Synthesis, Characterization, and Electrocatalytic Activity toward Methanol Oxidation of Carbon-Supported Pt(x)-(RuO(2)-M)(1-x) Composite Ternary Catalysts (M = CeO(2), MoO(3), or PbO(x))

Carbon-supported platinum is commonly used as an anode electrocatalyst in low-temperature fuel cells fueled with methanol. The cost of Pt and the limited world supply are significant barriers for the widespread use of this type of fuel cell. Moreover, Pt used as anode material is readily poisoned by carbon monoxide produced as a byproduct of the alcohol oxidation. Although improvements in the catalytic performance for methanol oxidation were attained using Pt-Ru alloys, the state-of-the-art Pt-Ru catalyst needs further improvement because of relatively low catalytic activity and the high cost of noble Pt and Ru. For these reasons, the development of highly efficient ternary platinum-based catalysts is an important challenge. Thus, various compositions of ternary Pt(x)-(RuO(2)-M)(1-x)/C composites (M = CeO(2), MoO(3), or PbO(x)) were developed and further investigated as catalysts for the methanol electro-oxidation reaction. The characterization carried out by X-ray diffraction, energy-dispersive X-ray analysis, transmission electron microscopy, X-ray photoelectron spectroscopy, and cyclic voltammetry point out that the different metallic oxides were successfully deposited on the Pt/C, producing small and well-controlled nanoparticles in the range of 2.8-4.2 nm. Electrochemical experiments demonstrated that the Pt(0.50)(RuO(2)-CeO(2))(0.50)/C composite displays the higher catalytic activity toward the methanol oxidation reaction (lowest onset potential of 207 mV and current densities taken at 450 mV, which are 140 times higher than those at commercial Pt/C), followed by the Pt(0.75)(RuO(2)-MoO(3))(0.25)/C composite. In addition, both of these composites produced low quantities of formic acid and formaldehyde when compared to a commercially available Pt(0.75)-Ru(0.25)/C composite (from E-Tek, Inc.), suggesting that the oxidation of methanol occurs mainly by a pathway that produces CO(2) forming the intermediary CO(ads).

Electronic structure and optical properties of BaMoO(4) powders

Barium molybdate (BaMoO(4)) powders were synthesized by the co-precipitation method and processed in microwave-hydrothermal at 140 degrees C for different times. These powders were characterized by X-ray diffraction (XRD), Fourier transform Raman (FT-Raman), Fourier transform infrared (FT-IR), ultraviolet-visible (UV-vis) absorption spectroscopies and photoluminescence (PL) measurements. XRD patterns and FT-Raman spectra showed that these powders present a scheelite-type tetragonal structure without the presence of deleterious phases. FT-IR spectra exhibited a large absorption band situated at around 850.4 cm(-1), which is associated to the Mo-O antisymmetric stretching vibrations into the [MoO(4)] clusters. UV-vis absorption spectra indicated a reduction in the intermediary energy levels within band gap with the processing time evolution. First-principles quantum mechanical calculations based on the density functional theory were employed in order to understand the electronic structure (band structure and density of states) of this material. The powders when excited with different wavelengths (350 nm and 488 nm) presented variations. This phenomenon was explained through a model based in the presence of intermediary energy levels (deep and shallow holes) within the band gap. (C) 2009 Elsevier B.V. All rights reserved.

Morphology and Blue Photoluminescence Emission of PbMoO(4) Processed in Conventional Hydrothermal

PbMoO(4) micro-octahedrons were prepared by the coprecipitation method at room temperature without the presence of surfactants and processed in a conventional hydrothermal at different temperatures (from 60 to 120 degrees C) for 10 min. These micro-octahedrons were structurally characterized by X-ray diffraction (XRD) and micro-Raman (MR) spectroscopy, and its morphology was investigated by field-emission gun scanning electron microscopy (FEG-SEM). The optical properties were analyzed by ultraviolet-visible (UV-vis) absorption spectroscopy and photoluminescence (PL) measurements. XRD patterns and MR spectra confirmed that the PbMoO(4) micro-octahedrons are characterized by a scheelite-type tetragonal structure. FEG-SEM micrographs points, out that these structures present a polydisperse particle size distribution in consequence of a predominant growth mechanism via aggregation of particles. In addition, it was observed that the hydrothermal conditions favored a spontaneous formation of micro-octahedrons interconnected along a common crystallographic orientation (oriented-attachment), resulting in self-organized structures. An intense blue PL emission at room temperature was observed in these micro-octahedrons when they were excited with a 350 nm wavelength. The origin of the PL emissions as well as its intensity variations are explained by means of a model based on both distorted [MoO(4)] and [PbO(8)] clusters into the lattice.

Effect of Different Solvent Ratios (Water/Ethylene Glycol) on the Growth Process of CaMoO(4) Crystals and Their Optical Properties

In this paper, calcium molybdate (CaMoO(4)) crystals (meso- and nanoscale) were synthesized by the coprecipitation method using different solvent volume ratios (water/ethylene glycol). Subsequently, the obtained suspensions were processed in microwave-assisted hydrothermal/solvothermal systems at 140 degrees C for 1 h. These meso- and nanocrystals processed were characterized by X-ray diffraction (X R I)), Fourier transform Raman (FT-Raman), Fourier transform infrared (FT-IR). ultraviolet visible (UV-vis) absorption spectroscopies, held-emission gun scanning electron microscopy (FEG-SEM). transmission electron microscopy (TEM). and photoluminescence (PL) measurements. X RI) patterns and FT-Raman spectra showed that these meso- and nanocrystals have a scheelite-type tetragonal structure without the presence of deleterious phases. FT-IR spectra exhibited a large absorption band situated at around 827 cm(-1), which is associated with the Mo-O anti-symmetric stretching vibrations into the [MoO(4)] clusters. FEG-SEM micrographs indicated that the ethylene glycol concentration in the aqueous solution plays an important role in the morphological evolution of CaMoO(4) crystals. High-resolution TEM micrographs demonstrated that the mesocrystals consist of several aggregated nanoparticles with electron diffraction patterns of monocrystal. In addition, the differences observed in the selected area electron diffraction patterns of CaMoO(4) crystals proved the coexistence of both nano- and mesostructures, First-principles quantum mechanical calculations based on the density functional theory at the B3LYP level were employed in order to understand the band structure find density of states For the CaMoO(4). UV-vis absorption measurements evidenced a variation in optical band gap values (from 3.42 to 3.72 cV) for the distinct morphologies. The blue and green PI. emissions observed in these crystals were ascribed to the intermediary energy levels arising from the distortions on the [MoO(4)] clusters clue to intrinsic defects in the lattice of anisotropic/isotropic crystals.

Ni catalysts with Mo promoter for methane steam reforming

NiO/Al(2)O(3) catalyst precursors were prepared by simultaneous precipitation, in a Ni:Al molar ratio of 3:1, promoted with Mo oxide (0.05, 0.5, 1.0 and 2.0 wt%). The solids were characterized by adsorption of N(2), XRD, TPR, Raman spectroscopy and XPS, then activated by H(2) reduction and tested for the catalytic activity in methane steam reforming. The characterization results showed the presence of NiO and Ni(2)AlO(4) in the bulk and Ni(2)AlO(4) and/or Ni(2)O(3) and MoO(4)(-2) at the surface of the samples. In the catalytic tests, high stability was observed with a reaction feed of 4:1 steam/methane. However, at a steam/methane ratio of 2: 1, only the catalyst with 0.05% Mo remained stable throughout the 500 min of the test. The addition of Mo to Ni catalysts may have a synergistic effect, probably as a result of electron transfer from the molybdenum to the nickel, increasing the electron density of the catalytic site and hence the catalytic activity. (C) 2009 Elsevier Ltd. All rights reserved.