The effect of incorporation of iron in cerium oxide structure on reducibility and catalytic performance for methane oxidation in diluted streams

Iron-doped ceria solids were synthesized through the citrate route. The incorporation of iron in the structure of ceria produced changes in the shape of the crystallites constituting the solid particles and the pore structure, improved the reducibility and enhanced the catalytic performance of the samples in the conversion of methane in diluted conditions. The solid with nominal composition Ce_0.9Fe_0.1O_1.95 was the most active catalyst in the oxidation of diluted methane streams, achieving 55% of methane conversion at 700 °C.     

TWC deactivation by lead: A study of the Rh/CeO2 system

The effect of the incorporation of low amounts of Pb to a Rh/CeO₂ catalyst on the typical reactions involved in Three Way Catalysts converters was investigated. The catalytic tests showed no deactivation for CO oxidation and NO reduction whereas for propylene oxidation the deactivation was evident and more intense with the increasing of Pb concentration. The interpretation of this behaviour is discussed on the basis of HREM combined with XEDS analysis, TPR and DRIFT spectroscopy of chemisorbed CO studies. The results provided clear evidence that Pb is not covering the Rh particles neither forming a new phase with Rh nor altering the dispersion of the Rh particles. They also suggest that a mixed oxide is formed in which lead cations are dissolved within the fluorite-like ceria structure. Thus, CeO₂ support may act as sink of Pb preventing the formation of a Rh–Pb compound that explains why the CO oxidation and the NO reduction were only slightly affected. However, an electronic interaction between the Rh particles and the Pb-containing CeO₂ support is evidenced by TPR and DRIFTS-CO chemisorption data: the Pb present in the Rh-CeO₂ interphase as a result of the Pb dissolution in the ceria framework is proposed to be responsible for the electronic donation to Rh metal particles. The possibility that the electronic interaction and/or the modification of the oxygen storage capacity (OSC) of the ceria, both caused by the Pb, bring about the deactivation in the oxidation of propylene is also discussed.     

Reverse water–gas shift reaction over ceria nanocube synthesized by hydrothermal method

A well-defined ceria nanocube with six (100) planes was successfully prepared by a facile hydrothermal method. Hydrogenation tests on the carbon dioxide, and several advanced analysis techniques, were used to investigate the catalytic performance of ceria nanocube for reverse water–gas shift (RWGS) and understand the governing reaction mechanism. The results demonstrated that the obtained ceria was a typical mesoporous material with a fluorite structure, and mainly had cerium with + 4 valence oxidation state. As-obtained ceria nanocube showed good performance for RWGS reaction, while nickel on ceria evidently promoted the hydrogenation of CO₂. An oxygen-transformation and metal-dissociation mechanism for RWGS reaction was proposed. The dissociation of carbon dioxide over ceria by directly oxidized oxygen vacancy was considered as a main reaction pathway of RWGS. Meanwhile, dissociated adsorption of CO₂ and hydrogen over nickel surface directly formed CO and supplied spillover hydrogen to nearby oxygen vacancies, respectively. The neighboring oxygen vacancies at the interface of nickel and ceria were considered as efficient active sites for CO₂ hydrogenation.     

Catalytic activity of ethylene oxidation over Au,Ag and Au–Ag catalysts: Support effect

Au, Ag and Au–Ag catalysts on different supports of alumina, titania and ceria were studied for their catalytic activity of ethylene oxidation reactions. An addition of an appropriate amount of Au on Ag/Al₂O₃ catalyst was found to enhance the catalytic activity of the ethylene epoxidation reaction because Au acts as a diluting agent on the Ag surface creating new single silver sites which favor molecular oxygen adsorption. The Ag catalysts on both titania and ceria supports exhibited very poor catalytic activity toward the epoxidation reaction of ethylene, so pure Au catalysts on these two supports were investigated. The Au/TiO₂ catalysts provided the highest selectivity of ethylene oxide with relatively low ethylene conversion whereas, the Au/CeO₂ catalysts was shown to favor the total oxidation reaction over the epoxidation reaction at very low temperatures. In comparisons among the studied catalysts, the bimetallic Au–Ag/Al₂O₃ catalyst is the best candidate for the ethylene epoxidation. The catalytic activity of the gold catalysts was found to depend on the support material and catalyst preparation method which govern the Au particle size and the interaction between the Au particles and the support.     

Ceria‐coated diesel particulate filters for continuous regeneration

The potential of diesel particulate filters wash‐coated with highly dispersed nano‐metric ceria particles for continuous regeneration has been investigated. To this end, catalytic filters were prepared, soot‐loaded (avoiding the formation of the cake layer), and regenerated—under isothermal conditions—at temperature ranging from 200–600°C. Results have shown that catalytic oxidation of soot starts from 300°C and, at all temperatures, the selectivity to CO₂ is higher than 99%. 475°C is the minimum temperature at which the filter is regenerated via catalytic path. At this temperature, the catalytic filter maintains substantially the same performance over repeated cycles of soot loading and regeneration, indicating that the thermal stability of ceria is preserved. This has been further confirmed by comparison between the outcomes obtained from characterization (X‐ray powder diffraction, N₂ adsorption at 77 K, Hg intrusion porosimetry, and scanning electron microscope/energy dispersive X‐ray analysis) of fresh filter and filter subjected to repeated regeneration tests. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3442–3449, 2017     

Growth of Pd particles in methanol synthesis over Pd/CeO2

The metal–support contact structure of Pd–CeO₂ changed with increasing the temperature of reduction. Upon high temperature reduction, severe sintering of Pd particles occurred, while sintering of the ceria support was marginal. The catalytic deactivation of the Pd/CeO₂ catalysts during methanol synthesis was caused by the further structural change in the Pd–CeO₂ contact under reaction conditions. Considerably large Pd particles (about 100 nm) were formed in the catalysts subjected to methanol synthesis, and there was a close correlation between the activity loss and the growth of the Pd particles. It was proposed that the structure of Pd–ceria contact shifted from small Pd clusters supported on ceria to sintered large Pd particles dispersed in a mass of ceria.     

The supported CeO2/Co3O4–MnO2/CeO2 catalyst on activated carbon prepared by a successive-loading approach with superior catalytic activity and selectivity for CO preferential oxidation in H2-rich stream

The supported CeO₂/Co₃O₄–MnO₂/CeO₂ catalyst on activated carbon (AC) prepared by a successive loading approach to support ceria, cobalt-manganese oxide and ceria on activated carbon exhibits superior catalytic activity and selectivity to Co₃O₄–MnO₂–CeO₂/AC prepared by a one-step loading for CO preferential oxidation in H₂-rich stream, although the same loading of Co, Mn and Ce was used, which illustrates that the addition of ceria doesn't always enhance catalytic performance in CO PROX reaction, and appreciate supporting method is essential. The superior catalytic activity and selectivity of developed catalyst can be ascribed to high reducibility, well dispersion, unique porous structure, and strong interaction between Co₃O₄–MnO₂ and CeO₂.     

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.     

改进柠檬酸络合法制备CuO-CeO2及其CO低温氧化催化性能

采用改进的柠檬酸络合法即以乙醇代替水作溶剂制备CuO-CeO₂催化剂，应用N₂物理吸附、XRD和H₂-TPR等技术对催化剂进行了表征，并采用微反-色谱装置考察了其对CO低温氧化反应的催化活性。结果表明，常规柠檬酸络合法所制备的CuO-CeO₂催化剂中仅存在一种与CeO₂相互作用较弱、粒子较大的CuO，而采用改进的柠檬酸络合法所制备的催化剂中除此之外还存在与CeO₂相互作用较强、粒子较小的高度分散的CuO，从而具有更高的CO低温氧化活性。     

Selective oxidation of soot over Cu doped ceria/ceria–zirconia catalysts

Copper doped ceria and ceria–zirconia mixed oxides were prepared using the citric acid sol–gel method. The temperature-programmed oxidation (TPO) results showed that the Cu modification helped to improve the activity and selectivity of ceria and ceria–zirconia for soot catalytic oxidation. The CO-TPR results showed that Cu–Ce had a better reducibility than pure ceria at low temperatures. After ageing at 800 °C for 20 h in flow air, CuO–CeO₂ showed the maximum soot oxidation rate at 378 and 519 °C under tight and loose contact conditions, respectively, achieving a nearly 100% selectivity to CO₂ production. This effect may be attributed to the existence of well dispersed copper oxide species strongly interacting with the ceria surface, which may decrease the activation energy of soot oxidation. A conceivable mechanism of this synergetic effect was proposed.     

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.     

Catalytic Carbon Oxidation in The Presence of Cerium Oxide: Experimental Study and Modeling of The Effect of Oxygen Concentration

The catalytic combustion of carbon black (CB) used as a model of diesel soot in tight contact with a commercial ceria (CeO₂) was investigated. Oxygen mole fractions of 10, 5, and 1 % in the gas phase were tested in order to gain a better understanding of the redox properties of ceria and the mechanism of the catalytic oxidation of carbon black. Both isothermal and temperature programmed runs are performed to extract kinetic parameters, such as activation energy and reaction order with respect to oxygen partial pressure. The experimental data are used to propose a model of CB oxidation in the presence of CeO₂ allowing the simulation of carbon oxidation.     

Reducibility of supported gold (III) precursors: influence of the metal oxide support and consequences for CO oxidation activity

The origin of CO oxidation performance variations between three different supported Au catalysts (Au/CeO₂, Au/Al₂O₃, Au/TiO₂) was examined by in situ XAFS and DRIFTS measurements. All samples were prepared identically, by deposition-precipitation of an aqueous Au(III) complex with urea, and contained the same gold loading (~1 wt %). The as-prepared supported Au(III) precursors exhibited different reduction behaviour during exposure to the CO/O₂/He reaction mixture at 298 K. The reducibility of the Au(III) precursor was found to decrease as a function of the support material in the order: titania > ceria > alumina. The as-prepared samples were inactive catalysts, but Au/TiO₂ and Au/CeO₂ developed catalytic activity as the reduction of Au(III) to metallic Au proceeded. Au/Al₂O₃ remained inactive. The developed catalytic CO oxidation activity at 298 K varied as a function of the support as follows: titania > ceria > alumina ~ 0. The EXAFS of samples pretreated in air at 773 K and in H₂ at 573 K reveals the presence of only metallic particles for Au/TiO₂ and Au/Al₂O₃. Au(III) supported on CeO₂ remains unreduced after calcination, but reduces during the treatment with H₂. CO oxidation experiments performed at 298 K with the activated samples show that the presence of metallic gold is necessary to obtain active catalysts (Au/CeO₂ is not active after calcination) and that the reducible supports facilitate the genesis of active catalysts, while metallic gold particles on alumina are not active.     

Characterization of ceria films on α-Al2O3(0001) and polycrystalline zirconia using O2 TPD with labeled 18O2

The nature of weakly bound oxygen on ceria films was studied using temperature-programmed desorption with labeled ¹⁸O₂. For α- Al₂O₃(0001)-supported ceria, a desorption feature between 800 and 1300 K is shown to result from partial reduction of ceria. However, this oxygen accounts for only a small fraction of the total oxygen in the ceria film and isotopic labeling studies suggest that this oxygen does not exchange freely with the remaining oxygen in the film. In contrast, results for zirconia-supported ceria demonstrate that much more oxygen desorbs in the low temperature regime below 1300 K, and that there is significant isotope exchange throughout the ceria film and with the zirconia substrate. Finally, exposure of reduced, zirconia-supported ceria to water at 670 K resulted in reoxidation of the ceria film. Oxygen from ceria was then shown to react with CO adsorbed on supported Rh particles, completing the catalytic cycle for the water-gas-shift reaction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Catalytic oxidation of saturated hydrocarbons on multicomponent Au/Al2O3 catalysts: Effect of various promoters

The performance of unpromoted and MO_x-(M: alkali (earth), transition metal and cerium) promoted Au/Al₂O₃ catalysts have been studied for combustion of the saturated hydrocarbons methane and propane. As expected, higher temperatures are required to oxidize CH₄ (above 400 °C), compared with C₃H₈ (above 250 °C). The addition of various MO_x to Au/Al₂O₃ improves the catalytic activity in both methane and propane oxidation. For methane oxidation, the most efficient promoters to enhance the catalytic performance of Au/Al₂O₃ are FeO_x and MnO_x. For C₃H₈ oxidation a direct relationship is found between the catalytic performance and the average size of the gold particles in the presence of alkali (earth) metal oxides. The effect of the gold particle size becomes less important for additives of the type of transition metal oxides and ceria. The results suggest that the role of the alkali (earth) metal oxides is related to the stabilization of the gold nanoparticles, whereas transition metal oxide and ceria additives may be involved in oxygen activation.     

A novel electrocatalytic polyaniline electrode for methanol oxidation

Pt clusters were electrodispersed on polymeric films to obtain catalytic electrodes for methanol oxidation. The electrodeposit was built up by applying either a constant potential or a repetitive square wave potential routine. The performance of the electrodes was followed by measuring the stripping peak potential of adsorbed CO, each assembly metal/Pani/Pt being characterized by SEM and EDAX. Polymeric electrodes, modified with Pt electrodeposited by the programmed potential variation had a better electrocatalytic activity for CO and methanol oxidation. The novel tailored electrode is the result of a balance between a particular morphology and the number of particles of the catalytic material on the conductive polyaniline matrix.     

Influence of preparation conditions of nano-crystalline ceria catalysts on the total oxidation of naphthalene, a model polycyclic aromatic hydrocarbon

Nano-crystalline ceria catalysts prepared by homogeneous precipitation with urea were tested for the total oxidation of naphthalene, a model polycyclic aromatic hydrocarbon (PAH). Systematic variation of preparation conditions, including calcination temperature, calcination time and aging time, resulted in differences in surface area, reducibility, morphology and crystallite size of the CeO₂ catalyst and hence differences in catalytic performance. A combination of high surface area, small crystallite size and high oxygen defect concentration was found to favor the efficiency of the ceria catalysts for the total oxidation of naphthalene. Optimum preparation conditions for this study included: aging time of 12 h, calcination temperature of 500 °C and a calcination time of 6 h.     

On the performance of porous platinized tungsten trioxide electrodes in the electrochemical oxidation of hydrogen

The performance of porous Pt-containing H₂ electrodes is found to be influenced by mass-transport effects. PtWO₃ electrodes are about twice as active as electrodes containing commercial PtC fuel cell catalysts in hydrogen oxidation. From our catalytic hydrogenation studies it follows that this difference in performance needs not to be caused by a synergistic effect between Pt and WO₃ with respect to H₂ oxidation. A more likely cause is a difference in effective conductivity, leading to reduced ohmic losses in PtWO₃ electrodes as compared with PtC electrodes.During this study we also investigated systems related to PtWO₃ in hydrogen oxidation. No interesting catalysts were found, however.     

An innovative approach to electro-oxidation of dopamine on titanium dioxide nanotubes electrode modified by gold particles

Au/TiO₂/Ti electrodes were prepared by galvanic deposition of gold particles from an acidic bath containing KAu(CN)₂ in the presence of a citrate buffer onto TiO₂ nanotubes layer on titanium substrates. Titanium oxide nanotubes were fabricated by anodizing titanium foil in a DMSO fluoride-containing electrolyte. The morphology and surface characteristics of Au/TiO₂/Ti electrodes were investigated using scanning electron microscopy and energy-dispersive X-ray, respectively. The results indicated that gold particles were homogeneously deposited on the surface of TiO₂ nanotubes. The nanotubular TiO₂ layers consist of individual tubes of about 40–80 nm diameters. The electro-catalytic behavior of Au/TiO₂/Ti electrodes for the dopamine electro-oxidation was studied by cyclic voltammetry and differential pulse voltammetry. The results showed that Au/TiO₂/Ti electrodes exhibit a considerably higher electro-catalytic activity toward the oxidation of dopamine. The catalytic oxidation peak current showed a linear dependence on dopamine concentration and a linear calibration curve was obtained in the concentration range of 0.5–2.5 mM of dopamine.     

Preferential CO oxidation in H2-rich gas mixtures over Au/doped ceria catalysts

Nanosized gold catalysts supported on doped ceria were prepared by deposition–precipitation method. A deep characterization study by HRTEM/EDS, XRD, FT-Raman, TPR and FTIR was undergone in order to investigate the effect of ceria modification by various cations (Sm³⁺, La³⁺ and Zn²⁺) on structural and redox properties of gold catalysts. Doping of ceria affected in different way catalytic activity towards purification of H₂ via preferential CO oxidation. The following activity order was observed: Au/Zn–CeO₂ > Au/Sm–CeO₂ > Au/CeO₂ > Au/La–CeO₂. The differences in CO oxidation rates were ascribed to different concentration of metallic gold particles on the surface of Au catalysts (as confirmed by the intensity of the band at 2103 cm⁻¹ in the FTIR spectra collected during CO–O₂ interaction). Gold catalysts on modified ceria showed improved tolerance towards the presence of CO₂ and H₂O in the PROX feed. The spectroscopic experiments evidence enhanced reactivity when PROX is performed in the presence of H₂O already at 90 K.