- Email: [email protected]
SiO2 catalysts for the methane decomposition reaction
international journal of hydrogen energy 35 (2010) 12091–12097
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Effect of Ca, Ce or K oxide addition on the activity of Ni/SiO2 catalysts for the methane decomposition reaction Beatriz Zapata a, Miguel A. Valenzuela b,*, Jorge Palacios b, Enelio Torres-Garcia a a b
Instituto Mexicano del Petro´leo, Programa de Procesos y Reactores, Eje C. 152, Me´xico, D.F., C.P. 07730, Mexico Instituto Polite´cnico Nacional-ESIQIE, Lab. Cata´lisis y Materiales, Zacatenco, Me´xico, D.F., C.P. 07738, Mexico
article info
abstract
Article history:
To increase the activity and stability of Ni/SiO2 catalysts, a series of Ni–Ca, Ni–K and Ni–Ce
Received 14 April 2009
promoted catalysts were prepared by successive impregnations. The textural properties,
Received in revised form
reducibility and catalytic performance in the methane decomposition reaction were
8 August 2009
investigated. The catalyst containing 30 wt.% Ni and 30 wt.% cerium oxide greatly
Accepted 14 September 2009
increased the conversion of methane (90% of equilibrium value) and improved the stability,
Available online 30 October 2009
whereas the Ni–K and Ni–Ca were less active and stable than the Ni/SiO2 catalyst. The results suggest that Ce addition prevents the sintering of nickel particles during reduction
Keywords:
process maintaining a random distribution between the silica and cerium oxide improving
Hydrogen production
the distribution and migration of deposited carbon.
Methane decomposition
ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Nickel–ceria–silica catalyst
1.
Introduction
The demand for hydrogen is ever increasing due to its use in chemical processing, electronics, food processing, metal manufacturing and fuel cells [1]. Hydrogen production, from water-splitting by using solar light or other renewable energy sources, is not competitive from an economical point of view [2]. Therefore, in the near future hydrogen production will continue to depend on fossil fuels, mainly natural gas [3]. Commonly, most of the industrial hydrogen production is based on the steam reforming process which is a source of significant CO2 emissions into the atmosphere [4]. Catalytic partial oxidation and autothermal reforming combined with CO2-sequestration are alternatives to the conventional steam reforming process [5,6]. The catalytic decomposition of methane (CDM) can be used to obtain COx-free hydrogen and has become an interesting research topic recently [7–15]. It produces pure hydrogen and carbon and there is no necessity for the separation of hydrogen from other gases, such as COx in the conventional processes.
The most studied catalysts for the CDM are nickel supported on: SiO2, TiO2, graphite, ZrO2, SiO2–Al2O3, Al2O3, MgO–SiO2, MgO, ZnAl2O4 and Ce–SiO2 among others [7–15]. Although other metals such as Fe, Pt, Pd, Cr, Ru, Mo or W have been tested in the CMD as well [7,13]. The catalysts based in noble metals have been found to be less sensitive to carbon deposition; however, due to their high cost it is more practical to develop Ni-based catalysts with high performance and high resistance to carbon deposition. The activity and life of the supported Ni catalysts for CDM strongly depend on the particle size of Ni metal and the textural properties of the support [10,13,15]. Unfortunately, rapid deactivation of Ni-based catalyst results at temperatures in excess of 600 C, leading to a low yield of hydrogen. The catalyst deactivation occurs when the metallic particles are encapsulated by non-reactive carbon compounds [16]. Several efforts have been done to increase the catalyst lifetime [17]. To improve the activity and stability of the catalysts and minimize their deactivation, bimetallic catalysts such as Ni–Cu have been reported [18]. In addition, a best
* Corresponding author. Tel: þ5255 57296000 55112, E-mail address: [email protected] (M.A. Valenzuela). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.09.072
12092
international journal of hydrogen energy 35 (2010) 12091–12097
Table 1 – Textural properties obtained by nitrogen physisorption of the catalysts. Specific surface Pore volume Average pore ˚) (mL/g) diameter (A area (m2/g)
Catalyst Ni–CaO/SiO2 Ni–K2O/SiO2 Ni–CeO2/SiO2 Ni/SiO2
212 192 126 249
0.43 0.40 0.21 0.48
81 85 66 77
Experimental
2.1.
Catalysts preparation
Catalyst
Ni/CaO/SiO2 Ni/K2O/SiO2 Ni/CeO/SiO2 Ni/SiO2
distribution of deposited carbon can be obtained when Ni is impregnated on supports that have been modified with alkaline metals such as Li, K or Ca [19]. In the present work, the effect of CaO, K2O or CeO2 as promoters of Ni supported on silica on the activity and stability during the CMD at 580 C was mainly studied. The catalysts were prepared by successive impregnations (first promoter oxide, then, Ni) with calcination between each step. Nitrogen physisorption, temperature-programmed reduction (TPR), X-ray diffraction (XRD), hydrogen chemisorption, Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM) were the characterization techniques used.
2.
Table 2 – Hydrogen chemisorption analyses of Ni and Nipromoted catalysts after reduction pre-treatment in hydrogen flow at 580 8C.
Silica (Davisil) of 100–200 mesh with 480 m2/g, particle size of 6 nm, pore volume of 6.75 mL/g and 99% purity was used as support. Ni and the promoted catalyst were prepared by impregnation using the corresponding nitrates dissolved in ethanol (Ni and Ni–Ca nitrates) and water (Ni–K and Ni–Ce). A typical preparation of Ni/SiO2 catalyst was as follows: 10 g of SiO2 was placed in a ball flask and then 60 cm3 of the alcoholic nitrate solution containing 30 wt.% Ni was added. The mixture was maintained under stirring during 30 min and the excess of alcohol was eliminated in a rotavapor at 95 C. The powder was dried at 90 C by 16 h and finally calcined at 500 C, 4 h. The
Avg. part. size (nm)
Metallic surface (m2/gcat)
Metallic dispersion (%)
958 318 5 155
0.2 0.6 45.4 1.3
0.1 0.3 22.7 0.6
promoted catalysts were prepared by successive impregnations. The promoter was firstly added following the above described procedure. The nominal amount of Ni and the promoter was 30 wt.%, 3 wt.% (Ca and K) and 30 wt.% for Ce, respectively.
2.2.
Characterization techniques
Nitrogen physisorption was carried out in a Micromeritics ASAP-2405. Previously, the samples were degasified at 300 C and vacuum during 18 h with a nitrogen flow. TPR experiments were performed in a Micromeritics TPD/TPR–2900 apparatus using a mixture of 5 vol. of H2% in argon with a heating rate of 5 C/min from room temperature to 800 C. The pre-treatment of the sample consisted in an oxidation at 400 C for one hour and heating rate of 10 C/min. X-ray diffraction patterns were recorded on a Bruker D8 Discover diffractometer using Cu Ka radiation operated at 40 kV and 30 mA. Hydrogen chemisorption analyses were carried in a Micromeritics TPD/TPR-2920 by pulses method. The catalysts were pre-treated with a pure H2 flow at 580 C and then cooled at room temperature with an Ar flow. H2 chemisorption analyses were performed at 25 C. The Raman spectra were recorded at room temperature using an Yvon Jobin Horiba (T64000) spectrometer equipped with a confocal microscope (Olympus, BX41) with a laser 514.5 nm at a power level of 10 mW. The spectrometer is equipped with a CCD detector, which is Peltier-cooled to 243 K to reduce thermal noise. The samples were studied by high-resolution
Ni
398
Ni-K2O/SiO2
Ni-CaO/SiO2 Ni-CeO2/SiO2
366
Intensity (a.u.)
H2 consupmtion (a.u.)
471
Ni CeO2 CeO2
Ni-CeO2/SiO2 CeO2
428
Ni-CaO/SiO2
Ni
NiO
Ni-K2O/SiO2 NiO
502
Ni/SiO2 Ni/SiO2 10
0
100
200
300
400
500
600
700
800
Temperature (°C) Fig. 1 – TPR profiles of Ni and Ni-promoted catalysts.
20
30
40
50
60
70
80
2 Theta, degrees
Fig. 2 – XRD patterns of Ni and Ni-promoted catalysts after reduction pre-treatment in hydrogen flow at 580 8C.
international journal of hydrogen energy 35 (2010) 12091–12097
atmospheric pressure. In all experiments, 0.05 g of catalyst and a total flow rate of 100 mL/min were used. The feed stream was a mixture of high purity methane (8 mol% CH4) diluted in argon. The gaseous effluent was analyzed by a GC on line using two detectors (TCD for hydrogen and FID for methane).
90 80
Conversion (%)
70 60
Ni-CeO2/SiO2
50
NiCaO/SiO2 Ni-K2O/SiO2
40
3.
Ni/SiO2
30 20 10 0 0
30
60
90
120
150
180
Reaction time (min)
Fig. 3 – Deactivation profiles of the catalysts at 580 8C and a mixture of 8% CH4/Ar. transmission electron microscopy (HRTEM), the micrographs were obtained in a JEOL JEM-2200FS with Schottky type field emission gun, operating at 200 kV, and point to point resolution of 0.19 nm. HRTEM digital images were obtained using a CCD camera and Digital Micrograph Software from GATAN. In order to prepare the materials for observation, the powder samples were dispersed in ethanol and supported on lacey formvar/ carbon copper grids.
2.3.
12093
Catalytic evaluation
1575
1348
Methane decomposition was carried out using a continuous flow stainless steel fixed bed reactor operated at 580 C at
Intensity (a.u.)
1610
Ni-CaO/SiO2
Ni-CeO2/SiO2
Ni-K2O/SiO2
1300
1400
1500
1600
Raman Shift
(cm-1)
1700
1800
Fig. 4 – Changes in Raman spectra in the range of 1200– 1800 cmL1 of different spent catalysts.
Results and discussion
Table 1 shows the textural properties of the prepared catalysts. In general, the addition of promoters did not modify the adsorption isotherm and the hysteresis loop of that of the reference catalyst (Ni/SiO2). However, a significant reduction in surface area was observed with the addition of promoters to Ni/SiO2, being more critical with cerium oxide due to its major amount (30 wt.%). TPR is used extensively for characterizing both nickel and nickel-doped catalysts [12,15,16]. Fig. 1 shows the TPR profiles of Ni/SiO2 and the promoted catalysts. Pure NiO is reduced at 400–420 C (dotted line in Fig. 1). Generally, for supported Ni catalysts the lower temperature peaks are attributed to the reduction of bulk NiO particles without interaction with the support, while the higher temperature peaks are assigned to the reduction of NiO particles with chemical interaction with the support or other surface compounds [20]. The Ni/SiO2 catalyst presented two reduction peaks at 370 and 520 C (Fig. 1), which can be attributed to bulk NiO (small particles) and NiO in intimate contact with the support. The Ni–K catalyst showed only one hydrogen consumption peak at 398 C, representing the characteristic reduction of stoichiometric nickel oxide. This means that the presence of potassium oxide on the surface on silica inhibits the interaction of Ni with silica which explains the presence of only one reduction peak attributed to bulk NiO. A quite similar reduction behavior was observed with the Ni–Ca catalyst, though the mean reduction peak was slightly shifted to 440 C. This may indicate that both CaO and K2O are homogeneously distributed on silica and when Ni is deposited on the modified support (i.e., CaO/SiO2 and K2O/SiO2) the characteristic property of a non-reducible behavior is obtained, and then NiO is easily reduced in comparison with the Ni/SiO2 catalyst. The Ni–Ce catalyst presented a different reduction behavior, a wide reduction band beginning at 370 C and finishing at around 600 C, with a maximum at 471 C was observed (Fig. 1). Usually, in Ni/CeO2 catalysts, the reduction profiles shows three zones of hydrogen consumption between 225–270 C, 300–400 C and 750–900 C, assigned to the reduction of adsorbed oxygen, NiO aggregated on the surface of CeO2 and bulk CeO2, respectively [21]. As is well known, the incorporation of Ni2þ into the lattice of CeO2 can cause charge unbalanced and lattice distortion [21]. Therefore, Ni– O–Ce solid solution is formed during Ni–Ce catalyst preparation favoring the reduction at low temperature of Ni and Ce [21]. Due to no low and high temperature reduction peaks were observed in the Ni/CeO2 catalyst, free small NiO particles aggregated on the surface of CeO2 are converted into metallic Ni particles. The results of hydrogen chemisorption of different catalysts are presented in Table 2. It is shown that the promoters do not
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international journal of hydrogen energy 35 (2010) 12091–12097
Fig. 5 – TEM images of the carbon formed by methane decomposition over spent catalysts: (a) Ni/SiO2 and (b) Ni–K.
increase the low Ni dispersion of Ni/SiO2 catalyst, except with the addition of Ce. This strange behavior could be explained in terms of the amount of Ce added (30 wt.%) which prevents the sintering of Ni particles during the reduction process. Fig. 2 shows the XRD patterns of the catalysts reduced at 580 C. In all samples appeared the characteristic peaks corresponding to metallic Ni planes (111), (200) and (222) at 2q ¼ 44.5, 51.8 and 76.4 , respectively. However, a small amount of NiO was found in all samples probably due to the formation of NiO– MO (M ¼ K, Ca or Ce) solid solutions difficulting NiO reduction. In
the case of Ni–Ce catalyst, broad peaks corresponding to CeO2 (fluorite structure) were clearly detected, though, one peak at 2q ¼ 33 , which was detected in the calcined catalyst (not shown here) did not appear in the reduced sample. This result could be an evidence of the formation of NiO–CeO2 solid solution during the reduction process. Fig. 3 shows the deactivation profiles of the catalysts at 580 C reaction temperature. The Ni–Ce catalyst presented methane conversion values closely to that of the equilibrium (P ¼ 1 atm, [CH4] ¼ 8 mol% and T ¼ 580 C). However, it was
international journal of hydrogen energy 35 (2010) 12091–12097
12095
Fig. 6 – TEM images of the carbon formed by methane decomposition over spent Ni–Ce catalyst.
deactivated quickly and the ratio of methane conversion after 1 h reaction time between initial conversion (stability factor, SF ¼ XCH4/XCH4o) was 0.77. After this deactivation step, the methane conversion was maintained constant. As can be seen in Fig. 3, the initial conversion of Ni–Ca and Ni–K was almost ´ of 0.375 and 0.125 respectively. The referthe same, with S Fs ence catalyst (Ni/SiO2) was slightly more active than the Ni–Ca and Ni–K and its initial conversion was half of the Ni–Ce. The SF for this catalyst was 0.44 taken after being reached its steady-state conversion (t ¼ 90 min). From these results, it is clear that the addition of calcium or potassium oxides did not have a beneficial effect compared with the Ni/SiO2 catalyst. At this point, it can be speculated that the presence of Ca and K oxides on the surface of silica inhibits the activity of Ni particles and also these particles are not resistant to deactivation of deposited carbon. On the contrary, the Ni–Ce catalyst was highly active and stable even at high load of cerium oxide (30 wt.%) which is explained in terms of the formation of small Ni particles highly dispersed on surface ceria. The Raman spectroscopy provides important information on the carbon structure and nature. Therefore, we would like to focus our attention on the main frequency range corresponding to carbonaceous Raman response. Fig. 4 shows the Raman spectra of carbons after methane decomposition over Ni catalysts. In general, there are two broad peaks at around 1348 and 1575 cm1. The former is usually ascribed to an A1g-
type mode called the disorder (‘‘D’’ band) and the latter is attributed to the stretching mode of the individual sheets in graphite (E2g mode, the so-called ‘‘G’’ band of graphite) [22]. The appearance of these two bands is in agreement with the finite size of the graphite particles and the ratios among these bands provide detailed information about the changes in the structure of carbon deposited [23]. These results indicate that the carbonaceous deposits become a mixture of order and disordered carbons and they can be considered as sensitive and insensitive from catalytic point of view [24]. Additionally, it is interesting to note the presence of a shoulder peak at 1610 cm1 (D0 band) typical of defective graphite-like materials and may be also related to the decay of the continuous carbon network into separate very small clusters. The catalytic activity results presented in Fig. 3 could indicate, in the case of the Ni–Ce catalyst, that the decrease in methane conversion from 88 to 67% in 1 h of reaction time, acquires a pseudo stable configuration after this time. This result suggests that the carbon deposited can be responsible for the rapid deactivation and an emerging stabilization of the small Ni particles by means of some kind of reaction between the carbon and surface Ni. Then, carbon could just play a role as stabilizing or participating in a synergetic effect that enhance the properties of the catalyst. Fig. 5 shows the TEM images of the deposited carbon on the Ni/SiO2 and Ni–K catalysts after 3 h of reaction time. A great
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international journal of hydrogen energy 35 (2010) 12091–12097
difference in the morphologies of the deposited carbon and metallic particles could be observed. For the Ni/SiO2 sample, the carbon was mainly as nanofiber form with sizes ranging from 20 to 40 nm. The nickel particles were located at the tips of the carbon nanofibers, and the diameter of the nickel particles was almost the same as those of the growing nanofibers. The smaller Ni particles (ca. 20 nm) were the most tended to form carbon nanofibers, some relatively medium size Ni particles (ca. 40 nm) and large (60 nm) were encapsulated by graphitic carbon that greatly inhibited the contact of Ni particles with methane. This confirms that the particle size of metallic nickel plays an important role in the process of methane decomposition [9,10,21]. It also provides evidence that large Ni particles cannot carry out the methane decomposition reaction, thus explaining the low activity of this catalyst. It can be seen that several Ni particles departed from the fibrous structures are aggregated over the deposited carbon. The Ni–K sample exhibited three Ni particle sizes similar than those of the Ni/SiO2 sample and several largest K2O particles (w70 nm) was also observed. In this case, carbon is formed like small clusters aggregates together with big particles of Ni and K2O and most of small Ni particles were encapsulated by large aggregated of deposited carbon. Therefore, the Ni–K sample showed the lowest stability of all catalysts. The TEM image of the spent Ni–Ce catalyst is shown in Fig. 6. The carbons were in nanofiber form as well as graphite form. In general, the Ni particle size was less than 10 nm associated with big irregular particles (50–100 nm), probably CeO2 particles, which confirms the results obtained by hydrogen chemisorption (Table 2). Some spherical dark particles together with irregular particles encapsulated inside the carbon nanofibers can be seen. However, in this catalyst the deactivation behavior was only significant at short reaction times (1 h) and then a high stability was observed.
4.
Conclusions
The catalytic behavior of the Ni/SiO2 catalysts in methane decomposition was found to be strongly dependent of the kind of promoter used. Ni–K and Ni–Ca catalysts were less active and stable than the Ni–Ce catalyst. TPR results indicated a different reduction behavior with each catalyst. Ni–K and Ni–Ca catalysts did not show evidence of chemical interaction. The interaction ways of Ni–Ce catalyst was found to determine the Ni particle size, morphology and the reactivity with deposited carbon. The Ni–SiO2, Ni–K and Ni–Ca catalysts were easily covered by graphitic carbon inhibiting the interaction with the methane and consequently giving rise to a lower activity a rapid and permanent deactivation. On the contrary, in the Ni–Ce catalyst the small Ni particles located at the top of the carbon nanofiber act as useful catalytic active sites for the methane decomposition continuously.
Acknowledgements The authors thank Gabriela Gomez Gasga and Claudia Rueda Martinez for their technical work in XRD and H2 chemisorption
measurements, respectively. The technical assistance in HRTEM by Paz del Angel Vicente is gratefully appreciated.
references
[1] Barreto L, Makihira A, Riahi K. The hydrogen economy in the 21st century: a sustainable development scenario. Int J Hydrogen Energy 2003;28:267–84. [2] Otsuka K, Kobayashi S, Takenaka S. Hydrogen–deuterium exchange studies on the decomposition of methane over Ni/ SiO2. J Catal 2001;200:4–9. [3] Muradov NZ, Vezirog˘lu TN. From hydrocarbon to hydrogen– carbon to hydrogen economy. Int J Hydrogen Energy 2005;30: 225–37. [4] Rostrup-Nielsen JR. Fuels and energy for the future: the role of catalysis. Catal Rev 2004;46:247–70. [5] York AP, Xiao TC, Green ML. Brief overview of the partial oxidation of methane to synthesis gas. Top Catal 2003;22:345–58. [6] Dias JAC, Assaf JM. Autothermal reforming of methane over Ni/g–Al2O3 catalysts: the enhancement effect of small quantities of noble metals. J Power Sources 2004;130:106–10. [7] Poirier MG, Zapundzhiev C. Catalytic decomposition of natural gas to hydrogen for fuel cells applications. Int J Hydrogen Energy 1997;22:429–33. [8] Steinberg M. Production of hydrogen and methanol from natural gas with reduced CO2 emission. Int J Hydrogen Energy 1998;23:419–25. [9] Zhang T, Amiridis MD. Hydrogen production via the direct cracking of methane over silica-supported nickel catalysts. Appl Catal A General 1998;167:161–72. [10] Takenaka S, Ogihara H, Yamanaka I, Otsuka K. Decomposition of methane over supported-Ni catalysts: effects of the supports on the catalytic lifetime. Appl Catal A General 2001;217:101–10. [11] Choudhary TV, Aksoylu E, Goodman. Nonoxidative oxidation of methane. Catal Rev 2003;45:151–203. [12] Valenzuela MA, Gonzalez O, Cordova I, Flores S, Wang JA. Hydrogen production by methane decomposition on nickel/ zinc aluminate catalysts. Chem Eng Trans 2004;4:61–6. [13] Otsuka K, Takenaka S. Production of hydrogen from methane by a CO2 emission-suppressed process: methane decomposition and gasification of carbon nanofibers. Catal Surveys Asia 2004;8:77–90. [14] Couttenye RA, De Vila MH, Suib SL. Decomposition of methane with autocatalytically reduced nickel catalyst. J Catal 2005;233:317–26. [15] Gonzalez OA, Valenzuela MA, Wang JA. Catalytic decomposition of methane over cerium-doped Ni catalysts. Mater Res Soc Symp Proc 2006;885E:49.1–6. [16] Li Y, Zhang B, Xie X, Liu J, Xu Y, Shen W. Novel Ni catalysts for methane decomposition to hydrogen and carbon nanofibers. J Catal 2006;238:412–24. [17] Navarro RM, Pen˜a MA, Fierro JLG. Hydrogen production reactions from carbon feedstocks: fo´sil fuels and biomasa. Chem Rev 2007;107:3952–91. and references therein. [18] Pinilla JL, Suelves I, La´zaro MJ, Moliner R, Palacios JM. Activity of NiCuAl catalyst in methane decomposition studied using a thermobalance and the structural changes in the Ni and the deposited carbon. Int J Hydrogen Energy 2008;33:2515–24. [19] Roh HS, Jun KW, Dong WS, Chang JS, Park SE, Joe II Y. Highly active and stable Ni/Ce–ZrO2 catalyst for hydrogen production from methane. J Mol Catal A Chemical 2002;181:137–42. [20] Venugopal A, Naveen Kumar S, Ashok J, Hari Prasad D, Durga Kumari V, Prasad KBS, et al. Hydrogen production by catalytic decomposition of methane over Ni/SiO. Int J Hydrogen Energy 2007;32:1782–8.
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[21] Li Y, Zhang B, Tang X, Xu Y, Shen W. Hydrogen production from methane decomposition over Ni/CeO2. Catal Comm 2006;7:380–6. [22] Jorio A, Pimenta MA, Souza Filho AG, Saito R, Dresselhaus G, Dresselhaus MS. Characterizing carbon nanotube samples with resonance Raman scattering. New J Physics 2003;5:139.1–139.17.
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[23] Nakamizo M, Kammereck R, Walker PL. Laser Raman studies on carbons. Carbon 1974;12:259–67. [24] Rodriguez-Gattorno G, Matinez-Hernandez A, AlemanVazquez LO, Torres-Garcia E. Structural and thermal study of carbon–modified molybdenum sub-oxide catalysts. Appl Catal A General 2007;321:117–24.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Effect of Ca, Ce or K oxide addition on the activity of Ni/SiO2 catalysts for the methane decomposition reaction Beatriz Zapata a, Miguel A. Valenzuela b,*, Jorge Palacios b, Enelio Torres-Garcia a a b
Instituto Mexicano del Petro´leo, Programa de Procesos y Reactores, Eje C. 152, Me´xico, D.F., C.P. 07730, Mexico Instituto Polite´cnico Nacional-ESIQIE, Lab. Cata´lisis y Materiales, Zacatenco, Me´xico, D.F., C.P. 07738, Mexico
article info
abstract
Article history:
To increase the activity and stability of Ni/SiO2 catalysts, a series of Ni–Ca, Ni–K and Ni–Ce
Received 14 April 2009
promoted catalysts were prepared by successive impregnations. The textural properties,
Received in revised form
reducibility and catalytic performance in the methane decomposition reaction were
8 August 2009
investigated. The catalyst containing 30 wt.% Ni and 30 wt.% cerium oxide greatly
Accepted 14 September 2009
increased the conversion of methane (90% of equilibrium value) and improved the stability,
Available online 30 October 2009
whereas the Ni–K and Ni–Ca were less active and stable than the Ni/SiO2 catalyst. The results suggest that Ce addition prevents the sintering of nickel particles during reduction
Keywords:
process maintaining a random distribution between the silica and cerium oxide improving
Hydrogen production
the distribution and migration of deposited carbon.
Methane decomposition
ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Nickel–ceria–silica catalyst
1.
Introduction
The demand for hydrogen is ever increasing due to its use in chemical processing, electronics, food processing, metal manufacturing and fuel cells [1]. Hydrogen production, from water-splitting by using solar light or other renewable energy sources, is not competitive from an economical point of view [2]. Therefore, in the near future hydrogen production will continue to depend on fossil fuels, mainly natural gas [3]. Commonly, most of the industrial hydrogen production is based on the steam reforming process which is a source of significant CO2 emissions into the atmosphere [4]. Catalytic partial oxidation and autothermal reforming combined with CO2-sequestration are alternatives to the conventional steam reforming process [5,6]. The catalytic decomposition of methane (CDM) can be used to obtain COx-free hydrogen and has become an interesting research topic recently [7–15]. It produces pure hydrogen and carbon and there is no necessity for the separation of hydrogen from other gases, such as COx in the conventional processes.
The most studied catalysts for the CDM are nickel supported on: SiO2, TiO2, graphite, ZrO2, SiO2–Al2O3, Al2O3, MgO–SiO2, MgO, ZnAl2O4 and Ce–SiO2 among others [7–15]. Although other metals such as Fe, Pt, Pd, Cr, Ru, Mo or W have been tested in the CMD as well [7,13]. The catalysts based in noble metals have been found to be less sensitive to carbon deposition; however, due to their high cost it is more practical to develop Ni-based catalysts with high performance and high resistance to carbon deposition. The activity and life of the supported Ni catalysts for CDM strongly depend on the particle size of Ni metal and the textural properties of the support [10,13,15]. Unfortunately, rapid deactivation of Ni-based catalyst results at temperatures in excess of 600 C, leading to a low yield of hydrogen. The catalyst deactivation occurs when the metallic particles are encapsulated by non-reactive carbon compounds [16]. Several efforts have been done to increase the catalyst lifetime [17]. To improve the activity and stability of the catalysts and minimize their deactivation, bimetallic catalysts such as Ni–Cu have been reported [18]. In addition, a best
* Corresponding author. Tel: þ5255 57296000 55112, E-mail address: [email protected] (M.A. Valenzuela). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.09.072
12092
international journal of hydrogen energy 35 (2010) 12091–12097
Table 1 – Textural properties obtained by nitrogen physisorption of the catalysts. Specific surface Pore volume Average pore ˚) (mL/g) diameter (A area (m2/g)
Catalyst Ni–CaO/SiO2 Ni–K2O/SiO2 Ni–CeO2/SiO2 Ni/SiO2
212 192 126 249
0.43 0.40 0.21 0.48
81 85 66 77
Experimental
2.1.
Catalysts preparation
Catalyst
Ni/CaO/SiO2 Ni/K2O/SiO2 Ni/CeO/SiO2 Ni/SiO2
distribution of deposited carbon can be obtained when Ni is impregnated on supports that have been modified with alkaline metals such as Li, K or Ca [19]. In the present work, the effect of CaO, K2O or CeO2 as promoters of Ni supported on silica on the activity and stability during the CMD at 580 C was mainly studied. The catalysts were prepared by successive impregnations (first promoter oxide, then, Ni) with calcination between each step. Nitrogen physisorption, temperature-programmed reduction (TPR), X-ray diffraction (XRD), hydrogen chemisorption, Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM) were the characterization techniques used.
2.
Table 2 – Hydrogen chemisorption analyses of Ni and Nipromoted catalysts after reduction pre-treatment in hydrogen flow at 580 8C.
Silica (Davisil) of 100–200 mesh with 480 m2/g, particle size of 6 nm, pore volume of 6.75 mL/g and 99% purity was used as support. Ni and the promoted catalyst were prepared by impregnation using the corresponding nitrates dissolved in ethanol (Ni and Ni–Ca nitrates) and water (Ni–K and Ni–Ce). A typical preparation of Ni/SiO2 catalyst was as follows: 10 g of SiO2 was placed in a ball flask and then 60 cm3 of the alcoholic nitrate solution containing 30 wt.% Ni was added. The mixture was maintained under stirring during 30 min and the excess of alcohol was eliminated in a rotavapor at 95 C. The powder was dried at 90 C by 16 h and finally calcined at 500 C, 4 h. The
Avg. part. size (nm)
Metallic surface (m2/gcat)
Metallic dispersion (%)
958 318 5 155
0.2 0.6 45.4 1.3
0.1 0.3 22.7 0.6
promoted catalysts were prepared by successive impregnations. The promoter was firstly added following the above described procedure. The nominal amount of Ni and the promoter was 30 wt.%, 3 wt.% (Ca and K) and 30 wt.% for Ce, respectively.
2.2.
Characterization techniques
Nitrogen physisorption was carried out in a Micromeritics ASAP-2405. Previously, the samples were degasified at 300 C and vacuum during 18 h with a nitrogen flow. TPR experiments were performed in a Micromeritics TPD/TPR–2900 apparatus using a mixture of 5 vol. of H2% in argon with a heating rate of 5 C/min from room temperature to 800 C. The pre-treatment of the sample consisted in an oxidation at 400 C for one hour and heating rate of 10 C/min. X-ray diffraction patterns were recorded on a Bruker D8 Discover diffractometer using Cu Ka radiation operated at 40 kV and 30 mA. Hydrogen chemisorption analyses were carried in a Micromeritics TPD/TPR-2920 by pulses method. The catalysts were pre-treated with a pure H2 flow at 580 C and then cooled at room temperature with an Ar flow. H2 chemisorption analyses were performed at 25 C. The Raman spectra were recorded at room temperature using an Yvon Jobin Horiba (T64000) spectrometer equipped with a confocal microscope (Olympus, BX41) with a laser 514.5 nm at a power level of 10 mW. The spectrometer is equipped with a CCD detector, which is Peltier-cooled to 243 K to reduce thermal noise. The samples were studied by high-resolution
Ni
398
Ni-K2O/SiO2
Ni-CaO/SiO2 Ni-CeO2/SiO2
366
Intensity (a.u.)
H2 consupmtion (a.u.)
471
Ni CeO2 CeO2
Ni-CeO2/SiO2 CeO2
428
Ni-CaO/SiO2
Ni
NiO
Ni-K2O/SiO2 NiO
502
Ni/SiO2 Ni/SiO2 10
0
100
200
300
400
500
600
700
800
Temperature (°C) Fig. 1 – TPR profiles of Ni and Ni-promoted catalysts.
20
30
40
50
60
70
80
2 Theta, degrees
Fig. 2 – XRD patterns of Ni and Ni-promoted catalysts after reduction pre-treatment in hydrogen flow at 580 8C.
international journal of hydrogen energy 35 (2010) 12091–12097
atmospheric pressure. In all experiments, 0.05 g of catalyst and a total flow rate of 100 mL/min were used. The feed stream was a mixture of high purity methane (8 mol% CH4) diluted in argon. The gaseous effluent was analyzed by a GC on line using two detectors (TCD for hydrogen and FID for methane).
90 80
Conversion (%)
70 60
Ni-CeO2/SiO2
50
NiCaO/SiO2 Ni-K2O/SiO2
40
3.
Ni/SiO2
30 20 10 0 0
30
60
90
120
150
180
Reaction time (min)
Fig. 3 – Deactivation profiles of the catalysts at 580 8C and a mixture of 8% CH4/Ar. transmission electron microscopy (HRTEM), the micrographs were obtained in a JEOL JEM-2200FS with Schottky type field emission gun, operating at 200 kV, and point to point resolution of 0.19 nm. HRTEM digital images were obtained using a CCD camera and Digital Micrograph Software from GATAN. In order to prepare the materials for observation, the powder samples were dispersed in ethanol and supported on lacey formvar/ carbon copper grids.
2.3.
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Catalytic evaluation
1575
1348
Methane decomposition was carried out using a continuous flow stainless steel fixed bed reactor operated at 580 C at
Intensity (a.u.)
1610
Ni-CaO/SiO2
Ni-CeO2/SiO2
Ni-K2O/SiO2
1300
1400
1500
1600
Raman Shift
(cm-1)
1700
1800
Fig. 4 – Changes in Raman spectra in the range of 1200– 1800 cmL1 of different spent catalysts.
Results and discussion
Table 1 shows the textural properties of the prepared catalysts. In general, the addition of promoters did not modify the adsorption isotherm and the hysteresis loop of that of the reference catalyst (Ni/SiO2). However, a significant reduction in surface area was observed with the addition of promoters to Ni/SiO2, being more critical with cerium oxide due to its major amount (30 wt.%). TPR is used extensively for characterizing both nickel and nickel-doped catalysts [12,15,16]. Fig. 1 shows the TPR profiles of Ni/SiO2 and the promoted catalysts. Pure NiO is reduced at 400–420 C (dotted line in Fig. 1). Generally, for supported Ni catalysts the lower temperature peaks are attributed to the reduction of bulk NiO particles without interaction with the support, while the higher temperature peaks are assigned to the reduction of NiO particles with chemical interaction with the support or other surface compounds [20]. The Ni/SiO2 catalyst presented two reduction peaks at 370 and 520 C (Fig. 1), which can be attributed to bulk NiO (small particles) and NiO in intimate contact with the support. The Ni–K catalyst showed only one hydrogen consumption peak at 398 C, representing the characteristic reduction of stoichiometric nickel oxide. This means that the presence of potassium oxide on the surface on silica inhibits the interaction of Ni with silica which explains the presence of only one reduction peak attributed to bulk NiO. A quite similar reduction behavior was observed with the Ni–Ca catalyst, though the mean reduction peak was slightly shifted to 440 C. This may indicate that both CaO and K2O are homogeneously distributed on silica and when Ni is deposited on the modified support (i.e., CaO/SiO2 and K2O/SiO2) the characteristic property of a non-reducible behavior is obtained, and then NiO is easily reduced in comparison with the Ni/SiO2 catalyst. The Ni–Ce catalyst presented a different reduction behavior, a wide reduction band beginning at 370 C and finishing at around 600 C, with a maximum at 471 C was observed (Fig. 1). Usually, in Ni/CeO2 catalysts, the reduction profiles shows three zones of hydrogen consumption between 225–270 C, 300–400 C and 750–900 C, assigned to the reduction of adsorbed oxygen, NiO aggregated on the surface of CeO2 and bulk CeO2, respectively [21]. As is well known, the incorporation of Ni2þ into the lattice of CeO2 can cause charge unbalanced and lattice distortion [21]. Therefore, Ni– O–Ce solid solution is formed during Ni–Ce catalyst preparation favoring the reduction at low temperature of Ni and Ce [21]. Due to no low and high temperature reduction peaks were observed in the Ni/CeO2 catalyst, free small NiO particles aggregated on the surface of CeO2 are converted into metallic Ni particles. The results of hydrogen chemisorption of different catalysts are presented in Table 2. It is shown that the promoters do not
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Fig. 5 – TEM images of the carbon formed by methane decomposition over spent catalysts: (a) Ni/SiO2 and (b) Ni–K.
increase the low Ni dispersion of Ni/SiO2 catalyst, except with the addition of Ce. This strange behavior could be explained in terms of the amount of Ce added (30 wt.%) which prevents the sintering of Ni particles during the reduction process. Fig. 2 shows the XRD patterns of the catalysts reduced at 580 C. In all samples appeared the characteristic peaks corresponding to metallic Ni planes (111), (200) and (222) at 2q ¼ 44.5, 51.8 and 76.4 , respectively. However, a small amount of NiO was found in all samples probably due to the formation of NiO– MO (M ¼ K, Ca or Ce) solid solutions difficulting NiO reduction. In
the case of Ni–Ce catalyst, broad peaks corresponding to CeO2 (fluorite structure) were clearly detected, though, one peak at 2q ¼ 33 , which was detected in the calcined catalyst (not shown here) did not appear in the reduced sample. This result could be an evidence of the formation of NiO–CeO2 solid solution during the reduction process. Fig. 3 shows the deactivation profiles of the catalysts at 580 C reaction temperature. The Ni–Ce catalyst presented methane conversion values closely to that of the equilibrium (P ¼ 1 atm, [CH4] ¼ 8 mol% and T ¼ 580 C). However, it was
international journal of hydrogen energy 35 (2010) 12091–12097
12095
Fig. 6 – TEM images of the carbon formed by methane decomposition over spent Ni–Ce catalyst.
deactivated quickly and the ratio of methane conversion after 1 h reaction time between initial conversion (stability factor, SF ¼ XCH4/XCH4o) was 0.77. After this deactivation step, the methane conversion was maintained constant. As can be seen in Fig. 3, the initial conversion of Ni–Ca and Ni–K was almost ´ of 0.375 and 0.125 respectively. The referthe same, with S Fs ence catalyst (Ni/SiO2) was slightly more active than the Ni–Ca and Ni–K and its initial conversion was half of the Ni–Ce. The SF for this catalyst was 0.44 taken after being reached its steady-state conversion (t ¼ 90 min). From these results, it is clear that the addition of calcium or potassium oxides did not have a beneficial effect compared with the Ni/SiO2 catalyst. At this point, it can be speculated that the presence of Ca and K oxides on the surface of silica inhibits the activity of Ni particles and also these particles are not resistant to deactivation of deposited carbon. On the contrary, the Ni–Ce catalyst was highly active and stable even at high load of cerium oxide (30 wt.%) which is explained in terms of the formation of small Ni particles highly dispersed on surface ceria. The Raman spectroscopy provides important information on the carbon structure and nature. Therefore, we would like to focus our attention on the main frequency range corresponding to carbonaceous Raman response. Fig. 4 shows the Raman spectra of carbons after methane decomposition over Ni catalysts. In general, there are two broad peaks at around 1348 and 1575 cm1. The former is usually ascribed to an A1g-
type mode called the disorder (‘‘D’’ band) and the latter is attributed to the stretching mode of the individual sheets in graphite (E2g mode, the so-called ‘‘G’’ band of graphite) [22]. The appearance of these two bands is in agreement with the finite size of the graphite particles and the ratios among these bands provide detailed information about the changes in the structure of carbon deposited [23]. These results indicate that the carbonaceous deposits become a mixture of order and disordered carbons and they can be considered as sensitive and insensitive from catalytic point of view [24]. Additionally, it is interesting to note the presence of a shoulder peak at 1610 cm1 (D0 band) typical of defective graphite-like materials and may be also related to the decay of the continuous carbon network into separate very small clusters. The catalytic activity results presented in Fig. 3 could indicate, in the case of the Ni–Ce catalyst, that the decrease in methane conversion from 88 to 67% in 1 h of reaction time, acquires a pseudo stable configuration after this time. This result suggests that the carbon deposited can be responsible for the rapid deactivation and an emerging stabilization of the small Ni particles by means of some kind of reaction between the carbon and surface Ni. Then, carbon could just play a role as stabilizing or participating in a synergetic effect that enhance the properties of the catalyst. Fig. 5 shows the TEM images of the deposited carbon on the Ni/SiO2 and Ni–K catalysts after 3 h of reaction time. A great
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difference in the morphologies of the deposited carbon and metallic particles could be observed. For the Ni/SiO2 sample, the carbon was mainly as nanofiber form with sizes ranging from 20 to 40 nm. The nickel particles were located at the tips of the carbon nanofibers, and the diameter of the nickel particles was almost the same as those of the growing nanofibers. The smaller Ni particles (ca. 20 nm) were the most tended to form carbon nanofibers, some relatively medium size Ni particles (ca. 40 nm) and large (60 nm) were encapsulated by graphitic carbon that greatly inhibited the contact of Ni particles with methane. This confirms that the particle size of metallic nickel plays an important role in the process of methane decomposition [9,10,21]. It also provides evidence that large Ni particles cannot carry out the methane decomposition reaction, thus explaining the low activity of this catalyst. It can be seen that several Ni particles departed from the fibrous structures are aggregated over the deposited carbon. The Ni–K sample exhibited three Ni particle sizes similar than those of the Ni/SiO2 sample and several largest K2O particles (w70 nm) was also observed. In this case, carbon is formed like small clusters aggregates together with big particles of Ni and K2O and most of small Ni particles were encapsulated by large aggregated of deposited carbon. Therefore, the Ni–K sample showed the lowest stability of all catalysts. The TEM image of the spent Ni–Ce catalyst is shown in Fig. 6. The carbons were in nanofiber form as well as graphite form. In general, the Ni particle size was less than 10 nm associated with big irregular particles (50–100 nm), probably CeO2 particles, which confirms the results obtained by hydrogen chemisorption (Table 2). Some spherical dark particles together with irregular particles encapsulated inside the carbon nanofibers can be seen. However, in this catalyst the deactivation behavior was only significant at short reaction times (1 h) and then a high stability was observed.
4.
Conclusions
The catalytic behavior of the Ni/SiO2 catalysts in methane decomposition was found to be strongly dependent of the kind of promoter used. Ni–K and Ni–Ca catalysts were less active and stable than the Ni–Ce catalyst. TPR results indicated a different reduction behavior with each catalyst. Ni–K and Ni–Ca catalysts did not show evidence of chemical interaction. The interaction ways of Ni–Ce catalyst was found to determine the Ni particle size, morphology and the reactivity with deposited carbon. The Ni–SiO2, Ni–K and Ni–Ca catalysts were easily covered by graphitic carbon inhibiting the interaction with the methane and consequently giving rise to a lower activity a rapid and permanent deactivation. On the contrary, in the Ni–Ce catalyst the small Ni particles located at the top of the carbon nanofiber act as useful catalytic active sites for the methane decomposition continuously.
Acknowledgements The authors thank Gabriela Gomez Gasga and Claudia Rueda Martinez for their technical work in XRD and H2 chemisorption
measurements, respectively. The technical assistance in HRTEM by Paz del Angel Vicente is gratefully appreciated.
references
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