Partial oxidation of methane using silver/gadolinia-doped ceria composite membranes

Article pubs.acs.org/cm

Novel Composite Cermet for Low-Metal-Content Oxygen Separation Membranes Enrique Ruiz-Trejo,*,† Paul Boldrin,† Alexandra Lubin,† Farid Tariq,† Sarah Fearn,‡ Richard Chater,‡ Stuart N. Cook,‡ Alan Atkinson,‡ Robert I. Gruar,§ Christopher J. Tighe,§ Jawwad Darr,§ and Nigel P. Brandon† †

Department of Earth Science and Engineering, Imperial College London, London SW7 2AZ, United Kingdom Department of Materials, Imperial College London, London SW7 2AZ, United Kingdom § Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, United Kingdom ‡

ABSTRACT: A dense composite of silver and Ce0.8Sm0.2O2−δ (Ag-CSO) was manufactured from ceramic nanoparticles coated by electroless deposition of silver. At 700 °C, a 1mm-thick membrane of the composite delivered an excellent oxygen permeation rate from air with a value of 0.04 μmol cm−2 s−1, using argon as the sweep gas and 0.17 μmol cm−2 s−1 using hydrogen. The low sintering temperature of the CSO nanoparticles allows the use of Ag rather than Pt or Pd and reduces the amount of metal needed for electronic conductivity to just 5.6 vol %, which is lower than any value reported in the literature. Oxygen diffusivity measurements confirmed that the oxygen migration remained high in the composite, with an increase in surface exchange coefficient of three orders of magnitude over Gd-doped ceria. The ease of membrane fabrication, combined with encouraging oxygen permeation rates, demonstrate the promise of the material for high-purity oxygen separation below 700 °C.

1. INTRODUCTION Pure oxygen is an important industrial gas. It is used in applications where air is not suitable, such as coal or biomass gasification, syngas production for gas-to-liquids processes, or semiconductor device fabrication; in the future, oxyfuel combustion in carbon capture and storage power plants may provide an even larger potential market. In gasification and syngas production, the presence of nitrogen results in the production of NOx compounds, which need gas cleaning systems to be removed, while in semiconductor device fabrication, high-purity oxygen is required to eliminate the detrimental effects of nitrogen on the properties of semiconductors. Substantial effort is taken in these processes to produce pure oxygen; for example, in a typical Fischer− Tropsch plant, the oxygen separation adds $8.8/bbl to the cost of the final product,1 while oxygen separation for oxyfuel combustion decreases the efficiency of the plant by 8%−10%.2 The advantages of ceramic oxygen separation membranes over rival technologies such as pressure swing adsorption and cryogenic separation include the extremely high purity of the oxygen produced, the continuous nature of the production, the potential for integration into existing processes, and the easy scalability, facilitating oxygen generation from laboratory scale to industrial scale. The potential and practicalities for incorporating ceramic membranes into industrial processes have been discussed elsewhere.3,4 © XXXX American Chemical Society

Oxygen can be separated from air using an oxygen-ionconducting electrolyte, such as yttria-stabilized zirconia (YSZ), as an electrically driven oxygen pump, or by using a passive mixed ionic-electronic membrane that provides both oxygen transport and electronic conductivity.3,5−8 A mixed ionicelectronic membrane can either be a composite with an electronically conductive phase and an oxygen ionic conductive phase, or a single-phase mixed conducting material such as La 1−xSrxCo1−yFeyO3‑δ (LSCF)9 or Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF).10 In this paper, we focus on the dual-phase mixed ionic-electronic systems (see Figure 1). Of course, other features, such as stability under a gradient in the chemical potential of oxygen,11 fast gas−solid reactions, stability in CO2containing environments, nonreactivity of components, ease of fabrication, and long-term operation, are also required. The two-phase option features the advantage common to all composites, that properties such as mechanical expansion coefficient and fast surface exchange coefficient can be tuned by altering the ratio between the components or changing the surface microstructure. Attractive materials for a dual-phase separation membrane are rare-earth-doped cerias and silver. Below 600 °C, the ceriaReceived: December 20, 2013 Revised: June 12, 2014

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Figure 1. In mixed ionic−electronic membrane systems, oxygen from air is incorporated into the lattice. The oxygen diffuses across the gasimpermeable membrane driven by the difference in partial pressure of oxygen between the two sides of the membrane. Oxygen is then released as molecular oxygen leaving behind oxygen vacancies and electrons. Oxygen migrates mainly as oxygen ions through the ionic material and electrons mainly through the electronic conductor network.

at a rate of 400 mL min−1 (the supercritical water is in the inner pipe). The CSO nanoparticles are instantly precipitated in the flow, and the resulting slurry is reduced by centrifugation until the solids settle, and the wet solids are then cleaned by placing them in dialysis bags and replacing the water with deionized water until the conductivity approaches that of the clean water itself. Finally, the particles are freeze-dried. The particles were then used without any further treatment. The nanoparticles were suspended and coated with silver using Tollens’ reaction, as described in the original work22 or as described in any comprehensive chemistry textbook. Briefly, a solution of AgNO3 is mixed with concentrated KOH resulting in a precipitate, which is then cleared by adding a few drops of NH4OH; the nanoparticles of CSO are then suspended in the clear solution using an ultrasonic bath. Finally, silver is precipitated by adding a diluted solution of dextrose dropwise (dextrose is used as the reducing agent). The deposition of silver began after 1 min and finished within 5 min. The precipitate was then rinsed and centrifuged 5 times to eliminate the remaining salts. The dry, clean, coated powders were then pelletized, isostatically pressed at 265 MPa for 1 min, and sintered at 1100 °C for 4 h. The surface of the sintered pellets was polished to homogenize the surface. A nominally pure CSO dense sample was sintered from the nanoparticles at 1200 °C for 1 h. As a control, a few grams of commercial Ce0.9Gd0.1O1.90 (CGO, Fuel Cell Materials) were also coated with silver in the same way and sintered at 1100 °C for 4 h. The density and open porosity was measured via the Archimedes’ method, using deionized water as the immersion fluid after previously boiling the samples for 1 h, to ensure that all air bubbles are eliminated. Electron Microscopy. A field-emission-gun scanning electron microscopy (FEG-SEM) system (Gemini, Model 1525) was used for imaging of the nanoscale materials. Images of fracture surfaces of the membranes were collected to confirm

based compounds are the best oxygen-ion conductors that are stable over a wide range of partial pressures of oxygen12 while silver is the least-expensive of the noble metals, metallic under oxidizing and reducing conditions, and unreactive with ceria.13 In addition, silver in itself has a high oxygen diffusivity,14 while, under reducing conditions, ceria exhibits electronic conductivity.15,16 An ideal pressure-driven oxygen separation membrane requires gas tightness and sufficient electronic conductivity provided by a percolating electronic network.17,18 The use of these two materials is hindered by the difficulty in combining them into a dense membrane, since conventionally produced doped cerias require sintering at 1300 °C or higher,19 while silver has a high agglomeration and volatility above 1000 °C.20 We report successfully creating a composite membrane with samaria-doped ceria (CSO) and silver using doped ceria nanoparticles21 with high sinterability and coating them with silver via a simple chemical reduction step.22 This method allows the composite to be densified at 1100 °C, thereby reducing the loss and agglomeration of silver. The resulting pellets have been characterized by focused-ion-beam scanning electron microscopy (FIB-SEM) and X-ray diffractometry (XRD), their oxygen diffusivity is measured by secondary-ion mass spectrometry (SIMS), and their performance as oxygen separation membranes is monitored by gas chromatography (GC).

2. EXPERIMENTAL SECTION The nanoparticles were produced using a continuous hydrothermal flow synthesis reactor. The full details of the reactor and process have been reported previously.23 Briefly, a solution containing ammonium cerium nitrate (0.4 M) and samarium nitrate (0.1 M) was pumped at a rate of 200 mL min−1 at a pressure of 24.1 MPa, and a solution of KOH (0.5 M) was pumped at a rate of 200 mL min−1; these two streams meet at a T-piece, before the combined stream was mixed in a confined jet mixer with a stream of supercritical water at 450 °C flowing B

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experiments, the concentrations of nitrogen, oxygen, and hydrogen were measured. During data analysis, it was assumed that any nitrogen present in the sweep gas mixture comes from leaks from air. By measuring the amount of nitrogen present, we were therefore able to calculate the amount of oxygen present due to leaks and subtract this from the measured oxygen concentration in the sweep gas mixture. When hydrogen was used as the sweep gas, the consumption of hydrogen as measured by GC was used to determine the amount of oxygen permeated, assuming that all hydrogen reacts with oxygen to form water. As in the previous case, the unwanted leaks from air were taken into consideration by measuring the amount of nitrogen present in the sweep gas.

the dense nature of the samples. Energy-dispersive X-ray (EDX) analysis on the samples was performed with a JEOL Model 6400 electron microscope in areas of 100 μm2. X-ray Diffraction. The powders and consolidated samples were analyzed with an X’Pert PRO MRD X-ray diffraction (XRD) system and the composition was estimated using the Chung method.24 The oxygen isotope exchange depth profile (IEDP)25 technique was used to measure the oxygen surface exchange and diffusion properties of the composite at 550 °C and pO2 values of 200, 75, and 25 mbar. All exchanged samples were measured by time-of-flight secondary-ion mass spectrometry (ToF-SIMS) using an IONTOF TOF SIMS V instrument, equipped with a bismuth liquid metal ion source (LMIS) pulsed gun incident at 45°. A 25 keV Bi+ primary ion beam of ∼0.3 pA current was used to generate the secondary ions using the bunched-burst alignment mode (seven pulses) for analysis, and a Cs+ beam (2 keV) incident at 45° for sputtering.26 The achieved lateral resolution is in the region of a few hundred nanometers. SIMS tracer oxygen-18 (18O−) and oxygen-16 (16O−) ion images were acquired over various analytical areas, with 256 × 256 pixels. Tomography. The Ag-CSO sample was analyzed in a FEI Helio FIBSEM. A Ga ion source at 15 kV was used to sputter trenches around a selected region of interest and expose a suitable face of the specimen for electron imaging and tomography.27 This exposed face was then imaged with the electron column at 5 kV. The electron imaging process was repeated with slices removed at 51 nm intervals until a final three-dimensional (3D) volume of ∼25 μm × 7.5 μm × 15 μm was acquired with voxel sizes of 25 nm (X/Y) and 51 nm (Z). A large suitable subsection of this data (∼14 μm × 7.5 μm × 12 μm) was subsequently reconstructed for further analysis, following segmentation in a manner detailed in prior studies.27−29 Image analysis was also used to align the slices and apply foreshortening corrections prior to reconstruction. Conductivity. The van der Paw method was used to measure the electrical conductivity of percolating composites from room temperature to 600 °C. Impedance spectroscopy (Autolab PGSTAT302) with an FRA module was used for a nominally pure CSO sample from 100 °C to 650 °C to determine bulk and grain-boundary conductivity. The electrochemical impedance spectroscopy response was measured in the frequency range from 0.1 Hz to 1 MHz and a potential of a 20 mV (root-mean-square, rms), and the results were fitted to two resistor-constant phase-element-equivalent circuits to separate grain boundary from bulk response. Permeation. A pellet of the Ag-CSO composite with a surface area of 2.54 cm2 and thickness of 1 mm was attached at the end of an alumina tube and sealed with an alumina-based ceramic sealant, such that one side of the pellet was exposed to a flow of air, while the other could be swept with argon or hydrogen. The assembly was placed into a furnace, heated at 500 °C, and left overnight at this temperature. The air was fed at 130 mL min−1 at normal temperature and pressure (NTP), while the other side was swept with zero-grade argon or 5% H2 in argon at a rate of 100 mL min−1 (NTP). The sweep gas mixture was analyzed by a gas chromatography (GC) system (micro-GC Varian CP-4900) attached downstream. Immediately prior to the permeation experiment, the GC system was calibrated with a mixture of 1% nitrogen in argon and a mixture of 2% oxygen in argon (both supplied by BOC). During the

3. RESULTS AND DISCUSSION Synthesis and Characterization. This is the first report of the synthesis of Ce0.8Sm0.2O1.9 (CSO) nanoparticles produced using a continuous hydrothermal flow synthesis (CHFS).30 The CHFS process involves the reaction between a flow of supercritical water with a flow of room temperature aqueous metal salts, which, when brought together in a engineered mixer, result in the formation of nanoparticle oxides within less than a second.31 Recently, the CHFS process was scaled up to a pilot plant capable of producing over 1 kg of nanoparticles per hour.23,31 Because of the rapid and direct precipitation of the oxides and relatively high temperature and pressure conditions, the nanoparticles are highly crystalline and therefore can be used without further heat treatmentsand, in fact potentially even without drying, making them ideal for our purposes. Ceria,32 as well as ceria that has been doped with various metals, have been produced previously using CHFS (e.g., Zr,33 Y,33 Zn23) with particle sizes in the region of 5 nm and surface areas between 150 m and 200 m2 g−1. The measured surface area of the CSO used was 168 m2 g−1, which corresponds to a spherical particle size of 5.1 nm. XRD analysis showed the particles to be phase-pure CSO, with a lattice parameter of 5.4343 Å, which is comparable to the expected values of 5.4330 Å (from Inorganic Crystal Structure Database (ICSD) No. 028792) and a crystallite size of 4.1 nm (from the Scherrer equation). After coating with silver using Tollens’ reaction, the XRD of the composite powders showed no secondary phases and no inter-reaction between the two component materials. This was also the case after sintering, in agreement with previous work.13,17 Figure 2 shows the XRD

Figure 2. XRD pattern of Ag-coated CSO powder and a sintered AgCSO composite. C

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pattern of the silver-coated CSO and the sintered Ag-CSO composite. The measured lattice parameter for silver is 4.0710 Å, slightly smaller than the expected 4.0858 Å (ICSD No. 064996). Figure 3 shows an agglomerate of CSO particles

Figure 3. Micrograph of an agglomerate of CSO particles coated with silver.

coated by nanoparticles of silver metal. The size of this particle indicates that the silver coating is on agglomerates of CSO rather than on individual nanoparticles. Figures 4a and 4b show the microstructure of a fracture surface of a sintered pellet. The composite is dense, with no major pores visible in the fracture face. All crystal grains are in the range of 100−500 nm. Figure 4a indicates the presence of grains ∼100 nm in size. Nanoparticles of doped cerias can sinter efficiently at short times and low temperatures,16,34 and it is known that the grain growth stops after a certain limit.19 As a comparison, composite powders were produced by coating commercial CGO powder with silver using the same method. Pellets pressed with these composite powders disintegrated after firing at 1100 °C, while those fired at 1300 °C were mechanically stable but porous and nonconducting, because of the loss of silver. This highlights the fact that the nanoparticles used were essential for the production of nonporous, conducting silver−ceria composites. The composition of the pellet was measured by XRD, EDX, and FIB-SEM (Table 1). XRD and EDX analyses give average compositions over relatively large areas (>100 μm), while FIBSEM can measure the local composition. XRD and EDX measurements were in fairly good agreement, indicating an average silver content well below 10 vol %, in contrast to previous work, which has metal contents above 30 vol %. The density of the sintered composite was 7.07 g cm−3, compared to an expected theoretical solid density of 7.40 g cm−3. A closer look at Figure 4b reveals two types of regions: a silver-rich region and a silver-poor region. Each was further analyzed by FIB-SEM tomography. Tomography. Figures 5a and 5b show a 3D reconstruction of the microstructure using FIB-SEM tomography with a voxel size ca. 25 nm in two regions of the specimen, showing silverrich and silver-poor regions, respectively. In both cases, the majority of the volume is composed of dense continuous CSO, with nonpercolating pores. In the silver-rich regions (Figure 5a), these nonpercolating pores, ca. 1−2 μm in size, often occur interspersed with or near lathelike silver sheets. The total silver

Figure 4. (a) Fracture surface of the Ag-CSO composite, showing the high contact between the different grains. (b) Cross section of a membrane free of microcracks or large pores.

content here is ∼11.8 vol %. In this region, the pore volume accounts for only 0.3 vol %, the percolating silver comprises 83.2% of total silver, with 16.8% of the silver being nonpercolating and trapped completely within the ceramic matrix. The silver particles appear to have sizes that are