We report the microwave synthesis and characterization of Au and Pd nanoparticle catalysts supported on CeO2, CuO, and ZnO nanoparticles for CO oxidation. The results indicate that supported Au/CeO2 catalysts exhibit excellent activity for low-temperature CO oxidation. The Pd/CeO2 catalyst shows a uniform dispersion of Pd nanoparticles with a narrow size distribution within the ceria support. A remarkable enhancement of the catalytic activity is observed and directly correlated with the change in the morphology of the supported catalyst and the efficient dispersion of the active metal on the support achieved by using capping agents during the microwave synthesis. The significance of the current method lies mainly in its simplicity, flexibility, and the control of the different factors that determine the activity of the nanoparticle catalysts.
procedures, elemental analysis, tables of DFT results, and notes on cell design and construction (PDF).
Single-crystal particles of the layered natrotantite, i.e., Na 2 Ta 4 O 11 , were prepared within a K 2 SO 4 /Na 2 SO 4 flux for flux-to-reactant molar ratios from 12:1 to 1:1 at a reaction temperature of 1000 °C for 2 h. Depending on the conditions, the flux reactions yielded crystals of Na 2 Ta 4 O 11 that ranged in size from ∼100 nm to ∼1000 nm. The highest and lowest flux amounts yielded more isolated single crystals with sharper facets and surfaces, whereas intermediate flux amounts yielded more aggregates of particles with smooth and rounded surface features. All products were characterized by UV−vis diffuse reflectance techniques and were found to exhibit an indirect bandgap size of ∼4.1−4.3 eV and a larger direct bandgap transition of ∼4.5 eV. When the crystals are suspended in aqueous solutions and irradiated by ultraviolet light, they exhibit stable photocatalytic rates for hydrogen production of ∼13.. The higher photocatalytic rates are found for the single crystals with the highly faceted and nanoterraced surfaces. Electronic structure calculations based on density functional theory confirm the lowest-energy bandgap transition is indirect and between the Γ and M k-points in the valence and conduction band states, respectively. The bandgap excitation is found to result in delocalization of the excited electrons over a layer of condensed TaO 7 pentagonal bipyramids, which is a relatively unexplored structural feature for photocatalytic metal oxides.
The p-type semiconductor Cu 5 Ta 11 O 30 has been investigated for the effect of Cu extrusion on its crystalline structure, surface chemistry, and photoelectrochemical properties. The Cu 5 Ta 11 O 30 phase was prepared in high purity using a CuCl-mediated flux synthesis route, followed by heating the products in air from 250 to 750 °C in order to investigate the effects of its reported film preparation conditions as a p-type photoelectrode. At 650 °C and higher temperatures, Cu 5 Ta 11 O 30 is found to decompose into CuTa 2 O 6 and Ta 2 O 5 . At lower temperatures of 250 to 550 °C, nanosized Cu II O surface islands and a Cu-deficient Cu 5−x Ta 11 O 30 crystalline structure (i.e., x ∼ 1.8(1) after 450 °C for 3 h in air) is found by electron microscopy and Rietveld structural refinement results, respectively. Its crystalline structure exhibits a decrease in the unit cell volume with increasing reaction temperature and time, owing to the increasing removal of Cu(I) ions from its structure. The parent structure of Cu 5 Ta 11 O 30 is conserved up to ∼50% Cu vacancies but with one notably shorter Cu−O distance (by ∼0.26 Å) and concomitant changes in the Ta−O distances within the pentagonal bipyramidal TaO 7 layers (by ∼0.29 Å to ∼0.36 Å). The extrusion and oxidation of Cu(I) to Cu(II) cations at its surfaces is found by X-ray photoelectron spectroscopy, while magnetic susceptibility data are consistent with the oxidation of Cu(I) within its structure, as given by Cu I(5−2x) Cu II x Ta 11 O 30 . Polycrystalline films of Cu 5−x Ta 11 O 30 were prepared under similar conditions by sintering, followed by heating in air at temperatures of 350 °C, 450 °C, and 550 °C, each for 15, 30, and 60 min. An increasing amount of copper deficiency in the Cu 5−x Ta 11 O 30 structure and Cu II O surface islands are found to result in significant increases in its p-type visiblelight photocurrent at up to −2.5 mA/cm 2 (radiant power density of ∼500 mW/cm 2 ). Similarly high p-type photocurrents are also observed for Cu 5 Ta 11 O 30 films with an increasing amount of CuO nanoparticles deposited onto their surfaces, showing that the enhancement primarily arises from the presence of the CuO nanoparticles which provide a favorable band-energy offset to drive electron−hole separation at the surfaces. By contrast, negligible photocurrents are observed for Cu-deficient Cu 5−x Ta 11 O 30 without the CuO nanoparticles. Electronic structure calculations show that an increase in Cu vacancies shifts the Fermi level to lower energies, resulting in the depopulation of primarily Cu 3d 10 -orbitals as well as O 2p orbitals. Thus, these findings help shed new light into the role of Cu-deficiency and Cu II O surface islands on the p-type photoelectrode films for solar energy conversion systems.
New p-type polycrystalline films of semiconducting Cu5Ta11O30 and Cu3Ta7O19 were prepared on fluorine-doped tin oxide (FTO) glass starting from their CuCl-flux synthesis as highly faceted micrometer-sized particles. The particles were annealed on FTO at 400–500 °C, followed by a mild oxidation in air at between 250 and 550 °C. In an aqueous 0.5 M Na2SO4 electrolyte solution (pH = 6.3), the films exhibit strong cathodic photocurrents under irradiation by visible and/or ultraviolet light, which increased with higher annealing and oxidation temperatures owing to increased p-type carrier concentration and better electrical contact between particles. Thermogravimetric analyses show that the oxidation treatments result in an oxygen uptake at concentrations of ∼3 × 1020 cm–3 at 250 °C, to ∼4 × 1021 cm–3 at 550 °C, with the higher temperatures leading to the decomposition of the film. The Cu5Ta11O30 and Cu3Ta7O19 bulk powders exhibit band-gap sizes of ∼2.59 and ∼2.47 eV, respectively, and show an onset of their cathodic photocurrents at wavelengths of ∼500–550 nm. Mott–Schottky measurements of their flat-band potentials have been used to determine the valence band positions at approximately +1.06 and +1.19 V versus RHE (pH = 6.3), and thus conduction band positions of about −1.53 and −1.28 V for Cu5Ta11O30 and Cu3Ta7O19, respectively. The band positions are thus suitably located for the photon-driven reduction and oxidation of water. The highest observed incident photon-to-current efficiencies (IPCE %) for hydrogen production were ∼5% at 350 nm and ∼1–2% at 500–600 nm. Electronic structure calculations based on density functional theory methods show that the conduction band states are delocalized within layers of TaO7 pentagonal bipyramids, whereas the valence band states originate within layers of linearly coordinated Cu(I) cations. The lowest-energy band-gap transitions involve a metal-to-metal charge transfer between Cu(I) and Ta(V) cations in these two types of layers. Compared to other Cu(I) oxides, these structures possess sufficiently disperse bands for high carrier mobility within these layers, and thus the strong cathodic photocurrents of the films.
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