铈锆比对低贵金属Pt＋Rh/Ce0.3＋xZr0.6－xY0.1O1.95＋Al2O3三效催化剂性能的影响

用共沉淀法制得一系列铈锆比不同的Ce_0.3＋xZr_0.6－xY_0.1O_1.95储氧材料, 并用于制备了一系列低贵金属Pt＋Rh/Ce_0.3＋xZr_0.6－xY_0.1O_1.95＋Al₂O₃三效催化剂. 用比表面、程序升温还原以及X射线衍射对该系列催化剂进行表征, 结果发现, 催化剂的活性与催化剂中贵金属的还原性能密切相关, 低铈储氧材料比高铈储氧材料更有利于促进贵金属还原, 因而含低铈储氧材料催化剂的活性明显优于含高铈储氧材料催化剂的活性, Pt＋Rh/Ce_0.35Zr_0.55Y_0.1O_1.95＋Al₂O₃的活性最佳, 对HC, CO和NO的起燃温度最低分别为: 235, 175, 200 ℃. 样品经1000 ℃水热老化之后, 贵金属Pt被烧结而发生迁移, 使得催化剂的活性及还原性能变差, 含低铈材料的催化剂的抗老化性能优于含高铈材料的催化剂, 其中Pt＋Rh/Ce_0.35Zr_0.55Y_0.1O_1.95＋Al₂O₃的抗老化性能最好.     

Role of yttrium loading in the physico-chemical properties and soot combustion activity of ceria and ceria–zirconia catalysts

Ce_1−xY_xO₂ and Ce_0.85−xZr_0.15Y_xO₂ mixed oxides have been prepared by 1000 °C-nitrates calcination to ensure thermally stable catalysts. The physico-chemical properties of the mixed oxides have been studied by N₂ adsorption at −196 °C, XPS, XRD, Raman spectroscopy and H₂-TPR, and the catalytic activity for soot oxidation in air has been studied by TG in the loose and tight contact modes. Yttrium is accumulated at the surface of Ce_1−xY_xO₂ and Ce_0.85−xZr_0.15Y_xO₂, and this accumulation is more pronounced for the former formulation than for the latter, because the deformation of the lattice due to zirconium doping favours yttrium incorporation. Yttrium and zirconium exhibit opposite effects on the surface concentration of cerium; while zirconium promotes the formation of cerium-rich surfaces, yttrium hinders the accumulation of cerium on the surface. For experiments in tight contact between soot and catalyst, all the Ce_1−xY_xO₂ catalysts are more active than bare CeO₂, and Ce_0.99Y_0.01O₂ is the most active catalyst. The benefit of yttrium doping in catalytic activity of ceria can be related to two facts: (i) the Y³⁺ surface enrichment hinders crystallite growth; (ii) the surface segregation of Y³⁺ promotes oxygen vacancies creation. High yttrium loading (x = 0.12) is less effective than low dosage (x = 0.01) because yttrium is mainly accumulated at the surface of the particles and hinders the participation of cerium in the soot oxidation reaction, which is the active component. For the mixed oxides with formulation Ce_0.85−xZr_0.15Y_xO₂ (operating in tight contact) the effect of zirconium on the catalytic activity prevails with respect to that of yttrium. For experiments in loose contact between soot and catalyst, the catalytic activity depends on their BET surface area, and the catalysts Ce_0.85−xZr_0.15Y_xO₂ (BET = 10–13 m²/g) are more active than the catalysts Ce_1−xY_xO₂ (BET = 2–3 m²/g). In the loose contact mode, the yttrium doping and loading have a minor or null affect on the activity, and the stabilising effect of the BET area due to zirconium doping prevails.     

General and facile synthesis of ceria-based solid solution nanocrystals and their catalytic properties

Uniform Ce_1−xZr_xO₂ (x=0.2–0.8) nanocrystals with ultra-small size were synthesized through a thermolysis process, facilitated by the initial formation of precursor (hydrated (Ce,Zr)-hydroxides) at low temperature. TEM, XRD, EDAX, and Raman spectra were employed to study the formation of the solid solutions with various Ce/Zr ratios. Ultraviolet–visible (UV–vis) spectra showed that the ratios of Ce³⁺ to Ce⁴⁺ in both surface and bulk for the as-prepared Ce_1−xZr_xO₂ nanocrystals increased with the zirconium content x. The well-distributed Zr and Ce in the hydrated (Ce,Zr)-hydroxides before their thermolysis became the crucial factor for the structural homogeneity of the products. In addition, this strategy was extended to the synthesis of Ce_1−xGd_xO_1−x/2, Ce_1−xSm_xO_1−x/2, and Ce_1−xSn_xO₂ solid solutions. Catalytic measurements indicated that the ceria-based catalysts were active for CO oxidation at temperatures beyond 250 °C and the sequence of catalytic activity was Ce_0.5Zr_0.5O₂>Ce_0.8Zr_0.2O₂>Ce_0.2Zr_0.8O₂>Ce_0.5Sm_0.5O_1.75.     

Highly Dispersed Ce x Zr1−x O2 Nano-Oxides Over Alumina, Silica and Titania Supports for Catalytic Applications

We have been exploring the utilization of supported ceria and ceria–zirconia nano-oxides for different catalytic applications. In this comprehensive investigation, a series of Ce _x Zr_1−x O₂/Al₂O₃, Ce _x Zr_1−x O₂/SiO₂ and Ce _x Zr_1−x O₂/TiO₂ composite oxide catalysts were synthesized and subjected to thermal treatments from 773 to 1073 K to examine the influence of support on thermal stability, textural properties and catalytic activity of the ceria–zirconia solid solutions. The physicochemical characterization studies were performed using X-ray diffraction (XRD), Raman spectroscopy (RS), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HREM), thermogravimetry and BET surface area methods. To evaluate the catalytic properties, oxygen storage/release capacity (OSC) and CO oxidation activity measurements were carried out. The XRD analyses revealed the formation of Ce_0.75Zr_0.25O₂, Ce_0.6Zr_0.4O₂, Ce_0.16Zr_0.84O₂ and Ce_0.5Zr_0.5O₂ phases depending on the nature of support and calcination temperature employed. Raman spectroscopy measurements in corroboration with XRD results suggested enrichment of zirconium in the Ce _x Zr_1−x O₂ solid solutions with increasing calcination temperature thereby resulting in the formation of oxygen vacancies, lattice defects and oxygen ion displacement from the ideal cubic lattice positions. The HREM results indicated a well-dispersed cubic Ce _x Zr_1−x O₂ phase of the size around 5 nm over all supports at 773 K and there was no appreciable increase in the size after treatment at 1073 K. The XPS studies revealed the presence of cerium in both Ce⁴⁺ and Ce³⁺ oxidation states in different proportions depending on the nature of support and the treatment temperature applied. All characterization techniques indicated absence of pure ZrO₂ and crystalline inactive phases between Ce–Al, Ce–Si and Ce–Ti oxides. Among the three supports employed, silica was found to stabilize more effectively the nanosized Ce _x Zr_1−x O₂ oxides by retarding the sintering phenomenon during high temperature treatments, followed by alumina and titania. Interestingly, the alumina supported samples exhibited highest OSC and CO oxidation activity followed by titania and silica. Details of these findings are consolidated in this review.     

Effect of Transition Metals (Cu, Co and Fe) on the Autothermal Reforming of Methane over Ni/Ce0.2Zr0.1Al0.7Oδ Catalyst

The transition metals (Cu, Co, and Fe) were applied to modify Ni/Ce0.2Zr0.1 Al0.7Oδcatalyst. The effects of transition metals on the catalytic properties of Ni/Ce0.2Zr0.1 Al0.7Oδautothermal reforming of methane were investigated. The Ni-supported catalysts were characterized by XRD, TPR and XPS. Tests in autothermal reforming of methane to hydrogen showed that the addition of transition metals (Cu and Co) significantly increased the activity of catalyst under the conditions of lower reaction temperature, and Ni/Cu0.05Ce0.2Zr0.1Al0.65Oδwas found to have the highest conversion of CH4 among all catalysts in the operation temperatures ranging from 923 K to 1023 K. TPR, XRD and XPS measurements indicated that the cubic phases of CexZr1-xO2 solid solution were formed in the preparation process of catalysts. Strong interaction was found to exist between NiO and CexZr1-xO2 solid solution. The addition of Cu improved the dispersion of NiO, inhibited the formation of NiAl2O4, and thus significantly promoted the activity of the catalyst Ni/Cu0.05Ce0.2Zr0.1Al0.65Oδ.     

Methanol conversion over CuO supported on CeO<Subscript>2</Subscript>-ZrO<Subscript>2</Subscript>: An in situ IR spectroscopic study

The main reactions yielding hydrogen are the recombination of hydrogen atoms on copper clusters and methyl formate decomposition. Methyl formate results from the interaction between the linear methoxy group and the formate complex located on CuO. The source of CO₂ appearing in the gas phase is the formate complex, and the source of CO is methyl formate. The rates of methoxy group conversion and product formation over supports (ZrO₂, CeO₂, Ce_0.8Zr_0.2O₂) and copper-containing catalysts (5%Cu/CeO₂, 5%Cu/ZrO₂, 2%Cu/Ce_0.8Zr_0.2O₂, 2%Cu/Ce_0.1Y_0.1Zr_0.8) are compared. The dominant process in methoxy group conversion over the supports and copper-containing catalysts is methanol decomposition to H₂ and CO and to H₂ and CO₂, respectively. The methoxy group conversion rate is proportional to the H₂ and CO₂ formation rate and is determined by the concentration of supported copper.     

合成方法对Rh/CeO2-ZrO2-La2O3催化剂的结构、表面性能及DeNOx活性的影响

用沉积沉淀法合成两种不同系列的CeO2-ZrO2-La2O3混合氧化物(ZrO2和La2O3沉积CeO2粒子(标记为A-x)以及CeO2和La2O3沉积ZrO2粒子(标记为B-x)),并用作Rh催化剂的载体。XRD、拉曼、TPR、XPS和O2脉冲等表征结果显示出不同的沉积顺序将导致不同的结构和氧化还原性能,且B-x具有更高的氧迁移性、储氧能力和表面Ce浓度。当其负载Rh后,Rh/B-x催化剂具有更高的NO和CO转化率及N2选择性,且Ce的最佳含量为50at%。这可能归因于Rh负载于富铈表面形成更多有利于NO分解的表面Ce3+活性位。     

Physicochemical properties of the surface of the (Y,La0.1)Ce x Zr1? x O2?δ( x = 0.1–0.7) and Y0.1Pr0.3Zr0.6O2?δ complex oxide systems

The physicochemical properties of the surface of the Y_0.1Ce _x Zr_1−x O_2−δ, La_0.1Ce _x Zr_1−x O_2−δ (x=0.1–0.7), and Y_0.1Pr_0.3Zr_0.6O_2−δ. complex oxide systems were studied using IR and X-ray photoelectron spectroscopies. An appreciable enrichment of the surface of the solids in rare-earth-metal cations (cerium or praseodymium) during the synthesis was revealed. While cations are uniformly spread over the surface of cerium-zirconium solid solutions, the Y_0.1Pr_0.3Zr_0.6O_2−δ surface is covered by the clusters or even a phase of praseodymia. Reductive treatment in hydrogen with subsequent reoxidation results in the segregation of cerium ions on the Y_0.1Ce_0.3Zr_0.6O_2−δ surface at a temperature as low as 770 K. Original Russian Text ? A.N. Kharlanov, L.N. Ikryannikova, V.V. Lunin, A. Yu. Stakheev, 2007, published in Zhurnal Fizicheskoi Khimii, 2007, Vol. 81, No. 7, pp. 1271–1277.     

Autothermal Reforming of Methane over Ni Catalysts Supported on CuO-ZrO2-CeO2-Al2O3

Ni catalysts supported on various mixed oxides of Al2O3 with rare earth oxide and transitional metal oxides were synthesized. The studies focused on the measurement of the autothermal reforming of methane to hydrogen over Ni catalysts supported on the mixed oxide ZrxCe30-xAl70Oδ (x=5, 10, 15). The catalytic performance of Ni/Zr10Ce20Al70Oδ was better than that of other catalysts. XRD results showed that the addition of Zr to Ni/Ce30Al70Oδ prevented the formation of NiAl2O4 and facilitated the dispersion of NiO. Effects of CuO addition to Zr10Ce20Al70Oδ were also investigated. The activity of Ni catalyst supported on CuO-ZrO2-CeO2-Al2O3 was somewhat affected and the Ni/Cu5Zr10Ce20Al65Oδ showed the best catalytic performance with the highest CH4 conversion, yield of H2, selectivity for H2 and H2/CO production ratio in operation temperatures ranging from 650 to 750℃.     

NO Reduction Over Noble Metal Ionic Catalysts

In last 40 years, catalysis for NO _x removal from exhaust gas has received much attention to achieve pollution free environment. CeO₂ has been found to play a major role in the area of exhaust catalysis due to its unique redox properties. In last several years, we have been exploring an entirely new approach of dispersing noble metal ions in CeO₂ and TiO₂ for redox catalysis. We have extensively studied Ce_1−x M _x O_2−δ (M = Pd, Pt, Rh), Ce_1−x−y A _x M _y O_2−δ (A = Ti, Zr, Sn, Fe; M = Pd, Pt) and Ti_1−x M _x O_2−δ (M = Pd, Pt, Rh, Ru) catalysts for exhaust catalysis especially NO reduction and CO oxidation, structure–property relation and mechanism of catalytic reactions. In these catalysts, lower valent noble metal ion substitution in CeO₂ and TiO₂ creates noble metal ionic sites and oxide ion vacancy. NO gets molecularly adsorbed on noble metal ion site and dissociatively adsorbed on oxide ion vacancy site. Dissociative chemisorption of NO on oxide ion vacancy leads to preferential conversion of NO to N₂ instead of N₂O over these catalysts. It has been demonstrated that these new generation noble metal ionic catalysts (NMIC) are much more catalytically active than conventional nano crystalline noble metal catalysts especially for NO reduction.     

NOx-Sorbing Metal Oxides, MnOx–CeO2. Oxidative NO Adsorption and NOx–H2 Reaction

(n)MnO_x–(1–n)CeO₂ binary oxides have been studied for the sorptive NO removal and subsequent reduction of NO_x sorbed to N₂ at low temperatures (150 °C). The solid solution with a fluorite-type structure was found to be effective for oxidative NO adsorption, which yielded nitrate (NO^– ₃) and/or nitrite (NO^– ₂) species on the surface depending on temperature, O₂ concentration in the gas feed, and composition of the binary oxide (n). A surface reaction model was derived on the basis of XPS, TPD, and DRIFTS analyses. Redox of Mn accompanied by simultaneous oxygen equilibration between the surface and the gas phase promoted the oxidative NO adsorption. The reactivity of the adsorbed NO_x toward H₂ was examined for MnO_x–CeO₂ impregnated with Pd, which is known as a nonselective catalyst toward NO–H₂ reaction in the presence of excess oxygen. The Pd/MnO_x–CeO₂ catalyst after saturated by the NO uptake could be regenerated by micropulse injections of H₂ at 150 °C. Evidence was presented to show that the role of Pd is to generate reactive hydrogen atoms, which spillover onto the MnO_x–CeO₂ surface and reduce nitrite/nitrate adsorbing thereon. Because of the lower reducibility of nitrate and the competitive H₂–O₂ combustion, H₂–NO reaction was suppressed to a certain extent in the presence of O₂. Nevertheless, Pd/MnO_x–CeO₂ attained 65% NO-conversion in a steady stream of 0.08% NO, 2% H₂, and 6% O₂ in He at as low as 150 °C, compared to ca. 30% conversion for Pd/–Al₂O₃ at the same temperature. The combination of NO_x-sorbing materials and H₂-activation catalysts is expected to pave the way to development of novel NO_x-sorbing catalysts for selective deNO_x at very low temperatures.     

Structure and Catalytic Properties of Ceria-based Nickel Catalysts for CO<Subscript>2</Subscript> Reforming of Methane

Carbon dioxide reforming (CDR) of methane to synthesis gas over supported nickel catalysts has been reviewed. The present review mainly focuses on the advantage of ceria based nickel catalysts for the CDR of methane. Nickel catalysts supported on ceria–zirconia showed the highest activity for CDR than nickel supported on other oxides such as zirconia, ceria and alumina. The addition of zirconia to ceria enhances the catalytic activity as well as the catalyst stability. The catalytic performance also depends on the crystal structure of Ni–Ce–ZrO₂. For example, nickel catalysts co-precipitated with Ce_0.8Zr_0.2O₂ having cubic phase gave synthesis gas with CH₄ conversion more than 97% at 800 °C and the activity was maintained for 100 h during the reaction. On the contrary, Ni–Ce–ZrO₂ having tetragonal phase (Ce_0.8Zr_0.2O₂) or mixed oxide phase (Ce_0.5Zr_0.5O₂) deactivated during the reaction due to carbon formation. The enhanced catalytic performance of co-precipitated catalyst is attributed to a combination effect of nano-crystalline nature of cubic Ce_0.8Zr_0.2O₂ support and the finely dispersed nano size NiO _x crystallites, resulting in the intimate contact between Ni and Ce_0.8Zr_0.2O₂ particles. The Ni/Ce–ZrO₂/θ–Al₂O₃ also exhibited high catalytic activity during CDR with a synthesis gas conversion more than 97% at 800 °C without significant deactivation for more than 40 h. The high stability of the catalyst is mainly ascribed to the beneficial pre-coating of Ce–ZrO₂ resulting in the existence of stable NiO _x species, a strong interaction between Ni and the support, and an abundance of mobile oxygen species in itself. TPR results further confirmed that NiO _x formation was more favorable than NiO or NiAl₂O₄ formation and further results suggested the existence of strong metal-support interaction (SMSI) between Ni and the support. Some of the important factors to optimize the CDR of methane such as reaction temperature, space velocity, feed CO₂/CH₄ ratio and H₂O and/or O₂ addition were also examined.     

Total Oxidation of Propene and Toluene on Copper/Yttrium Doped Zirconia

The catalytic oxidation of propene and toluene has been investigated on pure ZrO₂, pure Y₂O₃, and ZrO₂ doped with 1, 5, and 10 mol % Y₂O₃ in the presence or absence of copper (0.5, 1, and 5 wt%). A synergetic effect has been detected since ZrO₂ and Y₂O₃ exhibit significantly lower activities than the mixed oxides. The higher surface areas, related to structural change from mononoclinic (ZrO₂) to tetragonal (ZrO₂–;;Y₂O₃), partly explained the higher activity of ZrO₂–;;Y₂O₃. However, it has been shown that the number of anionic vacancies, created by the substitution of Zr⁴⁺ by Y³⁺, in yttria-stabilized zirconia solids depends on the yttrium contents. Their effect on propene and toluene oxidation activity is significant. The anionic vacancies should induce better activity of the ZrO₂—5 mol % Y₂O₃ catalyst with or without copper, which presents the higher number of Zr³⁺ species. This support should favor the formation of CuO particles, which should be the most active catalytic sites in the studied reaction.     

Toluene combustion over Pd-based monolithic catalysts with a novel Ce x Zr 1?X O2 washcoat

A novel Ce _x Zr _1−x O₂ washcoat was prepared by impregnation, which acts as a host for the active Pd component to prepare a series of Pd-based monolithic catalysts for toluene combustion. The redox behavior and catalytic activity depend on the molar ratio of Ce to Zr.     

Farbe und Konstitution bei anorganischen Feststoffen, 14. Mitt.: Die Lichtabsorption des zweiwertigen Kupfers in oxidischen Wirtsgittern

Zusammenfassung Um die Beziehungen zwischen der Lichtabsorption des zweiwertigen Kupfers nach isomorphem Einbau in ein oxidisches Wirtsgitter und dessen Konstitution aufzufinden, wurde Cu²⁺ in oktaedrischer (Cu _x Mg _1–x TiO₃, Cu _x Cd _1–x TiO₃, Cu _x Mg _1–x CaSiO₄, Cu _x Mg _1–x CaGeO₄, Cu _x Mg _2–x SiO₄, Cu _x Mg _2–x GeO₄) und tetraedrischer Koordination (Cu _x Zn_2–x SiO₄, Cu _x Mg _1–x Cr₂O₄) spektralphotometrisch untersucht. Die Farbkurven besitzen mindestens 2 Absorptionsbanden (Kristallfeldbanden) im längerwelligen und eine oft gut ausgeprägte Elektronenübergangsbande (charge transfer) im kürzerwelligen Spektralbereich. In einigen Fällen ist noch eine zweite Elektronenübergangsbande als Schulter zu erkennen. Es wurden auch Cu-haltige 2,3- und 2,4-Spinelle spektralphotometrisch untersucht (Cu _x Mg _1–x Al₂O₄, Cu _x Mg _1–x Ga₂O₄, Cu _x Cd _y Zn _1–x–y Al₂O₄, Cu _x Mg _2–x SnO₄, Cu _x Mg _2–x TiO₄, Cu _x Zn _1–x MgTiO₄, Cu _x Mg _1–x Cd _y TiO₄). Es zeigte sich, daß Cu²⁺ immer auf Tetraeder- und Oktaederlücken verteilt ist. Eine Aufweitung des Wirtsgitters durch isomorphen Einbau größerer Kationen bewirkt nicht immer eine IR-Verschiebung der Banden, sondern in einigen Fällen (Spinellphasen) auch eine UV-Verschiebung. Eine Sonderstellung nimmt das ägyptisch-Blau CuCaSi₄O₁₀ ein, da hier das Cu²⁺ von 4 O_2– in planarer Anordnung umgeben ist. Die Farbkurve weist 3 Maxima auf im Einklang mit der Kristallfeldtheorie.

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In order to find out relations between the lightabsorption of bivalent copper isomorphously incorporated into an oxidic host lattice and the constitution of this lattice, the spectrum of Cu²⁺ has been investigated in octahedral (Cu _x Mg_1–x TiO₃, Cu _x Cd _1–x TiO₃, Cu _x Mg _1–x CaSiO₄, Cu _x Mg _1–x CaGeO₄, Cu _x Mg _2–x SiO₄, Cu _x Mg _2–x GeO₄) and tetrahedral coordination (Cu _x Zn _2–x SiO₄, Cu _x Mg _1–x Cr₂O₄). The colour curves show at least 2 absorption bands within the region of longer wave length and a charge transfer band often well developed in the range of shorter wavelength. In some cases also a second charge transfer band becomes conspicuous as a shoulder. Copper containing 2,3- and 2,4-spinels have been also investigated (Cu _x Mg _1–x Al₂O₄, Cu _x Mg _1–x Ga₂O₄, Cu _x Cd _y Zn _1–x–y Al₂O₄, Cu _x Mg _2–x SnO₄, Cu _x Mg _2–x TiO₄, Cu _x Zn _1–x MgTiO₄, Cu _x Mg _1–x Cd _y Zn _1–y TiO₄). From the colour curve one can infer that Cu²⁺ occupies in the spinels always tetrahedral as well as octahedral interstices. A widening of the lattice does not effect always a shifting of the absorption bands towards IR but in some cases (spinel phases) also the inverse shifting will occur. An exceptional case represents the egyptian blue CuCaSi₄O₁₀ since in this lattice the Cu₂₊ are surrounded by four O^2– in a coplanar arrangement. The colour curve shows three absorption bands in agreement with the crystal field theory.

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Mit 20 Abbildungen     

Mixed insoluble acidic salts of tetravalent metals VI. Crystalline mixed zirconium-titanium arsenate and hafnium-titanium arsenate

Mixed crystalline zirconium-titanium arsenates Zr_xTi(_1–x) (HAsO₄)₂·H₂O and harnium-titanium arsenates Hf_xTi(_1–x) (HAsO₄)₂ H₂O (where x=0–1) of various compositions with different Zr/Ti or Hf/Ti ratios have been prepared. The compounds have been characterized by X-ray, thermal analysis, chemical analysis, and pH-titrations. For powder X-rays, the compounds appear to be homogeneous crystalline materials with structures similar to -ZrP, -TiP or -ZrTiP. The thermograms show a loss of approximately two moles of H₂O, followed by a small loss of O₂ with probable formation of ZrO₂, As₂O₃, HfO₂, As₂O₃, ZrO₂, As₂O₅, HfO₂, As₂O₅ from which As₂O₃ and As₂O₅ are practically sublimed in two steps. The exchange capacities at r.t. are generally similar to -ZrP and -TiP but are found to be much lower in value.     

Cyclohexane transformations over metal oxide catalysts. 1. Effect of the nature of metal and support on the catalytic activity in cyclohexane ring opening

The activities of monometallic Pt-, Ru-, and Rh-containing catalysts supported on Al₂O₃, Al₂O₃—F, SiO₂, WO₃/ZrO₂, and La₂Î₃/ZrO₂, in cyclohexane ring opening to form n-hexane were studied. The most active catalyst is Rh/Al₂O₃. Cyclohexane hydrogenolysis to n-hexane also occurs over the Pt/Al_;>2O₃ and Pt/La₂Î₃/ZrO₂ catalysts. Ring opening over the Ru catalysts proceeds at significantly lower temperatures (210—230 °C) than over the Pt and Rh catalysts (350—400 °C), but the ruthenium systems are less selective for n-hexane formation than Rh/Al₂O₃ catalysts. The effects of acid-basic properties of the support and the reaction conditions on the activities of the catalytic systems in cyclohexane ring opening was studied.     

Alkoxy-derived fine powders of simple and complex oxides

Fine powders of ZrO₂, ZrO₂-Y₂O₃, BaTi_1–x Zr _x O₃ have been obtained by hydrolysis of alkoxides. For preparation of ZrO₂-based materials precipitation from the partially hydrolyzed alkoxides solutions (sols) has been used. Barium titanate-based materials were obtained by hydroxide-alkoxide route with quick hydrolysis of titanium butoxide by excess of water on the first stage. The effect of the hydrolysis conditions on the size, shape, and specific surface area of the oxide powders has been discussed. Some considerations on comparison of the two hydrolytic techniques have been suggested.     

Preparation of inorganic nanoparticles in oil-in-water microemulsions: A soft and versatile approach

In this review, the synthesis of inorganic nanoparticles using oil-in-water (O/W) microemulsions as confined reaction media is discussed. Synthesis using (O/W) microemulsions has been demonstrated for a great variety of inorganic nanoparticles: metallic (Pt, Pd, Rh, Ag), single metal oxides (CeO₂, ZrO₂, TiO₂, Fe₂O₃), mixed and doped metal oxides (Ce_0.5Zr_0.5O₂, Ce_0.99Eu_0.01O₂, Zr_0.99Eu_0.01O₂, and Fe₂Mn_0.5Zn_0.5O₄), semiconductors (PbS, CdS, Ag₂S, ZnS, CdSe, PbSe, Ag₂Se), fluorides (CaF₂, YF₃, NdF₃, PrF₃), phosphates (CePO₄, HoPO₄), and chromates (BaCrO₄ and PbCrO₄). There are two synthetic strategies: 1) the use of oil-in water (O/W) microemulsions, in which the precursor is an ionic salt which is dissolved in the continuous aqueous phase; and 2) use of O/W microemulsions, in which the precursor is an organometallic salt dissolved in the oil droplets of the microemulsion. The latter approach keeps more resemblance to the typical W/O microemulsion reaction method, as it has the greatest level of precursor confinement.     

Correlation between acidic properties of nickel catalysts and catalytic activities for ethylene dimerization and butene isomerization

Catalytic activities of NiO–SiO₂ for ethylene dimerization and butene isomerization run parallel when the catalysts are activated by evacuation at elevated temperatures, giving two maxima in activities. The variations in catalytic activities are closely correlated to the acidity of NiO–SiO₂ catalysts. Catalytic activities of NiO–TiO₂ catalysts modified with H₂SO₄, H₃PO₄, H₃BO₃, and H₂SeO₄ for ethylene dimerization and butene isomerization were examined. The order of catalytic activities for both reactions was found to be NiO–TiO₂/SO₄^2- >> NiO–TiO₂/PO₄^3-NiO–TiO₂/BO₃^3- > NiO–TiO₂/SeO₄^2-> NiO–TiO₂, showing clear dependence of catalytic activity upon acid strength. The high catalytic activity of supported nickel sulfate for ethylene dimerization was related to the increase of acidity and acid strength due to the addition of NiSO₄. The asymmetric stretching frequency of the S=O bonds for supported NiSO₄ catalysts was related to the acidic properties and catalytic activity. That is, the higher the frequency, the larger both the acidity and catalytic activity. For NiSO₄/Al₂O₃–ZrO₂ catalyst, the addition of Al₂O₃ up to 5 mol% enhanced catalytic activity for ethylene dimerzation and strong acidity gradually due to the formation of Al–O–Zr bond. The active sites responsible for ethylene dimerization consist of a low-valent nickel, Ni⁺, and an acid, as evidenced by the IR spectra of CO adsorbed on NiSO₄/ -Al₂O₃ and Ni 2p XPS.