Solid State Transport and Hydrogen Permeation in the System Nd5

Article pubs.acs.org/cm

Solid State Transport and Hydrogen Permeation in the System Nd5.5W1−xRexO11.25−δ Sonia Escolástico,† Simona Somacescu,‡ and José M. Serra*,† †

Instituto de Tecnología Química (Universidad Politécnica de ValenciaConsejo Superior de Investigaciones Científicas), Avenida de los Naranjos s/n, 46022 Valencia, Spain ‡ “lie Murgulescu” Institute of Physical Chemistry, Romanian Academy, Spl. Independentei 202, 060021 Bucharest, Romania S Supporting Information *

ABSTRACT: Nd5.5WO11.25−δ is a mixed proton−electron conducting oxide, which shows an important mixed conductivity and stability in moist CO2 environments. However, the H2 fluxes obtained with this material are not high enough in order to apply them as H2 separation membranes in industrial applications. Re6+ cation presents similar ionic radii than W6+ and Re6+ can be reduced to different oxidation states under the operating conditions typical for hydrogen membrane separation. This fact leads to the improvement of the electronic conductivity and produce the generation of oxygen vacancies with the subsequent increase in the ionic conductivity. This work presents the synthesis as nanosized powders as well as the structural and electrochemical characterization of mixed conducting materials based on the system Nd5.5W1−xRexO11.25−δ where x = 0, 0.1, 0.5 and 1. The evolution of the crystalline structure and the shrinkage behavior are studied as a function of the sintering temperature. Total conductivity in reducing and oxidizing environments is studied systematically for samples sintered at 1350 °C. The H/D isotopic effect and the hydration influence are also analyzed by means of DC-electrochemical measurements. H2 permeation is carried out for the selected compound, Nd5.5W0.5Re0.5O11.25−δ, in the range of 700−1000 °C, obtaining a peak H2 flux value of 0.08 mL·min−1·cm−2. The reduction of the Re cation in this compound under reducing conditions is investigated by TPR and XPS. Finally, the stability of this material under CO2-rich gas stream was evaluated by measuring H2 permeation using a CO2 containing atmosphere as sweep gas. KEYWORDS: proton conductor, hydrogen membrane, hydrogen membrane, Ln6WO12, mixed conductor, tungstate

1. INTRODUCTION Hydrogen separation at high temperature is an interesting process for a broad variety of industrial processes in energy and chemical production. The integration in catalytic membrane reactors (CMR) may allow the process efficiency to be substantially increased. A typical example is the potential separation of hydrogen in gasification power plants (IGCC) plants, which would make it possible the efficient implementation of CO2 capture and storage technologies based on precombustion schemes.1 Both concepts, CCS technologies and CMR, could result in a more sustainable chemistry industry with less energy consumption, lower pollution, and enhanced performance in terms of separation, selectivity, and yield.2 The hydrogen permeability and catalytic activity at high temperatures of mixed proton−electronic materials based on ceramic materials, make these compounds potential candidates for use in catalytic reactions, such as steam reforming,3 water−gas shift, or dehydroaromatization of methane.4 In the above-mentioned applications, CO2 and H2O are usually present in the reaction mixture. Consequently, a requisite for mixed protonic electronic conductors is the stability in CO2 and H2O containing atmospheres, in addition to high proton con© 2013 American Chemical Society

ductivity combined with high electronic conductivity, that is, high ambipolar conductivity. Tungstates based compounds, commonly formulated as Ln6WO12, present an important mixed conductivity5 as well as a remarkable stability in moist CO2 environments.6 Hydrogen permeation of Nd5.5WO11.25−δ6b and La5.5WO11.25−δ6a,7 membranes have been reported; however, the hydrogen flows reached are not high enough in order to apply them in industrial processes. In recent works, partial substitution in the lanthanide position has allowed to improve the hydrogen flow obtained with Nd5.5WO11.25−δ8 and an important increase in the hydrogen flow was obtained by doping in the hexavalent metal position with Mo.8,9 Re cation presents an ionic radii very similar to that of W, 0.55 Å when is present in its oxidation state +6 and the coordination number is 7.10 Additionally, there are several studies where Ln6ReO1211 based materials have been studied obtaining fluorite phase materials. The latter two facts suggest that Re could be a good candidate for partial W Received: August 21, 2013 Revised: December 6, 2013 Published: December 10, 2013 982

dx.doi.org/10.1021/cm402821w | Chem. Mater. 2014, 26, 982−992

Chemistry of Materials

Article

degassed under Ar flow for 1 h and then were subjected to reduction under H2/Ar (1/9) flow, and heating rate of 10 °C/min till 1000 °C. The H2 consumption was measured by a TCD (thermal conductivity detector). TPR was also performed by using a grinded piece of the Nd5.5W0.5Re0.5O11.25−δ membrane sintered at 1550 °C. X-ray photoelectron spectroscopy (XPS) surface analysis was carried out on PHI Quantera equipment with a base pressure in the analysis chamber of 10−9 Torr. The X-ray source was monochromatized Al Kα radiation (1486.6 eV) and the overall energy resolution is estimated at 0.6 eV by the full width at half-maximum (fwhm) of the Au4f7/2 photoelectron line (84 eV). Although the charging effect was minimized by using a dual beam (electrons and Ar ions) as neutralizer, the spectra were calibrated using the C1s line (BE = 284.8 eV) of the adsorbed hydrocarbon on the sample surface (C−C or (CH)n bonds). As this spectrum was recorded at the start and the end of each experiment the energy calibration during experiments was quite reliable. The photoelectron lines were fitted by using the Voigt functions and imposed the required constraints on the relative intensities of the doublet transitions as well as on the spin−orbit parameters. To improve the energy resolution, the pass energy was minimized to 55 eV for collecting Re4f spectra at the expense of lower intensity. The “in situ” cleaning procedure by Ar ion etching was not carried out in order to prevent changes in the surface chemistry induced by preferential sputtering and/or induced reduction. Although we focused on maximizing the fit quality by minimizing the χ2 parameter, the most important priority was the physicochemical background found behind the mathematical procedures (more experimental details are available in Supporting Information). Nd5.5W0.5Re0.5O11.25−δ was selected in order to perform hydrogen permeation measurements and the used membrane consisted of a dense (density about 99%) 900 μm thick disc with diameter 15 mm sintered at 1550 °C with both disk sides coated by screen printing with a 20 μm layer of a Pt ink with the aim of improving the surface catalytic activity. Permeation measurements were accomplished on a double chamber quartz reactor following the same methodology than that reported elsewhere.6b,c,14 Hydrogen was separated from a mixture of H2−He using argon as sweep gas (both feed and sweep saturated in water at 25 °C). An atmosphere composed by 85% Ar and 15% CO2 was also used as sweep gas in order to confirm the stability of the membrane in operation in CO2-containing atmospheres. The gas flow rates used were 100 mL·min−1 for feed and 150 mL·min−1 for sweep. Feed and sweep humidification was accomplished by saturation of Milli-Q water at 20 °C. The hydrogen content in the permeate side was analyzed using micro-GC Varian CP-4900 equipped with Molsieve5A and PoraPlot-Q glass capillary modules.

substitution in the system Nd5.5WO11.25−δ in order to obtain a fluorite structure. Furthermore, Re reducibility, that is, Re cation is present in different oxidation states (typically +6,+5, +4) in mixed oxides, may increase the electronic conductivity and generate additional oxygen vacancies as previously suggested for the La5.5WO11.25−δ compound.12 This work presents the structural and electrochemical characterization of mixed protonic-electronic conducting materials based on the system Nd5.5W1−xRexO11.25−δ (x = 0, 0.1, 0.5, and 1). The evolution of the crystalline structure and shrinkage behavior are studied as a function of the sintering temperature in the range from 900 to 1500 °C. Total conductivity is studied in different environments for samples sintered at 1350 °C, analyzing also the H/D isotopic effect by DC-electrochemical measurements. The reducibility of these samples was investigated by temperature-programmed reduction and XPS. Hydrogen permeation is studied for the selected compound Nd5.5W0.5Re0.5O11.25−δ in the range of 700−1000 °C and stability in operation conditions is demonstrated using a CO2-rich gas stream as sweep gas.

2. EXPERIMENTAL SECTION Nd5.5W1−xRexO11.25−δ (x: 0.1, 0.5 and 1) materials were synthetized by the citrate-complexation route modified to stabilize W-, Re-, and Ndcontaining ions.13 A molar excess (10%) of B-position ions (W and Re) was employed to achieve proper phase purity. Neodymium oxide was dissolved in concentrated nitric acid and citric acid was subsequently added to the solution at a molar ratio 1:2 cation charge to citric acid. A solution with tungsten and rhenium precursors was prepared complexing it with citric acid at the same ratio. The resulting solution was gradually concentrated at 150 °C and followed by foaming. The resulting product is subsequently calcined in air at 900 °C to remove carbonaceous material and promote the formation of fluorite nanocrystallites. This nanosized powder was used for the preparation of pressed bars and discs for further transport characterization. Correct pure phase was checked by XRD analysis which was carried out on a CubiX FAST equipment using Cu Kα1,2 radiation and an X′Celerator detector in Bragg−Brentano geometry. XRD patterns were recorded in the 2θ range 20° to 70° and analyzed using Highscore Philips software. Electrical conductivity measurements were performed by four-point DC technique supplying the constant current by a programmable current source (Keithley 2601) while the voltage drop through the sample was detected by a multimeter (Keithley 3706). Rectangular bar samples used in conductivity measurements were accomplished using the Nd5.5W1−xRexO11.25−δ materials as calcined at 900 °C for 10 h and uniaxially pressed at 100 MPa. The pressed samples (40 × 5 × 4 mm3) were sintered in air at 1350 °C for 4 h. The density of the samples was around 80% and 90% for bars sintered at 1150 and 1350 °C, respectively. Bars were contacted with silver paste and wire. Conductivity tests were performed in different atmospheres studying the pO2 influence, the hydration, and H/D isotopic effect in oxidizing and reducing conditions: 1. Effect of pO2: in oxidizing conditions using different dry concentrations of O2/Ar atmospheres from calibrated gas cylinders (Linde) and in reducing conditions using 5% H2/95% He, 5% D2/95% He and 100% H2 wet (2.5% H2O) atmospheres from calibrated gas cylinders (Linde). 2. Hydration and isotopic effect (H/D): in oxidizing conditions using dry O2, O2 saturated with H2O (2.5%) and O2 saturated with D2O (2.5%) and in reducing conditions using 5% H2 in He (dry conditions), 5% H2 in He saturated in H2O (2.5%), 5% D2 in He (dry conditions), and 5% D2 in He saturated in D2O (2.5%). Temperature-programmed reduction (TPR) was performed using a 2910 Micromeritics equipment. Powders sintered at 1350 °C were

3. RESULTS 3.1. Structural Characterization. The evolution of the crystalline symmetry of the compounds was studied as a function of the Re content (x) and the sintering temperature. The XRD patterns of the Re containing compounds pressed as bars and subsequently sintered at 1150 and 1350 °C are plotted in Figure 1. The samples sintered at 1350 °C were afterward used in the total conductivity measurements. Doped compounds present cubic symmetry regardless of the sintering temperature and the Re content as inferred from Figure 1, where no peaks corresponding to the tetragonal or rhombohedral symmetry can be detected, at least to the limit of XRD analysis. However, undoped Nd 5.5 WO 11.25−δ compound presents cubic, tetragonal, or a mix of both symmetries depending on the specific synthesis method and the sintering temperature.15 In contrast, compounds from the Ln6ReO12 system where Ln = Ho, Er, Tm, Yb, and Lu were studied by Hartmann et al.11 who reported that Ln6ReO12 compounds present fluorite structure with rhombohedral symmetry. This symmetry was stable if the molar ratio between the hexavalent metal cation and rare earth cation was at least 0.605. In 983



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Figure 2. Shrinkage behavior for Nd5.5W1−xRexO11.25−δ compounds as a function of temperature.

in which was possible performed the measurements). In summary, the shrinkage behavior of the starting nano-oxides powders allows highly dense membranes (density about 99%) to be obtained at similar temperatures as for Nd5.5WO11.25−δ while minimizing the possible evaporation of Re and/or W with the consequent effects in transport and structural properties. 3.2. Electrochemical Characterization. Different conductivity measurements were performed in order to study the transport properties of Re doped-compounds. The selected conditions were described above. Conductivity as a Function of pO2. The first experiment run entailed the variation of pO2 in (i) dry oxidizing and (ii) wet reducing conditions aiming to qualitatively determine the contribution of the different charge carriers to the total conductivity. Figure 3 shows the total conductivity data of Nd5.5W1−xRexO11.25−δ compounds with x = 0, 0.1, and 0.5 as a function of the partial pressure of oxygen (pO2) under oxidizing conditions at different temperatures between 600 and 800 °C. Depending on the Re content and the operation temperature, it is possible to distinguish diverse conduction regimes, that is, the nature of the prevailing charge carrier transport. The total conductivity of the undoped compound follows a pO1/4 2 dependence indicating that the conductivity is dominated by p-type electronic transport under oxidizing conditions. The total conductivity of doped compound with x = 0.1 exhibits a power law dependence on pO2 with a 1/6 slope at high pO2. This relationship can be ascribed to the transition between a pO2 range where the compound is predominantly a p-type electronic conductor (σ ∝ pO1/4 2 ) and a range where the transport is mainly controlled by the transport of the ionic charge carriers (σ ∝ pO02). On the other hand, the drop in the conductivity values can be related to the reduction of the p-type electronic charge carriers via the vacancy generation (expressed in eq 1) in agreement with the power law dependence observed. 1 O2 + vO·· ↔ OOx + 2h (1) 2 At high concentrations of Re (x = 0.5), the total conductivity presents different behaviors with regard of the pO2 and temperature. Three regimes can be distinguished at 800 °C: (1) σ ∝ pO1/6 2 in the range 0.21−1 atm indicating that electronic conductivity prevails and the contribution of the ionic conductivity is important (transition area); (2) σ ∝ pO02 in

Figure 1. XRD diffraction patterns for the Nd5.5W1−xRexO11.25−δ compounds when x = 0.1, 0.5 and 1 sintered at 1150 and 1350 °C pressed as bars. Diffraction peaks corresponding to the sample holder material are indicated as *.

addition, the achievement of the total dissolution of Re in the Nd5.5WO11.25−δ matrix even at the highest W tungsten substitution values (x = 1) should be highlighted. Indeed, Re solubility in the compound La5.5WO11.25−δ is very limited, that is, x ≤ 0.2.9c,12 However, when the compounds were calcined at 1350 °C a slight peak is detected around 30°, which corresponds to the (001) reflection of Nd2O3 with hexagonal symmetry. This minor impurity is not expected to affect the electrochemical measurements. However, no Nd2O3 traces were detected when compounds were calcined at 1550 °C as can be observed in Supporting Information Figure S1 where XRD patterns for Nd5.5W0.5Re0.5O11.25−δ as powder and as a membrane treated at 1550 °C. Cell parameters for Nd5.5W1−xRexO11.25−δ compounds sintered at 1150, 1350, and 1550 °C are summarized in Table 1. Cell volume increases with the content of Re in the material, whereas it slightly decreases with sintering temperature. Table 1. Cell Parameters Calculated for Nd5.5W1−xRexO11.25−δ Compounds Sintered at 1150, 1350 and 1550 °C as Determined by XRD a (Ǻ ) sintering temperature x Nd5.5W1−xRexO11.25−δ

1150 °C

1350 °C

1550 °C

0.1 0.5 1

5.47 5.48 5.50

5.46 5.47 5.50

5.45 5.46 -

The shrinkage behavior was studied by using pellets with a diameter 13 mm and uniaxially pressed at 301 MPa for 1 min from the powder sintered at 900 °C. Subsequently sintering (all the samples were simultaneously sintered) was carried out in stagnant air at different temperatures, measuring the resulting sample diameter. From these values, shrinkage curves were obtained (Figure 2). Re doped-compounds present similar sintering activity than undoped compound at high temperature. However, when W ions are totally substituted by Re ions, the compound suffers the highest shrinkage observed (in the range 984



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Figure 3. Total conductivity for Nd5.5W1−xRexO11.25−δ compounds with x = 0, 0.1 and 0.5 as a function of the oxygen partial pressure (atm) under dry oxidizing conditions. The dotted lines are a guide for the eye.

Figure 4. Total conductivity for the Nd5.5W1−xRexO11.25−δ compounds where x = 0, 0.1 and 0.5 as a function of the pO2 under wet reducing atmospheres. The dotted lines are a guide for the eye.

the range 0.21−10−4 atm that can be ascribed to the mainly ionic conductivity; and (3) total conductivity is increasing with ) in the range decreasing oxygen partial pressure (σ ∝ pO−1/6 2 10−4−5 × 10−5 atm. This regime could be related to the rise in the n-type conductivity and/or the increase in the ionic contribution due to the vacancy formation. At 700 °C the area where the compound is predominantly ionic extends from pO2 = 1 atm to pO2 = 10−4 atm with an increase in the conductivity at lower pO2. Finally, at 600 °C three areas can be observed: (1) the conductivity drops with the increase of pO2 (pO2 = 1− 0.21 atm) that can be related with the low p-type electronic conductivity and an insufficient vacancy generation due to the low temperature and high pO2; (2) the compound is mainly ionic conductor in the range pO2 = 0.21−10−4 atm; and (3) the conductivity slightly increases at lower pO2. Conductivity measurements for Nd5.5ReO11.25−δ are not included in the Figure 3 due to the low stability that this compound presents upon thermal cycling. Nd5.5ReO11.25−δ was obtained as a fluorite but it suffered a progressive degradation during the conductivity measurements until its total destruction (minor fluorite phase and a mixture of different oxides were detected by XRD in the resulting powder). This breakdown can be related to the above-mentioned stability criteria established by Hartmann et al.11 The other compounds did not suffer any degradation or formation of secondary phases with time or measurement conditions (oxidizing and reducing atmospheres), as it was confirmed by XRD analysis after the testing experiments. Total conductivity in wet reducing conditions as a function of the pO2 has been plotted in Figure 4. At 800 °C, n-type conductivity prevails with respect to the ionic conductivity in both substituted compounds (similar behavior as for the undoped compound), as can be deduced from the drop in the conductivity with increasing pO2 and from the negligible H/D

isotopic effect. One should be note that the total conductivity exhibits a power law dependence on pO2 with a −1/6 slope. This relationship can be an indication of the transition area from pure ionic transport to predominant n-type electronic conduction. At 600 °C, the corresponding conductivity of Re doped-compounds presents a power law dependence on pO2 with a higher slope than undoped compound. This behavior can be related to the higher n-type electronic conductivity contribution when Re content increases, which is in agreement with the drop in the H/D isotopic effect magnitude. Indeed, the isotopic effect is negligible in Nd5.5W0.5Re0.5O11.25−δ compound at both measured temperatures due to large rise in the n-type conductivity. Total conductivity as a function of the Re concentration at two selected temperatures (600 and 800 °C) in wet 5% H2 and 5% D2 in He has been plotted in Figure 5 with the aim to discern the effect of the Re introduction on the predominant transport mechanisms. From this plot, the following trends with increasing Re contents can be inferred: (i) an increase in the total conductivity, e.g. the conductivity is increased by 1 order of magnitude when x augments from 0.1 to x = 0.5; and (ii) a decrease in the apparent H/D isotope effect, which is produced by the enhancement of the n-type electronic and oxygen ionic conductivity associated to the Re reduction (see Figure 8). Hydration and Isotopic Effect Study. The second set of experiments involved variation of temperature in different atmospheres. The temperature dependence of the total conductivity in dry O2, O2 saturated with H2O, and O2 saturated with D2O is presented in Figure 6. The undoped compound presents lower conductivity in wet atmospheres than in dry atmospheres. This fact is related to the decrease in the p-type conductivity when the sample is hydrated following the eq 2 that consumes oxygen vacancies minimizing the 985



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effect (σH2+H2O > σH2 and σD2+D2O > σD2) and from the isotopic effect (σH2+H2O > σD2+D2O) perceptible in Figure 7. On the contrary, at higher temperatures, oxygen ionic and n-type electronic conductivity are predominant as can be inferred from the negligible H/D isotopic effect and the higher conductivity value in dry atmospheres (2.0 × 10−26 atm) than in wet atmospheres (3.1 × 10−20 atm, less reducing conditions than dry atmospheres). Furthermore, the slope of the curves in wet conditions increases at temperatures above 700 °C, which indicates the onset of a new conduction mechanism, whose apparent activation energy has a similar value as those observed under dry conditions. The drop in protonic contribution can be reconciled with the exothermic character of the hydration reaction (eq 2), which results in the equilibrium shift toward dehydration with increasing temperatures. For the Nd5.5W0.5Re0.5O11.25−δ compound, the higher magnitude of the conductivity in dry atmospheres suggests that electrons and/or oxygen ions are the predominant charge carriers within the studied range of temperatures, and this fact can be related with the Re reduction behavior (see Figure 8) as it was abovementioned. Note that the apparent activation energy is relatively low and this effect should be associated with a predominant n-type electronic conductivity with respect to the oxygen-ion contribution (with higher apparent activation energy16). 3.3. Rhenium Reduction Study. Aiming to understand the evolution of the Re oxidation states in Nd5.5W1−xRexO11.25−δ compounds under reducing conditions and the possible relation between the increase in (a) the electronic and oxygen ion conductivity and (b) the Re content, temperature-programmed reduction (TPR) experiments were accomplished in dry hydrogen. Figure 8 presents the hydrogen consumption for Nd5.5W1−xRexO11.25−δ compounds (where x = 0, 0.1, 0.5 and 1) as powders sintered at 1350 °C and Nd5.5W0.5Re0.5O11.25−δ membrane sintered at 1550 °C. An important reduction peak can be observed from 200 to 600 °C depending on the Re concentration in the sample for the materials sintered as powder. For Nd5.5W0.5Re0.5O11.25−δ and Nd5.5ReO11.25−δ, a second reduction peak can be observed at temperatures above 900 °C. Previous works carried out with

Figure 5. Total conductivity for the Nd5.5W1−xRexO11.25−δ compounds as a function of the Re content at 600 and 800 °C under wet reducing atmospheres (H2 and D2, both diluted in 95% He and saturated at room temperature with H2O and D2O, respectively).

formation of electron holes via the mechanism expressed in eq 1. H 2O + vO·· + OOx ↔ 2OH·O

(2)

Nevertheless, in the Re doped-compounds, the conductivity values are not dependent on water vapor pressure and this effect can be ascribed to the higher oxygen-ion transport contribution that possess as it was deduced from results in Figure 3. Due to this oxygen ionic contribution and the corresponding higher oxygen vacancy concentration, the drop in the p-type electronic conductivity is compensated by the rise in the protonic conductivity, which produces very similar total conductivity values in the measured atmospheres regardless of the water partial pressure. Total conductivity in reducing conditions (5% H2, 5% H2+H2O, 5% D2, and 5% D2+D2O) as a function of the inverse temperature is plotted in Figure 7. The conductivity behavior of the Nd5.5W0.9Re0.1O11.25−δ is very similar to that corresponding to the undoped compound. At temperatures below 700 °C, the protonic conductivity prevails as deduced from the hydration

Figure 6. Total conductivity as a function of inverse of temperature under dry O2, O2+H2O, and O2+D2O (H2O and D2O saturated at room temperature) for Nd5.5W1−xRexO11.25−δ compounds. 986



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Figure 7. Total conductivity for Nd5.5W1−xRexO11.25−δ compounds as a function of inverse temperature in H2, D2, H2+H2O, and D2+D2O (H2 and D2 diluted in 95% He) atmospheres.

temperatures. In fact, the first reduction peak shifts to higher temperatures when Re concentration increases and this fact should be related to the structure stability that compels Re cations to remain in +6 oxidation state. As a summary, the reduction behavior (Figure 8) observed for the materials sintered as powder could be: (1) reduction of Re6+ (in agreement with the XPS results) to Re5+/Re4+ occurs at temperatures around 200−600 °C while (3) Re+5/4 reduction to Re0 takes place at much higher temperatures (>900 °C). However, the reduction behavior for the pressed Nd5.5W0.5Re0.5O11.25−δ compound sintered at 1550 °C differs from that observed for the same composition sintered as powder. A broad peak was observed from 250 to 600 °C that can be ascribed to the reduction of Re7+/6+ species coupled with the formation of additional oxygen vacancies and electron carriers and an intense peak appears above 950 °C. This difference between the behavior of powder sample and pressed sample (dense membrane) is an indication of the lower reducibility of the pressed sintered membranes. As an additional study of the reduction of the material, XPS analysis was used to determine quantitatively the chemical states concentrations of the elements present on the surface and corroborate the hypothesis extracted from the TPR analysis. XPS measurements were carried out using fresh and treated Nd5.5W0.5Re0.5O11.25−δ membrane sintered at 1550 °C. Treated membrane was exposed to an atmosphere composed of

Figure 8. Temperature-programmed reduction experiment in dry H2 (10% in Ar) for Nd5.5WO11.25‑δ, Nd5.5W1−xRexO11.25−δ compounds sintered at 1350 °C and Nd5.5W0.5Re0.5O11.25−δ membrane sintered at 1550 °C.

supported Re catalysts reported that the Re cations were reduced when exposed to H2 containing atmospheres in the range from 200 to 400 °C (with a huge influence of the support nature).17 However, in this work, Re is incorporated in the fluorite structure and the reduction occurs at much higher

Figure 9. Nd3d (a), W4f and Re4f (b), and O1s (c) superimposed XPS spectra for the fresh membrane Nd5.5W0.5Re0.5O11.25−δ (outer surface) as sintered at 1550 °C and after H2 treatment. 987



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Figure 10. O1s deconvoluted spectra for the Nd5.5W0.5Re0.5O11.25−δ pressed membrane (outer surface) as sintered at 1550 °C (a) and H2 treated (b). Deconvoluted high resolution band-like Re4f spectra as sintered (c) and after reduction at 900 °C (d).

Table 2. XPS Data: The Binding Energies and the Experimental Re Oxidation States Relative Concentrations Obtained after Deconvolution (Nd5.5W0.5Re0.5O11.25−δ Pressed Membrane Sintered at 1550 °COuter Surface) relative percentages of the Re oxidation states (error ±5%)

binding energy (eV) sample

C1s

O1s

NdWRe (sintered)

284.8

NdWRe (Hz)

284.8

I, 528.7 II, 530.8 III, 531.7 I, 528.9 II, 530.7 III, 531.8

Nd 3d 5/2

Nd4d

W 4f 7/2

982.3

121.4

35.1

982.3

121.4

35.1

Re 4f7/2 Re4+

42.8

100% H2 at 900 °C for 6 h and subsequently quenched. Nd and W cations do not suffer any reduction as can be deduced from the superimposed XPS spectra (Figure 9a, b). The perfect overlapping of these spectra suggests a quite similar chemical behavior of Nd and W before and after reduction. However, Re cations and O anions suffer some transformation as can be observed in the modification of the XPS high-resolution spectra (Figure 9b, c) when the sample is treated in reducing atmospheres. The analysis of the O1s band (Figure 10a, b) reveals the presence of three O species in Nd5.5W0.5Re0.5O11.25−δ material: (I) oxygen bonded in the lattice of the sample mostly to Nd3+ ions; (II) oxygen bonded in the lattice to W6+ and presumably Re+n ions; and (III) OH groups due to the hydration of these mixed oxides. These assignments are in line with the XPS analysis and deconvolution performed using reference standards Nd2O3 and WO3 (Supporting Information Figure S2). The O1s deconvoluted spectra for Nd5.5W0.5Re0.5O11.25−δ, before and after reduction (Figure 10a, b) shows a higher intensity of the component I relative to the component II. This is interpreted as follows: rhenium reduction leads to the decrease in the number Re−O bonds (via oxygen vacancy formation), which increases the ratio between the signal of species I and II. Moreover, a relative increase in the signal ascribed to OH is

Re 4f7/2 Re5+

Re 4f7/2 Re6+

Re 4f7/2 Re7+

44.3

45.3

46.4

43.7

44.5

46.2

Re4+

6.7

Re4+ Re5+

19.5

Re5+

Re6+

Re7+

22.7

60.5

16.8

48.0

20.7

5.2

observed, which suggests the increase in the surface concentration of protons, which is in agreement with the increased amount of oxygen vacancies susceptible to be hydrated, and this last aspect is strongly related with the ionic transport properties. On the other hand, the reduction effect on the Re oxidation states after an accurate and consistent assignment of the Re oxidation states is clearly visible (Figure 10c, d). The deconvolution procedure allows the contribution of the mixture (7+, 6+, and 5+) oxidation states to be accurately distinguished, for the as-sintered sample. The presence of rhenium in different oxidation states has been previously reported in Pt−Re/TiO2 and Pt−Re/ZrO2 catalysts18 that show a mixture of Re4+, Re6+, and Re7+, and the presence of Re5+ was detected in other mixed oxides.19,20 After reduction at 900 °C the contribution from cations Re4+ (42.8 eV) becomes more important in agreement with prior reports.21 The binding energies (BEs) of the most prominent XPS transitions (C1s, O1s, Nd3d5/2, Nd4d, W4f7/2, and Re4f7/2) for the sample before and after reduction are presented in Table 2 together with the Re oxidation states relative concentrations. This XPS study reveals that the reduction process on the outermost layer of the samples was effective only for Re chemistry. 988



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Figure 11. Deconvoluted high resolution band-like Re4f spectra of the bulk membrane as sintered and after reduction at 900 °C recorded on the fracture cross-section.

Figure 12. Hydrogen flow for Nd5.5W0.5Re0.5O11.25−δ membrane as a function of temperature and feeding with different pH2 (both sides of the membrane humidified) (a). Hydrogen flow for Nd5.5W0.5Re0.5O11.25−δ and Nd5.5WO11.25−δ as a function of the inverse temperature feeding 50% H2 with both sides of the membrane humidified (b).

The influence of pH2 was analyzed, with both sides of the membrane humidified and the obtained hydrogen flows as a function of the temperature are represented in Figure 12a. Hydrogen flow improves with the rise in temperature and hydrogen concentration in the feed stream as it is postulated by Wagner equation, reaching values up to 0.08 mL·min−1·cm−2 at 1000 °C. Hydrogen flow is 3-fold higher than the obtained for the undoped compound and it is very similar to that obtained with Ce, Eu, Pr, and Tb doped compounds at 1000 °C.9a,23 However, the hydrogen flow obtained at 850 °C, 0.03 mL· min−1·cm−2, is higher than those obtained with the abovementioned materials. Hydrogen flows for Nd5.5W0.5Re0.5O11.25−δ and Nd5.5WO11.25−δ (feeding 50% H2) as a function of inverse temperature are plotted in Figure 12b, where an important improvement in the hydrogen flow obtained by doping with Re is clearly observed. As shown in Figure 12b, hydrogen flow permeation presents a classic Arrhenius behavior. Hydrogen permeation process can be limited by the bulk ion diffusion or the surface exchange kinetics. In the studied material, the bulk ion (protons or oxygen ions) diffusion controls the H2 permeation flux under the current conditions due to the comparatively higher electronic conductivity within the studied temperature range. Furthermore, no limitations due to surface exchange reactions are expected due to the Pt layer deposition on both membrane surfaces that enhances the H2 oxidation/ reduction reactions. On the other hand, in the current conditions (both sides of the membrane humidified), hydrogen production can occur via two different transport mechanisms due to the coexistence of the transport of both oxygen ions and protons:6,9a (1) transport of protons through the membrane

These results are in agreement with the TPR analysis obtained for Nd5.5W0.5Re0.5O11.25−δ, where the broad peak observed from 250 to 600 °C was attributed to the reduction of Re7+/6+ to Re5+/4+ . No Re0 was observed due to the treatment temperature, below the onset Re5+/4+ reduction temperature 950 °C for the pressed sample. Differences between the Re oxidation states in the surface and the bulk have been reported for different rhenium compounds.22 For this reason, additional measurements were performed in the same membrane by analyzing the fracture cross-section. Figure 11 shows the corresponding Re4f XPS spectra for the sintered sample (a) and after H2 treatment (b). The as sintered sample presents a mixture of Re5+ and Re6+ (62%) whereas Re4+ is observed after the reduction treatment. Moreover, these results confirm that Re7+ cations are exclusively present on the outer surfaces (layer thickness < 10 nm) of the as-sintered membrane. 3.4. Hydrogen Permeation. Finally, hydrogen permeation was measured using the Nd5.5W0.5Re0.5O11.25−δ membrane. This material was selected due to its high total conductivity in reducing conditions. In all the measurements both sides of the membrane were humidified (pH2O = 0.025 atm). These conditions were selected due to the improvement of the hydrogen permeation previously observed in different tungstates when the membrane is hydrated. This increase in the proton transport through the oxide hydration is ascribed to the formation of two protonated oxygen atoms (lattice or interstitial oxygen), which are the essential charge carriers involved in hydrogen transport. 989



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Figure 13. Hydrogen flow for Nd5.5W0.5Re0.5O11.25−δ as a function of the temperature using as sweep gas: (1) 100% Ar, (2) 85% Ar/15% CO2 (a). Hydrogen flow for Nd5.5W0.5Re0.5O11.25−δ as a function of the time using as sweep gas an atmosphere composed by 85% Ar/15% CO2 (b). Both sides of the membrane humidified in both experiments.

Figure 14. SEM micrographs of Nd5.5W0.5Re0.5O11.25−δ fracture cross-section membrane sintered at 1550 °C before (a) and after (b, c, d) permeation measurements.

from the higher pH2 (hydrogen feed) to the lower pH2 side (Ar sweep) and (2) hydrogen production by water splitting in the sweep side due to the oxygen diffusion from the higher pO2 (Ar sweep) to the lower pO2 (hydrogen feed). In the permeation results of the Nd5.5WO11.25−δ membrane (Figure 12b), a change in the apparent activation energy (Ea) can be observed, which suggests a change in the limiting transport mechanism and consequently in the ambipolar conductivity, as previously observed for other similar membranes.6b The Ea value of 1.2 eV at high temperatures may be ascribed to the predominant hydrogen production by water splitting whereas the value of 0.98 eV at low temperature may be related with the higher proton transport contribution to the total hydrogen flow obtained. In contrast, the Re dopedcompound presents a constant apparent activation energy value

of 0.99 eV within the studied range of temperature; that is, the nature of the predominant ion conduction mechanism does not vary with the temperature and the hydrogen flow obtained in permeation measurements might be attributed to the mainly hydrogen production by water splitting reaction. These conclusions are in agreement with the minor protonic character of this compound as deduced from the conductivity measurements. In this case, this behavior can be attributed to the high reducibility of the Re cations, providing a much higher oxygen vacancy concentration in the analyzed temperature range that facilitates the oxygen ion transport with fairly variation with the temperature while n-type electronic may exceed the magnitude of the ionic conductivity, as inferred from the conductivity results (Figure 4, left-hand side). 990



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limit of XRD analysis for the membrane sintered at 1550 °C (see also Supporting Information Figure S1). The hydrogen flow values obtained for the Nd5.5W0.5Re0.5O11.25−δ membrane are higher than those previously reported for the most studied mixed protonic electronic materials as they are zirconates,14 cerates,24 and solid solutions of cerates and zirconates. 25 In addition, Nd5.5W0.5Re0.5O11.25−δ membrane presents the advantage of the higher stability in CO2 and acid gases atmospheres as compared with cerates and the lower sintering temperatures as compared with zirconates. Challenging applications for this kind of membranes comprise the integration in high-temperature membrane reactors where hydrogen and oxygen atoms may be removed or injected in a process, typically under very reducing conditions. A particular interesting application is the use in gasification processes, for example, IGCC plants where precombustion CO2 capture technology can be implemented while high purity hydrogen is produced. The advantages of this type of membranes are their lower price compared with precious metal-based membranes and their higher thermal, chemical, and mechanical resistance. However, substantially higher permeation flows have to be obtained in order to upscale and apply them in real industrial applications. Apart from compositional dependence tackled in this work, the hydrogen permeation can be improved by tailoring the membrane architecture, that is, by decreasing the membrane thickness, using supports with engineered porosity, and applying catalytic coatings for targeted surface reactions. Specifically, the proton permeation can be improved by reducing the bulk diffusion resistance although it involves the use of a support layer necessary leading asymmetric structured membranes. Planar asymmetric membranes made of La6−xWOy, which pertains to the tungstates family, have been obtained.26 The geometry and module design is a pivotal issue to achieve the required membrane area per volume of reactor for practical application.27 In this context, membranes can be produced with planar, tubular, hollow fiber, or even honeycomb geometries. Further, the company CerPoTech (Norway) produces La6−xWOy at commercial scale by means of spray pyrolysis.

The chemical stability of hydrogen separation membranes in H2O and CO2 is essential for their industrial application. Therefore, hydrogen permeation tests were performed using a mixture composed of 85% Ar and 15% CO2 as sweep gas with both sides of the membrane humidified. The resulting hydrogen flow is slightly lower than using Ar as sweep gas (Figure 13a). This effect is reversible and can be attributed to the CO2 competitive adsorption on the membrane surface, that is, affecting the surface exchange reaction. The measurements at each temperature remained stable at least for 2 h. Hydrogen permeation flows were recovered when CO2 was switched by Ar indicating that the adsorption is reversible. Finally, hydrogen permeation was measured for 15 h at 850 °C using 85% Ar and 15% CO2 as sweep gas and a rise in hydrogen flow can be observed with time (Figure 13b), probably due to a higher hydration of the membrane. In conclusion, these experiments provide a first proof of the membrane stability at work in CO2 rich environments. The integrity of the membrane was further investigated via SEM and XRD analysis. The fresh membrane is totally dense before permeation measurements with some occluded porosity as can be observed in Figure 14a. After permeation measurements (Figure 14 b−d) the membrane remains completely dense and some occluded porosity is also observed. However, bright nanoparticles are detected after the hydrogen permeation, presumably ascribed to Re reduction at high temperature (Figure 14 b, c) and the possible segregation of these nanoparticles might be in agreement with the TPR results (where the reduction to Re0 was attributed to an intense peak observed above 950 °C). This reduction was not observed by XPS due to the lower temperature of the sample reduction in that case (900 °C). Furthermore, a curious morphology in the grain fracture appears for the tested sample (detail in Figure 14d), and this can be related to the formation of dislocations and stacking faults. Re0 precipitates and dislocations were previously observed in the La5.5W0.8Re0.2O11.25−δ compound, as inferred from TEM analysis of FIB lamellas.12 In addition, membrane was measured by XRD before and after hydrogen measurements. The material presents single fluorite structure with cubic symmetry, and no secondary phases can be observed after the permeation experiments (Figure 15). Re segregation (Re0) was not observed by XRD and this may be attributed to the low concentration of nanoparticles which is lower than the detection limit of the diffractometer. Note that no impurities can be detected to the

4. CONCLUSIONS Nd5.5W1−xRexO11.25−δ (where x = 0.1, 0.5 and 1) materials were synthetized by the sol−gel method. The fluorite structure with cubic symmetry was obtained regardless of both Re concentration (x) introduced in the sample and sintering temperature. Total conductivity study was performed in oxidizing and reducing atmospheres as a function of pO2 and the hydration degree. In reducing atmospheres, typical operation conditions in the final membrane application, Re incorporation allows the n-type electronic and oxygen ionic conductivity to be substantially increased, despite of the drop in the relative protonic contribution. This enhanced electron and oxygen-ion transport is attributable to a mechanism, which involves the Re6+ reduction as inferred from TPR and XPS measurements. Furthermore, the total conductivity becomes 2 orders of magnitude higher than the undoped compound under these conditions. Hydrogen permeation measurements were performed using Nd5.5W0.5Re0.5O11.25−δ, which presents the highest total conductivity in reducing conditions. The hydrogen flow

Figure 15. Nd5.5W0.5Re0.5O11.25−δ membrane sintered at 1550 °C before and after hydrogen permeation measurements. Diffraction peaks corresponding to the sample holder material are indicated as *. 991



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G.; Escolastico, S.; Solis, C.; Serra, J. M. Inorg. Chem. 2013, 52 (18), 10375−86. (10) Shannon, R. Acta Crystallogr., Sect. A 1976, 32 (5), 751−767. (11) Hartmann, T.; Ehrenberg, H.; Miehe, G.; Wltschek, G.; Fuess, H. J. Solid State Chem. 1999, 148 (2), 220−223. (12) Escolastico, S.; Seeger, J.; Roitsch, S.; Ivanova, M.; Meulenberg, W. A.; Serra, J. M. ChemSusChem 2013, 6 (8), 1523−1532. (13) Escolástico, S.; Vert, V. B.; Serra, J. M. Chem. Mater. 2009, 21 (14), 3079−3089. (14) Escolástico, S.; Ivanova, M.; Solís, C.; Roitsch, S.; Meulenberg, W. A.; Serra, J. M. RSC Adv. 2012, 2 (11), 4932−4943. (15) (a) McCarthy, G. J.; Fisher, R. D.; Johnson, G. G., Jr.; Gooden, C. E. J. Solid State Chem., Proceedings of the 5th Materials Research Symposium National Bureau of Standards, Special Publication, 1972, 397−411. (b) Trunov, V. K. Russ. J. Inorg. Chem. 1968, 13, 4. (16) Itoh, N.; Stoneham, A. M. Radiat. Eff. Defects Solids 2001, 155 (1−4), 277−290. (17) (a) Yuan, Y.; Iwasawa, Y. J. Phys. Chem. B 2002, 106 (17), 4441−4449. (b) Mitra, B.; Gao, X.; Wachs, I. E.; Hirt, A. M.; Deo, G. Phys. Chem. Chem. Phys. 2001, 3 (6), 1144−1152. (18) Iida, H.; Igarashi, A. Appl. Catal. A 2006, 303 (2), 192−198. (19) Wiebe, C.; Gourrier, A.; Langet, T.; Britten, J.; Greedan, J. J. Solid State Chem. 2000, 151 (1), 31−39. (20) Bazuev, G.; Chupakhina, T.; Korolyov, A. J. Alloys Compd. 2009, 486 (1), 88−92. (21) (a) Bazuev, G. V.; Chupakhina, T. I.; Korolyov, A. V.; Kuznetsov, M. V. Mater. Chem. Phys. 2010, 124 (2−3), 946−951. (b) Retuerto, M.; Jimenez-Villacorta, F.; Martinez-Lope, M. J.; Huttel, Y.; Roman, E.; Fernandez-Diaz, M. T.; Alonso, J. A. Phys. Chem. Chem. Phys. 2010, 12 (41), 13616−13625. (22) Nikonova, O. A.; Capron, M.; Fang, G.; Faye, J.; Mamede, A.-S.; Jalowiecki-Duhamel, L.; Dumeignil, F.; Seisenbaeva, G. A. J. Catal. 2011, 279 (2), 310−318. (23) Escolástico, S.; Schroeder, M.; Serra, J. M. Submitted, 2013. (24) (a) Song, S. J.; Wachsman, E. D.; Rhodes, J.; Dorris, S. E.; Balachandran, U. Solid State Ionics 2004, 167 (1−2), 99−105. (b) Matsuka, M.; Braddock, R. D.; Matsumoto, H.; Sakai, T.; Agranovski, I. E.; Ishihara, T. Solid State Ionics 2010, 181 (29−30), 1328−1335. (25) Li, J.; Yoon, H.; Wachsman, E. D. J. Membr. Sci. 2011, 381 (1− 2), 126−131. (26) Weirich, M.; Gurauskis, J.; Gil, V.; Wiik, K.; Einarsrud, M.-A. Int. J. Hydrogen Energy 2012, 37 (9), 8056−8061. (27) Gallucci, F.; Fernandez, E.; Corengia, P.; van Sint Annaland, M. Chem. Eng. Sci. 2013, 92 (0), 40−66.

obtained with this compound was significantly higher with regard to undoped compound; that is, values up to 0.08 mL· min−1·cm−2 were reached at 1000 °C for a membrane thickness of 0.9 mm. Furthermore, stability in operation was demonstrated by measuring hydrogen permeation using CO2-rich sweep gas stream. Both properties high hydrogen permeation and remarkable stability under CO2 atmospheres make Nd5.5W0.5Re0.5O11.25‑δ an appealing membrane for practical application in high-temperature catalytic membrane reactors (CMR).

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental details on XPS analysis. Previous studies on XPS characterization of Rhenium compounds. Figure S1: XRD patterns for the Nd 5.5 W0.5Re0.5O 11.25−δ compound as powder and as membrane calcined at 1550 °C. Figure S2: O1s deconvoluted spectra for the standards Nd2O3 and WO3 high purity commercial powders. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +34.963877809. Tel: +34.9638.79448. E-mail: jmserra@ itq.upv.es. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the Spanish Government (Grant Nos. JAE-Pre 08-0058, ENE2011-24761, CSD-2009-0050, and SEV2012-0267) and the Helmholtz Association of German Research Centers through the portfolio topic MEM-BRAIN is kindly acknowledged. The authors are indebted to S. Jiménez for sample preparation.

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REFERENCES

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Solid State Transport and Hydrogen Permeation in the System Nd5

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