Surinderjit Singh Bhella

Studies on Chemical Stability and Electrical Properties of Proton Conducting Perovskite-Like Doped BaCeO3

The chemical stability and electrical properties of three promising perovskite-related structures BaCe 0.8 Gd 0.15 Pr 0.05 O 3―δ , BaCe 0.85 Sm 0.15 O 3―δ , and BaCe 0.85 Eu 0.15 O 3―δ were tested in air, humidified N 2 and H 2 , as well as in D 2 O + N 2 . Powder X-ray diffraction studies confirmed the formation of a cubic perovskite-like structure. The change in the lattice constant was consistent with B-site substitution in BaCeO 3 . All the investigated compounds formed barium carbonate in CO 2 at elevated temperatures and were found to be chemically unstable in boiling H 2 O. The data showed that these three compounds are chemically stable in humidified CH 4 at 800°C; however, at 600°C, the formation of barium carbonate was observed. The electrical conductivity in wet N 2 and/or H 2 was found to be higher than that in the D 2 O-containing atmosphere, confirming proton conduction in the doped BaCeO 3 . The Gd + Pr co-doped BaCeO 3 showed the highest total conductivity of 2.58 × 10 ―2 S cm ―1 in H 2 + 3% H 2 O at 700°C with an activation energy of 0.36 eV in the temperature range of 450―700°C.

Revisiting Tungsten Trioxide Hydrates (TTHs) Synthesis - Is There Anything New?

We report a very simple precipitation route to prepare a layered perovskite-type structure, tungsten trioxide hydrate (TTH), with the nominal chemical formula of WO(3) x 1.3 H(2)O (identical with 1/2H(2)W(2)O(7) x 1.6 H(2)O), using aqueous Na(2)WO(4) and SrCl(2). Our investigation shows that the concentration of HCl used to dissolve the SrCl(2) plays a crucial role in the stabilization of different structure types of layered TTHs. Highly acidic SrCl(2) (dissolved in 9 M HCl) solution yields an orthorhombic layered TTH of WO(3) x 2 H(2)O, while SrCl(2) dissolved in 3 M HCl appears to give an A-site-deficient Ruddlesdon-Popper (RP) related double-perovskite-type layered structure (DOLS-TTH). A well-known scheelite-type structure is obtained under weakly basic conditions (pH = 10.3 for Na(2)WO(4(aq)), 7.0 for SrCl(2(aq))). Previously, RP-type a DOLS of H(2)W(2)O(7) x 0.58 H(2)O was prepared, using an acid-leaching method, from the corresponding n = 2 member of the layered Aurivillius phase (AP) Bi(2)W(2)O(9). Powder X-ray diffraction showed the formation of layered RP DOLS with a large d spacing approximately 12.5 A, which is consistent with acid-leaching (Kuto et al. Inorg. Chem. 2003, 42, 4479-4484; Wang et al. J. Solid State Chem. 2007, 180, 1125-1129) and exfoliation (Schaak et al. Chem. Commun. 2002, 706-707) methods for synthesized TTHs. The proposed DOLS-TTH structure of newly prepared TTHs was further confirmed by an intercalation reaction using n-octylamine (C8A). A transmission electron microscopy study showed the formation of nanosized particles, and scanning electron microscopy coupled with energy dispersive X-ray analysis showed the absence of Na and Sr in the air-dried, as-precipitated products under acidic conditions. The bulk electrical (proton) conductivity of presently prepared TTHs was found to be on the order of 10(-4)-10(-3) S/cm at room temperature in wet N(2).

Facile conversion of layered Ruddlesden-Popper-related structure Y2O3-doped Sr2CeO4 into fast oxide ion-conducting fluorite-type Y2O3-doped CeO2.

The present work shows a new solid- and gas-phase reaction technique for the preparation of a fast oxide-ion-conducting Y(2)O(3)-doped Ce(1-x)Y(x)O(2-delta) (x = 0.1, 0.2) (YCO), which involves the reaction of layered (Ruddlesden-Popper K(2)NiF(4)-type) structure Y(2)O(3)-doped Sr(2)CeO(4) (YSCO) with CO(2) at an elevated temperature and subsequent acid-washing. A powder X-ray diffraction study revealed the formation of a single-phase cubic fluorite-type YCO for the CO(2)-reacted and subsequent acid-washed product. Energy dispersive X-ray analysis showed the absence of Sr in the CO(2)-treated and subsequent acid-washed product, confirming the transformation of layered YSCO into YCO. The cubic lattice constant was found to decrease with increasing Y content in YCO, which is consistent with the other YCO samples reported in the literature. The scanning electron microscopy study showed smaller-sized particles for the product obtained after CO(2)- and acid-washed YCO samples, while the high-temperature sintered YCO and the precursor YSCO exhibit larger-sized particles. The bulk ionic conductivity of the present CO(2)-capture-method-prepared YCO exhibits about one and half orders of magnitude higher electrical conductivity than that of the undoped CeO(2) and was found to be comparable to those of ceramic- and wet-chemical-method synthesized rare-earth-doped CeO(2).

In-situ powder X-ray diffraction investigation of reaction pathways for the BaCO(3)-CeO(2)-In(2)O(3) and CeO(2)-In(2)O(3) systems.

We report the first in-situ powder X-ray diffraction (PXRD) study of the BaCO(3)-CeO(2)-In(2)O(3) and CeO(2)-In(2)O(3) systems in air over a wide range of temperature between 25 and 1200 degrees C. Herein, we are investigating the formation pathway and chemical stability of perovskite-type BaCe(1-x)In(x)O(3-delta) (x = 0.1, 0.2, and 0.3) and corresponding fluorite-type Ce(1-x)In(x)O(2-delta) phases. The potential direct solid state reaction between CeO(2) and In(2)O(3) for the formation of indium-doped fluorite-type phase is not observed even up to 1200 degrees C in air. The formation of the BaCe(1-x)In(x)O(3-delta) perovskite structures was investigated and rationalized using in-situ PXRD. Furthermore the decomposition of the indium-doped perovskites in CO(2) is followed using high temperature diffraction and provides insights into the reaction pathway as well as the thermal stability of the Ce(1-x)In(x)O(3-delta) system. In CO(2) flow, BaCe(1-x)In(x)O(3-delta) decomposes above T = 600 degrees C into BaCO(3) and Ce(1-x)In(x)O(2-delta). Furthermore, for the first time, the in-situ PXRD confirmed that Ce(1-x)In(x)O(2-delta) decomposes above 800 degrees C and supported the previously claimed metastability. The maximum In-doping level for CeO(2) has been determined using PXRD. The lattice constant of the fluorite-type structure Ce(1-x)In(x)O(2-delta) follows the Shannon ionic radii trend, and crystalline domain sizes were found to be dependent on indium concentration.

Synthesis, Structure, Chemical Stability, and Electrical Properties of Nb-, Zr-, and Nb-Codoped BaCeO3 Perovskites

We report the effect of donor-doped perovskite-type BaCeO(3) on the chemical stability in CO(2) and boiling H(2)O and electrical transport properties in various gas atmospheres that include ambient air, N(2), H(2), and wet and dry H(2). Formation of perovskite-like BaCe(1-x)Nb(x)O(3±δ) and BaCe(0.9-x)Zr(x)Nb(0.1)O(3±δ) (x = 0.1; 0.2) was confirmed using powder X-ray diffraction (XRD) and electron diffraction (ED). The lattice constant was found to decrease with increasing Nb in BaCe(1-x)Nb(x)O(3±δ), which is consistent with Shannons ionic radius trend. Like BaCeO(3), BaCe(1-x)Nb(x)O(3±δ) was found to be chemically unstable in 50% CO(2) at 700 °C, while Zr doping for Ce improves the structural stability of BaCe(1-x)Nb(x)O(3±δ). AC impedance spectroscopy was used to estimate electrical conductivity, and it was found to vary with the atmospheric conditions and showed mixed ionic and electronic conduction in H(2)-containing atmosphere. Arrhenius-like behavior was observed for BaCe(0.9-x)Zr(x)Nb(0.1)O(3±δ) at 400-700 °C, while Zr-free BaCe(1-x)Nb(x)O(3±δ) exhibits non-Arrhenius behavior at the same temperature range. Among the perovskite-type oxides investigated in the present work, BaCe(0.8)Zr(0.1)Nb(0.1)O(3±δ) showed the highest bulk electrical conductivity of 1.3 × 10(-3) S cm(-1) in wet H(2) at 500 °C, which is comparable to CO(2) and H(2)O unstable high-temperature Y-doped BaCeO(3) proton conductors.

Sintering Effects on Proton Conductivity of Ta-Doped Ba2 ( CaNb ) 2O6 and its Reactivity with SOFC Cathodes

The effect of preparation temperature on electrical conductivity of Ba2(Ca 0.75 Nb 0.59 Ta 0.66 )O 6-δ , Ba2(Ca 0.75 Nb 0.66 Ta 0.59 )O 6-δ , and Ba2(Ca 0.79 Nb 0.66 Ta 0.55 )O 6-δ was investigated in air and humidified N 2 and O 2 . Powder X-ray diffraction (PXRD) showed the formation of double-perovskite-type structure. Among the compounds investigated, Ba2(Ca 0.79 Nb 0.66 Ta 0.55 )O 6-δ showed the highest proton conductivity of 3.7 and 5.3 X 10 -4 S/cm at 550°C in wet N 2 , respectively, for a 1400 and 1500°C sintered sample, while no change in conductivity was observed in air. PXRD showed that Ba 2 (Ca 0.79 Nb 0.66 Ta 0.55 )O 6-δ is stable against chemical reaction with La 0.8 Sr 0.2 MnO 3 (LSM) and Sm 0.5 Sr 0.5 CoO 3 (SSC) electrodes at 800 and 1000°C. Chemical compatibility was further confirmed by energy-dispersive X-ray (EDX) analysis. The ac impedance employing Pt, LSM, and SSC electrodes on Ba2(Ca 0.79 Nb 0.66 Ta 0.55 )O 6-δ showed that the area specific polarization resistance (ASPR) decreased in wet atmospheres compared to that of air. Unlike the oxide ion system, the ASPR was found to be much higher for the presently investigated proton system, suggesting that proton conductivity at the electrolyte and electrode interfaces or water effusion through microspores appears to control the ASPR. Among the electrodes tested, based on ac impedance studies, the LSM appears to be a better electrode compared to SSC for Ta-doped Ba 3 Ca 1+x Nb 2-x O 9-δ electrolyte.

Capture of sulfur dioxide from Claus tail gas using fiber-like alumina-based adsorbents

This paper describes the preparation of alumina fibers doped with CaO, MgO and La2O3 and reports their use as SO2 adsorbents. These materials were characterized using electron microscopy, powder X-ray diffraction and infrared analysis and were examined for their ability to capture SO2 selectively from gas mixtures containing large quantities of H2O, CO2 and some O2 at temperatures in excess of 353 K. Overall, it was found that these adsorbents could remove SO2 selectively and that they could be regenerated by treatment with an H2S-containing gas at approximately 600 K. Adsorption capacity was retained over several cycles. Fourier transform infrared analysis showed that SO2 was adsorbed as free SO2 and also in a combined form as sulfite and sulfate species. In the regeneration step, the adsorbed sulfur species were reduced to elemental sulfur and H2S or were desorbed as SO2. It is proposed that the chemistry described here could be applied to design of a process for capture of all sulfur species typically found in a Claus-based sulfur recovery system.