Surface Area Ceria Research Articles

Unique effect of surface area of support on propene combustion over Pd/ceria

The effects of calcination temperature and surface area of ceria support on the propene oxidation over Pd/ceria were investigated. Very unique dependence of the oxidation activity of Pd/ceria on surface area was observed, i.e. lower surface area of ceria is more preferable for the higher catalytic activity depending on the pretreatment conditions. The structure of supported Pd species was characterized by Pd K-edge EXAFS. Isolated Pd ions were stable on ceria with high surface area after the propene oxidation, which was caused by the easy oxidation of Pd species and formation of Pd–O–Ce bond. On the other hand, moderately dispersed Pd metal particles were abundant on ceria with low surface area. It was suggested that the weaker interaction with sintered ceria is suggested to be preferable for the formation of active Pd metal particles by the reduction at low temperatures.

Optimization of the synthesis parameters of high surface area ceria nanopowder prepared by surfactant assisted precipitation method

Response surface methodology (RSM) has been used to optimize the critical parameters responsible for higher surface area of ceria nanopowder prepared by surfactant assisted precipitation method. A three-level central composite design (CCD) was used to optimize pH, CTAB/metal molar ratio and calcination temperature. A quadratic model between response and the independent parameters was developed and the response surface model was tested with analysis of variance (ANOVA). The optimum operating conditions determined were a pH value of 9.4, CTAB/metal molar ratio of 0.5 and calcination temperature of 266 °C. Under these optimal conditions maximum surface area of 158 m 2/g has been achieved.

On the role of lattice/surface oxygen in ceria–zirconia catalysts for diesel soot combustion

A series of ceria and ceria–zirconia catalysts with varying composition and surface area have been systematically investigated in the oxidation of soot in the range of temperature 600 K < T < 800 K. The samples have been characterized by their textural and structural properties and formation of homogeneous solid solutions has been confirmed. The activity in soot combustion has been measured both in the presence and absence of gas-phase O 2 in order to characterize the contribution of surface and bulk properties of the materials. It was shown that the number of active surface oxygen linked to ceria plays a major role, while oxygen storage capacity, governed by introduction of zirconia, can be important when and where oxidation of soot particles is operated in the absence or defect of gaseous oxygen. The role of Zr in stabilizing surface area of ceria (thus increasing the number of surface active sites) is also important when evaluating the effect of composition on soot oxidation activity.

Surfactant assisted synthesis of high surface area ceria modified mesoporous tetragonal zirconia powder and its chromium adsorption study

Mesoporous zirconia modified by ceria has been synthesized through a surfactant-assisted route by using zirconyl oxychloride and ceric ammonium nitrate as precursors and dodecylamine as a template. The as-synthesized inorganic-template hybrid possesses high surface area (275 m 2 g −1) and large pore volume (0.52 cm 3 g −1). A template-free mesoporous zirconia–ceria with a decreased surface area of 80 m 2 g −1 and pore volume of 0.07 cm 3 g −1 was formed by heat treatment of the as-synthesized matrix at 500 °C for 2 h. The introduction of as small as 2 mol% ceria into the zirconia lattice has helped in stabilizing the technologically important tetragonal (t-) phase of zirconia up to 800 °C. The sample obtained even after calcination at 500 °C exhibits high adsorption capacities, ∼85% for chromium(VI) ions from aqueous solutions without any adjustment of pH, at room temperature, and within a period as short as 1 h. These present studies show that the mesoporous zirconia modified by ceria could be considered as a potential adsorbent for removing toxic Cr(VI) ions from aqueous solutions.

The effect of support reducibility on the stability of Co/CeO 2 for the oxidative steam reforming of ethanol

Ceria plays an active catalytic role in removing carbon from the catalyst by a support-mediated cleaning mechanism. Increasing the support surface area using a novel preparation method led to improved catalyst stability and a lower coking rate. DRIFTS of adsorbed ethanol shows that oxygen from the support facilitates the formation of acetate intermediate species, thus demonstrating the ability of the support to donate oxygen. This oxygen may come from either O adatoms by adsorption of O 2 at vacancies, whereby the cerium atoms involved are Ce 4+. Or, it may come from vacancy-associated Type II bridging OH groups, where the cerium atoms involved are Ce 3+. The higher oxygen/OH group mobility of high ceria surface area promotes the mechanism of carbon removal, which in turn contributes to the high stability of Co/CeO 2 catalyst.

High-throughput approach to the catalytic combustion of diesel soot II: Screening of oxide-based catalysts

Following the development of a high-throughput (HT) methodology for the evaluation of diesel soot oxidation catalysts in a 16 parallel channels reactor, a library of over 60 catalysts was tested under optimized conditions. The catalyst compositions were chosen to include solids which specific properties like oxygen storage capacity, oxygen mobility and ionic conductivity. The key parameters for high activity appear related to the presence of active and mobile surface oxygen species, and to an appropriate catalyst particle size in order to favour the number of contacts with the soot. In contrast, high oxygen storage capacity and bulk oxygen ion mobility do not appear as relevant properties for high catalytic activity. Nine new formulations were found to perform better than the reference catalyst “high surface area (HSA) ceria” (Rhodia).

Formation of Synthesis Gas from Propane Oxidation over Pt-on-Ceria: Effect of Ceria Surface Area, Reaction Temperature, and Oxygen/Fuel Ratio

The formation of synthesis gas from propane oxidation has been investigated on 1.0% Pt/CeO2 catalysts in a fixed-bed flow reactor over a temperature range of 200−800 °C at atmospheric pressure using ∼90% inert. Three kinds of ceria, with different surface areas, have been used as the catalytic support for this reaction. The catalysts were characterized using X-ray diffraction (XRD), temperature-programmed reduction (TPR), temperature-programmed oxidation (TPO), H2 chemisorption, and BET surface area. For the catalysts with the highest surface area, a large decrease in BET surface area has been observed after calcinations at high temperature. The oxidation of propane occurs with two sets of products. At low temperatures (T ≤ 500 °C), propane oxidizes exclusively into CO2 and H2O with little H2. Above 500 °C, O2 is completely consumed, and the selectivities of CO and H2 increase steadily with increase in temperature. In the region 500−700 °C, high-surface-area catalysts show higher activity and selectivity ...

Superior copper promoted bimetallic catalysts for chemoselective hydrogenation of ortho-chloro-nitrobenzene

In the present study, highly dispersed copper promoted bimetallic nanocomposite oxides with superior catalytic performance for vapor phase chemoselective hydrogenation of ortho -chloro-nitrobenzene were successfully prepared by deposition–precipitation and impregnation methods and characterized using different techniques. XRD and TPR results revealed the presence of small and extremely well dispersed metal oxide particles over the high surface area ceria–silica support. TEM analysis confirmed existence of nanocrystals with an average particle size of 7 nm. Among different catalysts employed, CuO–NiO/CS exhibited high conversion (96.6%), high product selectivity (98%), and excellent stability.

Copper Promoted Cobalt and Nickel Catalysts Supported on Ceria−Alumina Mixed Oxide: Structural Characterization and CO Oxidation Activity

Catalytic activity of CoO, NiO, CuO−CoO, and CuO−NiO nanocrystalline mono- and bimetallic catalysts over a thermally stable and high surface area ceria−alumina mixed oxide support was evaluated for oxidation of carbon monoxide at normal atmospheric pressure and lower temperatures. The content of Co or Ni in the respective monometallic catalysts was 10 wt % and the Cu-promoted samples contained 5 wt % each. These catalysts were prepared by a wet impregnation procedure and the CeO2−Al2O3 support was obtained by a deposition precipitation method. The synthesized catalysts were characterized by BET surface area, X-ray diffraction (XRD), energy dispersive X-ray microanalysis (EDX), temperature-programmed reduction (TPR), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) techniques. XRD patterns of 773 K calcined samples revealed the presence of metal oxide phases, namely, CeO2, CuO, NiO, and Co3O4 in the respective catalysts. However, after calcination at 1073 K, the XRD lines ...

STUDY ON THE SYNTHESIS OF MESOPOROUS CERIA IN THE PRESENCE OF GLUCOSE AND ACRYLAMIDE

Mesoporous ceria with high specific surface area have been prepared using eerie ammonium nitrate and non-surfactants organic compounds glucose, acrylamide via hydrothermal method. The influence of the main synthesis variables have been studied, including cerium ion valence, composition of raw materials, hydrothermal temperature and time. XRD, TEM, TG and nitrogen adsorption techniques have been used to characterize the samples. Results indicate that cerium ion valence has the most important effect on surface area of mesoporous ceria. Cerium ion valence changing from +3 to +4 causes higher surface area of samples. Moreover the coexistence of glucose and acrylamide in the synthesis is essential to obtain samples with high surface areas, whereas surface areas of samples are lower in the absence of glucose or acrylamide. TEM shows that the mesoporous ceria are composed of three-dimensional disordered primary nanoparticles. The average size of primary particles and mesopores are both a few nanometers.

Effect of hydrothermal treatment and silica on thermal stability and oxygen storage capacity of ceria–zirconia

Hydrothermal digestion of freshly precipitated cerium–zirconium hydroxides was used to synthesize nanocrystalline ceria–zirconia. This simple method produced ceria–zirconia with higher surface area and thermal stability than the untreated oxide. After calcination at 1000°C, the surface area of the hydrothermally synthesized samples was ∼11–12m2/g while the untreated Ce0.5Zr0.5O2 had only 4.2m2/g. The continuous dissolution and reprecipitation of hydroxides during hydrothermal treatment is postulated to a more defect-free structure which is able to withstand loss of surface area when exposed to high temperatures. In addition, these nanocrystalline oxides were more reducible than the untreated oxide. While the addition of silica to ceria–zirconia further increased the surface area and oxygen storage capacity, the oxides suffered severe loss of surface area after calcination to 1000°C. A silica-rich overlayer was formed which decreased the oxygen storage capacity as compared to silica-free ceria–zirconia. The oxygen storage capacity shows a strong dependence on the surface area for values below 50m2/g but diffusion of oxygen from the bulk becomes limiting for high surface area ceria–zirconia.

Liquid phase hydrogenation of crotonaldehyde over Au/CeO 2 catalysts

Three Au/CeO 2 catalysts, with different specific surface area (80, 150 and 240 m 2/g) were tested for the hydrogenation of crotonaldehyde in liquid phase. Reactions were carried out with molecular hydrogen as the reductant, in a batch reactor using isopropanol as the solvent, at 80 °C and 1 MPa. The high surface area catalyst (Au/CeO 2, 240 m 2/g) showed a selectivity towards crotyl alcohol of 29%. This value was much lower than the one previously obtained for the same sample under gas phase conditions (73%) [B. Campo, C. Petit, M. Volpe, J. Catal. 242 (2006) 162]. Under liquid phase the promotional effect of high surface area ceria is lost. The low and middle surface area samples (80 and 150 m 2/g respectively) showed meager selectivity to carbonyl hydrogenation due to both the low promotional effect of the support and the relatively large size of gold particles. For the three catalysts the formation of propanol was observed, which decreases catalysts selectivity to crotyl alcohol.

A simple synthesis of catalytically active, high surface area ceria aerogels

Abstract A simple synthetic route for preparing high surface area, structurally stable ceria aerogels is presented. Ceria aerogels were doped with 1% palladium: a water-gas shift (WGS) active metal. A reduced Pd/ceria aerogel and an as-prepared Pd/ceria aerogel are compared to determine the effect of pretreatment conditions on WGS activity. The BET surface area of the as-prepared ceria aerogel was 345 m 2 /g. CO chemisorption data and high-resolution TEM images indicate that the particle size of palladium on the reduced Pd/ceria sample was approximately 15 nm. X-ray photoelectron spectroscopy (XPS) data establish that the fraction of Ce 3+ from Ce 2 O 3 was approximately the same for both reduced and as-prepared samples. XPS data also indicate that approximately 44% of the palladium on the reduced Pd/ceria sample was converted to the metallic form. Each catalyst was tested for WGS activity. The reduced Pd/ceria sample exhibited the greatest WGS activity as compared with the other samples used in this study.

Ultrasonic-induced synthesis of high surface area colloids CeO2–ZrO2

The nanostructures of high surface area ceria–zirconia colloids have been successfully synthesized via a sonochemical method in the presence of polyethylene glycol 600. Their structural characteristics have been investigated using powder XRD, FESEM, BET surface area, TG, and other techniques. The average size of CeO2–ZrO2 nanoparticles is estimated to be 3.7 nm using Debye–Scherrer’s equation. The BET analysis indicates that colloids CeO2–ZrO2 have a remarkably high surface area of 226 m2 g−1.

Surfactant-Controlled and Microwave-Assisted Synthesis of Highly Active Ce x Zr1−x O2 Nano-Oxides for CO Oxidation

Uniform nanosized, mesoporous and high specific surface area ceria–zirconia (1:1 mole ratio) solid solutions were synthesized employing a poly-block copolymer surfactant combined with microwave or thermal treatment. For comparison purpose an identical composition CexZr1−xO2 mixed oxide was also prepared by a conventional coprecipitation method. The surface and bulk structure of the synthesized samples were investigated using X-ray diffraction (XRD), small angle X-ray scattering (SAXS), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), BET surface area and BJH pore size distribution (PSD) methods. The catalytic activity was evaluated for CO oxidation at normal atmospheric pressure. The CexZr1−xO2 solid solutions obtained through surfactant use exhibited high specific surface area and mesoporosity. The XRD measurements revealed the presence of cubic Ce0.75Zr0.25O2 and Ce0.6Zr0.4O2 phases in the case of conventional coprecipitation and surfactant controlled synthesized samples, respectively. The Raman measurements revealed existence of more oxygen defects in the surfactant-controlled and microwave treated sample. The SEM images suggested that all samples consist of typical spherical agglomerates with almost uniform size within the nanometer size range. The TEM measurements revealed nanosized crystallites in a narrow range between 4 and 8 nm, and densely packed in the case of conventional precipitation and microwave treated samples. Interestingly, the CexZr1−xO2 solid solution obtained by surfactant-controlled method and treated with microwave radiation exhibited better CO oxidation activity than other samples. Enhanced activity of surfactant-controlled and microwave treated sample is correlated with its unique physicochemical characteristics.

Reduction behavior of nanoparticles of Ce 0.8Zr 0.2O 2 produced by different approaches

High surface area ceria–zirconia solid solutions with nominal composition Ce 0.8Zr 0.2O 2 were prepared by co-precipitation and microemulsion techniques. The samples were characterized by XRD, temperature programmed reduction and N 2 physisorption after various calcination treatments. Both syntheses led to the formation of a single cubic phase. The reduction kinetics of Ce 0.8Zr 0.2O 2 was studied by static volumetric hydrogen adsorption. A change in the rate-controlling regime was observed with the temperature increase. The apparent activation energies determined between 473 and 523 K were 83 and 70 kJ mol - 1 for the Ce 0.8Zr 0.2O 2 produced by co-precipitation and microemulsion, respectively. In the high temperature range (523–673 K) both samples showed an apparent activation energy of 30 kJ mol - 1 . Considering the relatively high hydrogen pressure used during the volumetric measurements and the high reduction degree reached, the reduction rate of Ce 0.8Zr 0.2O 2 can be interpreted as bulk diffusion limited at 673 K, whereas at 473 K the process is surface step controlled.

Effect of specific surface area on oxygen storage capacity (OSC) and methane steam reforming reactivity of CeO2

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.

Effect of Specific Surface Area and Zr Doping Content on Oxygen Storage Capacity (OSC) and Methane Steam Reforming Reactivity of CeO2-ZrO2

This paper investigated the effect of specific surface area and Zr doping content on oxygen storage capacity (OSC) and methane steam reforming reactivity of CeO2 and Ce-ZrO2. It was found that the specific surface area of ceria and the doping of Zr present important role on the oxygen storage capacity (OSC) and the reactivity toward methane steam reforming. After calcination at 900{degree sign}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 and better reforming reactivity 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 of ceria prepared by SF was 2084 mmol/g, whereas those of ceria prepared by TP and PP were 1724 and 781 mmol/g, respectively. According to the reactivity toward methane steam reforming, after purging in 3 kPa methane at 900 C for 8h, the methane conversion for ceria prepared by SF was approximately 38%, whereas those of ceria prepared by TP and PP were 22%.

Steam Reforming of Ethanol over Ni on High Surface Area Ceria Support: Influence of Redox Properties on the Catalyst Stability and Product Selectivities

The steam reforming of ethanol over Ni on high surface area CeO2 support, synthesized by a surfactant-assisted approach, (Ni/CeO2 (HSA)) were studied under solid oxide fuel cell (SOFC) operating conditions. The catalyst provides significantly higher reforming reactivity and excellent resistance toward carbon deposition compared to Ni/Al2O3 and Ni on conventional ceria (Ni/CeO2 (LSA)) under the same conditions. At the temperature above 800{degree sign}C, the main products from the reforming processes over Ni/CeO2 (HSA) were H2, CO, and CO2 with small amount of CH4 depending on the inlet ethanol/steam ratio, whereas high hydrocarbon compounds i.e., C2H4 and C2H6 were also observed from the reforming of ethanol over Ni/CeO2 (LSA) and Ni/Al2O3 in the range of conditions studied (700-1000{degree sign}C). The excellent ethanol reforming performances of Ni/CeO2 (HSA) in terms of stability, reactivity, and product selectivities are due to the high redox property of CeO2 (HSA). During the ethanol reforming process, in addition to the reactions on Ni surface, the gas- solid reactions between the gaseous components presented in the system (C2H5OH, C2H6, C2H4, CH4, CO2, CO, H2O, and H2) and the lattice oxygen (Ox) on ceria surface also take place. Among these redox reactions, the reactions of adsorbed surface hydrocarbons with the lattice oxygen (Ox) can eliminate the formation of high hydrocarbons (C2H6 and C2H4), which easily decompose and form carbon species on Ni surface.

Steam reforming of ethanol with co-fed oxygen and hydrogen over Ni on high surface area ceria support

Ethanol steam reforming with/without co-fed oxygen and hydrogen over Ni on high surface area (HSA) CeO2 support, synthesized via a surfactant-assisted method, (Ni/CeO2 (HSA)) was studied under solid oxide fuel cell (SOFC) operating conditions for later application as an in-stack reforming catalyst. The catalyst provides considerably higher reforming reactivity and excellent resistance towards carbon deposition in comparison with Ni/Al2O3 and Ni on conventional ceria (Ni/CeO2 (low surface area; LSA)). At the temperature above 800 °C, the main products from the reforming processes over Ni/CeO2 (HSA) were H2, CO, and CO2 with some amount of CH4 depending on the inlet steam/ethanol and co-fed reactant (i.e. O2 and H2)/ethanol ratios, whereas high hydrocarbon compound i.e. C2H4 was also observed from the reforming of ethanol over Ni/CeO2 (LSA) and Ni/Al2O3. An addition of O2 (as oxidative steam reforming) and H2 significantly reduced the degree of carbon deposition. The presence of both reactants also promoted the conversions of hydrocarbon presented in the system (i.e. CH4 and C2H4) to CO and H2. The major consideration of these additions is the suitable co-fed reactant/C2H5OH ratio. The presence of too high oxygen concentration could oxidize Ni particles to NiO, which resulted in a lower reforming reactivity, and also combusts H2 to H2O. The suitable O2/C2H5OH molar ratio for the oxidative steam reforming of Ni/CeO2 was 0.4, which is less than that of Ni/Al2O3. An addition of too high hydrogen content slightly decreased the catalyst activity, which could be due to the active site competition of nickel particle and the inhibition of gas–solid redox reactions between the gaseous hydrocarbon components with the lattice oxygen (OOx) on the surface of CeO2 support in the case of Ni/CeO2.