Partial oxidation of methane over Rh/supported-ceria catalysts: Effect of catalyst reducibility and redox cycles

Partial oxidation of methane (POM) was studied over Rh/(Ce_0.56Zr_0.44)O_2−x, Rh/(Ce_0.91Gd_0.09)O_2−x, Rh/(Ce_0.71Gd_0.29)O_2−x and Rh/(Ce_0.88La_0.12)O_2−x. The effect of catalyst reducibility and redox cycles was investigated. It was found that the type of doped-ceria support and its reducibility played an important role in catalyst activity. It was also observed that redox cycles had a positive influence on H₂ production, which was enhanced as the number of redox cycle increased. Results of carbon formation are discussed as a function of ionic conductivity. Temperature programmed reduction (TPR) profiles, BET surface area, ionic conductivity and XRD patterns were determined to characterize catalysts. Catalytic tests revealed that of the materials tested, Rh/(Ce_0.56Zr_0.44)O_2−x was the most active material for the production of syngas, which correlates with its TPR profile. It was observed that doping CeO₂ with Zr, rather than with La or Gd caused an enhanced reducibility of Rh/supported-ceria catalysts.     

Syn-gas generation in the absence of oxygen and isotopic exchange reactions over Rh & Pt/doped-ceria catalysts

Syn-gas generation in the absence of oxygen by methane decomposition offers an interesting route to decrease reactor size and cost because methane is the only reactant in the gas phase. In this work, several catalysts were studied, Rh/CeO₂, Pt/CeO₂, Rh/(Ce_0.91Gd_0.09)O_2−x, Pt/(Ce_0.91Gd_0.09)O_2−x, Rh/γ-Al₂O₃ and Pt/γ-Al₂O₃ for methane reforming in the absence of gaseous oxygen. Rhodium showed a superior catalytic activity and selectivity with respect to Pt. This catalytic behavior may be due to the strong metal-support interaction, associated with the formation of mixed metal–oxide species at the interface. The addition of Gd³⁺ to ceria lowered the required temperatures for catalyst activation with respect to the un-doped material. Conversely to oxygen ion conducting materials, which showed a high selectivity for syn-gas generation, the non-oxygen conducting catalysts did not generated carbon monoxide. These results may be correlated to their oxygen storage capacity and ionic conductivity. Since gaseous oxygen was not delivered to the reactor, it is clear that the only source of oxygen was the catalyst. During the isothermal isotopic oxygen exchange experiments over Pt/(Ce_0.91Gd_0.09)O_2−x and Pt/γ-Al₂O₃, results illustrated that oxygen in the gas phase was exchanged with the oxygen from the catalyst. Three different molecules were detected ¹⁶O–¹⁸O, ¹⁶O–¹⁶O and ¹⁸O¹⁸O. A higher amount of oxygen was exchanged over Pt/(Ce_0.91Gd_0.09)O_2−x with respect to Pt/γ-Al₂O₃. It is proposed that mainly lattice and surface oxygen were exchanged over Pt/(Ce_0.91Gd_0.09)O_2–x and Pt/γ-Al₂O₃, respectively. It is also suggested that two types of reaction mechanisms take place, the simple and multiple hetero-exchange with the participation of one and two catalyst oxygen atoms, respectively. Similarly to methane reforming, lower temperatures were required for the oxygen exchange experiments over Rh than over Pt, as illustrated by results of the temperature-programmed exchange reactions. In summary, the properties of doped ceria may open new catalytic routes for oxidation reactions without gaseous oxygen because post-oxidation can restore its oxygen storage capacity.     

Catalytic partial oxidation of methane and isotopic oxygen exchange reactions over O labeled Rh/Gadolinium doped ceria

¹⁸O labeled catalysts, Rh/(Ce_0.91Gd_0.09)O_2−x (Rh/GDC10) and Rh/γ-Al₂O₃ (Rh/ALU) were used to study the catalytic partial oxidation of methane (CPOM) and oxygen isotopic exchange reactions. During the CPOM tests, higher C¹⁸O than C¹⁶O concentrations were observed over ¹⁸O labeled Rh/GDC10 than Rh/ALU, which is explained by the higher oxygen storage capacity and oxygen mobility of the former catalyst. Similarly, Rh/GDC10 showed higher oxygen exchange rates than Rh/ALU during the isotopic exchange experiments. The oxygen exchange between the gas phase and the solid is limited by the oxygen mobility in/on the catalyst. This catalytic behavior is due to the fact that ceria has two stable oxidation states, Ce³⁺ and Ce⁴⁺ and the addition of Gd³⁺ to ceria lattice enhanced the oxygen mobility by the creation of oxygen vacancies. These higher oxygen exchange rates also correlate to higher concentrations of C¹⁸O than C¹⁶O during the CPOM experiments. Pulse experiments suggest that the reaction mechanism for the CPOM on Rh/GDC10 occurred through a mixed (direct and indirect) mechanism. The direct mechanism assumes that H₂ and CO are primary reaction products formed in the oxidation zone at the catalyst entrance. Thus, CO formed from the reaction between lattice oxygen in Rh/GDC10 and adsorbed atomic carbon. CO₂ is formed through an indirect mechanism, where CH₄ reacts with O₂ to form CO₂ and H₂O. CO forms through the reactions between 1) CO₂ and CH₄ and 2) CH₄ and H₂O.     

Hydrogen production over Ni/ceria-supported catalysts by partial oxidation of methane

The catalytic performance of Ni dispersed on ceria-doped supports, (Ce_0.88La_0.12) O_2-x, (Ce_0.91Gd_0.09) O_2-x, (Ce_0.71Gd_0.29) O_2-x, (Ce_0.56Zr_0.44) O_2-x and pure ceria, was tested for the catalytic partial oxidation of Methane (CPOX). The catalysts were characterized by Brunauer Emmett Teller (BET), X-ray diffraction (XRD), temperature programmed reduction (TPR) and temperature programmed oxidation (TPO). Ni/ (Ce_0.56Zr_0.44) O_2-x showed higher Hydrogen production than the Ni/Gadolinium-doped catalysts, which may be due to its higher reducibility and surface area. By enhancing the support reducibility in Ni/doped-ceria catalysts, their catalytic activity is promoted because the availability of surface lattice oxygen is increased, which can participate in the formation of CO and H₂. It was also found that Ni/(Ce_0.56Zr_0.44) O_2-x showed higher catalytic performance after redox pretreatments. Similarly, a higher amount of H₂ or O₂ was consumed during hydrogenation and oxidation pretreatments, respectively. This may be correlated to re-dispersion of metallic particles and changes on the metal-support interface. In addition, it was observed that the ionic conductivity of Ni/(Ce_0.56Zr_0.44) O_2-x had an effect on the amount of carbon formed during the CPOX reaction at oxygen concentrations lower than the stoichiometric required, O/C ratios lower than 0.6. Its high oxygen mobility may have accelerated the surface oxidation reactions of carbon by reactive oxygen species, thus, inhibiting carbon growth on the catalyst surface.     

Effects of Ce/Zr composition on nickel based Ce(1−x)ZrxO2 catalysts for hydrogen production in sulfur–iodine cycle

In this study, 5%Ni/Ce_(1−x)Zr_xO₂( x = 0, 0.2, 0.5, 0.8, 1) catalysts were prepared by sol–gel method and tested for hydrogen iodide (HI) decomposition in sulfur–iodine (SI) cycle. The effects of zirconia incorporation were subsequently examined by a series of characterization methods. In comparison with the blank test, all of the catalysts, particularly Ni/Ce_0.8Zr_0.2O₂, remarkably enhance the HI decomposition. Therefore, adding a small amount of zirconia results in the highest catalytic activity, which could be attributed to the following three effects: increase in oxygen mobility and oxygen vacancy, stronger interaction between NiO and the supports. The oxygen mobility and oxygen vacancy are dominant in the HI decomposition. The strong interaction between NiO and the supports accelerates the oxygen transfer from the bulk to surface, which could also enhance the decomposition. However, excessive zirconia content has negative effects, which is due to the decrease in oxygen mobility and surface area. The poor thermal stability in zirconia-rich supports also restricts catalytic performance. In addition, a hypothetic mechanism of HI catalytic decomposition over Ni/Ce_(1−x)Zr_xO₂ is proposed.     

Deoxygenation of oleic acid over Ce(1–x)Zr(x)O2 catalysts in hydrogen environment

Ce_(1–x)Zr_(x)O₂ catalysts were prepared by co-precipitation method for deoxygenation (DO) of oleic acid. The CeO₂/ZrO₂ ratio was systematically varied to optimize Ce_(1–x)Zr_(x)O₂ catalysts. Ce_0.6Zr_0.4O₂ exhibited the highest oleic acid conversion as well as high selectivity to C₉ ∼ C₁₇ compounds (diesel fuel range) at the reaction temperature of 300 °C. The high activity/selectivity of Ce_0.6Zr_0.4O₂ catalyst was correlated to its reducibility, oxygen storage capacity and crystallite size.     

Low temperature hydrogen production by catalytic steam reforming of methane in an electric field

Catalytic steam reforming of methane in an electric field (electroreforming) at low temperatures such as 423 K was investigated. Pt catalysts supported on CeO₂, Ce_xZr_1−xO₂ solid solution and a physical mixture of CeO₂ and other insulators (ZrO₂, Al₂O₃ or SiO₂) were used for electroreforming. Among these catalysts, Pt catalyst supported on Ce_xZr_1−xO₂ solid solution showed the highest activity for electroreforming (CH₄ conv. = 40.6% at 535.1 K). Results show that the interaction among the electrons, metal loading, and catalyst support was important for high catalytic activity on the electroreforming. Catalytic activity of the electroreforming increased in direct relation to the input current. Characterizations using X-ray diffraction (XRD), temperature programmed reduction with H₂ (H₂-TPR), and alternate current (AC) impedance measurement show that the catalyst structure is an important factor for activity of electroreforming.     

Water–gas shift reaction over Pt and Pt–CeOx supported on CexZr1−xO2

The water–gas shift (WGS) reaction was examined over Pt and Pt–CeO_x catalysts supported on Ce_xZr_1−xO₂ (Ce_0.05Zr_0.95O₂, Ce_0.2Zr_0.8O₂, Ce_0.4Zr_0.6O₂, Ce_0.6Zr_0.4O₂, Ce_0.7Zr_0.3O₂ and Ce_0.8Zr_0.2O₂) under severe reaction conditions, viz. 6.7 mol% CO, 6.7 mol% CO₂, and 33.2 mol% H₂O in H₂. The catalysts were characterized with several techniques, including X-ray diffraction (XRD), CO chemisorption, temperature-programmed reduction (TPR) with H₂, temperature-programmed oxidation (TPO), inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and bright-field transmission electron microscopy (TEM). Among the supported Pt catalysts tested, Pt/Ce_0.4Zr_0.6O₂ showed the highest WGS activity in all temperature ranges. An improvement in the WGS activity was observed when CeO_x was added with Pt on Ce_xZr_1−xO₂ supports (x = 0.05 and 0.2) due to intimate contact between Pt and CeO_x species. Based on CO chemisorptions and TPR profiles, it has been found that the interaction between Pt species and surface ceria-zirconia species is beneficial to the WGS reaction. A gradual decrease in the catalytic activity with time-on-stream was found over Pt and Pt–CeO_x catalysts supported on Ce_xZr_1−xO₂, which can be explained by a decrease in the Pt dispersion. The participation of surface carbonate species on deactivation appeared to be minor because no improvement in the catalytic activity was found after the regeneration step where the aged catalyst was calcined in 10 mol% O₂ in He at 773 K and subsequently reduced in H₂ at 673 K.     

Sm0.2(Ce1−xTix)0.8O1.9 modified Ni-yttria-stabilized zirconia anode for direct methane fuel cell

Sm_0.2(Ce_1−xTi_x)_0.8O_1.9 (SCTx, x = 0-0.29) modified Ni-yttria-stabilized zirconia (YSZ) has been fabricated and evaluated as anode in solid oxide fuel cells for direct utilization of methane fuel. It has been found that both the amount of Ti-doping and the SCTx loading level in the anode have substantial effect on the electrochemical activity for methane oxidation. Optimal anode performance for methane oxidation has been obtained for Sm_0.2(Ce_0.83Ti_0.17)_0.8O_1.9 (SCT0.17) modified Ni-YSZ anode with SCT0.17 loading of about 241 mg cm⁻² resulted from four repeated impregnation cycles. When operating on humidified methane as fuel and ambient air as oxidant at 700 °C, single cells with the configuration of SCT0.17 modified Ni-YSZ anode, YSZ electrolyte and La_0.6Sr_0.4Co_0.2Fe_0.8O₃-Sm_0.2Ce_0.8O_1.9 (LSCF-SDC) composite cathode show the polarization cell resistance of 0.63 Ω cm² under open circuit conditions and produce a peak power density of 383 mW cm⁻². It has been revealed that the coated Ti-doped SDC on Ni-YSZ anode not only effectively prevents the methane fuel from directly impacting on the Ni particles, but also enhances the kinetics of methane oxidation due to an improved oxygen storage capacity (OSC) and redox equilibrium of the anode surface, resulting in significant enhancement of the SCTx modified Ni-YSZ anode for direct methane oxidation.     

Electrical conductivity and thermal expansion of neodymium-ytterbium zirconate ceramics

(Nd_1−xYb_x)₂Zr₂O₇ (0 ≤ x ≤ 1) ceramics were prepared by pressureless-sintering to obtain dense bulk materials. The electrical conductivity of (Nd_1−xYb_x)₂Zr₂O₇ was investigated by complex impedance spectroscopy over a frequency range of 20 Hz to 2 MHz from 723 to 1173 K in air. A high-temperature dilatometer was used to analyze thermal expansion coefficient of (Nd_1−xYb_x)₂Zr₂O₇ in the temperature range of 373-1523 K. The measured electrical conductivity obeys the Arrhenius relation. The grain conductivity of each composition in (Nd_1−xYb_x)₂Zr₂O₇ gradually increases with increasing temperature. A decrease of about one order of magnitude in grain conductivity is found at all temperature levels when the Yb content increases from x = 0.3 to x = 0.5. The highest electrical conductivity value obtained in this work is 9.32 × 10⁻³ S cm⁻¹ at 1173 K for (Nd_0.7Yb_0.3)₂Zr₂O₇ ceramic. (Nd_1−xYb_x)₂Zr₂O₇ ceramics are oxide-ion conductors in the oxygen partial pressure range from 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels. Thermal expansion coefficients of (Nd_1−xYb_x)₂Zr₂O₇ gradually decrease with increasing ytterbium content at identical temperature levels.     

A highly coking-resistant solid oxide fuel cell with a nickel doped ceria: Ce1−xNixO2−y reformation layer

A single phase mixed oxide ion-electron conducting electrochemical catalyst of Ce_1−xNi_xO_2−y is employed as an anode functional reformation layer for a coking-resistant solid oxide fuel cell (SOFC) based on oxide ion conducting electrolyte operated in methane and ethanol. The high catalytic activity of Ce_1−xNi_xO_2−y oxide for fuel reformation is demonstrated by the excellent cell performances in various fuels at relatively low temperatures (550–650 °C). The fast oxygen ions flux and formed steam at anode side are also found to be favorable for hydrocarbon reformation to promote the cell performance and long term stability. At 650 °C, maximum power densities of 415 and 271 mW cm⁻² are achieved in methane and ethanol respectively. The resistance against carbon deposition is significantly improved with stable voltage output in a long-term durability operation.     

Structured catalysts for the conversion of liquefied petroleum gas to hydrogen-rich gas and for anode off-gas afterburning

A number of mixed oxide Ce_0.75Zr_0.25O₂ supports were prepared, tested for reproducibility, and characterized by physicochemical methods. The most reproducible preparation method was adapted for depositing the mixed oxide on a FeCrAlloy mesh substrate coated by a protective alumina layer. Based on the obtained structured Ce_0.75Zr_0.25O₂/θ-Al₂O₃/FeCrAl support, the Rh/Ce_0.75Zr_0.25O₂/Al₂O₃/FeCrAl and Pt/Ce_0.75Zr_0.25O₂/Al₂O₃/FeCrAl catalysts were prepared and tested in the reactions of partial oxidation of LPG and deep oxidation of anode gases, respectively. Rh/Ce_0.75Zr_0.25O₂/Al₂O₃/FeCrAl provided complete conversion of LPG into synthesis gas of a composition close to the equilibrium one. Pt/Ce_0.75Zr_0.25O₂/Al₂O₃/FeCrAl provided complete conversion of all components of the anode gases at GHSV = 20,000–40,000 h⁻¹; however, at higher GHSV values, methane conversion decreased. The studies on the effect of methane content on deep oxidation of anode off gases showed that methane conversion began to decrease at a 1.5-fold excess of methane and dropped to 50% at a 10-fold excess of methane.     

Effect of ZrO2 on the performance of Co–CeO2 catalysts for hydrogen production from waste-derived synthesis gas using high-temperature water gas shift reaction

In this study, highly active and stable CeO₂, ZrO₂, and Zr_(1-x)Ce_(x)O₂-supported Co catalysts were prepared using the co-precipitation method for the high-temperature water gas shift reaction to produce hydrogen from waste-derived synthesis gas. The physicochemical properties of the catalysts were investigated by carrying out Brunauer-Emmet-Teller, X-ray diffraction, CO-chemisorption, Raman spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and H₂-temperature-programmed reduction measurements. With an increase in the ZrO₂ content, the surface area and reducibility of the catalysts increased, while the interaction between Co and the support and the dispersion of Co deteriorated. The Co–Zr_0.4Ce_0·6O₂ and Co–Zr_0.6Ce_0·4O₂ catalysts showed higher oxygen storage capacity than that of the others because of the distortion of the CeO₂ structure due to the substitution of Ce⁴⁺ by Zr⁴⁺. The Co–Zr_0.6Ce_0·4O₂ catalyst with high reducibility and oxygen storage capacity exhibited the best catalytic performance and stability among all the catalysts investigated in this study.     

Reduction reactivity of CeO2-ZrO2 oxide under high O2 partial pressure in two-step water splitting process

The O₂-releasing reaction under the air with the reactive ceramics of CeO₂-ZrO₂ oxides which can be applied to solar hydrogen production via a two-step water splitting cycle using concentrated solar thermal energy was investigated. CeO₂-ZrO₂ oxides were synthesized by polymerized complex method at different Ce:Zr molar ratio. The solid solubility of ZrO₂ in fluorite structure of CeO₂ was in good agreement with the initial content of Zr ions at the preparation in CeO₂-ZrO₂ oxide. The O₂-releasing reaction in air with CeO₂-ZrO₂ oxides was studied. Different solid solubility (0%, 10%, 20%, 30%) of ZrO₂ in CeO₂ were examined. The amount of O₂ gas evolved in the reaction with Ce_1−xZr_xO₂ (0 ? x ? 0.3) solid solutions was more than that with CeO₂, and the largest yield of 2.9 cm³/g was exhibited at x = 0.2 (Ce_0.8Zr_0.2O₂) for an O₂ release at 1500 °C in air. The reduced cerium ion in Ce_0.8Zr_0.2O₂ was about 11%, which is seven times higher than that with CeO₂. The optical absorption and luminescence spectra of the CeO₂-ZrO₂ oxide obtained before and after the O₂-releasing reaction suggest that the reduction of Ce⁴⁺ with formation of oxygen defect in the air. The enhancement of the O₂-releasing reaction with CeO₂-ZrO₂ oxide is found to be caused by an introduction of Zr⁴⁺, which has smaller ionic radius than Ce³⁺ or Ce⁴⁺, in the fluorite structure.     

Nickel zirconia cerate cermet for catalytic partial oxidation of ethanol in a solid oxide fuel cell system

Ni + Ce_xZr_1−xO₂ (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) cermets were synthesized and their catalytic performance for partial oxidation of ethanol (POE) reaction was studied. The structure, reducibility properties and carbon deposition behavior of the various catalysts were investigated. Among the various catalysts, Ni + Ce_0.8Zr_0.2O₂ displayed the best catalytic activity in terms of H₂ selectivity and also the highest coking resistance. The fuel cell with Ni + Ce_0.8Zr_0.2O₂ catalyst layer delivered a peak power density of 692 mW cm⁻² at 700 °C when operating on ethanol–O₂ gas mixtures, comparable to that applying hydrogen fuel. The fuel cell also showed an improved operation stability on ethanol–O₂ fuel for 150 h at 700 °C. Ni + Ce_0.8Zr_0.2O₂ is promising as an active and coke-tolerant catalyst layer for solid oxide fuel cells operating on ethanol-O₂ fuel, which makes it highly attractive by applying biofuel in an SOFC system for efficiency electric power generation.     

H2 production from a single stage water–gas shift reaction over Pt/CeO2, Pt/ZrO2, and Pt/Ce(1−x)Zr(x)O2 catalysts

To develop a single stage water–gas shift reaction (WGS) catalyst for compact reformers, Pt/CeO₂, Pt/ZrO₂, and Pt/Ce_(1−x)Zr_(x)O₂ catalysts have been applied for the target reaction. The CeO₂/ZrO₂ ratio was systematically varied to optimize Pt/Ce_(1−x)Zr_(x)O₂ catalysts. Pt/CeO₂ showed the highest turnover frequency (TOF) and the lowest activation energy (E_a) among the catalysts tested in this study. It has been found that the reduction property of the catalyst is more important than the dispersion for a single stage WGS. Pt/CeO₂ catalyst also showed stable catalytic performance. Thus, Pt/CeO₂ can be a promising catalyst for a single stage WGS for compact reformers.     

Preparation and evaluation of Ca3−xBixCo4O9−δ (0< x ≤ 0.5) as novel cathodes for intermediate temperature-solid oxide fuel cells

The misfit compounds Ca_3−xBi_xCo₄O_9−δ (x = 0.1–0.5) were successfully synthesized via conventional solid state reaction and evaluated as cathode materials for intermediate temperature-solid oxide fuel cells. The powders were characterized by X-ray diffraction, scanning emission microscopy, X-ray photoelectron spectroscopy, thermogravimetry analysis and oxygen-temperature programmed desorption. The monoclinic Ca_3−xBi_xCo₄O_9−δ powders exhibit good thermal stability and chemical compatibility with Ce_0.8Sm_0.2O_2−γ electrolyte. Among the investigated single-phase samples, Ca_2.9Bi_0.1Co₄O_9−δ shows the maximal conductivity of 655.9 S cm⁻¹ and higher catalytic activity compared with other Ca_3−xBi_xCo₄O_9−δ compositions. Ca_2.9Bi_0.1Co₄O_9−δ also shows the best cathodic performance and its cathode polarization resistance can be further decreased by incorporating 30 wt.% Ce_0.8Sm_0.2O_2−γ. The maximal power densities of the NiO/Ce_0.8Sm_0.2O_2−γ anode-supported button cells fabricated with the Ce_0.8Sm_0.2O_2−γ electrolyte and Ca_2.9Bi_0.1Co₄O_9−δ + 30 wt.% Ce_0.8Sm_0.2O_2−γ cathode reach 430 and 320 mW cm⁻² at 700 and 650 °C respectively.     

A new nickel–ceria composite for direct-methane solid oxide fuel cells

Various Ni–La_xCe_1−xO_y composites were synthesized and their catalytic activity, catalytic stability and carbon deposition properties for steam reforming of methane were investigated. Among the catalysts, Ni–La_0.1Ce_0.9O_y showed the highest catalytic performance and also the best coking resistance. The Ni–La_xCe_1−xO_y catalysts with a higher Ni content were further sintered at 1400 °C and investigated as anodes of solid oxide fuel cells for operating on methane fuel. The Ni–La_0.1Ce_0.9O_y anode presented the best catalytic activity and coking resistance in the various Ni–La_xCe_1−xO_y catalysts with different ceria contents. In addition, the Ni–La_0.1Ce_0.9O_y also showed improved coking resistance over a Ni–SDC cermet anode due to its improved surface acidity. A fuel cell with a Ni–La_0.1Ce_0.9O_y anode and a catalyst yielded a peak power density of 850 mW cm⁻² at 650 °C while operating on a CH₄–H₂O gas mixture, which was only slightly lower than that obtained while operating on hydrogen fuel. No obvious carbon deposition or nickel aggregation was observed on the Ni–La_0.1Ce_0.9O_y anode after the operation on methane. Such remarkable performances suggest that nickel and La-doped CeO₂ composites are attractive anodes for direct hydrocarbon SOFCs and might also be used as catalysts for the steam reforming of hydrocarbons.     

Effect of catalyst preparation on Au/Ce1−xZrxO2 and Au–Cu/Ce1−xZrxO2 for steam reforming of methanol

We tested 3 wt% gold (Au) catalysts on CeO₂–ZrO₂ mixed oxides, prepared by co-precipitation (CP) and the sol–gel (SG) technique, for steam reforming of methanol (SRM). Uniform Ce_1−xZr_xO₂ solid solution was dependent on the Zr/Ce ratio, where the incorporation of Zr⁴⁺ into the Ce⁴⁺ lattice with a ratio of 0.25 resulted in smaller ceria crystallites and better reducibility, and was found to be efficient for SRM activity. The catalytic activity was suppressed when the ratio was ≥0.5, which led to the segregation of Zr from solid solution and sintering of Au nanoparticles. It was found that the CP technique produced better catalysts than SG in this case. For the bimetallic catalysts, the co-operation of Au–Cu supported on Ce_0.75Zr_0.25O₂ (CP) exhibited superior activities with complete methanol conversion and low CO concentration at 350 °C. Furthermore, the size of the alloy particle was strongly dependent on the pH level during preparation.     

Effect of Gd and Yb co-doping on structure and electrical conductivity of the Sm2Zr2O7 pyrochlore

SmYb_1−xGd_xZr₂O₇ (0 ≤ x ≤ 1.0) ceramics were pressureless-sintered at 1973 K for 10 h in air. The relative density, structure and electrical conductivity of SmYb_1−xGd_xZr₂O₇ ceramics were investigated by the Archimedes method, X-ray diffraction, scanning electron microscopy and impedance spectroscopy measurements. SmYb_1−xGd_xZr₂O₇ (0 ≤ x ≤ 0.5) ceramics exhibit a defect fluorite-type structure, while SmYb_1−xGd_xZr₂O₇ (0.7 ≤ x ≤ 1.0) ceramics have a pyrochlore-type structure. The measured values of the electrical conductivities obey the Arrhenius relation. The grain conductivity of each composition in SmYb_1−xGd_xZr₂O₇ ceramics increases with increasing temperature from 723 to 1173 K. The grain conductivity of SmYb_1−xGd_xZr₂O₇ ceramics gradually increases with increasing gadolinium content at identical temperature levels. An increase of about one order of magnitude in grain conductivity is found at all temperature levels when the gadolinium content increases from 0.5 to 0.7. SmYb_1−xGd_xZr₂O₇ ceramics are oxide-ion conductors in the oxygen partial pressure range of 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels. The highest grain conductivity value obtained in this work is 2.69 × 10⁻² S cm⁻¹ at 1173 K for SmGdZr₂O₇ ceramic.