Copper oxide catalysts supported on ceria for low-temperature CO oxidation

Copper oxide catalysts supported on ceria were prepared by wet impregnation method using finely CeO₂ nanocrystals, which was derived from alcohothermal synthesis, and copper nitrate dissolved in the distilled water. The catalytic activity of the prepared CeO₂ and CuO/CeO₂ catalysts for low-temperature CO oxidation was investigated by means of a microreactor-GC system. The samples were characterized using BET, XRD, SEM, HRTEM and TPR.     

Comparison of CuO/Ce0.8Zr0.2O2 and CuO/CeO2 Catalysts for Low-temperature CO Oxidation

CuO/Ce_0.8Zr_0.2O₂ and CuO/CeO₂ catalysts were prepared via a impregnation method characterized by using FT-Raman, XRD, XPS and H₂-TPR technologies. The catalytic activity of the samples for low-temperature CO oxidation was investigated by means of a microreactor-GC system. The influence of the calcination temperature and different supports on the catalytic activity was studied.     

Gold supported CeO2/Al2O3 catalysts for CO oxidation: influence of the ceria phase

A series of low loading gold supported ceria/alumina catalysts have been prepared by the deposition–precipitation method, varying the pH of the synthesis. The catalysts were characterised by means of XRD, TEM, S_BET, XRF and UV–Vis techniques, and their catalytic activity towards CO oxidation in the absence and in presence of water in the stream, were tested. It has been found that in this low loading gold catalysts, where the metallic particles are far away one from another and the oxygen transportation is not the limiting step of the reaction, the electronic properties of the ceria phase and the structure of the metal-support perimeter more than the diameter of the gold nanoparticles is the determinant factor in the catalytic performances of the solid.     

Supported copper–ceria catalysts for low temperature CO oxidation

In this investigation, CuO/CeO₂–M_xO_y (M_xO_y = Al₂O₃, ZrO₂ and SiO₂) nanocomposite oxide catalysts were prepared by deposition-precipitation and wet impregnation methods, and evaluated for CO oxidation. Catalysts were characterized by XRD, TEM, UV–vis DRS, BET surface area and H₂-TPR techniques. The synthesized catalysts exhibited high specific surface area, and uniform particle size distribution over the supports. The nanocrystalline texture of mixed metal oxides is clearly evidenced by TEM analysis. TPR and XRD results revealed synergetic interactions between copper oxide and ceria. Among various catalysts investigated, the CuO/CeO₂–Al₂O₃ combination exhibited excellent CO oxidation activity with T_1/2 = 374 K and 100% CO conversion at below 420 K.     

Preparation, Characterization and Catalytic Activity for CO Oxidation of SiO2 Hollow Spheres Supporting CuO Catalysts

Silica hollow spheres were synthesized by sol–gel process using carbon microspheres as templates, and used as supports for CuO/SiO₂ catalysts. The samples were characterized by TEM, nitrogen adsorption–desorption, XRD and TPR, and furthermore, the catalytic performance for CO oxidation was approached. The results indicated that the catalytic activity of CuO supported on SiO₂ hollow spheres exhibited much higher as compared to that supported on commercial SiO₂. Enhancement of the catalytic activity may be attributed to the fact that the unique hollow spherical texture should facilitate the formation of main active species and gas diffusion in catalysts.     

CuO–CeO2/Al2O3 /FeCrAl monolithic catalysts prepared by sol-pyrolysis method for preferential oxidation of carbon monoxide

This work describes a new technique, sol-pyrolysis method, for depositing CuO–CeO₂on FeCrAl honeycomb supports. The monolithic catalysts prepared by the method presented good adhesion stability in ultrasonic and thermal shock tests. The principle of the deposition and the role of the support were studied and analyzed by SEM, XRD, TG-DTA, TPR and XPS techniques. The results showed that the active components adhered to the support via three stages. High surface energy of the crystal nuclei and the interaction between the active components and the support promoted adhesion stability. Moreover, the presence of the support influenced distribution and interaction of the active components, but had no obvious effect on catalytic performance. The CuO–CeO₂/Al₂O₃/FeCrAl monolithic catalysts were applied for the preferential oxidation of carbon monoxide in rich-hydrogen gases and revealed high activity and good selectivity under the presence of 15%CO₂ and 10%H₂O.     

Tape casting of nanocrystalline ceria gadolinia powder

A ceramic ceria gadolinia solid solution membrane for solid oxide fuel cells was fabricated by tape casting using a nanopowder of 37 nm average particle size. A novel combination of solvent and dispersant was used to disperse the nanoparticles. The polymer was added in a dilute stage to guarantee a homogeneous distribution. After casting a remarkable densification of the cast tape suspension from a solid loading of 20 up to 42 vol.% was observed during drying. The green tape was sintered to >92% theoretical density and was dense towards perfusion. The resulting grain size in the sintered specimen still was <200 nm.     

Synthesis and characterization of Cu-doped ceria nanopowders

Nanopowdered solid solution Ce_1−xCu_xO_2−γ samples (0 ≤ x ≤ 0.15) were synthesized by self-propagating room temperature synthesis (SPRT). Raman spectroscopy and XRD at room temperature were used to study the vibration properties of these materials as well as the Cu solubility in ceria lattice. The solubility limit of Cu²⁺ in CeO₂ lattice was found to be lower than published in the literature. Results show that obtained powders with low dopant concentration are solid solutions with a fluorite-type crystal structure. However, with Cu content higher than 7.5 mass%, the phase separation was observed and two oxide phases, CeO₂ and CuO, coexist. All powders were nanometric in size with high specific surface area.     

Effect of Pretreatment Atmosphere on CuO/TiO2 Activities in NO + CO Reaction

Using TiO₂ as carrier, CuO/TiO₂ catalysts with different CuO loading were prepared by the impregnation method. The catalytic activities in NO+CO reaction were examined with a micro-reactor gas chromatography reaction system and the methods of TPR, XPS and NO-TPD. It was found that the catalytic activities were affected by pretreatment atmosphere, i.e. H₂ atmosphere > reduction–reoxidation > 10%CO/He > reaction gas (fresh sample). NO decomposition was better by low-valence Cu species than by high-valence Cu species, i.e. Cu⁰>Cu⁺>Cu²⁺. The XPS results indicated that Cu species on CuO/TiO₂ were Cu⁰, Cu⁺, normal Cu²⁺(Cu²⁺(I)) and chain-structured Cu²⁺(Cu²⁺(II)) as –Cu–O–Ti–O–. The activities of Cu²⁺(II) were much higher than that of Cu²⁺(I), but both species were very unstable in the reaction atmosphere and easily reduced by CO, which accounted for the variable activities of fresh catalysts with increasing reaction temperature. In NO+CO reaction, the redox process was a cycle of Cu⁺–Cu²⁺(I) at low reaction temperature but was a cycle of Cu⁰–Cu⁺ at high reaction temperature. As shown by NO-TPD, high catalytic activities could be attributed to the following factors, e.g. oxygen caves on the catalyst’s surface after pretreatment with H₂ and reduction–reoxidation, formation of Cu⁰ after pretreatment with H₂, and increment of Cu species dispersion and formation of Cu²⁺(II) after pretreatment with reduction–reoxidation.     

Effect of chlorine on Ir/CeO2 catalyst behavior for preferential CO oxidation

Effect of chlorine on Ir/CeO₂ catalyst behavior for preferential CO oxidation is investigated by high-resolution transmission electron microscopy, X-ray photoemission spectroscopy, and diffuse reflectance infrared spectroscopy. The presence of chlorine favors the dispersion of Ir particles. On ceria support, the replacement of the lattice oxygen by chloride ions would produce CeOCl species, which could hinder the formation of hydroxyl groups and carbonate and/or carboxylate species on the ceria surface. These features could explain the decreased activity of the Cl-containing Ir/CeO₂ sample.     

Methanol synthesis pathway over Cu/ZrO2 catalysts: a time‐resolved DRIFT 13C‐labelling experiment

X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) have been used to characterize a series of Cu/Ce/Al₂O₃ catalysts. Catalysts were prepared by incipient wetness impregnation using metal nitrate and alkoxide precursors. Catalyst loadings were held constant at 12 wt% CuO and 5.1 wt% CeO₂. Mixed oxide catalysts were prepared by impregnation of cerium first, followed by copper. The information obtained from surface and bulk characterization has been correlated with CO and CH₄ oxidation activity of the catalysts. Cu/Al₂O₃ catalysts prepared using Cu(II) nitrate (CuN) and Cu(II) ethoxide (CuA) precursors consist of a mixture of copper surface phase and crystalline CuO. The CuA catalyst shows higher dispersion, less crystalline CuO phase, and lower oxidation activity for CO and CH₄ than the CuN catalyst. For Cu/Ce/Al₂O₃ catalysts, Ce has little effect on the dispersion and crystallinity of the copper species. However, Cu impregnation decreases the Ce dispersion and increases the amount of crystalline CeO₂ present in the catalysts, particularly in Ce modified alumina prepared using cerium alkoxide precursor (CeA). Cerium addition dramatically increases the CO oxidation activity, however, it has little effect on CH₄ oxidation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Shape effect of microstructured CeO2 with various morphologies on CO catalytic oxidation

Novel 3D-like CeO₂ microflowers and microspheres were synthesized by facile precipitation methods and tested for CO oxidation. Characterization by UV-Vis, PL, XPS and H₂-TPR revealed that 3D-like ceria show great superiority for favoring formation of oxygen vacancies, which in turn leads to higher oxygen mobility and reducibility. A direct relationship between the band gap of ceria and its catalytic activity towards CO oxidation was established. The activity and thermal stability of 3D-like ceria were much better than that of CeO₂ nanoparticles. The results indicated that the highly active ceria could be obtained by turning their shapes with a high concentration of oxygen vacancies.     

High temperature desulfurization over nano-scale high surface area ceria for application in SOFC

In the present work, suitable absorbent material for high temperature desulfurization was investigated in order to apply internally in solid oxide fuel cells (SOFC). It was found that nano-scale high surface area CeO₂ has useful desulfurization activity and enables efficient removal of H2S from feed gas between 500 to 850°C. In this range of temperature, compared to the conventional low surface area CeO₂, 80–85% of H2S was removed by nano-scale high surface area CeO₂, whereas only 30–32% of H2S was removed by conventional low surface area CeO₂. According to the XRD studies, the product formed after desulfurization over nano-scale high surface area CeO₂ was Ce₂O₂S. EDS mapping also suggested the uniform distribution of sulfur on the surface of CeO₂. Regeneration experiments were then conducted by temperature programmed oxidation (TPO) experiment. Ce₂O₂S can be recovered to CeO₂ after exposure in the oxidation condition at temperature above 600°C. It should be noted that SO₂ is the product from this regeneration process. According to the SEM/EDS and XRD measurements, all Ce₂O₂S forming is converted to CeO₂ after oxidative regeneration. As the final step, a deactivation model considering the concentration and temperature dependencies on the desulfurization activity of CeO₂ was applied and the experimental results were fitted in this model for later application in the SOFC model.     

Morphology-dependent redox and catalytic properties of CeO2 nanostructures: Nanowires, nanorods and nanoparticles

The redox features and the catalytic activities of ceria nanowires, nanorods and nanoparticles were comparatively studied. The morphology-dependent phenomenon is closely related to the nature of the exposed crystal planes. The CeO₂ nanoparticles mainly expose the stable {1 1 1} plane on the surface, whereas the rod-shaped nanostructures preferentially expose the reactive {1 1 0} and {1 0 0} planes, giving higher oxygen storage capacity and catalytic activity for CO oxidation. Although both the CeO₂ nanorods and the CeO₂ nanowires predominantly expose the reactive {1 1 0} and {1 0 0} planes, the CeO₂ nanowires favor to expose a large proportion of active planes on the surface, resulting in a much higher activity for CO oxidation than the nanorods.     

Synthesis and characterization of CuO/TiO2 catalysts for low-temperature CO oxidation

In this paper, the CuO/TiO₂ catalysts prepared by the deposition–precipitation (DP) method were extensively investigated for CO oxidation reaction. The structural characters of the CuO/TiO₂ catalysts were comparatively investigated by TG-DTA, XRD, and XPS measurements. It was shown that the catalytic behavior of CuO/TiO₂ catalysts greatly depended on the TiO₂-support calcination temperature, the CuO loading amount and the CuO/TiO₂ catalysts calcination temperature. CuO supported on the anatase phase of TiO₂-support calcined at 400 °C showed better catalytic activity than those supported on TiO₂ calcined at 500 and 700 °C. Among all our investigated catalysts with CuO loading from 2% to 12%, the catalyst with 8 wt% CuO loading exhibited the highest catalytic activity. The optimum calcination temperature of the CuO/TiO₂ catalysts was 300 °C. The XRD results indicated that the catalytic activity of the CuO/TiO₂ catalysts was related to the crystal phase and particle size of TiO₂ support and CuO active component.     

Quantification of microstructural features in gadolinia-doped ceria containing co-additives by digital image analysis

A digital image processing and analysis method has been developed to evaluate the microstructural features of chemically synthesized gadolinia-doped ceria powders containing small amounts of co-dopants. The effects of particles/clusters size, porosity and grain size distribution were examined in detail and compared to those parameters of the standard composition without co-dopants. In addition, the effect of the co-dopants in the grain growth was clarified. Sr²⁺ and Bi³⁺ act as sintering aid for densification of the standard composition. The lowest mean grain size along with higher fraction of porosity was obtained for Na⁺ addition.     

Hydrogen production by oxidative steam reforming of methanol using ceria promoted copper–alumina catalysts

The performance of different Cu/CeO₂/Al₂O₃ catalysts of varying compositions is investigated for the oxidative steam reforming of methanol (OSRM) in order to produce the hydrogen selectively for polymer electrolyte membrane (PEM) fuel cell applications. All the catalysts were prepared by co-precipitation method and characterized for their surface area, pore volume and oxidation–reduction behavior. The effect of various operating parameters studied are as follows: reaction temperature (200–300 °C), contact-time (W/F = 3–15 kg_cat s mol^− 1) and oxygen to methanol (O/M) molar ratio (0–0.5). The steam to methanol (S/M) molar ratio = 1.5 and pressure = 1 atm were kept constant. Among all the catalysts studied, catalyst Cu–Ce–Al:30–20–50 exhibited 100% methanol conversion and 179 mmol s^− 1 kg_cat^− 1 hydrogen production rate at 280 °C with carbon monoxide formation as low as 0.19%. The high catalytic activity and hydrogen selectivity shown by ceria promoted Cu/Al₂O₃ catalysts is attributed to the improved specific surface area, dispersion and reducibility of copper which were confirmed by characterizing the catalysts through temperature programmed reduction (TPR), CO chemisorption, X-ray diffraction (XRD) and N₂ adsorption–desorption studies. Reaction parameters were optimized in order to produce hydrogen with carbon monoxide formation as low as possible. The time-on-stream stability test showed that the Cu/CeO₂/Al₂O₃ catalysts were quite stable.     

ZrO_2负载Cu催化剂对CO低温催化氧化研究

用溶胶-凝胶-超临界干燥法制备了纳米氧化锆。采用沉淀法制备氧化锆负载铜催化剂。制备的催化剂用X-ray射线衍射(XRD),透射电镜(TEM),比表面积(BET)和H2-TPR等进行了表征。研究了催化剂的焙烧温度和负载比例对CO转化效率的影响,其最佳焙烧温度为650℃,Zr与Cu的最佳物质的量比是10:8。获得催化剂在温度为68℃具有催化活性,176℃时CO的转化率达到50%,较好地实现了ZrO2负载Cu在较低温度下对CO的催化。     

Low-temperature activity for CO oxidation over diesel oxidation catalysts studied by High Throughput Screening and DRIFT spectroscopy

We have investigated the low-temperature activity for CO oxidation for a series of platinum catalysts supported on Al₂O₃, TiO₂, ZSM-5, CeO₂ and ZrO₂-CeO₂. The results show major differences in activity, due to the support for Pt, especially in the presence of water. Improved activity over ceria containing samples in presence of water is likely due to the water-gas shift (WGS) reaction. Studies with in situ IR spectroscopy suggest a surface formate mechanism for the WGS reaction on Pt/CeO₂.     

Controlled fabrication of Sn/TiO2 nanorods for photoelectrochemical water splitting

In this work, we investigate the controlled fabrication of Sn-doped TiO₂ nanorods (Sn/TiO₂ NRs) for photoelectrochemical water splitting. Sn is incorporated into the rutile TiO₂ nanorods with Sn/Ti molar ratios ranging from 0% to 3% by a simple solvothermal synthesis method. The obtained Sn/TiO₂ NRs are single crystalline with a rutile structure. The concentration of Sn in the final nanorods can be well controlled by adjusting the molar ratio of the precursors. Photoelectrochemical experiments are conducted to explore the photocatalytic activity of Sn/TiO₂ NRs with different doping levels. Under the illumination of solar simulator with the light intensity of 100 mW/cm², our measurements reveal that the photocurrent increases with increasing doping level and reaches the maximum value of 1.01 mA/cm² at −0.4 V versus Ag/AgCl, which corresponds to up to about 50% enhancement compared with the pristine TiO₂ NRs. The Mott-Schottky plots indicate that incorporation of Sn into TiO₂ nanorod can significantly increase the charge carrier density, leading to enhanced conductivity of the nanorod. Furthermore, we demonstrate that Sn/TiO₂ NRs can be a promising candidate for photoanode in photoelectrochemical water splitting because of their excellent chemical stability.