Ce Interaction Research Articles

New insights on the Cu–Ce interaction in Cu/Ce catalysts based on ZSM-5 and Beta for the preferential oxidation of carbon monoxide in excess hydrogen

Cu-Ce-modified zeolites are shown to be effective in preferential CO oxidation (CO-PROX) in H2-rich stream. Catalysts containing copper and cerium based on zeolites ZSM-5 (Si/Al = 15, 28 and 40) and Beta (Si/Al = 19) are synthesized by sequential incipient wetness impregnation and tested in total (CO-TOX) and preferential oxidation of CO. The influence of zeolite framework type, Si/Al ratio, metal loading and order of metal introduction on the formation of active sites and their activity and selectivity are studied. Based on XRD, TEM, XPS, adsorbed CO DRIFTS and H2-TPR studies it is shown that of the key importance is synergistic interaction of copper and cerium not only on the surface, but in the zeolite channels. Cu+ incorporated in CeO2 and mixed copper-cerium oxo/hydroxocations can both be active sites in CO oxidation. Wider channels of Beta framework favor the penetration of metal cations, but not formation of necessary active sites. The presence of CuO phase in the catalysts negatively affects the stability and selectivity in CO-PROX. ZSM-5-based catalyst containing largest amount of oxocations (2.6 wt% Cu, 10 wt% Ce, Si/Al = 15) makes it possible to completely remove CO from H2-rich mixture in CO-PROX at 150–190 °C.

Cerium(III) and 5-methylisophthalate-based MOFs with slow relaxation of magnetization and photoluminescence emission.

Two novel Ce(III) metal organic frameworks (MOFs) with formulas [Ce(5Meip)(H-5Meip)]nGR-MOF-17 and [CeCl(5Meip)(DMF)]nGR-MOF-18 (5Meip = 5-methylisophthalate, DMF = N,N-dimethylformamide) have been synthesized, forming 3-dimensional frameworks. Magnetic measurements show that both compounds present field-induced slow magnetic relaxation under a small applied dc field. For GR-MOF-17, the temperature dependence of relaxation times is best described by a Raman mechanism, whereas for GR-MOF-18, relaxation occurs through a combination of Raman and local-mode pathways. Moreover, when avoiding short Ce⋯Ce interactions by magnetic dilution in GR-MOF-17@La and GR-MOF-18@La, only the local-mode mechanism is responsible for magnetic relaxation. Photophysical studies show the occurrence of ligand-centred luminescence in both compounds and phosphorescence emission at low temperature for GR-MOF-17.

Effect of interaction degree between Mn and Ce of MnOX-CeO2 formulation on NO reduction and o-DCB oxidation performed simultaneously

MnOX-CeO2 catalysts were prepared by different methods in order to assess the influence of Mn and Ce interaction on the catalytic activity of NO reduction and 1,2-dichlorobencene (o-DCB) oxidation carried out simultaneously. Changes on catalytic properties were evaluated by XRD, Raman, N2-physisorption, H2-TPR and NH3-TPD. The samples prepared by co-precipitation and sol-gel exhibited the best catalytic activity over the whole temperature range. These preparation methods provide high interaction between Mn and Ce at surface and bulk, which was evidenced by the formation of mixed oxide phase. According to characterisation, the enhancement of catalytic activity with Mn and Ce interaction is associated to the promotion of structural defects, which improve redox and acid properties. Oxidative capability is also improved as a consequence of the oxygen vacancies generated by the presence of those structural defects. On the other hand, the promotion of Mn and Ce interaction favours the production of N2O in NO reduction at a lower temperature, and also leads to o-DCB total oxidation by decreasing CO production.

Freeze-drying optimized Ce–P/γ-Al2O3 coating on ceramic-supported Pd catalysts for CO2 methanation and CO oxidation

Ce–P/γ-Al2O3 coatings applied with the freeze-drying technique were deposited on foam ceramics before being further prepared as Pd catalysts. According to the research, phosphoric acid replaced the hydroxyl on the surface of γ-Al2O3 to produce polyhydroxy P-containing sites that readily attracted cerium oxide. The palladium oxide was then embedded into the cerium oxide vacancy. The impacts of the drying technique on the catalysts were investigated, and the results indicated that freeze drying created a special coating with a large specific surface area and uniform mesoporous structure by inhibiting nanoparticle agglomeration. H3PO4's altering action was augmented to boost the number of active sites and the percentage of weakly acidic sites. Due to better P–Ce interactions, more Ce3+ was added into CeO2, which created numerous oxygen vacancies. By adjusting the P-containing sites, the dispersion of cerium oxide and the active phase could be controlled, which aided the activation and spillover of hydrogen. Furthermore, the capacity for oxygen adsorption and desorption was enhanced, and the amount of adsorbed oxygen species increased. As a result, the freeze-drying technique improved the advantages of the Ce–P/γ-Al2O3 coating, increasing the conversion and selectivity of CO2 methanation to 81% and 94%, respectively, and lowering the complete conversion temperature of CO oxidation to 116 °C. This research has a guiding influence on the development of foam ceramic-loaded palladium catalysts in the fields of energy and the environment.

New insights into the relationship between Cu–Ce interaction and reactive Cu species in CuOx-CeO2 catalysts for preferential oxidation of carbon monoxide in excess hydrogen

A group of CuOx-CeO2 catalysts for preferential oxidation of carbon monoxide (CO-PROX) is designed by constructing different distribution status of Cu species on similar CeO2 support to adjust the Cu–Ce interaction strength. Cycled temperature programmed reduction by hydrogen (H2-TPR) indicates the strength of Cu–Ce interaction can impact the amount and redox properties of surface highly dispersed CuOx and strongly bonded Cu-[Ox]-Ce species, which respectively determines the catalytic performance at low and high temperature. Temperature programed desorption of H2 (H2-TPD) reveals the presence of large amounts of Cu-[Ox]-Ce species can inhibit H2 adsorption and dissociation, which increases the O2 selectivity toward CO oxidation. Moreover, temperature programmed desorption of CO2 (CO2-TPD) and in situ diffuse reflectance infrared Fourier transform spectra (DRIFTs) illustrates that H2O and CO2 influences the low-temperature catalytic performance mainly by competitive adsorption with CO on surface highly dispersed reactive Cu+ sites. This work aims to shed light on the underlying Cu–Ce interaction regime, and guide the design of high-performance CuO–CeO2 catalysts in realistic reaction conditions.

Catalytic ignition of CO over CuCeZr based catalysts: New insights into the support effects and reaction pathways

Self-sustained catalytic combustion is a promising strategy to remove CO from the off-gas produced during steelmaking, where the potential catalysts are bulk copper-cerium-zirconium mixed oxides or those supported on TiO2 or ZSM-5 substrates. In this study, the effects of the catalyst support on the CO catalytic ignition performance and reaction pathways were investigated by FTIR coupled with a novel in-situ cell, together with the state-of-the-art characterization techniques. The Infrared (IR) transmission cell equipped with a magnetically driven system, could effectively prevent overlaps between active intermediate peaks (Cu+-CO and Cu+(CO)2) and gaseous CO peaks. The Cu+ cations located at the phase interface are the main active sites. The Cu and Ce interactions lead to the formation of solid solutions of CuCe0.75Zr0.25Oδ (CuCeZr). The monocarbonyls [Cu+-CO] are the dominant species during CO oxidation, and the vacancies in the solid solutions are occupied by oxygen, accelerating the oxygen cycle. The TiO2 or ZSM-5 supports promote copper dispersion over CuCe0.75Zr0.25Oδ/TiO2 (CuCeZr/T) and CuCe0.75Zr0.25Oδ/ZSM-5 (CuCeZr/Z) catalysts, which can be attributed to their high surface areas (168.2 and 346.3 m2/g, respectively), while the Cu-Ce interactions are less relevant. Hence, CO oxidation mainly occurs at the phase interface between copper oxide and TiO2/ZSM-5. Dicarbonyls [Cu+(CO)2] are the main intermediates for the CuCeZr/T and CuCeZr/Z catalysts, and the Cu2+ species are reduced to form dicarbonyls that also take part in the oxidation process. Although a well copper dispersion enhances the activity of individual copper sites on the CuCeZr/T and CuCeZr/Z catalysts, considering the redshift of the carbonyl bands and the increase in CO adsorption, the close interactions and high contents of Cu and Ce favor the local accumulation of heat and mass transfer over bulk CuCeZr, leading to the ignition of CO at low temperatures.

Improved NH3-SCR activity by the cooperation of FeVO4 and CeO2 nanosheet catalysts: structure and mechanism

The cooperation of FeVO4 and CeO2 nanosheets significantly improves NH3-SCR activity via the nanosheet structure with lattice defects and Fe–V–Ce interaction.

In situ investigation of the mechanochemically promoted Pd–Ce interaction under stoichiometric methane oxidation conditions

A stable Pd0/Pd2+ arrangement generated on Pd/CeO2 prepared by mechanochemistry enables high methane activation rates in the absence of excess oxygen being available.

Cerium-promoted nickel catalysts supported on yttrium-doped γ-alumina for carbon dioxide methanation

The catalytic performance of the Ni/Al, Y-doped Ni/Al-xY, and Ce-promoted Ni-yCe/Al-15Y catalysts for CO2 methanation was examined. The CO2 conversion and CH4 yield increased in the order of Ni/Al < Ni/Al-5Y < Ni/Al-25Y < Ni/Al-15Y at temperatures below 350 °C. The reduction degree, H2 uptake, and total CO2 adsorption uptake increased in this order for the Ni/Al-xY catalysts, suggesting that doping γ-Al2O3 with a moderate amount of Y improved the interaction between the nickel and the support. When the Ni/Al-15Y catalyst was promoted by Ce with different contents (0–5 wt%), the catalyst with 2 wt% Ce exhibited the highest CO2 conversion (92.5%) and CH4 yield (92.2%) at ≤ 350 °C. As evidenced by catalyst characterization, cerium promoter results in a synergetic Ni–Ce interaction over the combined Al-15Y support, leading to the formation of finely dispersed metal species and numerous active sites for splitting of the stable C–O bond.

Surface insights into MnOx-based catalysts containing metal oxides for the selective catalytic reduction of NOX with NH3

The MnOx/TiO2-based NOX reduction catalyst showed moderate performance at low temperature and had the disadvantage of low N2 selectivity. Herein, the effect of Sm and Ce interactions on MnOx-based catalysts in selective catalytic reduction was investigated. Sm increased the redox properties of the MnOx catalyst and the ratio of Mn4+. Meanwhile, Ce enhanced the additional redox trait of the Sm-modified catalyst. When exposed to SO2/O2, Sm and Ce-modified MnOx not only suppressed performance degradation, but also increased N2 selectivity. Sm and Ce suppress the formation of N2O by inhibiting the formation of N2O4 intermediate species. Additionally, regardless of exposure to SO2/O2, Sm/Ce modified MnOx is an LH-ER mechanism (Langmuir-Hinshelwood and Eley-Rideal mechanism), but in the case of MnOx, it is suggested to be mainly an ER mechanism when exposed to SO2/O2. From these results, a strategy for improving the N2 selectivity/SCR performance of MnOx/TiO2-based catalysts with SO2 resistance was provided.

Co supported on Cex[Al2O3]0.5-x as an effective catalyst for low-temperature CO oxidation: Effect of calcination temperature

The low-temperature behavior of CO oxidation was investigated using 0.1 mol Ce doped Co supported on alumina catalysts prepared using co-precipitation followed by deposition precipitation method. The impact of calcination temperature on CO oxidation with kinetic aspects has been studied in detail and correlated to the characterizations. The Ce doped CoAl catalyst calcined at optimum temperature (550 ⁰C) shoed formation of more facile Co3+, Ce3+, and adsorbed species. The Co, Ce, and alumina interaction leads to the formation of optimum penta and hexa coordinated alumina. The physicochemical properties of the catalyst have been studied in detail using XRD, FTIR, Raman, TPR, XPS, and 27Al NMR and correlated to the catalytic activity.

A new chiral rare-earth organic framework constructed by tartaric acid: synthesis, structural, thermal investigations and optical properties.

Hydrothermal reactions of rare-earth cerium with L-tartaric acid afford a new coordination polymer, namely, poly[[triaqua(μ4-L-tartrato)cerium(III)] chloride], {[Ce(C4H4O6)(H2O)3]Cl}n, (1). The structure was determined by single-crystal X-ray diffraction analysis and further characterized by IR and UV-Vis spectroscopy, powder X-ray diffraction and thermogravimetric analysis. Single-crystal X-ray diffraction analysis revealed that the compound is a new two-dimensional (2D) double-layered structure with one-dimensional left-handed helical chains. The different intermolecular interactions were confirmed using Hirshfeld surface analysis and molecular fingerprint plots. Molecular 2D fingerprint plots quantify the different interactions and highlight that H...H (24.8%), H...O/O...H (26.3%), Cl...H/H...Cl (19.6%), O...O (13.8%) and Ce...O/O...Ce (13.6%) interactions account for 99.8% of all contacts. Additionally, the photoluminescence properties of the compound were investigated in the solid state.

Operando XAS Study of Pt-Doped CeO2 for the Nonoxidative Conversion of Methane

The methane to olefins, aromatics, and hydrogen (MTOAH) process via Pt/CeO2 catalysts poses an attractive route to improve yield and stability for the direct catalytic conversion of methane. In this study, two sets of samples, one composed of PtOx single sites on ceria and the other with additional Pt agglomerates, were prepared. Both sets of samples showed enhanced catalytic activity for the direct conversion of methane exceeding the performance of pure ceria. Pulsed reaction studies unraveled three reaction stages: reduction of the ceria support during activation, an induction phase with increasing product formation, and finally, stable running of the catalytic reactions. The reduction of ceria was confirmed by X-ray absorption spectroscopy (XAS) after conducting the MTOAH reaction. Operando X-ray absorption spectroscopy at challenging reaction temperatures of up to 975 °C in combination with theoretical simulations further evidenced an increased Pt–Ce interaction upon reaction with CH4. Analysis of the extended X-ray absorption fine structure (EXAFS) spectra proved decoration and encapsulation of the Pt particles by the CeO2/Ce2O3 support or a partial Ce–Pt alloy formation due to the strong metal–support interaction that developed under reaction conditions. Moreover, methyl radicals were detected as reaction intermediates indicating a reaction pathway through the gas-phase coupling of methyl radicals. The results indicate that apart from single-atom Pt sites reported in the literature, the observed Pt–Ce interface may have eased the activation of CH4 by forming methyl radicals and suppressed coke formation, significantly improving the catalytic performance of the ceria-based catalysts in general.

Highly efficient K-doped Mn–Ce catalysts with strong K–Mn–Ce interaction for toluene oxidation

In this study, Kx-Mn-Ce catalysts prepared by sol–gel method were investigated for toluene oxidation. Compared with Mn–Ce, the catalytic performance of Kx-Mn-Ce was further improved. X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and Raman analyses demonstrate that K ions enter the lattice of CeO2 and disperse uniformly. The results of X-ray photoelectron spectroscopy (XPS), H2-temperature programmed reduction (H2-TPR), and O2-temperature programmed desorption (O2-TPD) analyses indicate that there is a strong interaction between K, Mn and Ce; the charge compensation effect would be induced when K ions enter the lattice of CeO2, which leads to more oxygen vacancies due to the generation of more Ce3+. Toluene-TPD shows that K-doping enhances the activation ability of toluene. Among all catalysts, K0.1-Mn-Ce shows the highest concentration of Mn4+, Ce3+, Osur, and redox ability, resulting in higher low-temperature catalytic activity. Additionally, the results of stability and water resistance also prove that K0.1-Mn-Ce catalyst possesses excellent stability and water resistance.

High jolt-resistance monolithic CuO–CeO2/AlOOH/Al-fiber catalyst for CO-PROX: Influence of AlOOH/Al-fiber calcination on Cu–Ce interaction

Micro-reactors for the preferential oxidation of CO in H 2 -rich stream (CO-PROX) are attractive for PEMFCs employed in portable electronic devices and automobiles, but the jolt is inevitable, which makes micro-reactors necessitate high jolt resistance. The monolithic structured catalyst could effectively resolve these problems. Herein, we employed the thin-felt monolithic Al-fiber substrate to fabricate the CuO–CeO 2 /AlOOH/Al-fiber catalyst for the CO-PROX reaction. This catalyst was prepared via first growing AlOOH nanosheets onto the Al-fiber surface by steam oxidation method, followed by depositing CuO–CeO 2 onto the AlOOH/Al-fiber support. The preferred catalyst delivered 100% CO conversion and 81% O 2 selectivity at 140 °C with a gas hourly space velocity of 12,000 mL g −1 h −1 , and particularly, performed stably for 120 h at the changeable temperatures of 120–160 °C. This work provides a strategy to tailor a qualified monolithic catalyst that couples the promising jolt resistance and catalytic performance at 120–160 °C. • The monolithic CuO–CeO 2 /AlOOH/Al-fiber catalyst was fabricated for CO-PROX. • This catalyst exhibits both good performance and high jolt resistance. • As-grown AlOOH nanosheets on Al-fiber enable high Cu–Ce dispersion and interaction. • The calcination of AlOOH/Al-fiber leads to decreased activity and hydroxyl amount.

Key Properties and Parameters of Pd/CeO2 Passive NOx Adsorbers

In this paper, a series of Pd/CeO2 catalysts prepared by different synthesis routes and showing different morphological and textural properties have been investigated for passive NOx adsorption (PNA) applications. The results obtained by NOx adsorption/desorption tests demonstrated that NOx storage capacity and NOx storage efficiency of Pd/CeO2 materials depend strictly on their surface area, whereas the morphology of the support and the Pd deposition method do not seem to play a key role. In contrast, the Pd deposition method does impact the dynamics of NOx desorption by affecting the amount of NOx desorbed at different temperatures. This seems to be connected to Pd–Ce interactions at the nanoscale that favor NOx desorption at higher temperatures suitable for PNA application. These findings are relevant in designing and optimizing the properties of Pd/CeO2 materials for their function as passive NOx adsorbers.

Effect of support preparation method on water-gas shift activity of copper-based catalysts

Reducing the effect of global warming and increasing the needs for fuel sources appoint hydrogen utilization in fuel cell technology a very promising demand. A step for required pure hydrogen production for this purpose is the water-gas shift (WGS) reaction and the design of well performing but cost effective WGS catalysts, the latter being of growing research interest. In the present study catalysts containing copper (3 wt%) supported on ceria/alumina (30 wt% ceria) and Y-doped ceria/alumina (1 wt% Y2O3 regarding ceria content) were prepared by impregnation (IM) and by mechanochemical mixing (MM). The effect of support synthesis methods on the performance in the WGS reaction was discussed based on catalyst characterization by means of XRD, HRTEM, XPS, and H2-TPR methods. It was established that the effect of Y-doping was strongly dependent on the method of support preparation being positive for MM and unfavourable in the case of impregnation. The IM method caused the formation of a superficial phase with CuO, Y-dopant, and ceria in close location. Additionally, a higher amount of oxygen vacancies was related to non-reducible Y3+ ions that interrupted Cu–oxygen vacancy–Ce interaction thus leading to both lower reducibility and WGS activity. The presence of yttria phase located on ceria-free alumina in the case of MM support decreased the negative influence of Ce3+ replacement by Y3+ ions. A positive effect of separate Y2O3 could explain the best WGS performance observed. Long-term WGS tests showed good stability thus making this catalyst promising composed mainly of alumina and low amount of active phases. The study contributes to the development of an advantageous formulation of well-performing and economically profitable WGS catalysts for efficient improving of hydrogen purity for small-scale applications.

Ru0.05Ce0.95O2 Solid Solution Derived Ru Catalyst Enables Selective Hydrodeoxygenation of m‐Cresol to Toluene

AbstractSelective hydrodeoxygenation (HDO) of biomass lignin‐derived phenolic compounds to aromatics is a demanding reaction. Oxophilic Ru based catalysts can catalyze direct deoxygenation but suffer from competing C−C hydrogenolysis to CH4 at 350 °C and hydrogenation of phenyl ring at 250 °C under atmospheric pressure. In this work, a solid solution Ru0.05Ce0.95O2 catalyst was hydrothermally synthesized, characterized and tested for HDO of m‐cresol. The Ru species is highly dispersed in the solid solution as isolated atoms and/or subnanometer clusters as a result of strong Ru−O−Ce interaction, and the catalyst shows high concentration of oxygen vacancies. In contrast to nonselective Ru/SiO2, Ru0.05Ce0.95O2 selectively catalyzes deoxygenation to toluene while significantly inhibits C−C hydrogenolysis and hydrogenation. Moreover, the overall intrinsic HDO rate on Ru0.05Ce0.95O2 is 2.2 and 8.8 times higher than those on Ru/CeO2 and Ru/SiO2, respectively. The Ru−OV−Ce sites were proposed to be the active sites for deoxygenation. Inhibition of C−C hydrogenolysis and phenyl hydrogenation can be ascribed to the reduced size of the Ru ensembles due to the strong interaction in the solid solution.

Co-regulation of dispersion, exposure and defect sites on CeO2 (111) surface for catalytic oxidation of Hg0

The additional cost of Hg0 capture in coal-fired power plants has facilitated the demand for environmental pollutants mitigation material for Hg0 oxidation to Hg2+ for an ultra-low Hg emission technology. Herein, a suite of CeO2-based catalysts were investigated aiming at ultra-low gaseous Hg0 emission at coal-fired power stations. Gaseous elemental mercury is feasible to be catalytically oxidized to Hg2+. The co-regulated dispersion, exposure and defect sites on CeO2 (111) surface with 2 wt% Ce and 8 wt% Mo compositions on γ-Al2O3 was found to be the most promising catalyst demonstrating a high catalytic oxidation efficiency, a broad operating temperature range and a low activation energy. Specifically, it is shown that the oxidation of Hg0 on the Ce-based catalysts can be enhanced by the addition of Mo (up to 8 wt%) via promoting the CeO2 (111) surface dispersion and exposure. Moreover, insights into the Ce and Mo synergistic interactions showed that it facilitated the formation of defect-containing surface sites. Besides, the co-regulation of dispersion, exposure and defect sites on CeO2 (111) surface was further studied by DFT calculations. This study provides a feasible approach in optimization of CeO2-based catalysts for catalytic oxidation of Hg0 to achieve efficient removal of environmental pollutants.

Towards the Effect of Pt0/Ptδ+ and Ce3+ Species at the Surface of CeO2 Crystals: Understanding the Nature of the Interactions under CO Oxidation Conditions

AbstractWe synthesized CeO2 nanowires and nanocubes showing {110}+{100} and {100} surfaces predominantly, respectively, once the different surface energies play a crucial role in the behavior of Ce4+/Ce3+ reversibility. We found out that Pt/CeO2 nanowires presented more Ce3+ content, oxygen vacancies, and atomically dispersed Pt, indicating a stronger Pt−Ce interaction. In contrast, the Pt/CeO2 nanocubes presented a higher contribution of Ptδ+ species, suggesting a well‐controlled Pt particle size (∼1 nm) and significant interaction with ceria, with oxygen species more available at the surface. Thus, we suggest that the ceria‘s different surface energies may lead to different Pt distributions of species over the supports. H2‐reduction treatment led to changes in Pt structure that showed a better catalysis performance for the Pt/CeO2 nanowires essentially, supported by XPS and CO‐DRIFTS. Nevertheless, this step did not cause improved activity to the point of overcoming the nanocubes‐based catalyst, and the reasons were fully discussed. Herein, we propose that catalysts′ performance depends on a complex combination of several materials′ characteristics. These features may lead to different reaction pathways depending on the pre‐treatment of the samples.