Pt-based hollow nanoparticles were investigated as electrocatalysts for the oxygen reduction reac... more Pt-based hollow nanoparticles were investigated as electrocatalysts for the oxygen reduction reaction (ORR) in acid electrolyte. The electrocatalysts were synthesized via Ni or Co diffusion/dissolution, induced by the vacancy-mediated Kirkendall effect, during electrochemical potential cycling of Ni@Pt and Co@Pt core-shell nanoparticles in acid media. The nanoparticles were characterized by high resolution transmission electron microscopy, in situ X-ray absorption spectroscopy and X-ray diffraction measurements. The results show substantial differences in nanoparticle structure/composition and in the activity for the ORR, depending on the nature of the non-noble metal in the nanoparticle core. The Pt hollow nanostructures showed higher specific catalytic activity than that of the state-of-the-art Pt/C electrocatalyst. This was attributed to three main effects: (i) hollow-induced lattice contraction in the multilayer Pt shells, (ii) mismatch-induced lattice contraction of the thick Pt shell by the remaining Ni or Co atoms and (iii) a ligand effect, due to the electronic interaction of Pt with the remaining Ni or Co atoms in the Pt multilayers of the hollow structure. These three effects caused a Pt d-band center down-shift, which decreased the adsorption strength of oxygenated reaction intermediates and spectators, thus increasing the ORR rate.
We report for the first time effects of altering the amount of Pd supported on high-surface area ... more We report for the first time effects of altering the amount of Pd supported on high-surface area carbon (Pd/C). Cyclic voltammetry in 0.1 M H 2 SO 4 using Pd/C electrocatalysts with distinct metal-to-carbon ratio, Pd black and a Pd wire electrodes reveals unambiguously metal loading effects, such as a decrease in the peak potential for the reduction of palladium oxide (PdO), an increase of the charges of desorption of hydrogen, and formation/ reduction of PdO with the Pd content. Such effects need to be taken into account when designing Pd-based Fuel Cells electrocatalysts.
Abstract In contrast to the expected behavior for metallic nanoparticles (NPs) where a contractio... more Abstract In contrast to the expected behavior for metallic nanoparticles (NPs) where a contraction of the lattice parameter occurs as the crystal size decreases, palladium NPs experience both lattice contraction and dilation. Such dual behavior has not been discussed in detail so far but it is of paramount importance given Pd is widely used in catalysis, electrocatalysis, gas sensors, and many other areas, and all these applications are strongly affected by structural change at the NP surface. Herein, we address the issue investigating both lattice contraction and dilation of Pd NPs synthesized by chemical reduction and also by compiling several results already published in the literature. It was found that when the synthesis is carried out in the absence of a substrate, dilation of the unit cell was detected; in contrast, if the synthesis is carried out in the presence of a substrate (carbon and RuO2), lattice contraction is observed. Lattice dilation was ascribed by the incorporation of interstitial atoms, such as H, C, O, which is suppressed on supported catalysts likely by a spillover effect from the metallic phase to the substrate, thus yielding to lattice contraction. Furthermore, we propose a new methodology to assess the total variation of the lattice parameter in the case of lattice dilation taking into consideration the expected contracted values for nanosized particles. To illustrate the importance of such new approach, it was possible to show a particle-size dependence of the carbon solubility on Pd nanostructures based on literature data.
Halide perovskite CsPbBr 3 quantum dots (QDs) were synthesized via supersaturated recrystallizati... more Halide perovskite CsPbBr 3 quantum dots (QDs) were synthesized via supersaturated recrystallization process and deposited on the surface of TiO 2 microtubes forming local-nanoheterostructures. Structural, morphological, and optical characterizations confirm the formation of heterostructures comprised of TiO 2 microtube decorated with green-emitting CsPbBr 3 nanocrystals. Optical characterizations reveal the presence of two band gap energies corresponding to CsPbBr 3 (2.34 eV) and rutile-TiO 2 (2.97 eV). Time-resolved photoluminescence decays indicate different charge dynamics when comparing both samples, revealing the interaction of CsPbBr 3 QDs with the microtube surface and thus confirming the formation of local nanoheterostructures. The voltage-current measurements in the dark show an abrupt decrease in the electrical resistivity of the CsPbBr 3 /TiO 2 heterostructure reaching almost 95% when compared with the pristine TiO 2 microtube. This significant increasing in the electrical conductivity is associated with charge transfer from perovskite nanocrystals into the semiconductor microtube which can be used to fine tune in its electronic properties. Besides controlling the electrical conductivity, decoration with semiconducting nanocrystals makes the hollow heterostructure photoluminescent which can be classified as a multifunctionalization in a single device.
The aim of the present work was to study the surface chemistry, microstructure and local corrosio... more The aim of the present work was to study the surface chemistry, microstructure and local corrosion processes at the decarburized layer of the SAE 9254 automotive spring steel. The samples were austenitized at 850 and 900 °C, and oil quenched. The microstructure was investigated using confocal laser scanning microscopy and scanning electron microscopy. The surface chemistry was analyzed by X-ray photoelectron spectroscopy. The local electrochemical activity was probed by scanning electrochemical microscopy. Microstructural characterization and XPS analysis indicate a dependence of the local electrochemical processes with the steel microconstituents and Si oxides in the decarburized layer.
A new series of hydrogen-bonded metallotecton networks 6−9 of the general formula [M(2) 2 (NO 3) ... more A new series of hydrogen-bonded metallotecton networks 6−9 of the general formula [M(2) 2 (NO 3) 2 ] were obtained from the reaction of 6-pyridin-2-yl-[1,3,5]-triazine-2,4-diamine 2 with transition-metal ions [M: Co(II), Ni(II), Cu(II), and Zn(II)]. Their supramolecular networks and associated properties were characterized by single-crystal and powder X-ray diffraction, IR, solid-state UV−vis spectroscopy, and thermogravimetric analysis associated with differential scanning calorimetry. On the basis of standard patterns of coordination involving 2,2′-bipyridine and simple derivatives, compound 2 binds transition-metal ions with predictable constitution and the diaminotriazinyl (DAT) groups serve orthogonally to ensure the intermetallotecton interactions by hydrogen bonding according to well-established motifs I−III. As expected, compound 2 formed octahedral 2:1 metallotectons with M(NO 3) 2 , and further self-assembled by hydrogen bonding of the DAT groups to produce pure, crystalline, homogeneous, and thermally stable materials. In these structures, nitrate counterions also play an important role in the cohesion of intermetallotectons to form two-dimensional and three-dimensional networks. These results illustrated the effectiveness of the synthetic approach to create a wide range of novel ordered materials with controllable architectures and tunable properties achieved by varying the central metal ion. Crystal morphologies of 6−9 were also investigated by scanning electron microscopy and calculation using Bravais−Friedel−Donnay−Harker method from their single-crystal structure.
The in situ formation of nanostructured aluminum hydroxides on the surface of alumina particles, ... more The in situ formation of nanostructured aluminum hydroxides on the surface of alumina particles, which can work as inorganic binder, was reported in this paper. The effect of the suspension pH during milling of alumina powder and subsequent hydrothermal treatment for the hydroxide formation and microstructure was depicted. Under acidic pH condition, the formation of hydroxides was not observed. When the pH of suspension changed from acidic to basic during milling, bayerite [Al(OH)3] nanoparticles were formed, but only a fraction of this hydroxide was converted to boehmite (AlOOH) during subsequent hydrothermal treatment. The aluminum hydroxide and oxyhydroxide formed in this condition improved the smoothness of extruded rods and the strength of presintered segments. For the powder milled under basic pH condition, the mechanochemically formed bayerite was completely converted into boehmite nanoparticles during the hydrothermal treatment. The presence of boehmite nanoparticles contrib...
ABSTRACT Magnesium (magnesium hydride) nanocomposites have been widely investigated for hydrogen ... more ABSTRACT Magnesium (magnesium hydride) nanocomposites have been widely investigated for hydrogen storage. As a general feature, these nanocomposites use high purity (expensive) catalysts to improve the hydrogen sorption kinetics of magnesium. In this manuscript we tested a low cost raw material (ferroniobium alloy) as catalyst for hydriding/dehydriding magnesium nanocomposites. The MgH2 + ferroniobium nanocomposites were synthesized by high-energy ball-milling and characterized by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray microanalysis. The hydrogen absorption and desorption curves were measured in an automated Sieverts' type apparatus. We observed that ferroniobium alloy is an effective catalyst for hydrogen sorption, mainly with ferroniobium as coarse granulates. The hydrogen sorption behavior of MgH2 + ferroniobium nanocomposites was compared to those of Mg + high purity Nb, Fe or (Fe + Nb) catalysts. It was found that the high purity metals are slightly more active catalysts than ferroniobium alloy for hydrogen sorption in Mg. This behavior is discussed regarding the thermodynamic features of the catalyst/matrix nano-interfaces formed during ball-milling.
Transition metal carbides are interesting materials with a singular combination of properties, su... more Transition metal carbides are interesting materials with a singular combination of properties, such as high melting points, high hardness, good transport properties and relatively low costs, which makes them excellent candidates for several technological applications. The possible applications of NbC carbide remained unexplored as it was in the past expensive and available in limited volumes. In order to guide investigations of the applicability of NbC, a deeper understanding of the physical properties of this carbide is fundamental. In this review paper, key physical properties of NbC are compiled with emphasis on its chemical bonding, a careful description of the C-Nb phase diagram, the phases formed and the crystal structures. Thermal properties are discussed and correlated with the intrinsic and extrinsic features of NbC. Finally, elastic properties are discussed.
In this paper we report that cold rolling could drastically improve hydrogen desorption kinetics ... more In this paper we report that cold rolling could drastically improve hydrogen desorption kinetics under microwave irradiation. Samples of metal hydride powders (TiH2, ZrH2, and MgH2) in as-received conditions and after cold rolling were microwave irradiated in a vacuum using a simple experimental setup. After irradiation, the samples were characterized by X-ray diffraction in other to evaluate the effectiveness of microwave heating. The diffraction patterns indicated that only MgH2 could be fully decomposed (dehydrided) in the as received state. TiH2 was only partially decomposed while no decomposition was observed for ZrH2. However, cold rolling the hydride powders prior to microwave heating led to a significant improvement of hydride decomposition, resulting in the complete dehydriding of TiH2 and extensive dehydriding of ZrH2. These results clearly indicated the positive effects of cold rolling on the microwave assisted desorption of the investigated binary hydrides.
ABSTRACT Metal matrix composites have been considered promising candidates as nuclear fuels for p... more ABSTRACT Metal matrix composites have been considered promising candidates as nuclear fuels for pressurized water reactors and also for nuclear waste management. Among others, Zircaloy is considered an excellent alternative for metallic matrix in such composites due to its excellent mechanical properties, high thermal conductivity and high corrosion resistance at operating temperatures. For manufacturing these fuels, a necessary step is the production of Zircaloy powder to be used as raw material. A feasible route to produce powders of refractory metals and alloys like Zircaloy is the hydriding and dehydriding process. For this type of processing route, hydrogen absorption and desorption should be performed at the lowest temperature and pressure possible in order to reduce the processing costs. In this paper, we investigated the hydrogen sorption kinetics of Zircaloy and the effect of cold rolling on the reaction rate. It was found that cold rolling greatly increases the hydrogenation kinetics and drastically reduces the dehydrogenation temperature.
Pt-based hollow nanoparticles were investigated as electrocatalysts for the oxygen reduction reac... more Pt-based hollow nanoparticles were investigated as electrocatalysts for the oxygen reduction reaction (ORR) in acid electrolyte. The electrocatalysts were synthesized via Ni or Co diffusion/dissolution, induced by the vacancy-mediated Kirkendall effect, during electrochemical potential cycling of Ni@Pt and Co@Pt core-shell nanoparticles in acid media. The nanoparticles were characterized by high resolution transmission electron microscopy, in situ X-ray absorption spectroscopy and X-ray diffraction measurements. The results show substantial differences in nanoparticle structure/composition and in the activity for the ORR, depending on the nature of the non-noble metal in the nanoparticle core. The Pt hollow nanostructures showed higher specific catalytic activity than that of the state-of-the-art Pt/C electrocatalyst. This was attributed to three main effects: (i) hollow-induced lattice contraction in the multilayer Pt shells, (ii) mismatch-induced lattice contraction of the thick Pt shell by the remaining Ni or Co atoms and (iii) a ligand effect, due to the electronic interaction of Pt with the remaining Ni or Co atoms in the Pt multilayers of the hollow structure. These three effects caused a Pt d-band center down-shift, which decreased the adsorption strength of oxygenated reaction intermediates and spectators, thus increasing the ORR rate.
We report for the first time effects of altering the amount of Pd supported on high-surface area ... more We report for the first time effects of altering the amount of Pd supported on high-surface area carbon (Pd/C). Cyclic voltammetry in 0.1 M H 2 SO 4 using Pd/C electrocatalysts with distinct metal-to-carbon ratio, Pd black and a Pd wire electrodes reveals unambiguously metal loading effects, such as a decrease in the peak potential for the reduction of palladium oxide (PdO), an increase of the charges of desorption of hydrogen, and formation/ reduction of PdO with the Pd content. Such effects need to be taken into account when designing Pd-based Fuel Cells electrocatalysts.
Abstract In contrast to the expected behavior for metallic nanoparticles (NPs) where a contractio... more Abstract In contrast to the expected behavior for metallic nanoparticles (NPs) where a contraction of the lattice parameter occurs as the crystal size decreases, palladium NPs experience both lattice contraction and dilation. Such dual behavior has not been discussed in detail so far but it is of paramount importance given Pd is widely used in catalysis, electrocatalysis, gas sensors, and many other areas, and all these applications are strongly affected by structural change at the NP surface. Herein, we address the issue investigating both lattice contraction and dilation of Pd NPs synthesized by chemical reduction and also by compiling several results already published in the literature. It was found that when the synthesis is carried out in the absence of a substrate, dilation of the unit cell was detected; in contrast, if the synthesis is carried out in the presence of a substrate (carbon and RuO2), lattice contraction is observed. Lattice dilation was ascribed by the incorporation of interstitial atoms, such as H, C, O, which is suppressed on supported catalysts likely by a spillover effect from the metallic phase to the substrate, thus yielding to lattice contraction. Furthermore, we propose a new methodology to assess the total variation of the lattice parameter in the case of lattice dilation taking into consideration the expected contracted values for nanosized particles. To illustrate the importance of such new approach, it was possible to show a particle-size dependence of the carbon solubility on Pd nanostructures based on literature data.
Halide perovskite CsPbBr 3 quantum dots (QDs) were synthesized via supersaturated recrystallizati... more Halide perovskite CsPbBr 3 quantum dots (QDs) were synthesized via supersaturated recrystallization process and deposited on the surface of TiO 2 microtubes forming local-nanoheterostructures. Structural, morphological, and optical characterizations confirm the formation of heterostructures comprised of TiO 2 microtube decorated with green-emitting CsPbBr 3 nanocrystals. Optical characterizations reveal the presence of two band gap energies corresponding to CsPbBr 3 (2.34 eV) and rutile-TiO 2 (2.97 eV). Time-resolved photoluminescence decays indicate different charge dynamics when comparing both samples, revealing the interaction of CsPbBr 3 QDs with the microtube surface and thus confirming the formation of local nanoheterostructures. The voltage-current measurements in the dark show an abrupt decrease in the electrical resistivity of the CsPbBr 3 /TiO 2 heterostructure reaching almost 95% when compared with the pristine TiO 2 microtube. This significant increasing in the electrical conductivity is associated with charge transfer from perovskite nanocrystals into the semiconductor microtube which can be used to fine tune in its electronic properties. Besides controlling the electrical conductivity, decoration with semiconducting nanocrystals makes the hollow heterostructure photoluminescent which can be classified as a multifunctionalization in a single device.
The aim of the present work was to study the surface chemistry, microstructure and local corrosio... more The aim of the present work was to study the surface chemistry, microstructure and local corrosion processes at the decarburized layer of the SAE 9254 automotive spring steel. The samples were austenitized at 850 and 900 °C, and oil quenched. The microstructure was investigated using confocal laser scanning microscopy and scanning electron microscopy. The surface chemistry was analyzed by X-ray photoelectron spectroscopy. The local electrochemical activity was probed by scanning electrochemical microscopy. Microstructural characterization and XPS analysis indicate a dependence of the local electrochemical processes with the steel microconstituents and Si oxides in the decarburized layer.
A new series of hydrogen-bonded metallotecton networks 6−9 of the general formula [M(2) 2 (NO 3) ... more A new series of hydrogen-bonded metallotecton networks 6−9 of the general formula [M(2) 2 (NO 3) 2 ] were obtained from the reaction of 6-pyridin-2-yl-[1,3,5]-triazine-2,4-diamine 2 with transition-metal ions [M: Co(II), Ni(II), Cu(II), and Zn(II)]. Their supramolecular networks and associated properties were characterized by single-crystal and powder X-ray diffraction, IR, solid-state UV−vis spectroscopy, and thermogravimetric analysis associated with differential scanning calorimetry. On the basis of standard patterns of coordination involving 2,2′-bipyridine and simple derivatives, compound 2 binds transition-metal ions with predictable constitution and the diaminotriazinyl (DAT) groups serve orthogonally to ensure the intermetallotecton interactions by hydrogen bonding according to well-established motifs I−III. As expected, compound 2 formed octahedral 2:1 metallotectons with M(NO 3) 2 , and further self-assembled by hydrogen bonding of the DAT groups to produce pure, crystalline, homogeneous, and thermally stable materials. In these structures, nitrate counterions also play an important role in the cohesion of intermetallotectons to form two-dimensional and three-dimensional networks. These results illustrated the effectiveness of the synthetic approach to create a wide range of novel ordered materials with controllable architectures and tunable properties achieved by varying the central metal ion. Crystal morphologies of 6−9 were also investigated by scanning electron microscopy and calculation using Bravais−Friedel−Donnay−Harker method from their single-crystal structure.
The in situ formation of nanostructured aluminum hydroxides on the surface of alumina particles, ... more The in situ formation of nanostructured aluminum hydroxides on the surface of alumina particles, which can work as inorganic binder, was reported in this paper. The effect of the suspension pH during milling of alumina powder and subsequent hydrothermal treatment for the hydroxide formation and microstructure was depicted. Under acidic pH condition, the formation of hydroxides was not observed. When the pH of suspension changed from acidic to basic during milling, bayerite [Al(OH)3] nanoparticles were formed, but only a fraction of this hydroxide was converted to boehmite (AlOOH) during subsequent hydrothermal treatment. The aluminum hydroxide and oxyhydroxide formed in this condition improved the smoothness of extruded rods and the strength of presintered segments. For the powder milled under basic pH condition, the mechanochemically formed bayerite was completely converted into boehmite nanoparticles during the hydrothermal treatment. The presence of boehmite nanoparticles contrib...
ABSTRACT Magnesium (magnesium hydride) nanocomposites have been widely investigated for hydrogen ... more ABSTRACT Magnesium (magnesium hydride) nanocomposites have been widely investigated for hydrogen storage. As a general feature, these nanocomposites use high purity (expensive) catalysts to improve the hydrogen sorption kinetics of magnesium. In this manuscript we tested a low cost raw material (ferroniobium alloy) as catalyst for hydriding/dehydriding magnesium nanocomposites. The MgH2 + ferroniobium nanocomposites were synthesized by high-energy ball-milling and characterized by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray microanalysis. The hydrogen absorption and desorption curves were measured in an automated Sieverts' type apparatus. We observed that ferroniobium alloy is an effective catalyst for hydrogen sorption, mainly with ferroniobium as coarse granulates. The hydrogen sorption behavior of MgH2 + ferroniobium nanocomposites was compared to those of Mg + high purity Nb, Fe or (Fe + Nb) catalysts. It was found that the high purity metals are slightly more active catalysts than ferroniobium alloy for hydrogen sorption in Mg. This behavior is discussed regarding the thermodynamic features of the catalyst/matrix nano-interfaces formed during ball-milling.
Transition metal carbides are interesting materials with a singular combination of properties, su... more Transition metal carbides are interesting materials with a singular combination of properties, such as high melting points, high hardness, good transport properties and relatively low costs, which makes them excellent candidates for several technological applications. The possible applications of NbC carbide remained unexplored as it was in the past expensive and available in limited volumes. In order to guide investigations of the applicability of NbC, a deeper understanding of the physical properties of this carbide is fundamental. In this review paper, key physical properties of NbC are compiled with emphasis on its chemical bonding, a careful description of the C-Nb phase diagram, the phases formed and the crystal structures. Thermal properties are discussed and correlated with the intrinsic and extrinsic features of NbC. Finally, elastic properties are discussed.
In this paper we report that cold rolling could drastically improve hydrogen desorption kinetics ... more In this paper we report that cold rolling could drastically improve hydrogen desorption kinetics under microwave irradiation. Samples of metal hydride powders (TiH2, ZrH2, and MgH2) in as-received conditions and after cold rolling were microwave irradiated in a vacuum using a simple experimental setup. After irradiation, the samples were characterized by X-ray diffraction in other to evaluate the effectiveness of microwave heating. The diffraction patterns indicated that only MgH2 could be fully decomposed (dehydrided) in the as received state. TiH2 was only partially decomposed while no decomposition was observed for ZrH2. However, cold rolling the hydride powders prior to microwave heating led to a significant improvement of hydride decomposition, resulting in the complete dehydriding of TiH2 and extensive dehydriding of ZrH2. These results clearly indicated the positive effects of cold rolling on the microwave assisted desorption of the investigated binary hydrides.
ABSTRACT Metal matrix composites have been considered promising candidates as nuclear fuels for p... more ABSTRACT Metal matrix composites have been considered promising candidates as nuclear fuels for pressurized water reactors and also for nuclear waste management. Among others, Zircaloy is considered an excellent alternative for metallic matrix in such composites due to its excellent mechanical properties, high thermal conductivity and high corrosion resistance at operating temperatures. For manufacturing these fuels, a necessary step is the production of Zircaloy powder to be used as raw material. A feasible route to produce powders of refractory metals and alloys like Zircaloy is the hydriding and dehydriding process. For this type of processing route, hydrogen absorption and desorption should be performed at the lowest temperature and pressure possible in order to reduce the processing costs. In this paper, we investigated the hydrogen sorption kinetics of Zircaloy and the effect of cold rolling on the reaction rate. It was found that cold rolling greatly increases the hydrogenation kinetics and drastically reduces the dehydrogenation temperature.
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