High-temperature Water–Gas Shift catalysts for hydrogen enrichment of a gas produced by biomass steam gasification

To improve the hydrogen content of a biomass steam gasification syngas, Water–Gas Shift Fe/CeO₂ catalysts supported on ceramic foams were developed. The impregnation of ceria as washcoat led to an increase in the support surface area (BET) and to the formation of well-dispersed iron particles (XRD and TPR) by iron oxide impregnation. Catalytic tests were performed at atmospheric pressure with minor pressure drops, under a gas mixture similar to that produced at the gasifier outlet. A satisfactory CO conversion and a large increase in H₂ content were reached by adjusting the operating parameters of the WGS and the catalysts’ composition. After-test characterizations indicated in situ catalysts activation with no over-reduction and a positive action of ceria on iron dispersion and sintering prevention.     

乙二醇溶剂热合成的CeO2的可逆氧化还原性及CO2捕获性能

利用乙二醇的还原性,采用乙二醇溶剂热法制备了表面具有丰富氧空穴的CeO₂-GST纳米晶,对其进行了X射线衍射、透射电镜、X射线光电子能谱、原位H₂还原-O₂氧化循环和CO₂原位红外漫反射表征,并研究了其可逆氧化还原性及CO₂捕获性能. 结果表明,与CeO₂-nanorod和柠檬酸溶胶法合成的CeO₂-CA样品相比,CeO₂-GST纳米晶具有最好的可逆氧化还原性能和循环稳定性,同时在50 ℃下具有最好的CO₂吸附性能（149 μmol/g）. 利用原位红外漫反射光谱研究了CO₂在还原CeO₂表面的吸附情况,发现CO₂主要以双齿碳酸盐和桥连碳酸盐两种形式吸附在CeO₂表面,其中桥连碳酸盐物种不稳定,He吹扫可脱附. 此外,CO₂在CeO₂-nanorod上还会生成稳定的甲酸盐和单齿碳酸盐物种.     

Co3O4/Ce0.8Pr0.2O2催化剂用于乙醇水蒸气重整反应的研究

采用共沉淀法制备了Co3O4/Ce0.8Pr0.2O2催化剂,并将其用于乙醇水蒸气重整制氢反应,考察了Co3O4负载量以及Pr掺杂对催化剂性能的影响.采用X射线衍射、程序升温还原、热重分析和透射电子显微镜对催化剂的结构和表面性质进行表征.结果表明,催化剂中部分Co进入到载体的晶格中,使载体发生畸变产生更多的氧空位;载体中Pr掺杂有利于生成更多的氧空位,提高了催化剂的抗积碳性能,同时Pr掺杂可以增强Co3O4与载体之间的相互作用,提高金属Co的抗烧结性能;15%Co3O4/Ce0.8Pr0.2O2催化剂具有最好的催化活性,在反应温度为400℃,空速80 000 mL/(g.h),n(H2O)∶n(EtOH)=3的条件下可将乙醇完全转化;10 h稳定性测试结果表明该催化剂具有较好的稳定性.     

Low-temperature and ambient-pressure synthesis and shape evolution of nanocrystalline pure, La-doped and Gd-doped CeO2

Nanocrystalline cuboidal ceria has been synthesized by low-temperature hydrothermal reaction of cerium nitrate hexahydrate with hexamethylene tetramine. The particles have been doped with La and Gd by adding aqueous solution of the nitrate salts of the metals to the reaction mixture. The pure and doped particles are cubic in crystal structure and 10-25 nm in size. The pure and La-doped ceria are cuboidal in morphology, whereas the Gd-doped particles are irregular in shape. High-resolution TEM imaging and image simulation indicates that atomic level steps are present on the particle surfaces. The particles are faceted parallel to the {1 1 1} and {1 0 0} crystallographic planes and a continuous switching takes place between the two possible surface facets. It appears that the surface energies of the {1 1 1} and {1 0 0} facets are quite similar in magnitude and the interplay of surface energy determines the particle shape. Chemically sensitive imaging and spectroscopy shows that the dopants are homogeneously distributed within the particles and that the oxidation state of Ce is a mixture of +3 and +4. No preferential segregation either of the dopant or the oxidation state was observed. However, since the facet switching does depend on the chemistry of the dopant, there must be an affect on the atomic scale.     

Vacancy formation and CO adsorption on gold-doped ceria surfaces

We study the effect of gold doping on oxygen vacancy formation and CO adsorption on the (1 1 0) and (1 0 0) surfaces of ceria by using density functional theory, corrected for on-site Coulomb interactions (DFT + U). The Au dopant substitutes a Ce atom in the surface layer, leading to strong structural distortions. The formation of one oxygen vacancy near a dopant atom is energetically “downhill” while the formation of a second vacancy around the same dopant requires energy. When the surface is in equilibrium with gaseous oxygen at 1 atm and room temperature there is a 0.4 probability that no oxygen atom left the neighborhood of a dopant. This means that the sites where the dopant has not lost oxygen are very active in oxidation reactions. Above 400 K almost all dopants have an oxygen vacancy next to them and an oxidation reaction in such a system takes place by creating a second vacancy. The energy required to form a second vacancy is smaller on (1 1 0) than on (1 0 0). On the (1 1 0) surface, it is much easier to form a second vacancy on the doped surface than the first vacancy on the undoped surface. The energy required to form a second oxygen vacancy on (1 0 0) is comparable to that of forming the first vacancy on the undoped surface. Thus doping makes the (1 1 0) surface a better oxidant but it has a small effect on the oxidative power of the (1 0 0) surface. On the (1 1 0) surface CO adsorption results in formation of a carbonate-like structure, similar to the undoped surface, while on the (1 0 0) surface direct formation of CO₂ is observed, in contrast to the undoped surface. The Au dopant weakens the bond of the surrounding oxygen atoms to the oxide making it a better oxidant, facilitating CO oxidation.     

Sn interaction with the CeO2(1 1 1) system: Bimetallic bonding and ceria reduction

A tin layer 0.8 nm thick was deposited onto the CeO₂(1 1 1) surface by molecular beam deposition at a temperature of 520 K. The interaction of tin with cerium oxide (ceria) was investigated by X-ray photoelectron spectroscopy (XPS), ultra-violet photoelectron spectroscopy (UPS) and resonant photoelectron spectroscopy (RPES). The strong tin-ceria interaction led to the formation of a homogeneous bulk Ce-Sn-O mixed oxide system. The bulk compound formation is accompanied by partial Ce⁴⁺ → Ce³⁺ reduction, observed as a giant 4f resonance enhancement of the Ce³⁺ species. CeO₂ and SnO₂ oxides were formed after oxygen treatment at 520 K. The study proved the existence of strong Ce-Sn interaction and charge transfer from Sn to the Ce-O complex that lead to a weakening of the cerium-oxygen bond, and consequently, to the formation of oxygen deficient active sites on the ceria surface. This behavior can be a key for understanding the higher catalytic activity of the SnO_x/CeO_x mixed oxide catalysts as compared with the individual pure oxides.     

CO oxidation over ceria supported Au22 nanoclusters: Shape effect of the support

CO oxidation over ceria-supported Au₂₂ nanoclusters shows strong dependence on the support shape: the lattice oxygen in CeO₂ rods is more reactive than in the cubes and thus make rods a superior support for Au nanoclusters in catalyzing low temperature CO oxidation.     

Synthesis and catalytic properties of microemulsion-derived cerium oxide nanoparticles

The synthesis of cerium dioxide nanoparticles using an inverse microemulsion technique and precipitation method was investigated. Cerium hydroxide nanoparticles were synthesized by adding diluted ammonia to n-heptane–surfactant–cerium nitrate system. The micelle and particle size in the range of 5–12 nm were controlled by varying the molar water to surfactant ratio and analyzed by dynamic light scattering (DLS), small angle X-ray scattering (SAXS) and high-resolution transmission electron microscopy (HRTEM). Cerium hydroxide nanoparticles were isolated and subsequently treated at 100–600 °C to obtain nanoscale ceria. Crystallite sizes of cerium dioxide in the range of 6–16 nm were estimated by Scherrer analysis by X-ray diffraction (XRD) and HRTEM. The catalytic activity of particles annealed at 400 and 600 °C in soot combustion reactions was characterized by temperature-programmed oxidation (TPO) indicating a size-dependant activity. Crystallite sizes and catalytic stability of elevated ceria systems were tested in second combustion cycles.