Oxygen Storage Capacity Material Research Articles

In Situ Raman Spectroscopy as a Tool for Discerning Subtle Structural Differences between Commercial (Ce,Zr)O2-Based OSC Materials of Identical Composition

In situ Raman spectroscopy was used at temperatures in the 50–480 °C range under oxidizing (20% O2/He) and reducing (5% H2/He) flowing gas atmospheres to compare the spectra obtained for a series of industrial rare earth doped CexZr1−xO2−δ oxygen storage capacity (OSC) mixed metal oxide materials of identical at % composition, which were prepared by the same chemical synthesis route, in which one synthesis parameter of the aqueous chemistry was slightly varied. The Raman fingerprint of the anionic sublattice is very sensitive to O atom relocations within the bulk of the material matrix and to the pertinent defect topology in each case. A protocol of sequential Raman measurements and analysis was proposed to discern subtle differences between the oxygen vacancy and defect topologies of the examined materials. It can be concluded that for two materials under comparison for their structures, identical Raman spectra are obtained only if the procedures followed for their preparation are identical; a slight variation of one single parameter (e.g., in the aqueous chemistry stage) results in discernible differences in the Raman spectra. The proposed procedure can serve as a tool for proving or disproving infringement of IPR (Intellectual Property Rights) protected preparation methods of ceria-based mixed metal oxide materials.

CeO2–MO x (M: Zr, Ti, Cu) mixed metal oxides with enhanced oxygen storage capacity

The effect of composition on the thermal stability and oxygen storage capacity (OSC) of a series of CeO2–MOx (M: Zr, Ti, Cu) mixed oxides was investigated. X-ray diffraction, N2 adsorption–desorption analysis (BET), transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy were used to examine structural and microstructural properties. The total OSC of the fresh and aged mixed oxides were measured using thermogravimetric–differential thermal analysis at 600 °C, under alternating reductive and oxidative environment. Ceria doping was found to be effective toward an enhancement of its redox properties. The addition of Zr4+ into the CeO2 lattice was found to have the least effect in enhancing the catalytic properties. In contrast with other studies in the literature [1, 2], increasing ZrO2 content led to an increase of the OSC of the samples of up to 522 μmol O2 g−1 for 75 % Zr4+ content. On the other hand, the CeO2–CuO with (1:3) molar ratio showed an almost three times higher OSC value than CeO2–ZrO2, which is deemed in the literature as one of the most promising OSC materials. In this work, the highest OSC values (1565 μmol-O2 g−1) were obtained for the fresh sample with CeO2–CuO (25:75) composition. The CeO2–CuO (25:75) mixed oxide still showed the highest OSC values with an only 6 % drop of OSC after aging at 900 °C for 6 h among all aged catalysts. On the other hand, ZrO2 containing samples showed the best thermal resistance against aging.

Optimization of Ce0.6Zr0.4−x Al1.3x O2 solid solution based on oxygen storage capacity

High oxygen storage capacity (OSC) materials are very much demanded for catalytic applications and ceria is one of the promising OSC materials widely used in three-way catalysts. The objective of the present work is to overcome the inherent limitations of ceria by doping Al3+ and Zr4+ into its lattice and optimize the concentration of dopant based on its OSC. A solid solution of Ce0.6Zr0.4−x Al1.3x O2 (CZA) (0.0 ≤ x ≤ 0.4) was synthesized by sol–gel technique and the concentration of Zr and Al was optimized based on the maximum OSC of CZA. In all the samples, the lattice defects on the structure were observed and confirmed by HRTEM and Raman spectroscopy. The crystal size of CZA samples was observed to be varied from 2.7 to 7.5 nm. The crystallite growth of CZA was observed to be less compared to that of ceria–zirconia solid solution after the ageing treatment. The OSC of the sample was calculated by the cyclic heating and cooling process in the temperature range of 30–800 °C using Thermogravimetric analysis and the same was observed to be varied from 0.108 to 0.230 μmol g−1 of CeO2. The coefficient of thermal expansion, which was qualitatively used to confirm the OSC, was measured using the thermomechanical analyser and it was observed to be high in Ce0.6Zr0.2Al0.26O2 compared to that of Ce0.6Zr0.4O2 and the same was found to be significant after 600 °C. It is concluded that Ce0.6Zr0.2Al0.26O2 has the highest OSC among all the samples, which could be explored for the potential catalytic applications.

Zr-Ce-Pr-Cu-O 系酸化物の相平衡と酸素放出・吸収特性

Oxide materials with high oxygen storage capacity (OSC) at low temperatures, possessing high thermal stability at high temperatures are indispensable for advanced automotive exhaust catalyst. A candidate for the high efficient OSC materials must lie in the composition of (Zr2/8Cex/8Pr(6-x)/8)O2-y (x=0~6), because the small amount of Ce addition stabilizes a single phase of λ-(Zr0.25Pr0.75)O2-y with wide oxygen nonstoichiometry. In addition, noble catalytic metal, e.g., copper must be investigated instead of platinum. In the present work, the (Zr2/8Cex/8Pr(6-x)/8)O2-y compositions (x=0~6) with the addition of Cu have been investigated for the phase relationship and OSC behavior. The samples were first oxidized at 1473 K in air, reduced in H2 at 1373 K, and then oxidized again at 973 K in air. The oxide samples obtained in each situation were subjected to XRD, OSC analysis and TPD oxygen gas analysis by means of TG/DTA in the Ar+5% H2. When oxidized at 1473 K in air, the two-phases region between the λ-and C-phases, observed for (Zr2/8Cex/8Pr(6-x)/8)O2-y, disappeared and a pyrochlore-like phase appeared with the addition of Cu. That is, the addition of Cu led to the mutual solid solution of λ- and C-phases. When the samples (Zr2/8Cex/8Pr(6-x)/8)Cu0.5/8O2-y (x<3) were reduced at 1373 K in H2, C-rare earths-like phase appeared on (Zr2/8Cex/8Pr(6-x)/8)Cu0.5/8O2-y (x<3). With increasing the Ce content, the lattice parameter of the CaF2 unit varied continuously to those of λ- and C-phases. For the samples (Zr2/8Cex/8Pr(6-x)/8)Cu0.5/8O2-y (x=0∼6), the oxygen storage capacity was greatly improved at low temperatures as 573 K, probably due to the oxygen defect of pyrochlore-like phase.

A new concept in high performance ceria–zirconia oxygen storage capacity material with Al2O3 as a diffusion barrier

CeO2–ZrO2 solid solution ((Ce,Zr)O2) is an indispensable oxygen storage capacity (OSC) material for emission control in gasoline-fuelled automobiles. The high performance OSC material developed in this study is composed of Al2O3 as “a diffusion barrier” and (Ce,Zr)O2 particles in intervening layers on a nanometer scale, and is abbreviated as “ACZ”. The Brunauer–Emmett–Teller (BET) specific surface area (SSA) of ACZ after durability testing in air at 1000°C was 20m2/g, which is higher than that of conventional CZ (2m2/g) composed of (Ce,Zr)O2 without Al2O3. After heat treatment at 1000°C in air, the particle size of (Ce,Zr)O2 in ACZ was about 10nm and that without Al2O3 was one-half of the size in pure CZ. The OSC was roughly characterized by the total capacity (OSC-c1) and the oxygen release rate (OSC-r). In a fresh catalyst, ACZ and CZ had almost the same OSC-c1; however, the OSC-r of ACZ was twice as fast as CZ. After durability testing, the OSC-r of both ACZ and CZ were reduced significantly, but the OSC-r of ACZ was about five times as fast as CZ. While the OSC-c1 was hardly influenced by the (Ce,Zr)O2 crystallite size and Pt particle size on the supports, the OSC-r was influenced by both of these parameters. The improvement of the OSC-r in the fresh catalyst and inhibition of the decrease in the OSC-r after durability testing were achieved by suppression of particle growth of (Ce,Zr)O2 in ACZ by introducing Al2O3 as a diffusion barrier with resultant inhibition of sintering of Pt particles.

The effect of La2O3 on dynamic OSC over the Pd/OSC/Al2O3-based catalysts

In this study, fresh and aged Pd/(OSC–Al2O3) and Pd/(Al2O3–OSC–La2O3) metallic monoliths (OSC material Ce0.75Zr0.25O2) were used to find out the effect of La2O3 on the catalyst behaviour in dynamic oxygen storage capacity (OSC) measurements. In addition, the interaction of CO, NO and O2 reaction compounds over the studied catalysts was investigated in order to understand the effect of La2O3 in the oxidation and reduction reactions in lean automotive exhaust gas conditions. A FT-IR gas analyser was used to analyse the product gas composition. The presence of La2O3 on fresh and aged catalysts had negative effect on both the dynamic OSC and the activity of the catalyst. The reason for this is the different washcoat compositions between the studied catalysts which could explain the differences in BET surface areas.

Properties of cerium–zirconium mixed oxides partially substituted by neodymium: Comparison with Zr–Ce–Pr–O ternary oxides

CeO 2 doped with praseodymium, neodymium and/or zirconium atoms were prepared by coprecipitation and by the sol–gel method. Structural properties were investigated by in situ XRD and Raman spectroscopy while oxygen storage capacity (OSC) was measured by transient CO oxidation. All the compounds, except pure Nd 2O 3, have a fluorite-type structure as well as a Raman band at 560 cm −1 characteristic of the oxygen vacancies involving non-stoichiometric oxides. The lattice parameter under hydrogen, being dependent on the temperature, revealed two reduction mechanisms: one at a low temperature at the surface and another at a high temperature in the bulk. Ce–Nd binary oxides show a strong tendency towards crystallite aggregation, which reduces accessibility to gases and OSC properties. Zirconium improves the thermal resistance to sintering of both Ce–Nd and Ce–Pr oxides. The Zr–Ce–Pr–O followed by Zr–Ce–Nd–O compounds displaying high oxygen mobility at a low temperature, appear to be very promising for practical applications such as OSC materials.

Shape-Selective Synthesis and Oxygen Storage Behavior of Ceria Nanopolyhedra, Nanorods, and Nanocubes

Single-crystalline and uniform nanopolyhedra, nanorods, and nanocubes of cubic CeO2 were selectively prepared by a hydrothermal method at temperatures in the range of 100-180 degrees C under different NaOH concentrations, using Ce(NO3)3 as the cerium source. According to high-resolution transmission electron microscopy, they have different exposed crystal planes: {111} and {100} for polyhedra, {110} and {100} for rods, and {100} for cubes. During the synthesis, the formation of hexagonal Ce(OH)3 intermediate species and their transformation into CeO2 at elevated temperature, together with the base concentration, have been demonstrated as the key factors responsible for the shape evolution. Oxygen storage capacity (OSC) measurements at 400 degrees C revealed that the oxygen storage takes place both at the surface and in the bulk for the as-obtained CeO2 nanorods and nanocubes, but is restricted at the surface for the nanopolyhedra just like the bulk one, because the {100}/{110}-dominated surface structures are more reactive for CO oxidation than the {111}-dominated one. This result suggests that high OSC materials might be designed and obtained by shape-selective synthetic strategy.

The effect of barium on the catalytic behaviour of fresh and aged Pd-Ba-OSC/Al 2O 3 catalysts

In this study, fresh and laboratory-aged (rapid ageing hot in laboratory (RAHLAB)) Pd-Ba-OSC/Al 2O 3 and Pd-OSC/Al 2O 3-La 2O 3 catalysts were investigated. The effect of low barium content on the behaviour of the OSC material (Ce 1− x Zr x O 2 mixed oxides) was studied. The influence of the addition order of Ba, Pd, and OSC material in the catalyst washcoat was also examined. In a model reaction NO + CO, the catalytic activities and selectivities to N 2 and N 2O were analysed by the FT-IR technique. In addition, the effect of thermal ageing treatment on catalytic behaviour was investigated. The addition of barium was found to have several positive effects on the fresh Pd-Ba-OSC/Al 2O 3 catalysts. The catalytic activity (NO reduction by CO) was improved below the light-off temperature when barium was added into the catalyst. At 500 °C, the fresh catalysts were fully selective to N 2. The barium-containing catalysts had a bit higher dispersion values compared to the sample without barium. The high surface area over the fresh barium-containing catalysts could explain partly the improvements in the dynamic oxygen storage capacity (OSC). Besides, that the barium-containing catalysts were more active than the Pd/(OSC-La 2O 3-Al 2O 3) catalyst, it was also observed that the addition order of barium has an effect on the catalytic activity of fresh catalysts. The fresh (Pd-Ba-OSC)/Al 2O 3 and (Ba-Pd-OSC)/Al 2O 3 catalysts were more active than the Pd/(OSC-La 2O 3-Al 2O 3) catalyst. This indicates that the interaction of Pd, Ba, and OSC was better on these catalysts. For the aged catalysts, the addition order of Pd, Ba, and OSC had no obvious effect on differences in catalytic properties between Ba-containing samples. All the aged catalysts had high selectivities to N 2 at 500 °C. Characterisation results by XRD, chemisorption, and BET revealed that catalytic deactivation was occurring during the ageing process. This thermal deactivation was observed as a lower catalytic activity and a lower OSC compared to the fresh catalysts. The deactivation is due to the collapse of the surface area, sintering of Pd particles and solid–solid phase transitions observed in the bulk material. Therefore, Ba content (only ∼3 wt.%) in the catalyst was probably too small to stabilise the catalyst against sintering.

The reducing capability of palladium segregated from perovskite-type LaFePdO x automotive catalysts

In order to elucidate the activity of LaFePdO x -based automobile three-way catalysts at low-temperatures, the oxygen release capacity (ORC) and oxygen storage capacity (OSC) of LaFePdO x were measured by means of thermogravimetry (TG) in a hydrogen-containing atmosphere at 200–400 °C and NO reduction activity was examined at 400 °C by an NO-pulse technique. The catalyst weight-loss in reduction was attributed to oxygen release, leading to the formation of metallic Pd 0. About 40% of Pd 3+ in LaFe 0.95Pd 0.05O 3 was converted into Pd 0 at 200 °C and about 90% at 400 °C. ORC and OSC as observed for the LaFePdO x catalysts were found to be comparable to results seen when adding Ce-based oxides as an OSC material in a commercial catalytic converter. The reducing capability of Pd for NO reduction demonstrated by the reduced LaFePdO x catalysts can probably be attributed to the very fine dispersion of Pd 0 species segregated from LaFe 0.95Pd 0.05O 3 perovskite. The Pd 0 species segregated from LaFePdO x by reduction was assessed to be metallic Pd 0 particles with diameters smaller than 2 nm. Highly dispersed Pd 0 particles on the LaFePdO x catalyst were found to act as “breathing active sites” that combined high catalytic activity with OSC/ORC performance.

A catalytic NO reduction in periodic lean and rich excursions over Rh supported on oxygen storage capacity materials

Catalytic lean NO reduction system in periodic two steps, an operation in oxidizing conditions and a relatively short operation in reducing conditions, has been investigated over various honeycomb-shaped catalysts, which contain a noble metal and materials with oxygen storage capacity (OSC), using a continuous flow reactor system with a simulated gas mixture.Rh supported on an oxygen storage component could catalyze NO reduction in the periodic lean and rich excursions much more effectively than Rh supported on other supports. Reaction mechanisms of the NO reduction on Rh/oxygen storage component were deliberated. Results indicate that an OSC function would play an important role in the NO reduction on Rh/ceria. Furthermore, two kinds of reaction mechanisms have been proposed for the NO reduction on Rh/OSC material.