CeO2 Support Research Articles

Influence of composition and morphology of the active phase on the catalytic properties of cobalt-nickel catalysts in the steam reforming of ethanol

Potassium promoted monometallic and bimetallic cobalt-nickel catalysts with CeO2 support were characterized in the fresh, reduced and after steam reforming of ethanol (SRE) form. TEM studies of reduced catalysts showed the formation of Co0 or Ni0 forms of active phase with fcc structure in the monometallic catalysts and Co–Ni alloys (fcc) with different proportion Co and Ni in the bimetallic catalysts.Among all tested catalysts, the nickel catalyst indicated the smallest crystallites of active phase (8.4 nm), while the cobalt catalyst indicated the largest crystallites of active phase (12.2 nm). The more cobalt in the active phase of catalysts was, the larger the crystallite size of active phase were.The catalytic studies showed that the increase in the content of nickel in the active phase of catalysts increased the selectivity of the SRE to methane, while the increase in the content of cobalt in the active phase caused an increase in the production of acetaldehyde and acetone. Furthermore, the more nickel in the active phase was, the more stable catalysts were.After 24 h work of catalysts in the SRE, nickel and Co–Ni (fcc) active phases were stable forms under SRE conditions, while the cobalt active phase was observed in two forms: metallic cobalt (fcc) and cobalt oxide (CoO). Moreover, on the cobalt catalyst dominated the encapsulating carbon deposit, whereas on the nickel catalyst the fibrous deposit was dominated. Encapsulating carbon deposit cause faster deactivation of the catalyst than fibrous deposit which explained the various stability of both catalysts in the SRE.

Surface modification of LaMnO3 perovskite supported on CeO2 using argon plasma for high-performance reduction of NO

The ever-increasing industrial and motor vehicle activities led to the overwhelming emission of nitrogen oxides (NOx) to the atmosphere, posing a serious environmental challenge. Selective catalytic reactions, based on Perovskite-type metal oxide catalysts, are a proven method used to eliminate NOx compounds and prevent their discharge to the ecosystem. Herein, LaMnO3 nanocatalysts were synthesized by the sol-gel auto-combustion method and supported on CeO2 support using dry impregnation method and used in the simultaneous catalytic reduction process of NO and CO. As a novel idea, the prepared nanocatalysts were treated using argon non-thermal glow discharge plasma to alter the surface properties. The physicochemical properties of these particles were determined using FT-IR, XRD, EDX, FE-SEM, and BET analysis. The plasma treatment process managed to reduce surface impurities. Additionally, the dispersion of the perovskite nanoparticles over the ceria support and the specific surface area was improved, while the diameter of the nanocatalysts was declined. As a result, the plasma modified LaMnO3/CeO2 nanocatalysts improved the conversions of NO and CO by a maximum of 41.19 and 49.11 %, respectively. The produced nanocatalysts were highly stable during the reduction process and lost 3.94 % of their activity after 30 h of operation. The plasma-based surface modification process was utterly effective in boosting the catalytic performance, while it was relatively cheap and easy to carry out.

Anchoring Positively Charged Pd Single Atoms in Ordered Porous Ceria to Boost Catalytic Activity and Stability in Suzuki Coupling Reactions.

Single-atom (SA) catalysis bridging homogeneous and heterogeneous catalysis offers new opportunities for organic synthesis, but developing SA catalysts with high activity and stability is still a great challenge. Herein, a heterogeneous catalyst of Pd SAs anchored in 3D ordered macroporous ceria (Pd-SAs/3DOM-CeO2 ) is developed through a facile template-assisted pyrolysis method. The high specific surface area of 3DOM CeO2 facilitates the heavily anchoring of Pd SAs, while the introduction of Pd atoms induces the generation of surface oxygen vacancies and prevents the grain growth of CeO2 support. The Pd-SAs/3DOM-CeO2 catalyst exhibits excellent activity toward Suzuki coupling reactions for a broad scope of substrates under ambient conditions, and the Pd SAs can be stabilized in CeO2 in long-term catalytic cycles without leaching or aggregating. Theoretical calculations indicate that the CeO2 supported Pd SAs can remarkably reduce the energy barriers of both transmetalation and reductive elimination steps for Suzuki coupling reactions. The strong metal-support interaction contributes to modulating the electronic state and maintaining the stability of Pd SA sites. This work demonstrates an effective strategy to design and synthesize stable single-atom catalysts as well as sheds new light on the origin for enhanced catalysis based on the strong metal-support interactions.

Development of a Cs-Ru/CeO2 Spherical Catalyst Prepared by Impregnation and Washing Processes for Low-Temperature Decomposition of NH3: Characterization and Kinetic Analysis Results

Cl-free Ru/CeO2 spherical catalysts were prepared by incipient wetness impregnation and subsequent washing processes using RuCl3·3H2O and a commercial spherical CeO2 support. Cs addition improved c...

CO2 hydrogenation on Co/CeO2-δ catalyst: Morphology effect from CeO2 support

The Co/CeO2-δ catalysts with different morphology structure were prepared for CO2 catalytic hydrogenation reaction. The physical and chemical properties of these catalysts were characterized by H2-TPR, XRD, TEM, ICP and H2-TPD. The characterization results indicated that the different morphology structure of CeO2 support obviously influence the exposed crystal plane, and then affect the concentration of oxygen vacancies, the metal-support interaction and the dispersion of Co0 active species on the CeO2 support. In addition, the Co ions can be dissolved into the CeO2 lattice to form Ce–O–Co solid solution, which promotes the formation of Co0 active species, oxygen vacancies, and the Ce3+-□-Co0 structure, thus significantly affects the CO2 hydrogenation performance of Co/CeO2-δ catalysts. The exposed {110} and {100} crystal plane of CoCe140 catalyst with nano-rods structure ensure the excellent CO2 hydrogenation performance, including CO2 conversion is 48.7%, the CH4 and CO selectivity are 91.7% and 8.3%, respectively.

Recent advances in catalytic automotive emission control: Passive NO storage at low temperatures

AbstractIn the practical applications, a high portion of the NOx is emitted without purification in the cold start period of engine because the selective catalytic reduction (SCR) or NOx storage and reduction (NSR) catalysts are not warmed up. Passive NOx absorbers (PNA) were proposed earlier to store the NOx at low temperatures and release the stored NOx during warm‐up period when the SCR and NSR catalysts downstream are functional, while this concept just attracted great interests recently to meet the extra stringent requirement of governmental emissions regulations. A plant of PNA catalysts, showing different NO adsorption/release characteristics, was disclosed. This review summarizes the recent progress in the development of PNA catalysts. The characteristics of NO adsorption/release on precious metal (i.e., Pt, Pd) loading Al2O3 support catalysts, CeO2 support catalysts, and zeolite support catalysts are summarized and compared.

Hydrogen generation by ammonia decomposition over Co/CeO2 catalyst: Influence of support morphologies

The fabrication of nanomaterial is a crucial issue in heterogeneous catalysis for on-site generation of hydrogen in proton exchange membrane fuel cell to achieve excellent performance for ammonia decomposition, and their effects of morphologies are still mysterious in the structure-reactivity relationship. In order to disclose it, three kinds of CeO2 supports with three-dimensionally ordered mesoporous structure (3DOM), nanotubes (NT) and nanocubes (NC) were synthesized by nanocasting of a mesoporous silica KIT-6 template with cubic Ia3d symmetries, hydrothermal method with and without urea, respectively. Various characterization methods (XRD, BET, H2-TPR, CO-TPD, TEM and XPS) were used to characterize the structure-reactivity relationship of catalysts. The Co/CeO2-3DOM catalyst had higher H2 producing rate (4.2 mmol/min･gcat) than Co/CeO2-NC (3.5 mmol/min･gcat) and Co/CeO2-NT (3.2 mmol/min･gcat) under the reaction conditions of 500 °C and GHSV = 6000 mL/gcat･h. The Co/CeO2-3DOM catalysts presented Co nanoparticles with mean size of 5.2 nm, and the highest surface Co concentration (5.12%) and Ce3+/Ce4+ ratio (0.53). Its high activity is attributed to higher surface area and more surface oxygen vacancies. The specific surface area and surface oxygen vacancy are significantly affected by the morphology of CeO2 support. The more mechanism insight of the structure-activity relationship for ammonia decomposition has been revealed.

Carbon-CeO2 interface confinement enhances the chemical stability of Pt nanocatalyst for catalytic oxidation reactions

Noble metals are downsized to nano-/subnanoscale to improve their catalytic activity and atom-economy. However, the stabilities in chemical state and catalytic performance of these nanocatalysts often suffer during harsh conditions. For Pt nanoparticles (NPs) supported on CeO2, activated oxygen diffused from the support over-stabilizes the active sites of Pt, degrading its performance at mild temperature. In this work, Pt nanocatalysts with unique structure of triple-junction are synthesized by selectively growing Pt NPs on the carbon-CeO2 interface. Impressively, the Pt NPs exhibit much enhanced catalytic stability and high activity for CO oxidation at mild temperature. The enhancement is attributed to electron donation from graphitized carbon and the confinement effect from the high-density nanopores of the CeO2 support. The triple-junction of Pt-C-CeO2, combining the merits of CeO2 for activating O2 and electron donating capability of carbon, provides new inspiration to the fabrication of high-performance nanocatalysts.

Cyclodextrin ‐ Stabilized Palladium Nanoparticles on Ceria: Investigation of Support Interactions and Pivotal Promotion in the Suzuki‐Miyaura Reaction

AbstractThe present work is focused on the understanding of the CeO2 support interactions with cyclodextrin‐stabilized palladium nanoparticles (PdNPs) and the effect in the Suzuki‐Miyaura reaction outcome. Mostly, it is evidenced the strong influence of the cyclodextrin torus on the reactivity of PdNPs towards the ceria matrix. Structural characterization by X‐rays photoelectron spectroscopy (XPS) Raman spectroscopy, diffuse reflectance spectroscopy (DRS) and powder X‐rays diffraction (XRD, via Rietveld refinement) afforded to infer that palladium is being partially brought into the CeO2 structure. Transmission electron microscopies (TEM and STEM) visualizations allowed to evidence a bimodal size distribution of PdNPs (1–5 nm and 20–100 nm) over the catalysts even though particles up to 5 nm are probably the most responsible for the catalytic results reported herein.

Enhanced surface properties of CeO2 by MnOx doping and their role in mechanism of methane dry reforming deduced by means of in-situ DRIFTS

MnOx-doped CeO2 and CeO2 supports, and their corresponding catalysts with Ni impregnation were studied in this work. Surface and mechanistic properties of prepared catalysts have been investigated for the process of methane dry reforming with 1:1 pure gas stream. Peak shifts observed in XRD and Raman analyses confirmed the successful incorporation of MnOx into the CeO2 lattice. MnOx doping induces both lattice defects that generate oxygen vacancies, as well as restricts CeO2 crystal growth. TEM elemental mapping evidenced the uniform distribution of Ni on supports. The restriction of crystal growth correspondingly exhibited a huge impact on specific surface area of MnOx-doped catalyst. A possible reaction mechanism has been deduced with the help of in-situ DRIFTS experiments. The oxygen vacancies generated due to the lattice defects in CeO2 played a huge role in enhancing coke resistance. The carbon accumulation reduced from 8.6 to 1.7 wt.% with MnOx doping.

Effect of Support and Chelating Ligand on the Synthesis of Ni Catalysts with High Activity and Stability for CO2 Methanation

Carbon dioxide methanation was carried out over Ni-based catalysts on different supports and chelating ligands in microreactors. To investigate the influence of chelating ligands and supports, the Ni catalysts were prepared using different support such as CeO2, Al2O3, SiO2, and SBA-15 by a citric acid (CA)-assisted impregnation method. The properties of the developed catalysts were studied by X-ray diffraction (XRD), Transmission electron microscope (TEM), and X-ray photoelectron spectroscopy (XPS) measurement, and the results show that the addition of CA in the impregnation solution improved the dispersion, refines the particle size, and enhanced the interaction of nickel species. The catalytic performance of the developed Ni catalysts were evaluated by CO2 methanation in microreactors in the temperature range of 275 °C–375 °C under 12.5 bar pressure. All the catalysts exhibit high CO2 conversion and extremely high selectivity to methane. However, the catalysts prepared via CA-assisted method exhibited excellent activity and stability, compared with Ni catalysts prepared by a conventional impregnation method, which could be attributed to highly dispersed nickel particles with strong metal–support interaction. The activity of CO2 methanation followed the order of Ni/CeO2-CA > Ni/SBA-15-CA > Ni/Al2O3-CA > Ni/SiO2-CA > Ni/CeO2. The Ni/CeO2 catalysts have also been prepared using different chelating ligands such as ethylene glycol (EG), sucrose (S), oxalic acid (OA) and ethylene diamine tetra acidic acid (EDTA). Among the tested catalysts prepared with different support and chelating ligands, the Ni/CeO2 catalyst prepared via CA-assisted method gave superior catalytic performance and it could attain 98.6% of CO2 conversion and 99.7% methane selectivity at 325 °C. The partial reduction of the CeO2 support generates more surface oxygen vacancies and results in a high CO2 conversion and methane selectivity compared with other catalysts. The addition of CA as promoter favored the synergistic effect of Ni and support, which led to high dispersion, controls the size, and stabilizes the Ni nanoparticles. Furthermore, the Ni/CeO2-CA catalyst yields high CO2 conversion in a time-on-stream study due to the ability of preventing the carbon deposition and sintering of Ni particles under the applied reaction conditions. However, the Ni/Al2O3-CA and Ni/SBA-15-CA catalysts showed stable performance for 100 h of time on stream.

Effect of oxide supports on palladium based catalysts for NO reduction by H2-SCR

The present work shows the synthesis of various mixed oxide catalysts prepared by co-precipitation followed by impregnation of palladium metal. The catalysts are denoted as Pd/MgO-CeO2, Pd/ZrO2-CeO2, Pd/Al2O3-CeO2, and Pd/TiO2-CeO2 and examined for selective catalytic reduction (SCR) of NO with H2 in the presence of excess oxygen (6% O2). The properties of catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), BET, transmission electron microscopy (TEM), H2-temperature-programmed reduction (H2-TPR), and Raman spectroscopy. The synthesized catalysts exhibit notable NO reduction potential at 175 ℃ and in a wide temperature range from 100 to 400 ℃. Compared to bare CeO2 support, the Pd/ZrO2-CeO2 catalyst greatly enhances the SCR activity with a wide temperature range (100–400 ℃). The experimental results showed that the Pd/ZrO2-CeO2 catalyst had the best catalytic activity with 92 % NO reduction and a wide temperature reduction window (150–400 ℃) at the gas hour space velocity (GHSV) of 20,000 h−1.

Isotopic Oxygen Exchange Study to Unravel Noble Metal Oxide/Support Interactions: The Case of RuO2 and IrO2 Nanoparticles Supported on CeO2, TiO2 and YSZ

AbstractThe aim of this study is to unravel the mechanism of (noble metal oxide)/(active support) interactions for catalytic purposes. Hence, isotopic oxygen exchange (IOE) tests were performed on Iridium‐ and ruthenium‐based oxides supported on cerium oxide (CeO2), titanium oxide (TiO2), and yttria‐stabilized zirconia (YSZ). IOE tests demonstrated the metal oxide support involvement in the propane oxidation reaction, with YSZ‐based catalysts showing the highest exchange rate of oxygen, while CeO2 and TiO2‐based catalysts had a less diffusion of lattice oxygen in that order. This is related to the presence of extrinsic oxygen vacancies in the YSZ‐based catalysts and the reduction ability of the CeO2 and TiO2 supports. Despite the limitations on oxygen exchange in some of the noble/metal oxide catalysts, their catalytic performance was comparable to the ones that showed a high oxygen exchange. Therefore, active supports results in a higher engagement of oxygen from the support but not in a linear correlation with the catalytic performance of the metal/support.

Structure and activity of mixed VOx-CeO2 domains supported on alumina in cyclohexane oxidative dehydrogenation

Alumina-supported VOx-CeO2 materials are synthesized by atomic layer deposition (ALD) and evaluated in cyclohexane oxidative dehydrogenation (ODH). The order of deposition of the two metal oxides by ALD influences the size, geometry and oxidation state of the resulting surface species as indicated by Raman spectroscopy coupled with density functional theory (DFT) calculations and XPS. The individual contributions of VOx and CeO2 surface sites to the cyclohexane ODH reaction mechanism are investigated through a comparison of the catalytic activity of the supported mixed metal oxide materials, VOx/Al2O3, VOx/CeO2 and a bare CeO2 support. The highest activity is observed for CeO2, and different surface active sites are distinguished by introducing cyclohexane and cyclohexene in the reactant feed mixture. The catalytic behavior is dependent on the nature of VOS (S, Support) bonds, and small clusters of CeO2 on Al2O3 favor total oxidation, similar to exposed CeO2 sites in VOx/CeO2.

Bovine serum albumin templated porous CeO2 to support Au catalyst for benzene oxidation

Bovine serum albumin (BSA)-templated CeO2 was prepared by citrate sol-gel method and used as a support to synthesize Au catalyst for benzene oxidation. It was found that the Au/BSA-CeO2 with 3.2 nm Au nanoparticles (NPs) gave rise to superior catalytic performance for benzene oxidation in which the temperature for 90 % benzene conversion was only 210 °C. In addition, the catalytic activity of Au/BSA-CeO2 remained stable for 140 h of on-stream reaction. Besides, density functional theory calculation demonstrated how the porous structure of CeO2 influenced the oxygen vacancy generation, and the synergetic effects of Au NPs and CeO2 support facilitated the activation of benzene which was adsorbed on Au NPs prior to benzene oxidation. The catalysis of benzene oxidation over Au/CeO2 catalyst follows the dual-site mechanism. BAS-templated CeO2 support possessed hierarchical structure, abundant adsorbed oxygen species and synergic interaction with Au that determined the performance of Au/BSA-CeO2.

Characterization of new catalysts prepared by in-situ activation of Ce50Ni50-xAux intermetallic compounds for CO oxidation

In this study, catalysts for CO oxidation were prepared through in-situ activation of Ce50Ni50-xAux intermetallic compounds and characterized; these included Ce50Ni50 and Ce50Ni45Au5. Ribbon samples of intermetallic compounds were fabricated by single-roller melt spinning and then ground into powders for subsequent analysis. XRD patterns of the compounds were indexed as orthorhombic symmetry and no second phase was detected within the limits of low XRD resolution, indicating a homogeneous nature. Among the catalysts evaluated, the metal-oxide complex consisting of Ni–Au and CeO2, synthesized from the in-situ activation of Ce50Ni45Au5 compounds, showed a better catalytic activity for CO oxidation. In the presence of CO, Ce oxidation is thermodynamically favored while Ni oxidation is inhibited. The catalyst of the metal-oxide complex prepared from the Ce50Ni45Au5 compound revealed the CO conversions of 28.1%, 76.3%, 96.3% and 97.1% at reaction temperatures of 100 °C, 150 °C, 200 °C and 250 °C, respectively. SEM results revealed that a large quantity of fine Au–Ni particles were distributed on the CeO2 support after thermal treatment, which contributed to the high CO conversion over the Ce50Ni45Au5 compound at temperatures below 200 °C. Compared with the results in the literature, the high catalytic activity of the Ce50Ni45Au5 intermetallic compound, with a low Au loading of 7.6 wt% and a low surface area of 1.1 m2/g, displayed the most beneficial characteristics for practical applications.

Co-Supported CeO2Nanoparticles for CO Catalytic Oxidation: Effects of Different Synthesis Methods on Catalytic Performance

Hydrothermal and co-precipitation methods were studied as two different methods for the synthesis of CeO2nanocatalysts. Co/CeO2 catalysts supported by 2, 4, 6, or 8wt% Co were further synthesized through impregnation and the performance of the catalytic oxidation of CO has been investigated. The highest specific surface area and the best catalytic performance was obtained by the catalyst 4wt% Co/CeO2 with the CeO2 support synthesized by the hydrothermal method (4% Co/CeO2-h), which yielded 100% CO conversion at 130 °C. The formation of CeO2 nanoparticles was confirmed by TEM analysis. XRD and SEM-EDX mapping analyses indicated that CoOx is highly dispersed on the 4% Co/CeO2-h catalyst surface. H2-TPR and O2-TPD results showed that 4% Co/CeO2-h possesses the best redox properties and the highest amount of chemically adsorbed oxygen on its surface among all tested catalysts. Raman and XPS spectra showed strong interactions between highly dispersed Co2+ active sites and exposed Ce3+ on the surface of the CeO2 support, resulting in the formation of the strong redox cycle Ce4+ + Co2+↔ Ce3+ + Co3+.This may explain that 4% Co/CeO2-h exhibited the best catalytic activity among all tested catalysts.

Insights into facet-dependent reactivity of CuO–CeO2 nanocubes and nanorods as catalysts for CO oxidation reaction

Copper–ceria (CuO–CeO2) catalysts have been known to be very effective for the oxidation of CO, and their chemical behavior has been extensively studied during the last decades. However, the effect of different CeO2 crystal surfaces on the catalytic activity of CuO–CeO2 for the oxidation of CO is still unclear and should be further elucidated. In this study, we deposited 1 wt% Cu on mostly {100}-exposed CeO2 nanocubes (1CuCe NC) and mostly {110}-exposed CeO2 nanorods (1CuCe NR), respectively. Both 1CuCe NC and 1CuCe NR have been used as catalysts for the oxidation of CO and achieved 100% and 50% CO conversion at 130 °C, respectively. The differences in the catalytic activity of 1CuCe NC and 1CuCe NR were analyzed using temperature-programmed reduction of H2 and temperature-programmed desorption of CO techniques. The results confirmed the excellent reducibility of the 1CuCe NC catalyst, which was attributed to the weak interactions between Cu and the CeO2 support. Moreover, in situ diffuse reflectance infrared Fourier-transform spectroscopy studies indicated that the {100} planes of 1CuCe NC facilitated the generation of active Cu(I) sites, which resulted in the formation of highly reactive Cu(I)-CO species during the oxidation of CO. Both the excellent redox properties and effective CO adsorption capacity of the 1CuCe NC catalyst increased its catalytic reactivity.

Insight into Single-Atom-Induced Unconventional Size Dependence over CeO2-Supported Pt Catalysts

Summary Identification of the structure-performance relationship at the atomic level is vital for getting a deep understanding of the size-dependence behavior of metal catalysts. Here, an unconventional size dependence of Pt toward selective hydrogenation of p-chloronitrobenzene has been extensively investigated over CeO2 support. An upturned volcanic curve toward the selectivity of p-chloroaniline was found by decreasing the size of Pt from nanoparticles to single atoms. Differences on predominant orbitals among diverse coordination-environment Pt sites were identified to be the key factors influencing the modes of interaction with the C-Cl bond of p-chloroaniline. Specifically, electrostatic repulsion between non-bonding orbitals of Cl and predominant orbitals of Pt sites with high steric hindrance were speculated to be responsible for the suppression of dehalogenation and the high selectivity. This strategy to build correlation between the valence orbitals of active sites and their catalytic behavior could well be expanded to other structure-sensitive reactions and atomically dispersed catalysts.

The distributions of alkaline earth metal oxides and their promotional effects on Ni/CeO2 for CO2 methanation

To understand the role of alkaline sites played for CO2 methanation and develop better catalysts, the promoted Ni/M0.1Ce0.9Ox catalysts, in which the CeO2 are modified by different alkaline earth metal oxides with a M/Ce (M = Mg, Ca, Sr, Ba) molar ratio of 1/9, have been prepared. With the sol − gel method, different alkaline earth metal oxides present varied distribution states in the modified CeO2 support. MgO and CaO are predominantly dissolved into the CeO2 support lattice to form a pure solid solution structure, while SrO and BaO are dispersed as the carbonates on the CeO2 support surface. The modification of all the alkaline earth metal oxides on CeO2 can improve the intrinsic activity of the obtained supported Ni catalysts, whose activity follows the order of Ni/Ca0.1Ce0.9Ox > Ni/Sr0.1Ce0.9Ox > Ni/Mg0.1Ce0.9Ox > Ni/Ba0.1Ce0.9Ox > Ni/CeO2. H2-TPR, H2 adsorption–desorption and XPS results have revealed that the modification of alkaline earth metal oxides can improve Ni dispersion, induce more surface oxygen vacancies and increase both the amount and the strength of the moderate alkaline sites, which are the major factors determining the activity of the Ni/M0.1Ce0.9Ox catalysts for CO2 methanation. Ni/Ca0.1Ce0.9Ox, possessing the best Ni dispersion and the largest amount of the moderate alkaline sites, displays the highest activity among all the catalysts.