Preparation of platinum-iridium nanoparticles on titania nanotubes by MOCVD and their catalytic evaluation

Pt based catalysts are commonly used in several industrial processes involving hydrogenation and dehydrogenation reactions. New deposition methods as well as support materials are being investigated to generate new catalysts with superior catalytic activity. In this work, platinum-iridium (Pt-Ir) nanoparticles of about 5 nm in size were supported on titania (TiO₂) nanotubes by metal organic chemical vapor deposition (MOCVD). The TiO₂ nanotubes were prepared by an alkali hydrothermal method using sodium hydroxide solution at 100 °C, during 64.8 ks. Pt-Ir nanoparticles were obtained by controlling the MOCVD conditions at 400 °C and 66.6 kPa.Textural properties and particle size were investigated by nitrogen physisorption (BET method), X-ray diffraction, Raman spectroscopy and high resolution transmission electron microscopy. Catalytic activity was measured in cyclohexene disproportion as the test molecule for hydrogenation/dehydrogenation reactions. The TiO₂ nanotubes exhibit a considerable high surface area of about 425,000 m²/kg, however, after calcination at 400 °C their nanotubular morphology was partially transformed. In spite of this change, the 5 nm Pt-Ir nanoparticles supported on TiO₂ nanotubes were more active in the cyclohexene disproportion reaction than conventional Pt-Ir/alumina catalysts in the whole range of temperatures investigated (50–250 °C). Hydrogenation reactions (high selectivity to cyclohexane) predominate at temperatures below 150 °C.     

Oxide coatings modified with transition and rare-earth metals on aluminum and their activity in CO oxidation

Oxide catalysts, which, in addition to Al₂O₃ + SiO₂, contain from one (Ni) to four oxides or compounds of transition metals (Ni, Cu, Mn, and Co) and oxides or compounds of rare-earth elements (Ce, La), are produced on D16 aluminum alloy by plasma electrolytic oxidation combined with impregnation and subsequent annealing. The composites formed begin to catalyze the CO oxidation in a temperature range of from 100 to 300°C. The catalysts used can be arranged in the following series of decreasing catalytic activity: Ni-Cu-Mn-Co-Ce > Ni-Cu-Mn-Co-Ce-La ≈ Ni-Cu-Mn-Co > Ni-Cu-Mn > Ni-Cu > Ni. Oxygen compounds of Cu⁺, Cu²⁺, Mn⁴⁺, Co³⁺, Ce³⁺, and Ce⁴⁺, which seem to determine the catalytic properties of the oxide systems studied, are found on the surface and in the subsurface layer with a total thickness of ～6 nm of the most active Ni-Cu-Mn-Co-Ce catalyst.     

Synthesis of ultrafine (Ti, W, Mo, V)(C, N)–Ni composite powders by low-energy milling and subsequent carbothermal reduction–nitridation reaction

Ultrafine (Ti, W, Mo, V)(C, N)–Ni composite powders with globular-like particles of 50–300 nm were synthesized at static nitrogen pressure from oxides by a simple and cost-effective route which combines traditional low-energy milling plus carbothermal reduction–nitridation (CRN) techniques. Reaction path of the (Ti, W, Mo, V)(C, N)–Ni system was discussed by X-ray diffraction (XRD) and thermogravimetry–differential scanning calorimetry (TG–DSC), and microstructure of the milled powders and final products was studied by scanning electron microscopy (SEM) and transmission electron microscope (TEM), respectively. The results show that CRN reaction has been enhanced by nano-TiO₂ and nano-carbon powders. Thus, the preparation of (Ti, 15W, 5Mo, 0.2V)(C, N)–20Ni is at only 1300 °C for 1 h. During synthesizing reaction, Ni solid solution phase forms at about 700 °C and reduction–carbonization of WO₂ and MoO₂ occurs below 900 °C. The reactions of TiO₂ → Ti₃O₅, Ti₃O₅ → Ti(C, O) and Ti(C, O) → Ti(C, N) take place at about 930 °C, 1203 °C and 1244 °C, respectively.     

Oxidation behavior at 300–1000 °C of a (Mo,W)Si2-based composite containing boride

The oxidation behavior of a (Mo,W)Si₂ composite with boride addition was examined at 300–1000 °C for 24 h in dry O₂. The oxidation kinetics was studied using a thermobalance, and the oxide scales were analyzed using a combination of electron microscopy (SEM/EDX, FIB, BIB) and XRD. Accelerated oxidation was found to occur between 500 °C and 675 °C, with a peak mass gain at 625 °C. The rapid oxidation is attributed to the vaporization of molybdenum oxide that leaves a porous and poorly protective silica layer behind. At higher temperature (700–1000 °C) a protective scale forms, consisting of a dense SiO₂/B₂O₃ glass.     

Structures, electrical and thermal expansion properties of Sr-doped La2Mo2O9 oxide-ion conductors

The oxide-ion conductors (La_1−xSr_x)₂Mo₂O_9−δ (x = 0.01–0.08) were prepared by means of a conventional solid-state reaction. The effects of Sr doping for La site on the structures, electrical and thermal expansion properties of the oxide-ion conductor La₂Mo₂O₉ were investigated using X-ray diffraction, direct current four-probe method, thermal dilatometer and scanning electron microscopy, respectively. The results show that the lattice constants were first decreased, then increased, and decreased again with the increase of Sr doping content. The solid solubility of Sr in (La_1−xSr_x)₂Mo₂O_9−δ is x = 0.07. The sinterability of samples is markedly improved with the increase of Sr doping content. The sintered density of sample x > 0.07 is higher than 96% of its theoretical density. When x > 0.02, doping Sr in La₂Mo₂O₉ can inhibit the excessive growth of grains, thus increases the sintered density of samples. The structural transition temperature shifts to the low side with the increase of Sr doping content, and the phase transition is completely suppressed when the doping content reaches 0.07. The conductivity of sample increases with the increase of Sr doping content. The conductivity of sample x = 0.07 reaches a maximum of 0.078 S/cm and 0.101 S/cm at 800 °C and 850 °C, respectively. In this study, it was demonstrated that doping 7 mol% Sr for La site not only can completely suppress the structural phase transition in La₂Mo₂O₉, but also can effectively enhance electrical conductivity of samples at higher temperature.     

Forming of AlSi8Cu3Fe alloy in the semi-solid state

AlSi8Cu3Fe alloy is a general purpose die casting alloy widely used to manufacture automotive parts. Forming of this alloy in the semi-solid state could largely eliminate problems inherent in die casting process and thus offers a number of advantages. Low superheat casting was employed in the present work to produce non-dendritic AlSi8Cu3Fe alloy feedstock. The ingots thus obtained were thixoformed in a laboratory press after they were held at 570 °C for 5 min, yielding a microstructure with predominantly α-Al globules and interglobular Si particles. The thixoformed parts attain hardness values as high as 125 HB after T6 heat treatment, implying a considerable age hardening potential. However, with a reheating temperature range of 568–572 °C, thixoforming of AlSi8Cu3Fe alloy components under industrial conditions may be challenging.     

稀土与过渡金属改性TiO_2光催化降解甲基橙研究

采用溶胶-凝胶法制备稀土La和过渡金属Fe掺杂改性的Ti O2光催化剂用于降解甲基橙染料溶液,并借助X射线衍射(XRD)仪、X射线光电子能谱(XPS)以及固体光致发光光谱(PL)等测试技术对催化剂进行表征,研究La和Fe掺杂对Ti O2催化剂结构和光催化性能的影响规律。结果表明,掺杂稀土元素La、过渡金属Fe可有效降低Ti O2光催化剂的晶粒尺寸,提高锐钛矿相的含量,催化剂中均匀分散的镧物种、铁物种会增加Ti O2催化剂表面氧空位和缺陷浓度,Fe、La掺杂后Ti O2光催化剂对甲基橙的降解率分别提高约16%和约22%,且稀土La改性催化剂显示较好的循环稳定性。     

On the properties of the eutectic alloy Al3(Nb,Cr) + Cr(Al,Nb)

The eutectic alloy Al₃(Nb,Cr) + Cr(Al,Nb) forms an in situ composite and the Al₃Nb presents high specific strength and low oxidation rate that may be improved by the combination with other phases. The purpose of this work is to investigate physical, mechanical and oxidation properties of the eutectic alloy. Therefore, Rietveld analysis was carried out for furnace cooled and water quenched samples and oxidation tests were performed on directional solidified samples. Compressive tests were performed for the eutectic alloy and also for the Nb–74.8% Cr–24.6% Al alloy in the as-cast condition. The alloy presents 12.9% Cr(Al,Nb) at room temperature, retained from the transformation Cr(Al,Nb) to Al(Nb)Cr₂. The combination of Al₃Nb with Cr(Al,Nb) and Al(Nb)Cr₂ considerably improves mechanical behaviour, leading the yield strength to 1525 MPa at 800 °C and 925 MPa at 900 °C. The oxidation tests showed the formation of several oxides at all temperatures studied and that from 900 °C on alfa Al₂O₃ is formed both in air and O₂ except under O₂ at 1000 °C. It is believed that the Cr(Al,Nb) phase acts as an Al reservoir for the formation of the various Al₂O₃ scales.     

喷雾干燥法制备碳载铂钴镍合金催化剂及其甲醇催化氧化性能研究

本文以氯铂酸、氯化镍和硝酸钴为原料,XC-72炭黑为载体,通过雾化干燥法结合煅烧还原制备碳载铂基（PtCo Ni）分散性好的多元合金纳米粒子催化剂。重点研究表面经过改性的炭黑对合金纳米粒子形成和分散的影响规律,研究碳载PtCoNi（原子比为1:1:1）合金纳米粒子的甲醇催化氧化活性、抗CO中毒能力和耐久性,以及不同原子比对催化氧化甲醇活性和抗CO中毒能力的影响规律。研究结果表明,采用表面改性后的炭黑作为载体,制备的碳载铂基（PtCoNi）催化剂为合金纳米粒子,且纳米粒子在炭黑表面分散均匀,粒径分布在1-4nm,平均粒径为2.3nm;与商用的Pt/C催化剂相比,PtCoNi/C（原子比为1:1:1）催化剂具有更高的甲醇催化氧化活性、耐久性和抗CO中毒性;不同原子比铂基多元催化剂在催化氧化甲醇活性上的顺序为：PtCoNi/C>Pt3CoNi/C>Pt5CoNi/C,抗CO中毒性顺序为：PtCoNi/C>Pt3CoNi/C>Pt5CoNi/C。     

掺杂La2O3对CDPF催化性能的影响

本文在CDPF催化剂上,通过掺杂10、20和30 g/L不同浓度的La2O3的助剂制备了不同的CDPF样品,采用XRD、XPS、H2-TPR等表征方法以及活性评价技术,研究掺杂不同浓度La2O3的CDPF载体表面催化剂的理化特性和催化活性的关系。结果表明：随着La2O3的掺杂浓度的增加,样品的结晶度先增加再减小；样品表面的Pt原子浓度先降低再升高；样品对CO的氧化活性呈下降趋势；样品对C3H8的特征温度T10和T50均呈先增加再降低,然后再增加的趋势；样品对NO2产率的T10呈增加趋势。     

Effect of CrB2 addition on densification, properties and oxidation resistance of TiB2

This paper presents the results of detailed studies carried out on the densification of TiB₂ with CrB₂ as sinter additive by hot pressing. The dense compacts were characterized by measurement of hardness, indentation fracture toughness, flexural strength, coefficient of thermal expansion and electrical resistivity. Oxidation characteristics were investigated between 600 °C and 1000 °C and isothermal oxidation kinetics at 850 °C. Phase identification and surface morphology analysis of hot pressed and oxidized samples were done using XRD and SEM. A high density of 96.61% Τ.D was obtained with the addition of 2.5% CrB₂ by hot pressing at 1750 °C under 35 MPa pressure. Hardness values of composites with 2.5–10% CrB₂ were close to 24 GPa and fracture toughness in the range of 3–5 MPa m^1/2. Coefficient of thermal expansion of the composite with 10% CrB₂ was measured in the range of 6.21–7.43 × 10⁻⁶/K from room temperature to 1000 °C. Electrical resistivity of TiB₂ + 10%CrB₂ was measured as 32.83, 75.97 and 120 μΩ cm at 25 °C, 500 °C and 900 °C, respectively. Observed nature of oxidation was parabolic for all composites. Formation of continuous and thick glassy film was observed with increased CrB₂ content in the composite. TiO₂ and CrBO₃ phases were identified on the oxidized surface which are responsible for the improved oxidation resistance of this composite.     

Investigation of the influence of Ni powder size on microstructural evolution and the thermal explosion combustion synthesis of NiTi

A key microstructural feature that controls the sintering behavior of Ni + Ti powders was determined to be the transformation of alpha-Ti to beta-Ti during heating. The use of very fine Ni powders causes this transformation to occur at the eutectoid temperature (i.e., 765 °C). The use of coarse Ni powders causes a gradual beta-Ti transformation from 765 to 882 °C. At 950 °C a large volume fraction of beta-Ti remains in coarse Ni/Ti mixtures whereas in fine Ni/Ti mixtures this phase is almost eliminated. Further heating above 950 °C causes the beta-Ti to melt, initiating a large exothermic reaction in the coarse Ni/Ti mixtures (i.e., 158 J/g) at 980 °C. The use of fine Ni significantly reduces this reaction (i.e., 3 J/g). Consequently, Ni powder size, and its influence over beta-Ti content can be used to control the reactive sintering behavior of Ni + Ti mixtures.     

Structural characteristics and resistive switching properties of thermally prepared TiO2 thin films

Polycrystalline TiO₂ thin films were formed on Pt(1 1 1)/Ti/SiO₂/Si by thermal oxidation of Ti films with temperatures ranging from 600 °C to 800 °C. Results of Raman spectra testing indicate that the structure of the oxidized TiO₂ films is rutile phase. The resistance switching behaviors (RSB) have been confirmed in Pt/TiO₂/Pt structures. A stable RSB with a narrow dispersion of the resistance states and switching voltages was observed in the sample fabricated with the oxidation temperature of 600 °C. The resistance ratios of high resistance states to low resistance states are larger than 10³ with the set and reset voltage as low as 2.5 V and 0.6 V, respectively.     

Effect of a trace of Cr on intermetallic compound layer for tin–zinc lead-free solder joint during aging

Influence of Cr on growth of interfacial intermetallic compound (IMC) at the interface of Sn–9Zn/Cu substrate during aging at 85 °C/20%RH and 85 °C/85%RH for 500 h has been investigated. After aging treatment, IMC layer at the Sn–9Zn/Cu joint is much thicker than that at the Sn–9Zn–Cr/Cu joint. Estimation according to experimental data presents that IMC growth rate of Sn–9Zn–Cr/Cu interface is about 70–75% lower than that of Sn–9Zn/Cu interface.     

碱性条件下M/C(M=Pt, Pd, Ni, Ag, Au)对甲醇氧化电催化活性的对比研究

用化学还原法制备了M/C(M=Pt, Pd, Ni, Ag, Au) 5种纳米金属碳截催化剂,用XRD、XPS分别表征活性物的晶相结构、表面组成和价态形式,用TEM观察催化剂微观形貌,用循环伏安法测定不同催化剂对甲醇氧化的电催化活性。结果表明,催化剂中的纳米金属粒径在3~11 nm之间,纯度较高,在载体表面的分布较为均匀;Pd/C催化剂对甲醇氧化有较好的催化作用,Ni/C对甲醇氧化有一定的活性,而Ag/C和Au/C几乎没有活性。在1 mol·L-1CH3OH+1 mol·L-1NaOH条件下,Pd/C对甲醇氧化最高峰电流密度为890 mA·mg-1,约为Pt/C的66%,Ni/C对甲醇氧化最高峰电流密度为6.3 mA·mg-1,仅为Pt/C的0.46%。     

Microfibrous entrapment of small catalyst particulates for high contacting efficiency removal of trace CO from practical reformates for PEM H2-O2 fuel cells

Preferential oxidation (PROX) of CO in H₂ is the most efficient way to remove CO from a practical reformate stream for PEM H₂-O₂ fuel cells. Pt/Al₂O₃ has long been known as a suitable catalyst for this purpose. Over the conventional Pt/Al₂O₃ catalyst, however, PROX of CO in H₂ has been known to occur at temperatures above 150°C, and the maximum CO conversion usually takes place at about 200°C. In this study, the promotion of Pt/Al₂O₃ with a transition metal results in significantly enhanced catalytic performance in the temperature range of 25 to 150°C. The active reaction temperature window is enlarged to 25 to 200°C compared with a narrow window at about 200°C over the conventional Pt/Al₂O₃. A high void and a tailorable sintered microfibrous carrier consisting of 5 vol.% of 4 and 8 μm diameter Ni fibers is used to entrap 15 vol.% 150 to 250 μm diameter Al₂O₃ particulates. The alumina support particulates are uniformly entrapped into a sinter-locked, three-dimensional network of 4 and 8 μm Ni fibers. Promoter and Pt are then dispersed onto the microfibrous entrapped alumina support particles by the incipient-wetness impregnation method. The composite catalysts possess 80 vol.% voidage. At equivalent bed volumes, microfibrous entrapped catalysts achieve complete CO reduction (GC detection limit ∼40 ppm CO) at five times the higher gas hourly space velocity value compared with packed beds of 1 to 2 mm catalyst particles demonstrating ultrahigh contacting efficiency provided by the microfibrous entrapped catalysts. This paper was presented at the ASM Materials Solutions Conference & Show held October 18–21, 2004 in Columbus, OH.     

The phase equilibria of the Al–Ce–Ni system at 500 °C

The phase equilibria at 500 °C in the Al–Ce–Ni system in the composition region of 0–33.3 at.% Ce are investigated using XRD and SEM/EDX techniques applied to equilibrated alloys. The previously reported ternary phases and the variation of the lattice parameters versus the composition for different solid solution phases are investigated. It is confirmed that τ₂(Al₂CeNi) exists at 500 °C, while τ₃(Al₅Ce₂Ni₅) does not exist at 500 °C. A new compound τ₉ with composition of about Al₃₅Ce_16.5Ni_48.5 is found. The solubility of Ni in Al₁₁Ce₃ and αAl₃Ce is generally about 1 at.%, while the solubility of Ni in Al₂Ce is measured to be 2.7 at.%. The solubility of Ce in Al₃Ni, Al₃Ni₂, AlNi and AlNi₃ is all less than 1 at.%. The solubility of Al in CeNi₅, Ce₂Ni₇ and CeNi₃ is measured to be 30.4, 4.8 and 9.2 at.%, respectively, while there is no detectable solubility for Al in CeNi₂. A revised isothermal section at 500 °C in the Al–Ce–Ni system has been presented.     

Effect of sintering on microstructure and mechanical properties of nano-TiO2 dispersed Al65Cu20Ti15 amorphous/nanocrystalline matrix composite

Mechanically alloyed Al₆₅Cu₂₀Ti₁₅ amorphous alloy powder with or without 10 wt% nano-TiO₂ dispersion was consolidated by isothermal spark plasma sintering in the range 200–500 °C with pressure up to 50 MPa. Selected samples were separately cold compacted with 50 MPa pressure and sintered at 500 °C using controlled atmosphere resistance and microwave heating furnaces. Phase and microstructural evolution at appropriate stages of mechanical alloying/blending and sintering was monitored by X-ray diffraction and scanning and transmission electron microscopy. Measurement and comparison of relevant properties (density/porosity, microhardness and yield strength) of the sintered compacts suggest that spark plasma sintering is the most appropriate technique for developing nano-TiO₂ dispersed amorphous/nanocrystalline Al₆₅Cu₂₀Ti₁₅ matrix composite for structural application.     

Nanocrystalline spherical iron–nickel (Fe–Ni) alloy particles prepared by ultrasonic spray pyrolysis and hydrogen reduction (USP-HR)

FeCl₂ and NiCl₂ were used for synthesis of nanocrystalline spherical Fe–Ni alloy particles by ultrasonic spray pyrolysis and hydrogen reduction (USP-HR). Spherical ultrafine Fe–Ni particles were obtained by USP of aqueous solutions of iron–nickel chloride followed by thermal decomposition of generated aerosols in hydrogen atmosphere. Particle sizes of the produced Fe–Ni particles can be controlled by the change of the concentration of an initial solution. The effect of the precursor solution in the range of 0.05, 0.1, 0.2 and 0.4 M on the morphology and crystallite size of the Fe–Ni alloy particles are investigated under the conditions of 1.5 h running time, 900 °C reduction temperature, and 1.0 L/min H₂ volumetric flow rate. X-ray diffraction (XRD) studies and Scherrer crystallite size calculations show that the crystalline size was nearly 28 nm. Energy dispersive spectroscopy (EDS) was performed to determine the chemical composition of the particles. Transmission electron microscope (TEM) was used to confirm the crystalline size, that was determined using XRD results. Scanning electron microscopy (SEM) observations reveal that the precursor solution strongly influences the particle size of the synthesized Fe–Ni alloy particles. Spherical nanocrystalline Fe–Ni alloy particles in the range of 80 and 878 nm were obtained at 900 °C.     

A convenient thermal reduction–nitridation route to nanocrystalline molybdenum nitride (Mo2N)

Nanocrystalline molybdenum nitride (γ-Mo₂N) was synthesized via a thermal reduction–nitridation route by the reaction of metallic sodium with anhydrous molybdenum pentachloride and ammonium chloride in an autoclave at 550 °C. X-ray powder diffraction pattern indicated that the product was cubic Mo₂N, and the cell constant was a = 4.161 Å. Scanning electron microscopy image showed that it consisted of particles with an average size of about 30 nm. The product was also studied by BET and TGA. It had good thermal stability and oxidation resistance below 400 °C in air.