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.     

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.     

Effects of Pd particle size on the rates of oxygen back-spillover and CO oxidation under dynamic oxygen storage and release measurements over Pd/CeO2 catalysts

A kinetic mathematical model has been applied to investigate for the first time the effects of Pd particle size on the rates of oxygen back-spillover and CO oxidation during Oxygen Storage Capacity (OSC) measurements under dynamic conditions over Pd/CeO₂ catalysts in the 500–700 °C range. The dependence of the intrinsic rate constant k₁ of the CO oxidation reaction on PdO, and that of k ₂ ^app of the oxygen back-spillover from ceria to Pd/PdO on the palladium particle size was estimated by performing curve-fitting of the experimental CO and CO₂ pulse transient responses obtained. Activation energies of 8.0, 9.5 and 21.1 kJ/mol were calculated for the Eley–Rideal step of CO oxidation for the 1.3, 1.8 and 16.4 nm Pd particles, respectively, supported on CeO₂. The transient rates of CO oxidation and oxygen back-spillover were found to decrease with increasing Pd particle size.     

Water–Gas Shift Reaction Over Aluminum Promoted Cu/CeO2 Nanocatalysts Characterized by XRD, BET, TPR and Cyclic Voltammetry (CV)

A series of aluminum promoted Cu/CeO₂ nanocatalysts with aluminum content in the range of 0–5wt.% were prepared by co-precipitation method and examined with respect to their catalytic performance for the water–gas shift (WGS) reaction. The catalysts were characterized by XRD, BET, H₂-TPR and cyclic voltammetry (CV) techniques. The results indicate that catalytic activity increases with the aluminum content at first, but then decreases with the further increase of aluminum content. Hereinto, Cu/CeO₂ catalyst doped with 1 wt.% of aluminum shows the highest catalytic activity (CO conversion reaches 84.4% at 200 °C) and thermal stability for WGS reaction. Correlation to the results from above characterization, it is found that the variation of catalytic activity is in very agreement with that of the surface area, the area of peak γ (i.e., the reduction of surface copper oxide (crystalline forms) interacted with surface oxygen vacancies on ceria), and the area of peak C₂ and in cyclic voltammetry process), respectively. Enough evidence was found for the fact that the metallic copper (Cu⁰) interacted with surface oxygen vacancies on ceria is the active site for WGS reaction over Cu/CeO₂ catalysts.     

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.     

Structure-activity Relation of Fe2O3–CeO2 Composite Catalysts in CO Oxidation

A series of Fe₂O₃–CeO₂ composite catalysts were synthesized by coprecipitation and characterized by X-ray diffraction (XRD), BET surface area measurement, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Their catalytic activities in CO oxidation were also tested. The Fe₂O₃–CeO₂ composites with an Fe molar percentage below 0.3 form solid solutions with the CeO₂ cubic fluorite structure, in which the doped Fe³⁺ initially substitutes Ce⁴⁺ in fluorite cubic CeO₂, but then mostly locate in the interstitial sites after a critical concentration of doped Fe³⁺. With an Fe molar percentage between 0.3 and 0.95, the Fe₂O₃–CeO₂ composites are mixed oxides of the cubic fluorite CeO₂ solid solution and the hematite Fe₂O₃. XPS results indicate that CeO₂ is enriched in the surface region of Fe₂O₃–CeO₂ composites. The Fe₂O₃–CeO₂ composites have much higher catalytic activities in CO oxidation than the individual pure CeO₂ and Fe₂O₃, and the Fe_0.1Ce_0.9 composite shows the best catalytic performance. The structure-activity relation of the Fe₂O₃–CeO₂ composites in CO oxidation is discussed in terms of the formation of solid solution and surface oxygen vacancies. Our results demonstrate a proportional relation between the catalytic activity of cubic CeO₂-like solid solutions and their density of oxygen vacancies, which directly proves the formation of oxygen vacancies as the key step in CO oxidation over oxide catalysts.     

Gold nanoparticles on ceria: importance of O vacancies in the activation of gold

Synchrotron-based techniques (high-resolution photoemission, in-situ X-ray absorption spectroscopy, and time-resolved X-ray diffraction) have been used to study the destruction of SO₂ and the water-gas shift (WGS, CO + H₂O → H₂ + CO₂) reaction on a series of gold/ceria systems. The adsorption and chemistry of SO₂ was investigated on Au/CeO₂(111) and AuO _x /CeO₂ surfaces. The heat of adsorption of the molecule on Au nanoparticles supported on stoichiometric CeO₂(111) was 4–7 kcal/mol larger than on Au(111). However, there was negligible dissociation of SO₂ on the Au/CeO₂(111) surfaces. The full decomposition of SO₂ was observed only after introducing O vacancies in the ceria support. AuO _x /CeO₂ surfaces were found to be much less chemically active than Au/CeO₂(111) or Au/CeO_2−x (111) surfaces. In a separate set of experiments, in-situ time-resolved X-ray diffraction and X-ray absorption spectroscopy were used to monitor the behavior of nanostructured {Au + AuO _x }–CeO₂ catalysts under the WGS reaction. At temperatures above 250 °C, a complete AuO _x → Au transformation was observed with high catalytic activity. Photoemission results for the oxidation and reduction of Au nanoparticles supported on rough ceria films or a CeO₂(111) single crystal corroborate that cationic Au^δ+ species cannot be the key sites responsible for the WGS activity at high temperatures. The active sites in {Au + AuO _x }/ceria catalysts should involve pure gold nanoparticles in contact with O vacancies of the oxide.     

Interaction of SO2 with CeO2 and Cu/CeO2 catalysts: photoemission, XANES and TPD studies

CeO₂ and Cu/CeO₂ are effective catalysts/sorbents for the removal or destruction of SO₂. Synchrotron‐based high‐resolution photoemission, X‐ray absorption near‐edge spectroscopy (XANES), and temperature‐programmed desorption (TPD) have been employed to study the reaction of SO₂ with pure and reduced CeO₂ powders, ceria films (CeO₂, CeO_2−x, Ce₂O_3+x) and model Cu/CeO₂ catalysts. The results of XANES and photoemission provide evidence that SO₄ was formed upon the adsorption of SO₂ on pure powders or films of CeO₂ at 300 K. The sulfate decomposed in the 390–670 K temperature range with mainly SO₂ and some SO₃ evolving into gas phase. At 670 K, there was still a significant amount of SO₄ present on the CeO₂ substrates. The introduction of O vacancies in the CeO₂ powders or films favored the formation of SO₃ instead of SO₄. Ceria was able to fully dissociate SO₂ to atomic S only if Ce atoms with a low oxidation state were available in the system. When Cu atoms were added to CeO₂ new active sites for the destruction of SO₂ were created improving the catalytic activity of the system. The surface chemistry of SO₂ on the Cu‐promoted CeO₂ was much richer than on pure CeO₂. The behavior of ceria in several catalytic processes (oxidation of SO₂ by O₂, reduction of SO₂ by CO, automobile exhaust converters) is discussed in light of these results. This revised version was published online in July 2006 with corrections to the Cover Date.     

High throughput discovery of CO oxidation/VOC combustion and water–gas shift catalysts for industrial multi-component streams

High-throughput and combinatorial approaches have been applied to the discovery of catalysts for selective low temperature CO oxidation/VOC removal using mixed CO/propylene feeds, and for the water–gas shift (WGS) reaction using real post-reformer feeds containing CO, CO₂, H₂O and H₂. The screening approach was based on a hierarchy of qualitative and semi-quantitative primary screens for the discovery of hits, and quantitative secondary screens for hit confirmation, lead optimization and scale-up. For WGS, primary screening was carried out using scanning mass spectrometry. For CO oxidation and VOC removal, parallel IR thermography was the primary screen. Multi-channel fixed bed reactors equipped with imaging reflection FTIR spectroscopy or GC were used for secondary screening. Novel RuCoCe compositions were discovered and optimized for CO oxidation/VOC removal and the effect of doping was investigated for supported and bulk mixed oxide catalysts. For WGS, noble metal-free and Pt-doped CoFeRu mixed oxides as well as Pt on CeO₂ and Pt on CeO₂/ZrO₂ were investigated and a new synergistic PtFeCe ternary composition was discovered. In these cases oxygen vacancies in the ceria lattice are believed to play a key role in the strong and synergistic Pt–Ce interaction. Alkaline metal doping was found to enhance the selectivity towards WGS by suppressing the unselective methanation side reaction and to increase the low temperature catalytic activity.     

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.     

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.     

Enhancement of Methanol Oxidation Performance Over Pd/CeO2 Derived from MOF and Mechanism Investigation via In Situ Studies

A Pd/CeO₂-M with high methanol oxidation performance was prepared from metal–organic framework (MOF). The reaction results indicate that the Pd/CeO₂-M could achieve 100% methanol oxidation at 200 °C over the traditional Pd/CeO₂-C at 350 °C. The high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and Raman show that the Pd was uniformly distributed over the CeO₂ that was rich in oxygen vacancies. The methanol temperature program surface reaction (MeOH-TPSR) demonstrates that the Pd/CeO₂-M could effectively decompose methanol at a lower temperature with the assistance of oxygen vacancies compared with Pd/CeO₂-C. Methanol oxidation was conducted with in-situ Raman, CO diffuse reflectance infrared Fourier transformed spectroscopy (CO-DRIFTS) and in-situ DRIFTS to elucidate the reaction mechanism and establish the structure–activity relationship. The oxygen vacancies have participated in the assistance of methanol oxidation, which was revealed via in-situ Raman. The CO-DRIFTS indicates that the chemisorbed CO was more in linear form, which demonstrates that the Pd particles were over CeO₂-M with a smaller size. The in-situ DRIFTS experiments show the methanol could be further effectively oxidized to bidentate formate over Pd/CeO₂-M with the assistance of oxygen vacancies. The Pd/CeO₂-M catalysis and its enhancement mechanism studies provided a potential strategy in the VOC removal catalysis development.

Graphical Abstract

     

Support effects in the dissociation of CO on Rh/CeO2(111)

The adsorption and reaction of CO on Rh particles supported on stoichiometric and partially reduced CeO₂(111) surfaces was studied using a combination of HREELS and TPD. A fraction of the CO adsorbed on the supported Rh particles was found to undergo dissociation to produce adsorbed C and O atoms. TPD results for isotopically labeled CO demonstrated that O atoms produced by CO dissociation rapidly exchange with the oxygen in the ceria lattice. The fraction of adsorbed CO which dissociated was found to increase significantly with the extent of reduction of the CeO₂(111) surface, suggesting that oxygen vacancies on the surface of the support play a direct role in the CO dissociation reaction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Kinetic modeling of CO oxidation on Pt/CeO2 in a gradientless reactor

Cerium oxide is a major additive in three-way catalysts used in emission control of automobile exhaust. Pt/CeO₂ was studied in order to better understand the role of ceria in promoting CO oxidation reaction. The kinetics of carbon monoxide oxidation on Pt/cerium oxide catalyst, was studied over the temperature range 100–170°C. Steady state kinetic measurements of CO oxidation were obtained in a computer controlled micro-CSTR reactor. Activation energies were reported to vary between 39·5 and 51·2 kJ mol⁻¹. At low concentrations of either reactant (CO, O₂) and total conversion, the catalyst exhibited multiple steady states, similar to the multiplicity behavior of Pt/Al₂O₃. The total conversion was reached at 120°C. In comparison, the total conversion at low reactant concentrations was reached at a temperature of 148°C for the alumina-supported catalyst. Langmuir–Hinshelwood mechanisms gave a good fit to the data. However, no single rate expression could effectively describe the CO oxidation data over the whole concentration in the product of the CSTR reactor. The facts gathered indicate that oxygen adsorbed on interfacial Pt/Ce sites and ceria lattice oxygen provides oxygen for CO oxidation. Cerium oxide has been found to lower CO oxidation activation energy, enhance reaction activity and tends to suppress the usual CO inhibition effect.     

Defects mediated enhanced catalytic and humidity sensing performance in ceria nanorods

The content of defects and manipulation of oxygen vacancies in ceria (CeO₂) nanomaterials through doping have been widely used as effective strategies to rationally control its reactivity for diverse applications. In this work, a facile and simple fabrication method of CeO₂ nanorod crystals is demonstrated where; defects content and oxygen vacancies were tailored by doping with transition metal (Iron, Fe) ions. The effect of Fe-doping level (Ce_0.95Fe_0.05O₂ and %, Ce_0.9Fe_0.1O₂) on the structural, morphological, and optical properties of ceria nanorods was explored in detail. Undoped and Fe-doped CeO₂ nanorods were further explored for their use in photocatalytic activity and humidity sensing applications. The results revealed superior photocatalytic activity of Ce_0.9Fe_0.1O₂ by showing about 90% degradation of organic dye within 80 min of reaction time. Further, the same sample demonstrated excellent humidity sensing capabilities with faster response (~19 s) and recovery rate (~ 49 s) at 100% humidity conditions than other investigated samples. This work presents a systematic study on the fluencies of Fe doping in CeO₂ nanorod crystals to better understand the role of oxygen vacancies for tailoring their structural and optical properties for proficient environmental and sensing applications.     

Comparative Study on Partial Oxidation of Methane over Ni/ZrO2, Ni/CeO2 and Ni/Ce–ZrO2 Catalysts

The partial oxidation of methane has been studied by sequential pulse experiments with CH₄ O₂ CH₄ and simultaneous pulse reaction of CH₄/O₂ (2/1) over Ni/CeO₂, Ni/ZrO₂ and Ni/Ce–ZrO₂ catalysts. Over Ni/CeO₂, CH₄ dissociates on Ni and the resultant carbon species quickly migrate to the interface of Ni–CeO₂, and then react with lattice oxygen of CeO₂ to form CO. A synergistic effect between Ni and CeO₂ support contributes to CH₄ conversion. Over Ni/ZrO₂, CH₄ and O₂ are activated on the surface of metallic Ni, and then adsorbed carbon reacts with adsorbed oxygen to produce CO, which is composed of the main path for the partial oxidation of methane. The addition of ceria to zirconia enhances CH₄ dissociation and improves the carbon storage capacity. Moreover, it increases the storage capacity and mobility of oxygen in the catalyst, thus promoting carbon elimination.     

Catalysis by doped oxides: CO oxidation by AuxCe1−xO2

Density functional theory calculations for the CeO₂(111) surface doped with Au, Ag, and Cu show that the bond between the oxygen atoms and the oxide is weakened by presence of the dopant. In CO oxidation, doping of CeO₂ with Au allows the oxide to react readily with CO and make carbonates. These decompose to release CO₂ and form an oxygen vacancy on the surface. The vacancy adsorbs oxygen from the gas and weakens its bond, so that it reacts with CO to form a carbonate, which decomposes to release CO₂ and heal the oxygen vacancy. To be a good oxidation catalyst, a doped oxide must achieve a balance between two conflicting requirements: It must make surface oxygen reactive but not so much that it will hinder the healing of the oxygen vacancies created by the oxidation reaction.     

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.     

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.