular Sieve 13X column and a Molecular Sieve 5A column respectively for complete separation and analysis of the gaseous components. Systematic error was much less than the random experimental error shown less than 3%. The conversions of CH4 (XCH4 ) and CO2 (XCO2 ) were calculated as follows: XCH4 =
Cin − Cout × 100% Cin
(1)
XCO2 =
Cin − Cout × 100% Cin
(2)
3. Results and discussion 3.1. Physical and chemical properties of the NiMgAl series samples The physical and chemical properties of the as-synthesized NiMgAl series catalysts were summarized in Table 1. The ICP results show that the actual Ni content in the NiMgAl series samples is in line with the nominal content. The BET surface areas and the pore volume of the NiMgAl series sample in terms of the Mg/Al ratios are 176, 180, 190, 147, 101 m2 /g and 0.33, 0.34, 0.39, 0.27, 0.19 cm3 /g, respectively, in which the order of the Mg/Al ratios is as followed
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ο • ο
ο
•
••
NC-NiMg1Al3
NC-NiMg1Al1
NC-NiMg3Al1 ∗
∗
10
20
30
NC-NiMg5Al1
40 50 60 2 theta (degree)
70
80
90
600oC
NiMg1Al1
NiMg1Al3
NiMg1Al5
200
400
600
800 o
Temperature ( C)
(B)
Fig. 2. H2 -TPR profiles of NiMgAl series samples.
Δ-MgO ⊗−NiAl2O4 ∇-MgNiO2 -NiO
⊗
Δ ∇
⊗
Δ ∇
Intensity (cps)
4000
NiMg5Al1
NiMg3Al1
NC-NiMg1Al5
Intensity (cps)
•
0.03
ο ο
ο-Al(OH)3 ∗−Mg5(CO3)4(OH)2·3H2O
ο •−hydrotalcite
Response (mV)
5000
(A)
3
⊗
NiMg1Al5 NiMg1Al3 NiMg1Al1 NiMg3Al1
NiMg5Al1
10
20
30 40 50 60 2 theta (degree)
70
80
Fig. 1. XRD patterns of NiMgAl series samples (A) non-calcined and (B) calcined at 500 ◦ C for 10 h. Where NC denotes the sample is not calcined at 500 ◦ C for 10 h but just post drying at 110 ◦ C.
5/1, 3/1, 1/1, 1/3 and 1/5. The NiMg1Al sample has the biggest BET surface area and the maximum pore volume. 3.2. X-ray powder diffraction characterization X-ray powder diffraction patterns of the as-synthesized NiMgAl series sample derived from HTs with various Mg/Al ratios and fixed Ni content are shown in Fig. 1. As can be seen in Fig. 1A, the XRD profiles of the as-synthesized sample without calcining (prefix as NC) clearly show three intense and sharp diffraction peaks at 2 of 11.27◦ (0 0 3), 22.78◦ (0 0 6) and 34.46◦ (0 0 9), which are attributed to the hydrotalcite characteristic peaks. In spite of the diffraction peaks at 2 of 18.7◦ (0 0 1), 40.5◦ (1 1 1), 20.3◦ (1 0 0) and 53.0◦ (1 1 2) which could be ascribed to Al(OH)3 were observed in Al-rich samples of NC-NiMg1 Al3 and NC-NiMg1 Al5 , and the diffraction peaks at 2 of 15.3◦ (0 1 1) and 30.8◦ (3 1 0) assigned to Mg5 (CO3 )4 (OH)2 ·3H2 O phase could be observed over the Mg-rich NC-NiMg5 Al1 sample. Furthermore, the intensity of the diffraction peaks due to hydrotalcite is much lower over Al-rich samples than over Mg-rich samples, indicating that higher Mg/Al ratios (from 1/1 to 5/1) facilitates the formation of hydrotalcite. On the other hand, the results also suggest that the NC-NiMgAl series samples with various
Mg/Al ratios and fixed Ni content have a typical layered structure with 3R symmetry that consists in a rhombohedric arrangement [12,13,17,20–22]. Meanwhile, compared to the XRD pattern of the NC-NiMg1 Al1 sample, the XRD diffraction positions at 2 of 11.27◦ (0 0 3), 22.78◦ (0 0 6) and 34.46◦ (0 0 9) of the NC-NiMg5 Al1 and NCNiMg3 Al1 samples slightly systematic shift to lower 2 values due to the expansion of the lattice spacing [23,24], corresponding to the appearance of new crystal phase such as Mg5 (CO3 )4 (OH)2 ·3H2 O phase. Fig. 1B shows the diffraction patterns of the NiMgAl series sample after calcining at 500 ◦ C for 10 h. For Mg-rich samples, such as NiMg5 Al1 , NiMg3 Al1 and NiMg1Al1, reflections corresponding to MgO and/or MgNiO2 (2 = 43.1◦ , 62.5◦ and 37.2◦ ) were observed. However, due to the overlapping of these three diffraction peaks, it is difficult to discriminate them exactly from MgO or MgNiO2 [25–28]. While based on the following H2 -TPR characterizations, it could be deduced that nickel should be mostly in the form of MgNiO2 due to the absence of the reduction peak ascribed to NiO. For Al-rich samples, namely NiMg1 Al3 and NiMg1 Al5 , three characteristic peaks at 2 = 37.1◦ , 44.8◦ and 65.5◦ were observed, which could be ascribed to NiAl2 O4 spinal phase and/or NiO crystal phase. Nevertheless, the intensity of the peak at 2 of 44.8◦ was stronger than that of 2 of 37.1◦ in well consistence with the reflections of NiO phase, therefore the existence of NiO may clearly be confirmed in these samples. However, the characteristic peak at 2 of 65.5◦ could stem from NiAl2 O4 spinal phase, and this sufficiently indicates the coexistence of NiAl2 O4 spinal crystal phase and NiO crystal phases in these samples. 3.3. Reducibility of NiMgAl series sample H2 -temperature programmed reduction (H2 -TPR) experiments were performed to obtain information about the NiMgAl series sample under reductive atmosphere, the results were shown in Fig. 2. The reductive degree of Ni2+ species in these samples were calculated according to the amount of H2 consumption, and the results were summarized in Table 2. As can be seen in Fig. 2, one broad reduction peak with a maximum temperature between 600 and 850 ◦ C is clearly observed in all NiMgAl series samples, assigned to the reduction of the Ni2+ species from mixed oxide, such as MgNiO2 and NiAl2 O4 , in spite of the differences in the maximum temperature. This reduction temperature is higher than that required for pure NiO and is a typical feature of catalysts derived from hydrotalcites. Specifically, the NiMg1Al5 sample showed one
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Fig. 3. TEM images and particle size histograms of (A) NiMg5 Al1 , (B) NiMg1 Al1 and (C) NiMg1 Al5 . These samples were pretreated at 800 ◦ C for 30 min under H2 atmosphere.
strong reduction peak centered at 460 ◦ C, which could be ascribed to the reduction of NiO [10,29–31]. In addition, based on the XRD measurements (Fig. 1B), the diffraction peak due to NiO could be clearly observed over NiMg1 Al5 sample. Therefore, it is reasonable to ascribe the reduction peaks at lower temperatures to the reduction of bulk NiO [10,25,29,30]. The reductive degree of Ni2+ specie was calculated as a function of H2 consumption during TPR process [18,32], and the corresponding results are presented in Table 2. For the NiMg5 Al1 , NiMg3 Al1 and NiMg1 Al3 samples, the reductive degree of Ni2+ specie are quite similar, which are only approximately 40%. As mentioned above, nickel is mostly in the form of MgNiO2 in the Mg-rich samples. Correlated this with the reductive behaviors, it is indicated that nickel
in the form of MgNiO2 is very difficult to reduce, only ca. 40% of it could be reduced at 850 ◦ C. However, with increase the content of Al, the reductive degree increased apparently. It reached as high as ca. 96% for NiMg1 Al5 sample. As indicated by XRD measurements (Fig. 1B), nickel is in the form of NiO over NiMg1 Al5 , which was easily reduced at lower temperature around 400 ◦ C. This accounts for the highest reductive degree of NiMg1 Al5 sample. However, the lower reductive degree over other samples indicates that H2 reduction of nickel in form of MgNiO2 and NiAl2 O4 needs even higher temperature than 850 ◦ C [32–35]. Fig. 3 shows the TEM images of the reduced NiMg5 Al1 , NiMg1 Al1 and NiMg1 Al5 catalysts. The more TEM pictures are shown in Fig. S1. The dark spots should be ascribed to the Ni nanoparticles dispersed
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Table 2 H2 -TPR properties over NiMgAl series sample. Degree of reductiona (%) [18,32]
NiMg5 Al1 NiMg3 Al1 NiMg1 Al1 NiMg1 Al3 NiMg1 Al5
22.7 22.6 28.3 22.3 54.6
39.8 39.8 49.6 39.1 95.8
Degree of reduction =
Measured H2 consumption during H2 −TPR Calculated H2 consumption
× 100%
on the supports [13]. For the NiMg5 Al1 sample, the Ni particles dispersed in a wide size range from 6 nm to 17 nm, and the size of 10–11 nm shows the highest frequency (the particle size was obtained from the TEM measurement of 100 Ni-NPs). In the case of NiMg1 Al1 sample, it can be observed the dispersion of the Ni particles is more uniform. Moreover, the Ni particle size in range of 6–9 nm presents more than 50% of the whole distribution, showing that a smaller average particle size was obtained over NiMg1 Al1 . In contrast, TEM images of the sample with the lowest Mg/Al ratio showed the presence of aggregated Ni nanoparticles, with a size distribution from 5 nm to 30 nm. This result is consistent with XRD measurements. Supplementary Fig. S1 can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2015.07.037 3.4. DRM activity of the NiMgAl series sample Fig. 4 shows CH4 and CO2 conversions at 800 ◦ C over the NiMgAl series samples. In all case, the catalytic conversion of CH4 is lower than that of CO2 , indicating that CO2 is consumed in parallel with the reverse water gas shift reaction (RWGS) [13]. It is clear that the catalytic activity relies on the Mg/Al ratios of the sample. For instance, the optimum NiMg1 Al1 sample shows the highest conversion of CH4 (83%) and CO2 (90%), and the conversions remain stable across the test period. As indicated by XRD measurements, the NiMgAl sample with Mg/Al of 1 exhibit the highest diffraction peaks ascribed to hydrotalcite, suggesting that Mg/Al ratio of 1 is the most optimum ratio for the formation of hydrotalcite, which is in accordance with the most of previous reports [13]. Correlating this with the activity results, it is clear that hydrotalcite as precursor for the mixed oxide is crucial for achieving the good activity for the DRM reaction. Nevertheless, for the other samples, it is obvious that the higher Mg/Al ratios provide a better activity. The activity follows the order of NiMg5 Al1 > NiMg3 Al1 > NiMg1 Al3 > NiMg1 Al5 . The NiMgAl sample with the highest Mg/Al ratio of 5/1 gives the highest conversion, and the NiMgAl sample with the lowest Mg/Al ratio of 1/5 has the lowest activity. The results indicate that magnesium oxide also plays a key role for the DRM reaction. In order to further examine the stability of the NiMgAl series sample in the DRM reaction, the NiMg5 Al1 , NiMg1 Al1 and NiMg1 Al5 samples were chosen to test their stability at 800 ◦ C for 30 h and the results are shown in Fig. 5. Fig. 5A shows that the conversion of CH4 of the NiMg5 Al1 and NiMg1 Al1 samples are substantially maintained constant whereas that of the NiMg1 Al5 sample has a little decrease in the long investigation process from 70% to 67%. Meanwhile, from Fig. 5B, the conversions of CO2 of these samples are close analogy with the trend of the corresponding conversion of the CH4 . 3.5. X-ray powder diffraction characterization of the spent NiMgAl series catalysts Fig. 6 shows the XRD patterns of the spent NiMgAl series samples after the DRM reaction at 800 ◦ C for 6 h. It is clear that the diffraction
90
80
70 NiMg5Al1 NiMg1Al5 NiMg1Al1
60
NiMg1Al3 NiMg3Al1
50 1
2
3
4
5
Time on stream (h)
(B) 100 90 Conversion of CO 2 (%)
H2 consumption (cm3 g−1 cat )
Conversion of CH4 (%)
(A) 100
Sample
a
5
80
70 NiMg5Al1 NiMg1Al5 NiMg1Al1
60
NiMg1Al3 NiMg3Al1
50 1
2
3
4
5
Time on stream (h) Fig. 4. Conversion of CH4 (A) and conversion of CO2 (B) over NiMgAl series catalysts. Conditions: CH4 /CO2 = 1/1, WHSV = 80,000 ml/h gcat , 800 ◦ C and ambient pressure.
patterns are very similar with those of the calcined fresh samples. But there is a new diffraction peak observed over the spent samples, it is at 2 of 51.6◦ , which is ascribed to the diffraction peak of metallic nickel (2 0 0) [36,37]. As can be seen in the inset of Fig. 6, although Ni0 could be observed over all the spent samples, no matter for the Mg-rich or Al-rich samples, especially over the spent NiMg1 Al5 sample, the characteristic peak intensity of Ni0 is the strongest. This suggests that though nickel oxide could be reduced to metallic nickel during the DRM reaction, nickel oxide in the NiMg1 Al5 sample is the most unstable one, and easily aggregated to large particles. Additionally, this is in agreement with the characterizations for the fresh samples that there is segregated NiO particles observed over the NiMg1 Al5 sample as proved by XRD measurement and H2 -TPR results, which is easily to reduce and aggregate to large particles during the DRM reaction. But over other samples, most of nickel oxide is stabilized in form of the NiMgO2 or NiAl2 O4 , which is more difficult to reduce as indicated by H2 -TPR. 3.6. The coke deposition behavior over the spent NiMgAl series catalysts CO2 -temperature programmed oxidization (CO2 -TPO) is usually employed to evaluate the amounts and types of coke deposition on sample surface [38,39], and the results are shown in Fig. 7. It is clear that the CO2 -TPO profiles of the spent NiMg5 Al1 and the spent NiMg1 All samples exhibit only two peaks at ca. 600 and 800 ◦ C, indicating that there are two types coke formation in the spent Mg-rich
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6
100 1.4E-08
(A)
m/z=28
60
m/z=44 Intensity
Conversion of CH4 (%)
80
S-NiMg5Al1
m/z=44
40
NiMg5Al1 NiMg1Al1 NiMg1Al5
20
5
10
15
20
25
m/z=28 m/z=44
0 0
S-NiMg1Al1
m/z=28
30
S-NiMg1Al5
Time on stream (h)
200
400
600
800 o
(B)
Temperature ( C)
100
Fig. 7. CO2 -TPO profiles of the spent NiMgAl series samples in DRM reaction for 30 h. Reaction condition: 1% CO2 /Ar. “S” denotes spent sample in DRM reaction.
Conversion of CO2 (%)
80 Table 3 Relative amount of carbon deposition on the spent sample.
60
NiMg5Al1 NiMg1Al1 NiMg1Al5
20
0
5
10
15
20
25
30
Time on stream (h) Fig. 5. Conversion of CH4 (A) and conversion of CO2 (B) as a function of time on stream over NiMgAl series samples. Conditions: CH4 /CO2 = 1/1, WHSV = 80,000 ml/h gcat , 800 ◦ C and ambient pressure.
Ni(200)
200
♦-Ni Δ-MgO ∇-MgNiO2 ⊗−NiAl2O4
Intensity (cps)
4000
Peak temperature (◦ C)
NiMg5 Al1 NiMg1 Al1 NiMg1 Al5
600 600 600
Total amount of carbon deposition (mg/gcat h)
40
0
Intensity (cps)
Spent sample
Δ ∇
S-NiMg1Al5
⊗
⊗
S-NiMg1Al3
NiMg1Al5 NiMg1Al1 NiMg5Al1
48 49 50 51 52 53 54 55 2 Theta (degree)
♦
Δ∇
⊗
700
850 800 850
3.3 10.6 18.2
samples of NiMg5 Al1 and NiMg1 Al1 . According to literatures [39], the first one appears around 600 ◦ C, which is defined as ␣-carbon; and the other appearing around 800 ◦ C is designated -carbon. The ␣-carbon might be ascribed to the readily activated carbonaceous deposits, whereas the -carbon is probably less reactive toward CO2 than ␣-carbon. The ␣-carbon species should be formed preferentially during the initial stages of the reforming process, mainly via CH4 decomposition [38–40]. In contrast, the -carbon should be attributed to the relatively inert carbon [39–41], which is more stable and therefore is oxidized at higher temperatures. While over the spent Al-rich sample of NiMg1 Al5 , there is a very sharp CO generation observed at 700 ◦ C, which has been assigned to ␥-carbon and might be a kind of a metastable fiber carbon [42]. By integrating the area of the CO2 consumption attributed to oxidation of the deposited carbon, the results are summarized in Table 3. One can clearly see that the total amount of coke on the spent Al-rich NiMg1 Al5 sample is six times higher than that of Mg-rich sample of NiMg5 Al1 . The results indicate that the Mg-rich sample results in decrease of carbon deposition. 4. Discussion
S-NiMg1Al1 S-NiMg3Al1 S-NiMg5Al1 10
20
30
40
50
60
70
80
2 Theta (degree) Fig. 6. XRD patterns of the spent NiMgAl series sample which were performed at DRM conditions for 6 h. Conditions: WHSV = 80,000 ml/h gcat , CO2 /CH4 = 1, 800 ◦ C.
It is well recognized that the nickel particles grow in size as a function of time on stream and then larger crystallites of nickel promote coke formation causing catalyst deactivation. Consequently, it is still a challenge that the current commercial industrial nickelbased catalyst is modified by various means to maintain superior catalytic behavior at higher space velocity and lower operating temperature [6–9]. Nevertheless, hydrotalcites (HTs) is extensively applied in the field of industrial catalysis due to unique physical and chemical properties [12]. The structure of HTs can be accommodated wide variations in the Mg2+ /Al3+ molar ratio, the type of interlayer anions, and different 2+ and 3+ metallic cations. More
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recently, the containing Ni2+ catalyst derived from HTs used in DRM reaction is a research topic. Since Ni2+ ions are able to be randomly distributed in the layered structure and somewhat Ni2+ ions are probable able to be insulated by Mg and Al ions, it is believed that nickel aggregation will be minimized in HTs-derived catalyst. In the present paper, a series of NiMgAl mixed oxides with Mg/Al ratios from 5/1 to 1/5 derived from HTs were characterized and tested for the DRM reaction. The NiMg1 Al1 sample with Mg/Al ratio of 1/1 presents the most perfect hydrotalcite crystalline phase [12]. For Mg-rich samples (Mg/Al of 5/1 and 3/1), there is detected of Mg5 (CO3 )4 (OH)2 ·4H2 O, while for Al-rich samples (Mg/Al of 1/3 and 1/5), there is Al(OH)3 formed. After calcination, all these precursors changed into mixed oxides. It is found that nickel is mostly stabilized as MgNiO2 for Mg-rich samples, while there are coexistence of NiO and NiAl2 O4 and MgNiO2 for the Al-rich samples. As indicated by H2 -TPR studies, nickel in MgNiO2 is very difficult to reduce as evidenced by its higher reduction temperature and lower reductive degree. But for the Al-rich sample, especially for the NiMg1 Al5 sample, most of nickel oxide was reduced at ca. 400 ◦ C, which is in accordance with the reduction temperature for aggregated NiO [35,43,44]. Both XRD and H2 -TPR results indicated that nickel is in form of aggregated NiO over the NiMg1 Al5 sample, but it is mostly stabilized in form of NiAl2 O4 and MgNiO2 over the other samples [32–35,45–47]. Correlation the characterization results with activity measurement, it becomes easier to understand the performances of the catalysts. First of all, hydrotalcite as precursor is especially important to achieve an excellent catalytic performance, as evidenced by the highest activity of NiMg1 Al1 for the DRM reaction with the best perfect HTs phase formation, actually, it is in agreement with elsewhere results [13]. Next, it is found that higher Mg/Al ratio leads to higher activity. As we known, MgO is base oxide, and this should be helpful for CO2 activation. In addition, as shown by H2 -TPR, nickel is stabilized in form of MgNiO2 in Mg-rich samples, which is very difficult to reduce. Thus, this should be the reason for its higher activity as well as stability. In contrast, the most Al-rich sample of NiMg1 Al5 exhibits the lowest activity and the lowest stability. Characterizations for the spent catalyst show that there is apparent of metallic nickel only over the most Al-rich sample. Correlated the characterization for the fresh sample, it is clear that aggregated NiO was reduced to metallic Ni during the DRM process, and then metallic nickel sintered into large particle size which is not favorable for the DRM reaction but very active for coke deposition. Actually, this has been verified by CO2 -TPO results in well agreement with literature [39].
5. Conclusions A series of NiMgAl mixed oxide samples with Mg/Al ratios from 5/1 to 1/5 derived from LDHs were tested for the DRM reaction. There are two factors which influence the activity: the first is the formation of hydrotalcite precursor, and the second is higher Mg/Al ratios. Actually, these two factors both related to stabilize nickel during the DRM reaction performed at high temperature. It is also found that nickel could be better stabilized in form of MgNiO2 over Mg-rich samples. Better nickel dispersion even after the DRM reaction renders the NiMg1 Al1 catalyst as both good activity and stability for the DRM reaction.
Acknowledgements We thank the National Natural Science Foundation of China for funding (Nos. 21073024 and 21373037).
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Please cite this article in press as: Y. Zhu, et al., Effect of Mg/Al ratio of NiMgAl mixed oxide catalyst derived from hydrotalcite for carbon dioxide reforming of methane, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.07.037