Influence of the catalyst surface area on the activity and stability of Au/CeO2 catalysts for the low-temperature water gas shift reaction

The influence of the support surface area on the activity and stability/deactivation of Au/CeO₂ catalysts (2.7 wt% Au) in the water gas shift reaction in dilute water gas were investigated by kinetic measurements and in situ Diffuse Reflectance IR spectroscopy. For ceria support surface areas between 24 and 284 m² g⁻¹, the gold particle size is independent on the catalyst surface area (about 2.1 nm) up to 188 m² g⁻¹, and we found increased amounts of (i) Auⁿ⁺, (ii) Ce³⁺, (iii) OH groups, and (iv) carbon containing adsorbed side products such as formates and carbonates for increasing surface area supports. Consequences of these results on the mechanistic understanding of the reaction are discussed.     

Characterizations of composite cathodes with La0.6Sr0.4Co0.2Fe0.8O3?δ and Ce0.9Gd0.1O1.95 for solid oxide fuel cells

Composite cathodes with La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) and Ce_0.9Gd_0.1O_1.95 (GDC) are investigated to assess for solid oxide fuel cell (SOFC) applications at relatively low operating temperatures (650–800 °C). LSCF with a high surface area of 55 m²g⁻¹ is synthesized via a complex method involving inorganic nano-dispersants. The fuel cell performances of anode-supported SOFCs are characterized as a function of compositions of GDC with a surface area of 5 m²g⁻¹. The SOFCs consist of the following: LSCF-GDC composites as a cathode, GDC as an interlayer, yttrium stabilized zirconia (YSZ) as an electrolyte, Ni-YSZ (50: 50 wt%) as an anode functional layer, and Ni-YSZ (50: 50 wt%) for support. The cathodes are prepared for 6LSCF-4GDC (60: 40 wt%), 5LSCF-5GDC (50: 50 wt%), and 4LSCF-6GDC (40: 60 wt%). The 5LSCF-5GDC cathode shows 1.29 Wcm⁻², 0.97 Wcm⁻², and 0.47 Wcm⁻² at 780 °C, 730 °C, and 680 °C, respectively. The 6LSCF-4GDC shows 0.92 Wcm⁻², 0.71 Wcm⁻², and 0.54 Wcm⁻² at 780 °C, 730 °C, and 680 °C, respectively. At 780 °C, the highest fuel cell performance is achieved by the 5LSCF-5GDC, while at 680 °C the 6LSCF-4GDC shows the highest performance. The best composition of the porous composite cathodes with LSCF (55 m²g⁻¹) and GDC (5 m²g⁻¹) needs to be considered with a function of temperature.     

Synthesis and catalytic properties of nanostructured Me/Mn<Subscript>0.5</Subscript>Ce<Subscript>0.5</Subscript>O<Subscript>2</Subscript> in the oxidation of carbon monoxide

The nanostructured solid solution Mn_0.5Ce_0.5O₂ is synthesized to develop effective noble metal free catalysts for the detoxification of technogenic contaminants. Its chemical and phase compositions and textural characteristics are studied by differential thermal analysis, X-ray diffraction analysis, laser mass spectrometry, and low-temperature nitrogen adsorption. The activity of the solid solution in the oxidation of carbon monoxide is determined by the flow method within a temperature range of 20–300°C at atmospheric pressure, a gas hourly space velocity of 1800 h⁻¹ for the following gas mixture composition, vol %: CO, 3.6; O₂, 8.0; N₂, balance. The activity of Mn_0.5Ce_0.5O₂ is shown to be appreciably higher than the activity of MnO_x and CeO₂, and the temperature of 100% conversion is 92, 120, and 210°C, respectively. Using the solid solution as a support and the technique of impregnation, we synthesize the nanostructured catalysts Cu/Mn_0.5Ce_0.5O₂ and Ag/Mn_0.5Ce_0.5O₂, which manifest high activity in the oxidation of carbon monoxide: the temperature of 100% conversion is 77 and 85°C, respectively. The new catalysts could be of interest for the purification of industrial and motor vehicle wastes.     

Phase transformation in CeO2–Co3O4 binary oxide under reduction and calcination pretreatments

The CeO₂–Co₃O₄ binary oxide was prepared by impregnation of the high surface area Co₃O₄ support (S.A. = 100m² g⁻¹) with cerium nitrate (20 wt% cerium loading on Co₃O₄). Pretreatment of CeO₂–Co₃O₄ binary oxide was divided both methods: reduction (under 200 and 400 °C, assigned as CeO₂–Co₃O₄–R200 and CeO₂–Co₃O₄–R400 and calcination (under 350 and 550 °C, assigned as CeO₂–Co₃O₄–C350 and CeO₂–Co₃O₄–C550). The binary oxides were investigated by means of X-ray diffraction (XRD), nitrogen adsorption at −196 °C, infrared (IR), transmission electron microscopy (TEM), diffuse reflectance spectroscopy (DRS) and temperature programmed reduction (TPR). The results showed that the binary oxides pretreatment under low-temperatures possessed larger surface area. The cobalt phase of binary oxides also was transferred upon the treating temperature, i.e., the CeO₂–Co₃O₄–R200 binary oxide exhibited higher surface area (S.A. = 109m² g⁻¹) and the main phase was CeO₂,Co₃O₄ and CoO. While, the CeO₂–Co₃O₄–R400 binary oxide exhibited lower surface area (S.A. = 40m² g⁻¹) and the main phase was CeO₂, CoO and Co. Apparently, the optimized pretreatment of CeO₂–Co₃O₄ binary oxide can control both the phases and surface area.     

The catalytic activity for CO oxidation of CuO supported on Ce0.8Zr0.2O2 prepared via citrate method

CuO/Ce_0.8Zr_0.2O₂ catalysts were prepared by citrate method and used for carbon monoxide oxidation. The samples were characterized by XRD, XPS, BET and ICP-AES techniques. The catalytic properties of the catalysts were studied by using a microreactor-GC system. XRD analysis showed Ce_0.8Zr_0.2O₂ was cubic, fluorite structure for all the catalysts. The XPS indicated the valence of Ce atom was +4 and there were reduced copper species presented in the CuO/Ce_0.8Zr_0.2O₂ catalyst. The results showed that the CuO loadings, calcination temperature and calcination time affected the catalytic activity of the catalysts for low-temperature CO oxidation. For comparison, the catalytic activities of CuO/CeO₂ catalysts calcined at different temperatures were also studied. The results indicated that CuO/Ce_0.8Zr_0.2O₂ catalyst had better thermal resistance than CuO/CeO₂ catalyst and had inferior activity than the CuO/CeO₂ catalyst when they were both calcined at 600 °C.     

Highly active and coking resistant Ni/CeO2-ZrO2 catalyst for partial oxidation of methane

Nickel catalysts over the CeO₂-ZrO₂ solid solution were successfully prepared by the co-precipitation method for partial oxidation of methane. The structures of the catalysts were systematically examined by N₂ adsorption/desorption, CO chemisorption, X-ray diffraction (XRD) and H₂-TPR techniques. The catalytic performance and carbon deposition were investigated for partial oxidation of methane as well. The results showed that the Ni/CeO₂-ZrO₂ catalysts had a large BET area and fine Ni dispersion. By the co-precipitation method, Ni and CeO₂-ZrO₂ solid solution had strong interaction confirmed by the H₂-TPR analysis. The Ni/CeO₂-ZrO₂ catalysts showed high activity and stability and the Ni/Ce_0.25Zr_0.75O₂ exhibited the best activity and coking resistance among these catalysts. The catalytic activities and coking resistant behaviors of catalysts were affected by the surface and structural properties of the catalysts.     

Absorption Spectra and Bioactivity Behavior of Gamma Irradiated CeO2-Doped Bioglasses

The synthesis and characterization of some bioglasses based on Hench’s Bioglass® 45S5 with additions of CeO₂ (1.0, 2.0, or 3.0%) have been carried out. Two objectives have been focused upon; first, the effect of successive ionizing gamma irradiation on the undoped and CeO₂-doped bioglass samples has been evaluated with the aim of justifying the role of the rare earth oxide CeO₂ on gamma irradiation. The second objective was directed to test the bioactivity of such prepared CeO₂-doped samples after soaking for 1 month in a simulated body fluid at 37 °C. The results indicate that the additions of CeO₂ suppress, to a marked extent, the generation of radiation induced defects especially in the visible region. The bioactivity results show that the studied CeO₂-doped bioglass samples gave rise to a calcium phosphate surface layer upon immersion in a simulated body fluid for 1 month at 37 °C and the bioactivity extent was almost identical in the CeO₂ doping interval limit (1?→?3% CeO₂) to that of the undoped base Hench’s Bioglass. The presence of both Ce³⁺ and Ce⁴⁺ ions were confirmed by optical absorption spectra. Electron spin resonance (ESR) studies of gamma irradiated CeO₂-doped glasses indicate and confirm the dominance of Ce⁴⁺ in the bioglass compositions and its transformation to Ce³⁺ by high gamma irradiation.     

Bio-template synthesis of CeO2 ultrathin nanosheets for highly selective and sensitive detection of ppb-level p-xylene vapor

Highly selective of carcinogenic and flammable p-xylene vapor and its sensing detection through metal oxides-based sensors has recently attracted much attention. In this work, mesoporous CeO₂ nanosheets were synthesized by simple cerium nitrate impregnation and air calcination using rose petals as bio-template. The effect of calcination temperature on its microstructure, Ce³⁺/Ce⁴⁺ mole ratio, as well as sensing performance was investigated. The CeO₂-650 ultrathin nanosheets calcined at 650 °C are assembled by cross-linking nanoparticles with small size, which possess homogeneous mesoporous distribution and relatively large specific surface area. At 217 °C, the sensor fabricated from CeO₂-650 ultrathin nanosheets shows short response time (T_res = 5 s), high selectivity and response (S = 22.1) towards 100 ppm p-xylene vapor, and its limit of detection (30 ppb) is the lowest among reported sensors based on pure metal oxides. The good sensing performance mainly originate from the synergistic effect of intrinsic features of mesoporous CeO₂-650 ultrathin nanosheets, surface adsorbed oxygen control, oxygen vacancy defects induced by Ce³⁺ and biotemplate imprinting. Therefore, mesoporous CeO₂-650 ultrathin nanosheets could be utilized as candidate for the detection of trace p-xylene vapor.     

Synthesis,characteristics, and redox properties of Zr/Sn-doped CeO2 nanoparticles in H2 gas at high temperatures

Metal(M = Zr, Sn)-doped CeO₂ nanoparticles were synthesized by a hydrothermal process to develop PdO@M-doped CeO₂ catalysts. The average particle size of both M-doped CeO₂ nanoparticles was under 10 nm, whereas the particle size reduced as the dopant concentration increased in the M-doped CeO₂ nanoparticles. The largest specific surface area was 226 m²/g in the Zr-doped CeO₂ nanoparticles. The particle morphology showed a spherical shape in both M-doped CeO₂ nanoparticles. The PdO@M-doped CeO₂ catalysts were then prepared by adsorbing Pd(OH)₂ onto the surface of M-doped CeO₂ nanoparticles by a precipitation method and synthesizing the catalysts by calcining at 500 °C for 3 h. The H₂ consumptions of the PdO@M-doped CeO₂ catalysts were characterized as the oxygen storage capacity at various temperatures. The results show that the oxygen storage capacities of the PdO@M-doped CeO₂ catalysts are superior to that of the pure CeO₂ catalyst at temperatures higher than 550 °C. The oxygen storage capacity of the PdO@Sn-doped CeO₂ catalyst is better than that of the PdO@Zr-doped CeO₂ catalyst.     

TiO2 Supported Nano-Au Catalysts Prepared Via Solvated Metal Atom Impregnation for Low–Temperature CO Oxidation

The Ce _x Ti_1?x O₂ mixed oxides at different mole ratios (x=0.1–1.0) were prepared by co-precipitation of TiCl₄ and Ce(NO₃)₃. The structural and reductive properties of the Ce _x Ti_{1? x} O₂ were affected by calcination temperature. At x=0.1–0.3, CeTi₂O₆ phase was formed and mainly as amorphous after calcination at 650°C. At x=0.3, only CeTi₂O₆ was formed after calcination at 750°C and CeTi₂O₆ crystallized completely after calcination at 800°C. TPR analyses showed that the amount of H₂ consumption by Ce _x Ti_1?xO₂ (650°C) (except x=0.1) was greater than that by single CeO₂, and the valence of CeO₂was the lowest (+3.18) at x=0.3. CuO/Ce_0.3Ti_0.7O₂ was prepared by the impregnation method and catalytic properties were examined by means of a GC micro-reactor NO+CO reaction system, BET, TPR, XRD, XPS and NO-TPD. It was found that CuO/Ce_0.3Ti_0.7O₂ calcined at 650°C had the highest activity in NO+CO reaction with 100% NO conversion at reaction temperature of 300°C, and at 650°C Ce_0.3Ti_0.7O₂just began to crystallize. The catalytic activities were largely affected by the pre-treatment conditions. At low reduction temperature (100°C), CuO species was difficult to reduce. When high degree of reductions took place, both CuO species and Ce_0.3Ti_0.7O₂ reduced and thus a part of CuO species on the support surface would be covered. The XPS and NO-TPD analyses showed that CuO/Ce_0.3Ti_0.7O₂ had four NO absorption centers (Cu⁺, Cu²⁺(I), Cu²⁺(II) and Ce³⁺). The CuO species involving in NO+CO reaction included Cu²⁺(I) and Cu⁺, and CeO₂ species (Ce³⁺ and Ce⁴⁺).     

A novel CeO2–xSnO2/Ce2Sn2O7 pyrochlore cycle for enhanced solar thermochemical water splitting

A novel CeO₂–xSnO₂/Ce₂Sn₂O₇ pyrochlore stoichiometric redox cycle with superior H₂ production capacities is identified and corroborated for two‐step solar thermochemical water splitting (STWS). During the first thermal reduction step (1400°C), a reaction between CeO₂ and SnO₂ occurred for all the CeO₂–xSnO₂ (x = 0.05–0.20) solid compounds, forming thermodynamically stable Ce₂Sn₂O₇ pyrochlore rather than metastable CeO_2‐δ. Consequently, substantially higher reduction extents were achieved owing to the reduction of Ce^IV to Ce^III. Moreover, in the subsequent reoxidation with H₂O (800°C), H₂ production capacities increased by a factor of 3.8 as compared to the current benchmark material ceria when x = 0.15, with the regeneration of CeO₂ and SnO₂ and the concomitant reoxidation of Ce^III to Ce^IV. The H₂O‐splitting performance for CeO₂–0.15SnO₂ was reproducible over seven consecutive redox cycles, indicating the material was also robust. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3450–3462, 2017     

Reduction of CO<Subscript>2</Subscript> to CO via reverse water-gas shift reaction over CeO<Subscript>2</Subscript> catalyst

CeO₂ catalysts with different structure were prepared by hard-template (Ce-HT), complex (Ce-CA), and precipitation methods (Ce-PC), and their performance in CO₂ reverse water gas shift (RWGS) reaction was investigated. The catalysts were characterized using XRD, TEM, BET, H₂-TPR, and in-situ XPS. The results indicated that the structure of CeO₂ catalysts was significantly affected by the preparation method. The porous structure and large specific surface area enhanced the catalytic activity of the studied CeO₂ catalysts. Oxygen vacancies as active sites were formed in the CeO₂ catalysts by H₂ reduction at 400 °C. The Ce-HT, Ce-CA, and Ce-PC catalysts have a 100% CO selectivity and CO₂ conversion at 580 °C was 15.9%, 9.3%, and 12.7%, respectively. The highest CO₂ RWGS reaction catalytic activity for the Ce-HT catalyst was related to the porous structure, large specific surface area (144.9 m²?g^?1) and formed abundant oxygen vacancies.     

Visible light-irradiated degradation of alachlor on Fe-TiO<Subscript>2</Subscript> with assistance of H<Subscript>2</Subscript>O<Subscript>2</Subscript>

0.1 Fe/Ti mole ratio of Fe-TiO₂ catalysts were synthesized via solvothermal method and calcined at various temperatures: 300, 400, and 500 °C. The calcined catalysts were characterized by XRD, N₂-adsorption-desorption, UV-DRS, XRF, and Zeta potential and tested for photocatalytic degradation of alachlor under visible light. The calcined catalysts consisted only of anatase phase. The BET specific surface area decreased with the calcination temperatures. The doping Fe ion induced a red shift of absorption capacity from UV to the visible region. The Fe-TiO₂ calcined at 400 °C showed the highest photocatalytic activity on degradation of alachlor with assistance of 30 mM H₂O₂ at pH 3 under visible light irradiation. The degradation fitted well with Langmuir-Hinshelwood model that gave adsorption coefficient and the reaction rate constant of 0.683 L mg⁻¹ and 0.136 mg/L·min, respectively.     

Synergic enhancement effect of zirconium oxide,cerium oxide and iron or cobalt oxide on the formation of isobutene from CO and H2

Fe₂O₃ or CoO modified CeO₂-ZrO₂ catalysts lead to four times higher yield of hydrocarbons than ZrO₂ with C₄ hydrocarbons selectivity of more than 50% and isobutene selectivity of more than 80%. XRD and XPS measurements suggested that the interaction of Fe or Co oxide with CeO₂ causes the higher Ce³⁺ concentration with their higher oxidation state. Their combination with ZrO₂ synergistically causes the active and selective formation of isobutene.     

Synthesis of nanocrystalline solid solutions based on zirconia and hafnia

Nanosized (2–8 nm) amorphous powders of the solid solution based on zirconia and hafnia are synthesized through back coprecipitation upon treatment of gels at temperatures from +20 to −77°C. Heat treatment of these powders at temperatures up to 1000 and above 1100°C leads to the formation of cubic (fluorite type, O _h ⁵ = Fm3m) and tetragonal phases of the Zr₈₂Hf₁₀Y₃Ce₅O _x composition, respectively. It is revealed that a decrease in the synthesis temperature (from +20°C to −6°C) results in a decrease in the size of gel agglomerates from 30 to 1 μm. Recrystallization processes in the gels prepared using cryochemical treatment are developed very slowly in the temperature range 500–1200°C (the crystallite size does not exceed 25 nm). Original Russian Text ? T.I. Panova, V.B. Glushkova, A.E. Lapshin, 2008, published in Fizika i Khimiya Stekla.     

The influence of chelating agents on cerium oxide decorated on graphite synthesized by the hydrothermal route

Morphology features of cerium oxide nanoparticles, such as size and agglomeration, are important as a coating that improves corrosion resistance and as reinforcement in mechanical applications. In this work, the influence of two heat treatments (160° and 190 °C) in combination with three different chelating agents in the preparation of CeO₂ and CeO₂ decorated on graphite (CeO₂_Gr) nanoparticles is studied. The novelty of this work is that CeO₂_Gr was successfully prepared using the hydrothermal method. All the samples evaluated by X-ray diffraction exhibit a single fluorite-type structure in the cubic phase and Fm3m space group. The spherical harmonics method using the Fullprof Suite program was used to determine the average crystallite sizes, which were 9 nm for CeO₂ and 7 nm for CeO₂_Gr. Transmission electron micrographs for the prepared samples with citric acid showed non-agglomerate particles with homogeneous particle sizes and a quasi-spherical shape distribution. Raman spectra show a band centre at 600 cm^-1 associated with the presence of Frenkel-type oxygen vacancies that induced the reduction of Ce⁴⁺ to Ce³⁺. The analysis of X-ray photoelectron spectra corroborates the coexistence of Ce³⁺ and Ce⁴⁺ species for CeO₂ and CeO₂_Gr nanoparticles. This work forms new perspectives in the development of CeO₂ decorated on graphite prepared by the hydrothermal method to obtain composites not only for sensing applications and wastewater treatment but also for corrosion resistance and reinforcement materials.     

In-situ XPS Study on the Reducibility of Pd-Promoted Cu/CeO2 Catalysts for the Oxygen-assisted Water-gas-shift Reaction

Cu/CeO₂, Pd/CeO₂, and CuPd/CeO₂ catalysts were prepared and their reduction followed by in-situ XPS in order to explore promoter and support interactions in a bimetallic CuPd/CeO₂ catalyst effective for the oxygen-assisted water-gas-shift (OWGS) reaction. Mutual interactions between Cu, Pd, and CeO₂ components all affect the reduction process. Addition of only 1 wt% Pd to 30 wt% Cu/CeO₂ greatly enhances the reducibility of both dispersed CuO and ceria support. In-vacuo reduction (inside XPS chamber) up to 400 °C results in a continuous growth of metallic copper and Ce³⁺ surface species, although higher temperatures results in support reoxidation. Supported copper in turn destabilizes metallic palladium metal with respect to PdO, this mutual perturbation indicating a strong intimate interaction between the Cu–Pd components. Despite its lower intrinsic reactivity towards OWGS, palladium addition at only 1 wt% loading significantly improved CO conversion in OWGS reaction over a monometallic 30 wt% Cu/CeO₂ catalysts, possibly by helping to maintain Cu in a reduced state during reaction.     

Hierarchical,template-free self-assembly morphologies in CeO2 synthesized via urea-hydrothermal method

Nano-crystalline CeO₂ was synthesized via the urea-hydrothermal method without templates or structure-directing agents. The synthesis parameters Ce³⁺ to Ce⁴⁺ and urea to cation molar ratios, reaction temperature and reaction time were varied to analyze their effect on morphology, texture and reducibility. The analysis of the obtained morphologies provides strong evidence of a hierarchical and sequential template-free self-assembly process that evolves from shuttles to dumbbells to spheres. In all cases, the morphology of samples remains unchanged even after calcination at 500 °C. The presence of Ce⁴⁺ in the initial solution clearly provides the full self-assembly sequence and is decisive for obtaining non-hollow spheres of CeO₂ with high specific surface area and high pore volume. Besides, if only Ce³⁺ is present, typical CeOHCO₃ shuttle-like particles with orthorhombic structure are obtained. The use of Ce³⁺ in combination with Ce⁴⁺ produces partial sequences of the self-assembly process that provide a strong indication of the hierarchical sequence.The urea to cation molar ratio controls the nucleation process and proves to be crucial to obtain the self-assembly sequence. On the other hand, temperature and reaction time show a moderate effect on morphology.     

Low-Temperature Water Gas Shift Reaction on Combustion Synthesized Ce1−x Pt x O2−δ Catalyst

Catalytic activity of Pt²⁺ ion substituted CeO₂ synthesized by solution combustion method was tested for low-temperature water gas shift reaction in H₂ rich steam reformate. XPS studies show that Pt is dispersed as ions and there is no change in Pt oxidation state after the reaction. CO conversion is found to be maximum at 200 °C over Ce_1?x Pt _x O_2?δ catalysts without any methanation. The values of rate are 1.86 and 4.66 μmol/g/s at 125 and 150°C respectively with a dry gas flow rate of 6 Lh^?1 over 2% Pt/CeO₂.     

Ceria-based Catalysts for the Production of H2 Through the Water-gas-shift Reaction: Time-resolved XRD and XAFS Studies

Hydrogen is a potential alternate energy source for satisfying many of our energy needs. In this work, we studied H₂ production from the water-gas-shift (WGS) reaction over Ce_1?x Cu _x O₂ catalysts, prepared with a novel microemulsion method, using two synchrotron-based techniques: time-resolved X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS). The results are compared with those reported for conventional CuO _x /CeO₂ and AuO _x /CeO₂ catalysts obtained through impregnation of ceria. For the fresh Ce_1?x Cu _x O₂ catalysts, the results of XAFS measurements at the Cu K-edge indicate that Cu is in an oxidation state higher than in CuO. Nevertheless, under WGS reaction conditions the Ce_1?x Cu _x O₂ catalysts undergo reduction and the active phase contains very small particles of metallic Cu and CeO_2?x . Time-resolved XRD and XAFS results also indicate that Cu^δ+ and Au^δ+ species present in fresh CuO _x /CeO₂ and AuO _x /CeO₂ catalysts do not survive above 200 °C under the WGS conditions. In all these systems, the ceria lattice displayed a significant increase after exposure to CO and a decrease in H₂O, indicating that CO reduced ceria while H₂O oxidized it. Our data suggest that H₂O dissociation occurred on the O_vacancy sites or the Cu–O_vacancy and Au–O_vacancy interfaces. The rate of H₂ generation by a Ce_0.95Cu_0.05O₂ catalyst was comparable to that of a 5 wt% CuO _x /CeO₂ catalyst and much bigger than those of pure ceria or CuO.