Synthesis, Characterization, and Activity Pattern of NiâAl Hydrotalcite

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Synthesis, characterization and activity pattern of Ni-Al hydrotalcite catalysts in CO2 methanation Salvatore Abate, Katia Barbera, Emanuele Giglio, Fabio Deorsola, Samir BENSAID, Siglinda Perathoner, Raffaele Pirone, and Gabriele Centi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01581 • Publication Date (Web): 12 Jul 2016 Downloaded from http://pubs.acs.org on July 16, 2016

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Industrial & Engineering Chemistry Research

Synthesis, characterization and activity hydrotalcite catalysts in CO2 methanation

pattern

of

Ni-Al

Salvatore Abate1,*, Katia Barbera1, Emanuele Giglio2, Fabio Deorsola2, Samir Bensaid2, Siglinda Perathoner1, Raffaele Pirone2 and Gabriele Centi1. 1

Dipartimento di Ingegneria Elettronica, Chimica e Ingegneria Industriale, Università di Messina

and INSTM CASPE (Laboratory of Catalysis for Sustainable Production and Energy) - Viale F. Stagno D’Alcontres 31, 98165 Messina, Italy. 2

Department of Applied Science and Technology (DISAT), Politecnico di Torino, Corso Duca degli

Abruzzi 24, 10129 Torino, Italy. *Corresponding author. E-mail address: [email protected] Abstract Two nickel-aluminum hydrotalcite samples (HTLCs) were prepared by co-precipitation method at different pH values and investigated as catalysts for the hydrogenation of carbon dioxide. The newly synthesized samples have been compared with a reference alumina supported nickel-based commercial catalyst, with equal nickel content. The as-prepared and commercial samples were characterized by BET analysis, atomic adsorption spectroscopy (AAS), X-ray diffraction (XRD) and temperature-programmed techniques (H2-TPR and CO-TPD). Catalytic activity of the analyzed samples was investigated towards hydrogenation of CO2 at atmospheric pressure by varying reaction temperature between 250 and 400 °C. The maximum CO2-to-CH4 conversion value achieved by hydrotalcyte was ≈ 86% at 300 °C. The superior performance of HTLCs has been put in relationship with the major catalysts reducibility nature and with the higher metal surface area and metal dispersion. The stability of the HTLCs was investigated through long-term tests, resulting in a good stability in the reported reaction conditions.

Keywords: carbon dioxide, methanation, Ni-Al hydrotalcite, synthetic natural gas, power to gas. 1. Introduction In order to overcome global climate change issues and fossil fuels progressive depletion, an increasing penetration of renewable energy sources (RES) is required. The intermittency of electricity production from RES (e.g., wind and solar) is one of the key challenges that should be 1 ACS Paragon Plus Environment


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solved in order to drive their widespread diffusion. The electric power production from RES does not often coincide with end-users demand, implying a potential unbalance of the electricity network1-3. The logistics and energy penalties of CO2 capture and storage (CCS) imply major problems that prevent most of existing technologies from being economically viable on a large scale4-7. The use of synthetic fuels made from CO2 as carriers for excess renewable electricity offers a potential solution in order to manage fluctuating output of renewable energy and mitigating CO2 emissions at the same time8-11. Power-to-gas (PtG) represents a possible pathway for long-term and high-capacity renewable electricity storage. Excess electricity can be used for hydrogen production through water/steam electrolysis. However, H2 direct use is limited by safety issues (large flammability interval) and storage/transport challenges (e.g., low energy density, steel embrittlement). Hydrogen can react with carbon dioxide according to the Sabatier reaction: + 4 ⇄ + 2

(1)

This reaction enables the production of a synthetic (or substitute) natural gas (SNG) that could be directly injected in an already existing infrastructure as the natural gas distribution grid12. Power-togas concept has been demonstrated on an industrial scale in Audi motor company “e-gas” facility in Werlte (Germany) of 1000 metric tons/year production of SNG from concentrated CO2 obtained from a nearby biogas plant13. Indeed, a growing interest on Power-to-gas is testified by the increasing number of demo-plants in this field, including the HELMETH project within which this study was conducted. Extensive studies have been carried out on metal-based catalytic systems for the CO2 methanation reaction, namely group VIIIB metals (Fe, Ru, Co, Rh, Ir, Ni, Pd, Pt) supported on several oxides (e.g., SiO2, TiO2, Al2O3, ZrO2, CeO2 and Ce–Zr mixed oxides). So far, Ni, Ru and Rh have revealed as the most effective metals for this reaction14-17; Ru and Rh have been reported as the most selective towards methane18,19. Nickel-based catalysts are commonly used for the methanation process and cover the larger part of published works, since nickel it is a cheaper metal and therefore interesting from the commercial standpoint20,21. The crucial issue concerning the activity, selectivity and stability of catalysts is the nature of the support: the different interactions that can be established between the metal and the support shall influence the catalytic properties of the active metal sites22-23. An additional concern of Ni-based catalysts seems to be the deactivation at low temperature due to the interaction of the metal particles with CO and the formation of mobile nickel carbonyls that lead to the metal sintering24-26. The control of the synthesis protocol is crucial in order to synthesize a 2 ACS Paragon Plus Environment

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catalyst reducible at low temperature and able to retain to the initial properties during use. Many studies report that methane formation is related to the surface area of the nickel metal obtained when the catalyst is reduced. The highest surface area of metal and the highest activities are obtained when the nickel is produced as very small crystallites, usually below 100 Å in diameter. Among the viable procedures, the Ni-Al hydrotalcites synthesis is of great interest. Hydrotalcites (HTLCs) are constituted by mixed hydroxides of divalent/trivalent metals made up of polycations and have a layered structure. Calcination of HTLCs is an alternative to traditional chemical and physical methods for the fabrication of a wide variety of mixed metal oxide nano-composite materials. The calcination progressively induces dehydration, dehydroxylation, and loss of compensating anions and leads to acidic and basic mixed oxides with a high surface area27. Such materials have succeeded in methanol synthesis, SO2 abatement and alcohol oxidation28. Recently, hydrotalcite-like catalysts gained interest for their activity concerning carbon dioxide methanation29-31. Gabrovska et al.29 studied the effect of reduction temperature and Ni2+/Al3+ ratio on the catalytic activity of hydrotalcites with a strong hydrogen excess inlet mixture for CO2 removal. The catalyst with higher Ni-content presented higher conversion when reduction temperature is lower than 500 °C, because of the presence of readily reducible NiO species. On the other hand, the sample with lower Ni/Al ratio showed greater activity when reduction was carried out above 500 °C due to the higher reducibility of NiAl2O4 spinel phase and to the effect of aluminum amount on Ni sintering29. He et al.30 compared the catalytic performance of Ni-Al hydrotalcite (prepared at pH=10) and a conventional Ni/γ-Al2O3 sample prepared through incipient wetness impregnation. Hydrotalcite presented higher activity probably due to the better Ni dispersion, to the smaller Ni particles size and to a larger amount of basic sites. Wierzbicki et al31 investigated the effect of lanthanum as trivalent metal for CO2 methanation using low nickel (≈ 15% wt.%) content hydrotalcite-like catalysts. Samples contain Ni, Al, Mg and La as divalent/trivalent metals and have been prepared through co-precipitation method. The addition of lanthanum led to increased amount of basic sites and shifted the reduction of Ni-species towards lower temperature. In addition, activity experiments put in evidence the correlation between superior catalytic activity, higher basicity and enhanced reducibility31. This work aims to make a systematic comparison between two Ni-Al hydrotalcites (prepared at different pH values of 8.7 and 12, respectively) with high nickel content and a commercial catalyst. The main purpose is a complete, wide and systematic characterization of the aforementioned samples, in order to establish a correlation between the physicochemical properties and the catalytic activity with a stoichiometric reacting mixture (H2/CO2). Catalyst characterization included BET, 3 ACS Paragon Plus Environment


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AAS, XRD, TEM, H2-TPR, CO-TPD and TGA/DTG. In particular, we focused on bulk features and crystallographic properties, including the effect of both drying and calcination. In addition, the effect of metal-support interaction on reducibility, metal surface area and dispersion has been evaluated. The final target is the identification of the best catalyst(s), in term of stability and activity, for carbon dioxide methanation taking place at temperature lower than 400 °C, in order to minimize the economic impact of the methanation unit within a synthetic natural gas production system.

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2. Experimental 2.1. Materials Two Ni-Al hydrotalcite type catalysts were prepared by co-precipitation method, by using nitrates salts as precursors of Ni and Al, in alkaline solution. The first catalyst was prepared at a pH of 12 (Ni-Al 12), using an alkaline solution containing both NaOH and Na2CO3, at room temperature. The second catalyst was synthesized at a pH of 8.7 (Ni-Al 8.7), using an alkaline solution containing Na2CO3 only, at 65 °C. After precipitation, both samples were then aged at 65 °C for 18 h. The Ni content was fixed with an Al/(Al+Ni) = 0.25-0.27 for both catalysts, with a theoretical loading of 75-80 wt% of NiO. Samples were dried at 110°C overnight and further calcined at 450°C for 6 h in air. The catalytic performance of prepared samples was compared with a reference commercial catalyst, containing ca. 80 wt% of NiO. Table 1 summarizes the main details on composition and synthesis of the tested samples. 2.2. Catalysts characterization Elemental composition of the catalysts was evaluated by using Atomic Absorption Spectroscopy (AAS), using a Ni lamp operating at λ= 232 nm. Prior the measurement of the sample solution, three standards (2-3-4 ppm) were prepared, since this lamp emits in a linear range up to 4 ppm. The values of surface areas were obtained on calcined samples, calculated from Brunauer Emmett Teller (BET) equation from the adsorption branch of the isotherms, obtained at -196 °C on a Quantachrome sorption analyzer. Prior the measurements, samples were heated in N2 flow at 350 °C for 1 h. The micropore area and volume were evaluated by t-plot method. X-ray diffraction (XRD) analysis of the samples in the 2θ range 10–80° was performed by using a Bruker D2 Phaser diffractometer operating with Ni β-filtered CuKα radiation at 40 kV and 30 mA. The average crystallite size was calculated by X-ray diffraction line broadening using the Scherrer formula d = k λ/( ), where d represents the grain size; k = 0.89 is the Scherrer constant related to the shape and index (hkl) of the crystals; λ is the wavelength of the X-ray (Cu Kα, 1.54056 Å); θ is the diffraction angle of the peak; B stands for the full width at half-height of the peaks (in radiants) of (111) plane. Temperature-programmed reduction (TPR) measurements at 100–800 °C were performed in a continuous-flow apparatus using a U-quartz microreactor (inner diameter equal to 4 mm) fed with a 5% H2/Ar mixture flowing at 50 Nml/min and heated at the rate of 10 °C/min. A ca. 150 mg catalyst diluted with 400 mg of SiC sample was used, with H2 consumption monitored by a TCD quantitatively calibrated with a commercial NiO standard. 5 ACS Paragon Plus Environment


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Metal surface area (MSA) values and CO-Temperature Programmed Desorption were obtained by pulsing (1.0 ml) a 10% CO/He mixture titration at room temperature. Before measurements, catalysts were reduced in situ at 450°C in a 5% H2/Ar mixture flow (50 Nml/min) for 1 h. After reduction, the samples were flushed in He carrier at 450 °C for 15 min, and then cooled to room temperature. CO-TPD was then performed after saturation in the T range of 20-800 °C with a heating ramp of 20 °C/min under He flow (30 Nml/min). Ni morphology structure was investigated by transmission electron microscopy (TEM) using a Philips CM12 microscope (resolution 0.2 nm), provided with high resolution camera, at an accelerating voltage of 120 kV. Suitable specimens for TEM analyses were prepared by ultrasonic dispersion in isopropylic alcohol adding a drop of the resultant suspension onto a holey carbon supported grid. The thermo gravimetric and differential scanning calorimetry (TG-DSC) analyses were performed by using a Seteram Sensys Instrument. The analyses were carried out over 4-7 mg of sample with a heating rate of 5 °C/min from 30 to 700 °C under N2 flux (30 Nml/min). Prior the run, each sample was flushed with pure N2 (30 Nml/min) for 10 min. Moreover, an analysis of coke formation over the used catalysts was investigated by TGA/DTG in an oxidative atmosphere until 850°C, ramp 5°/min. 2.3. Experimental setup A scheme of the experimental test unit is shown in a previous article19 (Abate et al): all gases were bottled and came from cylinders. H2, CO2 and N2 were properly mixed by using calibrated and computer-controlled mass flow controllers (MFCs). The MFCs performance has been crosschecked by using a device for the volume flow measurement. The quartz reactor containing the catalyst bed was placed inside a controlled oven in order to maintain the temperature constant. A thermocouple was vertically put on the top of the catalytic bed in order to track and measure the reaction temperature. After the reaction, the gas mixture passed through a condenser and a water trap in order to remove the water and to protect the gas analyzer. In order to characterize the system performance (in terms of methane yield or carbon dioxide conversion) the gas composition before and after the reactor was measured. A system of two 3-ways valves allowed the by-pass of the oven-reactor section, enabling the analysis of both inlet and outlet gas mixture through a single device. For the gas analysis, a multi-channel analyzer (Emerson XStream) with capabilities to measure CO, CO2, CH4, H2 and O2 was used. The gas analyzer was equipped with non-dispersive infrared (NDIR) sensors for CO, CO2 and CH4 measurement, a


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thermal conductivity (TCD) detector for H2 and a paramagnetic sensor for O2. The analyzer had a web user interface allowing the sensors calibration and the data acquisition. 2.4. Activity tests The samples were initially prepared at powder size. They were pelletized and then manually ground and filtered through two sieves, allowing a particles size distribution in the range 500 - 900 μm. The commercial catalyst was available in cylindrical pellets (diameter: ≈3 mm, height: ≈3 mm): it was ground and selected with the same procedure. For all tests the catalyst load was fixed at a value of 600 mg, while the samples had a density of about 1 g/cm3: this means that the reaction volume was around 600 mm3. The catalytic bed had a diameter of ≈4 mm and height equal to ≈50 mm. All the samples were reduced at a temperature of 500 °C for 3 hours by passing through the bed an activating stream of 10 % H2 in N2. The oven temperature was varied between 250 and 400 °C with increasing step of 25 °C. During this series of experiments, the value read by the thermocouple will be considered as the proper reaction temperature during the results analysis. A single operating condition was kept until both the measured temperature and gas composition (from the gas analyzer) stabilized to a constant value. A stoichiometric mixture (10% H2 and 2.5% CO2, i.e. H2/CO2 = 4) highly diluted with N2 (87.5 %) has been fed to the catalytic bed. The overall inlet stream volume flow has been set equal to 200 Nml/min (‘normal’ condition refers to 0 °C and 1.013 bar). For all the samples the catalytic bed was ≈ 600 mm3, resulting in a GHSV = 20000 h-1. GHSV was then varied by increasing the inlet volume flow, while the catalytic bed volume was kept constant: 25000 and 30000 h-1 have been obtained with inlet volume flow equal to 250 and 300 Nml/min, respectively. Assuming the nitrogen conservation, it was possible to calculate the outlet flow rate for each involved gas by using the volume fraction obtained from the gas analyzer data logger. Then, it was also possible to verify the conservation of carbon, oxygen and hydrogen atoms between inlet and outlet. Especially the overall carbon conservation was useful in order to exclude side reactions as carbon deposition (e.g., through Boudouard equilibrium) and other carbon compounds formation (e.g., heavier hydrocarbons, alcohols, etc.). CH4 and CO2 flow rate values (FCH4 and FCO2) were useful in order to calculate carbon dioxide conversion (χCO2) and methane yield (ηCH4) through the following formulas:

=

, ,!"#

$% =

&',!"#

(2)

,

(3)

,



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2.5. Stability tests The two hydrotalcite samples (Ni-Al 8.7 and Ni-Al 12) and the commercial catalyst were tested continuatively for 1500 min (≈ 25 hours), in order to evaluate their stability and performance increase/decrease. Specifically, the tests was carried out at a fixed temperature of ≈ 300 °C and at GHSV of 20000 h-1, with a stoichiometric mixture (H2/CO2=4) at the pressure of 5 bar. The two samples were activated with pure hydrogen at 500 °C for 2 hours. The stability tests were performed by using a Microactivity Efficient equipment (Micromeritics) with two fixed bed continuous flow reactors. The amount of catalyst was 130 mg mixed with SiC in order to reach the desired catalytic bed volume.

3. Results and discussion 3.1. Catalysts characterization 3.1.1.

Bulk features

Comparison of XRD patterns of all the calcined samples are reported in Figure 1. Particularly, for HTLCs samples, XRD profiles between dried and calcined samples reveal significant differences. Dried HTLCs samples exhibited features commonly shown in layered structures: narrow, symmetric and intense reflections of the basal (003), (006), and (009) planes at low 2θ angles, while broader, small ones at higher angles for the non-basal (012), (015), and (018) planes. The (009) and (012) reflections overlap resulted in a broad signal between 32° and 38° 2θ angles, whereas the two reflections of (110) and (113) can be clearly distinguished around 2θ = 60°. For dried HTLCs samples, the lattice parameters a and c were calculated assuming a 3R packing of the layers. The crystallographic a parameter indicates the average cation-cation distance, while c value matches corresponds to three times the interlayer distance, and it is controlled by both the size/orientation of the interlayer anions and the electrostatic forces occurring between these latter and the layers. Nevertheless, this computation is not applicable in our case, since the (110) reflection at 61.07° is somewhat broad; for this reason, it has been proposed that c can be better determined by averaging the position of the diffraction peaks corresponding to: /3 =

)(**+),∗)(**.)

(4)

with reflections at 2θ angle = 11.7 and 23.4 corresponding to (003) and (006) planes. Values of a0 for the cubic unit cell were calculated using the equation: /* = (0111) ⋅ √3

(5) 8 ACS Paragon Plus Environment

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Upon calcinations, the reflections of NiO were the only observed, whereas no diffraction peaks corresponding to crystalline Al phase can be observed, owing to the good dispersion of Al species in the oxide matrices. This was a result of the removal of physically adsorbed water and the partial release of interlayer water in the c direction, deteriorating the basal reflections, causing a decrease in the interlayer spacing. The disappearance of the (006) and (009) planes indicated disorder in the stacking of the layers. For comparison, the XRD pattern of reference catalyst was also recorded, showing a similar diffraction pattern. For all the calcined samples, the reflections appearing at 2θ angle= 37.3, 43.20, 62.8, and 75.2, can be indexed as (111), (200), (220), and (311) crystal planes of the bulk NiO, respectively. All these diffraction peaks were typical of the face-centred cubic (FCC) crystalline structure of NiO, not only in peak position, but also in their relative intensity, in accordance with that of the standard spectrum (JCPDS, No. 04-0835). The calculated values are reported in Table 2, together with the average crystallite size (d) for all the other samples. According to the literature32, the thermal decomposition of HTLCs includes three main stages, i.e., (i) the loss of interlayer water, (ii) the decomposition of structural hydroxyl groups and finally (iii) the decomposition of interlayer carbonate anions. For the present compounds, these three decomposition steps were evaluated by determining the second derivative of the TGA curve. All the new reflections in the pattern indicate the presence of poorly crystallized spinel-like mixed oxide. As the calcination occurred at high temperature, the crystallinity of the oxide phase formed was enhanced, as indicated by sharpening of the XRD lines. The crystallite sizes of NiO was ca. 3.57 nm for HTLCs samples, which was calculated from measured values for the spacing of the (111) plane. The NiO lattice constant calculated from the XRD data is 4.1676 Å. d(111) value was calculated from Bragg’s law, while a0 for the cubic unit cell was calculated using the equation (5). Being this value well below than pure NiO (4.1773 Å), it would suggest the incorporation of aluminium inside the NiO lattice, since Al3+ ion is smaller than Ni2+. These data would suggest that no sintering of the mixed oxides occurred, and at low calcinations temperature, the product of HTLC decomposition was constituted by a NiO phase doped with aluminium, in the form of nickel aluminate aggregates. Such a skeleton is that ensuring the retention of morphology upon rehydration, even after calcination. The values of microporous volume and external surface area of the different Ni catalysts are given in Table 3. In general, it can be observed that the addition of Ni led to a decrease of the microporous volume. However, this reduction was not extremely pronounced meaning that, for all samples, the active sites of the catalysts were accessible to the reactant molecules.



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Figure 2 displays the isotherm hysteresis loops of all the samples under study. The reference sample shows a type I isotherm, characteristic of highly microporous materials. The irreversibility of the isotherm at very high relative pressure (P/P0> 0.9) could be assigned to the adsorption of nitrogen in interparticle pores. The alteration of the pore size for the two HTLC samples was due to the isomorphous substitution of Al atoms into NiO frameworks which led to significant contraction of walls and consequently, reduction of pores33. The TG diagrams for the two HTLC samples (Figure 3) showed two weight losses for which the inflection points coincided with the temperatures, corresponding to the minima in the DT traces, as typically for layered double materials, indicating loss of lamellar arrangement. The spinel formation took place while some carbonate was still present, indicating that the collapse of the layered structure occurred before complete decarbonation. Physically adsorbed water started to be removed from the as-synthesized material below 100 °C, whereas the release of some interlayer water was initiated, in the range 150-200 °C, inducing a decrease in the basal plane (003) reflection. The finalization of this process led to an intermediate metastable mixture of phases, where the dehydrated hydrotalcite coexisted with the solid solution of spinels. Similarly to what proposed by Belloto et al34, removal of interlayer water promotes the diffusion of the trivalent Al3+ cation to the interlayer in Ni-Al HTLCs. This process leads to different cation coordination, changing to tetrahedral involving three oxygen atoms of the layer and one apical oxygen atom of the interlayer. According to these authors, the sites formerly occupied by the trivalent cation are left vacant, and the dimensions and geometry of the octahedral layer are maintained35. The formation of an intermediate metastable phase, a mixture of the emerging mixed oxide phase and the dehydrated HTLC, suggests that at 200 °C, the hydrotalcite structure collapsed completely. The final decarbonation step at high temperature led to a stable mixed oxidic structure. 3.1.2.

Influence of MSI (metal-support interaction) on Metal Dispersion and Reducibility

Micrographs of all the reduced samples under study are shown in Figure 4. Reduction of Ni-Al hydrotalcite samples precursor yielded the catalyst with highly dispersed Ni round-shaped nanoparticles, despite of a higher NiO loading, displaced in the alumina environment. Statistical analysis shows (not reported here) that the size distribution of Ni nanoparticles was narrow with mean size of 4 nm, in good agreement with XRD and Chemisorption measurements. Commercial catalyst presented a more inhomogeneous distribution and shape of Ni particles. H2-Temperature Programmed Reductions (H2-TPR) allowed us to characterize the metal-support interaction nature, by elucidating the influence of one or more phases present on reducibility of NiO phase. Figure 5 shows the TPR pattern of all the samples under study. Pure NiO, which was here used as reference, gave a sharp reduction peak centered at 383 °C, with a component at 283 °C, in 10 ACS Paragon Plus Environment

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line with literature that reports to be associated to the reduction of NiO not bounded to any support36. All the other samples showed, in different extent, a broad reduction profile shifted at higher temperatures, revealing that reduction of reacted NiO with the support occurred and the overlapped peaks, refined using Gaussian-like functions, presented a two-stage reduction in the commercial catalyst and Ni-Al 12 samples, and a three-stages reduction for Ni-Al 8.7 sample, respectively. The shift of reduction peak maximum reflected the process of increasing mobile Ni2+ distribution over the surface, making the reduction more difficult due to increase of polarization effect of aluminum ions. Peak maxima profiles (ranging between 478 and 705 °C) were in fact attributed to disperse amorphous Ni2+ surface species differing in reducibility because of different Al3+ surrounding nickel ions. Particularly, the components with a peak maximum at 478 and 495 °C were assigned to the superficial oxide nanoparticles, weakly interacting with alumina support19,37, whereas peaks in the temperature range between 566 and 705 °C were attributed to Ni-Al mixed oxide and represented the majority of hydrotalcite species. The variation of both TPR and XRD data indicated a bimodal nature of nickel oxidic species, which consisted in an amorphous overlayer of NiO, interacting but not chemically bound with the support precursors of the spinel phase formed to some extent already in aqueous solution, thus leading to the creation of Ni-Al spinel during calcinations even at low temperature. The intense H2 consumption peak at around 760 °C was attributed to the reduction of Ni2+ ions incorporated into tetrahedral vacancies on the preferentially exposed (110) plane of γ-Al2O338. Finally, it is worth to mention that, despite of a similar morphology of the HTLC samples, H2-TPR revealed a different Ni reducibility: NiAl-12 presented only two components, while the NiAl-8.7 had also a lower temperature feature (at 495 °C), resembling a more isolated species. Two distinct methanation sites were identified by CO-TPD experiments. The sites have been assigned to i) Ni atoms bounded to another Ni atom and Ni atoms strongly interacting with alumina support (NiAl2O4) or ii) Ni atoms formed during reduction of isolated NiO. Not only the reducibility of NiO, but also metal dispersion is essential for the sake of the catalytic activity. Thus we performed CO-TPD measurements over reduced catalysts. The patterns are reported in Figure 6. It is possible to observe the presence of two groups of desorption peaks: the first one in the T-range = 100-400 °C and a second at T-range = 450-800 °C, with the commercial system having a more complex sites distribution. The first group, which was deconvoluted in two peaks for all the considered samples, can be attributed to the so called α-sites, i.e. smooth Ni crystal planes which molecularly adsorb CO. The likelihood of forming Ni(CO)4 was relatively low since no low-temperature ( Ni-Al 8.7 > Commercial catalyst. Moreover, all the considered samples presented the yield peak at ≈ 300 °C. Catalytic activity can be put in relation with some results obtained from catalyst characterization. Hydrogen consumption calculated from H2-TPR (Table 4) measurements can provide an interpretation of the conversion results: higher NiO reducibility corresponded to better activity. Commercial catalyst presented minor H2/NiO value than hydrotalcite samples. CO-TPD results (Table 4) can lead to further interpretation: the catalytic activity reflected both the metal surface area (MSA) and the dispersion (D) calculated values. Thus, higher MSA and dispersion can explain the superior performance obtained from activity test results. Hydrotalcite catalysts seem to represent a good choice for methanation. In particular, the bulk structure allows higher nickel amount (in wt%) without a metal dispersion decrease: this can lead to both higher catalytic activity and better metal use. As said, HTLCs showed a slight superior performance than a well-engineered commercial catalyst . An important obtained result was the high activity at low temperature (below 300 °C). In order to inject the produced synthetic gas into the natural gas distribution grid, a very high CO2-to-CH4 conversion (above 97%) should be achieved. If the CO2 hydrogenation is performed at low temperature, the methane production is thermodynamically favored and a lower number of methanation reactors are required in order to obtain a sufficient overall conversion. Furthermore, once the reaction temperature and the CO2 conversion (or CH4 yield) are fixed, a catalyst with higher activity leads to a minor catalyst load within the reactor, implying lower cost. Moreover, low reaction temperature is less risky for the catalyst stability during operation. Stability tests results for the two hydrotalcite samples and the commercial catalyst are presented in Figure 10. Both hydrotalcite type catalysts showed a satisfactory stability and acceptable performance during the reaction time in the reported conditions, while a slight deactivation was observed for the commercial catalyst. The stability test were performed at 300°C although the real temperature inside the catalytic bed was slight higher due to the exothermic reaction.



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Moreover, the spent catalysts were characterized by TGA/DSC in order to investigate the coke formation. The TGA profiles didn’t show any weight loss, typically in the temperature range 450700 as reported by Son et al43., for this reason it is not reported in the paper. 4. Conclusion Two Ni-Al hydrotalcite samples for carbon dioxide methanation have been synthesized via coprecipitation of nitrates salts in alkaline solution at different pH (Ni-Al 8.7 and Ni-Al 12). They have been studied and compared with a reference commercial catalyst consisting in Ni (75 wt%) on γ-Al2O3. All the samples have been characterized from a physico-chemical standpoint: surface area, elemental composition and average crystallite size have been estimated through BET analysis, atomic absorption spectroscopy (AAS) and X-ray diffraction (XRD) analysis, respectively. H2-TPR allowed the characterization of the metal-support interaction and the evaluation of NiO reducibility. For all the analyzed samples, the reduction of reacted NiO with the support occurred; the TPR profile deconvolution led to the identification of two (commercial catalyst and Ni-Al 12) or three peaks (Ni-Al 8.7). CO-TPD measurements over reduced catalysts were performed in order to evaluate metal dispersion. The analysis led to identify, for the as-synthesized catalysts, two groups of desorption peaks within the temperature ranges 100-400 °C and 450-800 °C, respectively. A test unit operating at atmospheric pressure was set up in order to evaluate the samples performance. The experimental campaign was carried out by testing, for each sample, the same amount of catalyst with similar particle size distribution, in the same operating condition (in terms of temperature range and inlet reacting mixture). Methane yield values resulted (in the region far from chemical equilibrium) with the following order: Ni-Al 12 > Ni-Al 8.7 > commercial catalyst. The higher conversion can be put in relation with the NiO reducibility, metal surface area (MSA) and nickel dispersion. HTLCs activity at low temperature overcame commercial catalyst’s one: hydrotalcites can be thus considered as a promising kind of new catalyst for methanation, potentially replacing the more conventional ones (nickel supported on various oxides).

Acknowledgements The research leading to these results has received funding from the European Union's Seventh Framework Programme for the Fuel Cells and Hydrogen Joint Technology Initiative under grant


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agreement n° 621210 for the HELMETH project (Integrated High-Temperature Electrolysis and Methanation for Effective Power to Gas Conversion).



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Tables Table 1. Main details concerning synthesis and composition for all the analyzed catalysts

Sample Ni-Al 8.7 (hydrotalcite) Ni-Al 12 (hydrotalcite) Commercial

synthesis co-precipitation

co-precipitation

precursor

support

Ni(NO3)2·6H2O

-

Al(NO3)3·9H2O Ni(NO3)2·6H2O

-

Al(NO3)3·9H2O

commercial

γ-Al2O3


precipitating agent Na2CO3 (65°C per 18 h) Na2CO3+NaOH (65°C per 18 h) -


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Table 2. Main results on bulk features calculated by XRD analysis: d is the average crystallite size, a is the crystallographic parameter and c is the average interlayer distance (this last parameter has sense only for hydrotalcites)

Sample

d (Å)

a(Å)

c(Å) dried

Ni-Al 8.7

35.7

4.17

29.07

Ni-Al 12

35.5

4.17

28.77

Commercial

41.6

4.18

-


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Table 3. Physicochemical properties from B.E.T. analysis.

Sample

Specific surface area (m2/gcat)

Amicro (m2/gcat)

Vtot (cm3/gcat)

Vmicro (cm3/gcat)

Dpore (nm)

Ni-Al 8.7

189

13.6

0.64

0.005

11.8

Ni-Al 12

184

21.8

0.59

0.009

11.7

Commercial

182

7

0.23

0.016

25.1



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Table 4. Main results from atomic adsorption spectroscopy (AAS), H2-TPR and CO-TPD.

Sample

NiOa ( wt%)

H2/NiOb (%)

MSAc (m2Ni/gcat)

Dd (%)

Ni-Al 8.7

75

84.3

42.2

13

Ni-Al 12

76

86.0

51.8

16

Commercial

76

69.4

28.4

9.7

a. b. c. d.

Calculated by AAS Calculated by H2-TPR measurements Calculated by CO-TPD in the T-range 100-400 °C. Calculated assuming a round shape of Ni particles.


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Figures

Figure 1. XRD pattern of the analyzed samples



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Figure 2. Schematic illustration of hysteresis in ink-bottle pores.


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5

a)

0

Weight loss (%)

5

65

0

60

-10 55 -15

Interlayer water removal

50

-20 45 -25 Layer dehydroxylation

-30

Decarbonation

0

100

200

300

400

500

600

c)

-5

60

-10 55 -15

Interlayer water removal

50

-20 45 Layer dehydroxylation

-30

35 700

Decarbonation

40

-35 0

100

200

Temperature (°C)

300

400

500

600

35 700

Temperature (°C) 10

5

-5

-25

Weight loss (%)

-20

5

-5

-30

-10 0

-15 -20

-5

-25

Corrected Heat Flow (mW)

0


-10 -15

d)

0 5

-5

10

5

b)

0

Weight loss (%)

65

-25

40

-35

70


-5

70


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Weight loss (%)

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-30

-35 0

100

200

300

400

500

600

-10 700

-35 0

100

200

Temperature (°C)

300

400

500

600

-10 700

Temperature (°C)

Figure 3.Thermogravimetric analysis profiles for the two synthesized hydrotalcite samples: a) NiAl-8.7 dried; b) NiAl-8.7 calcined; c) NiAl-12 dried and d) NiAl-12 calcined.



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Figure 4. HR-TEM micrographs of the analyzed samples


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Figure 5. H2-TPR profile of the considered catalysts.



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Figure 6. CO-TPD profile for the analyzed samples.


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Figure 7. Test repetition effect for a) Commercial catalyst and b) hydrotalcite (Ni-Al 12) sample. Second and third tests have been carried out immediately after the first one. Also hydrotalcite Ni-Al 8.7 presented no further activation/deactivation during the test repetition.



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Figure 8. Ni-Al 12: activity curves at different space velocities a)CH4 yield , b)CO2 conversion and CO selectivity


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Figure 9. Activity curves for all the tested samples and comparison with chemical equilibrium a)CH4 yield , b)CO2 conversion and CO selectivity: Ni-Al 8.7 and Ni-Al 12 represent the two synthesized hydrotalcites, while the third one is the commercial catalyst.



90

CO2 Conversion [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Ni-Al 12 Ni-Al 8.7 Commercial

85

80

75 0

10

20

Time [h] Figure 10. Stability performance of the as-synthesized hydrotalcite and commercial samples at 300 °C.


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Synthesis, Characterization, and Activity Pattern of NiâAl Hydrotalcite

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