Cerium Oxide Catalysts Research Articles

Promotional effect of surface fluorine species on CeO2 catalyst for toluene oxidation

In this study, fluorine (F) modified cerium oxide (Fx-Ce) catalysts were prepared by co-precipitation method to investigate the impact of fluorine species on CeO2 catalyst for toluene oxidation. Compared to bared CeO2, the catalysis activity of Fx-Ce was further improved. The temperature of toluene conversion (T90% and T50%) for F3-Ce decreased by ΔT90% = 30°C and ΔT50% = 15°C, respectively. The characterizations of H2-TPR, O2-TPD, Raman, XPS certified that there was more chemisorbed oxygen, oxygen vacancies and Ce3+ ions on the F3-Ce than bared CeO2. DFT calculations demonstrated that the formation energy of oxygen vacancy on F3-Ce is reduced by surface-florine, which promotes the formation of oxygen vacancy. Meanwhile, Toluene-TPD indicated that fluorine-doping increased the adsorption capacity of toluene on CeO2. According to the above-mentioned results, F3-Ce has higher oxygen vacancy concentration, more active oxygen, and larger toluene adsorption capacity. Thus it has the optimal catalytic activity for toluene oxidation.

Recent advances in performances and mechanisms of cerium-based oxide catalysts for catalytic combustion of soot particles released from diesel engines

<p indent=0mm>With the increasingly strict environmental protection regulations and the improvement of people’s awareness of environmental protection, the serious environmental pollution caused by soot particles from diesel vehicle exhaust has attracted people’s attention. Catalytic purification technology is one of the most effective ways to eliminate soot particles and the research and development of highly effective catalysts is one of the key factors for determining the application of this technology. In recent years, cerium-based oxide catalysts have been widely used for catalytic combustion of diesel soot particles due to their excellent oxidation-reduction performance, suitable surface acidity, and high oxygen storage/release capacity. The researches focusing on highly active essence of the cerium-based oxide catalysts in soot combustion are helpful to understand the fundamental theories of catalytic processes of soot combustion and provide scientific guidances for the optimization of existing catalysts and design of new-type catalysts. In this review, the recent research progress of cerium-based oxide catalysts for diesel soot combustion is summarized, including single cerium oxide catalyst and cerium-based oxide catalysts modified by rare earth metals, transition metals, noble metals and alkali (alkaline earth) metals. Meanwhile, the reaction mechanisms of above-mentioned catalysts are also summarized. Finally, the problems and development directions of cerium-based oxide catalysts for soot combustion are proposed and prospected.

Catalytic Neutralization of Naphthenic Acid from Petroleum Crude Oil by Using Cerium Oxide Catalyst and 2- Methylimidazole in Polyethylene Glycol

Background: The presence of relatively high naphthenic acid in crude oil may contribute to the major corrosion in oil pipelines and distillation units in crude oil refineries. Thus, high concentration naphthenic acid crude oil is considered to be of low quality and is marketed at lower prices. In order to overcome this problem, the neutralization method had been developed to reduce the TAN value in crude oil. In this study, crude oil from Petronas Penapisan Melaka was investigated. Methods: The parameters studied were reagent concentration, catalyst loading, calcination temperature, and reusability of the potential catalyst. The basic chemical used was 2- methylimidazole in polyethylene glycol (PEG 600) with concentration 100, 500 and 1000 ppm. Cerium oxide-based catalysts were supported onto alumina prepared with different calcination temperatures. Results: The catalyst was characterized by using Brunauer-Emmett-Teller (BET), Fourier Transform Infrared Spectroscopy (FTIR) and Thermogravimetry Analysis-Differential Thermal Gravity (TGA-DTG) to study the physical properties of the catalyst. The Ce/Al2O3 catalyst calcined at 1000°C was the best catalyst due to larger surface area formation which lead to an increment of active sites thus will boost catalytic activity. The result showed that the Ce/Al2O3 catalyst meets the Petronas requirement as the TAN value reduced to 0.6 mgKOH/g from the original TAN value of 4.22 mgKOH/g. Conclusion: The best reduction of TAN was achieved by using catalyst loading of 0.39% and reagent of 1000 ppm.

A Highly Selective and Stable Ruthenium‐Nickel Supported on Ceria Catalyst for Carbon Dioxide Methanation

AbstractThe performance of nickel, ruthenium, and nickel‐ruthenium impregnated on cerium oxide catalysts was tested in the methanation of carbon dioxide reaction. The nickel and ruthenium contents were 4 and 0.4 wt%, respectively. The properties of the catalysts were studied using elementary analysis, Brunauer‐Emmet‐Teller specific surface area measurements, X‐ray diffraction, temperature programmed reduction, temperature programmed desorption, H2 chemisorption and transmission electron microscopy. The results showed that the addition of ruthenium improves the catalytic performance by promoting the dispersion of nickel species over the surface of the cerium oxide support. The ruthenium‐nickel combination produced a stable catalyst that did not show any deactivation even after 75 hours on‐stream. At high feed gas total pressures (5 and 10 bar), the catalytic conversions of CO2 were close to the thermodynamic equilibrium values with a 100 % selectivity towards CH4 formation.

Synergistic catalysis by Mn promoted ceria for molecular oxygen assisted epoxidation

Mesoporous manganese doped cerium oxide catalysts were prepared by an inverse micelle method with different manganese loadings. These materials exhibited superior activity for epoxidation of alkenes with molecular oxygen. Highest active 10MnCe material showed an activity of 80 % conversion and >99 % selectivity for cyclooctene epoxidation. The fluorite crystal structure which facilitates the high oxygen mobility, low temperature reducibility, homogeneous distribution of manganese on ceria, nanoparticle nature, higher number of oxygen vacancies, low valent states of manganese and cerium, and the ability to generate reactive oxygen species of the manganese doped cerium oxide could be correlated to the high catalytic activity. The reaction followed first order kinetics with a rate constant of 0.31 h−1 at 100 °C, and the activation energy turned out to be 10.5 kJ/mol. The mechanistic study revealed that the reaction proceeds via a unique mechanism which involves in situ generated superoxide and singlet oxygen species.

Significant effect of multi-doped cerium oxide for carbon monoxide oxidation studies

Current work composed of transition metal substituted CeO2 catalysts for the catalytic conversion of CO to CO2. The cerium oxide catalysts were successfully prepared via solution combustion method. The prepared oxides were characterized using various techniques such as X-ray diffraction (XRD), Infra-red spectroscopy (IR), thermogravimetry-differential thermal analysis (TG/DTA), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and Brunauer-Emmett-Teller surface area (BET). Further, surface oxygen mobility and CO consumption capacity of the catalyst were studied using H2-Temperature programmed reduction (H2-TPR) and CO pulse titration, respectively. It was observed that substitutions of Mn, Cu and Ag in CeO2 system could produce more synergy interactions. The result of CO oxidation demonstrates the doping Mn, Cu and Ag in cerium oxide has increased the CO conversion rate and achieves 100% conversion at 95 °C as compared to other catalyst from the series. Such enhanced activity can be related to synergistic effect in the cerium oxide by addition of Mn, Cu and Ag. Overall reduction pattern of the catalysts and CO adsorption profile on the catalysts surface were in support of CO oxidation reaction.

Low content of samarium doped CeO2 oxide catalysts derived from metal organic framework precursor for toluene oxidation

A series of samarium (Sm) doped cerium oxide (x% Sm/CeO2) catalysts were prepared by pyrolysis of Sm containing Ce-based metal organic framework (Sm/Ce-MOF) precursor and applied for catalytic oxidation of toluene. The obtained catalysts were characterized by SEM, TEM, XRD, BET, Raman, XPS, H2-TPR, O2-TPD, and their catalytic activities were evaluated by oxidation of toluene over them. It was found that doping of Sm increased the concentration of oxygen vacancy as well as the low temperature reducibility, thereby improved the catalytic performance. Moreover, among the prepared Sm doped CeO2 catalysts, the 1% Sm/CeO2 catalyst showed the best performance for the oxidation of toluene with conversions of 50 % and 100 % at temperatures of 194 and 222 °C respectively under a WHSV of 60,000 mL/(g·h). In addition, the 1% Sm/CeO2 catalyst also exhibited excellent stability and high toleration to the moisture.

Water Promotion (or Inhibition) of Condensation Reactions Depends on Exposed Cerium Oxide Catalyst Facets

Nanoparticles with well-defined facets enable quantitative correlations between surface features and catalytic activity. Here, we explore the role of geometric and acid–base properties in the mechanism of aldol condensation catalyzed by ceria nanoshapes. In the crucial C–C coupling step, the two carbonyl adsorbates are bound to two adjacent cations. On the CeO2(110) plane, all atoms lie on the same layer, favoring the interaction between adsorbates and facilitating the bimolecular C–C coupling. Thus, the initial unimolecular deprotonation is rate-limiting. In contrast, the (100) and (111) planes have an open structure with O on the top layer and Ce on the layer below. The former O layer interferes between adsorbates linked to Ce sites, making the C–C coupling rate-limiting. Water helps in overcoming this spatial hindrance by remote bond polarization via H-bonds. Therefore, water promotion is only observed on those planes for which the bimolecular step is rate-limiting.

Evaluation of hydroxylammonium nitrate (HAN) decomposition using bifunctional catalyst for thruster application

Monopropellant thruster technology is at the cusp of revival with new age ionic liquids which are safe, less toxic and high performing emerging as substitutes for traditionally used hydrazine. One such promising candidate is Hydroxylammonium nitrate (HAN). Current study explores an iridium metal impregnated bifunctional variant of an earlier developed doped cerium oxide catalyst for HAN decomposition. The performance of bifunctional catalyst was compared against unsupported cerium oxide catalyst and conventional iridium over gamma alumina catalyst. Thermal analysis was the primary test used for performance evaluation and was subsequently followed up in a batch reactor to evaluate the durability of the catalyst to extreme high temperature exposure. Characterization of catalyst before and after decomposition reaction was comprehensively conducted using surface area analysis, SEM/EDS, XRD and XPS. Results did suggest promotional effect of iridium over doped ceria support as demonstrated through increased pressure build-up and rate of pressure-rise.

Hierarchically Structured CeO2 Catalyst Particles From Nanocellulose/Alginate Templates for Upgrading of Fast Pyrolysis Vapors.

Hierarchically structured porous materials often exhibit advantageous functionality for many applications including catalysts, adsorbents, and filtration systems. In this study, we report a facile approach to achieve hierarchically structured, porous cerium oxide (CeO2) catalyst particles using a templating method based on nanocellulose, a class of renewable, plant-derived nanomaterials. We demonstrate the catalyst performance benefits provided by this templating method in the context of Catalytic Fast Pyrolysis (CFP) which is a promising conversion technology to produce renewable fuel and chemical products from biomass and other types of organic waste. We show that variations in the porous structures imparted by this templating method may be achieved by modifying the content of cellulose nanofibrils, cellulose nanocrystals, and alginate in the templating suspensions. Nitrogen physisorption reveals that nearly 10-fold increases in surface area can be achieved using this method with respect to commercially available cerium oxide powder. Multiscale electron microscopy further verifies that bio-derived templating can alter the morphology of the catalyst nanostructure and tune the distribution of meso- and macro-porosity within the catalyst particles while maintaining CeO2 crystal structure. CFP experiments demonstrate that the templated catalysts display substantially higher activity on a gravimetric basis than their non-templated counterpart, and that variations in the catalyst architecture can impact the distribution of upgraded pyrolysis products. Finally, we demonstrate that the templating method described here may be extended to other materials derived from metal chlorides to achieve 3-dimensional networks of hierarchical porosity.

The degradation of phthalate esters in marine sediments by persulfate over iron–cerium oxide catalyst

This study investigated the degradation of phthalate esters (PAEs) in marine sediments by sodium persulfate (Na2S2O8, PS) activated by a series of iron–cerium (Fe-Ce) bimetallic catalysts (FCBCs). The surface structure and chemistry of the FCBCs were characterized by TEM, HRTEM, XRD, FTIR, BET and XPS. Results show successful synthesis of FCBC catalysts. Factors such as PS concentration, Fe to Ce molar ratio, catalyst dosage, and initial pH that might affect PAEs degradation were investigated. Results revealed that PAEs was degraded more effectively over FCBC with a Fe-Ce molar ratio of 1.5:1. Increase in Ce improved the catalytic activity of FCBC due to increase in oxygen storage capacity (OSC). Acidic conditions enhanced PAEs degradation with a maximum degradation of 86% at pH 2 and rate constant (kobs) of 1.5 × 10−1 h−1 when the PS and FCBC concentrations were to 1.0 × 10−5 M and 1.67 g/L, respectively. Di-(2-ethylhexyl) phthalate (DEHP) was a salient marker of PAE contamination in sediments. Dimethyl phthalate (DMP) and diethyl phthalate (DEP) were easier to degrade than DEHP, diisononyl phthalate (DINP), dioctyl phthalate (DnOP) and diisononyl phthalate (DIDP). The synergistic catalytic effect of Fe3+/Fe2+ and Ce4+/Ce3+ redox couples, in addition to electron transfer of oxygen vacancies, activated S2O82− to generate SO4− and HO radicals, which played the major role of PAEs degradation. 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin trapping EPR studies verified the crucial role of SO4− and HO in the oxidative degradation process. FCBC/PS oxidation exhibited high-performance for the remediation of PAEs-contaminated marine sediments.

Effect of Small Molecular Organic Acids on the Structure and Catalytic Performance of Sol–Gel Prepared Cobalt Cerium Oxides towards Toluene Combustion

Cobalt cerium oxide catalysts with small molecular organic acids (SOAs) as chelating agents were prepared via the sol–gel method and investigated for the complete oxidation of toluene. Four kinds of natural SOAs, i.e. malic acid (MA), citric acid (CA), glycolic acid (GA), and tartaric acid (TA), were selected. The effect of organic acids on the composition, structure, morphology and catalytic performance of metal oxides is discussed in details. The cobalt cerium oxides catalysts were characterized by various techniques, including TG–DSC, XRD, SEM–EDS, N2–adsorption and desorption, XPS, and H2–TPR analyses. The results show that the nature of organic acids influenced the hydrolysis, condensation and calcination processes, as well as strongly affected the textural and physicochemical properties of the metal oxides synthesized. The best catalytic activity was obtained with the CoCe–MA catalyst, and the toluene conversion reached 90% at 242 °C. This outstanding catalytic activity could be related to its textural, redox properties and unique surface compositions and oxidation states. In addition, the CoCe–MA catalyst also showed excellent stability in long–time activity test.

Room temperature CO2 reduction to solid carbon species on liquid metals featuring atomically thin ceria interfaces

Negative carbon emission technologies are critical for ensuring a future stable climate. However, the gaseous state of CO2 does render the indefinite storage of this greenhouse gas challenging. Herein, we created a liquid metal electrocatalyst that contains metallic elemental cerium nanoparticles, which facilitates the electrochemical reduction of CO2 to layered solid carbonaceous species, at a low onset potential of −310 mV vs CO2/C. We exploited the formation of a cerium oxide catalyst at the liquid metal/electrolyte interface, which together with cerium nanoparticles, promoted the room temperature reduction of CO2. Due to the inhibition of van der Waals adhesion at the liquid interface, the electrode was remarkably resistant to deactivation via coking caused by solid carbonaceous species. The as-produced solid carbonaceous materials could be utilised for the fabrication of high-performance capacitor electrodes. Overall, this liquid metal enabled electrocatalytic process at room temperature may result in a viable negative emission technology.

Environment-friendly synthesis of diethyl carbonate via ethyl carbamate alcoholysis over cerium oxide catalyst

A series of cerium oxide catalysts were prepared by simply calcination of nitrate cerium, and used for clean synthesis of diethyl carbonate (DEC) by ethyl carbamate (EC) alcoholysis. 51% conversion and 82% selectivity were obtained with catalyst CeO2-600. Almost the same activity as the fresh sample could be regained if the used catalyst was recalcined at 600 °C. XPS, XRD, CO2-TPD and BET study suggested that high surface area and the presence of abundant basic sites are important to achieve a higher catalytic performance.

3D Gold-Modified Cerium and Cobalt Oxide Catalyst on a Graphene Aerogel for Highly Efficient Catalytic Formaldehyde Oxidation.

A hard template method is used to prepare porous gold-doped cerium and cobalt oxide (Au-Cex Coy ) materials. A series of 3D Au-Ce x Coy /graphene aerogel (GA) composites is then fabricated by a facile heating method. The obtained catalysts possess a well-defined structure of ordered arrays of nanotubes and good performance in formaldehyde (HCHO) oxidation. The composition and surface elemental valence states of the catalysts are modulated by the Ce/Co molar ratio. The Au-Cex Coy catalyst and graphene oxide sheets are well compounded within 60 s through a diamine cross-linker to form 3D Au-Cex Coy /GA composites. In addition, the resulting catalyst of 3 wt% Au-Ce3 Co/GA achieves ≈55% conversion at room temperature and 100% conversion when the reaction temperature is raised at 60 °C. The synergistic effect between CeO2 and Co3 O4 promotes the migration of oxygen species and the activation of Au, which facilitates HCHO oxidation. The method used to prepare the 3D catalyst could be used to produce other catalytic materials with good replication of the template. In addition, these findings provide a simple method for rapid fabrication of catalyst/GA composites. The superior activity and stability of the 3D Au-Ce3 Co/GA catalyst make it potentially applicable in HCHO removal.

Synthesis and characterization of a mesoporous cerium oxide catalyst for the conversion of glycerol

CeO2 mesoporous materials were synthesized employing the “hard and soft template” technique. Ceria catalysts have been prepared by a nanocasting procedure using SBA-15 and KIT-6 silica-based templates and Pluronic P123 and F127 for the “soft template” technique. The mesoporous CeO2 was characterized by techniques such as N2 physisorption, which confirmed a mesoporous size pore system and transmission electron microscopy, TEM, which confirmed the material ordered pore structure. TGA/DTA analysis was used to determine the loss of weight compared to the temperature before and after the elimination of the template. On the other hand, by means of IR spectroscopy of the Ce-KIT-6 material, the partial elimination of the Si template after the NaOH treatment was confirmed by decreasing the peaks corresponding to the vibration bands of the Si-O-Si linkages. Finally, by temperature-programmed desorption (TPD) of NH3 and CO2, the acid and basic strength of the catalysts were determined, respectively. All samples exhibited much higher total basicity relative to total acidity. The analysis of TPR-H2 showed regions of low temperature (350-400ºC) where the reduction of the bulk is plausible to be initiated and of high temperature (550ºC) of superficial reduction of the ceria.The performance of the catalysts was evaluated in the dehydration reaction of glycerol in gas phase at 320 ºC. The mesoporous CeO2 material was selective to acetol and reached a 100% conversion of glycerol. The catalyst prepared by the “soft template” technique (CeO2-P123) was the most selective to acetol (76%) with a 12% selectivity to acrolein. With the catalyst prepared by the “hard template” technique (Ce-KIT-6), the selectivity towards acetol was 48% and 22% towards acrolein. The acid properties of CeO2 were found to be a determining factor for the selectivity to acetol and acrolein.

Ce-modified AlZr pillared clays supported-transition metals for catalytic combustion of chlorobenzene

In this article, we provided a one-step hydrothermal method to prepare composite AlZr pillaring agents, and synthesized AlZr-pillared clays (AlZr-PILC) via ion exchange. Compared with conventional methods, our method successfully shortened synthetic routes and greatly reduced consumption of the materials. Then, AlZr-PILC-supported manganese and cerium oxide catalysts were obtained by impregnation method. The compositions and properties of these catalysts were characterized by some technical means. The energy dispersive X-ray spectroscopy clearly shows the existence of Mn, Ce, and O, which indicates the successful loading of the active components on the surface of AlZr-PILC. Meanwhile, the results of X-ray diffraction (XRD) and N2 adsorption experiments demonstrate that the synthesized AlZr-PILC outperforms the raw clays (Na-mmt) and mononuclear Al-PILC in the catalytic combustion of chlorobenzene. XRD and high-resolution transmission electron microscopy also proves that the high activity of them is related to the high dispersion of the oxides and the exposure of more active sites. H2-temperature-programmed reduction shows that cerium can promote the redox cycles of the manganese system through the strong interactions between MnO2, CeO2 and AlZr-PILC. In particular, MnCe(9:1)/AlZr-PILC shows the best catalytic activity in the catalytic combustion of chlorobenzene.

Synthesis of heterogeneous catalyst for the production of biodiesel from soybean oil

This study explore the comparison of a suitable heterogeneous catalyst for conversion of triglyceride into fatty acid methyl ester. A series of heterogeneous cerium, manganese, and zinc oxide catalyst supported at mixture of cinder was prepared by co-precipitation and applied for conversion of triglyceride in oil to biodiesel using methanol as solvent. Results showed that a maximum TG conversion of 99% was obtained in the transesterification reaction catalyzed by CeSO 4 +MnSO 4 +K 2 CO 3 catalyst calcinated in 5 h at 600 o C under the optimal conditions as catalyst amount of 3%, Ce:Mn:K molar ratio of 1:1:1. The catalytic activity of catalyst at 70 oC reaction temperature was over 90% after 6h . The experimental data were satisfactorily predicted at 99% confidence level. However, due to high and efficient yield CeSO 4 +MnSO 4 +K 2 CO 3 catalyst was identified as the most potential catalyst.

Active sites contribution from nanostructured interface of palladium and cerium oxide with enhanced catalytic performance for alcohols oxidation in alkaline solution

Nanostructured interface is significant for the electrocatalysis process. Here we comparatively studied the electrooxidation of alcohols catalyzed by nanostructured palladium or palladium–cerium oxide. Two kinds of active sites were observed in palladium–cerium oxide system, attributing to the co-action of Pd–cerium oxide interface and Pd sites alone, by CO stripping technique, a structure-sensitive process generally employed to probe the active sites. Active sites resulting from the nanostructured interfacial contact of Pd and cerium oxide were confirmed by high resolution transmission electron microscopy and electrochemical CO stripping approaches. Electrochemical measurements of cyclic voltammetry and chronometry results demonstrated that Pd–cerium oxide catalysts exhibited much higher catalytic performances for alcohols oxidation than Pd alone in terms of activity, stability and anti-poisoning ability. The improved performance was probably attributed to the nanostructured active interface in which the catalytic ability from each component can be maximized through the synergistic action of bi-functional mechanism and electronic effect. The calculated catalytic efficiency of such active sites was many times higher than that of the Pd active sites alone. The present work showed the significance of valid nanostructured interface design and fabrication in the advanced catalysis system.

Water–Gas Shift Reaction over Ni/CeO2 Catalysts

This paper reports the results of a study of a water–gas shift reaction over nickel–ceria catalysts with different metal loading. Within this study, the overall CO conversion and observed kinetic behavior were investigated over the temperature range of 250–550 °C in different reactor configurations (fixed-bed and microchannel reactors). The quasi-steady state kinetics of the CO water–gas shift reaction was studied for fractions of Ni-containing cerium oxide catalysts in fixed-bed experiments at lab-scale level using a very dilute gas (1% CO + 1.8% H2O in Не). A set of experiments with a microchannel reactor was performed using the feed composition (CO:H2O:H2:N2 = 1:2:2:2), representing a product gas from methane partial oxidation. The results were interpreted using computational models. The kinetic parameters were determined by regression analysis, while mechanistic aspects were considered only briefly. Simulation of the WGS reaction in the microreactor was also carried out by using the COMSOL Multiphysics program.