Ceria Zirconia Mixed Oxides Research Articles

BASF automotive catalyst expansion

Steady-state, CO-oxidation kinetics at 515 K have been measured on model, Pd catalysts, prepared by vapor deposition of Pd onto either zirconia, praseodymia, ceria, or a ceria-zirconia mixed oxide. A second rate process (RE2), associated with both the metal and the oxide support and observed previously on ceria-supported catalysts in excess CO [7], was also found for Pd supported on ceria-zirconia, but neither zirconia nor praseodymia had any effect on CO oxidation under the conditions of our study. For ceria and ceria-zirconia, deactivation, through the loss of RE2, caused by high-temperature calcination, was examined, with the Pd added after calcination so that the metal particle size was not a factor in deactivation. For ceria, there was a strong dependence on calcination temperature, with almost complete loss of RE2 above 1170 K. XRD showed that the loss was accompanied by a large increase in the crystallite size. Results for ceria-zirconia showed that the loss in this case was more gradual, with CO oxidation activity due to RE2 maintained to much higher calcination temperatures. Taking the importance of RE2 as a measure of the ability of the catalyst to use oxygen from the oxide, the implications of these results with respect to oxide structure and the effect of aging on oxygen-storage properties of reducible oxides are discussed.

Anionic surfactant‐aided preparation of high surface area and high thermal stability ceria/zirconia‐mixed oxide from cerium and zirconium glycolates via sol–gel process and its reduction property

AbstractThis work is focused on the ceria zirconia mixed oxide prepared through a surfactant‐introduced synthesis method. High surface area nanoparticle mesoporous ceria/zirconia‐mixed oxide was successfully synthesized and characterized using various techniques. High surface area mesoporous fluorite‐structured CeO2ZrO2 was obtained from the elimination of surfactants upon calcination. A surface area in excess of 205.6 m2/g was obtained after calcination at 500 °C, and dropped to 75.96 m2/g by heating at 900 °C. Temperature‐programming reduction (TPR) results showed that the lowest reduction temperature was obtained from the sample containing 40% zirconia content. Copyright © 2008 John Wiley & Sons, Ltd.

One‐Step Hydrothermal Synthesis of Nanocrystalline Ceria‐Zirconia Mixed Oxides: The Beneficial Effect of Sodium Inclusion on Redox Properties

Doping of low levels of sodium onto the cation sites of ceria-zirconia mixed oxides allows reduction of the materials to take place at lower temperatures than undoped materials, a desirable property for practical applications in supports for redox catalysis. Synthesis is achieved in a single step under mild hydrothermal conditions directly from aqueous mixtures of metals salts at high pH. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2089/2007/c2474_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Characteristics and performance of M-doped cerium zirconium mixed oxide nanosized catalysts (M = Zn, Sn) in CO oxidation

Nanosized Ce0.7Zr0.3Mx oxide particles (M=Zn, Sn, x = 0.1, 0.3, 0.5) with 10–20 nm diameter were synthesized by a coprecipitation method. Their structure and morphology were characterized by XRD, FT-Raman and TEM. Zn did not incorporate well into Ce0.7Zr0.3 oxide, but Sn did. The catalytic performance of these particles for CO oxidation was investigated between 150–400°C under severe conditions. Ce0.7Zr0.3Sn0.5 had the best performance with the lowest light-off temperature range (175–210°C). A temperature hysteresis was observed in all catalysts studied.

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.

The catalytic activity of CuO–CeO2 mixed oxides for diesel soot oxidation with a NO/O2 mixture

Copper doped ceria and ceria–zirconia mixed oxides were synthesized using the citric acid sol–gel method. The temperature-programmed oxidation (TPO) results showed that the Cu modification improved the low-temperature activity and the selectivity to CO2 of ceria for soot oxidation in the presence of NO and excess oxygen even after ageing at 800 °C for 20 h in flow air. Meanwhile, not only the segregation of Cu and sintering of CuO, but also the separation of Ce- and Zr-rich phases worsened the activity of the Cu–Ce–Zr catalyst after the high-temperature calcination.

Some recent results on the correlation of nano-structural and redox properties in ceria-zirconia mixed oxides

Abstract Some recent results on the redox behaviour of thermally aged ceria-zirconia mixed oxides with Ce/Zr molar ratios typically ranging from 50/50 to 70/30 are briefly reviewed. In accordance with them, a tentative model allowing us to correlate ageing conditions, surface and bulk nano-structural properties of the oxides, and changes occurred in their redox behaviour is proposed. As revealed by the analysis of appropriate chemical studies and the nano-structural information provided with High Resolution Transmission (HREM) and High Angle Annular Dark Field-Scanning Transmission (HAADF-STEM) electron microscopies, the presence/absence of a pyrochlore-related κ-phase in the aged oxides plays a key role in determining their redox response. In the low-temperature region (Tred ≤ 773 K), the enhanced reducibility exhibited by the oxide resulting from a high-temperature reduction/mild re-oxidation ageing cycle (SR-MO sample) is interpreted as due to kinetic reasons, the occurrence of the κ-like phase in its surface being responsible for a faster H2 chemisorption, the rate controlling step of the overall reduction process. By contrast, in the high-temperature range (Tred ≥ 973 K), the observed differences of reducibility would have a thermodynamic origin, which may be correlated with the total amount of κ-like phase present in the aged sample.

Effect of the thermal pre-treatments on ceria–zirconia redox properties: An Eu3+ luminescence study

Abstract The luminescence of Eu 3+ was used to investigate the structural modification of Ce 0.20 Zr 0.79 Eu 0.01 O 1.995 mixed oxides after various redox treatments. The Eu dopant was initially introduced selectively on the surface or in the bulk of the ceria–zirconia mixed oxide. The Eu occupancy of the surface-related site is predominant after severe oxidation pre-treatments while the bulk-related site is preferred after severe reduction (TPR) followed by mild reoxidation (MO).

Ce–Zr–Sr ternary mixed oxides structural characteristics and oxygen storage capacity

The structural/textual and dynamic oxygen storage capacity (DOSC) of Ce–Zr–Sr ternary mixed oxides prepared by sol–gel method are investigated by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), electron spin resonance (EPR), X-ray photoelectron spectroscopy (XPS) and mass spectrometry with CO–O 2 transient pulses/CO pulses. The results show that DOSC is greatly promoted by Sr doping, especially after high temperature ageing, stability of DOSC is obviously enhanced by Sr doping. Sr doping sample investigated displays comparable high thermal stability with corresponding smaller crystallites and higher surface area using XRD and BET measurements. Meanwhile, no separated SrO species is detected on XRD patterns, surface and electronic properties are modified by Sr doping through XPS measurement showing as Ce 3+ proportion increasing, which is consistent with g factor value shifting observed on EPR spectra. Sr introduction in ceria–zirconia mixed oxides strongly modifies the textual and structural properties. Bulk oxygen migration is strongly altered and displays a large enhancement by Sr doping.

Correlation between the surface properties and deNOx activity of ceria-zirconia catalysts

The catalytic activities of ceria-zirconia mixed oxides CexZr1−xO2 (x=0.17, 0.42, 0.62 and 0.8) are determined by isothermal steady-state experiments using a representative mixture of exhaust gases of coal combustion. Results show that all supports are active in deNOx reaction in the presence of the previous hydrocarbons. However, their catalytic activity varies with the content of cerium and goes through a maximum for x=0.62, leading to 27% NOx consumption. This activity was correlated with physicochemical properties determined by a linear solvation energy relationship (LSER) approach. Moreover, a mechanism of HC assisted reduction of NO is proposed on ceria-zirconia supported catalysts. This mechanism is divided in three catalytic cycles involving: (i) the oxidation of NO into NO2, (ii) the reaction of NO2 and the hydrocarbons leading to RNOx species and CxHyOz, and finally (iii) the decomposition of NO assisted by these latter CxHyOz species.

Preparation and characterization of cerium oxide templated from activated carbon

The application of carbon templating to the preparation of cerium oxide and cerium-zirconium mixed oxide was investigated. Impregnation of a highly mesoporous activated carbon (Darco KB-B) with aqueous cerium nitrate, followed by calcination to remove the template, afforded nanocrystalline ceria with a surface area of up to 148 m 2 /g. Ceria-zirconia with surface area of 148 m 2 /g was similarly prepared. TEM studies on the ceria product revealed the presence of a polycrystalline phase, comprised of irregular aggregates of ceria crystallites of ca. 6.5 nm diameter. Use of activated carbon fibers (ACFs) as templates afforded ceria with a gross morphology resembling that of the fiber template. The lower surface areas (3-59 m 2 /g) of the resulting ceria reflect the mainly microporous nature of the ACFs; evidently the Ce nitrate solution is largely unable to penetrate the micropores. TEM showed the ceria fibers to be comprised of crystallites possessing a morphology similar to the Darco KB-B templated ceria.

Study on Catalysts with Rhodium Loading on Different Cerium-Zirconium Mixed Oxides

The catalysts with Rh loading on different cerium-zirconium mixed oxides were characterized by BET, H 2-TPR and OSC. The effects of different cerium-zirconium mixed oxides on catalytic performance and thermal stability of Rh loaded catalyst were studied. The results show that: (1) Rh can enhance cerium-zirconium mixed oxides OSC and catalytic reaction rates; (2) cerium-zirconium mixed oxides with high Ce contents and low Zr contents are more favorable to the stability of catalysts. Moreover, the contents of CeO 2 have important effect on catalysts characteristics, and the addition of some rare earth components, such as La, Pr and Nd also have some influences.

Structural investigation of Ce2Zr2O8 after redox treatments which lead to low temperature reduction

Structural investigations of various aliquots of Ce2Zr2O8 were conducted after application of well-defined redox treatments. The results indicate that specific redox procedures induce modifications in the ceria–zirconia mixed oxide, which lead to notable effects on the hydrogen uptake temperature during Temperature Programmed Reduction (TPR). Pyrochlore-type cation ordering is evident for an aliquot which exhibits a single low temperature peak during TPR. However, the cation ordering is not complete in this case. Furthermore, the analysis indicates less extensive pyrochlore-type cation ordering for an aliquot which shows a TPR profile in which there are two peaks, one at high temperature and one at low temperature. Finally, an aliquot that exhibits a single high temperature TPR peak shows no evidence of cation ordering. The trend in the promotion of reduction to low temperature is associated with the extent of the pyrochlore-type cation ordering.

Study of the temperature-programmed oxidative degradation of hydrocarbons over Ce-based catalysts by evolved gas analysis

The catalytic behaviour of ceria, zirconia and ceria–zirconia mixed oxides in the temperature-programmed degradation of toluene and n-hexane was analysed by means of evolved gas analysis (mass spectrometry). Pure cerium oxide resulted the most active catalyst in the oxidation of both compounds. This fact revealed the crucial role of the surface oxygen species in the decomposition of this type of hydrocarbons. The low affinity of CeO2 for H2O and CO2, the major oxidation products, may be also responsible for the observed highly active catalytic behaviour.

Role of CeO2–ZrO2 in diesel soot oxidation and thermal stability of potassium catalyst

Abstract Potassium nitrate catalysts supported on different oxides (CeO 2 , Ce 0.5 Zr 0.5 O 2 and ZrO 2 ) were prepared for diesel soot combustion. The ageing treatment was performed at 800 °C for 24 h and the catalytic activity was evaluated by a temperature-programmed oxidation technique. The results demonstrated that, compared with CeO 2 and ZrO 2 , Ce 0.5 Zr 0.5 O 2 presented good redox properties, a high surface area and available potassium-holding capacity at an elevated temperature. For aged K/Ce 0.5 Zr 0.5 O 2 , the combustion temperature of soot particle was 359 °C under tight contact conditions and 455 °C under loose contact conditions. Thus, ceria–zirconia mixed oxides were considered as good candidate supports for diesel soot oxidation catalysis.

Rh(1%)@CexZr1−xO2–Al2O3 nanocomposites: Active and stable catalysts for ethanol steam reforming

Rh(1%)@CexZr1−xO2–Al2O3 nanocomposites have been investigated as active and thermally stable catalysts for ethanol steam reforming. Preformed Rh nanoparticles have been efficiently protected from deactivation/sintering by a porous layer of nanocomposite oxides. Chemisorption and activity data confirm the good accessibility of the metal phase to the reaction mixture. No appreciable deactivation is observed after 160h of reaction at 873K. The ceria–zirconia mixed oxides favour reforming reactions, reduce coke formation and facilitate its removal. The alumina component is important to stabilize the ceria–zirconia mixed oxides, preventing their sintering.

Supported CuO/Ce1−x Zr x O2 catalysts for the preferential oxidation of CO in H2-rich gases

The catalytic properties of CuO supported on ceria or ceria-zirconia mixed oxides have been investigated in the preferential oxidation of CO in H2-rich gases. CuO/CeO2 shows very high activity towards the oxidation of CO with a light-off temperature of about 70°C. This catalyst is very selective for the oxidation of CO rather than of H2 in the low temperature region (70–120°C), while at higher temperatures, the oxidation of hydrogen begins, causing of a maximum of CO conversion to arise with increasing temperature.

Pd Supported on Ce/Zr Mixed Oxides in the Reduction of NO with Propylene in Oxidizing Conditions

The last generation of catalysts for automotive emissions has substituted the traditional ceria by cerium-zirconium mixed oxides, due to their increased oxygen storage capacity, selectivity to the desired products (mostly N2) and durability; while increasing the proportion of palladium, due to its interesting behavior. Using simplified feed-streams with only three components: propylene, NO and O2, plus the N2 carrier, and with data of integral reactor in the range 200–300ºC, we have performed experiments in order to obtain kinetic data with a 0.5 %Pd/Ce0.68Zr0.32O2 catalyst. The results have shown that, in the studied range, inlet reaction rates of reactants consumption and products formation can be explained by a Mars-Van Krevelen mechanism in which O2 and NO oxidize the support, and products formation takes place via reduction of the support by the rest of the reactants. Net formation of NO is observed in the low-temperature range, while the opposite is truth in the high-temperature range. Although this behavior has no effect during normal operating conditions in the automobile, it could during cold start up. Also, it is interesting to note that, at low temperature, a significant amount of NH3 —decreasing with temperature— and some N2O —with a maximum at intermediate temperature (around 275ºC)— are formed even though the conditions were highly oxidizing, provided that C3H6 is present. NO, on the other hand, is the precursor for NO2.

Characteristics in oxygen storage capacity of ceria–zirconia mixed oxides prepared by continuous hydrothermal synthesis in supercritical water

Continuous hydrothermal synthesis in supercritical water (supercritical synthesis) is a method to prepare metal oxide nanoparticles rapidly and continuously using supercritical water. Highly crystallized nanoparticles of homogeneous complex metal oxides as well as single metal oxides could be produced easily by the supercritical synthesis. Ceria–zirconia mixed oxide is widely used as oxygen storage material, for example, in the three-way catalysts for the clean up of automotive gases. Ceria–zirconia mixed oxides were prepared by the new supercritical synthesis method and also by the conventional co-precipitation method. The characteristics in oxygen storage capacity (OSC) of ceria–zirconia mixed oxide prepared by the supercritical synthesis were compared with those by the co-precipitation method. It was confirmed through physical characterizations with N2 adsorption, SEM, TPR and O2-uptake that the supercritical synthesis could lead to ceria–zirconia mixed oxides with higher thermal stability and better OSC due to its morphology.

Analysis of the simultaneous catalytic combustion of chlorinated aliphatic pollutants and toluene over ceria-zirconia mixed oxides

Abstract In the present work the oxidation of representative VOCs, such as toluene, 1,2-dichloroethane and trichloroethylene, present in trace amounts in air streams over a series of Ce x Zr 1− x O 2 oxides (CeO 2 , Ce 0.8 Zr 0.2 O 2 , Ce 0.68 Zr 0.32 O 2 , Ce 0.5 Zr 0.5 O 2 , Ce 0.15 Zr 0.85 O 2 , ZrO 2 ) as catalysts was investigated with focus on the difference in catalytic performance in the oxidation of single and chlorinated VOC/toluene mixtures, and on the mixture effects on product selectivity. In general, the efficiency for the single VOC destruction decreased in the following order: toluene > 1,2-dichloroethane > trichloroethylene. For chlorinated compounds the best performance was observed for the mixed oxides with 50 and 85 mol% of zirconia content, while ceria exhibited the best behaviour for toluene oxidation. With regard to chlorinated compounds the combination of surface acidity and accessible lattice oxygen appeared to control the catalytic performance of the mixed oxides. On the contrary, the combustion of toluene was essentially controlled by surface oxygen species. Notable ‘mixture effects’ on both activity and selectivity were noticed when a given chlorinated feed was decomposed in the presence of toluene. On one hand, mutual inhibition took place but the conversion of trichloroethylene was retarded much more severely. On the other hand, the destruction of toluene was less affected by the presence of 1,2-dichloroethane. Competitive adsorption played an important role in the inhibition detected with CeO 2 -based catalysts. However, no one species was able to dominate the adsorption sites and thereby completely prevent the adsorption and reaction of other species. On the other hand, HCl selectivity was greatly enhanced by the presence of toluene in the feed stream.