Effect of calcination atmosphere on microstructure and H2/CO2 separation of palladium-doped silica membranes
Graphical abstract
Introduction
Hydrogen has recently gained significant attention as an alternative energy carrier to alleviate the environmental problems [1]. It is currently produced primarily through the reformation of fossil fuels, typically natural gas [2]. Steam reforming and water gas shift (WGS) are the most industrial relevant method of reformation. However, the produced synthesis gas mainly consists of H2 and CO2, along with some minor contaminants such as CO, CH4 and other trace gas [2]. In order to obtain high purity hydrogen, separation of H2 from the gas mixture is necessary. Presently, research interest is oriented towards the development of H2/CO2 separation system of effective cost. Current methods for hydrogen separation are pressure swing adsorption [3], cryogenic distillation [4] and membrane separation [5], [6]. Membrane separation is an interesting alternative for several conventional separation processes, due to its inherent advantages such as low cost, high energy efficiency, simplicity and compactness. Many types of membranes for hydrogen separation include palladium/palladium alloy membranes [7], polymeric membranes [8], carbon molecular sieve membranes [9], zeolite membranes [10] and silica membranes [11], [12]. Among of them, palladium membranes and silica membranes are the most widely investigated.
For the palladium membranes, they have been proposed to produce high purity hydrogen due to their exclusive selectivity for hydrogen via the solution-diffusion mechanism [13]. However, the use of pure Pd has been limited due to its high cost and poor chemical tolerance in the presence common contaminants, such as CO and H2S [14]. The Pd-based binary or ternary alloys can support higher CO and H2S content but have a reduced permeance compared with pure Pd. Silica membranes, especially those derived from the sol-gel technique, are attractive for gas separation applications in view of their high porosity, high thermal stability and easily tunable pore size in the level of gas molecule dimensions [15]. Unfortunately, pure silica networks are unstable in steam and may lose their efficiency in gas separation [16] because humidity can induce the densification of microporous silica network by the breakage of siloxane bonding, the formation of silanol groups and subsequent recombination and rearrangement of them into the siloxane network [17]. This may lead to a deteriorated performance of the membranes, such as dramatic decrease of gas permeance and selectivity [15]. As the presence of steam is usual in the vast majority of reactions such as water gas shift, the silica-based composite membranes were studied to further improve the hydrothermal stability [18].
In the past two decades, many researchers have been devoted great efforts in the development and improvement of hydrothermal stability of silica membranes. The hydrothermal stability of silica membranes can be increased by the addition of organic templates with the added benefit of conferring further functionalities, such as increasing the hydrophobicity and tailoring the porosity of the silica membrane [19]. Originally, methyltriethoxysilane templated silica (MTES) membranes, which incorporated hydrophobic terminal methyl groups (SiCH3), showed much greater hydrophobicity than silica membranes, making MTES membranes an attractive and practical solution [20]. Later, polybenzimidazole (PBI) [21], bis (triethoxysilyl) ethane (BTESE) [22], isobutyl [23], trifluoropropyl [24], octyl [25], phenyl [26] and perfluorodecyl [15] have been reported to improve the hydrophobicity of silica composites. In recent years, further functionalities of the silica matrix have been explored by introducing metal/metal oxides, such as Mg [27], Al [28], Co [29], Pd [30] and Zr [31]. And binary metals such as PdCo [32], FeCo [33] and LaCo [34] were incorporated during sol preparation which has resulted in a wide range of beneficial effects. Cobalt oxide silica membrane showed a H2 permeance of 1.9 × 10−7 mol m−2 s−1 Pa−1 with a H2/CO2 permselectivity of more than 1500 [35]. Pd-SiO2 membranes were successfully fabricated using the sol-gel method, which were calcined at 550 °C in a hydrogen atmosphere. The Pd-SiO2 membrane (Si/Pd molar ratio = 3/1) showed a high H2 permeance of 5.0 × 10−7 mol m−2 s−1 Pa−1 and molecular sieving performance with the H2/N2 permeance ratio of 260 at 500 °C and steam pressure difference of 70 kPa [30]. For lanthanum cobalt silica membrane under reduction and oxidation cycles at 500 °C, the maximum steady-state He permeance reached 1.5 × 10−7 mol m−2 s−1 Pa−1 at the highest He/CO2 permselectivity of 196 [34].
It is well known that metallic Pd has extemely high solubility for hydrogen atoms [36], [37]. Combining the above two methods, Pd/SiO2 organic-inorganic membranes were fabricated in this work. The incorporation of hydrophobic methyl groups and metallic palladium is expected to improve the hydrothermal stability and H2 selectivity of silica membrane. Membrane properties such as permeation and selectivity depend on the microstructures of the membrane such as pore size and distribution, porosity and the affinity between permeating species and the pore walls [30]. However, the calcination atmosphere has an important influence on the microstructures of the membrane. By consulting concerned literatures, it is found that the effect of sintering atmospheres on the microstructure and permeability characteristics of palladium-doped silica membranes is vital but was rarely reported previously. In this work, on the base of the results from X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric-differential thermogravimetric (TG-DTG) and N2 sorption/desorption measurements, the influences of calcination atmosphere on the physical-chemical microstructures were investigated and presented here in detail. Subsequently, permeation tests for single gases were performed and are discussed.
Section snippets
Sol synthesis
The palladium-doped silica sol was synthesised from a sol-gel method with a final molar ratio of TEOS: MTES: PdCl2: EtOH: H2O: HCl = 1: 0.8: 0.08: 7.6: 7.2: 0.08. Briefly, the tetraethylorthosilicate (TEOS, p.a. grade, Xi’an chemical reagent Co. Ltd., China) and methyltriethoxysilane (MTES, grade 98%, Hangzhou Guibao chemical Co. Ltd., China) were added to the absolute ethanol (EtOH, grade 99.9%, Xi’an chemical reagent Co. Ltd., China) and vigorously stirred until homogeneous mixture was
Chemical structure analysis
The FTIR spectra of unsupported Pd/SiO2 materials non-calcined and calcined under various atmospheres are shown in Fig. 1. It could be reasoned that peak around 1467 cm−1 is assigned to the CH2 species. The two peaks at 2925 and 2978 cm−1 are assigned to the modes of CH3 while that at around 1276 cm−1 belongs to the vibration of SiCH3. The bands due to the SiO asymmetric stretching, SiO symmetric stretching and SiOSi bending vibrations were observed at 1050, 792 and 443 cm−1, respectively [38],
Conclusions
Pd/SiO2 materials and membranes with SiCH3 groups were prepared by sol-gel method. The influences of sintering atmospheres on the microstructures and H2/CO2 separation of Pd/SiO2 membranes were investigated extensively. The results showed that PdCl2 can be transformed into metallic Pd0 after calcination at 350 °C under N2 and H2 atmospheres while air atmosphere can lead to partial oxidation of metallic Pd0. H2 atmosphere was found to be effective in slowing down the decomposition of organic
Acknowledgement
The authors are grateful to the National Nature Science Foundation of China 21573171 and Shaanxi Province Key Research and Development Plan of China 2017GY-121 for financial supports.
References (46)
Hydrogen purification of synthetic water gas shift gases using microstructured palladium membranes
J. Power Sources
(2015)- et al.
Thermal effects on breakthrough curves of pressure swing adsorption for hydrogen purification
Int. J. Hydrogen Energ.
(2016) - et al.
Hydrogen isotope separation characteristics of cryogenic distillation column
Fusion. Eng. Des.
(1989) - et al.
Preparation and characterization of inorganic membranes for hydrogen separation
Int. J. Hydrogen Energ.
(2016) - et al.
Palladium-copper membranes for hydrogen separation
Sep. Purif. Technol.
(2017) - et al.
Inhibition of hydrogen flux in palladium membranes by pressure-induced restructuring of the membrane surface
J. Membr. Sci.
(2017) - et al.
Economic comparison of several membrane configurations for H2/N2 separation
Gas Sep. Purif.
(1989) - et al.
Gas separation performance of supported carbon molecular sieve membranes based on soluble polybenzimidazole
J. Membr. Sci.
(2017) - et al.
Ultra-thin MFI membranes for olefin/nitrogen separation
J. Membr. Sci.
(2017) - et al.
Studies of the effect of pressure and hydrogen permeance on the ethanol steam reforming reaction with palladium-and silica-based membranes
J. Membr. Sci.
(2012)