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Al2O3 catalyst in thermocatalytic decomposition of methane
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 7 2 0 5 e7 2 1 5
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Effect of metal additives on the catalytic performance of Ni/Al2O3 catalyst in thermocatalytic decomposition of methane Di Wang a,b, Jie Zhang b, Jiangbo Sun b, Weimin Gao a,*, Yanbin Cui b,** a
Key Laboratory of Material Surface Engineering of Jiangxi Province, College of Materials and Electromechanics, Jiangxi Science and Technology Normal University, Nanchang 330000, China b State Key Laboratory of Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
article info
abstract
Article history:
Thermocatalytic decomposition of methane is proposed to be an economical and green
Received 14 November 2018
method to produce COx-free hydrogen and carbon nanomaterial. In present work, 60 wt%
Received in revised form
Ni/Al2O3 catalysts with different additives (Cu, Mn, Pd, Co, Zn, Fe, Mg) were prepared by co-
24 January 2019
impregnation method to investigate promotional effects of these metal additives on the
Accepted 28 January 2019
activity and stability of 60 wt% Ni/Al2O3 and find out a really effective promoter for
Available online 22 February 2019
decomposition of methane. The catalyst was characterized by N2 adsorption/desorption, Xray diffraction, scanning electron microscopy, inductively coupled plasma optical emission
Keywords:
spectrometer and hydrogen temperature programmed reduction. While metal additives
Activity
(5 wt%) were added into 60 wt% Ni/Al2O3, the activity stability of 60 wt% Ni/Al2O3 was
Thermocatalytic decomposition
improved and the CH4 conversion of 60 wt% Ni/Al2O3 was also improved except Zn addi-
Methane
tion. Mn addition was found to improve the catalytic activity of 60 wt% Ni/Al2O3 signifi-
Hydrogen
cantly and the CH4 conversion of 5 wt% Mn-60 wt% Ni/Al2O3 was ~80%. Cu addition was
Ni-based catalyst
found to remarkably improve the catalytic stability of 60 wt% Ni/Al2O3 and the CH4 con-
Carbon nanofiber
version of 5 wt% Cu-60 wt% Ni/Al2O3 decreased from 61% to 45% after 250 min of reaction time. Carbon nanomaterials formed in the thermocatalytic decomposition process were characterized by X-ray diffraction, scanning electron microscopy, thermal gravimetric analyzer and Raman spectroscopy. Carbon deposits consist of amorphous carbon and carbon nanofibers. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The majority of current global energy needs is supplied by combustion of fossil fuels, which releases large quantities of
greenhouse gases (GHG) [1,2], especially carbon dioxide (CO2) and other harmful emissions to the atmosphere [3]. With the continuous depletion of fossil fuels, GHG, CO2 and other harmful emissions resulting from the combustion of fossil fuels cause serious environmental deterioration, such as
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (W. Gao), [email protected] (Y. Cui). https://doi.org/10.1016/j.ijhydene.2019.01.272 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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global warming, climate change and ocean acidification [4]. Hence, it is necessary to search for alternative sustainable, secure and environmentally energy sources. Hydrogen is considered as a clean energy source as it products water without CO2 on its combustion. The amount of energy produced (39.4 kW h kg1) during hydrogen combustion is three times higher than that evolved by any other fuel on a mass basis, e.g. liquid hydrocarbons (13.1 kW h kg1) [5]. There is no natural source for hydrogen. Hydrogen needs to be extracted from various sources (water or hydrocarbons) by decomposition or reformation methods [6,7] and transformed into electricity and other energy forms with low pollution [8]. Many methods were used to produce hydrogen, for instance, steam reforming of methane, partial oxidation, water electrolysis, and biomass gasification, etc [5,9]. Electrolysis of water and photocatalytic water splitting are common methods to produce hydrogen from water [10]. But the simultaneous production of oxygen needs a further separation and purification of hydrogen from oxygen. Besides, the electrolysis of water is associated with high cost and the photocatalytic water splitting suffers from the limitation of photo conversion efficiency [7,11]. The main commercial hydrogen production process is based on steam reforming and partial oxidation of hydrocarbons and carbonaceous feedstocks, such as natural gas, coal and petroleum [4,12e17]. Compared to other fossil fuels, natural gas is a better feed for the production of hydrogen [18e20], which is widely available [21], easy to handle, and having the highest hydrogen-carbon ratio. The most used industrial process for hydrogen production from methane is steam reforming and partial oxidation of methane (SRM and POM) [22]. However, these processes require more process energy and composed of complicated steps. Besides, a large amount of COx was released with hydrogen, which leads to a difficult separation of H2 and COx [3,23e25]. Therefore, a feasible method for hydrogen production must be adopted to replace the conventional hydrogen production methods. Thermalcatalytic decomposition (TCD) of methane is one step process for COx-free hydrogen production and pure hydrogen is the only one gaseous product [26]. Besides, carbon nanomaterials (carbon nanofibers (CNFs) and carbon nanotubes (CNTs)) were also formed as by-products. Carbon nanomaterials are a commercially valuable material and they are useful in many fields [27], such as gas storage, composite additives, catalyst support and catalyst, etc [3,5,10]. As the most economical hydrogen production method, TCD process has been extensively studied since 1960s [28e31]. TCD route is much simpler than SRM process. The complicated separation system is needed for SRM but it is no need for TCD. Due to its strong CeH bond (440 kJ/mol) and high symmetry of molecular structure, decomposition of methane can only occur at high temperature over 1300 C in the absent of catalyst [32e34]. Therefore, various catalysts (including metal and carbonaceous catalysts) are used to decrease the activation energy of methane decomposition, lower operation temperature, and get higher methane crack rates [35]. Heterogeneous metalbased catalysts were highly effective for hydrogen production and carbon deposition in the form of nanofilaments [36]. Ni, Co and Fe (Ni > Co > Fe) [3] gained major attention for the TCD reaction for their low cost, good catalytic activity and
stability [37,38]. Due to its distinctive 3 d-orbital structure [39], Ni is considered as a promising candidate in methane decomposition reaction within low reaction temperature (500e700 C). On the other hands, Ni catalysts without support are not fully active for TCD reaction due to the thermal sintering [40,41]. So the carrier plays a vital role in the performance and stability of Ni catalyst. Al2O3 has been generally used as catalyst support. It has good pore size distribution, large pore volume and specific surface area, and various crystalline forms. Ni-based catalysts are easily deactivated at high temperature due to carbon encapsulation on the external surface of catalyst. The accumulated carbon on Ni-based catalysts tends to encapsulated instead of growing as CNFs or CNTs [42]. Due to fast deactivation observed with Ni-based catalysts, different metal additions have been tested to enhance the catalytic activity and stability of Ni-based catalysts [43,44]. Wang [38] et al. studied methane decomposition over Ni/SiO2 and NieFe/SiO2 catalysts. The experimental results indicated that the lifetime of NieFe/SiO2 catalysts was much longer than Ni/SiO2 catalyst at high reaction temperature and the structure of the carbon filaments formed over Ni/ SiO2 and NieFe/SiO2 was quite different. Metal promoters play a significant role in the catalytic performance of the catalysts for the TCD of methane. In present work, seven additives (Cu, Mn, Pd, Co, Zn, Fe, Mg) were tested as promoters to improve the catalytic activity and stability of 60 wt% Ni/Al2O3 and find out a really effective promoter for TCD of methane. To the best of our knowledge, Mn, Zn and Mg promoted Ni/Al2O3 catalysts for TCD of methane was not reported before. Moreover, the catalytic performance of Ni/Al2O3 catalysts with seven different metal additions was reported in this work. The experimental results indicated that the catalytic activity and stability of 60 wt% Ni/Al2O3 was improved with metal addition (Cu, Mn, Pd, Co, Fe, Mg). Mn addition was found to enhance the catalytic activity of 60 wt% Ni/Al2O3 significantly and Cu addition was found to remarkably improve the catalytic stability of 60 wt% Ni/Al2O3.
Experimental Catalyst preparation In this study, a series of 5 wt% M-60 wt% Ni/Al2O3 (M ¼ Cu, Mn, Pd, Co, Zn, Fe or Mg) catalysts were prepared by conventional co-impregnation technique. The nickel and additives (Cu, Mo, Pd, Co, Zn, Fe or Mg) nitrates were dissolved in distilled water. The Mn promoted catalysts were prepared by impregnation with MnCl2$4H2O as precursor. The aqueous solution was mixed with g-Al2O3 at 90 C for 3 h. Then, the mixture was dried in an oven at 110 C overnight and calcined at 450 C for 4 h in a muffle furnace.
Catalyst testing TCD of methane was evaluated in a fixed-bed reactor under atmospheric pressure. For each test, 50 mg catalyst was placed in the middle of the reactor. The reactor was heated up from room temperature to 650 C with a ramping rate of 10 C/ min. The catalyst was reduced by 40 mL/min flow of H2/N2
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(volume ratio of 1:1) at 650 C for 2 h. Then, nitrogen was fed into the reactor to purge residual. A pure methane (99.99%) feed steam (20 mL/min) was introduced into the reactor to accomplish the TCD of methane at 650 C. The gaseous products were analyzed by online gas chromatography (GC9610) equipped with a thermal conductivity detector (TCD).
Characterization The textural properties of the fresh catalysts were measured by N2 adsorption/desorption at 196 C in a Micrometrics ASAP 2020 HD88 apparatus. Before adsorption, the samples (100 mg) were degassed at 200 C under vacuum condition for 10 h to eliminate moisture and impurities physically adsorbed on the surface of catalyst. The specific surface areas and pore volumes were calculated by applying the BET equation to the respective N2 adsorption isotherms. Powder X-ray diffraction (XRD) of fresh, reduced and used catalysts were obtained using a PANalytical's X'pert PRO MPD with a monochromatic Cu-Ka radiation operated at 40 kV and 40 mA. The analysis was performed from 5 to 90 with a step size of 0.02 /min. Hydrogen temperature programmed reduction (H2-TPR) measurement was carried out in a TPR apparatus with a TCD (AutoChem II2920). In each test, 100 mg sample was used and heated under Ar atmosphere (20 mL/min) at 200 C for 1 h to remove the absorbed carbonates and hydrates on the surface of fresh catalyst. And then the catalyst was cooled down to room temperature and a mixed gas flow of 10% H2/90% Ar (v/v, 40 mL/min) was passed through the cell. The H2-TPR profiles were recorded by heating the catalyst from room temperature to 800 C at a rate of 10 C/min under a flow rate of 40 mL/min of a 10% H2/90% Ar mixture. The morphological appearance of the catalyst and deposited carbon was studied with a scanning electron microscopy (SEM) (Hitachi SU8020) operated at 5 kV accelerating voltage. Raman spectra of the used catalysts and deposited carbon was performed at room temperature using Renishaw inVia Raman Microscope with a wavelength of 532 nm from 100 to 2000 cm1. The amount of deposited carbon on the catalyst was also analyzed using a thermal gravimetric analyzer (TGA, 7300) for the spent catalyst. 5 mg of a spent sample was heated under air flow (100 mL/min) from room temperature to 900 C at a rate of 10 C/min. The deposited carbon amount was calculated based on the weight loss. The actual amount of the metal additives in Ni/Al2O3 catalysts was determined by inductively coupled plasma optical emission spectrometer (ICP-OES) (Thermo Icap 6300). The catalyst was dissolved in nitric acid and diluted with deionized water to prepare a solution with a concentration of 10 ppm. The standard solution is diluted with deionized water to prepare solutions with concentration of 1, 10, 20, 50 ppm.
Results and discussion Catalyst characterization Fig. 1 was N2 adsorption/desorption isotherms and pore size distributions of fresh 5 wt% M-60 wt% Ni/Al2O3 catalysts
Fig. 1 e N2 adsorption/desorption isotherms and pore size distributions of fresh catalyst sample. (1) 60 wt% Ni/Al2O3, (2) 5 wt% Cu-60 wt% Ni/Al2O3, (3) 5 wt% Zn-60 wt% Ni/ Al2O3, (4) 5 wt% Co-60 wt% Ni/Al2O3, (5) 5 wt% Fe-60 wt% Ni/Al2O3, (6) 5 wt% Mn-60 wt% Ni/Al2O3, (7) 5 wt% Mg-60 wt % Ni/Al2O, (8) 5 wt% Pd-60 wt% Ni/Al2O3.
(M ¼ Cu, Mn, Pd, Co, Zn, Fe, Mg), respectively. Due to multilayer adsorption, capillary filling and condensation, there has a sharp increase in N2 adsorption/desorption isotherm of the catalysts when the relative pressure (p/p0) was higher than 0.6. This phenomenon was created by the condition at which the adsorption temperature was lower than the critical temperature of adsorbate [45]. According to IUPAC standard, gAl2O3 support exhibited type Ⅳ isotherm with H2 type hysteresis loop (see Supporting Information, Fig. S1), which depicted the mesoporous structure and cylindrical-shaped channels of g-Al2O3 [46]. The N2 adsorption-desorption curves of the catalysts were classified as type Ⅴ with H3 hysteresis loop, which revealed the catalysts have mesoporous structure and agglomerated particles with slit shaped and non-uniform sizes pores [46]. Corresponding BET surface areas, average pore diameters and pore volumes derived from Fig. 1 and Fig. S1 were summarized in Table 1. As shown in Table 1, g-Al2O3 has a high surface area (168 m2/g) and an average pore size of 12.76 nm, which was beneficial for the dispersion of metal additive at high loading [47]. After loading Ni on g-Al2O3 support, the specific surface area and pore volume of g-Al2O3 were
Table 1 e Structural properties of the support and catalysts. Catalyst
g-Al2O3 60 wt% Ni/Al2O3 5 wt% Fe-60 wt% Ni/Al2O3 5 wt% Mn-60 wt% Ni/Al2O3 5 wt% Cu-60 wt% Ni/Al2O3 5 wt% Co-60 wt% Ni/Al2O3 5 wt% Mg-60 wt% Ni/Al2O3 5 wt% Zn-60 wt% Ni/Al2O3 5 wt% Pd-60 wt% Ni/Al2O3
BET surface area (m2/g)
Pore volume (cm3/g)
Pore size (nm)
168.18 87.55 89.26 87.28 88.25 81.57 69.36 67.22 64.76
0.68 0.24 0.25 0.28 0.27 0.22 0.15 0.20 0.29
12.76 9.97 9.21 11.63 9.74 8.72 9.18 9.11 12.72
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decreased from 168 to 0.68 to 87.55 and 0.24, respectively. Compared with 60 wt% Ni/Al2O3, the pore volume of 5 wt% M60 wt% Ni/Al2O3 (M ¼ Fe, Mn and Cu) was increased after Fe, Mn and Cu were added. But the specific surface area of 5 wt% M-60 wt% Ni/Al2O3 (M ¼ Fe, Mn and Cu) has little change compared with 60 wt% Ni/Al2O3. Compared with 60 wt% Ni/ Al2O3, the average pore size of 5 wt% Mn-60 wt% Ni/Al2O3 was increased. After loading Pd, Co, Mg and Zn into Ni/Al2O3, the specific surface area, pore size and pore volume of 5 wt% M60 wt% Ni/Al2O3 (M ¼ Co, Mg and Zn) decreased. Compared with 60 wt% Ni/Al2O3, the specific surface area of 5 wt% Pd60 wt% Ni/Al2O3 was decreased but the pore size and pore volume were increased. H2-TPR is an important method to characterize the reducibility and metal-support interaction of catalyst. It can also be used to confirm the suitable reduction temperature of catalyst [48]. Fig. 2 was H2-TPR profiles of fresh 5 wt% M-60 wt % Ni/Al2O3 catalysts (M ¼ Cu, Mn, Pd, Co, Zn, Fe, Mg) which were calcined at 450 C. As shown in Fig. 2, a reduction peak at 418 C and a shoulder peak at 470 C were observed for 60 wt% Ni/Al2O3 catalyst. Generally, pure NiO reduction temperature was around 400e420 C. Therefore, the peak in H2-TPR curve at 418 C for 60 wt% Ni/Al2O3 catalyst was attributed to the reduction of bulk NiO particles which have no interaction with the support. The peak in H2-TPR curve at 470 C for 60 wt% Ni/Al2O3 catalyst was assigned to the reduction of NiO particles which has strong interaction with the support [49]. Thus, the two peaks of 60 wt% Ni/Al2O3 catalyst detected by H2-TPR profile were respectively ascribed to the reduction of bulk NiO and NiO in intimate contact with Al2O3 support [50]. Besides, there had none NiAl2O4 peaks in H2-TPR profile of 60 wt% Ni/Al2O3, which indicated that bulk NiAl2O4 was not formed. This was possibly due to the low calcination temperature (450 C) used in this study or to the fact that surface NiAl2O4 has a different
Table 2 e Summary of the H2-TPR results over the prepared catalysts. Sample
Peak temperatures at maximum ( C) Peak 1
Peak 2
418.5 206.2 230.1 343.4 354 397 410.5 430.7
471.4 344.7 388.5 390.1 637.6 e e e
60%Ni/Al2O3 5%Cue60%Ni/Al2O3 5%Coe60%Ni/Al2O3 5%Pde60%Ni/Al2O3 5%Mge60%Ni/Al2O3 5%Zne60%Ni/Al2O3 5%Fee60%Ni/Al2O3 5%Mne60%Ni/Al2O3
reducibility than the bulk [51]. For 5 wt% Cu-60 wt% Ni/Al2O3, the H2-TPR profiles shown two reduction peaks at around 210 C and 345 C. The tiny peak at 210 C was assigned to the reduction of cooper oxide [52]. The peak at 345 C could be related to the reduction of NiO with weak interaction with Al2O3 support. As shown in Fig. 2 and Table 2, the H2-TPR profile of 5 wt% Cu-60 wt% Ni/Al2O3 indicates that the addition of Cu into Ni/Al2O3 catalyst improved the reducibility of Ni/Al2O3 catalyst and shifted the reduction temperature of Ni/Al2O3 catalyst to lower temperatures. As shown in H2-TPR profile of 5 wt% Mg-60 wt% Ni/Al2O3, the reduction temperature of the catalysts was shifted to higher temperature. The H2-TPR profile of 5 wt% Mg-60 wt% Ni/Al2O3 catalyst exhibited two reduction peaks at 355 and 400e750 C, which could be attributed to the reduction of NiO with weak and strong interaction with catalyst support, respectively. For 5 wt% Mn-60 wt% Ni/Al2O3 catalyst, it is shown in H2-TPR curve that the reduction temperature of the catalyst did not change significantly compared with that of Ni/Al2O3 catalyst. The reduction peak centered at 430 C was attributed to the reduction of mixed oxide (NiO and MnxOy). The H2-TPR
Fig. 2 e H2-TPR profiles of fresh catalysts calcined at 450 C.
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Fig. 3 e XRD patterns of fresh (a) and reduced (b)catalysts. (1) 60 wt% Ni/Al2O3, (2) 5 wt% Cu-60 wt% Ni/Al2O3, (3) 5 wt % Zn-60 wt% Ni/Al2O3, (4) 5 wt% Co-60 wt% Ni/Al2O3, (5) 5 wt% Fe-60 wt% Ni/Al2O3, (6) 5 wt% Mn-60 wt% Ni/Al2O3, (7) 5 wt% Mg-60 wt% Ni/Al2O, (8) 5 wt% Pd-60 wt% Ni/ Al2O3.
profile of 5 wt% Zn-60 wt% Ni/Al2O3 was similar to that of Ni/ Al2O3 and reducibility of Ni-based catalyst was decreased with the addition of Zn. Two peaks were observed in H2-TPR profile of 5 wt% Pd-60 wt% Ni/Al2O3 catalyst at 340 C and 390 C, which could be attributed to the reduction of PdO and NiO, respectively. With the addition of Co, Fe, the reducibility of Ni-based catalysts was improved and the 5 wt% Co-60 wt% Ni/Al2O3 catalyst had lower reduction temperature compared with the FeeNi catalysts. XRD (Fig. 3) were used to clarify the crystallization of fresh and reduced 60 wt% Ni/Al2O3 and 5 wt% M-60 wt% Ni/ Al2O3 catalysts. For the fresh catalysts (Fig. 3a), the observed peaks at 37.2 , 43.3 , 62.8 , 75.4 and 79.5 were detected to NiO phase, which indicted that nickel nitrate was decomposed to NiO after the catalysts was calcined at 450 C. As can be seen in Fig. 3a, the diffraction peaks detected at 37.2 , 43.3 of 5 wt% Cu-60 wt% Ni/Al2O3 catalyst were assigned to NiO and NixCu(1-x)O phase with overlapped peaks. Meanwhile, MnO3 phase was detected for 5 wt% Mo-60 wt% Ni/ Al2O3 catalyst at 37.2 with low intensity which was due to
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its good dispersion. For 5 wt% Zn-60 wt% Ni/Al2O3 catalyst, the diffraction peaks at 37.2 , 75.4 and 79.5 can be related to NiO and NixZn(1-x)O phase with overlapped peaks. In the XRD pattern of 5 wt% Mg-60 wt% Ni/Al2O3 catalyst, the diffraction peaks at 37.2 , 62.8 , 75.4 and 79.5 can be attributed to MgO phase. None diffraction peaks related to MxOy compounds and M-Ni alloys were observed in 5 wt% M-60 wt% Ni/Al2O3 (M ¼ Co, Fe, Pd) catalysts, which may due to the good dispersion of Co, Fe and Pd. Fig. 3b was the XRD patterns of the catalysts reduced at 650 C under H2/N2 atmosphere. The diffraction peaks of Ni phases were detected at 2q ¼ 44.4 , 51.8 , 76.3 for the reduced catalysts, which revealed Ni2þ phase in the fresh catalysts were reduced to metallic Ni phases by reduction and Ni was the active site for the catalytic cracking of methane. For 5 wt% Fe-60 wt% Ni/Al2O3, the appearance of NiFe3 was confirmed by the observed diffraction peaks at 51.8 and 76.3 , which indicated that NieFe alloy was formed in the catalyst. The diffraction peak intensity of NiFe3 was low, which was not detected or covered by the diffraction peak of Ni. For other 5 wt% M-60 wt% Ni/Al2O3 (M ¼ Cu, Mn, Pd, Co, Zn, Mg) catalyst, no other species except Ni were detected, which indicated that the metal particles were became smaller and had better dispersion after reduction. The morphology of the fresh 60 wt% Ni/Al2O3 and 5 wt% M60 wt% Ni/Al2O3 (M ¼ Cu, Mn, Pd, Co, Zn, Fe, Mg) catalysts was studied by SEM (Fig. 4). As shown in Fig. 4, the catalysts exhibited a porous structure. The porosity could be aroused from the escape of gas during the calcination of catalyst [53]. For 60 wt% Ni/Al2O3 catalyst, NiO species were tended to agglomerate and formed large randomly-sized cluster due to the weak metal-support interaction [54]. The 5 wt% M-60 wt% Ni/Al2O3 (M ¼ Cu, Mn, Pd, Co, Zn, Fe, Mg) catalysts were found to be mainly exists as particles, which were more or less spherical in shape. The 5 wt% M-60 wt% Ni/Al2O3 (M ¼ Mn, Pd) catalyst was found to be mainly exists as spherical particles and relatively uniform in size. For the 5 wt% M-60 wt% Ni/ Al2O3 (M ¼ Cu, Zn, Co, Fe) catalysts, the SEM images showed the catalysts were agglomerated and the particle size was irregular. As shown in Fig. 4g, the 5 wt% Mg-60 wt% Ni/Al2O3 catalyst has floral structure which was formed with lamellar structure. Table 3 is the actually contents of the metal components in the catalysts. Due to the limitation of wet impregnation method, the actually content of the metal in catalyst has a certain deviation from the set value.
Catalysts performance The TCD reaction over Ni-based catalysts were carried out at 650 C with a CH4 flow rate of 20 mL/min. Fig. 5 shows the effect of different additives (Cu, Mn, Pd, Co, Zn, Fe, Mg) on the methane conversion with time on steam over different catalysts. The methane conversion of the 60 wt% Ni/Al2O3 catalyst was also presented in Fig. 5 for comparison. For 60 wt % Ni/Al2O3 catalyst, the methane conversion was only 50% at 12 min time on stream. Then, the 60 wt% Ni/Al2O3 catalyst deactivated rapidly and the methane conversion decreased to 10% with time in the region of 63e87 min. As the rate of carbon production at NieCH4 interface is higher than that of
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Fig. 4 e SEM images of the fresh catalysts. (a) 60 wt% Ni/Al2O3, (b) 5 wt% Cu-60 wt% Ni/Al2O3, (c) 5 wt% Zn-60 wt% Ni/Al2O3, (d) 5 wt% Co-60 wt% Ni/Al2O, (e) 5 wt% Fe-60 wt% Ni/Al2O3, (f) 5 wt% Mn-60 wt% Ni/Al2O3, (g) 5 wt% Mg-60 wt% Ni/Al2O3, (h) 5 wt % Pd-60 wt% Ni/Al2O3.
carbon transfer through Ni particle, carbon is nucleated and deposited on the surface of Ni particle [3,55]. The active sites of the 60 wt% Ni/Al2O3 catalyst are encapsulated by carbon with time on steam and the access of reactants to active sites are hindered. The lifetime of Ni-based catalyst in methane catalytic decomposition is relatively short, especially at temperatures higher than 600 C [56e62]. In our work, the
reaction was carried out at 650 C. Therefore, the 60 wt% Ni/ Al2O3 catalyst was deactivated quickly. Compared with 60 wt % Ni/Al2O3 catalyst, the methane conversion for the 5 wt%
Table 3 e The actual amount of the promoted metals characterized by ICP-OES. Sample 60%Ni/Al2O3 5%Cue60%Ni/Al2O3 5%Zne60%Ni/Al2O3 5%Coe60%Ni/Al2O3 5%Fee60%Ni/Al2O3 5%Mne60%Ni/Al2O3 5%Mge60%Ni/Al2O3 5%Pde60%Ni/Al2O3
Ni content (%) 54.34 56.17 70.46 73.50 64.40 61.66 66.04 69.23
Amount of the metal addition (%) e 3.69 3.75 4.22 3.69 3.81 3.72 3.88
(Cu) (Zn) (Co) (Fe) (Mn) (Mg) (Pd)
Fig. 5 e Effect of different 5 wt% M-60 wt% Ni/Al2O3 catalysts on methane conversion.
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Fig. 6 e XRD patterns of used catalysts after methane decomposition. (1) 60 wt% Ni/Al2O3, (2) 5 wt% Cu-60 wt% Ni/ Al2O3, (3) 5 wt% Zn-60 wt% Ni/Al2O3, (4) 5 wt% Co-60 wt% Ni/Al2O3, (5) 5 wt% Fe-60 wt% Ni/Al2O3, (6) 5 wt% Mn-60 wt % Ni/Al2O3, (7) 5 wt% Mg-60 wt% Ni/Al2O, (8) 5 wt% Pd-60 wt % Ni/Al2O3.
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M-60 wt% Ni/Al2O3 (M ¼ Cu, Mn, Pd, Co, Fe, Mg) catalysts except 5 wt% Zn-60 wt% Ni/Al2O3 was increased. The methane conversion was decreased slightly to 48% by the addition of Zn to Ni/Al2O3. Compared with 60 wt% Ni/Al2O3 catalyst, the methane conversion of 5 wt% M-60 wt% Ni/ Al2O3 (M ¼ Co, Fe and Mg) catalysts was increased to 56.7%, 52.6% and 56.3% by the addition of Co, Fe and Mg into Ni/ Al2O3 catalyst. The catalytic activity of 60 wt% Ni/Al2O3 was improved significantly by adding Mn and the methane conversion of 5 wt% Mn-60 wt% Ni/Al2O3 was increased to ~80%. As indicates in BET results (Table 1), 5 wt% Mn-60 wt% Ni/ Al2O3 catalyst has a relatively large specific surface area, pore volume and pore size. This may be the reason for the good activity of MneNi/Al2O3 compared with Ni/Al2O3 catalyst at the initial stage of methane TCD reaction. It is suggested that Mn promotes the movement of Ni cations to the surface of catalyst during reduction process [63]. The tendency of methane conversion with time on stream for 5 wt% Mn-60 wt % Ni/Al2O3 was similar to that of 60 wt% Ni/Al2O3 catalyst. The catalytic activities all decreased rapidly with time on stream. The stability of the 5 wt% M-60 wt% Ni/Al2O3 (M ¼ Co, Fe, Mg, Pd, Cu and Zn) catalysts was improved obviously by
Fig. 7 e SEM images of carbonaceous products formed on 60 wt% Ni/Al2O3 and 5 wt% M-60 wt% Ni/Al2O3 (M ¼ Cu, Mn, Pd, Co, Zn, Fe, Mg). (a) 60 wt% Ni/Al2O3, (b) 5 wt% Cu-60 wt% Ni/Al2O3, (c) 5 wt% Zn-60 wt% Ni/Al2O3, (d) 5 wt% Co-60 wt% Ni/Al2O, (e) 5 wt% Fe-60 wt% Ni/Al2O3, (f) 5 wt% Mn-60 wt% Ni/Al2O3, (g) 5 wt% Mg-60 wt% Ni/Al2O3, (h) 5 wt% Pd-60 wt% Ni/Al2O3.
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adding Co, Fe, Mg, Pd, Cu and Zn additives into Ni/Al2O3 catalyst. Compared with 60 wt% Ni/Al2O3, the 5 wt% M-60 wt % Ni/Al2O3 (M ¼ Co, Fe and Mg) catalysts have longer lifetime. The methane conversion of 5 wt% M-60 wt% Ni/Al2O3 (M ¼ Co, Fe and Mg) catalysts was over 30% at 3 h time of steam. It was reported that the bimetallic catalysts (such as NieFe) exhibited long lifetime [30,64e68]. The rate of carbon diffusion through iron was three times higher than that of Ni, which was beneficial to carbon transfer through active sites [65]. Therefore, active sites of 5 wt% Fe-60 wt% Ni/Al2O3 were available and the lifetime was increased by adding Fe into Ni/ Al2O3. The lifetime of the Ni-based catalysts was improved remarkably by adding of Cu and Pd into Ni/Al2O3. The methane conversion of 5 wt% Cu-60 wt% Ni/Al2O3 decreased from 61% to 45% after 250 min time of stream. The methane conversion of 5 wt% Pd-60 wt% Ni/Al2O3 catalyst decreased from 61% to 32% at 230 min time on stream. The Cu addition can be effective in facilitating the reducibility of Ni2þ species and prevents the formation of encapsulating carbon on Ni surface [69]. Consequently, the stability of 5 wt% Cu-60 wt% Ni/Al2O3 catalyst was prolonged. Saraswat et al. [25] found that the performance of NieCu catalyst has higher performance and stability than Ni catalyst in the TCD of methane. It was proved that the formation of mixed metal oxide is the reason for the high performance and stability of NieCu catalyst. Yokoyama et al. found that the rate of carbon diffusion in Pd was faster than that of Ni, which lead to fast carbon migration and long catalyst lifetime [70]. The methane conversion of 5 wt% Zn-60 wt% Ni/Al2O3 catalyst is similar to that of 60 wt% Ni/Al2O3 catalyst. But the lifetime of 5 wt% Zn-60 wt% Ni/Al2O3 catalyst was enhanced significantly by Zn addition. For example, the methane conversion of 5 wt% Zn-60 wt% Ni/Al2O3 decreased from 48% to 33% after 250 min time of steam. The addition of Zn into Ni/Al2O3 can increase the growth rate of carbon fiber or carbon nanotube and prevent the formation of amorphous carbon over active
sites [71]. Consequently, the lifetime of 5 wt% Zn-60 wt% Ni/ Al2O3 catalyst was prolonged.
Characterization of deposited carbon X-ray is an important tool for the evaluation of the deposited carbon over the catalysts. Fig. 6 was the XRD patterns of the used catalysts after the catalytic decomposition of methane. Two obviously diffraction peaks at 26.1 and 43.7 were detected for all catalysts, which can be attributed to the growth of graphitic-like carbon on the surface of catalysts. Meanwhile, the metallic nickel phase was detected at 2q ¼ 44.4 , 51.8 , 76.3 for the used catalysts. The produced carbon was collected at the end of the reaction and the morphology of the deposited carbon were characterized via SEM. Fig. 7 was the SEM images of the produced carbon formed on 60 wt% Ni/Al2O3 and 5 wt% M-60 wt% Ni/Al2O3 catalysts after methane decomposition. As shown in Fig. 7, CNFs were formed on the 5 wt% M-60 wt% Ni/Al2O3 (M ¼ Mg, Pd, Co, Cu and Mn, Zn, Fe) catalysts. A small amount of amorphous carbon was also found on the surface of CNFs. The diameter of CNFs varied from hundred to thousand nanometers and the length of CNF were also different. As shown in Fig. 7g, the CNFs formed on MgeNi/Al2O3 have clean surface and no amorphous carbon was found on the surface of CNFs. The diameter of CNFs deposited on CoeNi/Al2O3 catalyst were around 350 nm. For ZneNi/Al2O3 catalysts, the average diameter of the CNFs was ~1 mm. The CNFs formed on FeeNi/Al2O3 catalyst were very short. The average diameter of CNFs formed on FeeNi/Al2O3 were close to that of CNFs formed on ZneNi/Al2O3 catalyst. TG analysis is often used to characterized the purity and thermal stability of deposited carbon [72]. Fig. 8 was the weight loss of the deposited carbon over 5 wt% M-60 wt% Ni/Al2O3 catalysts. The overall weight loss of the used catalysts was due to the combustion of carbon in air. Different forms of carbon
Fig. 8 e Thermogravimetric curves of the nanocarbon deposited over the catalysts.
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have different oxidation temperatures and the weight loss in TG curves below 450 C is mainly related to the combustion of amorphous carbon [73]. As shown in Fig. 8, all TG curves only presented a single-step degradation. The weight loss in TG curves below 450 C was very low, which indicated only few amorphous carbon were deposited on catalysts. The sharp drop in TG curves between 500 and 680 C was due to the combustion of CNFs. The deposited carbon yield calculated from TG curves was depicted in Table 4. Compared with other catalysts, the largest weight loss of deposited carbon was obtained over 5 wt % Cu-60 wt% Ni/Al2O3, which was in good agreement with the high activity and long lifetime of CueNi/Al2O3 catalyst. It has been reported that Cu could improve the activity of Ni-based catalyst for methane decomposition and thus increase the accumulation of carbon on catalyst surface [69]. Raman spectroscopy is a useful method to characterize the crystallinity and graphitization of carbon material. Fig. 9 was Raman spectra of the deposited carbon on different 5 wt% M-60 wt% Ni/Al2O3 catalysts. Two distinct bands, D band (1350 cm1) and G band (1580 cm1), were observed in Raman spectra of each sample. The G band could be attributed to in-plane CeC stretching vibrations of graphite layers and the D band was ascribed to the structural
Table 5 e ID/IG value of carbon Raman spectra produced by methane decomposition over different Ni-based catalysts. Catalyst Sample 60 wt% Ni/Al2O3 5 wt% Mn-60 wt% Ni/Al2O3 5 wt% Cu-60 wt% Ni/Al2O3 5 wt% Mg-60 wt% Ni/Al2O3 5 wt% Fe-60 wt% Ni/Al2O3 5 wt% Pd-60 wt% Ni/Al2O3 5 wt% Zn-60 wt% Ni/Al2O3 5 wt% Co-60 wt% Ni/Al2O3
ID
IG
ID/IG
2231.335 2496.138 7826.59 2814.852 2367.979 2907.316 2446.479 6085.232
2119.153 2205.251 7042.49 2644.689 2398.423 2998.029 2609.139 6637.229
1.053 1.132 1.118 1.064 0.987 0.970 0.938 0.917
imperfection of graphite [74,75]. The ratio of ID/IG is used to measure the graphitization degree of deposited carbon on catalyst [58]. A small value of ID/IG indicated the graphitization degree of deposited carbon was high [76]. As shown in Table 5, the CNFs deposited on 5 wt% Co-60 wt% Ni/Al2O3 catalyst has highest graphitization degree and the CNFs deposited on 5 wt% Mn-60 wt% Ni/Al2O3 catalysts has lowest graphitization degree.
Conclusion Table 4 e The yield values of carbon nanomaterials. Catalyst Sample 60 wt% Ni/Al2O3 5 wt% Cu-60 wt% Ni/Al2O3 5 wt% Zn-60 wt% Ni/Al2O3 5 wt% Co-60 wt% Ni/Al2O3 5 wt% Fe-60 wt% Ni/Al2O3 5 wt% Mn-60 wt% Ni/Al2O3 5 wt% Mg-60 wt% Ni/Al2O3 5 wt% Pd-60 wt% Ni/Al2O3
residue (%)
weight loss (%)
Carbon yield (%)
21.9 4.1 30.9 11.5 50.4 30.1 28 22.7
78.1 95.9 69.1 88.5 49.6 69.9 72 77.3
356.62 2339.02 223.62 769.57 98.41 232.23 257.14 340.53
In this work, seven 5 wt% M-60 wt% Ni/Al2O3 catalysts (M ¼ Cu, Mn, Pd, Co, Zn, Fe, Mg) were prepared by coimpregnation method to investigate promotional effects of these metal additives on the activity and stability of 60 wt% Ni/Al2O3 for TCD of methane. The additives of 5 wt% M-60 wt % Ni/Al2O3 catalysts have a significant influence on methane conversion in TCD reaction. While the addition of metal additives (5 wt%), the activity stability of 60 wt% Ni/Al2O3 was improved and the CH4 conversion of 60 wt% Ni/Al2O3 was also improved except Zn addition. And Mn addition was found to improve the catalytic activity of 60 wt% Ni/Al2O3 significantly and the CH4 conversion of 5 wt% Mn-60 wt% Ni/ Al2O3 was ~80%. Cu addition was found to remarkably improve the catalytic stability of 60 wt% Ni/Al2O3 and the CH4 conversion of 5 wt% Cu-60 wt% Ni/Al2O3 decreased from 60% to 45% after 250 min of reaction time. CNFs were formed on all Ni-based catalysts. CNFs formed on MgeNi/Al2O3 have clean surface and no amorphous carbon was found on the surface of CNFs.
Acknowledgments
Fig. 9 e Raman spectra of as-grown carbon nanofibers generated by methane decomposition over different Nibased catalysts. (1) 60 wt% Ni/Al2O3, (2) 5 wt% Cu-60 wt% Ni/Al2O3, (3) 5 wt% Zn-60 wt% Ni/Al2O3, (4) 5 wt% Co-60 wt % Ni/Al2O3, (5) 5 wt% Fe-60 wt% Ni/Al2O3, (6) 5 wt% Mn60 wt% Ni/Al2O3, (7) 5 wt% Mg-60 wt% Ni/Al2O, (8) 5 wt% Pd60 wt% Ni/Al2O3.
This work has been supported by the Fund of State Key Laboratory of Multiphase Complex Systems (No. MPCS-2017-A-11) and Center for Mesoscience, Institute of Process Engineering, Chinese Academy of Sciences (No. COM2016A003).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.01.272.
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Effect of metal additives on the catalytic performance of Ni/Al2O3 catalyst in thermocatalytic decomposition of methane Di Wang a,b, Jie Zhang b, Jiangbo Sun b, Weimin Gao a,*, Yanbin Cui b,** a
Key Laboratory of Material Surface Engineering of Jiangxi Province, College of Materials and Electromechanics, Jiangxi Science and Technology Normal University, Nanchang 330000, China b State Key Laboratory of Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
article info
abstract
Article history:
Thermocatalytic decomposition of methane is proposed to be an economical and green
Received 14 November 2018
method to produce COx-free hydrogen and carbon nanomaterial. In present work, 60 wt%
Received in revised form
Ni/Al2O3 catalysts with different additives (Cu, Mn, Pd, Co, Zn, Fe, Mg) were prepared by co-
24 January 2019
impregnation method to investigate promotional effects of these metal additives on the
Accepted 28 January 2019
activity and stability of 60 wt% Ni/Al2O3 and find out a really effective promoter for
Available online 22 February 2019
decomposition of methane. The catalyst was characterized by N2 adsorption/desorption, Xray diffraction, scanning electron microscopy, inductively coupled plasma optical emission
Keywords:
spectrometer and hydrogen temperature programmed reduction. While metal additives
Activity
(5 wt%) were added into 60 wt% Ni/Al2O3, the activity stability of 60 wt% Ni/Al2O3 was
Thermocatalytic decomposition
improved and the CH4 conversion of 60 wt% Ni/Al2O3 was also improved except Zn addi-
Methane
tion. Mn addition was found to improve the catalytic activity of 60 wt% Ni/Al2O3 signifi-
Hydrogen
cantly and the CH4 conversion of 5 wt% Mn-60 wt% Ni/Al2O3 was ~80%. Cu addition was
Ni-based catalyst
found to remarkably improve the catalytic stability of 60 wt% Ni/Al2O3 and the CH4 con-
Carbon nanofiber
version of 5 wt% Cu-60 wt% Ni/Al2O3 decreased from 61% to 45% after 250 min of reaction time. Carbon nanomaterials formed in the thermocatalytic decomposition process were characterized by X-ray diffraction, scanning electron microscopy, thermal gravimetric analyzer and Raman spectroscopy. Carbon deposits consist of amorphous carbon and carbon nanofibers. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The majority of current global energy needs is supplied by combustion of fossil fuels, which releases large quantities of
greenhouse gases (GHG) [1,2], especially carbon dioxide (CO2) and other harmful emissions to the atmosphere [3]. With the continuous depletion of fossil fuels, GHG, CO2 and other harmful emissions resulting from the combustion of fossil fuels cause serious environmental deterioration, such as
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (W. Gao), [email protected] (Y. Cui). https://doi.org/10.1016/j.ijhydene.2019.01.272 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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global warming, climate change and ocean acidification [4]. Hence, it is necessary to search for alternative sustainable, secure and environmentally energy sources. Hydrogen is considered as a clean energy source as it products water without CO2 on its combustion. The amount of energy produced (39.4 kW h kg1) during hydrogen combustion is three times higher than that evolved by any other fuel on a mass basis, e.g. liquid hydrocarbons (13.1 kW h kg1) [5]. There is no natural source for hydrogen. Hydrogen needs to be extracted from various sources (water or hydrocarbons) by decomposition or reformation methods [6,7] and transformed into electricity and other energy forms with low pollution [8]. Many methods were used to produce hydrogen, for instance, steam reforming of methane, partial oxidation, water electrolysis, and biomass gasification, etc [5,9]. Electrolysis of water and photocatalytic water splitting are common methods to produce hydrogen from water [10]. But the simultaneous production of oxygen needs a further separation and purification of hydrogen from oxygen. Besides, the electrolysis of water is associated with high cost and the photocatalytic water splitting suffers from the limitation of photo conversion efficiency [7,11]. The main commercial hydrogen production process is based on steam reforming and partial oxidation of hydrocarbons and carbonaceous feedstocks, such as natural gas, coal and petroleum [4,12e17]. Compared to other fossil fuels, natural gas is a better feed for the production of hydrogen [18e20], which is widely available [21], easy to handle, and having the highest hydrogen-carbon ratio. The most used industrial process for hydrogen production from methane is steam reforming and partial oxidation of methane (SRM and POM) [22]. However, these processes require more process energy and composed of complicated steps. Besides, a large amount of COx was released with hydrogen, which leads to a difficult separation of H2 and COx [3,23e25]. Therefore, a feasible method for hydrogen production must be adopted to replace the conventional hydrogen production methods. Thermalcatalytic decomposition (TCD) of methane is one step process for COx-free hydrogen production and pure hydrogen is the only one gaseous product [26]. Besides, carbon nanomaterials (carbon nanofibers (CNFs) and carbon nanotubes (CNTs)) were also formed as by-products. Carbon nanomaterials are a commercially valuable material and they are useful in many fields [27], such as gas storage, composite additives, catalyst support and catalyst, etc [3,5,10]. As the most economical hydrogen production method, TCD process has been extensively studied since 1960s [28e31]. TCD route is much simpler than SRM process. The complicated separation system is needed for SRM but it is no need for TCD. Due to its strong CeH bond (440 kJ/mol) and high symmetry of molecular structure, decomposition of methane can only occur at high temperature over 1300 C in the absent of catalyst [32e34]. Therefore, various catalysts (including metal and carbonaceous catalysts) are used to decrease the activation energy of methane decomposition, lower operation temperature, and get higher methane crack rates [35]. Heterogeneous metalbased catalysts were highly effective for hydrogen production and carbon deposition in the form of nanofilaments [36]. Ni, Co and Fe (Ni > Co > Fe) [3] gained major attention for the TCD reaction for their low cost, good catalytic activity and
stability [37,38]. Due to its distinctive 3 d-orbital structure [39], Ni is considered as a promising candidate in methane decomposition reaction within low reaction temperature (500e700 C). On the other hands, Ni catalysts without support are not fully active for TCD reaction due to the thermal sintering [40,41]. So the carrier plays a vital role in the performance and stability of Ni catalyst. Al2O3 has been generally used as catalyst support. It has good pore size distribution, large pore volume and specific surface area, and various crystalline forms. Ni-based catalysts are easily deactivated at high temperature due to carbon encapsulation on the external surface of catalyst. The accumulated carbon on Ni-based catalysts tends to encapsulated instead of growing as CNFs or CNTs [42]. Due to fast deactivation observed with Ni-based catalysts, different metal additions have been tested to enhance the catalytic activity and stability of Ni-based catalysts [43,44]. Wang [38] et al. studied methane decomposition over Ni/SiO2 and NieFe/SiO2 catalysts. The experimental results indicated that the lifetime of NieFe/SiO2 catalysts was much longer than Ni/SiO2 catalyst at high reaction temperature and the structure of the carbon filaments formed over Ni/ SiO2 and NieFe/SiO2 was quite different. Metal promoters play a significant role in the catalytic performance of the catalysts for the TCD of methane. In present work, seven additives (Cu, Mn, Pd, Co, Zn, Fe, Mg) were tested as promoters to improve the catalytic activity and stability of 60 wt% Ni/Al2O3 and find out a really effective promoter for TCD of methane. To the best of our knowledge, Mn, Zn and Mg promoted Ni/Al2O3 catalysts for TCD of methane was not reported before. Moreover, the catalytic performance of Ni/Al2O3 catalysts with seven different metal additions was reported in this work. The experimental results indicated that the catalytic activity and stability of 60 wt% Ni/Al2O3 was improved with metal addition (Cu, Mn, Pd, Co, Fe, Mg). Mn addition was found to enhance the catalytic activity of 60 wt% Ni/Al2O3 significantly and Cu addition was found to remarkably improve the catalytic stability of 60 wt% Ni/Al2O3.
Experimental Catalyst preparation In this study, a series of 5 wt% M-60 wt% Ni/Al2O3 (M ¼ Cu, Mn, Pd, Co, Zn, Fe or Mg) catalysts were prepared by conventional co-impregnation technique. The nickel and additives (Cu, Mo, Pd, Co, Zn, Fe or Mg) nitrates were dissolved in distilled water. The Mn promoted catalysts were prepared by impregnation with MnCl2$4H2O as precursor. The aqueous solution was mixed with g-Al2O3 at 90 C for 3 h. Then, the mixture was dried in an oven at 110 C overnight and calcined at 450 C for 4 h in a muffle furnace.
Catalyst testing TCD of methane was evaluated in a fixed-bed reactor under atmospheric pressure. For each test, 50 mg catalyst was placed in the middle of the reactor. The reactor was heated up from room temperature to 650 C with a ramping rate of 10 C/ min. The catalyst was reduced by 40 mL/min flow of H2/N2
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(volume ratio of 1:1) at 650 C for 2 h. Then, nitrogen was fed into the reactor to purge residual. A pure methane (99.99%) feed steam (20 mL/min) was introduced into the reactor to accomplish the TCD of methane at 650 C. The gaseous products were analyzed by online gas chromatography (GC9610) equipped with a thermal conductivity detector (TCD).
Characterization The textural properties of the fresh catalysts were measured by N2 adsorption/desorption at 196 C in a Micrometrics ASAP 2020 HD88 apparatus. Before adsorption, the samples (100 mg) were degassed at 200 C under vacuum condition for 10 h to eliminate moisture and impurities physically adsorbed on the surface of catalyst. The specific surface areas and pore volumes were calculated by applying the BET equation to the respective N2 adsorption isotherms. Powder X-ray diffraction (XRD) of fresh, reduced and used catalysts were obtained using a PANalytical's X'pert PRO MPD with a monochromatic Cu-Ka radiation operated at 40 kV and 40 mA. The analysis was performed from 5 to 90 with a step size of 0.02 /min. Hydrogen temperature programmed reduction (H2-TPR) measurement was carried out in a TPR apparatus with a TCD (AutoChem II2920). In each test, 100 mg sample was used and heated under Ar atmosphere (20 mL/min) at 200 C for 1 h to remove the absorbed carbonates and hydrates on the surface of fresh catalyst. And then the catalyst was cooled down to room temperature and a mixed gas flow of 10% H2/90% Ar (v/v, 40 mL/min) was passed through the cell. The H2-TPR profiles were recorded by heating the catalyst from room temperature to 800 C at a rate of 10 C/min under a flow rate of 40 mL/min of a 10% H2/90% Ar mixture. The morphological appearance of the catalyst and deposited carbon was studied with a scanning electron microscopy (SEM) (Hitachi SU8020) operated at 5 kV accelerating voltage. Raman spectra of the used catalysts and deposited carbon was performed at room temperature using Renishaw inVia Raman Microscope with a wavelength of 532 nm from 100 to 2000 cm1. The amount of deposited carbon on the catalyst was also analyzed using a thermal gravimetric analyzer (TGA, 7300) for the spent catalyst. 5 mg of a spent sample was heated under air flow (100 mL/min) from room temperature to 900 C at a rate of 10 C/min. The deposited carbon amount was calculated based on the weight loss. The actual amount of the metal additives in Ni/Al2O3 catalysts was determined by inductively coupled plasma optical emission spectrometer (ICP-OES) (Thermo Icap 6300). The catalyst was dissolved in nitric acid and diluted with deionized water to prepare a solution with a concentration of 10 ppm. The standard solution is diluted with deionized water to prepare solutions with concentration of 1, 10, 20, 50 ppm.
Results and discussion Catalyst characterization Fig. 1 was N2 adsorption/desorption isotherms and pore size distributions of fresh 5 wt% M-60 wt% Ni/Al2O3 catalysts
Fig. 1 e N2 adsorption/desorption isotherms and pore size distributions of fresh catalyst sample. (1) 60 wt% Ni/Al2O3, (2) 5 wt% Cu-60 wt% Ni/Al2O3, (3) 5 wt% Zn-60 wt% Ni/ Al2O3, (4) 5 wt% Co-60 wt% Ni/Al2O3, (5) 5 wt% Fe-60 wt% Ni/Al2O3, (6) 5 wt% Mn-60 wt% Ni/Al2O3, (7) 5 wt% Mg-60 wt % Ni/Al2O, (8) 5 wt% Pd-60 wt% Ni/Al2O3.
(M ¼ Cu, Mn, Pd, Co, Zn, Fe, Mg), respectively. Due to multilayer adsorption, capillary filling and condensation, there has a sharp increase in N2 adsorption/desorption isotherm of the catalysts when the relative pressure (p/p0) was higher than 0.6. This phenomenon was created by the condition at which the adsorption temperature was lower than the critical temperature of adsorbate [45]. According to IUPAC standard, gAl2O3 support exhibited type Ⅳ isotherm with H2 type hysteresis loop (see Supporting Information, Fig. S1), which depicted the mesoporous structure and cylindrical-shaped channels of g-Al2O3 [46]. The N2 adsorption-desorption curves of the catalysts were classified as type Ⅴ with H3 hysteresis loop, which revealed the catalysts have mesoporous structure and agglomerated particles with slit shaped and non-uniform sizes pores [46]. Corresponding BET surface areas, average pore diameters and pore volumes derived from Fig. 1 and Fig. S1 were summarized in Table 1. As shown in Table 1, g-Al2O3 has a high surface area (168 m2/g) and an average pore size of 12.76 nm, which was beneficial for the dispersion of metal additive at high loading [47]. After loading Ni on g-Al2O3 support, the specific surface area and pore volume of g-Al2O3 were
Table 1 e Structural properties of the support and catalysts. Catalyst
g-Al2O3 60 wt% Ni/Al2O3 5 wt% Fe-60 wt% Ni/Al2O3 5 wt% Mn-60 wt% Ni/Al2O3 5 wt% Cu-60 wt% Ni/Al2O3 5 wt% Co-60 wt% Ni/Al2O3 5 wt% Mg-60 wt% Ni/Al2O3 5 wt% Zn-60 wt% Ni/Al2O3 5 wt% Pd-60 wt% Ni/Al2O3
BET surface area (m2/g)
Pore volume (cm3/g)
Pore size (nm)
168.18 87.55 89.26 87.28 88.25 81.57 69.36 67.22 64.76
0.68 0.24 0.25 0.28 0.27 0.22 0.15 0.20 0.29
12.76 9.97 9.21 11.63 9.74 8.72 9.18 9.11 12.72
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decreased from 168 to 0.68 to 87.55 and 0.24, respectively. Compared with 60 wt% Ni/Al2O3, the pore volume of 5 wt% M60 wt% Ni/Al2O3 (M ¼ Fe, Mn and Cu) was increased after Fe, Mn and Cu were added. But the specific surface area of 5 wt% M-60 wt% Ni/Al2O3 (M ¼ Fe, Mn and Cu) has little change compared with 60 wt% Ni/Al2O3. Compared with 60 wt% Ni/ Al2O3, the average pore size of 5 wt% Mn-60 wt% Ni/Al2O3 was increased. After loading Pd, Co, Mg and Zn into Ni/Al2O3, the specific surface area, pore size and pore volume of 5 wt% M60 wt% Ni/Al2O3 (M ¼ Co, Mg and Zn) decreased. Compared with 60 wt% Ni/Al2O3, the specific surface area of 5 wt% Pd60 wt% Ni/Al2O3 was decreased but the pore size and pore volume were increased. H2-TPR is an important method to characterize the reducibility and metal-support interaction of catalyst. It can also be used to confirm the suitable reduction temperature of catalyst [48]. Fig. 2 was H2-TPR profiles of fresh 5 wt% M-60 wt % Ni/Al2O3 catalysts (M ¼ Cu, Mn, Pd, Co, Zn, Fe, Mg) which were calcined at 450 C. As shown in Fig. 2, a reduction peak at 418 C and a shoulder peak at 470 C were observed for 60 wt% Ni/Al2O3 catalyst. Generally, pure NiO reduction temperature was around 400e420 C. Therefore, the peak in H2-TPR curve at 418 C for 60 wt% Ni/Al2O3 catalyst was attributed to the reduction of bulk NiO particles which have no interaction with the support. The peak in H2-TPR curve at 470 C for 60 wt% Ni/Al2O3 catalyst was assigned to the reduction of NiO particles which has strong interaction with the support [49]. Thus, the two peaks of 60 wt% Ni/Al2O3 catalyst detected by H2-TPR profile were respectively ascribed to the reduction of bulk NiO and NiO in intimate contact with Al2O3 support [50]. Besides, there had none NiAl2O4 peaks in H2-TPR profile of 60 wt% Ni/Al2O3, which indicated that bulk NiAl2O4 was not formed. This was possibly due to the low calcination temperature (450 C) used in this study or to the fact that surface NiAl2O4 has a different
Table 2 e Summary of the H2-TPR results over the prepared catalysts. Sample
Peak temperatures at maximum ( C) Peak 1
Peak 2
418.5 206.2 230.1 343.4 354 397 410.5 430.7
471.4 344.7 388.5 390.1 637.6 e e e
60%Ni/Al2O3 5%Cue60%Ni/Al2O3 5%Coe60%Ni/Al2O3 5%Pde60%Ni/Al2O3 5%Mge60%Ni/Al2O3 5%Zne60%Ni/Al2O3 5%Fee60%Ni/Al2O3 5%Mne60%Ni/Al2O3
reducibility than the bulk [51]. For 5 wt% Cu-60 wt% Ni/Al2O3, the H2-TPR profiles shown two reduction peaks at around 210 C and 345 C. The tiny peak at 210 C was assigned to the reduction of cooper oxide [52]. The peak at 345 C could be related to the reduction of NiO with weak interaction with Al2O3 support. As shown in Fig. 2 and Table 2, the H2-TPR profile of 5 wt% Cu-60 wt% Ni/Al2O3 indicates that the addition of Cu into Ni/Al2O3 catalyst improved the reducibility of Ni/Al2O3 catalyst and shifted the reduction temperature of Ni/Al2O3 catalyst to lower temperatures. As shown in H2-TPR profile of 5 wt% Mg-60 wt% Ni/Al2O3, the reduction temperature of the catalysts was shifted to higher temperature. The H2-TPR profile of 5 wt% Mg-60 wt% Ni/Al2O3 catalyst exhibited two reduction peaks at 355 and 400e750 C, which could be attributed to the reduction of NiO with weak and strong interaction with catalyst support, respectively. For 5 wt% Mn-60 wt% Ni/Al2O3 catalyst, it is shown in H2-TPR curve that the reduction temperature of the catalyst did not change significantly compared with that of Ni/Al2O3 catalyst. The reduction peak centered at 430 C was attributed to the reduction of mixed oxide (NiO and MnxOy). The H2-TPR
Fig. 2 e H2-TPR profiles of fresh catalysts calcined at 450 C.
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Fig. 3 e XRD patterns of fresh (a) and reduced (b)catalysts. (1) 60 wt% Ni/Al2O3, (2) 5 wt% Cu-60 wt% Ni/Al2O3, (3) 5 wt % Zn-60 wt% Ni/Al2O3, (4) 5 wt% Co-60 wt% Ni/Al2O3, (5) 5 wt% Fe-60 wt% Ni/Al2O3, (6) 5 wt% Mn-60 wt% Ni/Al2O3, (7) 5 wt% Mg-60 wt% Ni/Al2O, (8) 5 wt% Pd-60 wt% Ni/ Al2O3.
profile of 5 wt% Zn-60 wt% Ni/Al2O3 was similar to that of Ni/ Al2O3 and reducibility of Ni-based catalyst was decreased with the addition of Zn. Two peaks were observed in H2-TPR profile of 5 wt% Pd-60 wt% Ni/Al2O3 catalyst at 340 C and 390 C, which could be attributed to the reduction of PdO and NiO, respectively. With the addition of Co, Fe, the reducibility of Ni-based catalysts was improved and the 5 wt% Co-60 wt% Ni/Al2O3 catalyst had lower reduction temperature compared with the FeeNi catalysts. XRD (Fig. 3) were used to clarify the crystallization of fresh and reduced 60 wt% Ni/Al2O3 and 5 wt% M-60 wt% Ni/ Al2O3 catalysts. For the fresh catalysts (Fig. 3a), the observed peaks at 37.2 , 43.3 , 62.8 , 75.4 and 79.5 were detected to NiO phase, which indicted that nickel nitrate was decomposed to NiO after the catalysts was calcined at 450 C. As can be seen in Fig. 3a, the diffraction peaks detected at 37.2 , 43.3 of 5 wt% Cu-60 wt% Ni/Al2O3 catalyst were assigned to NiO and NixCu(1-x)O phase with overlapped peaks. Meanwhile, MnO3 phase was detected for 5 wt% Mo-60 wt% Ni/ Al2O3 catalyst at 37.2 with low intensity which was due to
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its good dispersion. For 5 wt% Zn-60 wt% Ni/Al2O3 catalyst, the diffraction peaks at 37.2 , 75.4 and 79.5 can be related to NiO and NixZn(1-x)O phase with overlapped peaks. In the XRD pattern of 5 wt% Mg-60 wt% Ni/Al2O3 catalyst, the diffraction peaks at 37.2 , 62.8 , 75.4 and 79.5 can be attributed to MgO phase. None diffraction peaks related to MxOy compounds and M-Ni alloys were observed in 5 wt% M-60 wt% Ni/Al2O3 (M ¼ Co, Fe, Pd) catalysts, which may due to the good dispersion of Co, Fe and Pd. Fig. 3b was the XRD patterns of the catalysts reduced at 650 C under H2/N2 atmosphere. The diffraction peaks of Ni phases were detected at 2q ¼ 44.4 , 51.8 , 76.3 for the reduced catalysts, which revealed Ni2þ phase in the fresh catalysts were reduced to metallic Ni phases by reduction and Ni was the active site for the catalytic cracking of methane. For 5 wt% Fe-60 wt% Ni/Al2O3, the appearance of NiFe3 was confirmed by the observed diffraction peaks at 51.8 and 76.3 , which indicated that NieFe alloy was formed in the catalyst. The diffraction peak intensity of NiFe3 was low, which was not detected or covered by the diffraction peak of Ni. For other 5 wt% M-60 wt% Ni/Al2O3 (M ¼ Cu, Mn, Pd, Co, Zn, Mg) catalyst, no other species except Ni were detected, which indicated that the metal particles were became smaller and had better dispersion after reduction. The morphology of the fresh 60 wt% Ni/Al2O3 and 5 wt% M60 wt% Ni/Al2O3 (M ¼ Cu, Mn, Pd, Co, Zn, Fe, Mg) catalysts was studied by SEM (Fig. 4). As shown in Fig. 4, the catalysts exhibited a porous structure. The porosity could be aroused from the escape of gas during the calcination of catalyst [53]. For 60 wt% Ni/Al2O3 catalyst, NiO species were tended to agglomerate and formed large randomly-sized cluster due to the weak metal-support interaction [54]. The 5 wt% M-60 wt% Ni/Al2O3 (M ¼ Cu, Mn, Pd, Co, Zn, Fe, Mg) catalysts were found to be mainly exists as particles, which were more or less spherical in shape. The 5 wt% M-60 wt% Ni/Al2O3 (M ¼ Mn, Pd) catalyst was found to be mainly exists as spherical particles and relatively uniform in size. For the 5 wt% M-60 wt% Ni/ Al2O3 (M ¼ Cu, Zn, Co, Fe) catalysts, the SEM images showed the catalysts were agglomerated and the particle size was irregular. As shown in Fig. 4g, the 5 wt% Mg-60 wt% Ni/Al2O3 catalyst has floral structure which was formed with lamellar structure. Table 3 is the actually contents of the metal components in the catalysts. Due to the limitation of wet impregnation method, the actually content of the metal in catalyst has a certain deviation from the set value.
Catalysts performance The TCD reaction over Ni-based catalysts were carried out at 650 C with a CH4 flow rate of 20 mL/min. Fig. 5 shows the effect of different additives (Cu, Mn, Pd, Co, Zn, Fe, Mg) on the methane conversion with time on steam over different catalysts. The methane conversion of the 60 wt% Ni/Al2O3 catalyst was also presented in Fig. 5 for comparison. For 60 wt % Ni/Al2O3 catalyst, the methane conversion was only 50% at 12 min time on stream. Then, the 60 wt% Ni/Al2O3 catalyst deactivated rapidly and the methane conversion decreased to 10% with time in the region of 63e87 min. As the rate of carbon production at NieCH4 interface is higher than that of
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Fig. 4 e SEM images of the fresh catalysts. (a) 60 wt% Ni/Al2O3, (b) 5 wt% Cu-60 wt% Ni/Al2O3, (c) 5 wt% Zn-60 wt% Ni/Al2O3, (d) 5 wt% Co-60 wt% Ni/Al2O, (e) 5 wt% Fe-60 wt% Ni/Al2O3, (f) 5 wt% Mn-60 wt% Ni/Al2O3, (g) 5 wt% Mg-60 wt% Ni/Al2O3, (h) 5 wt % Pd-60 wt% Ni/Al2O3.
carbon transfer through Ni particle, carbon is nucleated and deposited on the surface of Ni particle [3,55]. The active sites of the 60 wt% Ni/Al2O3 catalyst are encapsulated by carbon with time on steam and the access of reactants to active sites are hindered. The lifetime of Ni-based catalyst in methane catalytic decomposition is relatively short, especially at temperatures higher than 600 C [56e62]. In our work, the
reaction was carried out at 650 C. Therefore, the 60 wt% Ni/ Al2O3 catalyst was deactivated quickly. Compared with 60 wt % Ni/Al2O3 catalyst, the methane conversion for the 5 wt%
Table 3 e The actual amount of the promoted metals characterized by ICP-OES. Sample 60%Ni/Al2O3 5%Cue60%Ni/Al2O3 5%Zne60%Ni/Al2O3 5%Coe60%Ni/Al2O3 5%Fee60%Ni/Al2O3 5%Mne60%Ni/Al2O3 5%Mge60%Ni/Al2O3 5%Pde60%Ni/Al2O3
Ni content (%) 54.34 56.17 70.46 73.50 64.40 61.66 66.04 69.23
Amount of the metal addition (%) e 3.69 3.75 4.22 3.69 3.81 3.72 3.88
(Cu) (Zn) (Co) (Fe) (Mn) (Mg) (Pd)
Fig. 5 e Effect of different 5 wt% M-60 wt% Ni/Al2O3 catalysts on methane conversion.
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Fig. 6 e XRD patterns of used catalysts after methane decomposition. (1) 60 wt% Ni/Al2O3, (2) 5 wt% Cu-60 wt% Ni/ Al2O3, (3) 5 wt% Zn-60 wt% Ni/Al2O3, (4) 5 wt% Co-60 wt% Ni/Al2O3, (5) 5 wt% Fe-60 wt% Ni/Al2O3, (6) 5 wt% Mn-60 wt % Ni/Al2O3, (7) 5 wt% Mg-60 wt% Ni/Al2O, (8) 5 wt% Pd-60 wt % Ni/Al2O3.
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M-60 wt% Ni/Al2O3 (M ¼ Cu, Mn, Pd, Co, Fe, Mg) catalysts except 5 wt% Zn-60 wt% Ni/Al2O3 was increased. The methane conversion was decreased slightly to 48% by the addition of Zn to Ni/Al2O3. Compared with 60 wt% Ni/Al2O3 catalyst, the methane conversion of 5 wt% M-60 wt% Ni/ Al2O3 (M ¼ Co, Fe and Mg) catalysts was increased to 56.7%, 52.6% and 56.3% by the addition of Co, Fe and Mg into Ni/ Al2O3 catalyst. The catalytic activity of 60 wt% Ni/Al2O3 was improved significantly by adding Mn and the methane conversion of 5 wt% Mn-60 wt% Ni/Al2O3 was increased to ~80%. As indicates in BET results (Table 1), 5 wt% Mn-60 wt% Ni/ Al2O3 catalyst has a relatively large specific surface area, pore volume and pore size. This may be the reason for the good activity of MneNi/Al2O3 compared with Ni/Al2O3 catalyst at the initial stage of methane TCD reaction. It is suggested that Mn promotes the movement of Ni cations to the surface of catalyst during reduction process [63]. The tendency of methane conversion with time on stream for 5 wt% Mn-60 wt % Ni/Al2O3 was similar to that of 60 wt% Ni/Al2O3 catalyst. The catalytic activities all decreased rapidly with time on stream. The stability of the 5 wt% M-60 wt% Ni/Al2O3 (M ¼ Co, Fe, Mg, Pd, Cu and Zn) catalysts was improved obviously by
Fig. 7 e SEM images of carbonaceous products formed on 60 wt% Ni/Al2O3 and 5 wt% M-60 wt% Ni/Al2O3 (M ¼ Cu, Mn, Pd, Co, Zn, Fe, Mg). (a) 60 wt% Ni/Al2O3, (b) 5 wt% Cu-60 wt% Ni/Al2O3, (c) 5 wt% Zn-60 wt% Ni/Al2O3, (d) 5 wt% Co-60 wt% Ni/Al2O, (e) 5 wt% Fe-60 wt% Ni/Al2O3, (f) 5 wt% Mn-60 wt% Ni/Al2O3, (g) 5 wt% Mg-60 wt% Ni/Al2O3, (h) 5 wt% Pd-60 wt% Ni/Al2O3.
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adding Co, Fe, Mg, Pd, Cu and Zn additives into Ni/Al2O3 catalyst. Compared with 60 wt% Ni/Al2O3, the 5 wt% M-60 wt % Ni/Al2O3 (M ¼ Co, Fe and Mg) catalysts have longer lifetime. The methane conversion of 5 wt% M-60 wt% Ni/Al2O3 (M ¼ Co, Fe and Mg) catalysts was over 30% at 3 h time of steam. It was reported that the bimetallic catalysts (such as NieFe) exhibited long lifetime [30,64e68]. The rate of carbon diffusion through iron was three times higher than that of Ni, which was beneficial to carbon transfer through active sites [65]. Therefore, active sites of 5 wt% Fe-60 wt% Ni/Al2O3 were available and the lifetime was increased by adding Fe into Ni/ Al2O3. The lifetime of the Ni-based catalysts was improved remarkably by adding of Cu and Pd into Ni/Al2O3. The methane conversion of 5 wt% Cu-60 wt% Ni/Al2O3 decreased from 61% to 45% after 250 min time of stream. The methane conversion of 5 wt% Pd-60 wt% Ni/Al2O3 catalyst decreased from 61% to 32% at 230 min time on stream. The Cu addition can be effective in facilitating the reducibility of Ni2þ species and prevents the formation of encapsulating carbon on Ni surface [69]. Consequently, the stability of 5 wt% Cu-60 wt% Ni/Al2O3 catalyst was prolonged. Saraswat et al. [25] found that the performance of NieCu catalyst has higher performance and stability than Ni catalyst in the TCD of methane. It was proved that the formation of mixed metal oxide is the reason for the high performance and stability of NieCu catalyst. Yokoyama et al. found that the rate of carbon diffusion in Pd was faster than that of Ni, which lead to fast carbon migration and long catalyst lifetime [70]. The methane conversion of 5 wt% Zn-60 wt% Ni/Al2O3 catalyst is similar to that of 60 wt% Ni/Al2O3 catalyst. But the lifetime of 5 wt% Zn-60 wt% Ni/Al2O3 catalyst was enhanced significantly by Zn addition. For example, the methane conversion of 5 wt% Zn-60 wt% Ni/Al2O3 decreased from 48% to 33% after 250 min time of steam. The addition of Zn into Ni/Al2O3 can increase the growth rate of carbon fiber or carbon nanotube and prevent the formation of amorphous carbon over active
sites [71]. Consequently, the lifetime of 5 wt% Zn-60 wt% Ni/ Al2O3 catalyst was prolonged.
Characterization of deposited carbon X-ray is an important tool for the evaluation of the deposited carbon over the catalysts. Fig. 6 was the XRD patterns of the used catalysts after the catalytic decomposition of methane. Two obviously diffraction peaks at 26.1 and 43.7 were detected for all catalysts, which can be attributed to the growth of graphitic-like carbon on the surface of catalysts. Meanwhile, the metallic nickel phase was detected at 2q ¼ 44.4 , 51.8 , 76.3 for the used catalysts. The produced carbon was collected at the end of the reaction and the morphology of the deposited carbon were characterized via SEM. Fig. 7 was the SEM images of the produced carbon formed on 60 wt% Ni/Al2O3 and 5 wt% M-60 wt% Ni/Al2O3 catalysts after methane decomposition. As shown in Fig. 7, CNFs were formed on the 5 wt% M-60 wt% Ni/Al2O3 (M ¼ Mg, Pd, Co, Cu and Mn, Zn, Fe) catalysts. A small amount of amorphous carbon was also found on the surface of CNFs. The diameter of CNFs varied from hundred to thousand nanometers and the length of CNF were also different. As shown in Fig. 7g, the CNFs formed on MgeNi/Al2O3 have clean surface and no amorphous carbon was found on the surface of CNFs. The diameter of CNFs deposited on CoeNi/Al2O3 catalyst were around 350 nm. For ZneNi/Al2O3 catalysts, the average diameter of the CNFs was ~1 mm. The CNFs formed on FeeNi/Al2O3 catalyst were very short. The average diameter of CNFs formed on FeeNi/Al2O3 were close to that of CNFs formed on ZneNi/Al2O3 catalyst. TG analysis is often used to characterized the purity and thermal stability of deposited carbon [72]. Fig. 8 was the weight loss of the deposited carbon over 5 wt% M-60 wt% Ni/Al2O3 catalysts. The overall weight loss of the used catalysts was due to the combustion of carbon in air. Different forms of carbon
Fig. 8 e Thermogravimetric curves of the nanocarbon deposited over the catalysts.
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have different oxidation temperatures and the weight loss in TG curves below 450 C is mainly related to the combustion of amorphous carbon [73]. As shown in Fig. 8, all TG curves only presented a single-step degradation. The weight loss in TG curves below 450 C was very low, which indicated only few amorphous carbon were deposited on catalysts. The sharp drop in TG curves between 500 and 680 C was due to the combustion of CNFs. The deposited carbon yield calculated from TG curves was depicted in Table 4. Compared with other catalysts, the largest weight loss of deposited carbon was obtained over 5 wt % Cu-60 wt% Ni/Al2O3, which was in good agreement with the high activity and long lifetime of CueNi/Al2O3 catalyst. It has been reported that Cu could improve the activity of Ni-based catalyst for methane decomposition and thus increase the accumulation of carbon on catalyst surface [69]. Raman spectroscopy is a useful method to characterize the crystallinity and graphitization of carbon material. Fig. 9 was Raman spectra of the deposited carbon on different 5 wt% M-60 wt% Ni/Al2O3 catalysts. Two distinct bands, D band (1350 cm1) and G band (1580 cm1), were observed in Raman spectra of each sample. The G band could be attributed to in-plane CeC stretching vibrations of graphite layers and the D band was ascribed to the structural
Table 5 e ID/IG value of carbon Raman spectra produced by methane decomposition over different Ni-based catalysts. Catalyst Sample 60 wt% Ni/Al2O3 5 wt% Mn-60 wt% Ni/Al2O3 5 wt% Cu-60 wt% Ni/Al2O3 5 wt% Mg-60 wt% Ni/Al2O3 5 wt% Fe-60 wt% Ni/Al2O3 5 wt% Pd-60 wt% Ni/Al2O3 5 wt% Zn-60 wt% Ni/Al2O3 5 wt% Co-60 wt% Ni/Al2O3
ID
IG
ID/IG
2231.335 2496.138 7826.59 2814.852 2367.979 2907.316 2446.479 6085.232
2119.153 2205.251 7042.49 2644.689 2398.423 2998.029 2609.139 6637.229
1.053 1.132 1.118 1.064 0.987 0.970 0.938 0.917
imperfection of graphite [74,75]. The ratio of ID/IG is used to measure the graphitization degree of deposited carbon on catalyst [58]. A small value of ID/IG indicated the graphitization degree of deposited carbon was high [76]. As shown in Table 5, the CNFs deposited on 5 wt% Co-60 wt% Ni/Al2O3 catalyst has highest graphitization degree and the CNFs deposited on 5 wt% Mn-60 wt% Ni/Al2O3 catalysts has lowest graphitization degree.
Conclusion Table 4 e The yield values of carbon nanomaterials. Catalyst Sample 60 wt% Ni/Al2O3 5 wt% Cu-60 wt% Ni/Al2O3 5 wt% Zn-60 wt% Ni/Al2O3 5 wt% Co-60 wt% Ni/Al2O3 5 wt% Fe-60 wt% Ni/Al2O3 5 wt% Mn-60 wt% Ni/Al2O3 5 wt% Mg-60 wt% Ni/Al2O3 5 wt% Pd-60 wt% Ni/Al2O3
residue (%)
weight loss (%)
Carbon yield (%)
21.9 4.1 30.9 11.5 50.4 30.1 28 22.7
78.1 95.9 69.1 88.5 49.6 69.9 72 77.3
356.62 2339.02 223.62 769.57 98.41 232.23 257.14 340.53
In this work, seven 5 wt% M-60 wt% Ni/Al2O3 catalysts (M ¼ Cu, Mn, Pd, Co, Zn, Fe, Mg) were prepared by coimpregnation method to investigate promotional effects of these metal additives on the activity and stability of 60 wt% Ni/Al2O3 for TCD of methane. The additives of 5 wt% M-60 wt % Ni/Al2O3 catalysts have a significant influence on methane conversion in TCD reaction. While the addition of metal additives (5 wt%), the activity stability of 60 wt% Ni/Al2O3 was improved and the CH4 conversion of 60 wt% Ni/Al2O3 was also improved except Zn addition. And Mn addition was found to improve the catalytic activity of 60 wt% Ni/Al2O3 significantly and the CH4 conversion of 5 wt% Mn-60 wt% Ni/ Al2O3 was ~80%. Cu addition was found to remarkably improve the catalytic stability of 60 wt% Ni/Al2O3 and the CH4 conversion of 5 wt% Cu-60 wt% Ni/Al2O3 decreased from 60% to 45% after 250 min of reaction time. CNFs were formed on all Ni-based catalysts. CNFs formed on MgeNi/Al2O3 have clean surface and no amorphous carbon was found on the surface of CNFs.
Acknowledgments
Fig. 9 e Raman spectra of as-grown carbon nanofibers generated by methane decomposition over different Nibased catalysts. (1) 60 wt% Ni/Al2O3, (2) 5 wt% Cu-60 wt% Ni/Al2O3, (3) 5 wt% Zn-60 wt% Ni/Al2O3, (4) 5 wt% Co-60 wt % Ni/Al2O3, (5) 5 wt% Fe-60 wt% Ni/Al2O3, (6) 5 wt% Mn60 wt% Ni/Al2O3, (7) 5 wt% Mg-60 wt% Ni/Al2O, (8) 5 wt% Pd60 wt% Ni/Al2O3.
This work has been supported by the Fund of State Key Laboratory of Multiphase Complex Systems (No. MPCS-2017-A-11) and Center for Mesoscience, Institute of Process Engineering, Chinese Academy of Sciences (No. COM2016A003).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.01.272.
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