Insights into the dynamics of oxygen storage/release phenomena in model ceria–zirconia catalysts as inferred from transient studies using H2, CO and soot as reductants

Samples of ceria–zirconia pre-treated under various conditions have been used as catalysts in CO and soot oxidation under stationary and transient conditions, in the presence and in the absence of oxygen. Their behaviour has been compared with that observed under redox conditions in the presence of hydrogen (oxygen storage activity). All the catalysts are active in CO and soot oxidation. Under stationary conditions, the activity in CO oxidation depends on the amount of Ce present, with little contribution from the redox capacity of the support and is strongly influenced by surface area. When the reaction is carried out under transient conditions, especially with low-surface area samples, the performances of ceria–zirconia are higher than those of ceria, with a maximum in the middle composition range. Interestingly, a similar behaviour is observed in soot combustion, where the activity for low-surface area sample is dependent on composition. This suggests that oxygen from the support plays a key role also in the oxidation of large carbon particles under a fully oxidizing mixture.     

Preparation and characterization of high surface area niobia, ceria–niobia and ceria–zirconia

Niobia, zirconia, ceria–niobia and ceria–zirconia oxide nanoparticles are prepared by soft chemical routes and show valuable textural properties. The pore volume and specific surface area keep significant values even after calcination at 873 K. According to DRX and STEM-EDX measurements, solid solutions are obtained in the case of ceria–zirconia, whereas separate phases are identified in ceria–niobia; in the latter case, however, UV–Vis diffuse reflectance spectroscopy shows also the formation of defects (color centers) arising from a partial dissolution at the interface of the oxide phases.     

Oxidation entropies and enthalpies of ceria–zirconia solid solutions

The thermodynamic redox properties for a series of ceria–zirconia solid solutions have been measured by determining their oxidation isotherms between 873 and 1073 K. Isotherms were obtained using Coulometric titration and using O₂ titration of samples equilibrated in flowing mixtures of H₂ and H₂O. Samples having the following compositions were studied after calcinations at 973 and 1323 K: CeO₂, Ce_0.92Zr_0.08O₂, Ce_0.81Zr_0.19O₂, Ce_0.59Zr_0.41O₂, Ce_0.50Zr_0.50O₂, Ce_0.25Zr_0.75O₂, Ce_0.14Zr_0.86O₂, and ZrO₂. While the oxidation enthalpy for CeO₂ was between −750 and −800 kJ/mol O₂, the oxidation enthalpies for each of the solid solutions were between −500 and −550 kJ/mol O₂ and essentially independent of the extent of reduction. The shapes of the isotherms for the solid solutions were affected by the oxidation entropies, which depended strongly on the sample composition and the extent of reduction. With CeO₂, Ce_0.92Zr_0.08O₂, and Ce_0.14Zr_0.86O₂, the samples remained single-phase after calcination at 1323 K and the thermodynamic redox properties were unaffected. By contrast, Ce_0.59Zr_0.41O₂ formed two phases following calcination at 1323 K, Ce_0.78Zr_0.22O₂ (71 wt.%) and Ce_0.13Zr_0.87O₂ (29 wt.%); the isotherm changed to that which would be expected for a physical mixture of the two phases. A model is presented which views reduction of the solid solutions in terms of the local atomic structure, with the formation of “pyrochlore-like” clusters causing the increased reducibility of the solid solutions. Some of the changes in reducibility are associated with the number of sites from which oxygen can be removed in order to form pyrochlore-like clusters.     

Towards the comprehension of oxygen storage processes on model three-way catalysts

CeO₂- and Ce_0.63Zr_0.37O₂-supported noble metal catalysts were studied. Samples were fully characterized using TEM, XRD, N₂ adsorption and H₂ chemisorption. The oxygen storage process was investigated focusing on the evolution as a function of temperature of both the oxygen storage capacity (OSC) and the oxygen storage complete capacity (OSCC). Aging effect on OSC was also examined in details in the case of Rh catalysts. Finally, the major role of oxygen diffusion, partly influenced by the metal/support interface quality, was confirmed.     

Platinum group metals as catalysts in enantioselective 1-phenylpropane-1,2-dione hydrogenation

Different γ-Al₂O₃ supported Ir, Pd, Ru, Rh and Pt catalysts were tested in enantioselective 1-phenylpropane-1,2-dione hydrogenation using cinchona alkaloid modifiers. Activity and enantioselectivity over Ir and Ru catalysts were low. Pd catalyst was active in the hydrogenation of 1-phenylpropane-1,2-dione, however, the enantioselectivity over this catalyst was almost negligible. Over Pd hydrogenation proceeded mainly via hydrogenation of the C₁O₁ carbonyl group, which is attached to the phenyl ring. Hydrogenation over Pd did not proceed in the second hydrogenation step via an enol form as found for ethyl pyruvate hydrogenation over Pd. The structure-selectivity relationship and solvent effects are similar over Pt and Rh in the first hydrogenation step. However, in the second hydrogenation step of hydroxyketones to diols large mechanistical differences between Pt and Rh were observed. Although the activity over Rh catalysts was lower than over Pt after optimization the best result obtained with Rh/γ-Al₂O₃ (5754 Lancaster) was 60% ee in toluene at maximum yield of 28%, which makes Rh a promising metal for enantioselective hydrogenation.     

Methanol decomposition to synthesis gas at low temperature over palladium supported on ceria–zirconia solid solutions

The Ce_xZr_1−xO₂ solid solution was used as a support of a palladium catalyst for methanol decomposition to synthesis gas at low temperature. All Pd-containing catalysts tested in this study showed high selectivity to synthesis gas (over 96%). The Pd supported on the composite oxide with a Ce/Zr molar ratio of 4/1 exhibited the highest activity. Pd/Ce_0.8Zr_0.2O₂ (17 wt.%) (cop) (prepared by coprecipitation method) showed a conversion of 51.2% for the methanol decomposition at 473 K, which was higher than those over 17 wt.% Pd/CeO₂ (cop) (40.7%) and 17 wt.% Pd/ZrO₂ (cop) (24.3%) at 473 K. The 17 wt.% Pd/Ce_0.8Zr_0.2O₂ (cop) catalyst showed a higher BET surface area and smaller Pd particles than those of 17 wt.% Pd/CeO₂ (cop). Moreover, a more active Pd^σ+ state could be maintained by Zr⁴⁺ ion modification due to promotion of the oxygen mobility and enhancement of the reductibility and increase in the acid sites of the CeO₂ support. The 17 wt.% Pd/Ce_0.8Zr_0.2O₂ (cop) catalyst showed a much higher conversion (51.2%) than that over 17 wt.% Pd/Ce_0.8Zr_0.2O₂ (imp) (prepared by impregnation method) (17.2%) at 473 K. This is due to the 17 wt.% Pd/Ce_0.8Zr_0.2O₂ (cop) possessing many small Pd particles. The 17 wt.% Pd/Ce_0.8Zr_0.2O₂ (cop) catalyst showed an initial conversion of 51.2% at 473 K but the conversion decreased to 43.1% after 24 h on stream. This deactivation was attributed to carbonaceous deposit on the catalyst surface. The amounts of coke on the 17 wt.% Pd/Ce_0.8Zr_0.2O₂ (cop) catalyst were 0.9 wt.% after 24 h on stream at 473 K and 2.1 wt.% after 1 h on stream at 523 K.     

Effects of redox treatments on the structural composition of a ceria–zirconia oxide for application in the three-way catalysis

The influence of calcination and redox cycles on the structural modification and redox properties of a ceria–zirconia mixed oxide of nominal composition Ce_0.6Zr_0.4O₂ were investigated by XRD and Rietveld refinement, by BET measurement, TPR and OSC analyses. The material is characterized by high total OSC and retains this property after several redox and calcination cycles up to 1273 K, despite the loss of porosity and the decrease of surface area. The Rietveld analysis of the diffractograms allowed to establish that at least two solid solutions are present in the as-prepared sample: a cubic phase, space group Fm-3m, richer in cerium compared to the nominal composition, a tetragonal phase (t′) richer in zirconium. The first redox treatment modifies the composition towards two different solid solutions with composition closer to the nominal one: a cubic Ce_0.65Zr_0.35O₂ and a tetragonal (t′) Ce_0.47Zr_0.53O₂. No changes are noticed after four further redox cycles, while the successive calcination at 1273 K gives rise to a second cubic phase richer in cerium and to another tetragonal phase richer in zirconium. Noteworthy a further redox cycle reorganizes the sample to a similar situation as before the calcination treatment and this configuration is maintained for four redox cycles. This alternance is restored after calcination and successive four redox cycles. The total OSC measured after the above treatments is high and increases after the successive cycles.     

Characterisation of ceria–zirconia solid solutions after hydrothermal ageing

Ce_1−xZr_xO₂ (x=0–0.84) solid solutions prepared by co-precipitation were characterised after calcination at 700 or 900°C, and after hydrothermal ageing at 1000 or 1200°C. The solid solutions were formed at 700°C, and crystallise as cubic or tetragonal phases depending on their compositions. Despite the rather high surface areas obtained after calcination at 700°C, the sintering is important at 900°C, and tremendous after hydrothermal ageing at 1000°C. For all compositions between 16 and 83 mol% ceria, complete de-mixing of the solid solutions into two phases was observed after ageing at 1200°C: one Zr-rich, tetragonal phase, and one Ce-rich, cubic phase. XPS and ISS measurements show that the phase separation takes place with surface enrichment in Zr, the Zr-rich phase being formed at the periphery of the particles, whereas the core is composed of the Ce-rich phase.     

Performance of alumina-supported noble metal catalysts for the combustion of trichloroethene at dry and wet conditions

The performance of four different alumina-supported noble metal catalysts (0.5% of Pd, Pt, Rh and Ru, respectively) for the deep oxidation of trichloroethene (1000–2500 ppmV, WHSV = 55 h⁻¹) in air was studied in this work. Experiments were carried out at both dry and wet (20,000 ppm of H₂O) conditions. Catalysts were compared in terms of activity, selectivity for the different reaction products (CO₂, HCl, Cl₂, C₂Cl₄, CCl₄ and CHCl₃), and stability at reaction conditions.

As general trend, the activity of the catalysts decreases in the order Ru  Pd > Rh > Pt. Concerning to the effect of the water addition, no important effect on the catalyst activity was observed, except in the case of Pt, for which an increase of the catalytic activity was observed. Reaction mechanism (and hence product distribution) is very similar for Rh, Pd and Pt, being in these cases C₂Cl₄ the only organochlorinated by-product detected. In the case of Ru, the reaction mechanism seems to be quite different, CCl₄ and CHCl₃ being the main organic by-products.

Simple power-law kinetic expressions (first order on trichloroethene concentration for Pd, Rh and Ru, and zeroth order for Pt) provide fairly good fits for catalytic performance of the studied catalysts.

Finally, deactivation studies show that both formation of active metal chlorides (especially in the case of Rh) and fouling (especially for Pd and Pt) are the main deactivation causes.     

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

Rh(1%)@Ce_xZr_1−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 160 h of reaction at 873 K. 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.     

Impact of support oxide and Ba loading on the NOx storage properties of Pt/Ba/support catalysts: CO2 and H2O effects

A series of 1 wt.%Pt/_xBa/Support (Support = Al₂O₃, SiO₂, Al₂O₃-5.5 wt.%SiO₂ and Ce_0.7Zr_0.3O₂, x = 5–30 wt.% BaO) catalysts was investigated regarding the influence of the support oxide on Ba properties for the rapid NO_x trapping (100 s). Catalysts were treated at 700 °C under wet oxidizing atmosphere. The nature of the support oxide and the Ba loading influenced the Pt–Ba proximity, the Ba dispersion and then the surface basicity of the catalysts estimated by CO₂-TPD. At high temperature (400 °C) in the absence of CO₂ and H₂O, the NO_x storage capacity increased with the catalyst basicity: Pt/20Ba/Si < Pt/20Ba/Al5.5Si < Pt/10Ba/Al < Pt/5Ba/CeZr < Pt/30Ba/Al5.5Si < Pt/20Ba/Al < Pt/10BaCeZr. Addition of CO₂ decreased catalyst performances. The inhibiting effect of CO₂ on the NO_x uptake increased generally with both the catalyst basicity and the storage temperature. Water negatively affected the NO_x storage capacity, this effect being higher on alumina containing catalysts than on ceria–zirconia samples. When both CO₂ and H₂O were present in the inlet gas, a cumulative effect was observed at low temperatures (200 °C and 300 °C) whereas mainly CO₂ was responsible for the loss of NO_x storage capacity at 400 °C. Finally, under realistic conditions (H₂O and CO₂) the Pt/20Ba/Al5.5Si catalyst showed the best performances for the rapid NO_x uptake in the 200–400 °C temperature range. It resulted mainly from: (i) enhanced dispersions of platinum and barium on the alumina–silica support, (ii) a high Pt–Ba proximity and (iii) a low basicity of the catalyst which limits the CO₂ competition for the storage sites.     

Gold supported on ceria and ceria–alumina promoted by molybdena for complete benzene oxidation

New gold–molybdena catalysts supported on ceria and ceria–alumina in reaction of complete benzene oxidation were studied. The catalysts were characterized by means of XRD, TPR, XPS and Raman spectroscopy. High and stable catalytic activity was established in the temperature region 200–240 °C. The presence of gold causes a modification in ceria structure leading to an increase of Ce³⁺ and oxygen vacancies formation. The loading of Al³⁺ increases additionally the oxygen vacancies, while a tendency of decrease of Ce³⁺ amount was observed. The presence of alumina results also in a larger share of active oxygen species proved by analysis of O 1s XPS spectra. The differences in the activities within the starting temperature range (150–180 °C) and in the region of 100% conversion (200–240 °C) could be explained by supposing that in the LT region the electron transfer between nanosized gold and ceria particles via oxygen vacancies has a crucial role. In the HT region the oxygen mobility, provoked by the defective structure of ceria due to the presence of Al³⁺, becomes of prevailing importance. It was also concluded that alumina prevents the gold and ceria agglomeration, which is the main factor to avoid deactivation under extreme reaction conditions.     

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 20 m²/g, which is higher than that of conventional CZ (2 m²/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 10 nm 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.     

Cerium–terbium mixed oxides as alternative components for three-way catalysts: a comparative study of Pt/CeTbOx and Pt/CeO2 model systems

By using a combination of oxygen buffering capacity (OBC) and oxygen storage capacity (OSC) measurements, the redox behaviour of a Pt/CeTbO_x catalyst is compared to that of a classic model TWC system: Pt/CeO₂. The results reported here show that the redox efficiency of the Pt/CeTbO_x catalyst is much better, especially at low temperature operation conditions such as those occurring during the cold start of engines. The catalyst containing terbium also shows lower ‘light-off’ temperatures for both methane and carbon monoxide oxidation.     

Modification of the oxygen storage capacity of CeO2–ZrO2 mixed oxides after redox cycling aging

High surface area CeO₂–ZrO₂ mixed oxides were treated at 900–950°C either under wet air or under successive reducing and oxidizing atmospheres in order to study the evolution of the oxygen storage capacity (OSC) of these solids after different aging treatments. Several complementary methods were used to characterize the redox behavior: temperature programmed reduction (TPR) by H₂, TPO, magnetic susceptibility measurements to obtain the Ce³⁺ content, FT-IR spectroscopy of adsorbed methanol and a method to compare the oxygen buffering capacity (OBC) of the oxides.

All the results confirm that the mixed oxides exhibit better redox properties than pure ceria, particularly after aging. The enhancement in the OSC at moderate temperature has to be related to a deeper penetration of the reduction process from the surface into the under-layers. Redox cycling aging promotes the reduction at low temperature of all the mixed oxides, the improvement being much more important for low surface area aged samples. The magnitude of this effect does not depend on the BET surface areas which have similar values after cycling. This underlines the critical influence that the preparation and activation procedure have on the final OSC behaviors of the ceria–zirconia mixed oxides.     

Rh oxide reducibility and catalytic activity of model Pt–Rh catalysts

Pt and Rh were impregnated by different methods into the washcoat to investigate the differences in Rh oxidation state and catalytic activity of the samples. Both fresh and laboratory aged samples were studied. Clear differences in catalytic activity were noticed between the catalysts with different Pt and Rh addition methods. The best oxidation activity for fresh catalysts was achieved with the catalyst having both Pt and Rh deposited into the Ce–Zr mixed oxide. However, this state was observed to be unstable, and hence, this particular catalyst was dramatically deactivated in air ageing at high temperature. After ageing, the catalyst having both Pt and Rh impregnated lastly into the entire calcined washcoat matrix had the best activity in all three reactions, carbon monoxide and hydrocarbon oxidation and nitrogen oxide reduction. According to XPS studies, Rh was in easily reducible form in all the fresh samples. After ageing, the highest portion of reducible Rh was observed in the sample having also the best catalytic activity.     

Deactivation phenomena during catalytic wet air oxidation (CWAO) of phenol over platinum catalysts supported on ceria and ceria–zirconia mixed oxides

Catalytic wet air oxidation (CWAO) of aqueous solution of phenol was carried out with pure oxygen at 160 °C in a stirred batch reactor on platinum supported oxide catalysts (Pt/CeO₂^c calcined at 650 and 800 °C and Pt/Ce_xZr_1 − xO₂ with x = 0.90, 0.75 and 0.50). The catalysts were characterized before (BET, FT-IR spectroscopy, hydrogen chemisorptions, oxygen storage capacity (OSC)) and after reaction (TPO, elementary analysis, GC–MS and DTA–TGA). The results demonstrate a poisoning of the catalysts during CWAO reaction due to the formation of different forms of carbon deposit on the materials: carbonates and polymeric carbon species. This poisoning phenomenon is limited by the introduction of 50% of zirconium into ceria lattice for the catalysts presenting the lowest surface area. Polymeric deposits play a major role in the catalyst deactivation.     

Synthesis and characterization of doped nano-sized ceria–zirconia solid solutions

Two compositions Ce_0.50Zr_0.39La_0.04Y_0.07O_2−δ and Ce_0.25Zr_0.65La_0.04Y_0.06O_2−δ based on ceria-zirconia solid solutions were prepared as nanopowders using a continuous hydrothermal flow synthesis reactor, followed by either freeze-drying or hotplate-drying of the slurry. Each dried nanopowder was then subjected to 10 h heat-treatment at 1000 °C, 1100 °C or 1200 °C in air (to simulate accelerated ageing). The reducibility and hydrogen consumption of the oxidised samples were measured using temperature programmed reduction (TPR) up to 1000 °C. The effects of composition, drying method and heat-treatment temperature were evaluated on the TPR profiles of the materials. The powders were further investigated using a range of analytical methods including UV/Vis spectroscopy (which yielded colour data), Raman spectroscopy, powder X-ray diffraction, BET surface area measurements and X-ray photoelectron spectroscopy (XPS). Chemometric methods were used to investigate relationships between the spectroscopic and total oxygen storage capacity (OSC) data. Principal component analysis (PCA) was used to provide a simple interpretation of the effects of various synthesis and treatment parameters on Raman spectra. Principal component regression (PCR) was used to build regression models relating the Raman spectra and the temperature of hydrogen consumption peak at several set temperatures in the TPR. The total hydrogen consumption of the materials was generally high, while the drying and heat-treatment conditions appeared to have a significant effect on the final properties of the resulting powders, such as the surface area and total oxygen storage capacity.     

Effect of SO2 on the oxygen storage capacity of ceria-based catalysts

We have examined the effect of SO₂ poisoning on a series of catalysts having Pd supported on ceria, alumina, and ceria–zirconia. For pre-exposure of 20 ppm SO₂ at 673 K, we observed no changes in the light-off curves for CO oxidation on Pd/alumina. This pre-exposure of SO₂ to Pd/ceria resulted in a significant upward shift in the light-off curve, so that the poisoned Pd/ceria catalyst exhibited similar rates to that of Pd/alumina. Similar upward shifts were observed for the water–gas-shift reaction upon exposure of Pd/ceria or Pd/ceria–zirconia samples to SO₂. However, pulse-reactor data with alternating CO and O₂ pulses showed that SO₂ poisoning actually increased the amount of oxygen that could be transferred to and from the catalyst over the entire temperature range that was examined. The implication of these results for understanding the effect of SO₂ poisoning and the measurement of OSC are discussed.     

Effect of specific surface area on oxygen storage capacity (OSC) and methane steam reforming reactivity of CeO<Subscript>2</Subscript>

It was found from the work that the specific surface area of ceria presents an important role on the oxygen storage capacity (OSC), the reactivity toward methane steam reforming, and the resistance toward carbon formation of this material. After calcination at 900°C, ceria prepared by surfactant-assisted method (SF) was observed from the present work to have significantly higher surface area than those prepared by templating (TP) and precipitation (PP) methods; this material showed strong OSC with good reforming reactivity in terms of thermal stability and resistance toward carbon formation compared to others. In detail, the degree of OSC was measured by the number of hydrogen uptake from the temperature programmed reduction (TPR). It was found that the value of hydrogen uptake from the TPR-1 of ceria prepared by SF was 2084 mmol g⁻¹, whereas those of ceria prepared by TP and PP were 1724 and 781 mmol g⁻¹, respectively. In addition, it was also proven in the present work that the OSC of these materials are reversible, according to the temperature programmed oxidation (TPO) and the second time temperature programmed reduction (TPR-2) results. According to the reactivity toward methane steam reforming, after purging in 3 kPa methane and 9 kPa steam at 900°C for 8 h, the methane conversion at steady state of ceria prepared by SF was approximately 38% with very low amount of carbon formed on the surface (0.16 mmol g⁻¹), whereas those of ceria prepared by TP and PP were 22% (with the amount of carbon formation of 0.30 mmol g⁻¹) and 13% (with the amount of carbon formation of 0.33 mmol g⁻¹), respectively.