CeO2-supported Fe,Co and Ni toward CO2 hydrogenation: Tuning catalytic performance via metal-support interaction

The chemical transformation of CO₂ produces carbon compounds that can be used as precursors for the production of chemicals and fuels.Here,we investigated the activity and selectivity of the transition metals(Fe,Co,and Ni) supported on CeO₂ catalyst for CO₂ hydrogenation at atmospheric pressure.We found that Ni/CeO₂ shows the highest CO₂ conversion compared with Fe/CeO₂ and Co/CeO₂.Besides,Co/CeO₂ and Ni/CeO<...     

Effects of cerium doping position on physicochemical properties and catalytic performance in methanol total oxidation

The physicochemical properties of Pd and PdPt catalysts which possess different Ce doping position were investigated by techniques of TEM, XRD, N₂ adsorption-desorption, XPS and FT-IR. The catalytic performance for methanol total oxidation was examined to study the effects of Ce adding position. CeO₂Al₂O₃TiO₂ (CAT) catalysts that Ce is directly introduced into support show higher reactivity and CO₂ selectivity than CeO₂/Al₂O₃TiO₂ (Ce/AT) samples in which Ce is loaded by impregnation method. The characterization results reveal that the Ce doping position does not cause obvious otherness of basic crystalline phase and mesoporous structure of support. However, the Ce doping position affects the pore shapes of support and then influences the pore diameter. CAT catalysts possess more abundant adsorbed oxygen and more Ti³⁺ can transform the more gaseous oxygen into the active oxygen species on the catalyst surface, which is beneficial to the reaction. The AlOTi bridges in CAT facilitate the cooperation of Al and Ti species, which further speeds up the reaction rate.     

Performance of mesoporous CeO2-Cr2O3 mixed metal oxides applied to benzene catalytic combustion

In this paper,a hydrothermal method was used to prepare(Ce,Cr)-MOF with different Ce/Cr molar ratios and then a series of CeO₂-Cr₂O₃ mixed metal oxides(CeCr-MMO) with mesoporous structure were prepared by thermal decomposition of these MOFs at different temperatures.After a series of characterization techniques were applied to test the physicochemical properties of the materials,it is found that thermal decomposition temperature(TDT) and Ce/Cr molar ratios have i...     

Effect of impregnation strategy on catalytic hydrogenation behavior of PtCo catalysts supported on La2O2CO3 nanorods

Small Pt and Pt-Co nanoparticles (NPs) stabilized on La₂O₂CO₃ nanorods (LOC) were prepared by wet impregnation method, and probed in liquid-phase chemoselective hydrogenation of crotonaldehyde (CRAL) to crotyl alcohol (CROL). It is found that incorporation of Co atoms into Pt catalyst significantly improves the hydrogenation activity and desired selectivity to CROL as it destroys the Pt-lanthanum interfaces and results into the formation of Pt-Co particles. In addition, a close examination of catalyst surface and reactive performance suggests that the impregnation sequence of Pt and Co exerts great influence on the physicochemical property and the catalytic hydrogenation behavior of PtCo/LOC catalysts. As a result of the interaction between Pt and Co species, high alloying degree of Pt-Co NPs is obtained in the co-impregnated catalyst (Pt-Co/LOC), thus achieving the highest hydrogenation activity. The selective deposit of Co atoms onto the low-coordinated Pt sites leads to the smallest metal particle size and high dispersion of Pt-Co NPs over the Pt/Co/LOC, giving rise to the highest selectivity and yield to CROL.     

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 ℃.     

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...     

Rare-earth mediated dihydrogen activation and catalytic hydrogenation

This review covers H-H bond cleavage of dihydrogen(H_2) mediated by structurally well-defined rareearth metal(scandium,yttrium and lanthanides) complexes,and their applications in homogenous catalysis,such as catalytic hydrogenation of unsaturated organic molecules.Depending on the mechanism of the H-H bond cleavage,this review is organized in two parts:(1) σ-bond metathesis,and(2)non-σ-metathesis H_2 activation.The latter is a new trend in this research field and is the emphasis of this review.Converting H_2 into inorganic rare-earth polyhydride complexes,albeit their potential applications as hydrogen-storage materiel,is not in the scope of this review.     

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.     

Pseudo core-shell LaCoO_3@MgO perovskite oxides for high performance methane catalytic oxidation

Magnesia modified LaCoO_3 was prepared by a facile one-step sol-gel method and used for removal of dilute methane.Compared with the conventional doping technique,the obtained LaCoO_3@MgO-x exhibits pseudo core-shell structure and shows superior catalytic activity.The methane conversion exceeds90% at 532℃ on LaCoO_3@MgO-0.1,while only 60% of methane is conversed using the doped perovskite LaCo_(0.9)Mg_(0.1)O_3.The high catalytic performance of LaCoO_3@MgO-0.1 is mainly attributed to the adjustment of surface acid-base properties by the MgO shell structure.According to density functional theory(DFT) calculation,the methane is more likely to be adsorbed and cracked on LaCoO_3@MgO-0.1.The in situ DRIFTS shows that CH_3-O-CH_3 intermediate specie is formed.The pseudo core-shell structure also enhances the stability and the LaCoO_3@MgO-0.1 maintains high activity after working for 100 h.The above results demonstrate that surface modification by magnesia is an effective strategy for improving LaCoO_3 catalytic performance.     

Getting insights into gas-phase sulfation effect on catalytic performance of praseodymium oxides in NH3-SCR of NO

Sulfation treatment has been widely used to promote the catalytic performance of ceria(CeO₂) based catalysts for the selective catalytic reduction of NO by NH₃(NH₃-SCR of NO).Praseodymium oxide(PrO_x),another commonly used rare earth material with similar structural properties as CeO₂,also shows satistactory redox properties due to the facile redox cycle of Pr³⁺■Pr⁴⁺.In this work,gas phase sulfation treatment with varied...     

Rare earth elements: Properties and applications to methanol synthesis catalysis via hydrogenation of carbon oxides

Methanol (CH₃OH) is an important industrial chemical with a wide variety of uses. Industrial methanol synthesis catalysts are typically composed of Cu, Zn, and Al, but the use of catalysts incorporating rare earth elements has been shown to improve the catalytic performance. Due to their unique chemical and physical properties, the use of rare earth elements (scandium, yttrium, and the lanthanides) in catalysis in general has continued to increase over the past few decades, while the use of rare earth in methanol synthesis catalysts has not, despite often improving performance. The ability of several of the rare earth elements (Pr, Ce, Eu, Tb, Yb) to easily switch between oxidation states makes them beneficial for many different types of catalysts. However, for methanol synthesis the surface basicity is an important property, and the basic nature of the rare earth elements can be used to tune the basicity of catalysts. A small number of correlations between rare earth properties and catalytic performance have been observed, but often do not apply to other catalysts. Properties such as strength of basic sites, ionic radius, and electronegativity have been found to correlate with performance results such as activity or selectivity.     

Effect of PdO_x structure properties on catalytic performance of Pd/Ce_(0.67)Zr_(0.33)O_2 catalyst for CO,HC and NO_x elimination

Pd/Ce_(0.67)Zr_(0.33)O_2 catalyst was pretreated in different atmosphere respectively, and characterized by CO chemical adsorption, XPS, HR-TEM, H_2-TPR, Raman, OSC and in situ DRIFTS to investigate the effect of the structure properties of PdO_x species on the catalytic performance for CO, HC and NO_x elimination. The results show that Pd/CZ catalyst pretreated in air atmosphere has higher oxidation activity of HC due to having high Pd dispersion and strong interaction between PdO_x and CZ support. Pd/CZ-H catalyst pretreated in reducing atmosphere exhibits better catalytic performance of NO_x elimination because of having relatively big Pd particle size, more Pd species in metallic state and higher concentration of oxygen vacancies. While for the Pd/CZ-RG catalyst pretreated in reactant atmosphere, strong adsorption of HC species on the surface of catalysts would lead to a part of active sites being covered, which inhibits HC and NO conversions.     

Synthesis of mesoporous CeO2-MnOx binary oxides and their catalytic performances for CO oxidation

Mesoporous CeO2-MnOx binary oxides with different Mn/Ce molar ratios were prepared by hydrothermal synthesis and characterized by scanning electron microscopy （SEM）, N2 sorption, X-ray diffraction （XRD）, X-ray photoelectron spectroscopy （XPS） and H2 temperature-programmed reduction （H2-TPR）. The characterization results indicated that the CeO2-MnOx catalysts exhibited flower-like microspheres with high specific surface areas, and partial Mn cations could be incorporated into CeO2 lattice to form solid solution. The CeO2-MnOx catalysts showed better catalytic activity for CO oxidation than that prepared by the coprecipitation method. Furthermore, the CeO2-MnOx catalyst with Mn/Ce molar ratio of 1 in the synthesis gel （Ce-Mn-1） exhibited the best catalytic activity, over which the conversion of CO could achieve 90% at 135 ℃. This was ascribed to presence of more Mn species with higher oxida- tion state on the surface and the better reducibility over the Ce-Mn-I catalyst than other CeO2-MnOx catalysts.     

Effects of water on CO catalytic oxidation over Pd/CeO_2

The role of water in CO oxidation was investigated on Pd/CeO_2 with different morphologies(rods(R),cubes(C) and octahedrons(O)).Compared with the absence of water,CO oxidation activity increases 2 times in the presence of water on Pd/CeO_2-C;but a decrease is found on Pd/CeO_2-R.Catalyst characterization reveals that Pd is mainly in the form of solid solution(Pd_xCe_(1-x)O_(2-σ)) on Pd/CeO_2-R and a mixture of metal and Pd_xCe_(1-x)O_(2-σ) solid solution on Pd/CeO_2-C.The strong interaction between Pd and CeO_2-R results in the form of stable bidentate carbonates species;while the relatively weak interaction between Pd and CeO_2-C leads to the produce of unstable monodentate carbonates species.The effects of water on CO oxidation activity closely relate with the Pd chemical state and the types of carbonates species.Water restrains CO adsorption on Pd_xCe_(1-x)O_(2-σ) solid solution,but it has negligent effects on metallic Pd species.In the presence of water,bidentate carbonates species remains stable but the decrease in the amount of monodentate carbonates species is observed.     

Effect of loading content of copper oxides on performance of Mn-Cu mixed oxide catalysts for catalytic combustion of benzene

A series of Mn-Cu mixed oxide catalysts were prepared by precipitation method. The catalysts were characterized by N 2 adsorption-desorption, H 2 -TPR and XPS. When the loading ratio of manganese oxides to copper oxides was 8:2 or 7:3, the catalysts possessed better catalytic activity, and benzene was converted completely at 558 K. Results of H 2 -TPR showed that the loading of a small amount of copper oxides decreased the reduction temperature of catalysts. Results of XPS showed that the loading of a small amount of copper oxides increased the proportion of manganese and defective oxygen on the surface of catalysts, and stabilized manganese at higher oxidation state. And the catalyst with the loading ratio 7:3 was a little worse than 8:2, since the interaction between manganese oxides and copper oxides is too strong, copper oxides migrate to the surface of catalysts and manganese oxides in excess are immerged.     

Catalytic performance for methane combustion of supported Mn-Ce mixed oxides

A series of supported Mn-Ce mixed oxide catalysts were prepared by the impregnation method and used for the oxidation of methane. The catalysts were characterized by N2 adsorption （BET）, X-ray diffraction （XRD）, laser Raman spectrum （LRS）, and temperature programmed reduction （TPR） techniques. The XRD and LRS results confirmed the high dispersion of active components or formation of solid solution between manganese and cerium oxides in the bulk and on the surface of mixed oxide catalysts. The reducibility was remarkably promoted by the stronger synergistic interaction between the two oxides from H2-TPR measurements. As expected, all the experimental mixed oxide catalysts showed excellent activity for methane combustion at low temperature. Especially, for the catalyst with Mn-Ce ratio 3：7, methane conversion reached 92% at a temperature as low as 470 ℃.     

CO2 sensing properties and mechanism of PrFeO3 and NdFeO3 thick film sensor

The CO_2 sensing of PrFeO_3 and NdFeO_3 sensors were investigated. Experimental results show that the resistances for PrFeO_3 and NdFeO_3 in CO_2 gas are larger than those in air and the responses for PrFeO_3and NdFeO_3 sensors increase with an increase in room-temperature relative humidity. When exposed to1000 ppm CO_2, the response of PrFeO_3 thick film based on nano-powders annealed at 700℃can reach8.44 at 160℃for the background of wet air with 58%of room-temperature relative humidity (RH),which is much larger than the corresponding value (3.03) in wet air with 25%RH. The sensing response S of NdFeO_3 thick-film sensor based on nano-powders annealed at 600℃to 3000 ppm CO_2 at the operating temperature 200℃can reach 2.36 for the background of wet air with 72%RH, which is larger than the corresponding value (1.83) in the air with 25%RH. Compared with other CO_2 sensing materials, the PrFeO_3 sensor has larger response at lower operating temperature for CO_2 gas and may be used as a new CO_2 sensing material.     

Synergistic catalytic removals of NO,CO and HC over CeO2 modified Mn-Mo-W-Ox/TiO2-SiO2 catalyst

A series of Mn-Mo-W-O_x/TiO_2-SiO_2 catalysts was modified with CeO_2 using an extrusion molding method. The catalytic activities of the obtained catalysts were tested for the synergistic catalytic removals of CO, NO and C_3H_8. The ratio of catalyst composition on catalytic activities for NH_3-SCR was optimized, which reveals that the molar ratio of Ti/Si was 9:1 and the catalyst containing 1.5 wt% CeO_2 and 12 wt% Mn-Mo-W-O_x exhibits the best catalytic performances. These samples were characterized by XRD, N_2-BET, Py-IR, NH_3-TPD, SEM/element mapping, H_2-TPR and XPS, respectively. Results show that the optimal catalyst exhibits more than 99% NO conversion, 86% CO conversion and 100% C_3H_8 conversion under GHSV of 5000 h~(-1). In addition, the GHSV has little influence on removal of NO when it is less than 15,000 h~(-1). Furthermore, the addition of CeO_2 will enhance the surface acidity, increase Mn~(4+)concentration and inhibit the grain growth, which are favorable for the excellent catalytic performance.Anyway,the 1.5 wt% CeO_2-12 wt% Mn-Mo-W-O_x/TiO_2-SiO_2 possesses outstanding redox properties,abundant acid sites and high Mn~(4+) concentration, which provide a guarantee for synergistic catalytic removal of CO, NO and HC.     

Effects of precursor moisture and inert N_2 atmosphere calcinations on structure and properties of alumina modified CeZrLaNd mixed oxides

CeO_2-ZrO_2 mixed oxides are widely used in the three-way catalysts due to their unique reversible oxygen storage and release capacity. Large surface area, high oxygen storage capacity and good thermal stability of cerium zirconium mixed oxides are the key properties for the automotive catalysts so as to meet the strict emission regulations. In this work, alumina modified CeZrLaNd mixed oxides were prepared by a co-precipitation method. The effects of moisture in precursor and inert N2 atmosphere during calcinations on the structure and properties were investigated by Brunauer-Emmett-Teller(BET) surface area measurements, X-ray diffraction(XRD), scanning electron microscopy(SEM), transmission electron microscopy(TEM), hydrogen temperature-programmed reduction(H_2-TPR), oxygen storage capacity(OSC), Raman spectroscopy, and X-ray photoelectron spectroscopy(XPS). The results show that the moisture in precursor during calcinations increases the crystal grain size of the cerium zirconium mixed oxides, improving the thermal stability. And the aged surface area of sample after being calcined at1000 ℃ for 4 h reaches 68.8 m~2/g(5.7% increase compared with the common sample). The inert N2 atmosphere endows a great pore-enlarging effect, which leads to high fresh surface area of 148.9 m2/g(13.5% increase compared with the common sample) and big pore volume of 0.5705 mL/g. The redox and oxygen storage capacity are also improved by inert N2 atmosphere with high OSC value of 241.06μmolO_2/g(41.3% increase compared with the common calcination), due to the abundant formation of the crystal defects and oxygen vacancies.     

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.