Perovskite-related Oxynitrides in Photocatalysis

Over the last decades photocatalytic water splitting has become of increasing importance for fundamental and applied research, since the direct conversion of sunlight into chemical energy via the production of H2 has the potential to contribute to the world's energy needs without CO2 generation. One of the unsolved challenges consists of finding a highly efficient photocatalyst that is cheap, environmentally friendly, contains exclusively abundant elements, is (photo)chemically stable and absorbs visible light. Photocatalytic efficiency is closely connected to both structural properties like crystallinity, particle size and surface area and to electronic properties like the band gap and the quantum efficiency. Hence extensive control over a large parameter field is necessary to design a good photocatalyst. A material class where the structure-composition-property relations and the influence of substitution effects are well studied is the perovskite-type family of compounds. The perovskite-related oxynitrides belong to this very flexible compound family where many of the necessary characteristics for a photocatalyst are already given and some of the intrinsic properties like the band gap can be tuned within the same crystal structure by substitution. In this work we present materials' design concepts to improve the photocatalytic efficiency of a perovskite-type catalyst and describe their effects on the photocatalytic activity

Mesoporosity in Photocatalytically Active Oxynitride Single Crystals

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Optical and transport properties of LaTi1-xMx(O,N)3±δ (x=0; 0.1, M=Nb5+, W6+) thin films prepared by plasma ammonolysis

Oxynitride thin films of composition LaTi1−xMx(O,N)3±δ with x=0; 0.1 and M=Nb5+, W6+ were prepared by nitridation via microwave-induced plasma (MIP) ammonolysis, which allowed keeping the nitridation time short (16 min). The higher possible oxidation states of the B-site substituents (Nb5+, W6+) with respect to Ti4+ caused higher N content for LaTi0.9Nb0.1(O,N)3±δ and LaTi0.9W0.1(O,N)3±δ compared to LaTiO2N due to charge-compensation. XPS O 1s and N 1s core level and valence band spectra evidenced for increasing N content in the order LaTiO2N<LaTi0.9Nb0.1(O,N)3±δ<LaTi0.9W0.1(O,N)3±δ. All films were N deficient comparing with their stoichiometric formulas and assuming cations in highest oxidation states. Along with increasing N content, the bandgaps decreased from 2.4 to 2.2 and 2.1 eV for LaTiO2N, LaTi0.9Nb0.1(O,N)3±δ, and LaTi0.9W0.1(O,N)3±δ and the electronic conductivities increased along with the decreased bandgaps. The Seebeck coefficients at 860 K indicated the highest charge-carrier density for LaTi0.9W0.1(O,N)3±δ.Fil: Maegli, Alexandra E.. Eidgenössische Materialprüfungs- und Forschungsanstalt; SuizaFil: Sagarna, Leyre. Eidgenössische Materialprüfungs- und Forschungsanstalt; SuizaFil: Populoh, Sascha. Eidgenössische Materialprüfungs- und Forschungsanstalt; SuizaFil: Penkala, Bartosz. Eidgenössische Materialprüfungs- und Forschungsanstalt; SuizaFil: Otal, Eugenio Hernan. Eidgenössische Materialprüfungs- und Forschungsanstalt; Suiza. Consejo Nacional de Investigaciones Científicas y Técnicas. Unidad de Investigación y Desarrollo Estratégico para la Defensa. Ministerio de Defensa. Unidad de Investigación y Desarrollo Estratégico para la Defensa; ArgentinaFil: Weidenkaff, Anke. Eidgenössische Materialprüfungs- und Forschungsanstalt; Suiz

Structural and photocatalytic properties of perovskite-type (La,Ca)Ti(O,N)(3) prepared from A-site deficient precursors

Two series of oxide precursors for perovskite-type (La,Ca)Ti(O,N)(3) were prepared by adding Ca2+ to A-site deficient LaTiO3.5 heterogeneously (Ca2+-backfilling) or by substituting Ca2+ for La3+ in stoichiometric LaTiO3.5 homogeneously (Ca2+-substitution). Activity of the resultant (La,Ca)Ti(O,N)(3) for photocatalytic O-2 evolution was tested in the presence of an electron acceptor (Ag+) and a superior activity of Ca2+-backfilled LaTiO2N compared to Ca2+-substituted LaTiO2N and unsubstituted LaTiO2N was demonstrated. X-ray diffraction patterns of the precursor oxides revealed a higher degree of crystallinity in La1-xTiO3.5-3x/2 compared to La1-xCaxTiO3.5-x/2. The higher crystallinity in La1-xTiO3.5-3x/2 resulted in lower Ti3+ defect formation during the ammonolysis reaction. This was evidenced by the lower background absorption of the diffuse reflectance spectra in Ca2+-backfilled compared to Ca2+-substituted LaTiO2N. Structural refinement of the diffraction patterns revealed growing crystal sizes with the Ca2+ content, which was more pronounced for Ca2+-backfilled than for Ca2+-substituted LaTiO2N. Thus, the enhanced photocatalytic activity of Ca2+-backfilled LaTiO2N was related to the quality of the precursor oxides, which influenced the defect concentrations and crystallite sizes of the resulting oxynitrides

Chromium-induced deactivation of a commercial honeycomb noble metal-based CO oxidation catalyst

A commercially available honeycomb CO oxidation catalyst used to control the exhaust of a solid oxide fuel cell (SOFC) based power system has been characterized after prolonged use. X-ray fluorescence (XRF), X-ray diffraction (XRD), Raman spectroscopy, N2 physisorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were employed to determine the phase composition, the morphology and the chemical state of the various components. Besides sintering of the active phase, deactivation was found to occur mainly as the result of the deposition of chromium-containing species on the catalyst washcoat. These fouling species mainly appeared as highly crystalline Cr2O3 particles and could still maintain acceptable CO oxidation activity under dry atmosphere. However, the formation of surface chromium oxyhydroxide species was found to occur in the presence of water vapor, leading to significant catalyst deactivation

Mesoporosity in Photocatalytically Active Oxynitride Single Crystals

Mesoporosity in photocatalytically active oxynitride single crystals and single-crystalline zones has been investigated by transmission electron microscopy techniques including nanobeam diffraction, electron energy loss spectroscopy, electron tomography, and high-resolution imaging. Several particle morphologies of the perovskite-related oxynitride LaTiO₂N were synthesized by solid-state and polymer complex synthesis of the La₂Ti₂O₇ precursor followed by thermal ammonolysis. A detailed analysis of pore sizes, pore shapes, and lattice defects and the local analysis of oxidation states allowed correlation between morphology, synthesis procedures, chemical and crystal defects, and photocatalytic activity. A pore formation mechanism via lattice condensation is proposed, which is simultaneously linked to lattice defect formation processes. On the basis of mechanistic understanding of the transformation from oxide to oxynitride, mesoporosity, and hence the photocatalytic or photoelectrochemical properties of the material, can be tuned

Methane abatement under stoichiometric conditions on perovskite-supported palladium catalysts prepared by flame spray synthesis

Three-way catalysts (TWC) are the key technology to reduce emissions of pollutants from stoichiometric engines. Perovskite-type catalysts of general formula ABO3±δ (A = La, Y; B = Mn, Fe) containing 2 wt% Pd were produced by flame spray synthesis (FSS) using metal nitrate precursors. The structural properties of the catalysts were characterized by X-ray diffraction (XRD), surface area determination (BET) and transmission electron microscopy (TEM). Crystalline metal oxide nano-particles of 20 nm average size were accompanied by minority La2O3 and Y2O3 phases. The state of Pd in the catalysts was characterized using X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge spectroscopy (XANES) and CO adsorption by infrared spectroscopy. Metallic Pd coexisted with Pd in oxidation state +2 and higher on all fresh samples. TEM confirmed the presence of dispersed Pd particles 2-5 nm in diameter. Therefore, under the chosen synthesis conditions, FSS provides supported palladium nano-particles rather than a solid solution. PdO was the dominant Pd species after calcination at 700 °C. The TWC activity was tested in a simulated stoichiometric gas mixture comprising CH4, CO, NOx and O2. PdO in combination with YFeO3±δ exhibited the lowest temperature for CH4 oxidation (T50 = 450°C), which was ca. 100°C lower than that of the sample obtained by the conventional wet-chemical method. After cycling under reaction conditions up to 850°C, a large improvement of catalytic activity for CH4 oxidation was observed which associated with the formation of metallic Pd particles (ca. 20 nm) and the hexagonal → orthorhombic phase transition of YFeO3±δ

Perovskite-Type LaTiO2N Oxynitrides for Solar Water Splitting: Influence of the Synthesis Conditions

Oxynitrides with the nominal composition LaTiO2N were prepared from a La2Ti2O7 precursor by thermal ammonolysis of the oxide under an NH3 flow for different durations t (4 30 h). X-ray diffraction (XRD) indicated that phase-pure LaTiO2N samples were obtained when t 13 h. The material was further characterised by scanning electron microscopy (SEM), surface area measurements based on the BET method, thermogravimetric analysis (TGA) and UV-visible diffuse reflectance spectroscopy (DRS). The powders displayed an increased crystallite size and a decreased surface area with increasing t. The ratio of N/(N+O) increased with t from 0.22 0.27. Correlated to this nitrogen increase, a small variation of the bandgap energy was observed from 2.19 to 2.12 eV with t. All LaTiO2N samples oxidised H2O into O2 in the presence of an electron acceptor (Ag+). The O2 evolution was increased from 12 μmol/h (t = 13 h) to 22 24 μmol/h (16 h t 30 h). It was found that the nitrogen content and the amount of defects played a key role in the photocatalytic O2 evolution.ISSN:1876-610

LaTiO2N/In2O3 photoanodes with improved performance for solar water splitting

LaTiO2N photoanodes for solar water splitting were prepared by electrophoretic deposition and demonstrated the best photo-currents ever reported for this material. Further important enhancement of the performance was obtained by the use of a sputtered In2O3 overlayer

The influence of defects formed by Ca excess and thermal post-treatments on the persistent luminescence of CaTiO_3:Pr

Red emitting CaTiO3:Pr phosphors with a nominal composition of Ca0.998+xPr0.002TiO3+δ (0.02≤x≤0.04) were prepared by solid state reactions with different thermal post treatments and characterized by X-ray diffraction, transmission electron microscopy and photoluminescence. The Ca excess exhibited complete solubility up to 4% in the samples treated at 1400 °C but segregation in the form of Ruddlesden-Popper phases (Ca3Ti2O7 - Ca4Ti3O10) was observed in samples prepared at 1500 °C. The increase in temperature for stoichiometric samples showed a monotonic increase of decay time due to the reduction of non-radiative recombination defects. It was found that the Ca excess favored the formation of oxygen vacancies which are known to act as trap. In the samples treated at 1400 °C, 3% of Ca excess showed to be the best concentration to increase the decay time of persistent luminescence. For the samples treated at 1500 °C, the segregation of Ruddlesden-Popper phases left a constant amount of Ca soluble in all the CaTiO3 samples. This constant concentration of Ca caused the same density of defects and, consequently, the same decay time in all samples