Aging behavior and ionic conductivity of ceria-based ceramics: a comparative study

An understanding of high-temperature aging effects on the electrical properties of electrolytes is very important in selecting optimum compositions for practical applications. The aging behavior and mechanisms of doped zirconia ceramics have been extensively studied. However, little information is available regarding the aging behavior of ceria-based electrolytes. The present study has demonstrated that a high-temperature aging at 1000 °C has a significant effect on the ionic conductivity of the Y- or Gd-doped ceria (Ce_1−xY_xO_2−δ and Ce_1−xGd_xO_2−δ), especially in the case of the Gd doping. The aging behavior is characterized by a critical dopant concentration, above which the aging has a detrimental effect on the conductivity of the doped ceria ceramics. The aging behavior in the doped ceria cannot be explained using the aging mechanisms applied to the doped zirconia. Instead, the formation of the microdomains in the doped ceria has been acknowledged to be the main contribution to the aging behavior of the Y- or Gd-doped ceria ceramics. The formation ability of microdomains has been estimated to be in the order of La³⁺>Gd³⁺>Y³⁺, based on the degree of size mismatch between the dopant ion and Ce⁴⁺ ion. The critical dopant concentrations at which the microdomains start to form for La³⁺, Gd³⁺ and Y³⁺ in the doped ceria ceramics are x=0.15, 0.2 and 0.25, respectively. This critical dopant concentration is also an important indication: below which the conductivity is governed by only the association enthalpy, and above which the conductivity is dominated mainly by the microdomains rather than the association enthalpy.     

Role of Ceria in the Design of Composite Materials for Elemental Mercury Removal

The necessity to drastically act against mercury pollution has been emphatically addressed by the United Nations. Coal‐fired power plants contribute a great deal to the anthropogenic emissions; therefore, numerous sorbents/catalysts have been developed to remove elemental mercury (Hg⁰) from flue gases. Among them, ceria (CeO₂) has attracted significant interest, due to its reversible Ce³⁺/Ce⁴⁺ redox pair, surface‐bound defects and acid‐base properties. The removal efficiency of Hg⁰ vapor depends among others, on the flue gas composition and temperature. CeO₂ can be incorporated into known materials in such a way that the abatement process can be effective at different operating conditions. Hence, the scope of this account is to discuss the role of CeO₂ as a promoter, active phase and support in the design of composite Hg⁰ sorbents/catalysts. The elucidation of each of these roles would allow the integration of CeO₂ advantageous characteristics to such degree, that tailor‐made environmental solution to complex issues can be provided within a broader application scope. Besides, it would offer invaluable input to theoretical calculations that could enable the materials screening and engineering at a low cost and with high accuracy.     

Mesoporous mesocrystal Ce1−xZrxO2 with enhanced catalytic property for CO conversion

Novel mesoporous mesocrystal Ce_1−xZr_xO₂ was synthesized using acetate salt as inorganic species and P123 as surfactant. Transmission electron microscopy reveals that the wall framework consists of a single phase based on the face-centered cubic CeO₂ and the nanocrystals are highly oriented with the crystal axis [001] parallel to the pore channel if the Zr⁴⁺ molar fraction x is 0.3 or less. However, when the Zr⁴⁺ molar fraction is larger than 0.3, a mixture of cubic and tetragonal phases forms and the preferential crystal orientation disappears as revealed by XRD and Raman measurements. The formation mechanism is ascribed to the oriented attachment following the manner of coherent interface. The single phase solid solution at Zr⁴⁺ molar fraction 0.3 demonstrates the best catalytic performance for CO conversion due to the unique mesoporous mesocrystal structure with dominant exposure of highly active {200} planes and an enhanced redox property caused by adequate Zr⁴⁺ incorporation.     

A density functional theory study of formaldehyde adsorption and oxidation on CeO2(1 1 1) surface

The effects of different oxygen species and vacancies on the adsorption and oxidation of formaldehyde over CeO₂(1 1 1) surface were systematically investigated by using density functional theory (DFT) method. On the stoichiometric CeO₂(1 1 1) surface, the C-H bond rupture barriers of chemisorbed formaldehyde are much higher than that of formaldehyde desorption. On the reduced CeO₂(1 1 1) surface, the energy barriers of C-H bond ruptures are less than those on the stoichiometric CeO₂(1 1 1) surface. If the C-H bond rupture occurs, CO and H₂ form quickly with low energy barriers. When O₂ adsorbs on the reduced (1 1 1) surface (O₂/O_v species), the C-H bond rupture barriers of formaldehyde are greatly reduced in comparison with those on the stoichiometric CeO₂(1 1 1) surface. If O₂ adsorbs on oxygen vacancy at sub-layer surface, its oxidative roles on formaldehyde are much similar to that of O₂/O_v species.     

V2O5-CeO2/TiO2-ZrO2催化剂表征及NH3还原NOx性能

商业选择性催化还原(SCR)催化剂V2O5-WO3(MoO3)/TiO2存在反应温度窗口窄(300–400 oC)和SO3转化率高等缺点,同时占催化剂总质量80%以上的载体TiO2比表面积小,热稳定性差.已有研究发现TiO2-ZrO2固溶体具有较大的比表面积和较强的表面酸性, TiO2与ZrO2的摩尔比为1：1时其比表面积达到最大. CeO2作为SCR催化剂的组成部分,由于其优良的储氧和放氧能力受到广泛关注.研究表明, CeO2-CuO, Ce/Ti-Si-Al和Mo2O3(Co2O3)/Ce-Zr等催化剂具有优良的SCR脱硝活性,同时对V2O5-WO3/TiO2催化剂进行CeO2改性,可提高催化剂的抗SO2中毒能力.实际烟气组分中同时存在SO2和H2O,必定会导致催化剂硫酸盐中毒,而目前对含Ce催化剂的硫酸盐中毒情况研究较少,因此开发新型高效脱硝催化剂十分必要.前期我们研究了xCeO2-3%V2O5/TiO2-ZrO2催化剂,发现CeO2可以显著拓宽脱硝温度窗口,同时增强催化剂酸性位点,但是V2O5含量较高时对环境及人体健康均有较大危害.本文采用共沉淀法制备摩尔比为1：1的TiO2-ZrO2固溶体,用浸渍法负载不同摩尔比的CeO2和1%的V2O5,得到一系列V-xCe/Ti-Zr催化剂,结合X射线衍射(XRD)、比表面积测试(BET)、高分辨透射电镜(HRTEM)、程序升温还原(H2-TPR)、原位漫反射红外光谱(in situ DRIFTS)和程序升温脱附(NH3-TPD)等手段分析催化剂的晶相、活性物质分散程度、氧化还原性质及表面酸性,在200–450 oC范围内考察Ce掺杂催化剂选择性催化还原NOx的脱硝活性,并在250 oC测试催化剂在NH3+NO+O2+SO2+H2O气氛中的脱硝活性,研究催化剂抗硫酸盐中毒能力.研究发现,CeO2掺杂可以拓宽脱硝反应活性窗口, V-0.2Ce/Ti-Zr (摩尔比Ce：Ti =0.2)表现出最优的脱硝性能,在250–350oC范围内脱硝效率均在92%以上,同时与前期研究结果对比发现CeO2含量较高时会导致高温段NOx转化率下降. XRD和HRTEM结果表明,ZrO2的添加可以显著降低载体TiO2的结晶度,复合氧化物TiO2-ZrO2呈无定形态, CeO2高度分散于载体之上,并且催化剂以单晶形式存在. H2-TPR结果表明,CeO2能显著提高催化剂的还原能力,主要的还原反应发生在CeO2的α(200–430oC)和β(430–600 oC)还原峰上,总体而言, V-0.2Ce/Ti-Zr表现出最大的氢气消耗量,即其还原性最强.低V2O5负载有利于较低温度SCR反应, V-0.3Ce/Ti-Zr的钒氧化物还原峰强度最大,其次是V-0.2Ce/Ti-Zr. NH3-TPD测试发现V2O5/TiO2主要存在中强酸及强酸,而V2O5/TiO2-ZrO2主要是弱酸, CeO2负载后随着其含量提高,弱酸强度增加.结合氨气原位漫反射红外光谱发现, CeO2可以增加催化剂Br?nsted和Lewis酸位数量,同时出现反应中间物–NH2, V2O5的负载量较高会抑制1660 cm–1处Br?nsted酸吸收峰的出现. BET结果发现, TiO2-ZrO2和V2O5/Ti-ZrO2比表面积分别可达255.73和143.77 m2/g, V2O5/TiO2仅为66.1 m2/g,表明ZrO2的添加可以显著增大催化剂比表面积,进而有利于SCR反应进行,沉积的氧化物进入载体孔道导致催化剂比表面积降低. V2O5-xCeO2/TiO2-ZrO2表现出较强的抗SO2中毒能力,但是在H2O存在条件下脱硝活性较差,可能是生成的硫酸铵盐及亚硫酸盐阻塞催化剂孔道所致. SO2和H2O停止通入后, V2O5-0.3CeO2/TiO2-ZrO2活性恢复至原有水平, V2O5-0.2CeO2/TiO2-ZrO2恢复至最初的84%.对中毒催化剂进行不同反应温度下的活性测试,发现V2O5-0.2CeO2/TiO2-ZrO2在中温段反应活性显著降低,可能是由于Ce(SO4)2的形成所致,由于V2O5-0.3CeO2/TiO2-ZrO2的Ce含量较高,其在此温度范围内活性依旧较高.两者在高温段NOx转化率均较高,推测是V2O5开始发挥活性组分作用的缘故.     

Nano- and micro-powder of zirconia and ceria-supported cobalt catalysts for the steam reforming of bio-ethanol

The usefulness of nano- and micro-powders of ceria and zirconia as a support of the cobalt-based catalyst as well as additional modification of zirconia-supported cobalt catalysts with cerium for the production of hydrogen in the steam reforming of bio-ethanol (SRE) for fuel cell applications was studied. It was found that mainly different structural features of the nano- and micro-powder of ceria- and zirconia-supported cobalt catalysts are higher cobalt surface area and much smaller average sizes of cobalt crystallites for the catalysts with high-dispersed support. Similarly, the presence of high-dispersed ceria introduced to the zirconia-supported catalysts simultaneously with the deposition of cobalt increases their total and active surface area as well as decreases the size of cobalt crystallites, regardless of the initial dispersion of zirconia support. The results of the temperature-programmed reduction and the temperature-programmed desorption of hydrogen showed that small crystallites of the cobalt phase strongly interact with high-dispersed ceria.Ceria has a great influence on the effects of the SRE. For the nano-powder ceria-supported cobalt catalyst and for the nano-powder zirconia-supported catalysts with the cobalt-ceria active phase the yield of hydrogen formed from one molecule of ethanol supplied to the SRE process was the highest; even at the relatively low temperature of 420 °C it is close to 5.5 mol H₂/mol EtOH. At the same time there were achieved: complete conversion of ethanol, very close to that of water and 92%, 81-84%, 5-6% selectivities to hydrogen, carbon dioxide and carbon monoxide, respectively.     

Electrochemical performance of ceria-gadolinia electrolyte based direct carbon fuel cells

A direct carbon fuel cell offers a high efficiency alternative to traditional coal fired electrical power plants. In this paper, the electrochemical performance of electrolyte supported button cells with Gd₂O₃-doped CeO₂ (CGO) electrolyte is reported over the temperature range 600 to 800 °C with solid carbon as a fuel and He/CO₂ as the purge gases in the fuel chamber. The electrochemical characterisation of the cells was carried out by the Galvanostatic Current Interruption (GCI) technique and measuring V-I and P-I curves. Power densities over 50 mWcm^-2 have been demonstrated using carbon black as the fuel. Results indicate that at low temperatures around 600 °C, the direct electrochemical oxidation of carbon takes place. However, at higher temperatures (800 °C) both direct electrochemical oxidation and the reverse Boudouard reaction take place leading to some loss in fuel cell thermodynamic efficiency and reduced fuel utilisation due to the in-situ production of CO. In order to avoid reverse Boudouard reaction whilst maximising performance, an operating temperature of around 700 °C appears optimal. Further, the electrochemical performance of fuel cells has been compared for graphite and carbon black fuels. It was found that graphitic carbon fuel is electrochemically less reactive than relatively amorphous carbon black fuel in the DCFC when tested under similar conditions.     

Nanosized ceria-based ceramics: a comparative study

Ce_(1−x)Gd/Sm _x O_2−δ (x = 0.05–0.2, GDC/SDC) nanometric powder was prepared by glycine-nitrates combustion synthesis, by strictly following uniformity in the preparation route. The thermochemical properties of the obtained precursor were studied by TGA/DTA. Crystallization of the fluorite phase occurred on heating at 800 °C or higher temperature. The grain size of calcined powder was found to be about 15 nm. Densification was studied as a function of dopant content. SDC was found more sinterable than GDC. Crystal structure and microstructure were characterized by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). Electrical characterization was carried out using the impedance spectroscopy method in the frequency range of 50 Hz–13 MHz. The bulk conductivity of SDC is higher than GDC pellet for all concentration ranges. The results were analyzed by using the concept of change of the chemical bond ionicity due to the replacement of the host by dopant. Guest/host ionic size, valence mismatch ratio and their consequences are counted semiquantitatively into the configurational and thermal entropy.     

Sintering behavior and ionic conductivity of Ce<Subscript>0.8</Subscript>Gd<Subscript>0.2</Subscript>O<Subscript>1.9</Subscript> with a small amount of MnO<Subscript>2</Subscript> doping

A 20% GdO_1.5 doped ceria solid solution with a small amount of MnO₂ doping (≤5% molar ratio) was prepared via the mixed oxide method from high-purity commercial powders with grain size around 0.2–0.5 μm. X-ray diffraction analysis indicated that all the samples exhibited the fluorite structure, and no new phase was found. The data from dilatometeric measurements and scanning electron microscopy observations revealed that 1% Mn doping reduced the sintering temperature by over 150 °C, and enhanced the densification and grain growth. Mn doping has little effect on grain interior conductivity, but a marked deterioration in grain boundary behavior is observed. This leads to a lower total conductivity in comparison with the undoped Ce_0.8Gd_0.2O_2–δ. Therefore, for solid oxide fuel cells (SOFCs) with Mn-containing compounds as electrodes, optimization of electrode fabrication conditions is needed to prevent the formation of a lower conductivity layer at the electrode/electrolyte interface since Mn will diffuse from the electrode side to the electrolyte during fabrication and operation of SOFCs. Electronic Publication     

Y2O3,CeO2掺杂的Mg-PSZ陶瓷材料研究

采用传统的陶瓷工艺制备少量掺杂Ｙ２Ｏ３,ＣｅＯ２的Ｍｇ－ＰＳＺ陶瓷,材料在较低的温度下烧结致密并实现了微晶化,探讨了Ｙ２Ｏ３,ＣｅＯ２的复合稳定作用和１１００℃热处理过程对材料相组成、显微结构和力学性能的影响。经１１００℃适当时间热处理,Ｙ２Ｏ３,ＣｅＯ２的复合稳定作用有效报制亚共析分解反应发生,优化调整ｃ－ＺｒＯ２晶粒中的ｔ－ＺｒＯ２析出体成核长大过程。断裂特征为穿晶、沿晶兼有,相变增韧和微裂纹