based catalyst for carbon dioxide methanation and methane stea

Research Article www.acsami.org

Cite This: ACS Appl. Mater. Interfaces 2017, 9, 43553−43562

Template-Assisted Wet-Combustion Synthesis of Fibrous NickelBased Catalyst for Carbon Dioxide Methanation and Methane Steam Reforming M. Aghayan,†,* D.I. Potemkin,‡,§ F. Rubio-Marcos,∥ S.I. Uskov,‡,§ P.V. Snytnikov,‡,§ and I. Hussainova†,⊥,# †

Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia Boreskov Institute of Catalysis, Pr. Lavrentieva, 5, Novosibirsk 630090, Russia § Novosibirsk State University, Pirogova Street, 2, Novosibirsk 630090, Russia ∥ Instituto de Cerámica y Vidrio (ICV-CSIC), C/Kelsen, 5, 28049 Madrid, Spain ⊥ ITMO University, Kronverkskiy 49, St. Petersburg 197101, Russia # University of Illinois at UrbanaChampaign, 1206 West Green Street, Urbana, Illinois 61801, United States ‡

ABSTRACT: Efficient capture and recycling of CO2 enable not only prevention of global warming but also the supply of useful low-carbon fuels. The catalytic conversion of CO2 into an organic compound is a promising recycling approach which opens new concepts and opportunities for catalytic and industrial development. Here we report about templateassisted wet-combustion synthesis of a one-dimensional nickel-based catalyst for carbon dioxide methanation and methane steam reforming. Because of a high temperature achieved in a short time during reaction and a large amount of evolved gases, the wet-combustion synthesis yields homogeneously precipitated nanoparticles of NiO with average particle size of 4 nm on alumina nanofibers covered with a NiAl2O4 nanolayer. The as-synthesized core−shell structured fibers exhibit outstanding activity in steam reforming of methane and sufficient activity in carbon dioxide methanation with 100% selectivity toward methane formation. The as-synthesized catalyst shows stable operation under the reaction conditions for at least 50 h. KEYWORDS: Ni-based catalyst, combustion synthesis, nanofibers, carbon dioxide methanation, methane steam reforming activity and selectivity are far from perfection.6,7 Au, Ag, and Zn favor the production of CO.7 Pd, Rh, Mo, Re, and Au are less reactive and possess low selectivity, simultaneously producing CH4, CH3OH, and CO.8 Among various metal catalysts, Ni-, Ru-, Pd-, and Rh-based catalysts possess high activity and selectivity toward CO2 methanation due to their high ability to dissociate CO2.8,9 Although Ni-based catalysts frequently suffer from coke deposition, particle sintering, and interaction with CO, they are still the most common because of their low price and high activity. High sensitivity to surface carbon deposition may result in deactivation or blocking of a reactor. Catalyst deactivation, the loss over time of catalytic activity and/or selectivity, is a problem of great and continuing concern in the practice of industrial catalytic processes.10 Carbon deposition can be depressed by using highly dispersed active Ni sites.11 A limiting factor is possible aggregation of nanostructured nickel after several cycles at a

1. INTRODUCTION Recently, the global warming caused by the evolution of huge amounts of carbon dioxide has become a challenging problem. A way to decrease the concentration of carbon dioxide in the atmosphere is to curb its emission. Another way is recycling CO2 into organic fuels, which allows the prevention of climate change, at that same time satisfying future energy needs of people.1,2 Therefore, the catalytic conversion of CO2 into industrially valuable chemicals, such as methane, methanol, dimethyl ether, and syngas, is an approach, which opens new concepts and opportunities for catalytic and industrial development. Various homogeneous and heterogeneous catalysts are used to hydrogenate carbon dioxide.3,4 Although homogeneous catalysts show satisfactory activity and selectivity, the recovery and regeneration are still problematic for CO2 hydrogenation.3 The development of alternative heterogeneous catalysts with high stability and activity combined with cost-effectiveness for large-scale production is a grand challenge. The choice of catalyst highly influences the main products (e.g., CO, CH3OH, CH4, etc.) obtained by CO2 hydrogenation.5 Cu is reported to be one of those metals capable to reduce CO2; however, its © 2017 American Chemical Society

Received: June 7, 2017 Accepted: November 20, 2017 Published: November 20, 2017 43553

DOI: 10.1021/acsami.7b08129 ACS Appl. Mater. Interfaces 2017, 9, 43553−43562

Research Article

ACS Applied Materials & Interfaces

Figure 1. Mesoporous network of pristine alumina nanofibers. SEM of (a) top and (b) side views. (c) Differential pore-size distributions of the network measured by nitrogen adsorption technique.

Figure 2. Schematic representation of wet-combustion synthesis.

formation of a needed phase without an additional step of calcination.17 Atomic layer deposition (ALD) of nickel, recently reported elsewhere,18 allows the deposition of nickel nanoparticles onto a porous γ-Al2O3 support with development of NiAl2O4 spinel that subsequently reduces to metallic Ni in the dry reforming of methane (DRM) reaction process. However, ALD is a relatively expensive procedure to be widely accepted by industry. In this work, we have developed a novel method combining impregnation and solution combustion approaches to prepare a hierarchically structured nickel-based catalyst hosted on a mesoporous fibrous alumina support. On the basis of this method, the reactive solution is infiltrated by the mesoporous network of the highly aligned self-assembled alumina fibers of 10 ± 2 nm diameter because of the capillary forces. The wetted samples are subsequently heat-treated to ignite a self-sustaining combustion process. As a result, homogeneously distributed nanometer-sized particles of nickel are formed throughout the array of ceramic nanofibers because of the high reaction temperature achieved in a short time and a large amount of gases evolved during the process. Moreover, the Ni nanoparticles strongly interact with the alumina substrate and show higher stability as compared to the Ni nanoparticles prepared by other methods. Therefore, the main goal of the present work is 2-fold: (i) synthesis of a porous γ-Al2O3 supported Ni nanoparticles catalyst using the simple, cost-effective, and feasible wet-combustion approach; and (ii) study of the catalytic behavior of the corresponding catalyst under the conditions of carbon dioxide methanation, and the reverse methane steam reforming reactions. In addition, nickel-based catalysts are widely used in hydrocarbon reforming processes,

high temperature developed during the processes. To impede the aggregation, mesoporous catalyst supports are highly desirable. Well-organized nanoporous catalysts are of great interest because of highly ordered pore structure, high specific surface area, tailorable pore size, framework, and surface properties.9,12 An appropriate support has to improve the dispersion of active components, promote effective metal− support interactions, and retain the unique properties of the catalyst. Recently, ordered mesoporous silica was utilized as catalyst supports for the catalytic partial oxidation of methane to produce syngas and hydrogen.13 The 2D hexagonal ordered mesoporous structure is found to play a crucial role for homogeneous distribution of Ni nanocrystals during the preparation and revealed a good resistance against carbon deposition. The synthesis technique of catalysts is another crucial issue influencing the particle size and morphology of the catalyst, tuning its dispersion and wettability, governing possible interaction between the components, thus ultimately tuning the catalytic activity and durability of the system.14 Therefore, the method of preparation is of the greatest scientific and industrial importance. Many different techniques are employed for the preparation of a Ni-based nanocatalyst with enhanced properties. One of the most widely utilized approaches for preparation of a supported metal catalyst is an impregnation using aqueous solution of nickel nitrate hexahydrate.15,16 However, usually this method is followed by a long-term calcination process, which results in formation of large particles due to aggregation of nanoparticles. The solution combustion method is another effective approach to synthesize a wide variety of nanomaterials. The high temperature generated during the combustion process within several seconds facilitates 43554


Research Article


Figure 3. Morphology, structure, and composition of the Ni/ANF(F). (a) FESEM micrograph. (b) X-ray diffraction pattern of the prepared fresh catalyst [Ni/ANF (F)]. (c) HRTEM images. (d, e) EDX spectra of the Ni/ANF(F) composite, corresponding to the regions marked as 1 and 2 in part c. (f) Lattice fringe, the d-spacing at 1.25 Å corresponding to NiO phase. (g) d-spacing at 2.11 Å corresponding to NiAl2O4 spinel structure. current of 40 kV and 100 mA, respectively. The ICDD PDF-2 database was used for phase identification. Peak positions were fit assuming a Lorentz peak shape. The relative volume fractions of the constituents were calculated using the integrated intensities of phases obtained from a line profile analysis. The Ni/ANF(F) reduction process in H2 stream (40 scm3 min−1) was studied by in situ XRD in an Anton Paar XRK-900 hightemperature reactor chamber mounted on the Bruker D8 diffractometer. The XRD patterns were recorded over the angular range 30− 60° (2θ) with a step size of 0.05° and a time per step of 6.5 s using Cu Kα radiation. The XRD patterns were collected at 25, 300, 400, 500, 600, and 700 °C by stepwise temperature increase. Prior to each record, the catalyst was kept at constant temperature for 30 min. The morphology of the catalysts was examined using scanning electron microscopy (SEM Zeiss EVO MA 15) and transmission electron microscope (TEM/HRTEM, JEOL 2100F) operating at 200 kV and equipped with a field emission electron gun providing a point resolution of 0.19 nm. For preparation of the samples for TEM examination, a piece of specimen was carefully suspended in ethanol, which was dropped on a copper TEM grid with a carbon film support. The microscope coupled with an INCA x-sight energy-dispersive X-ray spectrometer (EDXS), from Oxford Instruments, was used for the chemical elemental analysis. The specific BET surface area (SBET) of the catalysts was determined from complete nitrogen adsorption isotherms at −196 °C (ASAP 2400 instrument). The porosity of the Ni/ANF(F) catalyst was characterized by N2 adsorption−desorption measurements obtained at −196 °C (Sorptomatic 1990, Thermo Electron). The reduction behavior and the interactions between the active phase and the support of the as-synthesized sample were examined by using temperature-programmed reduction of H2 (H2-TPR) using a thermogravimetric analyzer Netzsch TG 209 F1 Libra. Prior to the reduction, the sample was pretreated in He at 600 °C to drive away water or impurities. H2-TPR was carried out in 10 vol % H2/He stream in the temperature interval 25−800 °C (heating rate 10° min−1). Sample weight loss, H2, and H2O MS-signals were used to characterize

especially that of methane, which is one of the commonly used methods for syngas and hydrogen production.19,20 To the best of our knowledge, this is the first report on the use of the super-high aspect ratio (length-to-diameter ratio is ∼107) self-aligned fiber network supported Ni catalyst.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The mesoporous network of selforiented γ-alumina nanofibers with a single fiber diameter of 10 ± 2 nm and a length of several centimeters (Figure 1a,b) was produced by the liquid phase oxidation of the melted aluminum as described elsewhere.21 This nanostructured network with the specific surface area of 155 m2 g−1 (BET analysis22), and a distance between fibers of ∼10 nm, (Figure 1c) was employed as a support for nickel deposition. The nickel-based catalyst was prepared by infiltrating 2 g of the blocks of nanofibers by a reactive solution consisting of 5.7 g of nickel(II) nitrate hexahydrate [Ni(NO3)2·6H2O] (Sigma-Aldrich, ≥98.5) and 1.2 g of glycine (Sigma-Aldrich, ≥99) dissolved in 15 mL of the deionized water. The wetted network was aged at room temperature for 30 min and subsequently placed into a muffle furnace preheated to 400 °C for 30 min. The temperature−time history of the synthesis process was recorded by highly accurate MPAC IGAR 12LO digital 2-color pyrometers with a fiber optic and a response time of 2 ms. The schematic representation of the process of wet-combustion synthesis of the Ni-based catalyst is given in Figure 2 and detailed in ref 23. The prepared catalyst is denoted as Ni/ANF(F). 2.2. Morphological and Chemical Characterization. The chemical composition of the samples was determined by inductively coupled plasma atomic emission spectrometry (an Optima instrument). The identification of crystalline phases was carried out by X-ray diffraction using a Bruker diffractometer (D8) equipped with a LynxEye position-sensitive detector. The samples were ground by a mortar and pestle. The XRD patterns were recorded over the angular range 10−85° (2θ) with a step size of 0.1° and a time per step of 100 s using Cu Kα radiation (λ = 0.154 056 nm) with a working voltage and 43555


Research Article


Figure 4. Structure and composition of the dip-coated sample. (a) FESEM micrograph. (b) X-ray diffraction pattern. the reduction process. The gas composition at the outlet point of the thermogravimetric analyzer was analyzed by mass spectrometer QMS200 (Stanford Research Systems). 2.3. Study of Catalytic Activity. Catalyst activity for the carbon dioxide methanation (CDM) and steam reforming of methane (SRM) was evaluated in a flow reactor with an inner diameter of 6 mm and equipped with a K-type thermocouple installed in the middle of the catalyst bed for a continuous temperature measurement. A portion of 100 mg of the fibrous catalyst was placed in the reactor without additional pelletizing. The gas feed was regulated by the mass-flow controllers (Bronkhorst). The carbon dioxide methanation reaction was performed at temperatures between 275 and 425 °C using the gas composition as follows: 8.2 vol % CO2, 38.2 vol % H2, and 53.6 vol % Ar with the total weight hourly space velocity (WHSV) of 34 000 scm3 g−1 h−1. Prior to the experiments, the catalysts were activated in H2/Ar (10/90) at 500 °C for 4 h with a heating rate of 2.5 °C min−1. The catalyst after the CDM reaction was denoted as Ni/ANF(M). The steam reforming of methane was performed at a temperature of 650 °C with a feed gas composition of 33.3 vol % CH4 and 66.7 vol % H2O. The total feed flow rate was adjusted to WHSV of 45 000− 360 000 scm3 g−1 h−1. Prior the experiments, the catalysts were activated in H2/Ar (10/90) at 700 °C for 8 h at a heating rate of 2.5 °C min−1. After the SRM reaction, the catalyst was denoted as Ni/ ANF(R). An industrial natural gas steam reforming Ni-based catalyst AlfaAesar HiFUEL R110 (≈16 wt % Ni) was chosen as a reference sample. The catalyst was denoted as Ni-ref. Prior to the experiments, Ni-ref was activated in H2/Ar (10/90) at 700 °C for 8 h at a heating rate of 2.5 °C min−1. Afterward, its activity was evaluated in CDM and SRM reactions under the same conditions as for Ni/ANF. The reactants and products in the gas phase were detected on-line using gas chromatography (Chromos GC-1000) equipped with molecular sieve 5A, Porapak T and Porapak Q columns, and two thermal-conductivity detectors (TCD) and one flame-ionization detector (FID), respectively. The operation temperature was programmed from 60 to 120 °C. The separated components from the Porapak Q column were fed to a methanator (containing a reduced nickel catalyst) operated at 360 °C to convert the CO and CO2 to methane prior to analysis with a flame ionization detector (FID). Argon was used as a carrier gas. The combination of methanator and FID allowed highly sensitive analysis of CO, CO2, and hydrocarbons in the gas mixture. The detection limit of the CO, CH4, and CO2 was 1 ppm, and those of the H2 and H2O concentration were 1000 and 2000 ppm, respectively. The relative errors of CH4, CO, CO2, H2, and H2O concentration determinations were 0.5%, 0.5%, 0.5%, 1%, and 2%, respectively. During the experiments, the carbon balance was controlled with an accuracy of ±2%. The performance of the catalyst was characterized in terms of the CH4, CO, CO2, H2, and H2O outlet concentrations and their comparison with the equilibrium values for corresponding reaction conditions. Equilibrium compositions and all experimental conditions were calculated utilizing HSC Chemistry version 7 software by minimization of the Gibbs free energy of the system.

3. RESULTS AND DISCUSSION 3.1. Wet-Combustion Synthesis. Molecular-level mixing of nickel nitrate and glycine, a short (seconds scale) duration of

Figure 5. Temperature−time history of the combustion in the nickel nitrate−glycine−alumina system.

the process, a high cooling rate, and a huge amount of gases evolved during the process provide an effective nucleation and growth of the fine particles of nickel oxide. The mesoporous network of alumina enables distribution of the reactive solution all over the nanofibers due to capillary forces, while hydroxyl and epoxide functional groups of alumina nanofibers21 enhance the bonding between alumina and metals by covalent oxygen bonds bridging nickel oxide and alumina. Thus, the prepared catalyst possesses high surface area and well-distributed dispersion of the catalytic phase at high loadings. The as-synthesized fresh Ni-based catalyst maintains the fibrous structure of the highly aligned 1D alumina precursor (Figure 3a). However, the diameter of a single composite fiber reaches 15−30 nm. The BET surface area drops down from 155 to 108 m2 g−1. The porosity of the Ni/ANF(F) catalyst is estimated as 80% with pore size of about 8 nm. The diffractogram shown in Figure 3b indicates that crystallites are very small, and the crystallinity degree of the sample is rather low. The NiO and Al2O3 phases along with NiAl2O4 phase are well-distinguished. Figure 3c−e demonstrates the high-resolution TEM (HRTEM) images and the energydispersive X-ray (EDX) analysis indicating uniform distribution of NiO nanoparticles over the core−shell structured composite fiber. The ceramic fiber consists of a NiAl2O4 spinel core with an average diameter of 10 nm encapsulated by a layer of NiO nanoparticles with an average crystallite size of 4 nm. Fringes of 1.25 Å (Figure 3f) and 2.11 Å (Figure 3g) are observed in the 43556


Research Article


Figure 6. (a) Time dependencies of sample weight and temperature. (b) H2 and H2O MS-signals for Ni/ANF(F) H2-TPR.

mesoporous network in a reliable, fast, and cost-effective way without any further calcination. The well-structured channels in the network enhance driving infiltration of precursors and their homogeneous distribution. After the wet samples were placed into the preheated muffle, a self-sustaining combustion process is ignited, which lasts a few seconds (Figure 5). The pyrometric analysis reveals that, after intensive evaporation of water (stage 1), an exothermic reaction begins (stage 2) with the total reaction time on the order of 0.1 s reaching the maximum combustion temperature of ∼1270 °C for the system. This intensive combustion process is furthered with a moderate exothermic process caused by the oxidation of not fully oxidized contaminations by air oxygen (stage 3). The huge amount of gases generated during the combustion process results in formation of a nanoscaled product, while the high temperature developed during the short combustion process facilitates crystallization of the nanoparticles and formation of ternary oxide (NiAl2O4) between NiO and Al2O3. 3.2. Reducibility of Ni/ANF(F). Figure 6 represents time dependencies of sample weight and temperature, and H2 and H2O MS-signals for Ni/ANF(F) H2-TPR. The incipient weight loss and H2 uptake indicate that the reduction of the assynthesized catalyst [Ni/ANF(F)] starts at 280 °C. The whole reduction process is divided in two sequential steps: (i) at 280− 500 °C with maximum reduction speed, H2 uptake and H2O evolution at 425 °C; and (ii) at 500−800 °C with maximum reduction speed and H2 uptake at 720 °C. The whole weight loss after 2 h of reduction was ca. 8.7 wt %, indicating reduction of 85% of Ni2+ in the sample. It is probable that higher exposure time is necessary for complete reduction of Ni2+. This is in good agreement with the complexity of the NiAl2O4 spinel reduction process, which is kinetically labored.31,32 For clarification of the nature of the two-step reductive process of Ni/ANF(F), an in situ XRD study was carried out. Figure 7 represents the XRD patterns of the samples obtained by the reduction of Ni/ANF(F) at 25, 300, 400, 500, 600, and 700 °C during stepwise temperature increase in H2 stream. Two reduction steps are distinguished: (i) NiO reduction to Ni; and (ii) NiAl2O4 reduction to Ni and Al2O3. NiO reduction starts at 300 °C and completes at 500 °C, while NiAl2O4 spinel reduction starts at 500 °C and completes at 700 °C. The CSR (catalyst structure after reduction) of Ni phase does not change obviously during further reduction and remains 10 nm. According to the literature,33 NiAl2O4 spinel reduces at 800− 900 °C. However, in our study NiAl2O4 spinel species are much more reactive toward reduction, probably because of the unique structure of Ni/ANF material and nanometer-scale size of the spinel layer. On the basis of the obtained results, 500 and 700

Figure 7. XRD pattern of the Ni-based catalyst during the reduction process in H2 stream at 25, 300, 400, 500, 600, and 700 °C.

shell and the core, respectively, corresponding to the NiO (311) and NiAl2O4 (400) lattice spacing. Notably, the wet-combustion method can be used to decorate nanofibers with different metal oxides or even provide controllable interaction between a core and a shell.23,24 However, for control of the homogeneous distribution of metal oxides over the fibers, it is important to choose a suitable metal salt precursor and a fuel, which not only serves as a source of energy but also greatly influences the reaction pathway.17,25 Specifically, infiltrating the mesoporous network of alumina by nickel nitrate aqueous solution without adding any fuel and treating at similar condition (400 °C, 30 min) results in nonhomogeneously “clustered” NiO nanoparticles detached to the fibers as demonstrated in Figure 4. The formation of ternary oxide and uniformly distributed NiO nanoparticles is favored when glycine is added into the initial reactive solution. There are unified viewpoints that the deposition of NiO on a spinel (e.g., NiAl2O4, MgAl2O4, CaAl2O4, etc.) leads to the higher activity and dispersion.26−29 The formation of a layer of ternary oxides between metal oxides (specifically NiO and Al2O3) can greatly improve the catalytic stability of the catalyst. As it was suggested by Chick et al.,30 glycine not only serves as a fuel-initiating self-sustaining combustion reaction, being oxidized by nitrate ions, but also complexes with metal cations, increasing their solubility and preventing selective precipitation as water is evaporating. Synthesis of various nanosized materials as a result of self-propagating reactions in a solution of metalcontaining oxidizers and fuel(s) is a well-known approach of solution combustion synthesis. Combining the merits of dipcoating and solution combustion synthesis, we functionalize a 43557


Research Article


Figure 9. (a) Space velocity dependencies of the CH4, CO, CO2, H2, and H2O outlet concentrations and their comparison with equilibrium values for the SRM reaction at 650 °C over the Ni/ANF catalyst. (b) Comparison of Ni/ANF and Ni-ref performances. (c) Stability test for Ni/ANF at 650 °C and WHSV of 360 000 scm3 g−1 h−1. Feed gas composition: 33.3 vol % CH4 and 66.7 vol % H2O. Points, experiment; lines, equilibrium. Figure 8. (a, b) Temperature dependencies of (dark red) CO2, (blue) CH4, (red) H2O, and (black) H2 outlet concentrations and their comparison with equilibrium values for the CO2 methanation over Ni/ ANF and Ni-ref. (c) Stability test for Ni/ANF at 323 °C. Feed gas composition: 8.2 vol % CO2, 38.2 vol % H2, and 53.6 vol % Ar. WHSV = 22 500 scm3 g−1 h−1.

selectivity toward CH4, and high thermal stability due to high exothermicity of the CDM reaction. Figure 8 represents the temperature dependencies of the CH4, CO2, H2O, and H2 outlet concentrations, and their comparison with equilibrium values for the CDM reaction over the prepared catalyst. It is seen, that with increasing temperature CO2 and H2 concentration decrease, reach minimums at ca. 370 °C, which correspond to equilibrium H2 and CO2 concentrations, and then increase along with the equilibria. The opposite dependency is observed for CDM reaction products CH4 and H2O; their concentrations reach the maximum at 370 °C and then go down along with the equilibria. Thus, the CDM reaction over Ni/ANF under particular reaction conditions is under kinetic control at T < 370 °C and under thermodynamic control at T > 370 °C. The selectivity toward CH4 formation is 100%. The Ni/ANF

°C were chosen as reduction temperatures of Ni/ANF(F) to Ni/NiAl2O4−Al2O3 and Ni/Al2O3 systems, respectively. 3.3. Catalytic Activity in Carbon Dioxide Methanation. The Ni/ANF reduced at 500 °C is tested in the CO2 methanation reaction (CDM). The CDM reaction is thermodynamically limited, and it causes restriction on the CO2 conversion and CH4 concentration that could be reached: When the catalyst operation temperature is higher, the achieved CO2 conversion and CH4 concentration will be lower.34−36 Therefore, the requirements toward a catalyst of the CO2 methanation reaction are high activity at T < 300 °C, high 43558


Research Article


Figure 10. Morphology of (a) Ni/ANF(F), (b) Ni/ANF(M), and (c) Ni/ANF(R). (d) Pore-size distribution for Ni/ANF by the desorption branch of complete low-temperature N2-adsorption isotherms.

3.4. Catalytic Activity in Methane Steam Reforming. The Ni/ANF reduced at 700 °C is tested in the methane steam reforming reaction (SRM). The SRM reaction is also thermodynamically limited. The SRM catalysts are conventionally operated at 650−850 °C, 1−30 bar pressure, and WHSV of 3000−10 000 scm3 g−1 h−1,38 which provides maximal CH4 conversion and H2 concentration. The requirements toward a methane steam reforming catalyst require high activity at T of ca. 700 °C, high thermal stability, and good hydraulic properties. Figure 9a represents the space velocity dependencies of the CH4, CO, CO2, H2, and H2O outlet concentrations and their comparison with equilibrium values for the SRM reaction at 650 °C for the Ni/ANF catalyst. We studied the influence of space velocity on product distribution to find the range of flow rates in which the catalyst provides maximal CH4 conversion to syngas (i.e., equilibrium product distribution is reached). Ni/ ANF performance in SRM reactions is compared with that of the industrial Ni-ref catalyst in terms of WHSV range, in which the equilibrium product distribution could be reached. It is seen (Figure 9b), that the Ni-ref catalyst provides equilibrium conversion at 650 °C and WHSV ≤ 90 000 scm3 g−1 h−1, while the equilibrium product distribution over Ni/ANF is achieved at WHSVs in the range 45 000−270 000 scm3 g−1 h−1, which

Table 1. Surface Areas and Phase Composition for Ni/ANF catalyst Ni/ANF(F) Ni/ANF(M) Ni/ANF(R)

2

SBET, m g 108 66 45

−1

phase composition NiAl2O4, γ-Al2O3, NiO NiAl2O4, γ-Al2O3, Ni γ-Al2O3, Ni

catalyst exhibits stable operation under CDM conditions for at least 50 h (Figure 8c). Ni/ANF performance in CDM reactions was compared with that of the industrial Ni-ref catalyst. It is seen (Figure 8a,b), that, over the Ni-ref catalyst, product distribution does not reach equilibrium even at 400 °C. Thus, Ni/ANF advances Niref in terms of CDM performance, probably because of higher Ni content and dispersion. However, in general Ni/ANF shows relatively low CDM performance. For comparison, the Ni/ CeO2 catalyst reaches equilibrium product distribution at the same temperature of 360 °C, but at a WHSV of 72 000 scm3 g−1 h−1.35 Such a result is expectable, as low-temperature CO2 activation requires basic sites onto the catalyst surface, as was discussed in ref 37. In summary, Ni/ANF could be applied for CO2 methanation, but it is not the best catalyst for this reaction. 43559


Research Article


Figure 11. Phase evolution of Ni/ANF(F) after CDM and MSR reactions. (a) XRD pattern of synthesized Ni/ANF(F) composite Ni/ANF catalyst after moderate-temperature CDM reaction. (b) TEM images of NiO/ANF after moderate-temperature CDM reaction. (c) HRTEM images from part b. (d) XRD pattern of synthesized Ni/ANF(F) after moderate-temperature SRM reaction. (e) TEM images of NiO/ANF after moderatetemperature SRM reaction. (f) HRTEM images from part e.

The pore-size distribution of the samples measured by the analysis of desorption branch of complete low-temperature N2adsorption isotherms (Barrett−Joyner−Halenda model) demonstrates that the as-prepared Ni/ANF(F) has a broad poresize distribution in the range 3−100 nm with a flat maximum at 4−10 nm, Figure 10. However, the volume of pores of size 4− 20 nm decreases for the treated catalyst [Ni/ANF(M) and Ni/ ANF(R)], while the pore-size distribution in the range 20−100 nm remains constant. The XRD analysis reveals that the catalyst is composed of Ni0, γ-Al2O3, and NiAl2O4 after CDM reaction and of Ni0 and γ-Al2O3 after MSR reaction (Figure 11a,d, Table 1). HR-TEM analysis reveals that the particle size of Ni0 that existed in Ni/ ANF(R) (∼15 nm) is slightly smaller than the particle size of Ni0 that existed in Ni/ANF(M) (∼10 nm). The obtained data confirm the full reduction of NiAl2O4 spinel species in the Ni/ ANF sample after SRM reaction. The phase composition of Ni/ANF(R) is caused by a reduction process of NiO to Ni0 and a complete segregation of spinel NiAl2O4 resulting in the formation the γ-Al2O3 phase and the NiO phase which is reduced to Ni0. The NiO phase, which proceeds from segregation of spinel NiAl2O4, is reduced to the metallic nickel. 3.6. Ni/ANF as a Catalytic Material. The fibrous structure of the catalyst remains unchanged during reduction in hydrogen and catalytic tests even at the high temperature of the SRM reaction. Therefore, it can be concluded that the fibrous structure of the catalyst and its unique morphology provides stability at high-temperature reaction conditions. However, the XRD analysis showed the changes in phase composition of the initial Ni/ANF(F) material during reduction in hydrogen and catalytic tests. Reduction at 500 °C led to the reduction of NiO to metallic Ni and its slight sintering, while reduction at 700 °C leads to complete segregation of the spinel NiAl2O4 phase with formation of an additional amount of metallic Ni and its further agglomeration.

are very high values compared to data demonstrated for industrially used catalysts.38 Thus, Ni/ANF advances Ni-ref and other industrial catalysts in terms of SRM performance, probably because of its ability to support high Ni dispersion at high Ni loadings even at the temperatures of the SRM reaction. The Ni/ANF catalyst exhibits stable operation under SRM conditions for at least 50 h (Figure 9c). The WHSV of 270 000 scm3 g−1 h−1 corresponds to a H2 productivity of 19.6 kg H kg −cat1 h−1 while for conventional catalysts this value does 2

not exceed 1 kg H kg −cat1 h−1. 2

In summary, the Ni/ANF catalyst exhibited extraordinary performance in methane steam reforming and was fully stable under reaction conditions. 3.5. Catalyst Structure after Reaction. For elucidation of the influence of prereduction and reaction conditions on the catalyst structure, the Ni/ANF is analyzed prior to and after two cycles of reactions by a number of physical−chemical techniques, including SEM, BET, XRD, and TEM. Figure 10a illustrates the morphology of the prepared Ni/ANF(F) catalyst presenting a network of well-aligned 1D nanocomposites keeping the fibrous structure of alumina support. The asproduced catalyst possesses a mesoporous structure due to the extensive cross-linking reactions during synthesis. The wetcombustion approach appears to be an excellent method for fabrication of mesoporous catalysts without adding poreforming agents. Moreover, the catalysts morphology remains unchanged after the catalytic tests at harsh conditions (Figure 10b,c). The specific surface area of the catalyst decreases from 108 to 66 m2 g−1 after two cycles of CDM reaction at 400 °C (Table 1). However, after the high-temperature (700 °C) SRM reaction, the specific surface of the catalyst decreases to 45 m2 g−1 because of agglomeration of nickel particles and occlusion of pores. 43560


Research Article


MAT2013-48009-C4-1-P for their financial support and is also indebted to MINECO for a “Ramon y Cajal” contract (Ref. RyC-2015-18626), which is cofinanced with European Social Fund. This work was supported by Prototron foundation under NICAT project (Estonia) and the Estonian Research Council under PUT1063 (I.H.).

Nevertheless, the obtained average size of Ni nanoparticles (∼15 nm) is a good value for such a high temperature (700 °C) and high Ni content (33 wt %). There is an increasing interest in the development of catalytic membrane reactors and catalytic composite materials for moderate- and high-temperature reactions, such as water−gas shift reaction,10,11 methane steam10,11 and dry12,14 reforming, methane oxidative coupling,39 etc. The developed Ni/ANF system is a very promising material for catalytic membranes or as an active component for composite membranes and catalysts due to its high catalytic activity and thermal stability, low density, high gas permeability, as well as tunable preparation procedure.

■

(1) Aziz, M. A. A.; Triwahyono, J. S.; Ahmad, A. CO2 Methanation Over Heterogeneous Catalysts: Recent Progress and Future Prospects. Green Chem. 2015, 17, 2647−2663. (2) Hashimoto, K.; Yamasaki, M.; Meguro, S.; Sasaki, T.; Katagiri, H.; Izumiya, K.; Kumagai, N.; Habazaki, H.; Akiyama, E.; Asami, K. Materials for Global Carbon Dioxide Recycling. Corros. Sci. 2002, 44, 371−386. (3) Moret, S.; Dyson, P. J.; Laurenczy, G. Direct Synthesis of Formic Acid From Carbon Dioxide by Hydrogenation in Acidic Media. Nat. Commun. 2014, 5, 5017. (4) Chen, Y.; Choi, S.; Thompson, L. Low Temperature CO2 Hydrogenation to Liquid Products via Heterogeneous Cascade Catalytic System. ACS Catal. 2015, 5, 1717−1725. (5) Wu, J.; Wen, C.; Zou, X. L.; Jimenez, J.; Sun, J.; Xia, Y.; Fonseca Rodrigues, M. T.; Vinod, S.; Zhong, J.; Chopra, N.; Odeh, I. N. Carbon Dioxide Hydrogenation over a Metal-Free Carbon Based Catalyst. ACS Catal. 2017, 7, 4497. (6) Chang, Z.; Huo, S. J.; Zhang, W.; Fang, J.; Wang, H. The Tunable and Highly Selective Reduction Products on Ag@Cu Bimetallic Catalysts Towards CO2 Electrochemical Reduction Reaction. J. Phys. Chem. C 2017, 121, 11368. (7) Dong, C.; Fu, J.; Liu, H.; Ling, T.; Yang, J.; Qiao, S. Z.; Du, X. W. Tuning the selectivity and activity of Au catalysts for carbon dioxide electroreduction via grain boundary engineering: a DFT study. J. Mater. Chem. A 2017, 5, 7184−7190. (8) Abate, S.; Mebrahtu, C.; Giglio, E.; Deorsola, F.; Bensaid, S.; Perathoner, S.; Pirone, R.; Centi, G. Catalytic Performance of γAl2O3−ZrO2−TiO2−CeO2 Composite Oxide Supported Ni-Based Catalysts for CO2Methanation. Ind. Eng. Chem. Res. 2016, 55, 4451− 4460. (9) Fechete, I.; Vedrine, J. C. Nanoporous Materials as New Engineered Catalysts for The Synthesis Of Green Fuels. Molecules 2015, 20, 5638−5666. (10) Argyle, M. D.; Bartholomew, C. H. Heterogeneous Catalyst Deactivation and Regeneration: A review. Catalysts 2015, 5, 145−269. (11) Becerra, A. M.; Castro-Luna, A. E. An Investigation on the Presence of NiAl2O4 in a Stable Ni on Alumina Catalyst for Dry Reforming. J. Chil. Chem. Soc. 2005, 50, 465−469. (12) Taguchi, A.; Schüth, F. Ordered Mesoporous Materials in Catalysis. Microporous Mesoporous Mater. 2005, 77, 1−45. (13) Kim, D.; Park, G. A.; Lim, J.; Ha, K. S. Effects of Spatially Restricted Ni Nanocrystals within Ordered Mesoporous on the Production of Syngas. Chem. Eng. J. 2017, 316, 1011−1025. (14) Zhang, M.; Fan, J.; Chi, K.; Duan, A.; Zhao, Z.; Meng, X.; Zhang, H. Synthesis, Characterization, and Catalytic Performance of NiMo Catalysts Supported on Different Crystal Alumina Materials in the Hydrodesulfurization of Diesel. Fuel Process. Technol. 2017, 156, 446−453. (15) Munnik, P.; de Jongh, P. E.; de Jong, K. P. Recent Developments in the Synthesis of Supported Catalysts. Chem. Rev. 2015, 115, 6687−6718. (16) He, S.; Zhang, L.; He, S.; Mo, L.; Zheng, X.; Wang, H.; Luo, Y. Ni/SiO2 Catalyst Prepared with Nickel Nitrate Precursor for Combination of CO2 Reforming and Partial Oxidation of Methane: Characterization and Deactivation Mechanism Investigation. J. Nanomater. 2015, 2015, 1−8. (17) Manukyan, K. V.; Cross, A.; Roslyakov, S.; Rouvimov, S.; Rogachev, A. S.; Wolf, E. E.; Mukasyan, A. S. Solution Combustion Synthesis of Nano-Crystalline Metallic Materials: Mechanistic Studies. J. Phys. Chem. C 2013, 117, 24417−24427.

4. CONCLUSIONS A nickel-based catalyst with a never-available-before structure has been synthesized by a novel template-assisted wetcombustion method. The as-prepared nickel-based catalyst possesses a core−shell fibrous structure with a single fiber diameter of less than 50 nm and very high aspect ratio of 107. The alumina nanofiber supported Ni catalysts exhibit relatively low activity in carbon dioxide methanation (equilibrium product distribution is achieved at 360 °C and WHSV of 22 500 scm3 g−1 h−1) and outstanding activity in steam reforming of methane (WHSV of 45 000−270 000 scm3 g−1 h−1 at 650 °C). The advanced performance of Ni/ANF is associated with low density of the material, high gas permeability, high Ni content, and dispersion under hightemperature reaction conditions. The material is stable under carbon dioxide methanation and methane steam reforming reaction conditions keeping a fibrous structure of the support unchanged and preventing sintering of Ni nanoparticles. The Ni-ANF catalyst demonstrates high catalytic stability for at least 50 h under the reaction conditions. In general, the developed method is relevant for functionalization of mesoporous networks, including preparation of the supported catalysts since it allows controlling the size, morphology, and the structure of the deposited particles and their dispersion. The advantage in using this method is its simplicity and cost-effectiveness enabling synthesis in one step (functionalization or decoration) of any kind of mesoporous nanocomposites with tailored phase composition and morphology for different applications.

■

REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +372 55588124; +372 6203359. ORCID

M. Aghayan: 0000-0002-0435-1022 F. Rubio-Marcos: 0000-0002-2479-3792 I. Hussainova: 0000-0003-3081-2491 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful to Dr. O.A. Stonkus for assistance in TEM analysis and Dr. V.P. Pakharukova for assistance in XRD analysis. The work was partially supported by RFBR Grant 1633-60106 mol_a_dk. D.I.P. appreciates financial support from the Russian Federation President scholarship SP-922.2016.1. F.R.-M. expresses his thanks to the MINECO (Spain) project 43561


Research Article


(38) Rostrup-Nielsen, J. R.; Sehested, J.; Nørskov, J. K. Hydrogen and Synthesis Gas by Steam and CO2 Reforming. Adv. Catal. 2002, 47, 65. (39) Martínez, A.; Prieto, G.; Rollan, J. Nanofibrous γ-Al2O3 as Support for Co-Based Fischer−Tropsch Catalysts: Pondering the Relevance of Diffusional and Dispersion Effects on Catalytic Performance. J. Catal. 2009, 263, 292−305.

(18) Shang, Z.; Li, S.; Li, L.; Liu, G.; Liang, X. Highly Active and Stable Alumina Supported Nickel Nanoparticle Catalysts for Dry Reforming of Methane. Appl. Catal., B 2017, 201, 302−309. (19) Salhi, N.; Boulahouache, A.; Petit, C.; Kiennemann, A.; Rabia, C. Steam Reforming of Methane to Syngas Over NiAl2O4 Spinel Catalysts. Int. J. Hydrogen Energy 2011, 36, 11433−11439. (20) Kathiraser, Y.; Oemar, U.; Saw, E. T.; Li, Z.; Kawi, S. Kinetic and Mechanistic Aspects for CO2 Reforming of Methane over Ni Based Catalysts. Chem. Eng. J. 2015, 278, 62−78. (21) Aghayan, M.; Hussainova, I.; Gasik, M.; Kutuzov, M.; Friman, M. Coupled thermal analysis of novel alumina nanofibers with ultrahigh aspect ratio. Thermochim. Acta 2013, 574, 140−144. (22) Aghayan, M.; Gasik, M.; Hussainova, I.; Rubio-Marcos, F.; Kollo, L.; Kübarsepp, J. Thermal and Microstructural Analysis of Doped Alumina Nanofibers, Thermochim. Thermochim. Acta 2015, 602, 43−38. (23) Aghayan, M.; Rodríguez, M. A.; Kirakosyan, K.; Hussainova, I. The Template-Assisted Wet-Combustion Synthesis of Copper Oxide Nanoparticles on Mesoporous Network of Alumina Nanofibers,. Mater. Chem. Phys. 2017, 192, 138−146. (24) Aghayan, M.; Voltsihhin, N.; Rodríguez, M. A.; Rubio-Marcos, F.; Dong, M.; Hussainova, I. Functionalization of Gamma-Alumina Nanofibers by Alpha-Alumina via Solution Combustion Synthesis. Ceram. Int. 2014, 40, 12603−12607. (25) Stella, K. C.; Nesaraj, A. S. Effect of Fuels on the Combustion Synthesis of NiAl2O4 Spinel Particles. Iran. J. Mater. Sci. Eng. 2010, 7 (2), 36−44. (26) Sahli, N.; Petit, C.; Roger, A. C.; Kiennemann, A.; Libs, S.; Bettahar, M. M. Ni Catalysts from NiAl2O4 Spinel for CO2 Reforming of Methane, Catal. Catal. Today 2006, 113, 187−193. (27) Zhou, L.; Li, L.; Wei, N.; Li, J.; Basset, J. M. Effect of NiAl2O4 Formation on Ni/Al2O3 Stability During Dry Reforming of Methane. ChemCatChem 2015, 7, 2508−2516. (28) Du, Y. L.; Wu, X.; Cheng, Q.; Huang, Y. L.; Huang, W. Development of Ni-based Catalysts Derived from Hydrotalcite-Like Compounds Precursors for Synthesis Gas Production via Methane or Ethanol Reforming. Catalysts 2017, 7, 70. (29) Ha, Q. L. M.; Armbruster, U.; Atia, H.; Schneider, M.; Lund, H.; Agostini, G.; Radnik, J.; Vuong, H. T.; Martin, A. Development of Active and Stable Low Nickel Content Catalysts for Dry Reforming Of Methane. Catalysts 2017, 7, 157. (30) Chick, L. A.; Pederson, L. R.; Maupin, G. D.; Bates, J. L.; Thomas, L. E.; Exarhos, G. J. Glycine-Nitrate Combustion Synthesis of Oxide Ceramic Powders. Mater. Lett. 1990, 10, 6−12. (31) Hill, B. E.; Misture, S. T. Atomic Scale Mechanisms of the Reduction of Nickel−Magnesium Aluminate Spinels. J. Am. Ceram. Soc. 2013, 96, 3603−3608. (32) Chen, I.; Shiue, D. W. Reduction of Nickel-Alumina catalysts. Ind. Eng. Chem. Res. 1988, 27, 429−434. (33) Miletić, N.; Izquierdo, U.; Obregon, I.; Bizkarra, K.; AgirrezabalTelleria, I.; Bario, L. V.; Arias, P. L. Oxidative Steam Reforming of Methane over Nickel Catalysts Supported on Al2O3−CeO2−La2O3. Catal. Sci. Technol. 2015, 5, 1704−1715. (34) Le, T. A.; Kim, M. S.; Lee, S. H.; Kim, T. W.; Park, E. D. CO and CO2 Methanation over Supported Ni Catalysts. Catal. Today 2017, 293, 89−96. (35) Atzori, L.; Cutrufello, M. G.; Meloni, D.; Cannas, C.; Gazzoli, D.; Monaci, R.; Sini, M. F.; Rombi, E. Highly Active NiO-CeO2 Catalysts for Synthetic Natural Gas Production by CO2 Methanation. Catal. Today 2017; DOI: 10.1016/j.cattod.2017.05.065. (36) Rönsch, S.; Schneider, J.; Matthischke, S.; Schlüter, M.; Götz, M.; Lefebvre, J.; Prabhakaran, P.; Bajohr, S. Review on Methanation− From Fundamentals to Current Projects. Fuel 2016, 166, 276−296. (37) Wang, F.; He, S.; Chen, H.; Wang, B.; Zheng, L.; Wei, M.; Evans, D. G.; Duan, X. Active Site Dependent Reaction Mechanism over Ru/CeO2 Catalyst toward CO2 Methanation. J. Am. Chem. Soc. 2016, 138, 6298−6305. 43562


based catalyst for carbon dioxide methanation and methane stea

Recommend Documents