Surface Area Ceria Research Articles

Graphene oxide boosted high surface area CeO2 for humidity sensing

Precise measurement of humidity is of high significance in diverse fields like instrumentation, automated systems, agriculture and climatology. Herein as a first ever attempt, we report ceria-graphene oxide (CeO2-GO) composites as resistive type humidity sensor, that shows attractive response to moisture. High surface area ceria is synthesised via ammonia mediated precipitation from salt precursor, and CeO2-GO composites are achieved via hydrothermal route. The composites are well characterized using various analytical tools. Experiments using different mass ratios of GO in the composite show that there is an optimum amount of GO to realise maximum efficiency in the test parameters. It is further noted that the performance of the sensor is controlled by the active adsorption sites, which is proportional to surface area of the composite. An optimized CeO2-GO matrix yielded a response and recovery time of 19 s and 10 s respectively. The narrow hysteresis in moisture adsorption-desorption study and excellent stability for about 60 days are also observed for the system, warranting the successful utility of the system for practical applications.

Phenol hydrogenation to cyclohexanol catalysed by palladium supported on CuO/CeO2.

Hydrogenation of phenol to cyclohexanone/cyclohexanol is an important reaction in production of nylon-6, nylon-66 and in petroleum industry. Liquid phase phenol hydrogenation over Pd-CuO/CeO2 was carried out under mild conditions. Palladium impregnated over CuO/CeO2 synthesized by co-precipitation method showed excellent catalytic activity for phenol hydrogenation (99% conversion with 80% cyclohexanol yield) at 90ºC and 10bar H2 pressure in water. Commercial 10%Pd/C showed only 8% phenol conversion under identical conditions. The detailed characterization revealed significant improvement in surface area of ceria after addition of CuO and decrease in crystallite size with creation of defects in CeO2 lattice. XPS analysis showed Pd loading on CuO/CeO2 to cause hydrogen spillover on the surface leading to increase in the oxygen vacancies. The interaction of phenol with catalyst surface studied by detailed FTIR analysis, revealed activation of phenol on oxygen vacancy of ceria as phenoxide ion with perpendicular orientation of aromatic ring on catalyst surface.

Efficient transfer hydrodehalogenation of halophenols catalyzed by Pd supported on ceria

The transfer hydrodehalogenation (THD) of halophenols is efficiently catalyzed by palladium supported on high surface area ceria (Pd/CeO2) under mild conditions (65 °C) using isopropanol (iPrOH) as hydrogen source. The reactivity of 4-halophenols (4-X-PhOH) varies in the order 4-F-PhOH > 4-Cl-PhOH > 4-Br-PhOH > > 4-I-PhOH and appears to be controlled by the desorption of halides from the catalyst surface. Kinetic analysis of the reactions and temperature programmed surface reaction (TPSR) experiments indicate that oxidative addition of C-X bonds and H-abstraction from isopropoxide compete for the same active sites on Pd. The catalyst was able to conduct the THD of various hazardous pollutants and emerging contaminants (dichlorodiphenyltrichloroethane (DDT), pentachlorophenol, pentafluorophenol and triclosan).

Highly active Ni/CeO2 for the steam reforming of acetic acid using CTAB as surfactant template

A series of CTAB-templated Ni/Ce catalysts were synthesized by adding CTAB during the hydrothermal synthesis of ceria to improve the pore structure of catalysts. Their catalytic performance was evaluated in the steam reforming of acetic acid and the effects of CTAB concentration on the porous structures, reducibilities, morphology, and activity of catalysts were studied. The catalysts were characterized by BET, XRD, H2-TPR, XPS, HRTEM, in-situ DRIFTS, DTG, FTIR, and temperature-programmed reaction to elucidate the structure-activity relationship of the catalyst. The results showed that owing to the CTAB assistance, a high surface area of ceria could be achieved, which induced a better Ni dispersion with a smaller Ni size, strengthened the interaction between Ni and CeO2, and promoted the reducibility of Ni, obtaining higher activity and lower methane yield than Ni/Ce. Among these prepared samples, Ni/Ce–C6 showed the highest surface area and the best catalytic performance with a hydrogen yield of up to 82.5% even at a low temperature (550 °C). Owing to the stronger Ni-ceria interaction of Ni/Ce–C6, the lattice oxygen in ceria migrates easily to the Ni surface, interacts with the reaction intermediates, and thus improves the CO2/CO ratio in the products. Much more CO and CO2 and less CH4 were observed over Ni/Ce–C6 during the temperature-programmed reaction, indicating its high activity. In-situ DRFITS characterization demonstrated that the two types of catalysts had similar reaction intermediates but various adsorption and conversion abilities toward the acetic acid. More reaction intermediates were adsorbed at low temperatures and a higher conversion was obtained over Ni/Ce–C6 owing to its better Ni dispersion. The CTAB assistance inhibited the formation of amorphous carbon but facilitated the formation of graphitic carbon at ∼637 °C which did not induce catalyst deactivation.

Phase composition, morphology, properties and improved catalytic activity of hydrothermally-derived manganese-doped ceria nanoparticles

Manganese-doped ceria nanoparticles were prepared by hydrothermal synthesis and the prepared samples were thermally treated at 500 °C for 2 h. The samples were investigated using x-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), energy-dispersive x-ray spectroscopy (EDS), N2 adsorption and x-ray photoelectron spectroscopy (XPS). XRD revealed that nanocrystalline ceria is the main phase in all samples, while a romanechite-like phase (Na2Mn5O10) appears in the sample doped with 30% of Mn. TEM coupled with EDS exposed the presence of the same phase in the sample doped with 20% Mn. While ceria particles have spherical morphology and particle size ranging from 4.3 to 9.2 nm, the rare crystals of the romanechite-like phase adopt a tubular morphology with a length of at least 1 μm. However, the decrease in the ceria lattice constant and the EDS spectra of the ceria nanoparticles clearly indicate that a substantial amount of manganese entered the ceria crystal lattice. Manganese doping has a beneficial impact on the specific surface area of ceria. XPS measurements reveal a decrease in the Ce3+/Ce3+ + Ce4+ content in the doped samples which is replaced by Mn3+. Moreover, a drastic increase in adsorbed oxygen is observed in the doped samples which is the consequence of the increase in Mn3+ species that promotes oxygen migrations to the surface of the sample. Compared to the pure sample, the doped samples showed significantly higher catalytic activity for the process of toluene oxidation.

Atomically Dispersed Dopants for Stabilizing Ceria Surface Area

Ceria is known to be a very good catalyst as well as a support for oxygen transfer (oxidation) as well as for hydrogen transfer (hydrogenation and dehydrogenation) reactions. Many of these reactions occur at high temperatures where ceria is known to sinter, leading to loss of surface area. The thermal stability of ceria can be improved by the addition of dopants, but the location of the dopant atoms and the mechanisms by which ceria stabilization occurs are poorly understood. We show here that dopants located on the surface of ceria are remarkably effective at stabilizing ceria surface area. Keeping metal loading constant at 0.88 mol%, we found that surface area of the ceria aged at 800 °C in air for 5 h ranged from 45 m2/g to 2 m2/g. Strongly bound dopants in atomically dispersed form help to pin surface sites and lower the mobility of ceria.

Toward hydrogen economy: Selective guaiacol hydrogenolysis under ambient hydrogen pressure

The conversion of guaiacol into the nylon precursors, cyclohexanol and cyclohexanone, is catalyzed by palladium supported on high surface area ceria (Pd/HS CeO2) under mild conditions (100 °C, ≤1 bar H2) via sequential hydrodemethoxylation and hydrogenation. In contrast, the 2-methoxycyclohexanol side product is generated by direct guaiacol hydrogenation. Reaction selectivity is determined by competing CO bond hydrogenolysis versus arene hydrogenation. Hydrogenolysis selectivity increases as H2 pressure decreases, and over 80 % H2 is incorporated into the products at 1 bar H2. Higher reactivity of guaiacol than anisole implies that the hydroxyl group is essential in Pd/HS CeO2 catalysis. Participation of water in the conversion is evidenced by deuterium incorporation at non-exchangeable positions when the reaction is performed in D2O. The combination of near quantitative mass balance of H2, high recyclability, and use of water as a solvent offers a simple, green and efficient conversion of lignin-derived aromatics into commercial products.

Synthesis of Nickel‐Doped Ceria Catalysts for Selective Acetylene Hydrogenation

AbstractMetallic nickel is known to be an active, but not a selective hydrogenation catalyst for conversion of alkynes to alkenes. On the other hand, nickel oxide is not active. Recently, we have demonstrated that nickel doped into ceria provides an inexpensive catalyst for selective hydrogenation of acetylene in the presence of ethylene. Here, we evaluate various synthesis methods to achieve optimal selective hydrogenation performance. We examined incipient wetness impregnation, coprecipitation, solution combustion, and sol‐gel synthesis to study how the method of preparation affects catalytic structure and behavior. Sol‐gel synthesis, coprecipitation, and solution combustion synthesis methods favor nickel incorporation into the ceria lattice, while incipient wetness impregnation creates segregated nickel species on the ceria surface. For hydrogenation of acetylene, these nickel surface species lead to poor ethylene selectivity due to ethane and oligomer formation. However, when nickel is incorporated into the ceria lattice, ethane formation is prevented even while achieving 100 % conversion of acetylene. Coke formation is also significantly reduced on these catalysts compared to conventional nanoparticle counterparts. We conclude that sol‐gel synthesis provides the optimal method for creating a uniform dopant distribution within the high surface area ceria.

Oxidation State and Dielectric Properties of Ceria-Based Catalysts by Complementary Microwave Cavity Perturbation and X-Ray Absorption Spectroscopy Measurements

The three-way catalytic converter (TWC) has become an integral part of the modern exhaust gas aftertreatment for gasoline engines. Some years ago, radio frequency technology has been successfully tested for monitoring and controlling ceria based TWC oxygen storage by contactless measuring the dielectric catalyst properties inside a cavity resonator. Applying the cavity perturbation method, we present results on CeO2, the oxygen storage component of three-way catalysts, and on differently prepared Pt–ceria model catalysts and combine it with in situ X-ray absorption spectroscopic (XAS) measurements conducted at the Ce L3-edge. The dielectric properties of the oxidized and reduced Pt–ceria and pure ceria powders were determined at 1.2 GHz between 250 and 550 °C. As the experiments show, the reduction of the material changes both, the polarization and the occurring dielectric losses. The complementary in situ XAS measurements, which allow monitoring variations in the redox state of ceria, gave a precise evaluation of the ceria reduction and oxidation. In addition, a substantial improvement of the low-temperature reducibility of ceria in the presence of Pt was demonstrated by both measurement techniques, in agreement with earlier studies. Furthermore, time resolved XAS data obtained during reducing/oxidizing cycles at 250 °C and 350 °C unraveled the faster and more pronounced reduction of ceria at higher temperature. Both methods proved to be valuable methods to provide insight into the oxygen storage and release process of ceria based materials, and capable to discriminate the impact of the noble metal or the ceria surface area in great detail, even under dynamic conditions, which is highly relevant for studying the function of state-of-the art three-way catalysis and giving input for OSC modeling studies.

Anodic Layers Based on Doped-Ceria/Ni Cermet for Direct Ethanol Fuel Cells

Latest generation of solid oxide fuel cells (SOFCs) is a multilayer device in which several functional layers were incorporated to the anode/electrolyte/cathode configuration. Therefore, using an active layer that enables the fuel cell to run using a renewable fuel such as bioethanol is a reasonable strategy. Separating the catalytic and electrochemical reactions in different anodic layers has two main advantages: keeping the high-performance YSZ/Ni cermet and allowing an active and stable catalyst for ethanol reforming. Doped-cerium oxide is an attractive ceramic material for solid oxide fuel cells (SOFCs) because of the excellent electrochemical and catalytic properties and good compatibility with remaining cell components. In the present study, we have investigated the effect of different dopants on the properties of doped-ceria in Ni cermets aiming at using such cermets as the catalyst for ethanol internal reforming in SOFCs. High surface area ceria doped (10 mol%) with Gd, Y, Zr, and Nb were prepared by an hydrothermal method. The ceria-based solid solutions were mixed with Ni by a wet impregnation method to obtain 18 wt.% Ni relative fraction. Such relatively low Ni volume fraction (~15 vol.%) aims at minimizing coke formation while promoting electronic conductivity. The catalyst materials were deposited onto the anode of standard electrolyte-supported cells [(La,Sr)xMnO3 / YSZ / YSZ/Ni] and tested on dry ethanol. The fuel cell tests results were analysed along with data of both steam reforming and decomposition catalytic reactions for ethanol. Post-test analyses evidenced that water electrochemically generatedpromotes ethanol reforming, in agreement with catalytic tests in which doped-ceria/Ni showed no evidence of carbon deposits in the steam reforming reaction. On the other hand, analyses on spent catalysts of ethanol decomposition revealed carbon deposits. Nevertheless, no significant differences were observed for the studied dopants with the notable exception of niobium, which has a limited solubility in the ceria matrix. The experimental results confirm that upon fuel cell operation, ceria-Ni based catalysts promote alcohol steam reforming resulting in stable direct ethanol SOFCs.

A Simple Chemical Route toward High Surface Area CeO2 Nanoparticles Displaying Remarkable Radical Scavenging Activity

AbstractThis work reports a simple chemical route to synthesize high surface area ceria (CeO2) nanoparticles which exhibit remarkable radical scavenging activity. Synthesized CeO2 nanoparticles are characterized by thermogravimetric analysis (TGA), X‐ray diffraction (XRD), Fourier‐transform infrared spectroscopy (FTIR) and Raman spectroscopy, scanning electron microscopy (SEM), energy‐dispersive X‐ray analysis (EDX), transmission electron microscopy (TEM), and Brunauer‐Emmett‐Teller surface area (BET surface area) analyses. The characterization results reveal the formation of cubic phase of CeO2 with particles sizes of 10 nm and a remarkably high specific surface area of 236.8 m2/g. The nanoparticles are explored for 1,1‐diphenyl‐2‐picrylhydrazyl (DPPH) and hydroxyl radical scavenging activity that is to assess the antioxidant property of nanoceria by in‐vitro spectrophotometric approach.

Synthesis of Mesoporous CeMnO Materials and Catalytic Application for Selective Oxidation of Benzyl Alcohol by Molecular Oxygen

Mesoporous ceria materials doped with Mn (CeMnO) have been synthesized by a convenient soft-templating method. The mesoporous structures, chemical composition, and activity of lattice oxygen have been characterized by various techniques including N2 adsorption–desorption, XRD, TEM, FT-IR, XPS, and H2-TPR. The introduction of Mn has not only increased the surface area and pore size of ceria, but also enhanced the activity of lattice oxygen of ceria. In the selective oxidation of benzyl alcohol to benzaldehyde, the CeMnO samples exhibit good and stable catalytic activity, much higher than the value obtained over the pure mesoporous ceria sample.

Ceria coated silica particles: One step preparation and settling behaviour under the influence of colloidal and hydrodynamic interactions

Ceria-silica composite nanoparticles are used in a variety of areas including catalysis, biomedical applications, and the microelectronic industry. In this study, we explain the connection between colloidal forces of interaction during preparation of ceria-silica mixed colloidal particles, the stability of these suspensions, and the relationship between the stability and the functionality of mixed colloidal particles under different conditions. Short-term stability evolution of the mixed colloidal particles reveals that at the very early stages of the interaction of ceria and silica, the interaction rate increases until it reaches a maximum value, and then it decreases during the subsequent stage. More importantly, it is found that the first stage of the interaction becomes more dominant at lower relative surface area of ceria to silica. Derjaguin-Landau-Verwey-Overbeek theory of colloidal interaction is successfully used to explain short-term stability results. Moreover, it is found that mixed colloidal particles exhibit a long-term stability behaviour that not only depends on the colloidal forces of interaction between ceria and silica particles, but also is affected by hydrodynamic effects such as gravitational forces. For that reason, mixed colloidal particles exhibit long-term stability behaviours that are not the same as their short-term stability.

Role Of CO2 As a Soft Oxidant For Dehydrogenation of Ethylbenzene to Styrene over a High-Surface-Area Ceria Catalyst

Catalytic performance and the nature of surface adsorbates were investigated for high-surface-area ceria during the ethylbenzene oxidative dehydrogenation (ODH) reaction using CO2 as a soft oxidant. The high surface area ceria material was synthesized using a template-assisted method. The interactions among ethylbenzene, styrene, and CO2 on the surface of ceria and the role of CO2 for the ethylbenzene ODH reaction have been investigated in detail by using activity test, in situ diffuse reflectance infrared and Raman spectroscopy. CO2 as an oxidant not only favored the higher yield of styrene but also inhibited the deposition of coke during the ethylbenzene ODH reaction. Ethylbenzene ODH reaction over ceria followed a two-step pathway: ethylbenzene is first dehydrogenated to styrene with H2 formed simultaneously, and then CO2 reacts with H2 via the reverse water gas shift. The produced styrene can easily undergo polymerization to form polystyrene, which is a key intermediate for coke formation. In the abse...

Impact of cerium-based support oxides in catalytic wet air oxidation: Conflicting role of redox and acid–base properties

High surface area ceria is an excellent support of metals (Ru, Pt, etc.) for catalytic wet air oxidation reactions (CWAO). Due to the low solubility of oxygen in water, the rate of transfer of O2 from the gas phase toward the catalyst has a decisive impact on the whole oxidation process. The role of the ceria support would be to facilitate this oxygen transfer. Redox properties of ceria (generally measured by the oxygen storage capacity, OSC) have a great impact on this step. Attempts were made to modulate the redox properties of ceria by using mixed oxides: CeO2–ZrO2, CeO2–PrO2–ZrO2 or CeO2–TiO2. In doing so, acid–base properties of the supports are also modified. The role of the redox and acid–base properties on CWAO reactions (acetic acid, aniline and phenol) are reviewed. In most cases, acid–base properties can have a great impact on the catalyst stability (deactivation by carbonates or polymer deposit). Increasing the redox properties may have an adverse effect on the catalytic performances when acid–base properties bias the whole process. In some cases (phenol over CeO2–TiO2 supported catalysts), an adequate Lewis acidity is preferable to a high OSC of the catalyst.

High Surface Area Ceria for CO Oxidation Prepared from Cerium t-Butoxide by Combined Sol–Gel and Solvothermal Processing

CeO2 was synthesized by combined sol–gel and solvothermal processing of gels obtained from acetaldoximate-modified cerium(IV) t-butoxide in the presence of the non-ionic surfactant Pluronic F127. The use of cerium(IV) t-butoxide as precursor contrasts very favorably with the often used ceric ammonium nitrate and results in more reliable and tailorable properties of the final materials. The kind of post-synthesis treatment of the gels and the composition of the precursor mixture proved to be crucial for obtaining high surface area ceria with a high Ce3+ proportion. Calcination in air or under nitrogen was compared with solvothermal treatment in ethanol or water and a combination of solvothermal treatment and calcination. The obtained materials are composed of 3.5–5.5 nm ceria nanoparticles. The highest specific surface area of 277 m2/g was obtained after solvothermal treatment, and 180 m2/g when solvothermal treatment was followed by calcination in air to remove residual organic groups. The highest Ce3+ proportion was 18 % after solvothermal treatment in ethanol and additional calcination in air. CO oxidation on selected samples indicated that activity scaled with surface area and thus was largest for samples solvothermally treated in ethanol. The reaction rate of the best sample was about 75-times larger than that of commercial ceria.Graphical .

Low temperature carbon monoxide catalytic oxidation at the Pd/Ce–Zr–Al–Ox catalyst

The homogeneous chemical composition ceria–zirconia–alumina (Ce–Zr–Al–Ox) nano-alloy were successfully synthesized by surfactant-assisted parallel flow co-precipitation method and applied as supports for low temperature CO oxidation. The experiment conditions were studied in detailed. At 0.92 wt% Pd loading, 30,000 ppm CO could be completely oxidized to CO2 at 30 °C at a WHSV of 4,380 ml g−1 h−1 over the Pd/Ce–Zr–Al–Ox (nCe:nZr = 3:1) catalyst. Pd/Ce–Zr–Al–Ox catalysts were systematical studied by mean of BET, XRD and TEM analysis. XRD characterization showed that zirconium element entered into cubic structure of ceria and leaded to structure distortion. Addition of aluminum increased specific surface area of ceria–zirconia solid solution substantially. The average pore diameter of Ce–Zr–Al–Ox support palladium catalysts were the key impact factor for CO oxidation. When the Pd/Ce–Zr–Al–Ox catalysts had highly dispersed palladium nanoparticles, large average pore diameter, suitable surface area and pore volume, the activity of CO oxidation was the best.

Preparation of High Surface Area Mesoporous Ni<sub>2x</sub>Ce<sub>1-x</sub>O<sub>2</sub> (x = 0, 0.05, 0.13, 0.2) and Its Applications in Methane Steam Reforming

The present study proposes a novel reverse precipitation procedure, which is different from the conventional precipitation method, for the synthesis of ultra high surface area mesoporous CeO2 at various conditions. Mesoporous Ni2xCe1-xO2 (x = 0.05, 0.13, 0.2) is also synthesized for use in the methane steam reforming reactions. The physicochemical properties of the catalysts and catalytic activity tests for the steam reforming of methane are also investigated. In the present study, the preparation of high surface area mesoporous CeO2 is proposed, which successfully enables control over the surface area. The results showed that the highest surface area of mesoporous ceria reached 695 m2/g. Tests on long-term catalytic activities indicated that Ni0.4Ce0.8O2 exhibited a much greater stability and activity for the steam reforming of methane than the others. Further, the present work also found that the amount of mobile oxygen increased simultaneously as the nickel content in the proposed catalysts due to the synergistic effects of structural nickel species to the oxygen vacant sites at the vicinity of the surface. Using the proposed new method, mesoporous ceria can be utilized more extensively in many ways, including as a three-way catalyst, as an electrolyte of solid oxide fuel cells, in reforming reaction, and in some other practical applications.

Preferential oxidation of CO (PROX) over CoOx/CeO2 in hydrogen-rich streams: Effect of cobalt loading

PROX reaction over Co catalysts supported on ceria nano-particles was investigated with a special focus on the effect of Co loading. High surface area ceria nano-particles were prepared through a precipitation technique using cerium (III) nitrate hexahydrate and sodium hydroxide. Co incorporation was achieved using a wet impregnation method in aqueous media. Catalysts were characterized using X-ray diffraction, X-ray photoelectron spectroscopy, temperature-programmed reduction, dispersion measurements, and in situ diffuse reflectance Fourier transform infrared spectroscopy and X-ray absorption spectroscopy. Activity and selectivity of CoOx/CeO2 catalysts with different Co loadings showed higher Co loadings to favor higher CO oxidation at the expense of oxygen selectivity. Higher activation energies for H2 oxidation compared to CO oxidation were observed, regardless of the Co loading, implying a higher temperature sensitivity for the H2 oxidation reaction. The Co phase was identified as Co3O4 and was stable in the reducing environments present in PROX reactions.

Water gas shift reaction on carbon-supported Pt catalysts promoted by CeO 2

The low temperature water gas shift reaction has been studied over carbon-supported platinum catalysts promoted by ceria. To this end, CeO 2 has been dispersed over an activated carbon support at different loadings (20, 30 and 40 wt.%) in order to obtain a high ceria surface area. Then, platinum has been incorporated by impregnation with an acetone solution of [Pt(NH 3) 4](NO 3) 2. Two more catalysts have been prepared with aqueous and ethanolic solutions, respectively, to assess the effect of the solvent in the final catalysts. Pt/CeO 2 and Pt/C catalysts have been used as references. Catalysts have been characterized (N 2 adsorption at 77 K, TEM, H 2-TPR) and tested in reaction after reduction with H 2 at 473 K. The obtained results show that CO conversion increases with the amount of ceria loaded, from 20 to 40 wt.%. At this temperature, platinum supported on bulk CeO 2 is less active than its carbon-based counterparts. The best results are obtained for the catalysts with 40 wt.% CeO 2 prepared by aqueous impregnation of the platinum precursor, which achieves conversions higher than 70% at 573 K, and shows no deactivation under reaction at this temperature for 120 h. On the other hand, Pt/C shows a very low activity, this certifying the determinant role of ceria in this reaction. These results indicate that dispersion of ceria on activated carbon allows to obtain catalysts with superior performance than using bulk CeO 2 as support, and with a lesser amount of ceria.