Abstract: The relationship between the oxygen species of cerium-based oxygen carriers and catalytic behavior, namely the correlation between catalytic activity and surface lattice oxygen (OS-L) and that between catalytic stability and bulk lattice oxygen (OB-L), was investigated by using CH3SH and Ce1-xYxO2-δ (x = 0, 0.25, 0.50, 0.75, and 1.0) solid solutions as examples. Activity and stability experimental studies with corresponding XPS were performed to assess the role of definite surface oxygen in cerium-based oxygen carriers. The surface lattice oxygen (OS-L), rather than the surface adsorbed oxygen (OS-A), was observed to be responsible for the catalytic decomposition of CH3SH. Further, the difference in catalytic activity between CeO2 and Y-doped samples is closely associated with the insertion of Y3+ ion into the lattice of CeO2 leading to the loss of surface lattice oxygen (OS-L). H2-temperature programmed reduction (TPR), a specially designed H2-TPR, X-ray photoelectron spectroscopy, reaction product (CO and CO2) analysis, and oxygen storage capacity tests were performed to demonstrate the migration of bulk lattice oxygen, which was directly related to the catalytic stability of CeO2 and Y-doped catalysts. Direct evidences of the migration of bulk lattice oxygen over cerium-based oxygen carriers were obtained. Additionally, the migration rate of bulk lattice oxygen (OB-L) within Ce0.75Y0.25O2-δ was faster compared to the migration rate of bulk lattice oxygen (OB-L) of CeO2. Finally, improvements in catalytic stability are closely associated with the fact that bulk lattice oxygen (OB-L) participates in the decomposition of CH3SH through its faster migration to replenish surface lattice oxygen (OS-L). The factors that influenced the migration rate of bulk lattice oxygen (OB-L) were thus also subsequently investigated and discussed.

Abstract: Multicomponent rare earth oxide (REO) nanocrystalline powders containing up to seven equiatomic rare earth elements were successfully synthesized in a single-phase CaF2-type (Fm-3 m) structure. The addition of more than six elements resulted in the formation of a secondary phase. Annealing at 1000°C for 1 h led to the formation of a single-phase (Ia-3) even in the 7-component system. In the absence of cerium (Ce4+), secondary phases were observed irrespective of the number of cations or the extent of thermal treatment indicating that cerium cations played a crucial role in stabilizing the multicomponent REOs into a phase pure structure.

Abstract: Transition metal (Cerium, Cobalt, and Iron) doped cryptomelane-type manganese oxide (M-OMS-2) catalysts have been successfully synthesized and characterized. The different metal-ion-doped K-OMS-2 catalysts showed distinct differences in their ozone decomposition activity. Cerium-doped OMS-2 materials showed ozone conversion of 90% under RH = 90% and space velocity of 600,000 h−1. X-ray diffraction (XRD), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) data suggested that the as-synthesized M-OMS-2 materials were all crystalline with no segregated metal oxide impurities. ICP-OES and XAFS results confirmed that Co3+ and Fe3+ replace Mn3+ in the cryptomelane structure and Ce4+mainly replaces the K+ in the tunnel and partially replaces the Mn4+ in the framework of the cryptomelane structure. Because of the differences in the substitution sites and the ionic radius of dopants, the morphologies of the catalysts were different. The Mn3+content and number of surface defects play a key role during the decomposition of ozone. Ce-OMS-2 is a promising catalyst for purifying waste gases containing ozone under high-humidity conditions.

Abstract: A series of Cux-Ce0.5-x-Zr0.5 oxides catalysts with different Cu/Ce ratio were synthesized by citric acid method. The catalysts were characterized by XRD, BET surface area, H2-TPR, NH3-TPD, NO-TPD, XPS and in-situ DRIFTS. The synergistic effect between copper and cerium on the catalytic performance of Cux-Ce0.5-x-Zr0.5 for selective catalytic reduction of NO with ammonia was investigated. It was found that the Cu0.2-Ce0.3-Zr0.5 catalyst show the excellent SCR activity, N2 selectivity and H2O/SO2 durability in a low temperature range of 150–270 °C even at high gas hourly space velocity of 84,000 h−1. The strong interaction leads to the improvement of the acidity and the increase in the amount of active oxygen species (oxygen vacancy), which are responsible for the higher activity at low temperatures. The SCR reaction process over Cu0.2-Ce0.3-Zr0.5 was also examined using in-situ DRIFTS. The DRIFTS results indicate that abundant ionic NH4+ (Bronsted acid sites), coordinated NH3 on the Lewis acid sites, as well as highly active monodentate nitrate and bridging nitrate species were the key intermediates in the SCR reaction.

Abstract: Ceria has recently shown intriguing hydrogenation reactivity in catalyzing alkyne selectively to alkenes. However, the mechanism of the hydrogenation reaction, especially the activation of H2, remains experimentally elusive. In this work, we report the first direct spectroscopy evidence for the presence of both surface and bulk Ce–H species upon H2 dissociation over ceria via in situ inelastic neutron scattering spectroscopy. Combined with in situ ambient-pressure X-ray photoelectron spectroscopy, IR, and Raman spectroscopic studies, the results together point to a heterolytic dissociation mechanism of H2 over ceria, leading to either homolytic products (surface OHs) on a close-to-stoichiometric ceria surface or heterolytic products (Ce–H and OH) with the presence of induced oxygen vacancies in ceria. The finding of this work has significant implications for understanding catalysis by ceria in both hydrogenation and redox reactions where hydrogen is involved.

Abstract: Across the periodic table the trans-influence operates, whereby tightly bonded ligands selectively lengthen mutually trans metal–ligand bonds. Conversely, in high oxidation state actinide complexes the inverse-trans-influence operates, where normally cis strongly donating ligands instead reside trans and actually reinforce each other. However, because the inverse-trans-influence is restricted to high-valent actinyls and a few uranium(V/VI) complexes, it has had limited scope in an area with few unifying rules. Here we report tetravalent cerium, uranium and thorium bis(carbene) complexes with trans C=M=C cores where experimental and theoretical data suggest the presence of an inverse-trans-influence. Studies of hypothetical praseodymium(IV) and terbium(IV) analogues suggest the inverse-trans-influence may extend to these ions but it also diminishes significantly as the 4f orbitals are populated. This work suggests that the inverse-trans-influence may occur beyond high oxidation state 5f metals and hence could encompass mid-range oxidation state actinides and lanthanides. Thus, the inverse-trans-influence might be a more general f-block principle. The inverse-trans-influence has been shown to operate in high oxidation state actinide complexes. Here, the authors report tetravalent cerium, uranium and thorium bis(carbene) complexes with trans C=M=C cores where experimental and theoretical data also suggest the presence of an inverse-trans-effect.

Abstract: A new type of convenient, environmentally friendly, and recyclable nanocatalyst (abbreviated as MgAlCe-LDH@Au) was designed and successfully assembled by loading Au nanoparticles (Au NPs; ∼3 nm average diameter) on a MgAlCe-LDH support through an in situ reduction of HAuCl4 by NaBH4. The MgAlCe-LDH support was prepared by doping the Magnesium–Aluminum Layered Double Hydroxide (MgAl-LDH) with cerium ions. The obtained MgAlCe-LDH@Au nanocatalyst was fully characterized by conventional methods and possesses excellent properties, such as a narrow size distribution, a high structural stability, a large specific surface area, and a good distribution of the Au NPs. Besides, this nanocatalyst displays a very remarkable activity in the reductive degradation of 4-nitrophenol by NaBH4 with a rate constant (kapp) of 0.041 s−1 and a catalyst turnover frequency (TOF) of 1.2 × 106 h−1; the reactions proceed in aqueous medium at room temperature and atmospheric pressure. The MgAlCe-LDH@Au nanocatalyst can also be recycled, maintaining its original activity even after seven consecutive reaction cycles. Additionally, MgAlCe-LDH@Au is a highly efficient catalyst for the reductive degradation (discoloration) of common organic dyes, including methylene blue, methyl orange, Congo red, rhodamine B, and rhodamine 6G, resulting in up to 3.2 × 104 h−1 values of TOFs. For comparative purposes, a related Ce-free MgAl-LDH@Au material was assembled and tested as the catalyst. The superior activity of MgAlCe-LDH@Au over MgAl-LDH@Au or MgAlCe-LDH can be explained by the following factors: (1) LDH itself can act as a co-catalyst and the doping of MgAl-LDH with cerium ions increases the charge separation efficiency of surface electrons; (2) Ce ions can strongly interact with Au atoms, modifying their electronic structure, stabilizing the oxidation states, and enhancing the fixation of Au NPs and their dispersion. Furthermore, the achieved catalytic activity of the MgAlCe-LDH@Au nanocatalyst is significantly superior when compared with other state-of-the-art systems for the degradation of similar types of organic contaminants.

Abstract: Intra-granular Acicular Ferrite (IAF), as one of the most well-known desirable microstructure of ferrite with a chaotic crystallographic orientation, can not only refine the microstructure and retard the propagation of cleavage crack but also provide excellent combination of strength and toughness in steel. The effect of adding cerium on microstructure and controlling proper cerium-based inclusions in order to improve properties in low-carbon commercial steel (SS400) were investigated. The type of inclusions can be controlled by changing S/O ratio and Ce content. Without Ce modification, MnS is a dominate inclusion. After adding Ce, the stable inclusion phases change from AlCeO3 to Ce2O2S. The optimum amount of cerium, 0.0235 wt.%, lead in proper grain refinement and formation of cerium oxide, oxy-sulfide and sulfide inclusions. Having a high amount of cerium results in increasing the number of inclusions significantly as a result it cannot be effective enough and the inclusions will act like barriers for others. It is found that the inclusions with a size of about 4∼7 μm can serve as heterogeneous nucleation sites for AF formation. Thermodynamic calculations have been applied to predict the inclusion formation in this molten steel as well, which show a good agreement with experimental one.

Abstract: A cerium-based metal–organic framework (MOF; 1) with a UiO-66 (UiO: University of Oslo) framework topology was synthesized solvothermally by employing 3,4-dimethylthieno[2,3-b]thiophene-2,5-dicarboxylic acid as a ligand. The MOF was thoroughly characterized by X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction, infrared spectroscopy, and thermogravimetric and N2 sorption analyses. The activated material (1′) retained its structural integrity in water, acetic acid and 1 M HCl solution. XPS investigation reveals the presence of both Ce(III) and Ce(IV) ions in 1. Owing to the presence of mixed-valence cerium ions, 1′ was able to oxidize the chromogenic peroxidase substrate 3,3′,5,5′-tetramethylbenzidine (TMB) or 2,2′-azinobis(3-ethylbenzothizoline-6-sulfonic acid) (AzBTS) in the absence of an external oxidizing agent. Thus, it showed inherent oxidase-like catalytic properties. Inspired by the excellent oxidase-mimicking activity of 1′, a protocol was developed for the rapid colorimetric sensing of biothiols in NaAc buffer (0.2 M, pH = 4). The sensing ability of 1′ towards cysteine was also demonstrated in human blood plasma. Furthermore, the redox-active cerium ions enabled 1′ to exhibit excellent heterogeneous catalytic performance in aerobic oxidation catalysis of thiol compounds. The material is reusable (both as a sensor and as a catalyst), low-cost and highly stable, which renders it a promising candidate for the monitoring of biothiols in immunoassays and medical diagnosis as well as for industrial oxidation catalysis.

Abstract: A set of cerium-copper oxide catalysts with various Ce/Cu contents was synthesized using the solution combustion synthesis (SCS) technique. Catalytic activities of the prepared materials were tested for the CO oxidation, total oxidation of ethene and soot combustion. As a whole, the best performances in terms of both CO oxidation and ethene total oxidation were achieved for the binary oxide catalysts having Ce/Cu ratio ranging from 0.67 to 1.5. It has been observed that catalysts with CuOx clusters interacting with CeO2 are particularly effective for both the oxidation reactions. This confirms that CeOx and CuOx domains may cooperate synergistically, leading to higher oxidation activity because of the easier surface reducibility and more abundant structural defects (oxygen vacancies). On the other hand, the soot combustion activity increases as a function of the Ce-content up to 95 at.%. Indeed, the best soot oxidation catalyst exhibits copper highly dispersed into the ceria framework (Ce-O-Cu species), along with an abundant population of Cu+ species and H-bonded hydroxyl groups. Finally, the best performing powder catalysts were deposited on Silicon Carbide (SiC)-type monoliths through a novel synthesis and their catalytic activity was confirmed in a laboratory-scale pilot plant reactor. All the prepared catalysts were characterized by physico-chemical techniques, including XRD, FESEM, TEM, N2 physisorption at −196 °C, H2-TPR, XPS, FT-IR and micro-Raman spectroscopies.

Abstract: The methylene blue (MB) removal abilities of raw activated carbon and iron/cerium modified raw activated carbon (Fe-Ce-AC) by adsorption were researched and compared. The characteristics of Fe-Ce-AC were examined by N2 adsorption, zeta potential measurement, FTIR, Raman, XRD, XPS, SEM and EDS. After modification, the following phenomena occurred: The BET surface area, average pore diameter and total pore volume decreased; the degree of graphitization also decreased. Moreover, the presence of Fe3O4 led to Fe-Ce-AC having magnetic properties, which makes it easy to separate from dye wastewater in an external magnetic field and subsequently recycle. In addition, the equilibrium isotherms and kinetics of MB adsorption on raw activated carbon and Fe-Ce-AC were systematically examined. The equilibrium adsorption data indicated that the adsorption behavior followed the Langmuir isotherm, and the pseudo-second-order model matched the kinetic data well. Compared with raw activated carbon, the maximum monolayer adsorption capacity of Fe-Ce-AC increased by 27.31%. According to the experimental results, Fe-Ce-AC can be used as an effective adsorbent for the removal of MB from dye wastewater.

Abstract: Catalytic decomposition of gaseous ozone (O 3 ) over todorokite-type manganese dioxides (T-MnO 2 ) at room temperature and the effects of cerium modification were investigated. Catalytic activity and stability were greatly improved over Ce-modified MnO 2 (Ce-MnO 2 ), which increased with the increase of Ce/Mn atomic ratios from 0.06 to 0.28. The cerium modification made agglomerated MnO 2 particles transformed into small sheets of Ce-MnO 2 catalyst with Ce/Mn ratio of 0.28, accordingly increasing the specific surface area. Moreover, the crystal boundaries between MnO 2 and CeO 2 formed at high ratios of Ce/Mn. Larger surface area and the crystal boundaries between MnO 2 and CeO 2 resulted in the formation of more oxygen vacancies, which act as the active sites for O 3 decomposition. Besides, we found the improved catalytic stability was associated with the desorption of oxygen species occupying the positions of oxygen vacancies.

Abstract: Monometallic cerium layered double hydroxides (MCe-LDHs) were successfully synthesized for the first time through a simple approach. XRD, TEM, SEM, XPS, FT-IR, TG-DSC techniques and UV–vis diffuse reflectance spectroscopy were used to characterize the samples. The obtained MCe-LDHs showed typical layered structure composing of quasi-hexagonal platelets with side length of about 2 μm and thickness about tens of nanometers, and preserved its platelet morphology after heat treatment at up to 800 °C. Our study also revealed that the heat-treatments at different temperatures could be employed to tune the concentration ratio of Ce3+/Ce4+ and the surface area of those cerium oxide platelets, both of which play a key role in the photocatalytic activity towards photoreduction of CO2.

Abstract: Metal organic frameworks (MOFs) bearing multicarboxylate linkers are in great demand for designing robust heterogeneous catalysts. A new microporous Ce(III)-based metal organic framework (Ce2NDC3) has been synthesized under solvothermal conditions, which showed strong paramagnetism and a CO2 uptake capacity of 1.64 mmol g−1 (7.23 weight%) at 273 K. The Ce2NDC3 showed high catalytic activity in CO2 fixation for the synthesis of cyclic carbonates with a maximum yield of 92% at ambient temperature and pressure. This rare earth metal-based MOF has been well characterized by single crystal X-ray diffraction, PXRD, N2 adsorption/desorption, UHR-TEM, FESEM, FTIR, 13C MAS NMR and TGA. Here, we have carried out magnetic analysis, which revealed that the Ce(III) in this MOF exhibited 2F5/2 magnetism in the ground state. The Ce2NDC3 catalyst showed high recycling efficiency in CO2 fixation reactions, together with retention of the MOF structure after several rounds of reuse. Presumably, the presence of acidic Ce(III) metal ions and microporosity in the coordinated polymer network is responsible for the high catalytic activity.