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Hydrogen production by catalytic methane decomposition over yttria doped nickel based catalysts
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 ) 9 9 2 2 e9 9 2 9
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Hydrogen production by catalytic methane decomposition over yttria doped nickel based catalysts Meltem Karaismailoglu a,b,*, Halit Eren Figen a, Sema Z. Baykara a a
Chemical Engineering Department, Yildiz Technical University, Davutpasa Campus, Topkapi, 34210, Istanbul, Turkey b Department of Energy Science and Technology, Turkish-German University, Beykoz, 34820, Istanbul, Turkey
article info
abstract
Article history:
Catalytic methane decomposition can become a green process for hydrogen production. In
Received 24 September 2018
the present study, yttria doped nickel based catalysts were investigated for catalytic
Received in revised form
thermal decomposition of methane. All catalysts were prepared by sol-gel citrate method
21 December 2018
and structurally characterized with X-ray powder diffraction (XRD), scanning electron
Accepted 29 December 2018
microscopy-energy dispersive spectroscopy (SEM-EDS) and Brunauer, Emmet and Teller
Available online 28 January 2019
(BET) surface analysis techniques. Activity tests of synthesized catalysts were performed in a tubular reactor at 500 ml/min total flow rate and in a temperature range between 390 C
Keywords:
and 845 C. In the non-catalytic reaction, decomposition of methane did not start until
Methane
880 C was reached. In the presence of the catalyst with higher nickel content, methane
Catalytic decomposition
conversion of 14% was achieved at the temperature of 500 C. Increasing the reaction
Hydrogen production
temperature led to higher coke formation. Lower nickel content in the catalyst reduced the
Yttria catalyst
carbon formation. Consequently, with this type of catalyst methane conversion of 50% has been realized at the temperature of 800 C. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Renewable energy technologies offer clean alternatives to overcome the dependence on the fossil fuels and to reduce greenhouse gases which cause global warming and climate change [1,2]. An energy carrier such as hydrogen can provide clean energy, since its combustion only produces water vapor [3e5]. Combustion of fossil fuels leads to emission of greenhouse gases such as carbon dioxide (CO2). Hence, replacing fossil fuels with hydrogen reduces greenhouse gas emission and dependence on hydrocarbon based fuels. However, hydrogen is a secondary fuel, which is traditionally derived from sources such as biomass, natural gas, coal, and naphtha
[6,7]. Almost 96% of the total hydrogen production is provided from fossil fuels (refinery oil, natural gas, and coal) and 4% is generated by electrolysis of water [8]. Hydrogen can be produced from methane via steam reforming [9], partial oxidation [10] and catalytic decomposition [11]. The most common chemical process for producing hydrogen from fossil fuels is steam reforming. During the reaction of natural gas with steam at high temperature, hydrogen and carbon dioxide are generated [9,12]. Catalytic methane decomposition is another promising approach to produce hydrogen [13]. It can even become a green process without CO or CO2 emission, if total conversion can be achieved. In this process, methane is converted into
* Corresponding author. Chemical Engineering Department, Yildiz Technical University, Davutpasa Campus, Topkapi, Istanbul, Turkey. E-mail address: [email protected] (M. Karaismailoglu). https://doi.org/10.1016/j.ijhydene.2018.12.214 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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 ) 9 9 2 2 e9 9 2 9
carbon and hydrogen in an inert gas at elevated temperatures [14,15]. The chemical reaction is described by Eq. (1). CH4 (g) 4 2H2(g) þ C(s) DH 25 C ¼ 74.8 kJ/mol
(1)
Compared to steam methane reforming process (SMR), catalytic decomposition of methane requires less energy (for steam reforming DH 25 C ¼ 206.1 kJ/mol, for thermal methane decomposition DH 25 C ¼ 74.8 kJ/mol) [15e17]. In the thermal cracking process, a considerable amount of hydrogen production is possible at the temperatures above 1200 C [15,18]. Hence, using metal oxide catalyst is a convenient approach to decrease the reaction temperature. Several studies indicate that nickel (Ni), cobalt (Co) and iron (Fe) are the most commonly used transition metals in the preparation of methane decomposition catalysts [19e22]. Mostly, this type of catalysts is supported on alumina (Al2O3), silica (SiO2), magnesia (MgO), and titania (TiO2) for increasing catalyst specific surface area and thereby its activity [20,23,24]. In addition, doping Ni based catalysts by noble metals like Ir, Pt, Pd and Rh enhances the catalyst activity and stability [25]. Ni-based catalysts show higher activity compared to iron or cobalt based catalysts, but they are easily deactivated at high temperatures [19,25,26]. Therefore, modifying Ni-based catalysts with the aforementioned promoters or supports provide a convenient approach. Calafat et al. have studied FeeNi catalysts on ZrO2 in catalytic decomposition of methane at the temperature of 650 C, and they reported that occurrence of FeeNi alloy decreased the catalysts’ deactivation rate and increased catalytic activity [27]. Bayat et al. [28] have investigated the catalytic performance of NiePd/Al2O3 catalyst in the temperature range of 600e800 C, and they have concluded that the addition of Pd in Ni-based catalysts enhanced the catalytic activity and improved the catalyst lifetime. A Pd promoted Ni/MgAl2O4 catalyst was prepared with the sol-gel method by Pudukudy et al. [29] and its performance was tested at the temperature of 700 C. They reported that Pd as promoter enhanced the catalytic activity and improved its stability. NieTiO2 catalysts with varying nickel weight percentage from 10 to 40% were synthesized and tested for methane decomposition. The results indicated that this type of catalysts were active for 300 min [24]. In another work, Ni, NieCu and NieCueFe on Al2O3 catalysts were synthesized and tested at various temperatures. For the NieCu on Al2O3 catalyst the optimal temperatures for decomposition of methane were found to be in the range of 600e650 C. Addition of Fe to the NiCueAl2O3 catalyst increased the operation temperatures to 700e750 C [20]. Kogler et al. [30] have studied hydrocarbon dissociation over solid oxide fuel cell (SOFC) electrolyte yttria-stabilized zirconia (YSZ) and compared its activity with pure yttria (Y2O3) and zirconia. Methane dissociation on Y2O3 started above 800 C, and more amount of carbon was deposited on Y2O3 surface compared to YSZ and ZrO2, which was related to the catalyst activity in methane decomposition. In another study, catalytic methane partial oxidation was carried out over yttria promoted metallic nickel catalysts, and it was noted that the methane conversions were higher over the yttria promoted metallic Ni catalysts compared to those over the metallic Ni catalyst [31]. In the literature, application of Y2O3 doped Ni based catalysts have not been reported for
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methane conversion for hydrogen production so far. In the present work, Ni-based catalysts were doped with yttria due to its effect on improving catalyst activity and stability in catalytic methane decomposition. It is expected that yttria doped catalysts should have higher activity and stability compared to the pure nickel oxide catalyst.
Experimental Catalyst preparation Yttria doped nickel based catalysts were synthesized using the sol-gel method. 1 M precursor solution of nickel dinitrate hexahydrate (NiN2O6$6H2O, Alfa Aeser) and yttrium nitrate hexahydrate (Y(NO3)3$6H2O, ABCR GmbH) were prepared and citric acid was added. Following mixing overnight, 1 M ammonium carbonate (NH4)2CO3 solution was added for adjusting the pH to 6. The solution was evaporated at 80 C and the precursor solution became a viscous gel. The catalyst was oven dried at 210 C and calcined at 1000 C for 5 h. Synthesized samples, labeled as C, are given in Table 1.
Characterization of catalysts Crystal phases in each catalyst were determined by performing X-Ray Diffraction (XRD, Philips Panalytical X'Pert-Pro) analyses carried out with CuKa radiation. Micromeritcs Gemini VII BET Instrument was used for determining Brunauer-Emmett-Teller (BET) surface areas of the catalysts under N2 adsorption. Before BET measurements, each sample was degassed at 300 C. Scanning electron microscopy (SEM, ZEISS EVO LS 10) was used for determining the surface morphology and microstructure of the catalysts. Metal contents were studied with energy dispersive X-Ray spectroscopy (EDS). All samples were coated with gold and were held on the sample holder with carbon adhesive tape. Thermogravimetric analyses of spent catalysts were performed by EXSTAR TG/DTA 6300 thermal analyzer (SII Nanotechnology). Each sample was heated in the temperature range of 25e1000 C at a heating rate of 10 C/min under air flow. H2 temperature-programmed reduction (TPR) experiments were carried out in a stainless steel reactor interfaced to the quadrupole mass spectrometer (Hiden Analytical QGA). A 200 mg catalyst sample was loaded in the stainless steel reactor and heated by an electrical furnace to 1000 C at a rate of 3 C/min in a reducing gas of 5% H2 in nitrogen at a flow rate of 400 ml/min.
Catalytic decomposition of methane (CDM) The activity of synthesized catalysts was examined for H2 production from CH4 decomposition under atmospheric pressure. The CH4 decomposition was carried out in a stainless steel reactor. A catalyst sample of 200 mg, with a layer of quartz wool underneath, was loaded in the reactor. A gas mixture of 6% CH4 in N2 was passed through the catalyst at a flow rate of 500 ml/min at reaction temperatures varying from 390 to 845 C. The process flow diagram of the catalyst test
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Table 1 e XRD crystal phases and surface area measurements of the fresh catalysts. Code
NiO/Y2O3 Mole ratio
Crystal phases of fresh catalysts (Reference code)
Crystal phases of spent catalysts (Reference code)
BET (m2/g)
Ni (98-006-4989), C (98-007-6767) Y2O3 (98-015-3500) Ni (98-006-4989), Y2O3 (98-015-3500) Ni (98-006-4989), Y2O3 (98-015-3500), C (98-007-6767) Ni (98-006-4989), Y2O3 (98-015-3500) C (98-007-6767) SiO2 (98-007-0005)
1.8617
C1
1:0
NiO (98-007-6669)
C2 C3
0:1 1:1
C4
2.5:1
Y2O3 (98-015-3500) NiO (98-007-6669), Y2O3 (98-015-3500) NiO (98-007-6669), Y2O3 (98-015-3500)
C5
3.5:1
NiO (98-007-6669), Y2O3 (98-015-3500)
6.9793 2.6456 6.4254
4.6646
Fig. 1 e Process flow diagram of the catalyst test system for catalytic methane decomposition.
system for catalytic methane decomposition is given in Fig. 1. Before reaction, each catalyst was reduced with a gas mixture of 5%H2 in N2, and the reduction temperature was determined according to the TPR results. Reaction products were analyzed by the quadrupole mass spectrometer (Hiden AnalyticaL QGA). The accuracy levels of the instruments were: mass spectrometry (Hidden Analytical, ±0.003%), mass flow controller (Teledyne Hasting, ±1% of full scale), temperature controller (Ordel, ±0.2%) and K-type thermocouples (Pt% 10RhePt, ±1.5 C). The equation (Eq. (2)) for calculating the conversion of CH4, XCH4 is given below: XCH4 ð%Þ ¼
FCH4 ;inlet FCH4 ;outlet 100 FCH4 ;inlet
(2)
Results and discussion Catalyst characterization Following the synthesis of nickel oxide and yttrium oxide catalysts by using sol-gel method, samples were calcined at
the temperature of 1000 C. Considering the high calcination temperature, a combustion reaction is generated between nitrate and citrate anions which leads to the formation of yttrium oxide and nickel oxide [32]. Calculation of the required amounts of yttrium nitrate hexahydrate and nickel (II) nitrate hexahydrate is based on Eqs. (3) and (4). 18Y(NO3)3 þ 15C5H6O4 / 9Y2O3 þ 75CO2 þ 27N2 þ 45H2O
(3)
9Ni(NO3)2 þ 5C5H6O4 / 9NiOþ 25CO2 þ 9N2 þ 15H2O
(4)
Heating the precursor solution at 210 C leads to the conversion of citric acid (C6H8O7) to itaconic acid (C5H6O4) [32,33]. Hence, the citric acid is not present in the chemical equations (Eq. (3) and (4)). Crystalline structure of fresh and spent catalyst samples were determined by XRD analyses and crystal phases were listed in Table 1. XRD phases of all catalysts are presented in Fig. 2 which verified the existence of pure NiO and Y2O3 phases in all fresh bimetallic catalyst samples. For all fresh NiO loaded catalysts, the peaks at 2q ¼ 37.2, 43.2 and 62.8 were assigned to the presence of cubic NiO phase. With increasing NiO loading the peak intensities were higher as expected. The peaks at the 2q
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Fig. 2 e XRD patterns of the fresh (a) C1, (b) C2, (c) C3, (d) C4, (f) C5 and spent (a*) C1, (b*) C2, (c*) C3, (d*) C4 and (e*) C5 catalysts.
values of 20.6, 29.2, 33.9, 48.6 and 57.7 indicated that Y2O3 phase was formed in yttrium doped catalysts. Spent bimetallic catalyst samples contained elemental nickel, carbon (C) and Y2O3 in the metal oxide phase. XRD analyses proved the presence of Ni phases at 2q ¼ 44.6 and 52.0 for the catalysts except C2. Peaks at 2q ¼ 26.8 and 43.8 for C4 and C5 catalysts correspond to the carbon phase. Specific surface area (BET) measurements of fresh catalysts are given in Table 1, and Fig. 3 displays the SEM micrographs of the fresh and spent catalysts at 20 000X magnification. As shown in Fig. 3(a) the fresh NiO (C1) catalyst consists of spherically shaped particles. The SEM micrograph of Y2O3 catalyst (C2) depicted that this sample has a porous nature, which explains its higher specific surface area compared to that of the NiO catalyst (Table 1). Addition of Y2O3 into the NiO structure decreases the porous nature, and agglomerated
particles are formed as shown in Fig. 3(c). Hence, the C3 catalyst has a smaller specific surface area than pure Y2O3. The SEM micrographs of the fresh C4 and C5 catalysts (Fig. 3(d) and (e)) show that they have more porous structure compared to the C3 (Fig. 3(c)) with higher specific surface areas of 6.4254 and 4.6646 m2/g, respectively. Considering the studies about the effect of catalyst surface area on its activity reports on increase in catalysts activity directly proportional to increase in catalysts surface area available [34,35]. Solid carbon as a decomposition product was determined on spent catalysts surface in SEM analyses. TPR profiles of all synthesized catalysts are given in Fig. 4. As shown in Fig. 4(b), pure yttrium oxide (C2) has no reduction peak. Hence, the reduction peaks of yttria doped Ni based catalysts (C3, C4 and C5), around 360 C, may be due to the reduction of Ni2þ to Ni0 [31,36]. Moreover, the reduction peak
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Fig. 4 e H2-TPR profiles of the catalysts: (a) C1, (b) C2, (c) C3, (d) C4 and (e) C5.
of C3 has a lower intensity than C4 and C5 due to its lower nickel oxide content.
Catalytic decomposition of methane (CDM)
Fig. 3 e SEM micrographs of the synthesized (a) C1, (b) C2, (c) C3, (d) C4, (e) C5 and spent (a*) C1, (b*) C2, (c*) C3, (d*) C4, (e*) C5 catalysts at 20 000X magnification.
According to the experimental results, methane can be thermally decomposed into hydrogen and carbon at temperatures above 880 C. Hence, use of catalysts renders hydrogen production possible at lower reaction temperatures. Variation in CH4 conversions over the yttria doped nickel oxide catalysts at reaction temperatures increasing from 390 C to 845 C are presented in Fig. 5. Methane decomposition was not possible over the catalyst C2 (Y2O3) in this temperature range. Compared to yttria doped catalysts, pure nickel oxide (C1) exhibited strong deactivation. In literature, temperature levels for catalytic methane decomposition over Ni based catalysts are in the range of 550e900 C [37e39]. Over yttria doped nickel oxide catalyst lowering the reaction temperature to 390 C was possible. Fig. 5(a) shows the CH4 conversion over the C1, C4 and C5 catalysts at 390 C. The yttria doped catalysts C4 and C5 displayed higher activity through initial methane conversions of 9.6% and 5.9%, respectively, followed by a rapid drop in their activities. Fig. 5(b) presents the methane conversion over the same catalysts (C1, C4 and C5) at the reaction temperature of 500 C, where the highest methane conversion was 14%; indicating that an increase in reaction temperature enhanced the methane conversion due to the endothermic nature of the reaction. However, nickel oxide (C1) lost its activity in a run time of 12min and the immediate decrease in its activity can be explained in terms of the high amount of carbon formation and deposition on the active Ni surfaces [11]. Considering data available in literature, results obtained in the present work were comparable to those obtained at 500 C with a catalyst containing a similar Ni loading of 70% (mole ratio of NiO to that of support material) [40]. Maximum conversion obtained in the present work was 14% compared to the reported 6%. However, catalyst life time was considerably shorter. Fig. 5(c) displays the methane conversion over the nickel oxide (C1) and yttria doped nickel oxide (C4 and C5) catalysts at the temperature of 590 C. It was observed that the activity of the catalyst C4 decreased with the increase in
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Fig. 5 e Comparison of CH4 conversions: (a) over the C1, C4 and C5 catalysts at T ¼ 390 C, (b) over the C1, C4 and C5 catalysts at 500 C, (c) over the C1, C4 and C5 catalysts at T ¼ 590 C, (d) over the C4 and C5 catalysts at T ¼ 690 C, (e) over the C3, C4 and C5 catalysts at T ¼ 800 C, (f) over the C3 catalyst at T ¼ 845 C.
reaction temperature, which showed that 500 C was more suitable for carrying out methane decomposition over this catalyst. Similar result was obtained by Rastegarpanah et al. [41]. They tested Ni/xMgO$Al2O3 catalysts in methane decomposition at the temperatures of 600 and 675 C and reported that each catalyst lost its activity at 675 C faster than at 600 C, which indicated that 600 C is moderate as a reaction condition. C5 was an active catalyst with an initial methane conversion of 12.7% at 500 C, but the catalyst lost its activity in 35 min. After the catalyst was deactivated entirely at 500 C, increasing the reaction temperature to 590 C led to an increase in maximum methane conversion of 3% with better catalyst durability (Fig. 5(c)). Fig. 5(d) presents the methane conversions over the catalysts C4 and C5 at the temperature of 690 C. The maximum methane conversions obtained over C4 and C5 were 3% and 2.3%, respectively. Besides, conversions over both catalysts were followed by a steady state.
The yttria doped nickel oxide catalyst with the NiO/Y2O3 mole ratio of 1:1 (C4) exhibited no catalytic activity in the temperature range between 390 C and 790 C. As given in Fig. 5(e), the initial methane conversion over the catalyst C4 was around 50% at 800 C and after a short duration the catalyst lost its activity. The kinetic curves of methane conversions over the samples C4 and C5 were close to each other despite of the difference in their nickel oxide contents, and highest methane conversions over these catalysts were 8.3% and 7.7%, respectively. Fig. 5(f) indicates that C4 is still an active catalyst after increasing the temperature to 845 C, but with the initial methane conversion of 1.2%. In view of overall catalyst evaluation, C4 and C5 catalysts have displayed higher activity. Related to the agglomerated structure and lower specific surface area of the C3 catalyst, it has been expected that this catalyst would have a lower activity than the C4 and C5 catalysts. SEM micrographs of the
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Acknowledgement Project 213M368 supported by TUBITAK-MAG and project 2016-07-01-DOP01 supported by Research Fund of the Yildiz Technical University are gratefully acknowledged.
references
Fig. 6 e Comparison of weight loss percentage of spent C1, C2, C3, C4 and C5 catalysts.
spent C3 catalyst verified this theory with a less amount of filamentous carbon on C3 catalyst surface. In Fig. 2(d*) and 2 (e*), a high amount of filamentous carbon on the spent C4 and C5 catalysts particles was displayed, and it indicated that decomposition of methane over these catalysts was performed with a higher activity. Also, accumulated carbon on catalysts surface was determined by thermogravimetric analysis (TGA), and the results were shown in Fig. 6. There is a direct correlation between weight loss percentage and accumulated carbon on spent catalysts. The percentages of C4 and C5 catalysts were higher than C3 as expected. Hence, SEM micrographs were verified by TGA results.
Conclusion In the present study, NiO/Y2O3 catalysts were synthesized by sol-gel method and the effect of addition of yttria on catalyst performance was investigated. Catalytic tests were performed in the temperature range 390e845 C in a tubular reactor. The results indicated that yttria addition increased the catalyst stability and enhanced the catalyst activity at lower reaction temperatures. Pure nickel oxide is an inactive catalyst at the temperature of 845 C. Yttria addition improved its activity at elevated temperatures. Hence, in high temperature processes, using yttria doped nickel oxide catalysts can be a convenient approach. With the addition of yttrium oxide to the catalyst structure, an increase in specific surface area of the nickel oxide catalyst was observed (C3, C4 and C5). Experimental data indicated a correlation between surface area and catalytic activity of yttria doped nickel based catalysts. The C4 catalyst, which has the highest surface area of 6.4254 m2/g, exhibited a better activity compared to the C1 and C5 catalysts, which have surface areas of 2.6456 and 4.6646 m2/g, respectively. Conversion values obtained with the present catalyst (C4) were comparable (even higher) to those reported in the literature; however excess carbon coverage was encountered.
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Hydrogen production by catalytic methane decomposition over yttria doped nickel based catalysts Meltem Karaismailoglu a,b,*, Halit Eren Figen a, Sema Z. Baykara a a
Chemical Engineering Department, Yildiz Technical University, Davutpasa Campus, Topkapi, 34210, Istanbul, Turkey b Department of Energy Science and Technology, Turkish-German University, Beykoz, 34820, Istanbul, Turkey
article info
abstract
Article history:
Catalytic methane decomposition can become a green process for hydrogen production. In
Received 24 September 2018
the present study, yttria doped nickel based catalysts were investigated for catalytic
Received in revised form
thermal decomposition of methane. All catalysts were prepared by sol-gel citrate method
21 December 2018
and structurally characterized with X-ray powder diffraction (XRD), scanning electron
Accepted 29 December 2018
microscopy-energy dispersive spectroscopy (SEM-EDS) and Brunauer, Emmet and Teller
Available online 28 January 2019
(BET) surface analysis techniques. Activity tests of synthesized catalysts were performed in a tubular reactor at 500 ml/min total flow rate and in a temperature range between 390 C
Keywords:
and 845 C. In the non-catalytic reaction, decomposition of methane did not start until
Methane
880 C was reached. In the presence of the catalyst with higher nickel content, methane
Catalytic decomposition
conversion of 14% was achieved at the temperature of 500 C. Increasing the reaction
Hydrogen production
temperature led to higher coke formation. Lower nickel content in the catalyst reduced the
Yttria catalyst
carbon formation. Consequently, with this type of catalyst methane conversion of 50% has been realized at the temperature of 800 C. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Renewable energy technologies offer clean alternatives to overcome the dependence on the fossil fuels and to reduce greenhouse gases which cause global warming and climate change [1,2]. An energy carrier such as hydrogen can provide clean energy, since its combustion only produces water vapor [3e5]. Combustion of fossil fuels leads to emission of greenhouse gases such as carbon dioxide (CO2). Hence, replacing fossil fuels with hydrogen reduces greenhouse gas emission and dependence on hydrocarbon based fuels. However, hydrogen is a secondary fuel, which is traditionally derived from sources such as biomass, natural gas, coal, and naphtha
[6,7]. Almost 96% of the total hydrogen production is provided from fossil fuels (refinery oil, natural gas, and coal) and 4% is generated by electrolysis of water [8]. Hydrogen can be produced from methane via steam reforming [9], partial oxidation [10] and catalytic decomposition [11]. The most common chemical process for producing hydrogen from fossil fuels is steam reforming. During the reaction of natural gas with steam at high temperature, hydrogen and carbon dioxide are generated [9,12]. Catalytic methane decomposition is another promising approach to produce hydrogen [13]. It can even become a green process without CO or CO2 emission, if total conversion can be achieved. In this process, methane is converted into
* Corresponding author. Chemical Engineering Department, Yildiz Technical University, Davutpasa Campus, Topkapi, Istanbul, Turkey. E-mail address: [email protected] (M. Karaismailoglu). https://doi.org/10.1016/j.ijhydene.2018.12.214 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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carbon and hydrogen in an inert gas at elevated temperatures [14,15]. The chemical reaction is described by Eq. (1). CH4 (g) 4 2H2(g) þ C(s) DH 25 C ¼ 74.8 kJ/mol
(1)
Compared to steam methane reforming process (SMR), catalytic decomposition of methane requires less energy (for steam reforming DH 25 C ¼ 206.1 kJ/mol, for thermal methane decomposition DH 25 C ¼ 74.8 kJ/mol) [15e17]. In the thermal cracking process, a considerable amount of hydrogen production is possible at the temperatures above 1200 C [15,18]. Hence, using metal oxide catalyst is a convenient approach to decrease the reaction temperature. Several studies indicate that nickel (Ni), cobalt (Co) and iron (Fe) are the most commonly used transition metals in the preparation of methane decomposition catalysts [19e22]. Mostly, this type of catalysts is supported on alumina (Al2O3), silica (SiO2), magnesia (MgO), and titania (TiO2) for increasing catalyst specific surface area and thereby its activity [20,23,24]. In addition, doping Ni based catalysts by noble metals like Ir, Pt, Pd and Rh enhances the catalyst activity and stability [25]. Ni-based catalysts show higher activity compared to iron or cobalt based catalysts, but they are easily deactivated at high temperatures [19,25,26]. Therefore, modifying Ni-based catalysts with the aforementioned promoters or supports provide a convenient approach. Calafat et al. have studied FeeNi catalysts on ZrO2 in catalytic decomposition of methane at the temperature of 650 C, and they reported that occurrence of FeeNi alloy decreased the catalysts’ deactivation rate and increased catalytic activity [27]. Bayat et al. [28] have investigated the catalytic performance of NiePd/Al2O3 catalyst in the temperature range of 600e800 C, and they have concluded that the addition of Pd in Ni-based catalysts enhanced the catalytic activity and improved the catalyst lifetime. A Pd promoted Ni/MgAl2O4 catalyst was prepared with the sol-gel method by Pudukudy et al. [29] and its performance was tested at the temperature of 700 C. They reported that Pd as promoter enhanced the catalytic activity and improved its stability. NieTiO2 catalysts with varying nickel weight percentage from 10 to 40% were synthesized and tested for methane decomposition. The results indicated that this type of catalysts were active for 300 min [24]. In another work, Ni, NieCu and NieCueFe on Al2O3 catalysts were synthesized and tested at various temperatures. For the NieCu on Al2O3 catalyst the optimal temperatures for decomposition of methane were found to be in the range of 600e650 C. Addition of Fe to the NiCueAl2O3 catalyst increased the operation temperatures to 700e750 C [20]. Kogler et al. [30] have studied hydrocarbon dissociation over solid oxide fuel cell (SOFC) electrolyte yttria-stabilized zirconia (YSZ) and compared its activity with pure yttria (Y2O3) and zirconia. Methane dissociation on Y2O3 started above 800 C, and more amount of carbon was deposited on Y2O3 surface compared to YSZ and ZrO2, which was related to the catalyst activity in methane decomposition. In another study, catalytic methane partial oxidation was carried out over yttria promoted metallic nickel catalysts, and it was noted that the methane conversions were higher over the yttria promoted metallic Ni catalysts compared to those over the metallic Ni catalyst [31]. In the literature, application of Y2O3 doped Ni based catalysts have not been reported for
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methane conversion for hydrogen production so far. In the present work, Ni-based catalysts were doped with yttria due to its effect on improving catalyst activity and stability in catalytic methane decomposition. It is expected that yttria doped catalysts should have higher activity and stability compared to the pure nickel oxide catalyst.
Experimental Catalyst preparation Yttria doped nickel based catalysts were synthesized using the sol-gel method. 1 M precursor solution of nickel dinitrate hexahydrate (NiN2O6$6H2O, Alfa Aeser) and yttrium nitrate hexahydrate (Y(NO3)3$6H2O, ABCR GmbH) were prepared and citric acid was added. Following mixing overnight, 1 M ammonium carbonate (NH4)2CO3 solution was added for adjusting the pH to 6. The solution was evaporated at 80 C and the precursor solution became a viscous gel. The catalyst was oven dried at 210 C and calcined at 1000 C for 5 h. Synthesized samples, labeled as C, are given in Table 1.
Characterization of catalysts Crystal phases in each catalyst were determined by performing X-Ray Diffraction (XRD, Philips Panalytical X'Pert-Pro) analyses carried out with CuKa radiation. Micromeritcs Gemini VII BET Instrument was used for determining Brunauer-Emmett-Teller (BET) surface areas of the catalysts under N2 adsorption. Before BET measurements, each sample was degassed at 300 C. Scanning electron microscopy (SEM, ZEISS EVO LS 10) was used for determining the surface morphology and microstructure of the catalysts. Metal contents were studied with energy dispersive X-Ray spectroscopy (EDS). All samples were coated with gold and were held on the sample holder with carbon adhesive tape. Thermogravimetric analyses of spent catalysts were performed by EXSTAR TG/DTA 6300 thermal analyzer (SII Nanotechnology). Each sample was heated in the temperature range of 25e1000 C at a heating rate of 10 C/min under air flow. H2 temperature-programmed reduction (TPR) experiments were carried out in a stainless steel reactor interfaced to the quadrupole mass spectrometer (Hiden Analytical QGA). A 200 mg catalyst sample was loaded in the stainless steel reactor and heated by an electrical furnace to 1000 C at a rate of 3 C/min in a reducing gas of 5% H2 in nitrogen at a flow rate of 400 ml/min.
Catalytic decomposition of methane (CDM) The activity of synthesized catalysts was examined for H2 production from CH4 decomposition under atmospheric pressure. The CH4 decomposition was carried out in a stainless steel reactor. A catalyst sample of 200 mg, with a layer of quartz wool underneath, was loaded in the reactor. A gas mixture of 6% CH4 in N2 was passed through the catalyst at a flow rate of 500 ml/min at reaction temperatures varying from 390 to 845 C. The process flow diagram of the catalyst test
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Table 1 e XRD crystal phases and surface area measurements of the fresh catalysts. Code
NiO/Y2O3 Mole ratio
Crystal phases of fresh catalysts (Reference code)
Crystal phases of spent catalysts (Reference code)
BET (m2/g)
Ni (98-006-4989), C (98-007-6767) Y2O3 (98-015-3500) Ni (98-006-4989), Y2O3 (98-015-3500) Ni (98-006-4989), Y2O3 (98-015-3500), C (98-007-6767) Ni (98-006-4989), Y2O3 (98-015-3500) C (98-007-6767) SiO2 (98-007-0005)
1.8617
C1
1:0
NiO (98-007-6669)
C2 C3
0:1 1:1
C4
2.5:1
Y2O3 (98-015-3500) NiO (98-007-6669), Y2O3 (98-015-3500) NiO (98-007-6669), Y2O3 (98-015-3500)
C5
3.5:1
NiO (98-007-6669), Y2O3 (98-015-3500)
6.9793 2.6456 6.4254
4.6646
Fig. 1 e Process flow diagram of the catalyst test system for catalytic methane decomposition.
system for catalytic methane decomposition is given in Fig. 1. Before reaction, each catalyst was reduced with a gas mixture of 5%H2 in N2, and the reduction temperature was determined according to the TPR results. Reaction products were analyzed by the quadrupole mass spectrometer (Hiden AnalyticaL QGA). The accuracy levels of the instruments were: mass spectrometry (Hidden Analytical, ±0.003%), mass flow controller (Teledyne Hasting, ±1% of full scale), temperature controller (Ordel, ±0.2%) and K-type thermocouples (Pt% 10RhePt, ±1.5 C). The equation (Eq. (2)) for calculating the conversion of CH4, XCH4 is given below: XCH4 ð%Þ ¼
FCH4 ;inlet FCH4 ;outlet 100 FCH4 ;inlet
(2)
Results and discussion Catalyst characterization Following the synthesis of nickel oxide and yttrium oxide catalysts by using sol-gel method, samples were calcined at
the temperature of 1000 C. Considering the high calcination temperature, a combustion reaction is generated between nitrate and citrate anions which leads to the formation of yttrium oxide and nickel oxide [32]. Calculation of the required amounts of yttrium nitrate hexahydrate and nickel (II) nitrate hexahydrate is based on Eqs. (3) and (4). 18Y(NO3)3 þ 15C5H6O4 / 9Y2O3 þ 75CO2 þ 27N2 þ 45H2O
(3)
9Ni(NO3)2 þ 5C5H6O4 / 9NiOþ 25CO2 þ 9N2 þ 15H2O
(4)
Heating the precursor solution at 210 C leads to the conversion of citric acid (C6H8O7) to itaconic acid (C5H6O4) [32,33]. Hence, the citric acid is not present in the chemical equations (Eq. (3) and (4)). Crystalline structure of fresh and spent catalyst samples were determined by XRD analyses and crystal phases were listed in Table 1. XRD phases of all catalysts are presented in Fig. 2 which verified the existence of pure NiO and Y2O3 phases in all fresh bimetallic catalyst samples. For all fresh NiO loaded catalysts, the peaks at 2q ¼ 37.2, 43.2 and 62.8 were assigned to the presence of cubic NiO phase. With increasing NiO loading the peak intensities were higher as expected. The peaks at the 2q
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Fig. 2 e XRD patterns of the fresh (a) C1, (b) C2, (c) C3, (d) C4, (f) C5 and spent (a*) C1, (b*) C2, (c*) C3, (d*) C4 and (e*) C5 catalysts.
values of 20.6, 29.2, 33.9, 48.6 and 57.7 indicated that Y2O3 phase was formed in yttrium doped catalysts. Spent bimetallic catalyst samples contained elemental nickel, carbon (C) and Y2O3 in the metal oxide phase. XRD analyses proved the presence of Ni phases at 2q ¼ 44.6 and 52.0 for the catalysts except C2. Peaks at 2q ¼ 26.8 and 43.8 for C4 and C5 catalysts correspond to the carbon phase. Specific surface area (BET) measurements of fresh catalysts are given in Table 1, and Fig. 3 displays the SEM micrographs of the fresh and spent catalysts at 20 000X magnification. As shown in Fig. 3(a) the fresh NiO (C1) catalyst consists of spherically shaped particles. The SEM micrograph of Y2O3 catalyst (C2) depicted that this sample has a porous nature, which explains its higher specific surface area compared to that of the NiO catalyst (Table 1). Addition of Y2O3 into the NiO structure decreases the porous nature, and agglomerated
particles are formed as shown in Fig. 3(c). Hence, the C3 catalyst has a smaller specific surface area than pure Y2O3. The SEM micrographs of the fresh C4 and C5 catalysts (Fig. 3(d) and (e)) show that they have more porous structure compared to the C3 (Fig. 3(c)) with higher specific surface areas of 6.4254 and 4.6646 m2/g, respectively. Considering the studies about the effect of catalyst surface area on its activity reports on increase in catalysts activity directly proportional to increase in catalysts surface area available [34,35]. Solid carbon as a decomposition product was determined on spent catalysts surface in SEM analyses. TPR profiles of all synthesized catalysts are given in Fig. 4. As shown in Fig. 4(b), pure yttrium oxide (C2) has no reduction peak. Hence, the reduction peaks of yttria doped Ni based catalysts (C3, C4 and C5), around 360 C, may be due to the reduction of Ni2þ to Ni0 [31,36]. Moreover, the reduction peak
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Fig. 4 e H2-TPR profiles of the catalysts: (a) C1, (b) C2, (c) C3, (d) C4 and (e) C5.
of C3 has a lower intensity than C4 and C5 due to its lower nickel oxide content.
Catalytic decomposition of methane (CDM)
Fig. 3 e SEM micrographs of the synthesized (a) C1, (b) C2, (c) C3, (d) C4, (e) C5 and spent (a*) C1, (b*) C2, (c*) C3, (d*) C4, (e*) C5 catalysts at 20 000X magnification.
According to the experimental results, methane can be thermally decomposed into hydrogen and carbon at temperatures above 880 C. Hence, use of catalysts renders hydrogen production possible at lower reaction temperatures. Variation in CH4 conversions over the yttria doped nickel oxide catalysts at reaction temperatures increasing from 390 C to 845 C are presented in Fig. 5. Methane decomposition was not possible over the catalyst C2 (Y2O3) in this temperature range. Compared to yttria doped catalysts, pure nickel oxide (C1) exhibited strong deactivation. In literature, temperature levels for catalytic methane decomposition over Ni based catalysts are in the range of 550e900 C [37e39]. Over yttria doped nickel oxide catalyst lowering the reaction temperature to 390 C was possible. Fig. 5(a) shows the CH4 conversion over the C1, C4 and C5 catalysts at 390 C. The yttria doped catalysts C4 and C5 displayed higher activity through initial methane conversions of 9.6% and 5.9%, respectively, followed by a rapid drop in their activities. Fig. 5(b) presents the methane conversion over the same catalysts (C1, C4 and C5) at the reaction temperature of 500 C, where the highest methane conversion was 14%; indicating that an increase in reaction temperature enhanced the methane conversion due to the endothermic nature of the reaction. However, nickel oxide (C1) lost its activity in a run time of 12min and the immediate decrease in its activity can be explained in terms of the high amount of carbon formation and deposition on the active Ni surfaces [11]. Considering data available in literature, results obtained in the present work were comparable to those obtained at 500 C with a catalyst containing a similar Ni loading of 70% (mole ratio of NiO to that of support material) [40]. Maximum conversion obtained in the present work was 14% compared to the reported 6%. However, catalyst life time was considerably shorter. Fig. 5(c) displays the methane conversion over the nickel oxide (C1) and yttria doped nickel oxide (C4 and C5) catalysts at the temperature of 590 C. It was observed that the activity of the catalyst C4 decreased with the increase in
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Fig. 5 e Comparison of CH4 conversions: (a) over the C1, C4 and C5 catalysts at T ¼ 390 C, (b) over the C1, C4 and C5 catalysts at 500 C, (c) over the C1, C4 and C5 catalysts at T ¼ 590 C, (d) over the C4 and C5 catalysts at T ¼ 690 C, (e) over the C3, C4 and C5 catalysts at T ¼ 800 C, (f) over the C3 catalyst at T ¼ 845 C.
reaction temperature, which showed that 500 C was more suitable for carrying out methane decomposition over this catalyst. Similar result was obtained by Rastegarpanah et al. [41]. They tested Ni/xMgO$Al2O3 catalysts in methane decomposition at the temperatures of 600 and 675 C and reported that each catalyst lost its activity at 675 C faster than at 600 C, which indicated that 600 C is moderate as a reaction condition. C5 was an active catalyst with an initial methane conversion of 12.7% at 500 C, but the catalyst lost its activity in 35 min. After the catalyst was deactivated entirely at 500 C, increasing the reaction temperature to 590 C led to an increase in maximum methane conversion of 3% with better catalyst durability (Fig. 5(c)). Fig. 5(d) presents the methane conversions over the catalysts C4 and C5 at the temperature of 690 C. The maximum methane conversions obtained over C4 and C5 were 3% and 2.3%, respectively. Besides, conversions over both catalysts were followed by a steady state.
The yttria doped nickel oxide catalyst with the NiO/Y2O3 mole ratio of 1:1 (C4) exhibited no catalytic activity in the temperature range between 390 C and 790 C. As given in Fig. 5(e), the initial methane conversion over the catalyst C4 was around 50% at 800 C and after a short duration the catalyst lost its activity. The kinetic curves of methane conversions over the samples C4 and C5 were close to each other despite of the difference in their nickel oxide contents, and highest methane conversions over these catalysts were 8.3% and 7.7%, respectively. Fig. 5(f) indicates that C4 is still an active catalyst after increasing the temperature to 845 C, but with the initial methane conversion of 1.2%. In view of overall catalyst evaluation, C4 and C5 catalysts have displayed higher activity. Related to the agglomerated structure and lower specific surface area of the C3 catalyst, it has been expected that this catalyst would have a lower activity than the C4 and C5 catalysts. SEM micrographs of the
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Acknowledgement Project 213M368 supported by TUBITAK-MAG and project 2016-07-01-DOP01 supported by Research Fund of the Yildiz Technical University are gratefully acknowledged.
references
Fig. 6 e Comparison of weight loss percentage of spent C1, C2, C3, C4 and C5 catalysts.
spent C3 catalyst verified this theory with a less amount of filamentous carbon on C3 catalyst surface. In Fig. 2(d*) and 2 (e*), a high amount of filamentous carbon on the spent C4 and C5 catalysts particles was displayed, and it indicated that decomposition of methane over these catalysts was performed with a higher activity. Also, accumulated carbon on catalysts surface was determined by thermogravimetric analysis (TGA), and the results were shown in Fig. 6. There is a direct correlation between weight loss percentage and accumulated carbon on spent catalysts. The percentages of C4 and C5 catalysts were higher than C3 as expected. Hence, SEM micrographs were verified by TGA results.
Conclusion In the present study, NiO/Y2O3 catalysts were synthesized by sol-gel method and the effect of addition of yttria on catalyst performance was investigated. Catalytic tests were performed in the temperature range 390e845 C in a tubular reactor. The results indicated that yttria addition increased the catalyst stability and enhanced the catalyst activity at lower reaction temperatures. Pure nickel oxide is an inactive catalyst at the temperature of 845 C. Yttria addition improved its activity at elevated temperatures. Hence, in high temperature processes, using yttria doped nickel oxide catalysts can be a convenient approach. With the addition of yttrium oxide to the catalyst structure, an increase in specific surface area of the nickel oxide catalyst was observed (C3, C4 and C5). Experimental data indicated a correlation between surface area and catalytic activity of yttria doped nickel based catalysts. The C4 catalyst, which has the highest surface area of 6.4254 m2/g, exhibited a better activity compared to the C1 and C5 catalysts, which have surface areas of 2.6456 and 4.6646 m2/g, respectively. Conversion values obtained with the present catalyst (C4) were comparable (even higher) to those reported in the literature; however excess carbon coverage was encountered.
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