978 resultados para MOLYBDENA-ALUMINA CATALYSTS


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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

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Pós-graduação em Química - IQ

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Machining is one of the most commonly manufacturing processes used in the modern world, consuming millions of dollars annually. Because of this, it is crucial for the automotive industry to reduce costs on their heat-resistant alloy machining processes, such as compacted graphite iron (CGI), which has shown an increasing trend of its application in diesel engine blocks, brakes disks, among other applications, due to its superior mechanical properties to gray cast iron. Despite this advantage, its use is still limited due to its difficulty of machining, moreover, cutting tools are displayed as the main factor in increasing the machining cost. Seeking an alternative to a better machinability of CGI, this paper aims to study two types of ceramic tools developed in Brazil, and benchmark their performance by dry turning. For this, were used CGI class 450 and two tools: ceramic of silicon nitride (Si3N4) and alumina-based (Al2O3), with a cutting speed (Vc) of 300, 400 and 500 m / min; feed (f) of 0.2 mm / rev and depth of cut (ap) of 0.5 mm, using three replicates and starting with new cutting edges. The results showed that the Al2O3 tool had the best performance in Vc of 500 m / min, while the Si3N4 tool had the best results in Vc of 300 m / min. This can be explained by the tool of Si3N4 based include soft intergranular phase, called amorphous, while alumina has higher abrasion resistance due to its high refractoriness. The results make it clear that the tools have significant potential for machining of compacted graphite iron, being necessary a strict control of the cutting parameters used

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Machining is one of the most commonly manufacturing processes used in the modern world, consuming millions of dollars annually. Because of this, it is crucial for the automotive industry to reduce costs on their heat-resistant alloy machining processes, such as compacted graphite iron (CGI), which has shown an increasing trend of its application in diesel engine blocks, brakes disks, among other applications, due to its superior mechanical properties to gray cast iron. Despite this advantage, its use is still limited due to its difficulty of machining, moreover, cutting tools are displayed as the main factor in increasing the machining cost. Seeking an alternative to a better machinability of CGI, this paper aims to study two types of ceramic tools developed in Brazil, and benchmark their performance by dry turning. For this, were used CGI class 450 and two tools: ceramic of silicon nitride (Si3N4) and alumina-based (Al2O3), with a cutting speed (Vc) of 300, 400 and 500 m / min; feed (f) of 0.2 mm / rev and depth of cut (ap) of 0.5 mm, using three replicates and starting with new cutting edges. The results showed that the Al2O3 tool had the best performance in Vc of 500 m / min, while the Si3N4 tool had the best results in Vc of 300 m / min. This can be explained by the tool of Si3N4 based include soft intergranular phase, called amorphous, while alumina has higher abrasion resistance due to its high refractoriness. The results make it clear that the tools have significant potential for machining of compacted graphite iron, being necessary a strict control of the cutting parameters used

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Ion implantation of metal species into insulators provides a tool for the formation of thin, electrically conducting, surface layers with experimenter-controlled resistivity. High energy implantation of Pt and Ti into alumina accelerator components has been successfully employed to control high voltage surface breakdown in a number of cases. In the work described here we have carried out some basic investigations related to the origin of this phenomenon. By comparison of the results of alumina implanted with Ti at 75 keV with the results of prior investigations of polymers implanted with Pt at 49 eV and Au at 67 eV, we describe a physical model of the effect based on percolation theory and estimate the percolation parameters for the Ti-alumina composite. We estimate that the percolation dose threshold is about 4 x 10(16) cm(-2) and the maximum dose for which the system remains an insulator-conductor composite is about 10 x 10(16) cm(-2). The saturation electrical conductivity is estimated to be about 50 S/m. We conclude that the observed electrical conductivity properties of Ti-implanted alumina can be satisfactorily described by percolation theory. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3697900]

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In this paper, Co/CeO2 catalysts, with different cobalt contents were prepared by the polymeric precursor method and were evaluated for the steam reforming of ethanol. The catalysts were characterized by N-2 physisorption (BET method), X-ray diffraction (XRD), UV-visible diffuse reflectance, temperature programmed reduction analysis (TPR) and field emission scanning electron microscopy (FEG-SEM). It was observed that the catalytic behavior could be influenced by the experimental conditions and the nature of the catalyst employed. Physical-chemical characterizations revealed that the cobalt content of the catalyst influences the metal-support interaction which results in distinct catalyst performances. The catalyst with the highest cobalt content showed the best performance among the catalysts tested, exhibiting complete ethanol conversion, hydrogen selectivity close to 66% and good stability at a reaction temperature of 600 degrees C. (c) 2012 Elsevier B.V. All rights reserved.

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Catalysts containing mixtures of NiO, MgO and ZrO2 were synthesized by the polymerization method. They were characterized by X-ray diffraction (XRD), physisorption of N-2 (BET), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES), and then tested in the partial oxidation of methane (POM) in the presence of air (2CH(4):1O(2)) at 750 degrees C for 6 h. Among the ternary oxides, the catalyst with 40 mol% MgO showed the highest conversion rates in the catalytic processes, but also the highest carbon deposition values (48 mmol h (1)). The greater the amount of NiO-MgO solid solution formed, the higher was the conversion rate of reactants (CH4), peaking at 40 mol% of MgO. Catalysts with lower Ni content on the surface achieved a high rate of CH4 conversion into synthesis gas (H-2 + CO). The formation of more NiO-MgO solid solution seemed to inhibit the deactivation of Ni degrees during reaction. The values of the H-2/CO product ratio were generally found to be slightly lower than stoichiometric. (C) 2012 Elsevier Ltd. All rights reserved.

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The aim of this work was to perform a systematic study of the parameters that can influence the composition, morphology, and catalytic activity of PtSn/C nanoparticles and compare two different methods of nanocatalyst preparation, namely microwave-assisted heating (MW) and thermal decomposition of polymeric precursors (DPP). An investigation of the effects of the reducing and stabilizing agents on the catalytic activity and morphology of Pt75Sn25/C catalysts prepared by microwave-assisted heating was undertaken for optimization purposes. The effect of short-chain alcohols such as ethanol, ethylene glycol, and propylene glycol as reducing agents was evaluated, and the use of sodium acetate and citric acid as stabilizing agents for the MW procedure was examined. Catalysts obtained from propylene glycol displayed higher catalytic activity compared with catalysts prepared in ethylene glycol. Introduction of sodium acetate enhanced the catalytic activity, but this beneficial effect was observed until a critical acetate concentration was reached. Optimization of the MW synthesis allowed for the preparation of highly dispersed catalysts with average sizes lying between 2.0 and 5.0 nm. Comparison of the best catalyst prepared by MW with a catalyst of similar composition prepared by the polymeric precursors method showed that the catalytic activity of the material can be improved when a proper condition for catalyst preparation is achieved. (C) 2012 Elsevier B.V. All rights reserved.

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In this paper, we discuss the effects of catalyst load with respect to carbon powder for several Pt and Pb-based catalysts, using formic acid as a model molecule. The discussion is based on electrochemical tests, a complete morphological investigation and theoretical calculations. We show that the Pt and Pb-based catalysts presented activity in formic acid oxidation at very low catalyst loads (e.g., 0.5% in respect to the carbon content). Physical characterisations demonstrate that the electrodes are composed of separated phases of Pt and lead distributed in Pt nanometric-sized islands that are heterogeneously dispersed on the carbon support and Pb ultra-small particles homogeneously distributed throughout the entire carbon surface, as demonstrated by the microscopy studies. At high catalyst loads, very large clusters of Pb(x)O(y) could be observed. Electrochemical tests indicated an increase in the apparent resistance of the system (by a factor of 19.7 Omega) when the catalyst load was increased. The effect of lead in the materials was also studied by theoretical calculations (OFT). The main conclusion is that the presence of Pb atoms in the catalyst can improve the adsorption of formic acid in the catalytic system compared with a pure Pt-based catalyst. (C) 2011 Elsevier B.V. All rights reserved.

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Aim. This work tested the effect of the addition of Al2O3/GdAlO3 longitudinal fibers in different contents to veneering porcelain of two dental all ceramic systems. Methods: Fibers (0.5 mm diameter) obtained by the Laser Heated Pedestal Growth (LHPG) method were added to bar-shaped specimens made by veneer porcelain (monolayers) or both the veneer and the core ceramic (bilayers) of two all-ceramic systems: In-Ceram Alumina - glass infiltrated alumina composite (GIA) and In-Ceram 2000 AL Cubes - alumina polycrystal (AP) (VITA Zahnfabrik). The longitudinal fibers were added to veneering porcelain (VM7) in two different proportions: 10 or 17 vol%. The bars were divided into nine experimental conditions (n = 10) according to material used: VM7 porcelain monolayers, VM7/GIA, VM7/AP; and according to the amount of fibers within the porcelain layer: no fibers, 10 vol% or 17 vol%. After grinding and polishing the specimens were submitted to a three point bending test (crosshead speed = 0.5 mm/min) with porcelain positioned at tensile side. Data were analyzed by means of one-way ANOVA and a Tukey's test (alpha = 5%). Scanning electronic microscopy (SEM) was conducted for fractographic analysis. Results. Regarding the groups without fiber addition, VM7/AP showed the highest flexural strength (MPa), followed by VM7/GIA and VM7 monolayers. The addition of fibers led to a numerical increase in flexural strength for all groups. For VM7/GIA bilayers the addition of 17 vol% of fibers resulted in a significant 48% increase in the flexural strength compared to the control group. Fractographic analysis revealed that the crack initiation site was in porcelain at the tensile surface. Cracks also propagated between fibers before heading for the alumina core. Conclusions. The addition of 17 vol% of Al2O3/GdAlO3 longitudinal fibers to porcelain/glass infiltrated alumina bilayers significantly improved its flexural strength. 10 vol% or 17 vol% of fibers inclusion increased the flexural strength for all groups. (C) 2011 Elsevier Ltd. All rights reserved.

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The synthesis of nickel catalysts for industrial applications is relatively simple; however, nickel oxidation is usually difficult to avoid, which makes it challenging to optimize catalytic activities, metal loadings, and high-temperature activation steps. A robust, oxidation-resistant and very active nickel catalyst was prepared by controlled decomposition of the organometallic precursor [bis(1,5-cyclooctadiene)nickel(0)], Ni(COD)(2), over silica-coated magnetite (Fe3O4@SiO2). The sample is mostly Ni(0), and surface oxidized species formed after exposure to air are easily reduced in situ during hydrogenation of cyclohexene under mild conditions recovering the initial activity. This unique behavior may benefit several other reactions that are likely to proceed via Ni heterogeneous catalysis.

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One of the key objectives in fuel cell technology is to reduce Pt loading by the improvement of its catalytic activity towards alcohol oxidation. Here, a sol-gel based method was used to prepare ternary and quaternary carbon supported nanoparticles by combining Pt-Ru with Mo, Ta, Pb, Rh or Ir, which were used as electro-catalysts for the methanol and ethanol oxidation reactions in acid medium. Structural characterization performed by XRD measurements revealed that crystalline structures with crystallites ranging from 2.8 to 4.1 nm in size and with different alloy degrees were produced. Tantalum and lead deposited as a heterogeneous mixture of oxides with different valences resulting in materials with complex structures. The catalysts activities were evaluated by cyclic voltammetry and by Tafel plots and the results showed that the activity towards methanol oxidation was highly dependent of the alloy degree, while for ethanol the presence of a metal capable to promote the break of C-C bond, such as Rh, was necessary for a good performance. Additionally, the catalysts containing of TaOx or PbOx resulted in the best materials due to different effects: the hi-functional mechanism promoted by TaOx and a better dispersion of the catalysts constituents promoted by PbOx. (C) 2012 Elsevier B.V. All rights reserved.

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Catalysts containing 10%Co supported on CexZr1-xO2 (0 < x < 1) were applied to ethanol steam reforming reactions. The catalysts were characterized by Raman spectroscopy, XANES-H-2 and DRS-UV-Vis. The catalytic tests were conducted at 673, 773 and 873 K, with molar ratios of H2O:ethanol = 3:1. The ethanol conversion and H-2 selectivity were temperature dependent and the association of CeO2 with ZrO2 in the support led to show a low formation of CO, due to the higher mobility of oxygen. (C) 2012 Elsevier B.V. All rights reserved.

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The aim of this Account is to provide an overview of our current research activities on the design and modification of superparamagnetic nanomaterials for application in the field of magnetic separation and catalysis. First, an introduction of magnetism and magnetic separation is done. Then, the synthetic strategies that have been developed for generating superparamagnetic nanoparticles spherically coated by silica and other oxides, with a focus on well characterized systems prepared by methods that generate samples of high quality and easy to scale- up, are discussed. A set of magnetically recoverable catalysts prepared in our research group by the unique combination of superparamagnetic supports and metal nanoparticles is highlighted. This Account is concluded with personal remarks and perspectives on this research field.

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Objectives: To compare the biomechanical fixation and histomorphometric parameters between two implant surfaces: non-washed resorbable blasting media (NWRBM) and alumina-blasted/acid-etched (AB/AE), in a dog model. Material and methods: The surface topography was assessed by scanning electron microscopy, optical interferometry and chemistry by X-ray photoelectron spectroscopy (XPS). Six beagle dogs of similar to 1.5 years of age were utilized and each animal received one implant of each surface per limb (distal radii sites). After a healing period of 3 weeks, the animals were euthanized and half of the implants were biomechanically tested (removal torque) and the other half was referred to nondecalcified histology processing. Histomorphometric analysis considered bone-to-implant contact (BIC) and bone area fraction occupancy (BAFO). Following data normality check with the Kolmogorov-Smirnov test, statistical analysis was performed by paired t-tests at 95% level of significance. Results: Surface roughness parameters Sa (average surface roughness) and Sq (mean root square of the surface) were significantly lower for the NWRBM compared with AB/ AE. The XPS spectra revealed the presence of Ca and P in the NWRBM. While no significant differences were observed for both BIC and BAFO parameters (P>0.35 and P>0.11, respectively), a significantly higher level of torque was observed for the NWRBM group (P = 0.01). Bone morphology was similar between groups, which presented newly formed woven bone in proximity with the implant surfaces. Conclusion: A significant increase in early biomechanical fixation was observed for implants presenting the NWRBM surface.