Trans. Nonferrous Met. Soc. China 26(2016) 1373-1378

Preparation and oxygen permeability of Bi₂₆Mo₁₀O₆₉ porous layer modified ionic-electronic mixed conductor on air side

Tai-he WANG¹, Rong LI¹, Qiang ZHEN^1,2, Lin CHENG¹

1. Nano-Science and Technology Research Center, Shanghai University, Shanghai 200444, China;

2. School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China

Received 6 May 2015; accepted 17 November 2015

Abstract: Bi₂₆Mo₁₀O₆₉ nanopowder was prepared by hydrothermal method and used as a surface modification material for oxygen separation membrane to enhance oxygen permeability. Thermal decomposition behavior and phase variation of the precursor were investigated by thermal analyzer (TG-DSC) and high-temperature X-ray diffraction (HT-XRD). Bi₂₆Mo₁₀O₆₉ porous layer was coated on the air side of BaCo_0.7Fe_0.2Nb_0.1O_3-d (BCFN) oxygen permeable membrane by dipping method. In the partial oxidation experiment of coke oven gas (COG), the Bi₂₆Mo₁₀O₆₉-coated BCFN membrane exhibits higher oxygen permeability and CH₄ conversion than the uncoated BCFN membrane. When the thickness of BCFN membrane was 1 mm and the COG and air fluxes were 120 and 100 mL/min, the oxygen permeation flux reached 16.48 mL/(min·cm²) at 875 °C, which was 16.96% higher than the uncoated BCFN membrane. Therefore, Bi₂₆Mo₁₀O₆₉ porous layer on the air side will be promising modification coating on the oxygen permeability of BCFN membrane.

Key words: Bi₂₆Mo₁₀O₆₉; porous-coating; BaCo_0.7Fe_0.2Nb_0.1O_3-d (BCFN); oxygen permeation flux

1 Introduction

Mixed ionic and electronic conductors are considered as promising materials for partial oxidation of methane (POM) to synthesize syngas (H₂-CO   mixture) [1]. Pervoskite-type oxides (ABO₃), such as SrCo_0.8Fe_0.2O_3-δ (SCF) [2], Ba_1-xSr_xCo_0.8Fe_0.2O_3-δ  (BSCF) [3] and La_1-xSr_xFe_1-yM _yO_3-δ (LSFM) [4,5], display infinite oxygen selectivity from air and higher oxygen fluxes compared with other organic   membranes [6,7]. Among these materials, BaCo_0.7Fe_0.2Nb_0.1O_3-δ (BCFN) shows high oxygen permeability and better chemical stability under reducing atmosphere in oxygen permeation process [8,9]. It is reported that BCFN with thickness of 1 mm shows an oxygen permeation flux of 1.63 mL/(min·cm²) at    875 °C [10]. However, this value is still not high enough to satisfy the requirement for industrial application.

When the oxygen permeation is limited by bulk diffusion, decreasing the thickness of the membrane could enhance the permeation rate, but it will also lead to low mechanical strength. And for a thin membrane (L<=L_c , L_c is defined as the thickness of membrane that divides the bulk limit and surface limit), oxygen surface exchange rate will become a dominant factor for oxygen permeability [11,12]. Thus, surface modification is usually an effective technique to further increase the oxygen permeability. It was reported that for Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ membrane with a single GdBaCo₂O_5+δ layer on the helium side, the oxygen permeation flux could rise by 23% at 875 °C [13]. CHENG et al [14] reported that BCFN membrane coated with Ce_0.8Re_0.2 O_2-δ (Re=Sm, Gd) porous layer on the reductive side showed significant enhancement in oxygen permeation flux.

However, oxygen permeability is also improved by coating porous layer on air side of membrane [15,16]. SHEN et al [17] proposed that on air side of the membrane, if the coating layer had a faster oxygen adsorption rate constant than the membrane material, oxygen ions could accumulate in the coating layer, thus a

higher oxygen pressure could be formed on the surface and resulted in higher oxygen permeation flux. HONG  et al [18] investigated La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ (LSCF) dense ceramic with porous Sm_xCe_1-xO_2-δ layer and found obvious effects on surface exchange rate. The surface exchange coefficient increased almost linearly with ionic conductivity of Sm_xCe_1-xO_2-δ, since the Sm_xCe_1-xO_2-δ supplied additional free oxygen vacancies, which is critical for oxygen incorporation in LSCF. Therefore, high ionic conductivity and oxygen adsorption ability  are expected for a porous layer on the oxygen  membrane.

Recently, most research has focused on perovskite oxide and fluorite structure oxides, such as CeO₂-based material as porous layer. Compared with these materials, Bi₂O₃-based materials exhibit higher ionic conductivity. The binary oxide Bi₂₆Mo₁₀O₆₉ is a new family of oxygen ion conductors, and this type of compound displays a transference number almost unity for the oxygen ions in air [19,20]. However, Bi₂₆Mo₁₀O₆₉ was usually synthesized by solid reaction method with micron grain size. In this work, Bi₂₆Mo₁₀O₆₉ nanopowder was successfully prepared by hydrothermal method, and the decomposition behavior of precursor was also investigated by TG-DSC and high-temperature XRD (HT-XRD). The oxygen permeation flux was studied in partial oxidation experiment of coke oven gas (COG) with submicron Bi₂₆Mo₁₀O₆₉ porous layer coated on the air side of BCFN dense membrane.

2 Experimental

2.1 Sample preparation

Using analytically pure Bi₂O₃ and (NH₄)₆Mo₇O₂₄·4H₂O agents as raw materials, Bi₂₆Mo₁₀O₆₉ precursor was synthesized by hydrothermal method. Bi₂O₃ was dissolved in a nitric acid with Bi³⁺ concentration of 1.7 mol/L, while (NH₄)₆Mo₇O₂₄·4H₂O was dissolved in an ammonia solution with Mo³⁺ concentration of 0.9 mol/L. Then, the nitrate solution was dropwise added into the ammonia solution with continuous stirring, and the pH value of this mixed solution was controlled at 7. The mixed solution was poured into hydro-thermal reactor and kept at 180 °C for 20 h. After being filtered and dried at 80 °C, the precursor was calcined at 800 °C for 3 h to obtain Bi₂₆Mo₁₀O₆₉ nanopowders.

BCFN dense membrane with a thickness of 1 mm was prepared according to Ref. [14]. The slurry, a mixture of Bi₂₆Mo₁₀O₆₉ nanopowder, terpineol, carbon fiber, ethyl cellulose and polyethylene glycol, was coated on the air side of BCFN membranes by dipping method. The coated samples were heated at 600 °C for 3 h with a heating rate of 5 °C/min.

2.2 Characterization

Thermal decomposition behavior of Bi₂₆Mo₁₀O₆₉ precursor was investigated using composite thermal analyzer (STA409PC, Germany). The experiment was carried out in air from room temperature to 950 °C with a heat rate of 10 °C/min [19].

A D8 advance AXS Bruker diffractometer (Cu K_α radiation, Germany), equipped with a HTK 1200N Anton Paar high temperature chamber, was used to study the phase variation of the Bi₂₆Mo₁₀O₆₉ precursor during the heating process from room temperature to 925 °C. The diagrams were collected every 20 °C, in the 20°≤2q≤80° domain, with a 0.0148° step size and a counting time of 0.2 s per step.

Phases of Bi₂₆Mo₁₀O₆₉ nanopowder and Bi₂₆Mo₁₀O₆₉ coated BCFN membrane were characterized by X-ray diffraction (Rigaku, D/max-2550, Japan) in the range of 20°<=2θ<=80°, with a 0.02° step size and a 4 (°)/min scanning rate. The average crystalline size of Bi₂₆Mo₁₀O₆₉ nanopowder was calculated using the Scherrer equation D=0.89l/(Bcosq), where B is the full width at half maximum of Bragg peaks.

Micrograph of Bi₂₆Mo₁₀O₆₉ nanopowder was observed using transmission electron microscopy (JEOL, 200CX, Japan). The microstructure of surface and cross section of Bi₂₆Mo₁₀O₆₉ coated BCFN sample was observed by scanning electron microscopy (JEOL, 6700F, Japan).

2.3 Oxygen permeation experiment

The permeation properties of the disk shape membranes were investigated in vertical high temperature oxygen permeation apparatus, as shown in Fig. 1. The membrane (d=17 mm, t=1 mm) was sealed with sliver rings to quartz tubes on both sides, and the effective area of membrane exposed to gas was 1.3 cm². In the experiment, a total of 1.0 g of 0.4-0.8 mm Co_0.3Mg_0.7O powder was directly placed on the upper surface of membrane as catalyst. COG (31.8% CH₄, 57.705% H₂, 7.366% CO and 3.069% O₂) and the air passed through two quartz inner tubes to sweep the upper and lower surfaces of the membrane, respectively. The gas flow rates were controlled by mass flow controllers.

Fig. 1 Schematic of membrane reactor for oxygen permeation

Prior to the experiment, gas-tightness of membrane reactor was examined by detecting nitrogen in the outlet gas. On the permeation side of the membrane, the water was removed by Mg(ClO₄)₂, and H₂, CO, CO₂ and CH₄ in the outlet gas were analyzed by a Varian CP3800 gas chromatography (GC) with a thermal conductivity detector (TCD). According to the law of mass conservation, the oxygen permeation flux, CH₄ conversion and H₂ selectivity were calculated from H₂, CO, CO₂ and CH₄ concentrations and the flow rate of the inlet and outlet gas [14,21].

3 Results and discussion

3.1 Thermal decomposition behavior of Bi₂₆Mo₁₀O₆₉ precursor

Figure 2 shows the TG-DSC curves of Bi₂₆Mo₁₀O₆₉ precursor obtained by hydrothermal method. Before  480 °C, the mass loss of 2.7% was mainly caused by the release of adsorbed water and the burning of organic disperser, then by the decomposition of the small amount of hydroxide, corresponding to an endothermic peak at 430 °C. After 480 °C, the TG curve tends to be plat, no mass loss was observed. However, an endothermic peak occurred at 710 °C in the DSC curve caused by the phase variation.

Fig. 2 TG-DSC curves of Bi₂₆Mo₁₀O₆₉ precursor in heating process

The precursor was also studied by HT-XRD and the diffractograms are shown in Fig. 3. Orthorhombic phase Bi₂MoO₆ and cubic phase Bi₂O₃ were identified from room temperature to 440 °C. At 440 °C, the peaks of Bi₂MoO₆ and Bi₂O₃ started to change into a new phase. The disappearance of Bi₂MoO₆ was observed at 700 °C, and it was replaced by a monoclinic phase of Bi₂₆Mo₁₀O₆₉. This phase variation was also observed in DSC curve with an endothermic peak at 710 °C. At   800 °C, only monoclinic Bi₂₆Mo₁₀O₆₉ was observed, and this phase structure was kept when returned to room temperature.

Fig. 3 High temperature X-ray diffraction patterns of Bi₂₆Mo₁₀O₆₉ precursor

3.2 Characterization of Bi₂₆Mo₁₀O₆₉ nanopowder

According to the results of TG-DSC and HT-XRD, the precursor was calcined at 800 °C for 3 h to obtain Bi₂₆Mo₁₀O₆₉ nanopowder. Figure 4(a) gives the XRD pattern of Bi₂₆Mo₁₀O₆₉ nanopowder, as expected from high temperature XRD, pure Bi₂₆Mo₁₀O₆₉ phase was obtained. Calculated by the Scherrer equation, the average grain size was 70 nm. The TEM image of Bi₂₆Mo₁₀O₆₉ nanopowder is shown in Fig. 4(b). It shows that the particles are nearly spherical shape with grain size smaller than 100 nm, which is in good agreement with the calculated value from XRD.

Fig. 4 XRD pattern (a) and TEM image (b) of Bi₂₆Mo₁₀O₆₉ nanopowder

3.3 Characterization of Bi₂₆Mo₁₀O₆₉ porous layer coated BCFN membrane

Figure 5 shows the XRD patterns of Bi₂₆Mo₁₀O₆₉- coated BCFN and BCFN membranes, respectively. The BCFN membrane exhibits a cubic pervoskite structure. For Bi₂₆Mo₁₀O₆₉-coated BCFN sample, the main peaks correspond to both Bi₂₆Mo₁₀O₆₉ and pervoskite structure, and no impurity phase is observed, revealing that the coexistence of the BCFN and Bi₂₆Mo₁₀O₆₉ is stable.

Fig. 5 XRD patterns of Bi₂₆Mo₁₀O₆₉ coated BCFN and uncoated BCFN membrane

Figure 6 shows the microstructures of the surface and the cross section of Bi₂₆Mo₁₀O₆₉-coated BCFN sample. From Fig. 6(a), Bi₂₆Mo₁₀O₆₉ coating layer exhibits a porous structure, caused by the volatilization of the organic solution in the slurry. From Fig. 6(b), it can be seen that Bi₂₆Mo₁₀O₆₉ porous layer tightly contacts with BCFN membrane, and the thickness of porous layer is about 5 mm.

3.4 Oxygen permeability of Bi₂₆Mo₁₀O₆₉-coated BCFN membrane

Figure 7 presents the oxygen permeation flux (J_O2) of Bi₂₆Mo₁₀O₆₉-coated BCFN and uncoated BCFN membranes at 875 °C as a function of COG flow rate. The flow rate of air is 100 mL/min. The oxygen permeation flux increases with rising COG flow rate, which could be attributed to the lower oxygen partial pressure on the permeation side. Bi₂₆Mo₁₀O₆₉-coated BCFN exhibits higher oxygen permeation flux than uncoated BCFN membrane. When the COG flux was 120 mL/min, the oxygen permeation flux of Bi₂₆Mo₁₀O₆₉ coated BCFN reached 16.48 mL/(min·cm²), which was 16.96% higher than that of the uncoated BCFN membrane. The CH₄ conversion of BCFN membrane with and without Bi₂₆Mo₁₀O₆₉ coating is shown in Fig. 7. It can be seen that the Bi₂₆Mo₁₀O₆₉-coated BCFN membrane showed significant enhancement in CH₄ conversion. The average growth was 19.4%. When the COG flux was 60 mL/min, the CH₄ conversion of Bi₂₆Mo₁₀O₆₉-coated BCFN reached 99.6%, while that of the uncoated BCFN membrane was only 80%. At the meantime, the H₂ selectivities were 79.6% and 59%, respectively.

Fig. 6 SEM images of Bi₂₆Mo₁₀O₆₉ coated BCFN membrane

Fig. 7 Oxygen permeation flux and CH₄ conversion of Bi₂₆Mo₁₀O₆₉-coated BCFN and uncoated BCFN membrane (Reaction conditions: COG flow rate, 40-120 mL/min; air flow rate, 100 mL/min; membrane thickness, 1.0 mm; temperature, 875 °C)

The improvement in oxygen permeation flux on one hand relies on the extra surface area and active sites arising from the nanoparticles, on the other hand is owing to the coating layer, which may change the effective oxygen pressure on the surface of the membrane [14]. Therefore, the porous coating could accurate the process of oxygen reduction process.

4 Conclusions

1) Bi₂₆Mo₁₀O₆₉ precursor was prepared by hydrothermal method. Thermal decomposition behavior of the precursor was investigated by TG-DSC and high-temperature X-ray diffractometry, and Bi₂O₃ , Bi₂MoO₆ were found in the precursor.

2) With temperature increasing, Bi₂O₃ and Bi₂MoO₆ started to form a new phase, Bi₂MoO₆ disappeared at  700 °C, and monoclinic phase of Bi₂₆Mo₁₀O₆₉ was observed at 800 °C. After calcining the precursor at  800 °C for 3 h, Bi₂₆Mo₁₀O₆₉ nanopowder with grain size of 70 nm was obtained.

3) Bi₂₆Mo₁₀O₆₉ porous layer was coated on the air side of BCFN membrane by dipping method, and no reaction between BCFN and coating layer was observed.

4) In the partial oxidation experiment of COG, the Bi₂₆Mo₁₀O₆₉-coated BCFN exhibits higher oxygen permeability than uncoated BCFN. When the COG flux and air flux were 120 mL/min and 100 mL/min, the oxygen permeation flux reached 16.48 mL/(min·cm²), which was 16.96% higher than uncoated BCFN membrane. This means that Bi₂₆Mo₁₀O₆₉ porous layer on the air side can improve the oxygen permeability of ionic-electronic mixed conductor. However, the improving mechanism in oxygen permeation needs to be further studied.

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Bi₂₆Mo₁₀O₆₉多孔涂层空气侧改性混合离子导体的制备及其增进透氧性能

王太和¹，李 榕¹，甄 强^1,2，程 琳¹

1. 上海大学 纳米科学与技术研究中心，上海 200444；

2. 上海大学 材料科学与工程学院，上海 200072

摘  要：通过水热法合成Bi₂₆Mo₁₀O₆₉纳米粉体，并作为表面改性材料用于提高氧分离膜的透氧性能。通过TG-DSC 和高温XRD(HT-XRD)对前驱体的热分解行为及物相变化进行研究。采用浸渍法在BaCo_0.7Fe_0.2Nb_0.1O_3-δ (BCFN)的空气侧涂覆Bi₂₆Mo₁₀O₆₉ 多孔涂层。在焦炉煤气(COG)的部分氧化重整实验中，涂覆Bi₂₆Mo₁₀O₆₉ 多孔涂层的BCFN 膜的透氧量和CH₄转化率均高于无涂层的BCFN 膜。当BCFN透氧膜厚度为1mm，COG流量和空气流量分别为120 mL/min 和100 mL/min 时，875 °C透氧速率达到16.48 mL/(min·cm²)，比无涂层的BCFN 膜高16.96%。因此，空气侧Bi₂₆Mo₁₀O₆₉多孔涂层有望作为改性涂层提高BCFN膜的透氧性能。

关键词：Bi₂₆Mo₁₀O₆₉；多孔涂层；BaCo_0.7Fe_0.2Nb_0.1O_3-d (BCFN)；透氧速率

 (Edited by Xiang-qun LI)

Foundation item: Projects (51272154, 51472156) supported by the National Natural Science Foundation of China; Projects (sdcx2012033, sdcx2012062) supported by the Innovation Fund of Shanghai University, China; Project (14ZR1416400) supported by Special Research Foundation for Training and Selecting Outstanding Young Teachers of Universities in Shanghai, China; Project supported by Shanghai Science and Technology Committee, China

Corresponding author: Rong LI; Tel: +86-21-66137266; E-mail: li-rong@shu.edu.cn

DOI: 10.1016/S1003-6326(16)64241-3