Ce Zr Oxides Research Articles

Enhancement and sulfur poisoning mechanism of Ce doped ZrO2 catalyst in COS hydrolysis at low-temperature

In order to achieve efficient and longer-lifespan removal of carbonyl sulfide (COS), a high-performance catalyst for hydrolysis is needed urgently. In this study, Ce doped ZrO2 oxides are used to remove COS with or without H2O. The results showed that the doping of Ce significantly improves the removal of COS by ZrO2 and obtains a sulfur capacity of 155.4 mgS/g at 70 °C, but there is still accompanied by sulfur poisoning. The characterization reveals that CeO2 has a cubic phase, ZrO2 has a mixed phase of tetragonal and monoclinic, and Ce-doped Zr with different ratio of Ce/Zr forms a Ce-Zr solid solution. Ce and Ce-Zr oxides have Ce3+/Ce4+ pairs, adsorbed oxygen species and surface oxygen vacancies, while ZrO2 has adsorbed oxygen and surface oxygen vacancies, the presence of which can promote the dissociation of H2O and surface adsorption of COS. ZrO2 has abundant medium-strong basicity while CeO2 has abundant weak and strong basicity, and different proportions of Ce/Zr adjust the surface basic strength, basic amount and surface redox property of Ce-Zr solid solution, which changes the adsorption and hydrolysis of COS and H2O and related oxidation reactions on Ce and Zr, thereby changing the sulfur poisoning. Ce has abundant –OH groups, which can rapidly adsorb COS, leading to hydrolysis and direct oxidation of COS in the absence of H2O, while the OH groups on ZrO2 promote the hydrolysis of COS to H2S. Although suitable Ce-doped ZrO2 improves the removal efficiency of COS, it still inevitably leads to the sulfur poisoning, but the presence of Zr could change the sulfation degree of CeO2 and the Claus reaction.

Kinetic Relevance of Surface Reactions and Lattice Diffusion in the Dynamics of Ce–Zr Oxides Reduction–Oxidation Cycles

Reduction–oxidation cycles in oxides are ubiquitous in oxygen storage and transport, chemical looping processes, and fuel cells. O-atom addition and removal are mediated by coupling reactions of oxidants and reductants at surfaces with diffusion of O-atoms within oxide crystals, with either or both processes as limiting steps. CeO2–ZrO2 solid solutions (CZO) are ubiquitous in practice. They are used here to illustrate general experimental strategies and reaction–diffusion formalisms for nonideal systems that enable assessments of the kinetic relevance of the steps that mediate O-atom addition and removal in these materials; these experiments are described within the context of models that describe the driving forces for reaction and diffusion rigorously in terms of oxygen chemical potentials (μO). These strategies assess the rate consequences of varying the fluid phase redox potential, through changes in the identity and pressures of the reactants and products used in redox cycles (O2; CO/CO2; H2/H2O; N2O/N2), of introducing dispersed metal nanoparticles that capture and react lattice O-atoms in CZO using CO or H2, and of imposing intervening dwells without reaction within redox cycles. O-removal rates depend on reductant pressures, even when CO/CO2 and H2/H2O ratios are chosen to maintain the same surface μO if surface reactions were quasi-equilibrated. These data, taken together with significant rate enhancements in O-removal when Pt nanoparticles are present at CZO crystal surfaces and with similar rates before and after inert dwells, demonstrate that reduction rates by both CO and H2 are limited by surface reactions without the presence of consequential spatial gradients in μO within CZO crystals. In contrast, O-addition rates to partially reduced CZO crystals are similar for N2O and O2 reactants and are not affected by the presence of Pt nanoparticles; O-addition rates are significantly higher after intervening inert dwells during CZO oxidation, indicative of spatial gradients in μO, which relax during nonreactive periods. These methods and models, illustrated here for CZO redox cycles at conditions relevant to oxygen storage practice, allow systematic assessments of the kinetic relevance of lattice diffusion and surface reactions for systems that use solids for the reversible storage and release of atoms, irrespective of the identity of the solids or the atoms (e.g., O, H, N, and S).

Expediting Toluene Combustion by Harmonizing the Ce-O Strength over Co-Doped CeZr Oxide Catalysts.

Low-temperature catalytic degradation of volatile organic compounds (VOCs) by enhancing the activity of non-precious metal catalysts has always been the focus of attention. The mineralization of aromatic VOCs requires the participation of a large number of oxygen atoms, so the activation of oxygen species is crucial in the degradation reaction. Herein, we originally adjust the Ce-O bond strength in CeZr oxide catalysts by cobalt doping to promote the activation of oxygen species, thus improving the toluene degradation performance while maintaining high stability. Subsequent characterizations and theoretical calculations demonstrate that the weakening of the Ce-O bond strength increases the oxygen vacancy content, promotes the activation of oxygen species, and enhances the redox ability of the catalysts. This strategy also promotes the activation of toluene and accelerates the depletion of intermediate species. This study will contribute a strategy to enhance the activation ability of oxygen species in non-noble metal oxide catalysts, thereby enhancing the degradation performance of VOCs.

Copper–Cerium–Tin Oxide Catalysts for Preferential Oxidation of CO in Hydrogen: Effects of Synthesis Method and Copper Content

Copper was incorporated into the Ce-Sn and comparative Ce-Zr oxide supports by one-pot precipitation in the presence of CTAB template and by the impregnation of templated Ce-Sn and Ce-Zr oxides. The synthesized Cu-Ce-Sn and Cu-Ce-Zr catalysts were tested in the continuous-flow preferential oxidation of CO in hydrogen excess. The one-pot synthesized tin- and zirconium-doped catalysts demonstrated better CO conversion and CO2 selectivity than their impregnated counterparts. For the tin-modified ternary system that showed the best catalytic performance, the copper content was further optimized. The structure, reducibility, surface chemical state and textural properties of the catalysts were analyzed by SEM-EDX, XRD, H2-TPR, Raman spectroscopy, XPS and TEM. The nonmonotonic changes in the specific surface area, Cu+/Cu2+ ratio and ratio of lattice and non-lattice oxygen with increasing the Cu content are discussed in terms of copper distribution in the catalysts. The influence of the interaction between copper oxide species and the cerium–tin/cerium–zirconium oxide support on the performance of the ternary catalysts was thoroughly analyzed and discussed.

Ni and Ni–Co Catalysts Based on Mixed Ce–Zr Oxides Synthesized in Isopropanol Medium for Dry Reforming of Methane

Ni and Ni–Co Catalysts Based on Mixed Ce–Zr Oxides Synthesized in Isopropanol Medium for Dry Reforming of Methane

Synergetic promotion of 4Mn10Ce/γ-Al2O3 catalyst to Pt/xBa/Ce0.5Zr0.5O2 catalysts for NOx removal from diesel exhausts

In this investigation, Pt/xBa/Ce0.5Zr0.5O2 catalysts were prepared by deposition and incipient wetness impregnation method and the physio-chemical properties of samples were characterized with XRD and BET. The experiments were carried out to evaluate the effect of 4Mn10Ce/γ-Al2O3 on the removal of NOx over the Pt/xBa/Ce0.5Zr0.5O2 catalysts and NO-TPD was employed for determining the NOx adsorption. The results showed that the larger diffraction peak width of Ce-Zr oxides and specific area were found over Pt/15Ba/Ce0.5Zr0.5O2. Based on the NOx storage and storage-reduction experiments, it was found that the decline of NOx storage efficiency was attributed to the decomposition of nitrites and nitrates under high temperatures beyond 350℃, and the inhibition of reactions of NOx with Ba by the NO oxidation. NO2 was stored more easily and faster than NO, which contributed to NOx storage after 4Mn10Ce/γ-Al2O3 catalyst added. The stability of stored species was undermined at higher temperature due to H2 consumption by 4Mn10Ce/γ-Al2O3 catalyst leading to shortage of reductant and more reaction heat. The optimal concentration of O2 at around 7.5% can provide more OⅡ for the oxidation process of NO and improve the performance of NOx reduction.

Fabrication of Zr–Ce Oxide Solid Solution Surrounded Cu-Based Catalyst Assisted by a Microliquid Film Reactor for Efficient CO2 Hydrogenation to Produce Methanol

Currently, supported copper-based catalysts have been regarded as one of the most promising catalysts for catalyzing the hydrogenation of CO2 to synthesize methanol, due to their good selectivity t...

On the enhanced sulfur and coking tolerance of Ni-Co-rare earth oxide catalysts for the dry reforming of methane

Sulfur and coking tolerance of Ni-based dry reforming catalysts were examined. Catalysts utilizing both Ce/Zr and Ce/La oxide supports, some with additional Co, were tested. Long-term reaction runs were conducted with and without sulfur in the feed. Catalysts were also characterized by STEM, XPS, XAFS and XANES and CO chemisorption. Only catalysts where Co was also present, and supported on the Ce-Zr oxide, were capable of extended sulfur tolerance at >20 ppm sulfur. This tolerance, along with a greatly reduced coking rate, is linked to Co in intimate contact with Ni, the mixture existing as clusters anchored and influenced electronically by the oxide support. The activation of methane takes place on these sites. Larger metal aggregates formed by ripening during reaction appear to be spectators. The measured activation energies for dry reforming suggest that CO2 activation takes place at the oxide interface, and is a kinetically significant step.

Enhanced adsorption and desorption of Cr(VI) from aqueous solution using hydrous Ce1– x Zr x O2: Isotherm, kinetics and thermodynamic evaluation

Hydrous Ce1– x Zr x O2 (x = 0.25; 0.5; 0.75) mixed oxides were used as potential Cr(VI) adsorbents, expecting to take an advantage of benefits of their corresponding single oxides (e.g. high adsorption capacity) and to eliminate their weaknesses (e.g. limited reusability, long adsorption time). Chemical incorporation of ZrO2 into CeO2 lattice significantly changed the texture and the morphology of hydrous single oxides, enhancing their adsorptive and desorptive properties. Cr(VI) removal efficiency depended on Ce/Zr molar ratio, increasing in the following order: CeO2<Ce0.75Zr0.25O2<Ce0.5Zr0.5O2< ZrO2<Ce0.25Zr0.75O2. Hydrous Ce0.25Zr0.75O2 showed the highest adsorption capacity (ca. 35.5 mg g−1), desorption efficiency (ca. 34.5 mg g−1), short time needed to attain adsorption equilibrium (30 min) and chemical stability (90% of original Cr(VI) adsorption capacity in the third cycle). Cr(VI) adsorption on hydrous Ce1– x Zr x O2 oxides obeyed the Langmuir isotherm model and followed a pseudo-second-order kinetic model. The values of thermodynamic parameters indicated exothermic and spontaneous adsorption. For hydrous Ce–Zr oxides, adsorption was governed mainly by physisorption and ion exchange, making possible effective and fast regeneration. For hydrous CeO2 and ZrO2, Cr(VI) adsorption was not completely reversible. Reduction of Cr(VI) to Cr(III), on CeO2 surface, and formation of ≡Zr–OH2 +–HCrO4 − complex on ZrO2 surface, were also considered as suppliers of Cr(VI) removal.

Gold Nanoparticles Supported on Ce–Zr Oxides for Selective Hydrogenation of Acetylene

Gold nanocatalysts have been reported to be active in the selective hydrogenation of acetylene. In this paper, a series of Au/Ce1−xZrxO2 (x = 0.03 to 0.5) supported catalysts with different ratios of Ce to Zr were prepared and their catalytic performance for selective hydrogenation of acetylene was investigated. The structure and surface defects of the catalyst can be changed by altering the amount of Zr incorporation. Compared to both Au/CeO2 and Au/ZrO2, the synthesized Au/Ce1−xZrxO2 showed the higher catalytic activity with keeping almost 100% ethylene selectivity at 300 °C. The TEM observation indicated that the incorporation of Zr led to the formation of hollow spherical solid solutions. The specific surface area also has a certain influence on the catalytic activity. Considering the catalytic activity of Au/Ce1−xZrxO2 and their characterizations by XPS and Raman, the concentration of Ce4+ and the oxygen vacancy of the Ce1−xZrxO2 support play an important role in the selective hydrogenation of acetylene.

Synthesis and modification of Ce-Zr oxide compositions as photocatalysts

In the paper, Ce-Zr oxide compositions were prepared using the coprecipitation and mechanochemical mixing methods of the initial precipitated oxides CeO2 and ZrO2. The effects of mechanochemical treatment (MChT), hydrothermal treatment (HTT) and microwave treatment (MWT) on the structure and physicochemical properties of the obtained samples were investigated. The obtained materials were characterized using the N2 adsorption/desorption, XRD, thermogravimetry (TG, DTA) and UV–vis/DRS, XPS methods. The photocatalytic properties of the prepared materials were tested as the rhodamine B degradation under visible light. Moreover, their mechanocatalytic activity was investigated. As a result of mechanochemical, hydrothermal and microwave treatment, almost all the obtained materials were characterized by a larger specific surface area. The use of both hydrothermal and microwave treatments of the wet gel resulted in the formation of the solid solution CeO0,2ZrO0,8O2. The obtained samples exhibited an increased photo- and mechanocatalytic activity under visible light.

Honeycomb monolithic design to enhance the performance of Ni-based catalysts for dry reforming of methane

Supported Ni catalysts (4.5 wt%) using a Ce-Zr oxide (18/82 molar ratio and a ceria-rich surface) depicting advanced redox properties, were deposited by washcoating over cordierite honeycombs (230 and 400 cpsi). FIB-STEM unveiled nanostructure details otherwise undistinguishable by conventional techniques. The catalytic performance was evaluated in the dry reforming of methane at 700−900 °C, using a CH4:CO2 1:1 feedstock, and exploring high Weight Hourly Space Velocity (115−346 L g−1 h−1). The structured catalysts exhibited better performance than the corresponding powders, reaching values close to thermodynamic limits for both reactants conversion and H2/CO ratio, from 750 °C, and no deactivation was observed in prolonged experiments (24−48 h). This was related to both the high catalyst efficiency after being deposited with low loading on the cordierite and the intrinsic advantages of the monolithic reactor, like preventing from the kinetic control that operates in powdered samples under high WHSV or limiting the deactivation.

Structural, Textural, and Catalytic Properties of Ni-CexZr1−xO2 Catalysts for Methane Dry Reforming Prepared by Continuous Synthesis in Supercritical Isopropanol

A series of 5%Ni-CexZr1−xO2 (x = 0.3, 0.5, 0.7) catalysts has been prepared via one-pot solvothermal continuous synthesis in supercritical isopropanol and incipient wetness impregnation of CexZr1−xO2 obtained by the same route. The textural, structural, red-ox, and catalytic properties in methane dry reforming (MDR) of Ni-modified Ce-Zr oxides synthesized by two routes have been compared. It was shown by XRD, TEM, and Raman spectroscopy that the method of Ni introduction does not affect the phase composition of the catalysts, but determines the dispersion of NiO. Despite a high dispersion of NiO and near-uniform distribution of Ni within Ce-Zr particles observed for the one-pot catalysts, they have shown a lower activity and stability in MDR as compared with impregnated ones. This is a result of a low Ni concentration in the surface layer due to segregation of Ce and decoration of nickel nanoparticles with support species.

XNi/Ni0.05Ce0.20Zr0.75O2 Solid Solution over a CO2 Methanation Reaction

A Ni-modified ceria–zirconia support with Ni impregnation at different Ni-loading catalysts for a CO2 methanation reaction was systematically studied. The corresponding structures of each catalyst were characterized by Brunauer–Emmett–Teller surface area analysis, X-ray powder diffraction, X-ray fluorescence, H2 temperature-programmed reduction, CO2 temperature-programmed desorption, Fourier transform infrared, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and thermogravimetric/differential thermal analysis. The results showed that Ni-modified Ce–Zr oxide improved the basic properties as well as the oxygen vacancies. A gradual increase in Ni loading from 15 to 45 wt % was found to increase medium-strong basic sites, and surface Ce3+and Ni0 species along with oxygen vacancies favor high activity. The CO2 methanation activity was related to the amount of Ni loadings where the 45Ni/Ni005CZO catalyst was reported to be the most active catalyst. This is due to the high amount of Ni surface suitable for H2 activation and high medium basic sites to accommodate CO2 activation, leading to high catalytic activity.

Tailored Alkali Resistance of DeNOx Catalysts by Improving Redox Properties and Activating Adsorbed Reactive Species

SummaryIt is still challenging to develop strongly alkali-resistant catalysts for selective catalytic reduction of NOx with NH3. It is generally believed that the maintenance of acidity is the most important factor because of neutral effects of alkali. This work discovers that the redox properties rather than acidity play decisive roles in improving alkali resistance of some specific catalyst systems. K-poisoned Fe-decorated SO42−-modified CeZr oxide (Fe/SO42−/CeZr) catalysts show decreased acidity but reserve the high redox properties. The higher reactivity of NHx species induced by K poisoning compensates for the decreased amount of adsorbed NHx, leading to a desired reaction efficiency between adsorbed NHx and nitrate species. This study provides a unique perspective in designing an alkali-resistant deNOx catalyst via improving redox properties and activating the reactivities of NHx species rather than routinely increasing acidic sites for NHx adsorption, which is of significance for academic interests and practical applications.

SYNTHESIS OF COMPLEX OXIDE COMPOSITIONS OF COBALT–MANGANESE AND CERIUM–ZIRCONIUM AND THEIR CATALYTIC ACTIVITY IN THE DECOMPOSITION OF HYDROGEN PEROXIDE

Cobalt oxides and/or manganese and their com-position based on cerium and zirconium oxides (CeO2 : ZrO2 = 1:1 mol.%) with a content of up to 20 wt. % are synthesized. Samples of both individual oxides and complex oxide compositions were prepared by precipitation from solutions of am-monia (room temperature) or hexamethylenetet-ramine (80–90 °C) followed by heat treatment. Results of DTA show, that due to the calcination at 400 ° C (2 h), the obtained samples lose 17–22 wt. % corresponding to 2–3.8 molecules of water. According to the X-ray powder analysis, initially are formed hydroxide compounds of cobalt (CoO· xH2O) and manganese (MnO2·yН2О), which, after being heated at 400 °C for 2 hours, are converted into mixed oxides from the composition of Co3O4 and Mn3O4. The average particle size calculated by the Sherer equation is 18–30 nm.&#x0D; In the study of catalytic activity on the example of the reaction of the hydrogen peroxide decomposition, it was found that the obtained samples from the solution of GMTA show a greater ability to catalytically decompose hydrogen peroxide compared to samples obtained from the ammonia solution. In this case, the catalytic activity of dried samples is twice as high as roasted, regardless of the method of obtaining. Samples of oxide compo-sitions with deposited 5–10 wt. % of Ce–Zr oxides (1:1) exhibit the highest ability to decompose H2O2. In this case, samples of compositions obtained from the solution of GMTA, have a prolonged catalytic action, and when precipitation in the solution of ammonia, the reaction takes place quite actively during 4–5 days.&#x0D; Compositions formed from co-deposited or mechanically mixed hydroxocompounds of cobalt and manganese with 5 wt. % of CeO2–ZrO2 (1:1) deposited on them have different catalytic activity. In the case of mechanically mixed, it is 30% lower and with subsequent calcination at 400 °C, it is reduced by almost half, and with co-precipitation, the activity is quite high and does not change with heat treatment. In the case of obtaining samples of Co–Mn with Ce–Zr (1:1) deposited on them in excess of 10 wt. % the catalytic activity of the samples dried at 80 °C is equal to the activity of the co-deposited hydroxocompounds of cobalt and manganese and the calcination at 400 °C it reduces it by 30 %.&#x0D; The best ability for catalysis was found in samples CoO·xH2O + 5 wt. % MnO2·yН2О, СоO×хН2О + 10 wt. % CeO2:ZrO2 and СоO×хН2О–MnO2×yН2О, precipitated with the GMTA solution and dried at 80 °C. The besser catalytic properties revealed a sample of СоО×хН2О + 10 wt. % CeO2:ZrO2, which with-out stirring is capable of decomposing 1.2–1.4 dm3/g of hydrogen peroxide with a rapid reaction and in the experiment the volume of H2O2 reacted was 3.4 dm3/g.

Rapid screening of ternary rare-earth – Transition metal catalysts for dry reforming of methane and characterization of final structures

Several CeLa and CeZr oxides, with different transition metal additives, were studied for the dry reforming of methane. A rapid screening technique was developed to measure reforming and coking rates at low partial pressures. It is a good indicator of catalyst behavior at higher conversions and partial pressures. Following rapid screening, select catalysts were examined at longer times on stream. Those containing Ni and Co together were the most stable. Catalysts containing CeLa oxides lacked practicality, partly due to more reverse water-gas shift. Catalysts containing CeZr oxides fared better, with Ce/Zr = 3 (molar) showing the best stability for Ni-based catalysts. Reaction and deactivation results for Ni- and Ni/Co-containing catalysts could be explained partly in terms of DFT calculations for model surfaces, and partly in terms of spent catalyst characterizations. The Ni interacts strongly with the mixed oxides, even when it is in a mostly reduced state, as in CeZr.

Tailored metastable Ce-Zr oxides with highly distorted lattice oxygen for accelerating redox cycles.

Ceria-based catalysts are widely used in oxidation or oxidation-reduction reactions in the field of environmental science. Their catalytic functions are determined by their ability to exchange oxygen species with oxidants. The enhancement of oxygen release is desired since it is often the rate-determining step in redox cycles. Herein, we developed a lattice oxygen distortion method to enhance oxygen activation by quenching the Ce-Zr oxide nanoparticles formed from an extremely high temperature. This process can ensure the formation of solid solutions as well as avoiding atomic rearrangement during calcination, retaining the lattice oxygen at a metastable and disordered state without vacancies. Reduction, vacuum or metal deposition will easily induce oxygen release accompanied by vacancy creation. The metastable oxides can provide about 19 times more oxygen vacancies than traditional ones in a CO atmosphere. CO oxidation rates increased with increasing Zr content from 25 to 75% and achieved a new level, which is attributed to the acceleration of oxygen circulation via promoting oxygen release and supplying plenty of oxygen vacancies for redox cycles. This strategy is expected to be applied in the design and fabrication of improved oxygen storage materials.

Methane dry reforming over Ni catalysts supported on Ce–Zr oxides prepared by a route involving supercritical fluids

AbstractCe0.5Zr0.5O2mixed oxides were prepared in a flow reactor in supercritical isopropanol with acetylacetone as a complexing agent. Variation of the nature of the Zr salt and the temperature of synthesis affected the phase composition, morphology and specific surface area of oxides. X-ray diffraction and Raman spectroscopy studies revealed formation of metastable t” and t’ phases. Oxides are comprised of agglomerates with sizes depending on the synthesis parameters. Loading NiO decreases the specific surface area without affecting X-ray particle sizes of supports. Such sintering was the most pronounced for a support with the highest specific surface area, which resulted in the lowest surface content of Ni as estimated by X-ray photoelectron spectroscopy and in the formation of flattened NiO particles partially embedded into the support. The catalytic activity and stability of these samples in the dry reforming of methane were determined by the surface concentration of Ni and the morphology of its particle controlled by the metal-support interaction, which also depends on the type of catalyst pretreatment. Samples based on ceria-zirconia oxides prepared under these conditions provide a higher specific catalytic activity as compared with the traditional Pechini route, which makes them promising for the practical application.

The Synthesis of Ce1 – xZr x O2 Oxides in Supercritical Alcohols and Catalysts for Carbon Dioxide Reforming of Methane on Their Basis

The effect of the features of starting zirconium compounds on the solvothermal synthesis of mixed Ce–Zr oxides in a flow reactor using ethanol and isopropanol as a supercritical medium was studied. The phase composition and structural properties of the synthesized samples were investigated using X-ray diffraction analysis (XRD) and Raman spectroscopy. The morphology and textural properties were studied by transmission electron microscopy (TEM) and thermal desorption of nitrogen. It was found that the use of zirconium oxychloride or acetate as the starting substances results in the formation of a mixture of phases enriched with cerium or zirconium, whereas the use of zirconium butoxide with acetylacetone as a complexing agent allows obtaining a homogeneous solid solution. The catalytic properties of the synthesized oxides with supported Ni were tested in methane dry reforming (MDR). The catalyst containing a single-phase oxide synthesized in the presence of acetylacetone exhibited much higher activity, selectivity and coking stability.