Strong metal-support interaction in Ni/TiO2 catalysts: in situ EXAFS and related studies

In situ EXAFS and X-ray diffraction investigations of Ni/TiO₂ catalysts show that NiTiO₃ is formed as an intermediate during calcination of catalyst precursors prepared by the wet-impregnation method; the intermediate is not formed when ion-exchange method is used for the preparation. On hydrogen reduction, NiTiO₃ gives rise to Ni particles dispersed in the TiO₂(rutile) matrix. The occurrence of the anatase-rutile transformation of the TiO₂ support, the formation and subsequent decomposition/reduction of NiTiO₃ as well as the unique interface properties of the Ni particles are all factors of importance in giving rise to metal-support interaction. Active TiO₂(anatase) prepared from gel route gives an additional species involving Ni³⁺.     

Evidence of a metal-support interaction in sol-gel derived Cu-ZrO2 catalysts for CO hydrogenation

Sol-gel derived Cu/ZrO₂ catalysts have recently been shown to have high activity and selectivity toward methanol synthesis. TPR, TEM, in situ XRD and N₂O decomposition have now been used to characterize the active sites in such catalysts over a wide range of Cu concentration. Copper is shown to be in two forms: surface aggregates (or particulate) and dispersed copper in the ZrO₂ substitutional sites. The proportion of the former increases with an increasing Cu content, while the overall strength of the Cu-ZrO₂ interaction simultaneously decreases. The activity in CO/CO₂ hydrogenation showed no evident correlation with the total Cu surface area, but rather with the concentration of highly-dispersed form of copper. This is taken to indicate that the copper in the substitutional sites of ZrO₂ is predominantly responsible for and associated with the active sites on Cu/ZrO₂ for CO/CO₂ hydrogenation.     

The interaction of rutile (TiO2) surface with some catalytically active transition metal oxides during heating,studied by electrochemical methods

The interaction of the surface of a rutile monocrystal (110 oriented) with the oxides of some transition metals (V, Mo, Cr, Mn and Nb) during heating of the crystal with these oxides was investigated by electrochemical methods. In all cases insertion of the metal atoms in the rutile surface was observed, the degree of insertion depending strongly on the metal in question and the conditions of experiment. The interaction of rutile surface with metal oxides changes dramatically the conditions of charge transfer at the rutile surface, which may influence the course of catalytic reactions occurring at the surface of rutile-supported transition metal oxide catalysts. This revised version was published online in June 2006 with corrections to the Cover Date.     

Role of tungsten in supported Pt-W reforming catalysts Part II. Influence of the tungsten loading and metal-support interactions effect

Support Pt-W catalysts are studied for 3-methylhexane reforming. The increase in tungsten loading and the use of SiO₂ support instead of Al₂O₃ in Pt-W catalysts leads to the decrease in total activity and aromatization selectivity. X-ray Photoelectron Spectroscopy (XPS) has been applied to bimetallic Pt-W/Al₂O₃ and SiO₂ catalysts with different Pt/W ratios to try to explain the catalytic results. Observations lead to the conclusion that tungsten species are strongly anchored on the support in Pt-W/Al₂O₃ catalysts at low tungsten concentration. In this case, tungsten species are not reducible (oxidation state +VI) and not accessible catalytically; tungsten is hindered by small Pt particles. At large tungsten loadings, beyond the theoretical monolayer capacity of the support, a fraction of tungsten species migrates to the surface and becomes reducible. This fraction of tungsten-reducible species and large platinum particles are accessible on the surface, but another fraction of tungsten species strongly anchored on the support remains not accessible and not reducible. A model is proposed.     

Ethylene slurry polymerization using nickel diimine catalysts covalently-attached onto MgCl2-based supports

MgCl₂/alcohol adducts were recrystallized with alkyl aluminums and used as catalysts supports for nickel diimine complexes functionalized with amine groups. These supported catalysts were used to polymerize ethylene in a slurry reactor. The MgCl₂-based supported nickel diimine catalysts had higher activities than the equivalent SiO₂-supported nickel diimine catalysts, even without the use of activators. These catalysts made polyethylene with melting temperatures and molecular weights higher than those made with the equivalent homogeneous catalysts. Interestingly, the catalyst activity and polymer molecular weight could be controlled by changing the support composition. In addition, covalent chemical bonds between the functionalized nickel diimine complex and the MgCl₂-based supports avoided catalyst leaching during polymerization, leading to the production of polymer particles with good morphology. The mechanical strength of the resulting polymer particles, however, was lower than those made with SiO₂-supported nickel diimine catalysts.     

Effect of precursors on surface and catalytic properties of Fe/TiO2 catalysts

Titania‐supported iron (5 wt%) catalysts were prepared by a sol–gel method using different gelation pH and metal precursors (Fe(II) and Fe(III)). Characterization data of calcined catalysts revealed that, irrespective of the nature of the metal precursor, iron is present in all cases as ferric oxide. However, the crystalline phase exhibited by titania does depend on the metal precursor used. The catalytic activity of the catalysts, tested in the combustion of methane at atmospheric pressure, is not related to the dispersion of iron oxide. Thus, Fe³⁺ ions may be obtained in two extreme situations; one highly dispersed in which Fe³⁺ ions are placed in the titania network and another in which large Fe₂O₃ crystals are located on the surface of the catalyst. The former exhibits the best performance in the combustion of methane. © 2002 Society of Chemical Industry     

The behaviour of Ru/Fe2O3 catalysts and Fe2O3 supports in the TPR and TPO conditions

The texture of Fe₂O₃ support and Ru/Fe₂O₃ catalysts supported on iron oxides obtained from β-FeOOH (B) or δ-FeOOH (D) and their catalytic activity in WGSR were studied. Also their susceptibilities to reduction and reoxidation, were studied by TPR, using H₂ or CO and TPO methods. In the case of TPR_CO the composition of the reducing mixture containing traces of H₂O enabled investigation of water gas shift reaction (WGSR).

The catalysts from series D were found more readily reducible and oxidised than those from series B. The supported ruthenium enhanced the redox effects and caused the appearance of additional effects related directly to its presence. Depending on the kind of support and on ruthenium presence considerable differences in temperatures of WGSR onset were found. It is suggested that the susceptibility of the catalysts to reduction and oxidation is responsible for their activity in the WGSR.     

Supported honeycomb monolith catalysts for high-temperature ammonia decomposition and H2S removal

Catalysts that have potential in simultaneous removal of H₂S and NH₃ decomposition were developed. The monolith supports of high surface area and acceptable mechanical strength based on titania and silica–alumina precursors were prepared and tested. Preparation routine and composition of Mn, Fe and Cu oxides supported honeycomb catalysts have been optimized. Impregnated and washcoated monolith catalysts were tested in ammonia high-temperature decomposition.     

Evidence for strong metal-support interaction (SMSI) in Rh/TiO2 system

Rh(1 wt%)/TiO₂ samples were prepared by both incipient wetness and ion-exchange methods and were characterised by temperature programmed desorption (TPD), electron spin resonance (ESR), mass spectrometry (MS) and hydrogen chemisorption. The incipient wetness sample was found to be more favourable for the onset of SMSI state. The reduction of Ti⁴⁺ to Ti³⁺ during hydrogen spillover or due to the lattice oxygen (O^2–) deficiency seemed to be responsible for the SMSI state. A mechanistic pathway is proposed to explain the onset of SMSI behaviour.     

纳米TiO2粉末的液相法制备及光学吸收特性的研究

采用水解-均相沉淀法,在常温下即可制得纯锐钛型纳米TiO2粉末,对所制得的粉末进行了TEM、XRD、EDS、XPS和UV-VIS分析。结果表明:采用水解-均相沉淀法,通过添加NH4C l和H2SO4可在室温下制得粒径为32nm的纯锐钛型纳米TiO2粉末,并且粉末粒径均匀,尺寸分布较窄;纳米TiO2粉末对波长200~360 nm范围内的紫外线有较好的吸收作用,且吸收较均匀。     

Enhanced catalytic performance of Cu-and/or Mn-loaded Fe-Sep catalysts for the oxidation of CO and ethyl acetate

The Fe-modified sepiolite-supported Mn-Cu mixed oxide (CuxMny/Fe-Sep) catalysts were prepared using the co-precipitation method.These materials were characterized by means of the XRD,N2 adsorption-desorption,XPS,H2-TPR,and O2-TPD techniques,and their catalytic activities for CO and ethyl acetate oxidation were evaluated.The results show that catalytic activities of the CuxMny/Fe-Sep samples were higher than those of the Cu1/Fe-Sep and Mn2/Fe-Sep samples,and the Mn/Cu molar ratio had a distinct influence on catalytic activity of the sample.Among the CuxMny/Fe-Sep and Cu1Mn2/Sep samples,Cu1Mn2/Fe-Sep performed the best for CO and ethyl acetate oxidation,showing the highest reaction rate and the lowest T50 and T90 of 4.4 × 10-6 mmol.g 1.s-1,110,and 140 ℃ for CO oxidation,and 1.9 × 10-6 mmol.g-1.s-1,170,and 210 ℃ for ethyl acetate oxidation,respectively.Moreover,the Cu1 Mn2/Fe-Sep sample possessed the best lowtemperature reducibility and the lowest temperature of oxygen desorption as well as the highest surface Mn4+/Mn3+ and Cu2+/CuO atomic ratios.It is concluded that factors,such as the strong interaction between the Cu or Mn and the Fe-Sep support,good low-temperature reducibility,and good mobility of chemisorbed oxygen species,might account for the excellent catalytic activity of Cu1 Mn2/Fe-Sep.     

Effect of Addition of Ni metal catalyst onto the Co and Fe supported catalysts for the formation of carbon nanotubes

Production of novel porous material is a major target in current material science research due to its wide applications. As carbon nanotube (CNTs) is a one dimensional hollow structure it is also one of the promising materials in applications ranging from electronics to hydrogen storage medium. Catalytic chemical vapor deposition (CCVD) is a method whereby CNTs can be produced in large amount. Thus, in this work, we have synthesized CNTs via pyrolysis of acetylene using various supported transition-metal catalysts in a fixed-bed reactor. Scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to investigate the CNTs structure. The structures of nanotubes formed by acetylene pyrolysis were dependent on the catalysts used. It was found that alumina supported Ni/Fe catalyst inhibited the formation of CNTs growth while alumina supported Ni/Co catalyst gave high density of CNTs. However, nanotubes grown over alumina supported Ni/Fe catalyst were less dense due to the deactivation of the catalyst at the early stage of the pyrolysis process.     

An in situ high pressure FT-IR study of CO2/H2 interactions with model ZnO/SiO2, Cu/SiO2 and Cu/ZnO/SiO2 methanol synthesis catalysts

In situ FT-IR spectroscopy allows the methanol synthesis reaction to be investigated under actual industrial conditions of 503 K and 10 MPa. On Cu/SiO₂ catalyst formate species were initially formed which were subsequently hydrogenated to methanol. During the reaction a steady state concentration of formate species persisted on the copper. Additionally, a small quantity of gaseous methane was produced. In contrast, the reaction of CO₂ and H₂ on ZnO/SiO₂ catalyst only resulted in the formation of zinc formate species: no methanol was detected. The interaction of CO₂ and H₂ with Cu/ZnO/SiO₂ catalyst gave formate species on both copper and zinc oxide. Methanol was again formed by the hydrogenation of copper formate species. Steady-state concentrations of copper formate existed under actual industrial reaction conditions, and copper formate is the pivotal intermediate for methanol synthesis. Collation of these results with previous data on copper-based methanol synthesis catalysts allowed the formulation of a reaction mechanism.     

La and Ce loaded TiO2-ZSM-5 catalysts: Comparative characterization and photocatalytic activity investigations

Lanthanide ions (Ln:La³⁺ or Ce³⁺) loaded TiO₂ supported ZSM-5 catalysts were prepared and characterized by using X-ray diffraction (XRD), surface area (BET), UV–vis diffuse reflectance spectra (UV–vis DRS), atomic force microscopy (AFM) and scanning electron microscopy SEM with energy dispersive X-ray analysis (EDX) techniques. Photocatalytic activities of the supported catalysts were examined in the presence of methyl orange (MO) molecule under UV irradiation. La³⁺ loaded catalysts revealed higher dark adsorption capacities and decolorization percentages in comparison to Ce³⁺ incorporated ones. Effect of lanthanide ion contents and reuse performances of the catalysts were controlled. A tentative degradation mechanism of MO was postulated based on the electron-accepting ability of the lanthanide ions and hole-trapping duty of ZSM-5.     

Synthesis and characterization of several Ni/NiAl2O4 catalysts active for the 1,2,4-trichlorobenzene hydrodechlorination

Nickel spinels were synthesized in order to be used as supports of nickel catalysts for the hydrodechlorination of 1,2,4-trichlorobenzene in the gas phase. All the samples were structurally characterized using X-ray diffraction (XRD), BET, temperature-programmed reduction (TPR), scanning electron microscopy (SEM) and temperature-programmed desorption (TPD) techniques. The Ni/NiAl₂O₄ catalysts tested had high conversions and selectivities towards benzene. This is due to their reduction degree (XRD) and structural surface properties (BET area, SEM, TPD). The most active and selective catalyst yields 87% benzene at 523 K for a conversion of 82%. These Ni/spinel catalysts desorb large amounts of hydrogen at lower temperatures (<625 K) which can compete with the aromatic compounds to be adsorbed on the surface of the catalyst. The amount of low-temperature hydrogen and the fact that it competes with the aromatic molecules favour a more exhaustive hydrodechlorination.     

二氧化钛催化超声降解酸性品红溶液及纳米锐钛矿型与普通金红石型二氧化钛催化性能的比较

采用活化处理过的纳米锐钛矿型与普通金红石型TiO2为催化剂，研究了各种因素对酸性品红溶液降解反应的影响，结果表明：TiO2对超声降解酸性品红具有明显的催化作用，而且纳米锐钛矿型TiO2的催化效果好于普通金红石TiO2，在超声功率为50W，频率为40kHz，纳米锐钛矿型TiO2催化剂用量0.5g／L，品红溶液初始浓度为20mg／L的条件下，超声反应3小时，降解率达到85％以上。通过对数据进行动力学方程拟合，发现品红溶液的降解反应符合一级反应。     

金红石相纳米二氧化钛分散浆料制备及其复合电镀应用

以四氯化钛为原料低温水解 ,在液相中一步简单地制备了金红石相TiO2 ,并且原位制备了一定浓度的纳米TiO2 分散浆料。X射线衍射测试表明此TiO2 呈纯的金红石结构 ,透射电子显微镜观察发现此浆料分散性好 ,可直接在化学复合电镀中应用。采用常规的电镀过程在金属表面形成了TiO2 -Ni复合电镀层 ,比一般的镀镍层均匀、细致。     

FT-IR and TPD studies on support-metal and promoter-metal interaction. Pt and Pd catalysts

New CO absorption bands were observed on promoted Pd and Pt catalysts and were attributed to C and O coordination of CO. The possibility of selective coverage of some metal faces by the promoter is discussed. TPD and FT-IR experiments show that the promoters stabilize an acetyl species upon adsorption of acetaldehyde.     

Infrared and adsorption study of strong metal-support interaction in diluted platinum-alumina catalysts

Diffuse reflectance IR spectroscopy along with H₂ and CO adsorption measurements were applied to the study of 0.3% Pt/--Al₂O₃ catalyst precalcined in an air flow at 1023 K and then reduced at 773 and 973 K in hydrogen. Reduction of the catalyst at 973 K results in formation of supported metallic Pt particles in SMSI (strong metal-support interaction) state, as evidenced by the sharp decrease of Pt ability to chemisorb H, at room temperature as well as the significant lowering of thermal stability of linear Pt-CO complexes. Spectroscopically the effect of SMSI is manifested by the slight increase of the singleton frequency of linearly adsorbed CO from 2050 to 2060 cm^–1 together with the decrease of the dipole-dipole shift from 40 to 15 cm^–1 and by a large decrease of absorption band extinction.