Combined reforming of methane by carbon dioxide and water: Particle size effect of Ni–Mg nanoparticles

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 0 ( 2 0 1 5 ) 1 2 9 8 5 e1 2 9 9 2

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Combined reforming of methane by carbon dioxide and water: Particle size effect of NieMg nanoparticles Ali Nakhaei Pour*, Maryam Mousavi Department of Chemistry, Ferdowsi University of Mashhad, P.O. Box: 9177948974, Mashhad, Iran

article info

abstract

Article history:

The effect of NieMg particle size on product selectivity, catalyst activity and coke forma-

Received 19 May 2015

tion on Ni/MgO/a-Al2O3 catalyst were studied in simultaneous steam and CO2 reforming of

Received in revised form

methane to syngas. The NieMg particle sizes in the range of 13.7e29.7 nm were prepared

4 August 2015

via incipient wetness impregnation method using a-Al2O3 as support. Experimental results

Accepted 6 August 2015

for turn over frequency (TOF) show that the intrinsic catalyst performances were depen-

Available online 28 August 2015

dent on NieMg particle size. However, the products selectivities and H2/CO ration were found to be independent of NieMg particle size. The experimental results reveal that the

Keywords:

coke formation rates on the surface of catalysts are increased by increasing NieMg particle

CO2-steam reforming

size. Nevertheless, the correlated site carbon atom deposition rates (carbon atoms

Ni catalysts

deposited on one site of catalyst) is shown an opposite trend.

Methane

Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Syngas Size dependent

Introduction The production of synthesis gas for gas to liquid (GTL) process attracts an increasing interest in combined steam and carbon dioxide reforming of methane which is a feasible process to adjust the H2/CO ratios of syngas with various feed ratio of CH4, H2O and CO2 [1e4]. In general, the required H2/CO ratio of syngas depends on the target processes such as FischereTropsch and methanol synthesis [5,6]. When the conventional steam reforming of methane is applied for the syngas production for GTL process, need an additional process such as carbon dioxide reforming of methane and partial oxidation of methane to adjust the H2/CO ratio [1,2,7].

The CO2-reforming (dry reforming) of methane produced syngas with H2/CO ratio of about two, which is suitable for GTL process. In addition, the dry reforming reaction converts two greenhouse gases (CH4 and mainly CO2) into a valuable feedstock and may lead to the reduction of CO2 emissions. A greenhouse gas (GHG) is any gas in the atmosphere, which absorbs and re-emits heat, and thereby keeps the planet's atmosphere warmer than it otherwise would be [8]. The main GHGs in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide and ozone. GHGs occur naturally in the Earth's atmosphere, but human activities, such as the burning of fossil fuels, are increasing the levels of GHG's in the atmosphere, causing global warming and climate change. Carbon dioxide is the most common GHG emitted by human

* Corresponding author. Tel./fax: þ98 5138795457. E-mail addresses: [email protected], [email protected] (A. Nakhaei Pour). http://dx.doi.org/10.1016/j.ijhydene.2015.08.011 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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activities, in terms of the quantity released and the total impact on global warming [9,10]. Thus, the dry reforming for production of syngas is a key process in decreasing of carbon dioxide emission. However, the high carbon deposition is a problem in the CO2 reforming of methane [10]. Recent studies indicated that the carbon deposition is reduced when the steam and CO2 reforming reactions are carried out simultaneously [11,12]. In addition, the development of cokingresistant catalyst has potential significance to commercial utilization of CO2 and CO2 e steam reforming [2,7,11]. Addition of alkali promoters like as MgO may be decreased the coke formation of Ni-based catalysts [11,13,14]. Although the nickel catalysts are used in methane reforming, many new researches have been made to improve Ni-based reforming catalysts by improving the support, promoters, and the nickel particle size [15,16]. As reported in literature, a decrease in particle size of nickel catalysts in dry reforming of methane increased the activity of reforming catalysts and limit the formation of coke [17]. The role of active particle size has been prone as an important parameter in many catalytic processes [18e20]. Our previous studies showed that the NieMgO catalyst has good activity and selectivity in CO2-steam reforming of methane to syngas [21,22]. In the present work, the effect of NieMg particle size in the catalytic activity of NiOeMgO supported on a-Alumina was studied selectivity in CO2-steam reforming of methane to syngas. The size dependent evaluation of combined CO2 and H2O reforming of methane for catalyst activity, products selectivities and coke formation are evaluated in this work.

from the adsorption isotherm. The pore volume was measured using the volume of the adsorbed nitrogen (STP) at P/P0 ¼ 0.2. The crystal structure and the compositional homogeneity of the prepared catalysts were examined by X-ray diffraction (XRD) technique. The XRD spectrum of the catalysts were collected using an X-ray diffractometer, Philips PW1840 X-ray diffractometer, using monochromatized Cu/Ka radiation (40 kV, 40 mA) with scan rate of 0.02 (2q) per second from 10 to 80 . XRD peak identification was done by comparison to the JCPDS database software. For hydrogen chemisorption experiments, around 1 g of catalyst was reduced in situ using a flow of 4%(v/v) H2/N2 and GHSV ¼ 5.35 NLh1 g-cata1 from room temperature to 1223 K and then fixed in this temperature for 12 h in experimental set up (Fig. 1). The sample was then degassed at 623 K for 3 h under vacuum (106 mbar) and the chemisorption measurements were performed at 298 K and desorbed hydrogen was calculated using frontal chromatography technique. NieMg particle size estimations are based on a truncated octahedron geometry, with assuming complete reduction, semi spherical particles and a H/Ni adsorption stoichiometry factor of one [23]. The calculated results were listed in Table 1. Carbon deposition on spent catalysts was studied from room temperature to 1223 K by Temperature Programmed Oxidation (TPO) method in situ after reaction in experimental set up. The samples were heated from room temperature to 1223 K at the rate of 5 K/min, using 20% O2/N2 mixture with space velocity GHSV ¼ 5 NLh1 g-cata1. The produced CO2 in effluent of the reactor was analyzed using frontal chromatography technique and related to deposited carbons.

Experimental Catalyst preparation Crystalline a-Al2O3 was synthesized as a catalyst support from aluminum nitrate and ammonium carbonate. Before impregnation of nickel, the prepared a-Al2O3 support precoated with 1 wt.% of Mg via incipient wetness impregnation method of Mg(NO3)2$6H2O. After impregnation, samples were dried overnight at 400 K and calcined at 1200 K for 4 h. The supported Ni catalysts were prepared via incipient wetness impregnation of nickel on the MgO pre-coated a-Al2O3 support using Ni (NO3)2$6H2O as the Ni precursor compound. NieMgeO particle size in the range of 13.7e29.7 nm were obtained by varying the Ni loading (1e5 wt. %) at constant Mg loading 1 wt.%. The impregnated catalyst precursors were dried at 400 K overnight, and calcined at 1200 K for 4 h.

Catalyst characterization The BrunauereEmmetteTeller (BET) surface area, pore volume and the average pore diameter of the catalysts were determined by N2 physisorption using a Micromeritics ASAP 2010 automated system. The analysis was done using N2 adsorption at 77 K and the catalyst sample (0.5 g) was degassed at 373 K for1 h and then at 573 K for 2 h prior to analysis. Both the pore volume and the average pore diameter were calculated by BarreteJoynereHallenda (BJH) method

Reactor apparatus and activity e testing procedure Catalytic reaction runs were conducted in a fixed-bed quartz reactor with inner-diameter of 1.2 cm at atmospheric pressure. The feed consisted of carbon dioxide (>99.99%), steam and methane (>99.99%) with a molar ratio CH4/ (CO2 þ H2O) ¼ 0.87, CH4/CO2 ¼ 1 and CO2/H2O ¼ 6.4. In this study, the feed ratio in combined CO2 and H2O reforming of methane adjusted to obtain the syngas with H2/CO ¼ 1.08 involves the following reaction:

CH4 þ 0.16 H2O þ CO2 / 2.08 H2 þ 1.92 CO þ 0.08 CO2 þ 0.08 (1) H2O The flow of gases was controlled by electronic mass flowmeter (Brooks 5850) and water was added to the feed using a HPLC pump and a specially designed evaporator. A thermocouple was placed in the catalyst-bed to monitor the reaction temperature and another thermocouple showed the furnace temperature. A schematic diagram of the experimental setup is shown in Fig. 1. A one gram of catalyst samples were located in reaction zone of the reactor and the catalytic activity was studied in the reaction temperature of 1023 K at GHSV ¼ 20 NLh1 g-cata1 and atmospheric pressure. Prior to the reaction, the catalyst was first pre-reduced by flow of 4%(v/v) H2/N2 and GHSV ¼ 5.35 NLh1 g-cata1 from room

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Fig. 1 e Schematic of the experimental setup: (1) O2 trap; (2) Carbonyl trap; (3) Quartz reactor; (4) Thermocouple; (5) Catalyst; (6) Silica gel absorbent; (7) flow meter; (8) Gas chromatograph; (9) Flare; (10) Exit valve; (11) Pressure meters; (12) Electrical heater.

temperature to 1223 K and then fixed in this temperature for 12 h. The gas compositions of feed and products were analyzed by a gas chromatograph equipped with two sequent connected packed column, Propack Q and Molecular Sieve 5A, and argon was used as a carrier gas. The un-reacted water was eliminated by silicagel adsorbent in effluent of the reactor and measured from increasing of the absorbent weight.

Results and discussion BET surface area and crystalline structures in catalysts The BET surface areas of the catalysts were measured by nitrogen gas adsorption using a continuous flow method and the results are listed in Table 1. Table 1 shows the specific surface area and total pore volume of the fresh a-alumina support and impregnated catalysts after calcination. As it is

revealed, the specific surface area and total pore volume of the catalysts are decreased by increasing the Ni loading. XRD pattern of the fresh powdered catalysts after calcination are shown in Fig. 2. In XRD pattern of prepared catalysts, the a-Al2O3 support in most samples exhibited peaks at 2q of 25.6, 35.2, 37.8, 43.3, 46.2, 52.5, 57.5, 61.3, 66.5, 68.2, and 76.8, corresponding to the (012), (104), (110), (113), (202), (024), (116), (018), (214), (300), and (1010) surfaces, respectively. They were ascribed to the rhombohedral structure of a-Al2O3 and assigned to JCPDS file no. 46-1212 [24]. XRD pattern in Fig. 2 shows the formation of a solid solution between NiO and MgO on the alumina supported catalyst. In XRD pattern of prepared catalysts, the NieMgeO solid system exhibited peaks at 2q of 36.5, 43.1, 62.5, 75.2 and 78.9, corresponding to the (111), (200), (220), (311), and (222) spaces, respectively [25,26]. No other lines of Ni-containing phases are observed. These findings indicate unambiguously the formation of a solid solution NiOeMgO [14,27]. Since Ni2þ and Mg2þ have the same valence and quite close ionic radius values [r (Ni2þ) ¼ 0.07 nm and r (Mg2þ) ¼ 0.065 nm], the dimension of

Table 1 e BET surface are, Total pore volume, Ni and Mg loadings, surface Ni and Mg densities dispersions and particle sizes as determined by H2 chemisorption of the various catalysts. Mg loading (wt.%) 0 1.0 1.0 1.0 1.0 1.0 1.0 a b

Ni loading (wt.%)

BET surface area (m2/g)

Total pore volume (cm3/g)

Niurf. density (atom/nm)

NieMgsurf. density (atom/nm)

NieMg dispersionb (%)

dNieMgb (nm)

0a 1.0 1.8 2.6 3.4 4.2 5.0

11.2 10.9 10.6 10.3 10.1 9.8 9.5

0.161 0.158 0.155 0.153 0.150 0.147 0.145

0 9.2 16.5 23.8 31.1 38.5 45.8

0 31.3 38.6 45.9 53.2 60.6 67.9

0 7.5 6.1 5.1 4.4 3.9 3.5

0 13.7 16.9 20.1 23.3 26.5 29.7

a-Alumina support. From H2 chemisorption.

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(i.e. the density of Ni and Mg atoms at the surface of catalyst) and the hydrogen chemisorptions results. The density of Ni and Mg atoms at the surface of prepared catalysts after reduction are calculated from metal loading and dispersion of the Ni and Mg atoms. The surface atoms numbers of metals are depended on metals dispersion. Overall, Mg loading is 1 wt.% and nickel loadings varied from 1 wt.% to 5 wt.% in the final catalysts, thus the NieMg particle sizes were calculated from 13.7 to 29.7 nm (see Table 1). As shown in Table 1, when increasing the Ni loading, a limited but constant increase in NieMg particle size was observed. This growth probably results from an increasing metal surface density favoring NieMg particle growth. The nickel density of catalyst varied from 9.2 to 45.8 atom-Ni/nm2, and total atom density (Ni and Mg atoms) varied from 31.3 to 67.9 atom/nm2.

Catalyst activity

Fig. 2 e XRD pattern of the prepared catalysts after calcination. Ni loading 1 wt.% (a), 1.8 wt.% (b), 2.6 wt.% (c), 3.4 wt.% (d), 4.2 wt.% (e), 5 wt.% (f).

their crystal cells are very close to each other. In addition, the crystal structures of both NiO and MgO are belong to the NaCl lattice-type. Thus, the NiO and MgO components in the NieMgeO system is distinguishable form NixMg1-xO solid solution due to excellent mutual solubility between them [26]. Nevertheless, the incorporation of nickel to the structure of MgO produces the shift of the diffraction peaks to higher 2q values (lower crystalline plane distances) when the nickel content increases, as can be seen in Fig. 2. As no free NiO phase is observed, it appears that the quantities of Niincorporated are below the maximum amount which are required to quantitatively form the ideal NieMgeO solid solution. The increasing in intensity of diffraction peaks with increasing of nickel content indicated that there was an increase in crystallite size of the solid solution.

NieMg nanoparticle sizes Incipient wetness impregnation was used to prepare various a-alumina supported NieMg nanoparticles. The NieMg nanoparticle sizes were calculated from the Ni and Mg loading

The goal of the present work is to study the effect of the NieMg particle size on the Ni/Mg/a-Al2O3 catalyst activity in combined CO2 and H2O reforming of methane for syngas production. Table 2 listed the results for CH4, H2O and CO2 conversions and syngas (H2 þ CO) selectivity. All the catalysts exhibited nearly thermodynamic equilibrium for CH4 and CO2 conversions within a range of acceptable error. As shown in Table 2, by increasing the Ni loading, a constant increase in CH4, H2O and CO2 conversions was observed. Table 2 also summarizes the H2 and CO yields and H2/CO ratio over Ni/Mg/ a-Al2O3 catalysts with various Ni loading at 1023 K. These values are nearly to the thermodynamic equilibrium values. The selectivities to both H2 and CO are near to 100% because there are no side products in combined CO2 and H2O reforming of methane [26]. The H2/CO ratio was higher than expected value (1.08 in Eq. (1)) due to the occurrence of side reactions such as reverse water gas shift (RWGS) reaction (Eq. (2)) and methane decomposition (Eq. (3)) which consumed produced hydrogen [28].

CO2 þ H2 4 CO þ H2O

(2)

CH4 / 2H2 þ Cs

(3)

Table 2 e Feed conversion (mol%), products selectivity (%) and H2/CO molar ratio in products. Mg loading Ni loading (wt.%) (wt.%)

Conversion (mol.%) CO2 CH4 H2O

1.0 1.0 1.0 1.0 1.0 1.0

5.0 4.2 3.4 2.6 1.8 1.0

71 66 61 57 55 52

74 70 65 60 57 54

44 42 39 38 35 34

Selectivity H2/CO (%) ratio CO

H2

96 94 95 94 97 97

93 93 93 92 95 95

1.10 1.09 1.10 1.10 1.09 1.09

Reaction condition: GHSV ¼ 20 NLh1 g-cata1, T ¼ 1023 K, P ¼ 1 bar, Feed composition: CH4/(CO2 þ H2O) ¼ 0.87, CH4/CO2 ¼ 1 and CO2/ H2O ¼ 6.4.

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As shown in Table 2, the feed conversion is increased by Ni loading in catalyst. However, from these results it cannot be concluded that the combined CO2 and H2O reforming of methane is a structure sensitive reaction. Since metal catalyzed reactions are conducted on the surface of a catalyst, a higher weight-normalized activity obtained when surface-tovolume ratio increased. If the surface-specific activity (TurnOver Frequency, TOF) is independent from metal particle size, the increase in weight-based activity is linear toward the increase in metal surface area. In present work the TOF number (TOF, number of CH4, CO2 and H2O molecules converted per active site per second) was calculated using the number of active sites achieved from H2 chemisorption. The TOF results after 12e18 h on stream are illustrated in Fig. 3. As shown in Fig. 3, the TOF results increased from 5.87 to 9.2 s1, when NieMg particle size decreased from 29.7 to 13.7 nm, in opposite trend with the feed conversion (Table 2). Although, the catalyst with 5 (wt.%) Ni loading has the best catalyst activity in our Ni loading range. These results showed that the combined H2O and CO2 reforming of methane on Ni-based catalysts is a structure sensitive reaction. The apparent particle size effect in this study is at variance with that claimed by other investigators [15,17]. The structure sensitivity of combined H2O and CO2 reforming of methane on Ni-based catalysts can be explained by evaluation of reaction mechanism. The adsorption with decomposition and dissociation of CH4, H2O and CO2, resulting in increased the formation of the formate (HCOO) and hydroxyl intermediate [10]. In detail, the adsorption of CO2 on the catalyst surface sites forms carbonate (CO2 3 ) and hydrocarbonate (HCO 3 ) species which are reacted with H atoms (produced from the CH4 decomposition) to form formate intermediates. These intermediates subsequently decompose into CO and adsorbed OH groups. Thermodynamic analysis of the effect of the nanoparticle size on adsorption of reactants on the catalytic surface, shows that the Gibbs free energy of adsorption is increased by decreasing of catalyst particle size [20,29,30]. Thus, an increase in the TOF results for small

Fig. 3 e Site-time yields (TOF) calculated after 12e18 h on stream. (Reaction condition: GHSV ¼ 20 NLh¡1 g-cata¡1, T ¼ 1023 K, P ¼ 1 bar, Feed composition: CH4/ (CO2 þ H2O) ¼ 0.87, CH4/CO2 ¼ 1 and CO2/H2O ¼ 6.4).

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NieMg particles (Fig. 3) can be attributed to an increase in Gibbs free energy for adsorption of CH4, H2O and CO2 reactants.

Coke study Carbon deposition on the catalyst is a problem always encountered in syngas production by nickel catalysts [10,11,31]. It should be pointed out that the coke formation leads to catalyst deactivation, plugging of the reactor and break down of the catalyst. In addition, the stability of the catalysts are depends on the rate of coke formation. The combined CO2 and H2O reforming of methane reaction is prone to cause coke formation higher than steam reforming of the methane reaction due to the low H/C ratio in the reactant gas. The amount of coke formation during the combined CO2 and H2O reforming of methane can be estimated from the ratios of H/C and O/C in the reactant gas [10,12]. When the ratios of H/C and O/C are high, the coke formation on the surface of nickel catalysts is decreased. The deposition of carbon during methane reforming can be originated from either methane decomposition (reaction (3)) or CO disproportionation (Boudouard reaction (4)), which are thermodynamically favorable under the reaction conditions [31]:

2CO / CO2 þ Cs

(4)

These side reactions occur in parallel of main reactions at the surface of catalysts. The rate of carbon deposition can be estimated at the end of reaction by elimination of deposited coke using oxygen. These results are reported as carbon deposited per time-on-stream. The TPO profiles of the spent catalysts with different nickel loading are shown in Fig. 4. The results indicated three kinds of carbon species on the catalyst surface after the reaction. The first peak at lower temperature (<500 K) can be assigned to carbidic carbon [13,32]. The second peak at 650 K was assigned to amorphous carbon, which deposited on nickel sites. The third peak at higher temperatures (>800 K) was assigned to whisker-type carbon [6,33]. The results showed an increase in the area of the peaks in the TPO profiles with increasing nickel content. The amount of carbon deposited over spent catalysts is detected by the TPO measurements after reaction. The results are reported as a function of nickel loading on NiO/MgO/aAl2O3 catalyst in Fig. 5. As shown in the Fig. 5, the amount of coke deposited on the surface of spent catalysts is increased by increasing the nickel loading. The correlation between the amount of deposited carbon and the metal particle size has been thoroughly discussed in various former studies [34,35]. As reported in some previous works, the catalysts with small nickel particle size provide a lower amount of deposited carbon while the catalysts with large average particle size yield a high amount of carbon [36]. As shown in Fig. 5, by increasing of Ni loading (NieMg particle size), the coke formation rate is increased. However, if the number of carbon atoms deposited on one site of catalyst is calculated (Fig. 6), the opposite result is obtained. As shown in Fig. 6, by decreasing of catalyst particle size, the correlated

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Fig. 5 e The amount of deposited carbon (C) over spent catalysts by the TPO measurements against Ni loading wt.%.

H2. Coke formation results revealed that although the coke formation rate is increased by increasing NieMg particle size, but the correlated site carbon atom deposition rate is showed an opposite trend. Fig. 4 e The TPO profiles of the spent catalysts with different nickel loading. Ni loading 5 wt.% (a), 4.2 wt.% (b), 3.4 wt.% (c), 2.6 wt.% (d), 1.8wt.% (e), 1 wt.% (f).

site carbon atom deposition rate is increased. The higher catalyst activity and higher adsorption constant of reactants in smaller particle size maybe increased the site coke deposition rates.

Conclusion The effect of nickel particle on product selectivity, conversion and coke formation over Ni/MgO/a-Al2O3 catalyst was studied in simultaneous steam and CO2 reforming of methane to syngas. As no free NiO phase is observed, it appears that the quantities of Ni-incorporated are below the maximum amount required to quantitatively form the ideal NieMgeO solid solution. Within the range of 13.7e29.7 nm for NieMg nanoparticles, alumina-supported nickel catalysts were found to be dependent to the NieMg particle size based on experimental TOF results. Nevertheless, the products selectivities and H2/CO ratio were found to be independent of NieMg particle size. The H2/CO ratio was less than expected value (1.15) because of side reactions such as reverse water gas shift (RWGS) reaction and methanation reaction which consumed

Fig. 6 e The number carbon atoms deposited on one site of catalyst against NieMg particle size.

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Acknowledgments Financial support of the Ferdowsi University of Mashhad, Iran (2/33595-5/12/93) is gratefully acknowledged.

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