High-loaded nickel–alumina catalyst for direct CO2 hydrogenation into synthetic natural gas (SNG)

Fuel 113 (2013) 598–609

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High-loaded nickel–alumina catalyst for direct CO2 hydrogenation into synthetic natural gas (SNG) Sònia Abelló a,⇑, César Berrueco a, Daniel Montané a,b a b

Catalonia Institute for Energy Research (IREC), Bioenergy and Biofuels Area, C/Marcellí Domingo 2, Building N5, Universitat Rovira i Virgili, 43007 Tarragona, Spain Departament d’Enginyeria Química, Universitat Rovira i Virgili, Avda. Països Catalans 26, 43007 Tarragona, Spain

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 The CO2 methanation performance

over a high-loaded Ni–Al catalyst was investigated.  The Ni–Al mixed oxide exhibits high methanation activity.  Coprecipitation of metal precursors leads to high surface area mixed oxide.  Highly loaded and dispersed small Ni nanoparticles were achieved.  500 h-lifetime tests confirm the high activity/stability of the nickel-based catalyst.

a r t i c l e

i n f o

Article history: Received 18 April 2013 Received in revised form 7 June 2013 Accepted 10 June 2013 Available online 24 June 2013 Keywords: Carbon dioxide Hydrogenation Methanation Nickel–alumina

a b s t r a c t The methanation of carbon dioxide was carried out over a high-loaded Ni–Al mixed oxide catalyst (ca. molar ratio Ni/Al = 5), prepared by conventional coprecipitation of the metal precursors. This route makes possible to obtain multimetallic mixed oxides upon calcination with high metal loading and high surface area. X-ray powder diffraction (XRD), temperature-programmed reduction (TPR), transmission electron microscope (TEM) and thermogravimetric analysis were used for studies of the precursors and catalysts. Calcined samples were activated under H2 at 500 °C, leading to the formation of small metallic nickel particles (ca. 6 nm) dispersed on a high surface area Ni(Al)Ox matrix. Activity tests were conducted using different H2/CO2 molar ratios (3–5), WHSV (0.2–1.0 molCO2/(gcat h), temperatures (250–500 °C), and pressures (5–20 bar). Despite the high nickel loading (ca. 70 wt.%), which is theoretically thought to be counter-productive for the nickel-based catalyst performance, our Ni–Al activated catalyst exhibited high CO2 conversion, and rendered a CH4 selectivity very close to 1. This is originated by the formation of small metallic nickel crystallites (ca. 6 nm) dispersed over NiO–alumina upon partial reduction of the mixed oxide. The catalyst experiences complete reduction during reaction, which slightly increases the Ni crystallite size, but preserves the high activity in ca. 500 h lifetime tests even at high space velocity. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Widespread introduction of renewable power sources such as wind and sun energy, albeit beneficial from an environmental ⇑ Corresponding author. Tel./fax: +34 977 297 922. E-mail address: [email protected] (S. Abelló). 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.06.012

standpoint, poses the challenge of matching power demand and production, the latter being dependent on changing weather conditions. Mismatch between the effective power production capacity and instantaneous demand could be modulated through energy storage systems that accumulate excess power in valley periods (demand below production capacity), and release it to the grid during peak periods (demand above production capacity). Several

S. Abelló et al. / Fuel 113 (2013) 598–609

alternatives are being investigated but chemical storage appears to be the most suitable for very large-scale installations. More specifically, hydrogen production by electrolysis is a mature technology that could suit this purpose, but the use of hydrogen as energy carrier has severe drawbacks. The need to establish a new and costly infrastructure to distribute hydrogen, and the difficulties and inefficiencies associated with hydrogen storage and transportation favor its conversion into more efficient energy carriers such as methane (synthetic natural gas, SNG) [1]. CH4 can be stored and distributed safely in huge quantities through infrastructures that are already in place for natural gas (NG) [2]. Renewable hydrogen can then be used to synthesize SNG by hydrogenation of carbon oxides from different sources, namely:  Carbon dioxide captured from stationary emission sources [3] like power plants, cement kilns, and oil refineries.  Syngas, produced by gasification of residual biomass and dedicated non-food energy crops [4–6], which consists in a mixture of carbon oxides and hydrogen that is hydrogen-deficient to achieve complete conversion of the carbon oxides into methane.  Biogas from anaerobic digestion of wet biomass, which may contain up to 45% of carbon dioxide mixed with methane. Coupled to this, significant efforts are currently being done to decrease the amount of carbon dioxide emitted into the atmosphere by combustion of fossil fuels. This serious problem is preferentially tackled by caption and underground storage of CO2 [7,8]. However, CO2 turns out to be an important C1 building block to obtain high added value molecules, from a safe, economical and renewable carbon source, and its utilization also contributes to alleviate global warming. Therefore, the opportunity to transform H2 and CO2 into CH4 becomes a choice [9,10]. Sabatier and Senderens seem to be the pioneers for the synthesis of methane by passing a mixture of H2 and CO/CO2 over a reduced nickel catalyst in the temperature range 200–250 °C [11,12].

CO2 þ 4H2 ¢ CH4 þ 2H2 O ðDH R;298 ¼ 165 kJ=molCO2 Þ

ð1Þ

CO þ 3H2 ¢ CH4 þ H2 O ðDH R;298 ¼ 206 kJ=molCO Þ

ð2Þ

Since then, several reviews concerning CO2 transformation and catalytic CO2 hydrogenation have been published [1,13–16]. Both homogeneous and heterogeneous catalysts have been applied to CO2 hydrogenation [17,18]. Although homogeneous catalysts display suitable activity and selectivity, the regeneration of catalysts is not straightforward. In contrast, heterogeneous catalysts are preferred considering stability, separation, handling and reuse [1]. A vast number of studies have been performed over metalbased heterogeneous catalysts on the CO2 hydrogenation to methane, as Ru [19], Rh [20–23], Ni [2,24–29] supported on various solids like Al2O3, SiO2, TiO2, CeO2, or ZrO2. Supported nickel catalysts are the most widely investigated. In fact, the support is known to play an important role from what is generally called metal-support effect, which affects the bonding and the reactivity of the chemisorbed species. This ultimately determines the higher activity and selectivity towards methane [28,30]. Several works have been found that exemplify the performance of high-surface area nickelbased catalysts. Chang et al. concluded that the hydrogenation of nickel catalysts supported over rice husk (130 m2/g) is more effective than that of nickel catalysts prepared over SiO2 gel [31], using a nickel loading between 2.5 and 25 wt.%. Attending to the significant influence of the support and Ni loading on the dispersion of nickel particles, the research was addressed towards the preparation of highly dispersed metal supported catalysts, like Ni supported on rice husk ash-alumina with 15 wt.% of Ni [28], Ni/ MCM-41 with 1–3 wt.% of Ni [30], and NiO/SBA-15 with 10– 70 wt.% of NiO [32]. The metal particle size plays a key role in

599

the performance of nickel-based catalysts. In fact, the mean particle size of nickel increases with the concentration of metal in the catalyst. Increasing the nickel loading may increase the concentration of active sites, and therefore, higher CH4 yields can be expected [28]. However, an excessive amount of nickel may produce the opposite effect due to low dispersion and susceptibility to sintering [33]. Different studies reveal the impossibility to synthesize disperse metal particles below 10 nm by the most usual methods (i.e., impregnation and sol–gel) when the amount of metal in the catalyst is higher than 15 wt.% [34,35]. Recently, a Ni/ Al2O3 catalyst with high Ni loading (50 wt.%), synthesized by a solution combustion method, was stable during 50 h in a CO methanation test [36]. However, the average Ni particle size, as determined by TEM, was around 48 nm. A classical example for the formation of high surface area and high metal dispersion catalysts with small Ni particles is coprecipitation, which in turn, keeps the possibility to prepare materials with a high metal loading (>40 wt.%). Following this synthesis route, the mixed oxide matrix derived from calcination of the coprecipitated precursor is able to stabilize an active metal, which is finely integrated and dispersed. In this work we have studied the performance of a Ni–Al catalyst with extremely high Ni content (ca. 70 wt.%) but having small Ni crystallites in the methanation of CO2. Catalytic tests were conducted in a continuous fixed-bed reactor at different temperatures, pressures, H2/CO2 molar ratios and weight hourly space velocity (WHSV), and detailed characterization of the fresh and used solids was provided. 2. Experimental 2.1. Catalyst preparation A nickel–alumina precursor with a nominal Ni/Al molar ratio of 5 was prepared by coprecipitation at constant pH. This molar ratio was selected to guarantee a high Ni loading in the final catalyst. Aqueous solutions of the metal nitrates (1 M of Ni(NO3)26H2O and 1 M of Al(NO3)39H2O, ratio 5:1) and the precipitating agent (NaOH/Na2CO3, 1 M of each) were simultaneously fed into a polypropylene vessel by a 905 TitrandoÒ automated titrator (Metrohm AG) equipped with two 800 DosinoÒ dosing systems. The pH during precipitation in the stirred reactor vessel was maintained at a constant value of 9 (±0.1). After addition of the reactants, the product slurry was aged at 30 °C for 12 h under stirring, followed by filtration, washing and drying overnight at 80 °C. The as-synthesized product, denoted as Ni5Al–P was calcined in static air at 450 °C for 6 h using a heating rate of 5 °C/min, to yield Ni5Al–C. Prior to some characterization studies and catalytic tests (Section 2.3), the calcined sample was heated in N2 at 5 °C/min to 500 °C and reduced at this temperature in a mixture of 10 vol% H2 in N2 for 3 h (sample Ni5Al–R). The spent catalysts obtained after different reaction tests (sample Ni5Al–UV after reaction at different WHSV, Ni5Al–UP after reaction at different pressures, Ni5Al–ULT for a lifetime test reaction) were also characterized. 2.2. Catalyst characterization The chemical composition of the solid was determined by inductively coupled plasma–optical emission spectroscopy (ICP– OES) in a Spectro Arcos 165 spectrophotometer. Before analysis, the solids were dissolved in 1 wt.% HNO3 aqueous solution. Powder X-ray diffraction patterns were acquired in a Siemens D5000 diffractometer with Bragg–Brentano geometry using Ni-filtered Cu Ka radiation. Data were collected in the 2h range of 5–70° with an angular step of 0.05° and a counting time of 3 s per step. The phase content and crystallite sizes were calculated by a multiphase

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Rietveld refinement using the Diffrac-Plus Topas software (Bruker AXS GmbH, Germany, version 4.0). Spherical crystallite shape, negligible contribution of the lattice strain to the reflection width, and unimodal particle distribution were assumed [38]. Therefore, crystallite sizes obtained with this method are only estimations [37], but suitable to compare catalysts of the same type prepared with different compositions, and fresh and spent catalysts. Transmission electron microscopy (TEM) was carried out in a JEOL JEM-1011 microscope operated at 100 kV. A few droplets of the sample suspended in ethanol were placed on a carbon-coated copper grid followed by evaporation at ambient conditions. N2 adsorption data were measured on a Quantachrome QuadrasorbSI gas-adsorption analyzer. Prior to the measurements, the samples were degassed under vacuum at 120 °C (as-synthesized) and 300 °C (calcined, reduced and used) for 10 h, respectively. The skeleton density of selected solids was measured by helium pycnometry at 25 °C in a Quantachrome Ultrapycnometer. Prior to the measurement, the sample was dried at 120 °C for 12 h. Temperature-programmed reduction with hydrogen (H2-TPR) was measured in a ChemBet Pulsar TPR/TPD unit equipped with a thermal conductivity detector. Ca. 50 mg of Ni5–Al oxide obtained by calcination of the precursor at 450 °C were loaded in the U-quartz microreactor, pretreated in air (20 N mL/min) at 300 °C for 1 h, and cooled to 50 °C in the same atmosphere. The analysis was carried out in a mixture of 5 vol% H2 in N2 (20 N mL/min), ramping the temperature from 50 to 900 °C at 10 °C/min. Thermogravimetric analysis–differential scanning calorimetry (TGA–DSC) was measured in a Mettler Toledo TGA/DSC 1 microbalance coupled to a Pfeiffer OmniStar QMA/E 200 mass spectrometer. Analyses were performed in air (20 N mL/min) ramping the temperature from room temperature to 1000 °C at 5 °C/min. Masses m/z 18 (H2O) and m/z 44 (CO2) were monitored. 2.3. Activity tests The activity and selectivity of the catalyst were tested in a fully automated laboratory scale fixed-bed catalytic reactor (Microactivity Reference, PID Eng&Tech, Spain). Experiments were conducted using 50–200 mg of catalyst particles sieved to 200–300 lm, which were mixed with ca. 3 g of quartz chips sieved to the same particle sizes. The catalyst was loaded into the reactor tube and heated at 10 °C/min to 500 °C under N2 (90 N mL/min), and was then reduced in situ for 3 h at this temperature by adding 10 N mL/min of H2 (10% H2 concentration). After reduction, the flow rates of H2, CO2 and N2 were adjusted and the reaction test started; N2 was used as internal standard for the chromatographic analysis of the reaction products. Experiments were conducted using H2/ CO2 molar ratios of 3.0, 4.0 and 5.0, a constant N2/CO2 ratio of 1.0, and pressures of 5.0, 10.0 and 20.0 bar(g). Catalyst screening was developed isothermally at intervals of 50 °C in the range 500–250 °C; the tests were started at the highest temperature, with a holding time of 5 h at each temperature, and decreasing to the following value at 10 °C/min. At the end of the series, temperature was raised again to 500 °C at 10 °C/min and the catalyst tested for three more hours in order to check for deactivation. Space velocity (WHSV) was varied from 0.2 to 1.0 molCO2/(gcat h) in the same temperature range. A lifetime test was also conducted at 10 bar(g), 400 °C and a WHSV of 2.0 molCO2/(gcat h), with H2/CO2 and N2/CO2 molar ratios of 4.00 and 1.00, respectively. The product gas was cooled to 5 °C and the condensed water removed and measured in a scale connected to the unit control system. The composition of the dry gas was determined with an on-line gas microchromatograph (490 microGC, Agilent Technologies). Gas samples were automatically analyzed every 2.3 min along the screening experiments, and every 6 min during the lifetime tests. Carbon dioxide conversion (XCO2) and selectivity to the different products

(Sj) were calculated according to Eqs. (3) and (4), where FCO2_0 is the feed molar flow rate of CO2, Fj is the molar flow rate of product j at the reactor outlet, and nCj is the number of carbon atoms in compound j.

X CO2 ¼ 100   Sj ¼

  F CO2 0  F CO2 F CO2 0

nC j  F j F CO2 0  F CO2

ð3Þ

 ð4Þ

3. Results and discussion 3.1. Catalyst activation The molar Ni/Al ratio in the solid measured by ICP–OES was 5.4, slightly higher than the nominal ratio of 5. The XRD pattern of the as-prepared sample (Ni5Al–P) in Fig. 1 reveals the formation of a takovite-type structure as the only crystalline phase with very broad reflections. This belongs to the hydrotalcite family (Ni0.75Al0.25(OH)2(CO3)0.1250.5H2O, JCPDS 15-0087), containing mixed positively charged Ni and Al double hydroxide layers, separated by compensating anions (typically carbonates) and water molecules [39]. The characteristic reflections of the three basal (0 0 3), (0 0 6), an (0 0 9) planes were identified at ca. 11.8°, 22.9°, and 34.7° 2h, respectively. From the two typical non-basal (1 1 0) and (1 1 3) planes at higher 2h, only the first reflection was clearly discerned at 60.9° 2h, as a result of the low crystallinity of this sample. Although Ni–Al takovites can be prepared with Ni/Al molar ratios between 1 and 5.6 [40], the present coprecipitate is practically in the upper limit, and therefore, higher level of amorphicity is expected. No reflections associated with single metal hydroxides were identified. As typically reported for hydrotalcite-like compounds, the Ni5Al–P sample exhibited the platelet-like morphology of these layered compounds (Fig. 2) [41,42]. The estimated lateral length of these platelets was in the range of 25–40 nm with a thickness of 2–5 nm. Thermogravimetric analysis in air showed their typical two-step decomposition pattern (not shown), with a total weight loss of 33%, slightly higher than the expected weight loss of a takovite structure with molar ratio Ni/Al = 5 based on its stoichiometry. The higher total weight loss obtained is probably attributed to the presence of additional phases, amorphous in nature, formed during the synthesis of precursors. The N2 adsorption isotherms at 196 °C of the as-prepared sample shows the typical profile of a hydrotalcite-type compound (Fig. 3, left). The isotherm is type IIb with H3 hysteresis, which is characteristic of materials with slitshaped pores between aggregates of plate-like particles [43]. The

Fig. 1. XRD patterns of the as-synthesized takovite (Ni5Al–P) and the products of calcination (Ni5Al–C) and reduction (Ni5Al–R). Crystalline phases: (4) takovite, (j) Ni(Al)Ox and (s) Ni0.

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Fig. 2. Transmission electron micrographs of the different samples.

Fig. 3. N2 adsorption isotherms at 196 °C of the as synthesized takovite (Ni5Al–P) and the products of calcination (Ni5Al–C) and reduction (Ni5Al–R) (left) and H2-TPR profile of the Ni5Al–C catalyst (right).

sample was solely mesoporous, with a total pore volume of 0.12 mL/g, and a BET surface area of 20 m2/g (Fig. 3 and Table 1). The results of skeletal density, as determined by helium pycnometry, show that the Ni5Al sample had a density of qHe = 2.57 g/mL, which was slightly lower than that of the naturally occurring mineral takovite (Ni6Al2(OH)16(CO3,OH), qHe = 2.8 g/mL) [39]. This difference can be attributed to both the higher Ni/Al molar ratio in the solid, and the higher degree of amorphicity observed in our as-prepared sample, in support with the less crystalline nature of the solid as observed by XRD (Fig. 1). As shown in Fig. 1, the solids calcined at 450 °C showed characteristic reflections of the NiO structure (bunsenite, powder diffraction file 03-065-2901 from ICDD) as the only crystalline phase. No distinctive reflections associated with any crystalline form of alumina were detected in the sample. As reported elsewhere [44], the NiO reflections are shifted to higher 2h values than those corresponding to pure NiO, due to Al3+ substitution in the nickel oxide lattice and the formation of a solid solution, known as Ni(Al)Ox, despite the higher nickel content in the solid. Titulaer et al. argued that calcination of takovite proceeds via formation of disordered oxide spinel intermediate phase, which is composed of NiO containing some Al ions, and a nickel containing alumina phase, which

is a very poorly ordered Ni deficient spinel Ni1.dAl2+2d/304 [40]. These phases were also previously described by Alzamora et al. [45]. However, the appearance of a spinel phase was not evident by XRD at such low temperatures due to its amorphous character. To confirm this, a quantitative analysis of the phases present in the calcined sample was performed by Rietveld refinement (Table 1). Based on the crystalline phases, bunsenite was clearly the only phase. The average crystallite size obtained through the same refinement method on the calcined sample was estimated at 3.6 nm (Table 1). Besides, thermal decomposition of the takovite at 450 °C induces the transformation from lamellar to irregular nodular nanoparticles having a size distribution in the range 5– 10 nm in the derived Ni(Al)Ox mixed oxide (Fig. 2) [46]. The N2 adsorption isotherm of the calcined sample is shown in Fig. 3 (left), showing porosity in the mesopore range, with a hysteresis loop type H2. In fact, the porosity is greatly increased from that of the starting takovite, reaching a total pore volume of 0.28 mL/g and BET surface area of 151 m2/g. The temperature programmed reduction profile of the Ni5Al–C sample is shown in Fig. 3 (right). Similarly to previous observations over mixed oxides with lower Ni/Al molar ratios [47,48], the reduction of nickel occurred in the broad range of 400–850 °C, with a

Table 1 Characterization data of the samples. Sample

Ni5Al–P Ni5Al–C Ni5Al–R Ni5Al–UV Ni5Al–UP Ni5Al–ULT

Phase composition (%)

Crystallite size (nm) 0

NiO

Ni

– 100 56 0 0 0

– 0 44 100 100 100

Vp (mL/g)

SBET (m2/g)

0.13 0.28 0.29 0.18 0.14 0.15

37 151 118 65 44 89

0

NiO

Ni

– 3.63 1.54 – – –

– – 6.4 8.4 8.5 8.6

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Fig. 4. Catalyst temperature profile, carbon dioxide conversion and product selectivity towards methane, carbon monoxide and ethane during a typical catalyst screening experiment (H2/CO2 and N2/CO2 molar ratios of 4 and 1, WHSV of 0.8 molCO2/(gcat h) and 10 bar(g); initial reduction period of 3 h with 10 vol% H2 in N2 (100 N mL/min1).

symmetrical profile centered at 610 °C. The reduction of pure NiO typically shows a single peak located at 340–397 °C [49,50]. According to this, the reduction of nickel in the Ni(Al)Ox solid solution was hindered by the presence of aluminum, a similar effect occurring in other Ni(M3+)-oxides [44,45,48], thus causing a shift to higher reduction temperatures with respect to pure NiO. The broad temperature range may include the co-reduction of various nickel species interacting with aluminum, as substantiated by the presence of multiple structures in the model by Puxley et al. for nickel/alumina catalysts [47]. Reduction of the Ni5Al–C sample at 450 °C with the described protocol (Section 2.1) lead to the decrease of the Ni(Al)Ox reflections at the expense of Ni0 formation (Fig. 1), with two narrow reflections at 44° and 52° 2h (powder diffraction file 4-850 from ICDD). Based on the percentage of crystalline phases in Table 1, the reduction process was incomplete, showing a 56% of NiO. However, the catalyst was continuously reduced under reaction conditions (vide infra). The Ni0 average crystallite size, estimated by the Rietveld refinement method, was

6.4 nm. Transmission electron microscopy and N2 adsorption were carried out over the Ni5Al–R sample to verify whether the reduction treatment exerted any morphological and textural variations with respect to the oxide precursor. As shown in Fig. 2, the morphology of the calcined and reduced samples did not apparently change, showing the nodular nanoparticles already observed in the calcined sample. However, a noticeable increase in the size of the nanoparticles was observed, from ca. 5–10 nm in Ni5Al–C to 11–25 nm in Ni5Al–R. Typically, the growth of Ni particles is hindered by the presence of aluminum ions, as the latter ions diffuse to the surface of the crystallites and alumina phases nucleate. The presence of this alumina together with that already formed during calcination, prevent any further interparticle sintering; this has been postulated to be the reason for a direct relationship between the particle size of the nickel oxide rich phases of the calcined samples in takovite-based materials and those of the completely reduced materials [45]. However, in Ni5Al–R, the increment in particle size might be related to the low amount of aluminum in

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603

Fig. 5. Influence of reaction pressure on the average carbon dioxide conversion and product selectivity at 250–500 °C, and comparison with chemical equilibrium. All experiments performed at a WHSV of 0.8 molCO2/(gcat h), and H2/CO2 and N2/CO2 molar ratios of 4 and 1, respectively. Error bars for the average values are confidence intervals at the 99% probability level.

this catalyst, which is not able to hinder the growth of Ni upon reduction. The reduction treatment significantly altered the BET surface area of the sample, which decreased to 118 m2/g, while the shape of the N2 isotherm of both, calcined and reduced samples resembled to a large extent. 3.2. Catalyst screening Fig. 4 shows the typical temperature profile of a complete screening experiment and the variations in conversion and product selectivity with catalyst temperature (10 bar(g), WHSV of 0.8 molCO2/(gcat h) and a H2/CO2 molar ratio of 4). The catalyst was reduced in situ at 500 °C for 3 h under 10% H2 in nitrogen. Then the CO2 and H2 feeds were started and the catalyst was maintained at 500 °C for 5 h. As described in Section 2.3, the reaction temperature was then subsequently reduced to 450, 400, 350, 300 and 250 °C, being maintained at each value for 5 h. After that, temperature was raised again to 500 °C and the catalyst tested for additional 3 h to check for deactivation or changes in selectivity. Methane was always the major product, besides water, and small amounts of carbon monoxide and traces of ethane were detected as the sole secondary products. Carbon balance closure was always between 98% and 101%, regardless of catalyst temperature (not shown). The catalyst performed under stable conditions at the different tested temperatures, with the exception of the experiment conducted at 350 °C which required around 3 h of operation to attain stable conversion and selectivity. No significant deactivation was observed during the nearly 35 h that the experiment lasted in total, as evidenced from the last 3 h of testing, in which temperature was raised again to 500 °C and results identical to those of the beginning of the experiment were obtained. Data on conver-

sion and selectivity at each temperature were then averaged using only those points that corresponded to stable operation intervals (i.e., 5 h at most temperatures except for 350 °C, in which only data of the last 2 h were considered), and the confidence intervals for the average values calculated at the 99% probability level. This catalyst testing procedure was repeated at different combinations of pressure, feed ratio and space velocity. Pressures of 5, 10 and 20 bar(g) were tested at constant space velocity (WHSV of 0.8 molCO2/(gcat h)) using a stoichiometric feed ratio (H2/CO2 of 4). Fig. 5 shows that at temperatures above 400 °C, carbon dioxide conversion and selectivity to methane, carbon monoxide and ethane approached their corresponding equilibrium values. Below 400 °C carbon dioxide conversion did not reach equilibrium at this space velocity, which caused methane selectivity to be lower and carbon monoxide and ethane selectivity to be higher than their corresponding equilibrium values. This was especially noticeable at 5 bar(g), where carbon monoxide selectivity was as high as 5% at 300 °C and nearly 14% at 250 °C. Above 10 bar(g), however, pressure had little effect and very similar results were obtained at 10 and 20 bar(g). Therefore, 10 bar(g) was selected to study the effect feed composition and space velocity. The effect of the H2/CO2 molar feed ratio was tested at H2/CO2 molar ratios of 3, 4 and 5, maintaining a pressure of 10 bar(g) and a space velocity of 0.8 molCO2/(gcat h). Fig. 6 shows that conversion and selectivity approached their corresponding equilibrium values when temperature was above 400 °C, but below that, methane selectivity was slightly below equilibrium while carbon monoxide and ethane selectivity were significantly higher than equilibrium values. CO2 conversion increased with the partial pressure of H2 probably because reaction rate was promoted by a high concentration of adsorbed hydrogen atoms on the catalytic surface.

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Fig. 6. Influence of the feed H2/CO2 molar ratio on the average carbon dioxide conversion and product selectivity at 250–500 °C, and comparison with chemical equilibrium. All experiments performed at 10 bar(g), a WHSV of 0.8 molCO2/(gcat h), and a N2/CO2 molar ratio of 1. Error bars for the average values are confidence intervals at the 99% probability level.

Fig. 7. Influence of space velocity on the average carbon dioxide conversion and product selectivity at 250–500 °C, and comparison with chemical equilibrium. All experiments performed at 10 bar(g), and H2/CO2 and N2/CO2 molar ratios of 4 and 1, respectively. Error bars for the average values are confidence intervals at the 99% probability level.

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Fig. 8. Lifetime catalyst activity test (490 h): carbon dioxide conversion and product selectivity at 400 °C, 10 bar(g), a WHSV of 2.0 molCO2/(gcat h), and H2/CO2 and N2/CO2 molar ratios of 4 and 1, respectively.

Table 2 Comparison of catalytic performance in CO2 methanation over Ni-based alumina catalysts. Entry

Catalyst

Preparation

P (bar)

Ni (%)

dNi (nm)a

T (°C)

WHSV (mL/g h)

H2/CO2

XCO2 (–)

SCH4 (–)

Refs.

1 2 3 4 5 6 7 8 9 10 11 12

LaNi4Al Ni/Al2O3 Ni/Al2O3 Ni–5%Mo/Al2O3 Ni/RHA-Al2O3c Ni/c-Al2O3 Ni/Al2O3 Ni–2%CeO2/Al2O3 Ni/Al2O3 Ni–Fe/Al2O3 Ni/Al2O3 Ni5Al-Rd

Arc melting Coprecipitation Impregnation Impregnation Impregnation Impregnation impregnation Impregnation Sol–gel Coprecipitation Impregnation Coprecipitation

50 1 1 1 1 15 1 1 10 10 1 10

n.a. 25 15 15 15 (25) 10 15 15 40.4 30 10 69.1

n.a. n.a. n.a. n.a. 4.7 (10.5) n.a. n.a. n.a. 7.51 9.4 n.a. 6.4

400 235 250 250 500 380 300 300 220 220 400 400

3000 2400 b 22,250 22,250 30,000 11,000b 15,000 15,000 9600 9600 10,000 268,800

4 9 2 2 4 4 4 4 4 4 4 4

91.5 99 14.5 17.2 63 (66) 6.8 45 71 61.1 58.5 5 92.4 (83.5)

95 99.7 >97 >97 92 (90.9) 88.9 >99 >99 99.2 99.5 >99 >99 (>99)

[55] [27,56] [57] [57] [28] [58] [59] [59] [60] [61] [62] This work

n.a. Means not available. a Crystallite size determined from XRD. b Information not complete. c Data between brackets correspond to different experimental conditions. d Data between brackets correspond to results after a 490 h lifetime test.

The effect of space velocity was also investigated at 10 bar(g) with a stoichiometric feed ratio. Space velocity was varied from 0.2 to 1.0 molCO2/(gcat h) and the results are reported in Fig. 7. Equilibrium conversion and selectivity were achieved above 450 °C at all space velocities, whereas at 350 °C equilibrium was reached only at an WHSV below 0.4 molCO2/(gcat h). For temperatures below 350 °C equilibrium conversion was not reached, even at the lowest space velocity tested. As a general trend, below equilibrium conversion, the lower the conversion the lower the selectivity towards methane, and the larger the selectivity to carbon monoxide and ethane. This is consistent with the accepted mechanism for carbon dioxide

hydrogenation. Even if some aspects of the reaction pathways involved in carbon dioxide activation on nickel catalysts are still controversial [47], it is generally accepted that the mechanism involves formation of adsorbed CO as the main active intermediate through reverse water gas shift, which subsequently reacts with adsorbed hydrogen atoms to form CHx intermediate species, and methane [24,26,51–53]. A small fraction of the CO desorbed to the gas phase, especially at low pressure. This may be due to a lower coverage of the active surface with dissociated hydrogen atoms at low pressure, thus reducing the rate of hydrogenation of the adsorbed CO. Adsorbed CHx species may also dimerize through a secondary reaction path to yield trace amounts of ethane. This is

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Fig. 9. XRD patterns of the used catalysts after reaction at different WHSV (Ni5Al– UV), pressure (Ni5Al–UP) and lifetime test (Ni5Al–ULT). Crystalline phases: (s) Ni0.

consistent with ethane selectivity being only dependent on catalyst temperature and space velocity at non-equilibrium conversion, and nearly unaffected by the H2/CO2 feed ratio or reactor pressure. Ethane, however, was not a stable product since it was likely transformed into methane by the high activity of nickel catalysts in C–C rupture reactions. This was proven by the fact that at constant temperature, ethane selectivity decreased towards equilibrium selectivity when space velocity was lowered (data at 300 and 350 °C in Fig. 7). 3.3. Lifetime testing The screening study revealed that 400 °C was the most adequate temperature for the catalyst to be effective in carbon dioxide methanation. At lower temperature the reaction rate was significantly lower and equilibrium conversion could only be achieved at relatively low space velocity (i.e., below 0.4 molCO2/(gcat h)). Above 400 °C equilibrium limitations started to favor the formation of increasing amounts of carbon monoxide and ethane, thus diminishing carbon dioxide conversion and methane selectivity. This temperature was therefore selected to perform lifetime tests to establish the performance of the catalyst at a relevant time on stream. Pressure was maintained at 10 bar(g) since higher pressures do not offer a significant advantage from a selectivity standpoint. A stoichiometric relation of hydrogen and methane was used. An excess of hydrogen would improve conversion and reduce the formation of carbon monoxide, but a stoichiometric ratio is preferred for economic reasons due to the cost of hydrogen production. Fig. 8 shows the change in carbon dioxide conversion and product selectivity for an extended experiment of 490 h performed at the aforementioned conditions and a space velocity of 2.0 molCO2/ (gcat h), which was twice the highest space velocity tested during

Fig. 11. TGA-MS profiles during decomposition of the used catalyst after lifetime reaction test.

the screening experiments. The initial conversion (average of the first hour) was 92.4%, which is in full agreement with the equilibrium conversion, while measured selectivity was above 99.99% for methane and negligible amounts of carbon monoxide and ethane were produced. The catalyst experienced an exponential decay in activity that reduced conversion to an average value of 83.5% after 350 h of time on stream, a value that was maintained until the end of the experiment (490 h). Methane selectivity decreased to 99.7% after 350 h and showed a slow tendency to decrease afterwards, while carbon monoxide selectivity reached 0.29% at 350 h and tended to increase slightly beyond that. Ethane selectivity was not affected substantially, and remained around 103%. Overall, the experiment showed that the catalyst maintained a high activity and good selectivity towards methane after 490 h of uninterrupted time on stream, even at the unfavorable conditions of space velocity used in this test, which was twice the highest space velocity used in the catalyst screening tests, and at least an order of magnitude larger than the space velocities typically used on catalyst development studies. Table 2 summarizes the comparison of our results with a list of similar systems based on nickel–alumina catalysts to benchmark the catalytic performance attained with the high-loaded Ni5Al–R catalyst. Due to the high number of studies dealing with Ni catalysts for SNG, only those related to CO2 methanation have been

Fig. 10. Transmission electron micrographs of selected used samples. The scale bar applies to all micrographs.

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considered for the sake of comparison. Of course, a direct comparison cannot be performed accurately due to the variability in both the reaction conditions (pressure, temperature and space velocity) and the loading of nickel itself, which impact on the catalytic performance. A first analysis of the data indicates that methane selectivity is maintained above 99% in most cases, which reinforces the high CH4 production accomplished by nickel catalysts. Impregnation of alumina is typically the most extended preparation method for methanation catalysts. In these cases, nickel loadings between 10 and 25 wt.% are incorporated in the alumina support, giving rise to CO2 conversions below 71% (entry 8) for a cerium-doped nickel alumina catalysts. Entries 3 and 4, considering a 15 wt.% Ni/Al2O3 catalyst without and with Mo as promoter, respectively, induce low CO2 conversions at relatively high space velocity. As a general trend, no relationships are deduced between the activity and Ni crystallite size when catalysts are prepared by impregnation, with the exception of entry 5; in the latter case, small nickel crystallites can be obtained by impregnation of a rice husk–ash–alumina support with 25 wt.% of Ni, leading to CO2 conversions of 66% at a relatively high temperature (500 °C) and WHSV (30,000 mL/g h) [28]. Aside impregnation, coprecipitation has been eventually used to produce Ni–Al catalysts for CO2 methanation (entries 2, 9 and 10). The high CO2 conversion displayed in entry 2 despite the low reaction temperature is mainly a consequence of the low space velocity combined with a high nickel loading (25 wt.%). Further increase of nickel loading up to 40 wt.% (entry 9) causes a decrease in CO2 conversion at low reaction temperature (220 °C) and higher WHSV. It is interesting to note that catalysts in entries 9 and 10 exhibit a Ni crystallite size below 10 nm even their high nickel loading. In this work (entry 12), a Ni–Al catalyst prepared by coprecipitation with a high Ni loading (ca. 70 wt.%) was very active and selective at 400 °C, even if the reaction conditions were selected as the most drastic in terms of WHSV, being one order of magnitude higher than the space velocity reported by other authors. Furthermore, the activity was relatively high after a 490 h lifetime test, thus demonstrating that the preparation method and the activation procedure exert some influence on the formation of small Ni crystallites over a partially reduced NiO matrix, even the high Ni loading.

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N2 adsorption revealed a lower surface area in the used catalysts probably due to coke residues (not visible by XRD). The BET surface area of the original catalyst Ni5Al–R (118 m2/g) markedly decreased to 65 m2/g after reaction at different WHSV, 44 m2/g after reaction at different pressures (from 5 to 20 bar), and 89 m2/g after the lifetime reaction test (Table 1). These results indicate that subjecting the catalyst to variations in pressure (from 5 to 20 bar) exerts a stronger effect on its textural properties compared to the other parameters. No carbonaceous deposits were observed over the used catalysts by TEM, even after the reaction test of 490 h (Fig. 10). Besides, similar Ni particle sizes than those observed in Ni5Al–R were attained. This indicates the relatively high stability of the catalyst exposed to extreme reaction conditions, which was previously attributed to the presence of the alumina phase between the nickel particles [45]. In order to determine the presence of adsorbed species, the catalyst Ni5Al–ULT was characterized by thermogravimetry (Fig. 11). As expected from the gradual oxidation of metallic nickel upon heating in air, the sample showed a total weight gain of ca. 11%. Along the analysis it exhibited an initial weight loss below 200 °C, followed by a gradual weight increase at temperatures between 160 and 400 °C, and finally, a slight weight loss till the final temperature. As clearly shown in Fig. 11, H2O and CO2 were the only products of decomposition below 450 °C, both appearing at practically the same temperature. Besides, the theoretical weight gain for the oxidation of metallic nickel into NiO is around 21.4%, which is much higher than the value determined in the used sample (ca. 15%). This implies a ca. 6% of weight difference, which may include coke and water released upon thermal treatment of the used sample. Based on these results, changes monitored up to ca. 425 °C are a consequence of Ni0 oxidation matching in temperature with an eventual coke burning (not visible by TEM). Therefore, the results indicate that the moderate deactivation of the catalyst after the 490 h reaction test can be due to both slight formation of carbon deposits and virtual Ni sintering. Based on these results, highloaded nickel–aluminum mixed oxide derived from its corresponding coprecipitated precursor is an efficient catalyst for CO2 hydrogenation into methane.

3.4. Characterization of the spent catalysts 4. Conclusions It is well known that carbon deposition is one of the main reasons for catalyst deactivation during CO2 and CO methanation processes [54]. Also, sintering of Ni particles at high methanation temperature is responsible for activity decay over Ni catalysts [33]. In order to evaluate the changes in structure of the mixed oxide catalyst during the catalytic tests, selected used materials were subsequently characterized by XRD, TEM, N2 adsorption and TGA-MS. Fig. 9 shows the XRD pattern of several used catalysts subjected to variations of WHSV (Ni5Al–UV), pressure (Ni5Al–UP) and after the 490 h reaction test (Ni5Al–ULT). In all cases, metallic nickel reflections with much higher intensity compared to the fresh catalyst Ni5Al–R in Fig. 1 could be observed. No Ni(Al)Ox reflections were present over any of the used catalysts, which confirms the in situ reduction occurring upon time on stream. A lowintense amorphous phase could be only discerned in the Ni5Al– ULT, which was probably a result of some spinel (NiAl2O4) formation. The average Ni0 crystallite size of all samples amounted 8.4, 8.5 and 8.6 nm, respectively, which was slightly higher than the average size obtained for the catalyst precursor (6.4 nm). The similar crystallite size obtained after different conditions indicate that only a limited degree of sintering occurred, which may be responsible for the moderate deactivation observed during the lifetime test. [45]

A Ni(Al)Ox mixed oxide with high metal loading (molar ratio Ni/ Al = 5) has been found to be active, selective and stable catalyst for carbon dioxide methanation upon convenient activation (reduction). Coprecipitation of the metal precursors enables to incorporate high nickel content in the final oxide matrix, which also exhibits high surface area. A catalytic screening under different experimental conditions has been performed over the reduced catalyst. The amount of methane produced during carbon dioxide hydrogenation depends on temperature, pressure, H2/CO2 molar ratio and WHSV. Only small amounts of carbon monoxide and traces of ethane were detected as secondary products. Although an excessive amount of nickel is expected to induce low catalytic activity due to low dispersion and sintering, we have reported for the first time that despite the high amount of nickel (ca. 70 wt.%), partial reduction of the oxide leads to small metallic nickel crystallites of ca. 6 nm dispersed over NiO–alumina, which are active and selective for CO2 methanation. Subsequent in situ reduction upon time on stream slightly increases the Ni crystallite size, but the resulting high-loaded nickel-based catalyst displays high stability after lifetime tests of around 500 h. Besides, the remarkable activity and selectivity to methane is preserved under more extreme conditions.

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