Effect of hydration on the surface basicity and catalytic activity of Mg-rare earth mixed oxides for aldol condensation

Magnesium and rare earth mixed oxides (Mg₃REO_x (RE = La, Y, Ce)) were prepared and characterized by X-ray diffraction (XRD), N₂ adsorption–desorption, infrared spectra and microcalorimetry of CO₂. The results reveal that the Mg₃CeO_x catalyst is present in the form of Mg-Ce-O solid solution, while the Mg₃LaO_x and Mg₃YO_x catalysts are probably rare earth oxides dispersed on MgO surface. As a result, among the calcined Mg₃REO_x catalysts, the Mg₃CeO_x catalyst presents the highest rate constant for acetone aldolization, which is well correlated to its more homogeneous distribution of basic sites. In contrary, the Mg₃YO_x catalyst exhibit the lowest catalytic activity for acetone aldolization. Upon hydration pre-treatment, the basic properties on the surface of the Mg₃REO_x catalysts were changed markedly. The Mg₃YO_x catalyst after hydration treatment shows the highest amount of basic sites on catalyst surface, and then exhibits the highest activity among the hydrated Mg₃REO_x catalysts. These results make it possible to fine-tune basic sites for acetone aldolization.     

Novel promoting effects of cerium on the activities of NO_x reduction by NH_3 over TiO_2-SiO_2-WO_3 monolith catalysts

A series of catalysts were prepared by doping different loadings of CeO2 over TiO2-SiO2-WO3 and used for the selective catalytic reduction of NOx by NH3. The experimental results showed that the selective catalytic reduction（SCR） performance and SO2-resistant ability of TiO2-SiO2-WO3 were greatly enhanced by the introduction of cerium. The catalyst containing 10% CeO2 showed the highest NO conversion in a wide temperature range and good N2 selectivity with broad operation temperature window at the gas hourly space velocity（GHSV） of 30000 h–1, which was a very promising catalyst for NOx abatement from diesel engine exhaust. The catalysts were characterized by X-ray diffraction（XRD）, scanning electron microscopy with energy dispersive X-ray spectroscopy（SEM-EDS）, N2 adsorption-desorption（BET） and X-ray photoelectron spectroscopy（XPS）. The characterization results showed that the bigger pore radius, higher surface atomic concentration and dispersion of Ce and the abundant adsorbed oxygen on the surface of catalyst contributed to the best NH3-SCR performance of CeO2/TiO2-SiO2-WO3 catalyst containing 10% CeO2.     

Enhancing low-temperature NH3-SCR performance of Fe–Mn/CeO2 catalyst by Al2O3 modification

In the work, supported catalysts of FeO_x and MnO_x co-supported on aluminum-modified CeO₂ was synthesized for low-temperature NH₃-selective catalytic reduction (NH₃-SCR) of NO. Impressively, the SCR activity of the obtained catalyst is markedly influenced by the adding amount of Al and the appropriate Ce/Al molar ratio is 1/2. The activity tests demonstrate that Fe–Mn/Ce1Al2 catalyst shows over 90% NO conversion at 75–250 °C and exhibits better SO₂ resistance compared to Fe–Mn/CeO₂. Fe–Mn/Ce1Al2 shows the expected physicochemical characters of the ideal catalyst including the larger surface and increased active reaction active sites by controlling the amount of Al doping. Also, the better catalytic activity is well correlated with the present advantaged surface adsorption oxygen species, Mn⁴⁺ species, Ce³⁺ species and the enhanced reducibility of Fe–Mn/Ce1Al2, which is superior to the Fe–Mn/CeO₂ catalyst. More importantly, we further demonstrate that the amount and strength of surface acid sites are improved by Al-doping and more active intermediates (monodentate nitrate) is generated during NH₃-SCR reaction. This work provides certain insight into the rational creation of simple and practical denitration catalyst environmental purification.     

Promotion effect of niobium on ceria catalyst for selective catalytic reduction of NO with NH3

The CeO₂, Ce–Nb–O_x and Nb₂O₅ catalysts were synthesized by citric acid method and the promotion effect of Nb on ceria for selective catalytic reduction (SCR) of NO with NH₃ was investigated. The catalytic activity measurements indicate that the mixed oxide Ce–Nb–O_x presents a higher SCR activity than the single oxide CeO₂ or Nb₂O₅ catalyst. In addition, the Ce–Nb–O_x catalyst shows high resistance towards H₂O and SO₂ at 280 °C. The Raman, X-ray photoelectron spectra and temperature programmed reduction with H₂ results indicate that the incorporation of Nb provides abundant oxygen vacancies for capturing more surface adsorbed oxygen, which provides a superior redox capability and accelerates the renewal of active sites. Furthermore, the Fourier transform infrared spectra and temperature programmed desorption of NH₃ results suggest that niobium pentoxide shows high surface acidity, which is partly retained in the Ce–Nb–O_x catalyst possessing a high content of Lewis and Brønsted acid sites. Therefore, the incorporation of Nb improves both the redox and acidic capacities of Ce–Nb–O_x catalyst for the SCR reaction. Here, the redox behavior is primarily taken on Ce and the acidity is well improved by Nb, so the synergistic effect should exist between Ce and Nb. In terms of the reaction mechanism, in situ DRIFT experiments suggest that both NH₃ on Lewis acid sites and NH₄⁺ on Brønsted acid sites can react with NO species, and adsorbed NO and NO₂ species can both be reduced by NH₃. In the SCR process, O₂ primarily acts as the accelerant to improve the redox and acid cycles and plays an important role. This work proves that the combination of redox and acidic properties of different constituents can be feasible for catalyst design to obtain a superior SCR performance.     

CeO2-x modified Ru/γ-Al2O3 catalysts for ammonia decomposition reaction

Developing high-performance ammonia decomposition catalysts for preparing CO_x-free hydrogen shows great practical significance.Herein,CeO₂ is used as a promoter to modulate the metal-support interaction to enhance the catalytic performance of Ru/Al₂O₃ catalysts.A series of 1Ru/xCe-10AI(x=0.5,1,or 3)catalysts was prepared by a facile colloidal deposition method.We find that the optimized 1 Ru/1Ce-10Al catalyst exhibits excellent activity for the decompo...     

Effect of manganese and/or ceria loading on V_2O_5-MoO_3/TiO_2 NH_3 selective catalytic reduction catalyst

The effect of manganese and/or ceria loading of V_2 O_5-Mo_O_3/TiO_2 catalysts was investigated for selective catalytic reduction(SCR) of NO_x by NH_3.The manganese and/or ceria loaded V_2 O_5-MoO_3/TiO_2 catalysts we re prepared by the wetness impregnation method.The physicochemical characteristics of the catalysts were thoroughly characterized.The catalytic performance of 1.5 wt% V_2 O_5-3 wt% MoO_3/TiO_2(V1.5 Mo3/Ti) is greatly enhanced by addition of 2.5 wt% MnO_x and 3.0 wt% CeO_2(V1.5 Mo3 Mn2.5 Ce3/Ti) below450℃.Compared with the V1.5 Mo3/Ti catalyst with NO_x conversion of 75% at 275 ℃,V1.5 Mo3 Mn2.5 Ce3/Ti exhibits higher NO_x conversion of 84% with good resistance to SO_2 and H_2 O at a gas hourly space velocity value of 150000 h~(-1).The active manganese,cerium,molybdenum,and vanadium oxide species are highly dispersed on the catalyst surface and some synergistic effects exist among these species.Addition of MnO_x significantly enhances the redox ability of the cerium,vanadium,and molybdenum species.Addition of Ce increases the acidity of the catalyst.More active oxygen species,including surface chemisorbed oxygen,form with addition of Mn and/or Ce.Because of the synergistic effects,appropriate proportions of manganese in different valence states exist in the catalysts.In summary,the good redox ability and the strong acidity contribute to the high NH3-SCR activity and N2 selectivity of the V1.5 Mo3 Mn2.5 Ce3/Ti catalyst in a wide temperature range.And the V1.5 Mo3 Mn2.5 Ce3/Ti catalyst shows good resistance to H_2 O and SO2 in long-time catalytic testing,which can be ascribed to the highly sulfated species adsorbed on the catalyst.     

Barrier effect of SiO_2 shell over hollow CeO_2/CuO@SiO_2 catalysts for broadening temperature window of total CO conversion

The hollow inverse CeO_2/CuO@SiO_2 catalysts with different Ce/Cu mass ratios were synthesized by the two-step hydrothermal and incipient wetness impregnation methods,and characterized by multitechnique characterizations,such as SEM,TEM,XRD,H_2-TPR,XPS and N_2 adsorption-desorption techniques.It is found that the hollow shell is composed of CuO and SiO_2,and CeO_2 nanoparticles are coated on the surface of CuO@SiO_2 support.And the CeO_2/CuO@SiO_2 catalyst with the Ce/Cu mass ratios of 1:1 denoted as 1 CeO_2/CuO@SiO_2,which possesses a maximum amount of highly dispersed copper species and medium-sized CuO as well as the highest concentration of oxygen vacancies,exhibits the highest catalytic activity and widest full CO conversion window.The barrier effect of the SiO_2 shell effectively prevents the reduction of CuO species,which broadens temperature window of CO total conversion and enhances CO_2 selectivity above 155℃over the 1 CeO_2/CuO@SiO_2 catalyst in comparison with the CuO-CeO_2 and CeO_2-CuO catalysts.     

Synergistic effects of PtFe/CeO2 catalysts afford high catalytic performance in selective hydrogenation of cinnamaldehyde

Selective hydrogenation of unsaturated aldehydes remains a grand challenge in controlling chemoselectivity up to now.We synthesized a series of PtFe_x/CeO₂ catalysts,which were characterized by X-ray diffraction(XRD),transmission electron microscopy(TEM),X-ray photoelectron spectroscopy(XPS) as well as temperature-programmed-reduction by hydrogen(H₂-TPR).The catalytic performance of PtFe_x/CeO₂,including cinnamaldehyde(CAL) conversion and sele...     

Active manganese oxide on MnOx–CeO2 catalysts for low-temperature NO oxidation: Characterization and kinetics study

MnO_x–CeO₂ catalysts were synthesized to investigate the active sites for NO oxidation by varying the calcination temperature. XRD and TEM results showed that cubic CeO₂ and amorphous MnO_x existed in MnO_x–CeO₂ catalysts. High temperature calcination caused the sintering of amorphous MnO_x and transforming to bulk crystalline Mn₂O₃. H₂-TPR and XPS results suggested the valence of Mn in MnO_x–CeO₂ was higher than pure MnO_x, and decreased with the increasing calcination temperature. The turnover frequency (TOF) was calculated based on the initial reducibility according to H₂-TPR quantitation and kinetic study. The TOF results indicated that the initial reducibility of amorphous MnO_x with high valence manganese ions was equivalent to the active sites for NO oxidation. It can be inferred that the amorphous MnO_x plays a key role in low-temperature NO oxidation.     

Controllable synthesis of argentum decorated CuOx@CeO2 catalyst and its highly efficient performance for soot oxidation

In this paper, CuO_x@Ag/CeO₂ catalysts were synthesized by simple wet-chemical method and equal volume impregnation method. The obtained catalysts were subjected to soot temperature programmed oxidation (soot-TPO) activity tests and were further characterized by various techniques such as X-ray diffraction (XRD), transmission electron microscopy/high-resolution transmission electron microscopy (TEM/HR-TEM), N₂ physisorption, X-ray photoelectron spectroscopy (XPS) and H₂-temperature programmed reduction (H₂-TPR). The results show that CuO_x@Ag/CeO₂ synthesized presents well controlled core-shell structures, with nano-cube like Cu₂O as the core and Ag decorated polycrystalline CeO₂ grafting layers as the shell. Such core-shell structured CuO_x@Ag/CeO₂ can successfully construct a secondary oxygen delivery channel (CuO_x → CeO₂ → Ag) to effectively transfer bulk oxygen of the catalyst to the soot, resulting in its excellent soot oxidation activity compared to CuO_x@CeO₂. The potential benefiting effect by Ag introduction over Cu@Ag/Ce can be concluded as: (i) pumping lattice oxygen and accelerating gaseous O₂ dissociation to generate significantly increased active surface oxygen content; (ii) modulating a moderate surface oxygen vacancies concentration to maintain more highly active O₂^– species.     

Catalytic performance of Zr-doped CuO-CeO_2 oxides for CO selective oxidation in H_2-rich stream

Zr-doped CuO-CeO_2 catalysts for CO selective oxidation were designed and prepared by the hydrothermal method and coprecipitation. The experimental samples were characterized by means of N_2 adsorption-desorption isotherms, powder X-ray diffraction, temperature-programmed reduction and Xray photoelectron spectroscopy. It is observed that the catalyst prepared by hydrothermal method exhibits larger specific surface area, smaller crystalline size and higher dispersion of active components compared with those of the catalyst obtained by coprecipitation. Meanwhile, redox properties of copper oxide are improved significantly and highly dispersed copper species providing CO oxidation sites are present on the surface. Furthermore, adsorptive centers of CO and active oxygen species increase on the copper-ceria interfaces. The Zr-doped CuO-CeO_2 catalyst prepared by hydrothermal method possesses superior catalytic activity and selectivity for selective oxidation of CO at low temperature compared with those of the sample prepared by coprecipitation. The temperature corresponding to 50% CO conversion is only 73 ℃ and the temperature span of total CO conversion is expanded from 120 to 160 ℃.     

Preparation and application of ZnO doped Pt-CeO2 nanofibers as electrocatalyst for methanol electro-oxidation

ZnO doped Pt/CeO₂ nanocomposites were prepared by electrospinning and reduction impregnation. X-ray diffraction (XRD), transmission electron microscopy (TEM), energy disperse spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were employed to characterize the nanocomposites. It is observed that ZnO and CeO₂ form the hexagonal wurtzite phase and cubic fluorite phase in the nanocomposite, respectively, whilst Pt nanoparticles (NPs) with the number-averaged size of ca. 3.1 nm are uniformly distributed on the surface of nanofibers. The mass fraction of Pt NPs in the nanocomposites is about 10 wt%. The doping of ZnO is effective to promote reactive oxygen species, surface reaction sites and the interaction between Pt and oxides. The catalytic performance of nanocomposites was evaluated by the methanol electro-oxidation, indexed with the catalytic activity, stability of catalyst. As a result, it is found that the nanocomposite exhibits much higher activity and stability for methanol oxidation than the undoped Pt/CeO₂ catalyst.     

Regulating local coordination environment of rhodium single atoms in Rh/CeO2 catalysts for N2O decomposition

Rh single atom catalysts(SACs) have been insensitively investigated recently due to the maximum utilization efficiency of Rh,one of the most expensive precious metals.Although great efforts have been made in the development and application of Rh SACs,there are few reports on the precise control of the local coordination environment of Rh single sites on CeO₂ and their catalytic performance for N₂O decomposition.Herein,Rh/CeO₂ catalysts with different Rh-O coordin...     

Selective catalytic reduction of NOx with NH3 over core-shell Ce@W catalyst

An environmentally benign WO₃ wrapped cubic CeO₂ core–shell catalyst (Ce@W) was developed for the selective catalytic reduction of NO_x with NH₃. Compared with CeW particles prepared via the conventional co-precipitation method, this core–shell catalyst not only displays higher tolerance to SO₂ and H₂O, but also exhibits a wider activity temperature window of 250–450 °C, in which NO_x conversion and N₂ selectivity reaches 100%. The improved performance of Ce@W catalysts can be contributed to the strong interactions between CeO₂ (100) and WO₃, which generates more Ce³⁺ and surface chemisorbed oxygen. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) reveal that the more thermally stable Brønsted acid sites on Ce@W lead to its excellent high-temperature activity.     

Preparation of M/Ce1–xTixO2 (M=Pt,Rh, Ru) from sol-gel method and their catalytic oxidation activity for diesel soot

A series of Ce_1–xTi_xO₂ mixed oxide catalysts were synthesized by sol-gel method and then loading of noble metal (M = Pt, Rh, Ru) was used for soot oxidation. Ti-doped Ce_1–xTi_xO₂ catalysts (x is the molar ratio of Ti/(Ti + Ce) and ranges from 0.1 to 0.5) exhibit much better oxidation performance than CeO₂ catalyst, and the Ce_0.9Ti_0.1O₂ catalyst calcined at 500 °C has the best catalysis activity. Each noble metal (1 wt%) was supported on Ce_0.9Ti_0.1O₂ (M/C9T1) and the properties of the catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman, Brunauer–Emmett–Teller (BET) method, and H₂-temperature programmed reduction (H₂-TPR) results. Results show that the introduction of Ti into CeO₂ forming Ti-O-Ce structure enhances the catalytic activity and increases the number of oxygen vacancies at the catalyst surface. The noble metal is highly dispersed over Ce_0.9Ti_0.1O₂, and M/C9T1 catalysts present enhanced activity in comparison to Ce_0.9Ti_0.1O₂. It is found that noble metals can greatly increase the activity of the catalyst and the corresponding oxidation rate of soot can enhance the electron transfer capacity and oxygen adsorption capacity of the catalyst. A small amount of Ti doping in CeO₂ can significantly improve the activity of the catalyst, while a large amount of Ti reduces the performance of the catalyst because a large amount of Ti is enriched on the surface of the catalyst, which hinders the contact and reaction between the catalyst and the soot.     

Combustion synthesis of YAG:Ce phosphors via the thermite reaction of aluminum

Cerium-doped yttrium aluminum garnet(YAG:Ce) as a yellow phosphor for white light-emitting diodes(LEDs) was synthesized via a facile combustion method using Y_2 O_3, CeO_2, Al_2 O_3, Al,and NaClO_4 as raw materials. The combustion synthesis approach utilizes the strong exothermic oxidation of aluminum to realize a self-sustaining reaction. In this study, we investigated the effects of the ratios of Al_2 O_3 to AI,fluxes, and coprecipitated materials as raw materials on the luminescence properties of the synthesized YAG:Ce phosphors. When the amount of Al_2 O_3 x is varied, the combustion reaction proceeds at x ≤ 1.8,with x = 1.725 being the optimum condition for producing a high-performance product. When 5 wt%BaF2 is added, the luminescence intensity is significantly improved owing to a decrease of YAP(YAlO_3)formation with improved uniformity. However, the addition of CaF_2 and NaF does not improve the luminescence properties. To suppress the segregation of CeO_2, we used the coprecipitated material Y_2 O_3-CeO_2 as a raw material. Unlike with separate addition of Y_2 O_3 and CeO_2, Ce ions are uniformly distributed in the coprecipitated material, resulting in improved luminescence properties. The combination of BaF_2 and coprecipitated material significantly improves the internal quantum efficiency to83.0%, which is close to that of commercial phosphors.     

Highly dispersed CeO2–x nanoparticles with rich oxygen vacancies enhance photocatalytic performance of g-C3N4 toward methyl orange degradation under visible light irradiation

CeO₂/g-C₃N₄ photocatalysts have attracted tremendous attention in the photocatalytic degradation of organic pollutants. The design and construction of highly active CeO₂/g-C₃N₄ photocatalysts without harsh conditions are still challenging. Herein, highly dispersed CeO_2–x nanoparticles with rich oxygen vacancies were successfully precipitated on the surface of g-C₃N₄ under mild conditions. The fabricated CeO_2–x/g-C₃N₄ exhibits remarkable activity and stability for photocatalytic degradation of MO pollutant. The optimal rate constant of MO degradation over CeO_2–x/g-C₃N₄ is about 0.031 min^?1, which is three times higher than that of g-C₃N₄. A negligible activity decrease is observed after three cycling runs. The enhanced catalytic performance can be ascribed to the excellent dispersion of CeO_2–x with rich oxygen vacancies that benefit O₂ adsorption and visible light absorption. In addition, the proper band alignment between CeO_2–x and g-C₃N₄ is conducive to the highly efficient separation of photogenerated electron–hole pairs.     

Promoting effect of tantalum and antimony additives on deNOx performance of Ce3Ta3SbOx for NH3-SCR reaction and DRIFT studies

A superior Ce-Ta-Sb composite oxide catalyst prepared using homogeneous precipitation method exhibited excellent deNOx efficiency and nearly 100% N_2 selectivity with broad operation temperature window and better resistance to higher space velocity, meanwhile strong resistance to H_2 O and SO_2. This catalyst was systematically characterized using XRD, N_2 adsorption, SEM, TEM, XPS, ESR, Raman, H_2-TPR,NH3-TPD and in situ DRIFTS. There exists a synergistic effect between Ce, Ta and Sb species. It is further indicated that the prominent deNOx performance of the Ce3 Ta3 SbOx catalyst is attributed to the elevated Ce3+ concentrations, abundant active surface oxygen species, as well as surface acidity and reducibility,which is closely linked with the synergistic effect between Ce, Sb and Ta species. Results from DRIFTS reveal that the reaction mechanism of surface-adsorbed NH3 and NO_x species is linked to temperature,the L-H mechanism mainly occurs at low temperature(300 ℃),while the E-R mechanism occurs at high temperature(300 ℃). Overall,these findings indicate that Ce3 Ta3 SbOx is promising for NO_x practical abatement.     

Preparation and characterization of Ce1-x NixO2 as oxygen carrier for selective oxidation methane to syngas in absence of gaseous oxygen

A citric acid complex method was employed to prepare Ce/Ni mixed oxides with various Ce/Ni ratios useful for selective oxidation methane to syngas in the absence of gaseous oxygen, and the catalytic activity measurement was investigated in a fixed bed reactor at 800 °C. The prepared oxygen carriers were characterized by various characterization techniques such as TG-DSC, XRD and TPR. The results of TG-DSC indicated that the Ce_1-xNi_xO₂ precursor generated a stable phase after the heat-treatment at temperatures above 800 °C. The XRD characterization suggested that some Ce-Ni solid solution was formed when Ni²⁺ ions was incorporated into the lattice of CeO₂, and it led to the generation of O-vacancy which could improve the oxygen mobility in the lattice of oxygen carriers. It was found that Ce_0.8Ni_0.2O₂ gave the highest activity in the selective oxidation methane to syngas reaction, and the average methane conversion, CO and H₂ selectivity reached to 82.31%, 82.41% and 87.64%, respectively. The reason could be not only attributed to the fitting amount of NiO dispersed on the CeO₂ surface and bulk but also to actual lattice oxygen amount increased in oxygen carrier.     

In situ DRIFTS study of NOx adsorption behavior on Ba/CeO2 catalysts

A series of Ba/CeO2 catalysts with different Ba loading amounts were prepared by incipient wetness impregnation. Their NOx adsorption behaviors under NO and NO＋O2 conditions were investigated by in situ DRIFTS. It was found that NOx was ad-sorbed and stored in the form of nitrites and nitrates on both Ba and Ce sites on the surface of the catalysts. The less thermally stable BaCO3 was suggested to be the main active phase for NOx trapping. Ceria served primarily as an oxygen supplier in the absence of O2, and the reaction from nitrites to nitrates on Ba sites was the key step in this case. In the presence of O2, however, gaseous O2 became the main oxygen source. The NOx adsorption capacity of the catalyst was dominated by the Ba content. Moreover, the stability of ni-trites and nitrates formed on Ce sites was found to be lower than those formed on Ba sites which existed in the form of the ionic bar-ium nitrate species.