Ceria Zirconia Mixed Oxides Research Articles

Design of Specific Acid-Base-Properties in CeO2-ZrO2-Mixed Oxides via Templating and Au Modification

Ceria-zirconia mixed oxides and gold supported oxides exhibit very good thermal stability and catalytic activity, as well as great selectivity. This work has been focused on the controlled synthesis and characterization of cationic- and amphiphilic-templated ceria, zirconia, and ceria-zirconia mixed oxides from nitrate and iso-propoxide precursors, and ceria-zirconia mixed oxides modified with gold via the deposition precipitation method with urea. The characterization of the acidic and basic properties was carried out through two test reactions. A complete chemical and structural characterization of the materials was done using Atomic Absorption Spectroscopy (AAS), Brunauer-Emmet-Teller Surface Analysis (N2-BET), X-Ray Diffraction (XRD), NH3- Temperature Programmed Desorption (TPD)/CO2-TPD, and Fourier Transform Infrared Spectroscopy (FTIR). Template techniques led to the formation of high surface area mesoporous materials with high activity and thermal stability. In general, the acid sites density was decreased, whereas the basic site density was increased by modification with Au or incorporation of zirconia in case of mixed oxides.

Ceria–zirconia mixed oxides: Synthetic methods and applications

ABSTRACTThe primary objective of this review was to illustrate the significance of ceria–zirconia (CZ) mixed oxides as catalysts and catalyst supports as employed for a wide variety of catalytic applications both in the liquid and gaseous phases. In particular, we were interested in bringing together the recent literature pertaining to these mixed oxides with catalysis perspective. The most prominent application of CZ mixed oxides is in three-way catalysis (TWC) as oxygen storage and release material for several years by replacing cerium dioxide as it shows better efficiency and a high thermal stability. Doping with zirconium oxide, as it is alone a non-reducible oxide, makes the CZ mixed oxide a highly reactive, thermally stable, and more reducible with elevated oxygen storage capacity (OSC) that are important for TWC applications. Apart from the TWC use, the CZ mixed oxides have a huge number of applications, as a direct component or a support, ranging from water–gas shift reaction, reforming of hydrocarbons, dehydration of alcohols, CO2 utilization, catalytic combustion of pollutants, fine chemicals production, photocatalysis, and so on. All these applications are mainly dependent on three parameters of the mixed oxides, namely, OSC or redox nature, acid–base properties, and crystalline phases. Besides, most of the applications are influenced by the physical properties such as specific surface area, pore volume, pore diameter, crystallite size, and so on. In this review, many details pertaining to the synthesis of these mixed oxides by various conventional and non-conventional methods, their characterization by several techniques, and their application for various reactions of energy and environmental significance, as reported in the literature, are assessed.

Formation of NH3 and N2O in a modern natural gas three-way catalyst designed for heavy-duty vehicles: the effects of simulated exhaust gas composition and ageing

An increasing number of heavy-duty vehicles are using liquefied natural gas (LNG) as a fuel due to the expanding refuelling station network for LNG and lower overall emissions compared to diesel vehicles. The latest EURO VI regulation or natural gas fuelled vehicles set a limit for NH3 of 10 ppm, and N2O exhaust is expected to be restricted in Europe in the near future. Poisonous and corrosive NH3 and the greenhouse gas N2O are formed as by-products in a three-way catalyst used to minimize the emissions of stoichiometric heavy-duty engines. In this work, we studied how high temperature NH3 and N2O formed in modern, fresh and aged bimetallic Pd/Rh three-way catalysts in simulated exhaust gas. More precisely, the exhaust gas composition and temperature were examined. Decreases in NO concentration and increases in temperature lowered the formation of NH3 and N2O, whereas a decrease in CH4 concentration reduced only NH3 formation. According to Raman and powder X-ray diffraction experiments, the structure of the catalyst changed during the ageing, and this reputedly affected the function of cerium-zirconium mixed oxides and thus the formation of NH3 and N2O. Temperature programmed reduction (H2-TPR) measurements showed changes in cerium-zirconium mixed oxide performance after ageing supporting Raman spectroscopy findings. Catalyst ageing in oxidizing conditions increased the formation of N2O. This study showed that exhaust gas composition plays an important role in the formation of undesired NH3 and N2O emissions.

Support-induced effect on the catalytic properties of Pd particles in water denitrification: Impact of surface and structural features of mesoporous ceria-zirconia support

The support effect on the catalytic properties of palladium particles has been investigated in the reduction of nitrites by hydrogen at 20°C in batch conditions. The support material was composed of ceria-zirconia mixed oxide stabilized in the tetragonal structure according to an Evaporation Induced Self-Assembly method. In comparison, a co-precipitation method leads to inhomogeneity in composition related to the partial segregation of the cubic fluorite structure of CeO2. Further textural properties obtained for the two synthesis routes differ as well as changes in the reducibility. Surface analysis demonstrated a more extensive surface reduction of Ce4+ to Ce3+ on the series prepared by coprecipitation correlated to a greater stabilization of metallic Pd particles. However, the best catalytic performances were obtained on low loaded Pd samples supported on the ceria-zirconia support prepared by the Evaporation Induced Self-Assembly method which emphasizes the fact that catalytic properties cannot be simply explained by the stabilization of zero valent precious metal at the vicinity of anionic vacancies. Further calculations of TOF and interfacial rates were achieved and compared to the selectivity behavior suggesting that the localization of Pd in contact with the tetragonal or cubic structure of the solid solution and CeO2 could play a key role in determining the catalytic properties.

The effect of hydrogen peroxide on properties of Ce0.35Zr0.55La0.055Pr0.045O2 oxides and the catalytic performance used on Pd supported three-way catalyst

In this study, two series of cerium zirconium mixed oxides Ce0.35Zr0.55La0.055Pr0.045O2 were prepared under traditional co-precipitation and oxidation co-precipitation methods respectively. The physicochemical properties of the samples were compared under these two methods and assessed by XRD, Raman, BET, TEM, H2TPR, OSC, XPS and catalysts measurements. The formation of homogeneity phase structure can be facilitated by changing the precipitating properties of Ce3+ under oxidation co-precipitation method, which is helpful to enhance the homogeneity of Ce and Zr at atomic level. What's more, it is conducive to remove impurities Na+ and Cl− by oxidation co-precipitation with hydrogen peroxide. The catalysts activities are related to both the redox properties and the textural properties of mixed oxides. The Pd-only TWCs supported on the CZLP-H-F exhibits better catalytic performance and thermal stability with wider air/fuel ratio operation window, lower light-off and full conversion temperatures of C3H8 and NO. The homogeneity of phase structure for cerium zirconium mixed oxide can be predicted and deduced from detecting the atomic distribution uniformity of its precursor. So this work not only provides insights into the mechanisms for phase segregation of cerium zirconium mixed oxide, but also provides a guidance to improve homogeneity of cerium zirconium mixed oxide by adding additives.

Identifying the nature of the copper entities over ceria-based supports to promote diesel soot combustion: Synergistic effects

The aim of this research is to prepare catalysts exhibiting a wide range of copper incorporation/distribution/contact degrees on ceria-zirconia mixed oxide by preparing catalysts with a high degree of copper incorporation, well-dispersed copper on the surface/porosity and different degrees of physical interaction. Their catalytic activities towards diesel soot combustion and NO oxidation to NO2 were analyzed with the purpose to identify the nature of the copper entities over ceria-based supports to promote diesel soot combustion. The catalysts were characterized by N2 adsorption-desorption isotherms at −196°C, XRD, Raman spectroscopy, XPS, H2-TPR and reducibility tests under inert atmosphere. Activity tests were performed towards NO oxidation to NO2 (from 25°C to 700°C) and diesel soot combustion in NOx/O2 atmosphere, under ramp mode (from 25°C to 700°C) and isothermal conditions (450°C). Results reveal that copper in close contact with ceria greatly promotes its reducibility under H2 and He at low temperatures, as well as catalytic NO oxidation to NO2. Mainly, 2%Cu/ceria-zirconia catalyst prepared by incipient wetness impregnation shows the highest activity towards soot combustion at low temperatures (≤450°C), due to the observed synergistic copper-ceria interactions.

The effect of precipitation pH on thermal stability and structure of Ce0.35Zr0.55(LaPr)0.1O2 oxides prepared by co-precipitation method

Cerium zirconium-based oxygen storage material is indispensable in improving the catalytic efficiency for TWCs. However, the synthesis mechanism is seldom investigated with a method by RE/Zr chloride and sodium hydroxide. Therefore, in this study, Ce0.35Zr0.55(LaPr)0.1O2 oxides were prepared by co-precipitation method at different precipitation pH. The relationships between pH and physicochemical properties of Ce0.35Zr0.55(LaPr)0.1O2 oxides were elucidated. The results showed that the obtained Ce0.35Zr0.55(LaPr)0.1O2 oxides exhibited homogenous phase structure, best thermal stability and highest oxygen storage capability at pH = 10. In order to analysis the results, the colloidal nuclear reaction model was proposed and the rationality of the model was clearly illustrated by measuring zeta potential (ζ), impurities content of Na+ and Cl− and pH variations of the co-precipitation reaction system. It was proved that the cerium zirconium mixed oxides precursor's surface potential was changed from positively charged to negatively charged along with the pH values changed from low to high and had iso-electric point (IEP) at pH = 8. In addition, the pH values of all solutions were reduced differently after the precipitation reaction. The model may provide a good reference for preparation cerium zirconium mixed oxides with different composition under chloride raw material system.

Influence of Ce/Zr Ratio on the Synergistic Effect over CuCe1–xZrxOy/ZSM-5 Catalysts for the Self-Sustained Combustion of Carbon Monoxide

ABSTRACTThe self-sustained combustion of CO was studied over a series of CuCe1–xZrxOy/ZSM-5 catalysts with different Ce/Zr molar ratios to evaluate the correlation of their structural characteristics with catalytic performance. The obtained results demonstrated that the activity of the cerium-containing catalysts was much higher than that of the CuZr1/Z, with the activity of CO combustion following the order: CuZr1/Z < CuCe0.25Zr0.75/Z < CuCe0.5Zr0.5/Z < CuCe1/Z < CuCe0.75Zr0.25Z. After cerium addition, the cerium-zirconium mixed oxides formed promoted the copper species well dispersed on the catalyst surface. The high activities of cerium-containing catalysts were closely related to the synergetic effects among the copper, cerium, and zirconium species, as the surface copper species provided sites for CO adsorption and the cerium-zirconium mixed oxides promote the activation of oxygen. The activities were probably also attributed to some copper species present as substitutional defects in the ceria lattice, which were more reducible than the copper clusters, minicrystals, and bulk CuO particles. The CuCe0.75Zr0.25/Z possessed excellent redox capacity, adsorption/activation properties for O and CO species, and therefore induced a self-sustained combustion of CO at the lowest temperature than that of other CuCe1-xZrxOy/ZSM-5 catalysts.

The impact of synthesis method of CNT supported CeZrO2 and Ni-CeZrO2 on catalytic activity in WGS reaction

Carbon nanotube (CNT) supported catalysts containing ceria-zirconia mixed oxide (CeZrO2) and nickel were synthesized and tested in water gas shift (WGS) reaction. Physicochemical characterization including N2 adsorption, X-ray diffraction (XRD), scanning and transmission microscopy (SEM/TEM), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA) and temperature programmed reduction with H2 (H2-TPR), as well as catalytic tests of WGS reaction showed that the synthesis method had significant impact on composition, morphology, structural properties and catalytic performance of obtained hybrid materials. The catalysts obtained by co-precipitation of metal oxides (NiO and/or CeZrO2) on CNT walls demonstrated better dispersion of active phase and smaller particle size than catalyst obtained by depositing of powder CeZrO2 or Ni-CeZrO2. Moreover, the catalyst obtained by co-precipitation revealed better performance in WGS reaction; however, some CH4 formation was noticed over Ni-CeZrO2/CNT system. The role of CeZrO2 in catalysts performance in WGS as well as the importance of good metal-oxide contact were confirmed.

Catalytic reactions of coke with dioxygen and steam over alkaline-earth-metal-doped cerium-zirconium mixed oxides

A series of cerium-zirconium mixed oxides doped with alkaline earth metals was prepared by calcining a mixture of Ce0.8Zr0.2O2 and the nitrate salt of alkaline earth metals at 1173K. The results from X-ray diffraction, Raman spectroscopy and analytical electron microscopy indicated the alkaline earth compounds were dispersed on the surface of Ce0.8Zr0.2O2. The doped mixed oxides were evaluated in the catalytic reaction of solid coke under tight contact conditions with 10 vol.% O2 in He at 693K and with 100vol.% H2O at 993K using thermogravimetric analysis. For coke oxidation with O2, the weight loss was first order in coke, and the measured first order rate constant was proportional to the total surface area of catalyst at low catalyst loadings. For coke gasification with H2O, the weight loss was nearly zero order in coke, and the measured areal reaction rate was independent of the total surface area of catalyst at low catalyst loadings. The addition of alkaline earth metals inhibited coke oxidation with dioxygen but promoted coke gasification with steam.

The influence of H2O2 on the properties of CeO2-ZrO2 mixed oxides

A series of ceria-zirconia (CZ) mixed oxides were prepared by coprecipitation, in which different amounts of H2O2 were added to investigate the influence of H2O2 on the properties of CZ. Modifications in structure, oxygen storage capacity, reducibility and thermal stability were studied systematically. The X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) results revealed that the thermal stability of the material was dramatically improved by the addition of H2O2, which was mainly due to the increase of the solubility of ZrO2 in ceria from 4.7 to 69.3%. The study of UV Raman spectroscopy confirmed that the concentration of defect sites of CZ materials increased with increasing H2O2/Ce molar ratio. The material prepared with H2O2/Ce molar ratio of 2 (CZ-2) exhibited the highest structural stability, the solubility of which was 68.8 and 69.3% before and after thermal treatment, and it maintained a homogeneous single cubic phase even after aging treatment at 1000 °C for 3 h. Also the highest thermal stability of the oxygen storage capacity (OSC) was found in CZ-2, which showed the reduction rates of 80.4 and 76.4% before and after thermal treatment. Moreover, a reaction mechanism of H2O2 and metal ions in the mixed precursor solution was assumed, in which the cerium-peroxide bridge-zirconium complex structure was proposed.

Effect of the calcination temperature of cerium–zirconium mixed oxides on the structure and catalytic performance of WO3/CeZrO2 monolithic catalyst for selective catalytic reduction of NOx with NH3

The calcined temperature of the carrier obviously affected SCR activity of catalysts, WO3/Ce0.68Zr0.32O2-500 showed the best low-temperature NH3-SCR activity due to its more Lewis acid sites and stronger redox property.

The proportion of Ce4+ in surface of CexZr1-xO2 catalysts: The key parameter for direct carboxylation of methanol to dimethyl carbonate

In order to correlate the physico-chemical properties of CexZr1-xO2 catalysts with their catalytic activities for direct carboxylation of methanol to dimethyl carbonate (DMC), ceria-zirconia mixed oxides with the same nominal composition (Ce0.5Zr0.5O2), which were prepared by different methods, were applied. The catalysts were characterized by means of XRD, N2 adsorption, ICP-MS, NH3-TPD, CO2-TPD, Raman spectroscopy and XPS. Significant variations in their physico-chemical properties were observed by varying the preparation conditions. The results showed that the catalytic activity of the ceria-zirconia mixed oxides increased with improving the proportion of Ce4+, acidity and basicity of the catalyst surface. And the number of surface Ce4+ content is the most important factor.

Effect of calcination/reduction temperature of Ni impregnated CeO2–ZrO2 catalysts on hydrogen yield and coke minimization in low temperature reforming of ethanol

CeO2–ZrO2 was proven to be a quite effective catalyst support for steam reforming of ethanol (SRE). In this study, properties of Ni impregnated mesoporous CeO2–ZrO2 catalysts of different compositions were optimized by changing the calcination/reduction temperature. Increase of calcination/reduction temperature from 400 °C to 650 °C caused significant decrease in surface area and redox ability of the catalysts, yielding higher coke formation and lower hydrogen yields. Results proved that, very high hydrogen selectivity values with negligible coke formation can be achieved at a SRE reaction temperature of 450 °C, over the Ni impregnated ceria zirconia mixed oxide catalyst, which was also calcined/reduced at 450 °C. Considering its high hydrogen selectivity values with negligible coke formation, the catalyst containing a CeO2/ZrO2 ratio of 6:1 was found as the best catalyst for low temperature SRE. Achievement of high hydrogen yields with negligible coke formation at SRE temperatures as low as 400 °C was quite promising from the point of view of hydrogen economy.

Influence of pretreatment atmospheres on the performance of bimetallic Au-Pd supported on ceria-zirconia mixed oxide catalysts for benzyl alcohol oxidation

Monometallic Au, Pd and bimetallic Au-Pd catalysts supported on a Ce0.62Zr0.38O2 mixed oxide have been synthesized by the sol-immobilization method. An in-depth Scanning-Transmission Electron Microscopy (STEM) study has been performed to reveal the structural and chemical nature of the metal nanoparticle system present in these catalysts. Attention has been paid both to the evolution of the particle size distribution and the degree of Au-Pd interaction as a function of the treatment used to activate the catalysts. This characterization work has been complemented by results coming from other macroscopic techniques like Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES), N2 physisorption, Temperature Programmed Oxidation (TPO) or X-ray Photoelectron Spectroscopy (XPS). The whole set of characterization data evidences the intrinsic structural complexity of this type of bimetallic systems, in which a fraction of monometallic Au, monometallic Pd and bimetallic nanoparticles of varying compositions coexist. This picture, which clearly contrasts with other much more homogeneous situations described in previous literature on bimetallic catalysts, also indicates the requirement of combining both atomic scale and macroscopic techniques to understand the structure of these catalysts.Concerning catalytic performance, a synergistic effect between Au and Pd has been observed in the selective oxidation of benzyl alcohol on ceria-zirconia. Moreover, the catalytic activity of these bimetallic Au-Pd catalysts is higher than that on other commonly used supports, such as activated carbon or carbon nanotubes. The treatments in oxidative and inert atmospheres at 250°C improve the catalytic activity with respect to the fresh, un-pretreated, catalyst. Subsequent reduction of the oxidized bimetallic catalyst leads to an activity similar to that of the fresh catalyst, which is reduced at room temperature with H2 bubbles.

Structure and properties of cerium zirconium mixed oxide prepared under different precipitate aging processes

Oriented attachment and Ostwald ripening are two aging mechanisms of precipitation particles which may result in different crystallization mechanism of precipitates during the aging process. In this work, the effects of different aging process on the structure and properties of cerium zirconium mixed oxides were investigated. The results indicated that the mixed structure of 11.48% CeO2 phase and 88.52% Ce0.26Zr0.62(LaPr)0.12O2 solid solution phase were obtained under oriented attachment aging process. The rod-like CeO2 phase coexisted with spherical Ce0.26Zr0.62(LaPr)0.12O2 solid solution phase, which improved the surface area (64 m2/g) and pore volume (0.32 mL/g) of cerium zirconium mixed oxides after 1000 °C 4 h thermal treatment. However, through controlling the aging process, the Ce0.35Zr0.55(LaPr)0.10O2 solid solution with homogenous phase structure was generated by Ostwald ripening aging process, exhibiting higher oxygen storage capacity (501 µmol O2/g) and H2 consumption per gram (1378.3 µmol H2/g).

Synthesis of stable cerium zirconium oxide nanoparticle – Diesel suspension and investigation of its effects on diesel properties and smoke

One of the main problems of diesel engines is the harmful smoke in exhaust and the use of fuel borne catalysts is a good option for controlling the same. Cerium zirconium mixed oxide (CexZr(1−x)O2) nanoparticle is an excellent fuel borne catalyst. Two main issues with the addition of cerium zirconium mixed oxide nanoparticles in diesel are the lack of stability of catalytic nanoparticle in diesel suspension and the influence of these nanoparticles on various properties of diesel. The present work focuses on the synthesis of stable suspension of cerium zirconium mixed oxide nanoparticles in diesel and also on the effect of these nanoparticles on fuel properties, diesel engine performance and exhaust smoke. Cerium zirconium mixed oxide nanoparticle was synthesized by co precipitation method and characterized using Transmission electron microscope and Dynamic light scattering techniques. Catalytic activity of mixed oxide nanoparticles was compared by means of Temperature programmed reduction technique. Nanofluid was prepared by two step method, employing an ultrasonic shaker and oleic acid was used as surfactant to improve the stability of nanoparticle in diesel. The concentration of surfactant was varied from 0.01 to 0.1% by volume and the optimum value was determined by means of UV spectrum absorbance study. The dosing level of nanoparticles in diesel was varied from 2.5 to 20ppm and the optimum concentration of catalytic nanoparticles for maximum stability was determined based on zeta potential measurement. Various properties of diesel and modified diesel were determined as per ASTM standards. Load tests were carried out on a single cylinder four stroke diesel engine to investigate the effect of catalytic nanoparticles on diesel engine performance and smoke. Performance studies shows 31% reduction in exhaust smoke and 3% enhancement in brake thermal efficiency of diesel engine for a nanoparticle concentration of 17.5ppm.

Low-Temperature Selective Catalytic Reduction with NH&lt;sub&gt;3&lt;/sub&gt; over MnO&lt;sub&gt;x&lt;/sub&gt;-CeO&lt;sub&gt;2&lt;/sub&gt; Catalysts Supported on Nano Tetragonal Zirconia

The MnOx-CeO2/t-ZrO2 catalyst was prepared by impregnation with nano t-ZrO2 as the support. The influence of active component and reaction temperature on denitration performance of catalyst was investigated. The results showed that denitration efficiency improved as active component increased and reaction temperature rose. The denitration efficiency of 2.5% MnOx-CeO2/t-ZrO2 at 100°C was 68.1% while 15% MnOx-CeO2/t-ZrO2 was 97.4%. The results of XRD, BET and H2-TPR showed that surface structure of loaded catalyst was good for oxidation-reduction and denigration. NH3-TPD test demonstrated that NH3 was mainly adsorbed at Lewis acid sites on the surface of catalysts and became coordination NH3. Intermediate product NH2NO generated from reactions between coordination NH3 and NO which finally changed into N2 and H2O.NOx are potentially harmful to humans as a kind of primary pollutants. And NOx are the main cause of many environment problems, such as acid rain, surface ozone pollution and Particulate Matter 2.5[1]. The emission of NOx was 2337.8 tons in China in 2011 and that was 2275.4 tons[2]. The environmental situation is grim although the emission of NOx had begun decreasing. Emission standard of air pollutants for thermal power plants which came into effect on January 1, 2012 require the emission concentration of NOx under 100mg·m-3. The task is arduous.Selective catalytic reduction (SCR) of NOx with NH3 is the most promising method to remove NOx and catalysts with high activity play a decisive part in low temperature SCR technology. Many researches about metal oxide as SCR catalyst support have been reported recently, such as TiO2[3], Al2O3[4], activated carbon[5] and molecular sieve[6]. Zirconium oxide has attracted considerable attention recently as a catalyst support because of its special characteristics. Takahashi et al.[7] investigated the influence of the various compositions of TiO2 and ZrO2 on the NOx removal ability over a sulfur-treated NSR catalyst and came to a conclusion that ZrO2 support suppressed the solid phase reaction with potassium. Reddy et al.[8, 9] investigated structural characteristics of nanosized ceria-silica, ceria-titania, and ceria-zirconia mixed oxide catalysts and found these mixed oxides exhibit better redox properties than pure CeO2. YAN Zhi-yong et al.[10] reported that the existence of ZrO2 in catalysts can raise its specific area and enhance the dispersion of CeO2 on catalysts which results in high activity of the catalysts. CeO2/TiO2-ZrO2 catalyst has strong tolerance to water vapor and sulfur dioxide.It is well known that ZrO2 exists mainly in three polymorphs with monoclinic (m-ZrO2), tetragonal (t-ZrO2) and (c-ZrO2) cubic structures[11]. ZrO2 polymorphs have different amphoteric character of its surface hydroxyl groups. The crystalline phase of ZrO2 has a great effect on the structure, activity and selectivity of catalysts. Therefore, it is valuable to investigate the effects of nanocrystalline zirconia polymorphs on catalytic properties of MnOx-CeO2/t-ZrO2 Catalysts which few researchers have concerned about. In this study, we try to investigate catalytic activity and microstructure of SCR catalysts with manganese oxide and cerium oxide supported on t-ZrO2.

Synergistic effect of bimetallic Au-Pd supported on ceria-zirconia mixed oxide catalysts for selective oxidation of glycerol

A series of bimetallic Au-Pd catalysts supported on Ce0.62Zr0.38O2 mixed oxide has been synthesized using a simultaneous deposition-precipitation method. Different Au:Pd ratios were obtained maintaining Au loadings constant and varying Pd content. Elemental analysis, X-ray diffraction, N2 physisorption, X-ray photoelectron spectroscopy, scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) and X-ray energy dispersive spectroscopy (XEDS) techniques have been employed to characterize these catalysts. The catalytic activities for selective oxidation of glycerol over these bimetallic catalysts have been evaluated, a synergistic effect being observed. Thus, the bimetallic Au-Pd catalyst with the lowest Pd content was the one exhibiting the highest catalytic activity among all the prepared catalysts. Oxidation at 700°C of the bimetallic 2.2AuPd/Ce0.62Zr0.38O2 catalyst led to an increase of the catalytic activity for the selective oxidation of glycerol, which seemed to be due to the formation of a larger fraction of bimetallic Au-Pd alloy-type nanoparticles after oxidation at higher temperatures.

Hybrid plasma-catalytic methanation of CO2 at low temperature over ceria zirconia supported Ni catalysts

A hybrid plasma-catalytic system was used in for the hydrogenation of carbon dioxide (CO2) into methane (methanation) at atmospheric pressure and very low temperature using a dielectric barrier discharge (DBD) plasma reactor packed with Ni-CexZr1−xO2 catalysts. Three catalysts were prepared by a conventional wet impregnation method, using 15 wt% of Ni loading over ceria-zirconia mixed oxides having different Ce/Zr ratios. The physico–chemical features of both catalysts and supports were evaluated by means of X-Ray Diffraction (XRD), Temperature-Programmed Reduction of H2 (H2-TPR), Temperature Programmed-Desorption of CO2 (CO2-TPD) and Transmission Electron Microscopy (TEM). The methanation experiments in the absence or in the presence of plasma were carried out in the temperature range of 90–420 °C. The hybrid plasma 15NiCZ5842 catalyst combination was found to efficiently convert CO2 into methane even at low temperature. Indeed, CO2 conversions as high as 80%, together with 100% selectivity toward methane was measured in the presence of plasma at 90 °C. On the contrary in the absence of plasma, the same conversion and selectivity were only achieved at much higher temperatures around 300 °C, for the same catalyst.