Abstract
The aim of the present work is to evaluate the influence of La 6 W 2 O 15 secondary phase on the properties and integrity of La 6-x WO 12-? –based membranes. Structural, microstructural and thermo–chemical study was carried out evidencing significant crystallographic and thermal expansion anisotropy: the reason for poor thermo–mechanical stability of La 6 W 2 O 15 . Conductivity of La 6 W 2 O 15 was one to two orders of magnitude lower compared to the phase pure La 6-x WO 12-? in the range of 300 to 900 °C. The relaxation study showed that the hydration process was faster for the La 6 W 2 O 15 compared to the LWO phase, due to the higher electronic contribution to the total conductivity. Short–term stability tests in H 2 at 900 °C and in a mixture of CO 2 and CH 4 at 750 °C were conducted and material remained stable. Remarkable reactivity with NiO and YSZ at elevated temperatures was further evidenced compared to the relative inert behavior towards MgO and CGO. Keywords: La 6 W 2 O 15 , La 6-x WO 12-? , mixed protonic–electronic conductor, dense ceramic membrane, hydrogen separation.
Highlights
H2/CO2–gas separation by means of dense ceramic membranes is an advanced approach to realize the Carbon Capture and Storage (CCS) concept in the pre–combustion power plants (Jordal et al 2004, Fontaine et al 2007, Czyperek et al 2010) along with the production of H2 with high purity for a number of applications, e.g. electricity production, mobile applications, chemical industry, etc. ((a) Norby et al 2006)
Single phase La6W2O15 material was synthesized via the conventional solid state route from stoichiometric amounts of constituent oxides
Internal tensions are accumulated in the material, leading to cracked microstructure, as evidenced by the electron microscopy
Summary
H2/CO2–gas separation by means of dense ceramic membranes is an advanced approach to realize the Carbon Capture and Storage (CCS) concept in the pre–combustion power plants (Jordal et al 2004, Fontaine et al 2007, Czyperek et al 2010) along with the production of H2 with high purity for a number of applications, e.g. electricity production, mobile applications, chemical industry, etc. ((a) Norby et al 2006). High temperature proton conducting membranes could be integrated in, for example, catalytic membrane reactors, for non–oxidative coupling of methane and aromatization (Li et al 2002) making possible the improvement of the process selectivity and efficiency by means of the in–situ removal of hydrogen. In order to be efficiently implemented in H2–separation tasks, materials have to exhibit mixed protonic–electronic conductivity and significant mechanical and thermal stability (Meulenberg, Ivanova et al 2011, (a) Ivanova et al 2012). Haugsrud et al 2006, (b) Norby et al 2006, Mokkelbost et al 2008, Brandão et al 2011, Huse et al 2012, b. Most of them do not show appreciable levels of electronic conductivity, or exhibit too restricted levels of protonic conductivity under the operating conditions relevant for the H2–separation Acceptor–doped rare–earth oxides, e.g. Y2O3 (Norby et al 1984, Norby et al 1986), Ln2O3 (Balakrieva et al 1989, Gorelov et al 1990, (a) Norby et al 1992, (b) Norby et al 1992, Larring et al 1995, Larring et al 1997), rare–earth phosphates (Norby et al 1995, Amezawa et al 1998, Amezawa et al 2004), pyrochlores (Shimura et al 1996, Omata et al 1997, Omata et al 2004, Haugsrud et al 2005, (a) Eurenius et al 2010, (b) Eurenius et al 2010), ortho–niobates ((a) Haugsrud et al 2006, (b)
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