Ce Zr Mixed Oxides Research Articles

Hydrogen production by steam reforming of ethanol: Study of mixed oxide catalysts Ce 2Zr 1.5Me 0.5O 8: Comparison of Ni/Co and effect of Rh

One of the most interesting routes to produce energy from biomass is the production of hydrogen by bio-ethanol steam reforming. Deactivation, related to carbonaceous compounds deposition, is considered the principal inconvenience of the catalytic system based on transition metals supported on oxides. Carbon formation could be minimized using a support with good redox properties, a promoter, or enhancing the strong metal–support interaction. In this work, a catalytic system based on doped Ce–Zr mixed oxides with Co, Ni, Rh, Rh–Co or Rh–Ni is synthesized using a pseudo sol–gel method, characterized by X-ray diffraction (XRD) and thermo-programmed reduction (TPR), and evaluated as catalyst in the steam reforming of ethanol. Ethanol steam reforming and physicochemical characterization were performed in order to understand the active phase and the role of the promoter on the catalyst reduction state, the catalytic performance and the carbonaceous deposits formation.

Comparison of Cu–Ce–Zr and Cu–Zn–Al mixed oxide catalysts for water-gas shift

Cu–Zn–Al and Cu–Ce–Zr mixed oxide catalysts were prepared by two different methods, co-precipitation and flame spray pyrolysis. The performance of the catalysts was evaluated using the water-gas shift reaction with and without CO2 and H2 added to the feed. Cu–Ce–Zr catalysts are found not to be superior to Cu–Zn–Al catalysts in terms of initial activity and short-term stability. Their apparent activation energy appears to be less affected by increased concentrations of CO2 and H2.

Thermally stable ceria–zirconia catalysts for soot oxidation by O2

Abstract Ce–Zr mixed oxides calcined at 1000 °C are more active catalysts for soot oxidation than pure CeO 2 calcined at the same temperature, both in loose and tight contact between soot and catalyst. 1000 °C sinterised-CeO 2 presents a very low surface area (2 m 2 /g), a large crystal size (110 nm) and a lack of surface redox properties. Ce–Zr mixed oxides present higher BET surface areas (typically 17–19 m 2 /g), smaller crystal sizes and enhanced redox properties. The Zr molar fraction does not affect appreciably the catalytic activity of Ce–Zr mixed oxides in the range studied (Zr molar fraction from 0.11 to 0.51).

DRIFTS–XANES study of Pd-Ni/(Ce,Zr)O x/Al 2O 3 model automotive catalysts

Pd-Ni catalysts supported on Al 2O 3 and (Ce,Zr)O x /Al 2O 3, both in fresh and thermally aged states, were examined with the main aim of determining the effects of Ni and Ce-Zr mixed oxide on the catalytic activity for CO and C 3H 6 oxidation, and NO reduction reactions under stoichiometric conditions. For this purpose, catalytic activity results were analysed in conjunction with DRIFTS and XANES spectra recorded under reaction conditions to obtain information on the processes occurring in the catalysts during the course of the reactions. While a nickel-induced promotion of the formation of contacts between palladium and the Ce-Zr mixed oxides generally enhances the CO oxidation performance of the catalyst, it results detrimental to the NO reduction activity. This has been related to the higher difficulty in achieving the most active metallic palladium state for comparatively similar particle sizes in the presence of such contacts. Comparison of DRIFTS spectra under various reactant mixtures and between fresh and aged systems was shown to explain in a unique way the detrimental catalytic effects of hydrocarbon self-poisoning and sintering of the active components in the catalysts.

Dehydration of 4-methylpentan-2-ol over Ce xZr 1− xO 2/SiO 2 nano-composite catalyst

Surface stabilized nano-sized ceria–zirconia solid solution on silica support was synthesized through a soft chemical route using colloidal silica dispersion and nitrate precursors of cerium and zirconium. The dispersion and structural stability of the Ce x Zr 1− x O 2 solid solution after thermal treatments at 773 and 1073 K was investigated by means of X-ray diffraction (XRD), high-resolution transmission electron microscopy (HREM), X-ray photoelectron spectroscopy (XPS) and BET surface area techniques. Catalytic performance of the synthesized Ce x Zr 1− x O 2/SiO 2 sample was tested for the conversion of 4-methylpentan-2-ol to 4-methylpent-1-ene in the vapour phase under normal atmospheric pressure. From XRD measurements, the presence of cubic phases of the composition Ce 0.75Zr 0.25O 2, Ce 0.6Zr 0.4O 2 and Ce 0.5Zr 0.5O 2 were identified. The former phase was noted at 773 K and the later phases at 1073 K indicating enrichment of zirconium in the cubic fluorite type ceria lattice at higher calcination temperature. HREM results revealed existence of very small Ce–Zr mixed oxide crystallites of ∼5 nm size over amorphous silica at 773 K. No appreciable increase in the size of the crystallites was noticed even after calcination at 1073 K. XPS measurements indicated significant amount of Ce 3+ present in the samples. All the characterization techniques revealed that silica does not form any unwanted inert compounds with the dispersed Ce–Zr-oxides. The activity studies reveal that the Ce x Zr 1− x O 2/SiO 2 catalyst exhibits a high and stable activity in the time-on-stream experiments with good product selectivity to 4-methylpent-1-ene. Also a significant decrease in the reaction temperature corresponding to 50% conversion is noted over this catalyst.

Model bimetallic Pd-Ni automotive exhaust catalysts: Influence of thermal aging and hydrocarbon self-poisoning

Bimetallic Pd-Ni catalysts supported on Al2O3 and (Ce,Zr)Ox/Al2O3 were examined with respect to their catalytic performance for the elimination of CO, NO and C3H6 under stoichiometric conditions. The effects of a thermal aging treatment at 1273K, reactant competition in the presence of the hydrocarbon and the influence of the presence of nickel in the catalyst have been analysed by XRD, HREM, catalytic activity measurements and in situ DRIFTS spectroscopy. Self-poisoning effects, induced by the presence of the hydrocarbon in the reactant mixture, were identified as the main factor affecting the light-off activity. While a Ni-induced preferential interaction between Pd and the Ce-Zr mixed oxide component appears, in general terms, to be beneficial for the catalytic performance of the fresh (Ce,Zr)Ox/Al2O3-supported bimetallic catalyst, it is shown to be detrimental for the aged system as a consequence of a facilitated degradation of the (Ce,Zr)Ox component and encapsulation of the active palladium particles.

The effect of Ni in Pd–Ni/(Ce,Zr)O x/Al 2O 3 catalysts used for stoichiometric CO and NO elimination. Part 2: Catalytic activity and in situ spectroscopic studies

Pd–Ni catalysts supported on Al 2O 3, (Ce,Zr)O x /Al 2O 3, and (Ce,Zr)O x were examined with the principle objective of determining the effects of Ni on catalytic activity for CO oxidation and NO reduction reactions under stoichiometric conditions. Catalytic activity findings for the CO + O 2 and CO + O 2 + NO reactions were analyzed in conjunction with in situ DRIFTS and XANES results to obtain information on the processes occurring in the catalysts during the course of the reactions. The results reveal a significant dependence on the nature of the support in terms of the catalytic changes produced by nickel. In the absence of significant nickel-induced electronic perturbations of palladium, these are related to indirect effects on palladium distribution over the catalysts or to a certain impediment of the interactions between active palladium and Ce–Zr mixed-oxide components. Significant promotion of CO oxidation was observed for the (Ce,Zr)O x /Al 2O 3-supported catalyst, which reveals a relevant role for the particle size of the nanostructured Ce–Zr mixed oxide in this reaction.

The effect of Ni in Pd–Ni/(Ce,Zr)O x/Al 2O 3 catalysts used for stoichiometric CO and NO elimination. Part 1: Nanoscopic characterization of the catalysts

Pd–Ni catalysts supported on Al 2O 3, (Ce,Zr)O x /Al 2O 3, and (Ce,Zr)O x are characterized at a nanoscopic level using mainly electron microscopy-related techniques (HREM images, XEDS, and Z-contrast images/EELS spectra done with TEM and STEM instruments). The presence of rather homogeneous Ce–Zr mixed oxide nanostructures is revealed from analysis of HREM pictures. Analysis of the Pd–Ni/(Ce,Zr)O x /Al 2O 3 system shows the existence of preferential interactions of Pd and Ni with the (Ce,Zr)O x and Al 2O 3 components, respectively, as evidenced mainly by XEDS and confirmed by FMR. Dispersion states of the metals and particle size distributions of palladium are inferred from analysis of Z-contrast images and EELS spectra. The results are complemented by XPS and XAFS results used to analyse the chemical state of the metallic components in the catalysts and their dispersion over the different supports.

Effect of the presence of n-hexane on the catalytic combustion of chlororganics over ceria–zirconia mixed oxides

The complete oxidation of volatile organic compounds of different chemical natures, such as 1,2-dichloroethane, trichloroethylene and n-hexane, over Ce–Zr mixed oxide catalysts has been studied in a conventional fixed bed flow reactor. Catalytic conversion was found to depend on Ce/Zr molar content. The combination of surface acidity and accessible lattice oxygen appeared to control the catalytic performance of the mixed oxides. Thus, Ce 0.5Zr 0.5O 2 showed the highest combustion activity for the abatement of chlorohydrocarbons, whereas pure ceria was found to be the most active in the oxidation of the non-chlorinated compound. Destruction efficiency for the three VOCs decreased in the order: n-hexane > 1,2-dichloroethane > trichloroethylene. Significant ‘mixture effects’ were noticed when a given chlorinated feed was decomposed in the presence of n-hexane. On one hand, the destruction of the mixture induced an inhibition of the reactivity of each compound, leading to a marked increase in the ignition temperature. Competitive adsorption played an important role in the inhibition detected with CeO 2-based catalysts. On the other hand, HCl selectivity was greatly enhanced through the increased presence of water in the reaction atmosphere generated as a reaction product.

Catalysis in membrane reformers: a high-performance catalytic system for hydrogen production from methane

A palladium membrane reactor has been designed and sized to be applied to the dry reforming of methane for pure hydrogen production at a small scale. Three different parameters affecting the reactor operation have been adjusted to tune the reformer and optimise its performance: the extraction conditions, the CO 2/CH 4 ratio in the reactant mixture composition, and the reactants feed flow rate. By forcing H 2 extraction and adjusting the rates of hydrogen production in the reactor and hydrogen permeation through the Pd membrane, it has been possible to obtain very high CH 4 conversion for mixtures with CO 2/CH 4 ratios above unity. Reactant mixtures with CO 2/CH 4 ratios close to 2 offered the best results: high H 2 recovery yields (above 95%) and lower carbon deposition in the catalysts under the severe conditions imposed by the membrane reactor operation. Hydrogen extraction from the reaction side has been shown to enhance the carbon deposition rate on nickel-based reforming catalysts, in which the formation of low-reactivity carbon in the form of fibres has been observed. The dispersion of nickel on high oxygen mobility supports, such as Ce–Zr mixed oxides (Ce 0.5Zr 0.5O 2), which are chemically stable under reaction conditions, results in highly efficient catalysts capable of keeping their surface free of inactive carbon deposits. This kind of oxide provides an extra source of oxygen that equilibrates the net rates of carbon deposition and removal on the nickel surface, thus avoiding the accumulation of carbon in the catalyst.

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.

The effect of platinum on the reducibility of Rh oxides on CeZr modified alumina supported automotive catalysts

AbstractThe reducibility of Rh oxide on Rh‐only and PtRh catalysts was investigated. The catalyst washcoats were composed of alumina modified by La and CeZr mixed oxide. The catalyst ageing was performed in air at 1000°C for 3 h. In order to study the synergistic effect of Pt and Rh on the reducibility of Rh oxide, the propensity of the noble metal oxides to reduction was investigated by exposing the fresh and aged catalysts to 400 mbar of H2 at 300°C. XPS was used to observe the changes in chemical state of the noble metals and TEM and XRD were employed to determine the catalyst composition and morphology. Differences were revealed in the behaviour of Rh‐only and PtRh catalysts in H2‐reduction. Rhodium supported on alumina‐containing washcoats without Pt was found to react with the support at elevated temperatures to form a reduction‐resistive oxide phase. On the PtRh bimetallic catalyst Rh was found to be partly reducible after air ageing and the Rh 3d transition emerged with an unusually low binding energy after H2‐reduction. PtRh alloy particles were also formed in air ageing. Copyright © 2004 John Wiley & Sons, Ltd.

Effect of Ce–Zr mixed oxides on the thermal stability of transition aluminas at elevated temperature

Ce–Zr mixed oxides is a kind of promoted oxygen storage capability material widely used in three-way catalysts. Various contents (10, 20, 30 and 40%) of Ce–Zr were physically mixed into the pure γ-alumina for surface modification. The thermal stability of a series of Ce–Zr/Al 2O 3 supports was examined. Differential thermal analysis shows Ce–Zr mixed oxides increase the α-alumina transformation temperature up to 1222 °C. The powder X-ray diffraction studies show that minor θ-alumina phase still exists in the samples with ≥20% Ce–Zr doping content after aging at 1050 °C for 50 h. The best stabilizing effect is observed in the case of 20% Ce–Zr/Al 2O 3. Even after the accelerated aging test, it maintains an acceptable specific surface area (24 m 2 g −1) in relation with the delayed transformation of θ into α-alumina.

Metal–promoter interface in Pd/(Ce,Zr)O x/Al 2O 3 catalysts: effect of thermal aging

The behavior of Pd three-way catalysts promoted with Ce–Zr mixed oxides and supported on alumina subjected to thermal degradation treatments has been examined during light-off tests under stoichiometric CO+NO+O 2. Attention was paid to the region of aging temperature 1273–1373 K, where phase segregation of the ceria–zirconia promoter occurs. Characterization of the samples was performed using X-ray diffraction and Raman spectroscopy. In situ XANES/DRIFTS studies of the Pd chemical state at the surface and in the bulk of the materials were conducted to determine the noble metal response to the gas atmosphere during light-off. Electron paramagnetic resonance was used to analyze the promoter surface and to study the redox response of the materials in contact with the reactant gases. Thermal degradation appears to sinter both the noble metal and promoter components of the catalyst but also stabilizes oxidized Pd entities in contact with the promoter prior to the occurrence of the Ce–Zr phase segregation reaction. Additionally, it enriches the promoter surface in the lanthanide cation. The influence of these physicochemical phenomena on the catalytic properties of the Pd-based TWC is discussed.

The thermal stability and catalytic performance of Ce-Zr promoted Rh-Pd/γ-Al 2O 3 automotive catalysts

The noble metals (Rh, Pd) supported on alumina promoted with Ce-Zr mixed oxides were investigated as catalysts under diverse oxidants/reductants ratios of simulated automotive exhaust feed gas. Aging test was performed in air at 1000 °C for 20 h. Both fresh and aged catalysts, as well as alumina-supported Ce-Zr mixed powders, were characterized by means of X-ray diffraction (XRD), scanning electron microscope (SEM), Brunauer–Emmett–Teller (BET) and X-ray photoelectron spectroscopy (XPS). The results confirmed the oxygen storage capacity of Ce-Zr mixed oxides favored CO, C 3H 8 and NO conversions in stationary versus dynamic stoichiometric operation. The accelerated aging of the systems produced inconspicuous effect on the phase structure, specific surface area and chemical state of these samples. Accordingly, the aged samples still showed low light-off temperature and broad λ-window, which suggested the relative low relevance of the thermal degradation on the overall catalytic performance of the system.

NO reduction with ethanol on MoO 3/Al 2O 3 and CeO 2-ZrO 2-supported Pd catalysts

The reduction of NO with ethanol on MoO 3/Al 2O 3 and CeO 2-ZrO 2-supported Pd catalysts was studied. The Pd/CeO 2-ZrO 2 sample showed higher activity for the conversion of NO and higher selectivity for N 2 formation when compared to the Pd-MoO 3/Al 2O 3 sample. The CO chemisorption results showed that the CeZr mixed oxide chemisorbed a higher amount of CO, and this is related to a higher reducibility capacity of this compound when compared to MoO 3. Furthermore, TPD analysis of adsorbed NO and ethanol showed that the Pd/CeO 2-ZrO 2 catalyst has a higher ability of dissociating NO to N 2 and of decomposition of ethanol with the formation of CO 2. Also, TPSR experiments showed that on Pd-8Mo/Al 2O 3, ethanol competes with NO for adsorption sites on partially reduced molybdenum oxide. This was not the case for the Pd/Ce 0.75Zr 0.25O 2 catalyst, where both NO and ethanol adsorb and decompose on the partially reduced mixed oxide surface. These facts are probably related to the better performance of this catalyst in relation to the Pd-MoO 3/Al 2O 3 catalyst for the NO + ethanol reaction.

Characteristics of Ce-Zr mixed oxide films as buffer layer by controlling composition

YSZ films have been widely used as buffer layers for high T/sub c/ superconductor. However, it is necessary to investigate and develop another buffer layer with suitable, simple, and neat processing. Films of Ce-Zr mixed oxide were deposited by rf and dc magnetron co-sputtering. In sputtering process, dc power of Zr was fixed in 200 W while rf power of Ce was varied with 30 W, 50 W, 100 W, respectively. As-deposited (Ce/sub x/Zr/sub 1-x/)O/sub 2/ films were crystallized without post annealing. It was confirmed that the composition of the films could be controlled with controlling rf power of Ce target. The /spl Phi/ scan of XRD showed that all (Ce/sub x/Zr/sub 1-x/)O/sub 2/ films were [200] c-axis oriented. Three consecutive magnetron sputtering procedure for seed, CZO and cap layer for HTSC films using Ce, Zr and CeO/sub 2/ target were carried out on the Si[100] and Ni substrate successfully. It is suggested that sputtered and c-axis oriented (Ce/sub x/Zr/sub 1-x/)O/sub 2/ films can be a potential candidate to replacing YSZ buffer layer.

Characteristics of Textured Ce-Zr Mixed-Oxide Films as Buffer Layer for YBCO Coated Conductors

Single- and multi-layer (Ce1 − xZrx)O2 films (0 ≤ x ≤ 0.84) on Si (100) and polycrystalline Ni substrates were prepared using RF and DC magnetron co-sputtering. XRD of φ scan analysis showed that all (Ce1 − xZrx)O2 films were biaxially oriented with the c-axis perpendicular to the plane of the film. During sputtering, DC power to the Zr target was fixed at 200 W, while RF power to the Ce target was set at 30 W, 50 W, or 100 W. As-deposited ZrO2 film was amorphous and was crystallized by post-annealing. However, as-deposited (Ce1 − xZrx)O2 films were crystalline even when grown at room temperature and the structures of films were cubic or tetragonal depending on the Ce ion incorporation. It was found that multilayered CeO2/(Ce1 − xZrx)O2/CeO2 films could be deposited with a continuous compositional gradient in a sputtering batch. This layered CeO2/CZO/CeO2 structure can maintain its original texture after 800°C annealing and is therefore suitable for subsequent YBCO film growth. Furthermore, Ni diffusion is effectively blocked by the buffer layers just like the YSZ currently used in coated conductor fabrication.

High‐temperature‐aging‐promoted Rh oxides on supported automotive exhaust catalysts

AbstractThermal decomposition of noble metal oxide phases formed during aging in air on industrially manufactured γ‐Al2O3‐supported Rh catalysts was investigated. Some of the catalyst washcoats were modified by various stabilizers, promoters and oxygen storage components, such as La2O3, CeO2 and Ce–Zr mixed oxide. In order to investigate the effect of aging, stabilizers and manufacturing method on the thermal stability of the noble metal oxide phases formed, a stepwise thermal decomposition in high vacuum (pressure <10−7 Torr) was performed. X‐ray photoelectron spectroscopy (XPS) was used to observe the changes in chemical state after each annealing cycle. Our results show marked differences in the rate of reduction between different Rh catalysts. Disposition of the Rh oxides towards reduction was also investigated by exposing the catalysts to 400 mbar of H2 at 300°C for 30 min. After 3 h of aging in air at 1000°C an irreducible rhodium oxide, which was identified to be either Rh2O3 diffused into alumina or RhO2 in intimate contact with the alumina surface, was formed regardless of the Ce–Zr mixed‐oxide additive and manufacturing method of the catalyst. Copyright © 2002 John Wiley & Sons, Ltd.

Influence of thermal sintering on the activity for CO–O2 and CO–O2–NO stoichiometric reactions over Pd/(Ce, Zr)Ox/Al2O3 catalysts

A Pd/(Ce, Zr)Ox/Al2O3 catalyst has been studied to determine how thermal aging under air at 1273K influences its structural characteristics and catalytic properties for CO oxidation reactions under stoichiometric conditions, employing either O2 or an NO–O2 mixture as oxidants. XRD and TEM-EDS show that significant sintering of both Pd and Ce–Zr mixed oxide active components is produced upon aging. As a consequence, lower CO oxidation or NO reduction activities were observed in catalytic tests. In situ DRIFTS studies for both reactions indicate that the lower activity of the aged systems is not only due to the sintering of active components but also due to a consequent loss of (Ce, Zr)Ox-induced promoting effects on Pd activity.