Sulfur-Tolerant,Exsolved Fe–Ni Alloy Nanoparticles for CO Oxidation

Topics in Catalysis - Metallic nanoparticles exsolved at the surface of perovskite oxides have been recently shown to unlock superior catalytic activity and durability towards various chemical...     

TiO2@carbon core-shell nanostructure supports for platinum and their use for methanol electrooxidation

Nanostructures consisting of TiO₂ particles as a core and carbon as a shell (TiO₂@C) were prepared by heat treatment of TiO₂ nanoparticles at 700 or 900 °C in a methane atmosphere. X-ray diffraction and transmission electron microscopy showed that a carbon shell layer was formed whose thickness increased with increasing reaction temperature. These structures were used as supports for platinum nanoparticles and the hybrid particles exhibit improved catalytic activity and stability toward methanol electrooxidation compared to Pt on a carbon black (Vulcan XC-72R). It is likely that enhanced catalytic properties of the Pt on TiO₂@C could be due to the stability of the core-shell support in comparison with carbon black support.     

纳/微异构型钙钛矿氧化物电极在固体氧化物燃料电池的研究进展

开发高效、稳定、廉价的钙钛矿氧化物电极材料是固体氧化物燃料电池(SOFC)进一步商业化发展的关键。目前,研究重点仍集中在解决阳极积碳、硫毒化以及阴极氧还原(ORR)低温性能不佳等问题。最近,有研究报道,一些易还原过渡金属元素掺杂的钙钛矿可以在还原气氛中原位析出该金属并以纳米颗粒的形式"镶嵌"在钙钛矿表面形成"纳米金属–钙钛矿"复合结构。该方法制备的材料具有性能高、抗积碳能力强、可再生性好等优点。从钙钛矿氧化物本体的选择、A/B位掺杂、缺陷调整、以及拓扑离子交换、相变诱发等方面,总结了近年来关于构建纳米(析出金属颗粒)微米(钙钛矿氧化物母体)异质结构(统称纳微异构)钙钛矿氧化物纳米纤维复合电极的研究。此外,总结了具有纳米纤维状形貌的钙钛矿氧化物电极及其结构对于SOFC性能、稳定性的影响,最后提出了该类纳微异构材料的优势、不足和展望。     

Magnetic CoFe2O4 nanoparticles doped with metal ions: A review

Ceramic-magnetic nanoparticles (CMNPs) are attracting attention due to their various applications, especially in biomedical industries. Among them, spinel ferrite CMNPs have received considerable deliberations among different spinel metal oxides due to their fascinating characteristics. Spinel ferrite CMNPs are used for enhancement of the applicability of CMNPs without affecting the intrinsic advantages of iron oxide CMNPs. Spinel ferrites with doping agents have useful electrical and magnetic properties in various fields. Moreover, the replacement of metallic atoms in ferrites is promising to manipulate physical characteristics and improve their performance. Among different spinel ferrites, CoFe₂O₄ nanoparticles are the most investigated CMNPs. Furthermore, they are used as permanent magnets, magnetic recorders in high-density and micro-wave devices, and magnetic fluids. This study reviews the CoFe₂O₄ nanoparticles doped with various elements and their applications in various fields.     

Silver nanoparticles enhanced luminescence and stability of CsPbBr3 perovskite quantum dots in borosilicate glass

Series of silver nanoparticles (NPs) embedded CsPbBr₃ quantum dots (QDs) glass was synthesized via the melt-quench method. Ag NPs and CsPbBr₃ QDs coexist in the TEM image of the Ag-doped glass sample. Photoluminescence (PL) spectra show that the 0.1 molar ratio Ag₂O-doped sample had a PL intensity 2.37 times than the undoped sample. This increase is generated by localized surface plasmon resonance coupling between the Ag NPs and CsPbBr₃ QDs. Excessive Ag doping weakens the PL intensity due to spectral self-absorption of the Ag NP surface plasmon resonance (SPR). Self-adsorption of SPR is detrimental to luminescence properties because it increases the amount of photogenerated charge carriers, which proceed through nonradiative relaxation pathways. In addition, stability results of Ag NP-doped-CsPbBr₃ QD glass show that they have excellent stability. This study on Ag NP-doped-CsPbBr₃ QD glass provides a new idea for the future development of perovskite QD optoelectronic devices.     

Modification of grain boundary structure of SrTiO3 using hydroxyl additives

Strontium titanate (SrTiO₃) is a typical perovskite material due to its crucial properties such as a good dielectric constant and favorable conductivity. Early studies reported that hydroxyl (OH) groups can manipulate the shape of the SrTiO₃ surface and control the crystal growth by changing the surface energy. This concept may be used to increase the population of (111) surfaces to form more Σ3 grain boundaries, which have lower energy for high ionic conductivity. In this study, two common OH additives, glycerol and 1,2-propanediol, were used for preparation of SrTiO₃ powders. SrTiO₃ powders were synthesized by spray pyrolysis and then sintered to form polycrystalline SrTiO₃. The results showed that the additive-treated SrTiO₃ powders had a higher population of (111) planes and exhibited higher conductivity than did un-treated powder. In addition, the highest population of Σ3 grain boundaries was generated by 1,2-propanediol treatment (11.0 ± 0.8%) of SrTiO₃, followed by glycerol treatment (10.0 ± 0.3%) and no treatment (2.5 ± 0.2%), respectively.     

三维有序大孔钙钛矿金属氧化物作为高效燃烧催化剂的研究进展

三维有序大孔（3DOM）材料因为具有独特的结构、可调的组成、高稳定性、高电子传递能力以及高比表面积而引起很多学者的研究兴趣。本文主要介绍了3DOM钙钛矿材料作为催化剂或载体在催化碳烟、挥发性有机物和甲烷燃烧方面取得的成果,分析了制备条件、结构特征对催化性能的影响;指出了3DOM钙钛矿材料具有优异的催化燃烧稳定性、高分子吸附和扩散能力以及高氧迁移率,能够显著降低氧化还原温度和表观活化能,因此可以作为高效燃烧催化剂。最后提出了未来研究重点和发展方向,在未来的研究中应该开发出更多可以与贵金属相媲美的廉价的新型3DOM材料以拓展3DOM材料应用范围。另外,3DOM 材料表面性能的提升以及用于催化燃烧的作用机理需进一步深入研究。     

Effect of A-site metal on methane combustion on 2% Pd / AMn1-x Fe x O3 (A = Ba, La, Pr; x = 0.4, 0.6, 1) perovskites

Barium, lanthanum, and praseodymium perovskites were prepared by malic acid complexation. Surface areas of the La and Pr perovskites were between 17.1 and 21.6 m² g⁻¹. The moderate low surface areas (5.7 m² g⁻¹) observed for corresponding barium perovskites were due to the high calcination temperatures. The calcination temperature also affected the shapes and sizes of the perovskite particles. According to SEM images the nanoparticles of the La and Pr perovskites were spherical, whereas those of barium perovskites were flakes. The conversion of methane increased in the order of A-site metal Ba < Pr < La. The CH₄ conversion after SO₂ treatment correlated with size of the perovskite particles: the smaller the particles the better the activity. The highest methane conversion after SO₂ treatment was achieved with lanthanum perovskite with B-site metal combination Mn_0.4Fe_0.6.     

Nanoengineering catalyst supports via layer-by-layer surface functionalization

Recent progress in the layer-by-layer surface modification of oxides for the preparation of highly active and stable gold nanocatalysts is briefly reviewed. Through a layer-by-layer surface modification approach, the surfaces of various catalyst supports including both porous and nonporous silica materials and TiO₂ nanoparticles were modified with monolayers or multilayers of distinct metal oxide ultra-thin films. The surface-modified materials were used as supports for Au nanoparticles, resulting in highly active nanocatalysts for low-temperature CO oxidation. Good stability against sintering under high-temperature treatment was achieved for a number of the Au catalysts through surface modification of the support material. The surface modification of supports can be a viable route to control both the composition and structure of support and nanoparticle interfaces, thereby tailoring the stability and activity of the supported catalyst systems.     

The effect of the calcination temperature of LaFeO3 precursors on the properties and catalytic activity of perovskite in methane oxidation

In the synthesis of perovskite-type LaFeO₃ oxides iron and lanthanum nitrates were used as a precursors. The nitrates were dissolved in water, evaporated, crushed and calcined in temperature range of 650–850?°C. The obtained perovskites were applied as an active layer on monolithic catalysts for the oxidation of methane. The increase in the calcination temperature of the perovskite precursors from 650° to 850°C results in a reduction in the surface area of the powders from 10.1 to 4.2?m²/g. XRD studies revealed that calcination at 800–850?°C caused the formation of an almost homogeneous LaFeO₃ perovskite phase. A decrease in the La/Fe surface ratio from 12 to 5.2 with the rise in calcination temperature from 650° to 800°C was detected by XPS. EDX results confirmed that at 750–850?°C, the La/Fe ratio in the perovskite layer is close to the stoichiometric and amount to 1.01–1.03. The highest activity in methane oxidation was achieved when the LaFeO₃ perovskite was calcined at 700?°C. A further slight increase in the activity was noticed after H₂ treatment. As the calcination temperature of the perovskites is increased, the catalyst activity decreases due to a reduction in the specific surface area, despite the more complete LaFeO₃ perovskite phase formation.     

Surface modification of electron transport layers based on TiO2 nanorod boosts efficiency of perovskite solar cells

In this work, TiO₂ heterostructure thin films including rutile TiO₂ nanorods (TNRs) and anatase TiO₂ nanoparticles (TNPs) on fluorine-doped tin oxide (FTO) glass are fabricated by the hydrothermal method and are applied as electron transport layers (ETLs) in MAPbI₃-based perovskite solar cells (PSCs). To enhance the surface area of ETL, TNRs are first etched in acidic solution by another hydrothermal process for different reaction times before coating with TNPs. The morphological and structural properties of TNRs after etching are carefully investigated. Interestingly, the surface modification of TNR thin film by appropriate TNP deposition and etching improves significantly the efficiency of PSC devices by more than 1.6 times. To further improve the performance of PSC, phenyl-C61-butyric acid methyl ester (PCBM) is used to enhance the charge transfer efficiency at the ETL/perovskite interface, and the optimal PSC device shows the champion efficiency of 18.50% with low charge transfer resistance (11.56 ohms).     

Methane Combustion and CO Oxidation on Zirconia-Supported La,Mn Oxides and LaMnO3 Perovskite

ZrO₂-supported La, Mn oxide catalysts with different La, Mn loading (0.7, 2, 4, 6, 12, and 16 wt% as LaMnO₃) were prepared by impregnation of tetragonal ZrO₂ with equimolar amounts of La and Mn citrate precursors and calcination at 1073 K. The catalysts were characterized by X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and BET specific surface area determination. The redox properties were tested by temperature-programmed reduction (TPR), and the catalytic tests were carried out for methane combustion at 650–1050 K and for CO oxidation at 350–800 K. XRD revealed the presence of tetragonal zirconia with traces of the monoclinic phase. LaMnO₃ perovskite was also detected for loading higher than 6%. XAS and TPR experiments suggested that at high loading small crystallites of LaMnO₃, not uniformly spread on the zirconia surface, were formed; while at low loading, La, Mn oxide species interacting with the support, and hard to be structurally defined, prevailed. The catalysis study indicated that the presence of a perovskite-like structure is necessary for the development of highly active sites. Dilute catalysts were in fact poorly active even when considering the activity per gram of La, Mn perovskite-like composition. For methane combustion and CO oxidation, similar trends of the activity as a function of the loading point to a similarity of the active sites for the two reactions on the examined catalytic system.     

Nickel catalysts supported on carbon nanofibers: structure and activity in methane decomposition

Structures of Ni catalysts supported on filamentous carbon (CFC) produced by methane decomposition over coprecipitated Ni and Ni-Cu/alumina catalysts were studied by EXAFS and TEM. Thermal pre-treatment in N₂ at 350°C of samples impregnated by nickel nitrate precursor was found to produce either NiO or nickel carbide, Ni₃C, phase. This was explained by different reducing properties of the carbon nanofibers which depend on the surface structure. High stability of the Ni/C catalysts in methane decomposition reaction at 550°C was found with those prepared from only nickel chloride precursor, due to the formation of large (30-70 nm) Ni particles further leading to new carbon filaments growth. Data implies a common mechanism of the filamentous carbon deposition in all Ni-based catalysts, independent of the support (silica, alumina, carbon) being used. However, accumulation of filamentous carbon is strongly influenced by morphology and texture of the support. This revised version was published online in July 2006 with corrections to the Cover Date.     

The pressure-induced mechanical and optoelectronic behavior of cubic perovskite PbSnO3 via ab-initio investigations

The density functional theory based full-potential linear-augmented-plane-wave plus local-orbital method has been used to study the physical properties of PbSnO₃ in hypothetical cubic perovskite. An external pressure up to 40?GPa has been applied on PbSnO₃ to realize the variation in its electronic band structure and the subsequent optical properties. The stability of the PbSnO₃ has been investigated by the mechanical properties, the enthalpy of formation and Goldschmidt tolerance factor. Moreover, the Born criteria have been adopted to justify the mechanical stability of the PbSnO₃ perovskite. We show that the electronic bandgap of PbSnO₃ can be tailored from indirect to direct band gap at high symmetry (X-X) direction at an external pressure of magnitude ~ 26?GPa. The effect of pressure on the optical properties has been studied in terms of dielectric function, absorption, refraction, reflection, and optical loss factor. The application of hydrostatic pressure has shifted the maximum absorption toward the visible range, revealing that PbSnO₃ can be used for high- pressure optoelectronic applications.     

Sonochemically Prepared high Dispersed Ru/TiO2 Mesoporous Catalyst for Partial Oxidation of Methane to Syngas

Highly dispersed Ru nanoparticles on mesoporous TiO₂ have been synthesized by a one-step ultrasound assisted polyol reduction procedure. The catalysts have been characterized by XRD, TEM and HR-TEM, EDX, BET and TPR methods. It has been demonstrated that the sonochemical method reduces the Ru⁺³ ions creating a narrow size distribution of metallic nanoparticles deposited on the mesoporous support without damaging its pore structure. The nanoparticles of Ru are highly dispersed and stable because of their incorporation into the mesopores, and the strong metal-support interaction. The catalytic properties of Ru/TiO₂(MSP) have been tested in the partial oxidation of methane, and high activity and selectivity towards CO and H₂ have been demonstrated.     

Total Oxidation of Methane and Chlorinated Hydrocarbons on Zirconia Supported A1–xSrxMnO3 Catalysts

The catalytic behavior of ZrO₂ and ZrO₂ containing 8 mol‐% Y₂O₃ supported A_1–xSr_xMnO₃ (A = La, didymium) perovskites was studied in the total oxidation of methane, chloromethane and dichloromethane considering catalyst deactivation and byproduct formation. The perovskites are dispersed on the support surface; clusters with a perovskite‐like structure were formed. The supported catalysts are characterized by higher specific surface areas compared with the unsupported ones. Partial substitution of A‐site cations by Sr leads to an enhancement of the catalytic activity in the total oxidation of methane, but not in the total oxidation of chlorinated hydrocarbons (CHC). The catalytic activity of supported and unsupported catalysts is comparable in the total oxidation of methane in spite of the significantly lower perovskite content of the supported catalysts. In the CHC conversion the catalytic activity of the supported catalysts is higher than that of the unsupported ones.     

Characterization of in situ prepared nanocomposites of PS and TIO2 nanoparticles surface modified with alkyl gallates: Effect of alkyl chain length

Incorporation of surface modified TiO₂ nanoparticles into polystyrene (PS) matrix was achieved by in situ bulk radical polymerization of styrene. The surface of TiO₂ nanoparticles was modified with four amphiphilic esters of the gallic acid (octyl, decyl, lauryl and cetyl gallate), which have different lengths of hydrophobic alkyl chain (C8–C16). FTIR and UV‐Vis spectroscopy measurements confirmed the formation of a charge transfer complex between surface Ti atoms and gallic esters. Microstructural characterization of the synthesized nanocomposites revealed that the best dispersion of TiO₂ nanoparticles in PS was achieved when the TiO₂ surface was modified with octyl gallate. The presence of surface modified TiO₂ nanoparticles by different alkyl gallates does not have an influence on the molecular weight and glass transition temperature of PS matrix. On the other hand, thermal and thermo‐oxidative stability of PS were significantly improved by incorporation of surface modified TiO₂ nanoparticles. The most pronounced improvement of thermal and thermo‐oxidative stability was observed for TiO₂ nanoparticles surface modified with octyl gallate. POLYM. COMPOS., 2013. © 2013 Society of Plastics Engineers     

Synthesis of CeO2 nanoparticles with different morphologies and their properties as peroxidase mimic

The overall objective of this work was to identify and determine the controlling factors of catalytic activity of CeO₂ nanoparticles with different properties. Therefore, four kinds of CeO₂ nanoparticles are successfully synthesized and used as peroxidase mimics. The results reveal that the catalytic activity is related to specific surface area and surface oxygen (O_sur) content. For different CeO₂ nanoparticles, specific surface area is the leading factor of catalytic activity. The catalytic activity increases with the increasing specific surface area. If the CeO₂ nanoparticles have similar specific surface area, the less O_sur contains, the lower catalytic activity will be. Meanwhile, crystallinity is not the influence factor of catalytic activity. Bamboo-like CeO₂ (B-CeO₂) nanoparticles show excellent catalytic activity and the reaction conditions are optimized. Furthermore, the catalytic activity of B-CeO₂ nanoparticles decreases very slightly after 15 cycles of the catalysis experiment. Therefore, B-CeO₂ nanoparticles can serve as effective recyclable peroxidase mimics.     

Fabrication and investigation of a new highly humidity stable nanocrystalline perovskite,tetramethylammonium lead triiodide to be used in solar cells

Hybrid organic-inorganic halide perovskite (CH₃NH₃PbI₃) materials have broad applications such as photovoltaics, light emitting diodes, and sensors. However, one of the major drawbacks of these materials is their sensitivity to moisture. Here, a new promising perovskite nanocrystal: tetramethylammonium lead triiodide (CH3)₄NPbI₃ has been introduced and fabricated. Investigation of morphology, structural, optical and stability properties was done by XRD, SEM, TEM, and UV–visible. Characterization prove that the processed powder is in the form of nanoparticles and drop cast film has a hexagonal rod-like morphology. Crystal structure was determined by powder x-ray diffraction and electron diffraction pattern. It shows a perovskite crystal structure with a hexagonal unit cell. Powder XRD after 45 days suggests that this new compound is stable in humid conditions. Direct optical band gap of this material was obtained by transforming diffuse reflectance spectrum to Kubelka-Munk spectrum and it was 2.66 eV.     

Zirconia-supported LaCoO3 catalysts for hydrogen production by oxidative reforming of diesel: Optimization of preparation conditions

The influence of calcination temperature and precursor type used in the preparation of ZrO₂-supported LaCoO₃ catalyst on its behaviour for hydrogen production by oxidative reforming of diesel has been analyzed in terms of LaCoO₃ structure. Four samples have been prepared by wetness co-impregnation with cobalt and lanthanum salts and characterized by means of XRD, BET, SEM-EDX, TXRF and XPS. Physicochemical characterization shows a great influence of the nature of precursors and calcination temperature used in the synthesis on the textural, morphological, surface and structural properties of LaCoO₃ deposited over ZrO₂. The use of nitrate precursors and high calcination temperature leads to the formation of LaCoO₃ perovskite structures of high grain and crystallite size on ZrO₂ support. On the contrary, the catalyst prepared from acetate precursors and calcined at low temperature showed perovskite crystallites of lower size. For this sample, the smaller perovskite crystallites on the catalyst at the beginning of the reaction imply higher and more stable hydrogen production for short-term test aging test.