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At least 19 recordsLinked to original sources
PubMed2006

Hydrogen production from a combination of the water-gas shift and redox cycle process of methane partial oxidation via lattice oxygen over LaFeO3 perovskite catalyst.

A redox cycle process, in which CH4 and air are periodically brought into contact with a solid oxide packed in a fixed-bed reactor, combined with the water-gas shift (WGS) reaction, is proposed for hydrogen production. The sole oxidant for partial oxidation of methane (POM) is found to be lattice oxygen instead of gaseous oxygen. A perovskite-type LaFeO3 oxide was prepared by a sol-gel method and employed as an oxygen storage material in this process. The results indicate that, under appropriate reaction conditions, methane can be oxidized to CO and H2 by the lattice oxygen of LaFeO3 perovskite oxide with a selectivity higher than 95% and the consumed lattice oxygen can be replenished in a reoxidation procedure by a redox operation. It is suggested that the POM to H2/CO by using the lattice oxygen of the oxygen storage materials instead of gaseous oxygen should be possibly applicable. The LaFeO3 perovskite oxide maintained relatively high catalytic activity and structural stability, while the carbonaceous deposits, which come from the dissociation of CH4 in the pulse reaction, occurred due to the low migration rate of lattice oxygen from the bulk toward the surface. A new dissociation-oxidation mechanism for this POM without gaseous oxygen is proposed based on the transient responses of the products checked at different surface states via both pulse reaction and switch reaction over the LaFeO3 catalyst. In the absence of gaseous-phase oxygen, the rate-determining step of methane conversion is the migration rate of lattice oxygen, but the process can be carried out in optimized cycles. The product distribution for POM over LaFeO3 catalyst in the absence of gaseous oxygen was determined by the concentration of surface oxygen, which is relevant with the migration rate of lattice oxygen from the bulk toward the surface. This process of hydrogen production via selective oxidation of methane by lattice oxygen is better in avoiding the deep oxidation (to CO2) and enhancing the selectivity. Therefore, this new route is superior to general POM in stability (resistance to carbonaceous deposition), safety (effectively avoiding accidental explosion), ease of operation and optimization, and low cost (making use of air not oxygen).

Journal Article↗
PubMed2006

Unsteady-state direct partial oxidation of methane to synthesis gas in a fixed-bed reactor using AFeO3 (A = La, Nd, Eu) perovskite-type oxides as oxygen storage.

Direct partial oxidation of methane to synthesis gas on AFeO(3) (A = La, Nd, Eu) oxides by a novel sequential redox cyclic reaction in the absence of gaseous oxygen was investigated over a fixed-bed reactor. These oxides were prepared by the sol-gel method and characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques. XRD analysis showed that all AFeO(3) (A = La, Nd, Eu) oxides, calcined at 1173 K, are single-phase perovskites. The CH(4)-TPSR/MS and continuous reaction experiments indicated that the AFeO(3) (A = La, Nd, Eu) oxides provide mostly oxygen species, as the sole oxidant originated from lattice oxygen instead of gaseous oxygen, which can oxidize CH(4) to synthesis gas with high selectivity in the absence of gaseous oxygen. In terms of material economics and the amount of oxygen species for synthesis gas formation, the LaFeO(3) sample exhibits the best performance among these tested AFeO(3) oxides for synthesis gas production. The pulse experiments at different temperatures showed that the rate of oxygen migration during the CH(4) reaction with LaFeO(3) is strongly affected by the reaction temperature, and increases with rising temperature, which is favorable to much more CH(4) selective oxidation at high temperature. The two types of oxygen species are identified by experiments of continuous reactions and pulses, and confirmed by XPS. Methane can be converted selectively to synthesis gas by consumption of lattice oxygen, and general carbonaceous deposits on the catalyst surface do not occur under the appropriate reaction conditions by sequential redox cycles. The performance of selective oxidation of CH(4) to synthesis gas can be recovered by reoxidation using gaseous molecular oxygen; the LaFeO(3) oxide maintains relatively high catalytic activity and structural stability in redox atmospheres.

Journal Article↗
PubMed2006

Photocatalytic oxidation of propylene with molecular oxygen over highly dispersed titanium, vanadium, and chromium oxides on silica.

Photocatalytic oxidation of propylene with molecular oxygen at room temperature was investigated over various silica-supported metal oxides with low loading. The photocatalytic active site is assumed to be the isolated tetrahedrally coordinated metal oxides in the ligand-to-metal charge-transferred state, such as (Mdelta- -OLdelta+). Photocatalytic epoxidation of propylene into propylene oxide was promoted over silica-supported V and Ti oxides at steady state. Over silica-supported Cr oxide, the propylene oxide formation rate was remarkably decreased with the time course in the reaction. The oxidation state and the coordination environment of the supported Ti, V, and Cr oxide species were determined by diffuse reflectance UV-vis spectroscopy (DRS) and electron spin resonance (ESR). During the photocatalytic oxidation, the oxidation state of the Ti4+ species was not varied. On the other hand, the V5+ species was partially reduced to V4+ and the Cr6+ species was successively reduced to Cr5+ and Cr3+. An isotopic tracer study of the C3H6-18O2 reaction suggests the difference of the active oxygen species between TiO2/SiO2 and V2O5/SiO2. The active oxygen species on TiO2/SiO2 is derived from molecular oxygen. On the other hand, the photogenerated products on V2O5/SiO2 incorporate the lattice oxygen of the surface metal oxide species. It is suggested that the kinds of terminal ligand (hydroxyl or oxo) of the tetrahedrally coordinated metal oxides on silica decide the active oxygen species in the photocatalytic oxidation. A photoinduced hole center on the monohydroxyl (SiO)3Ti-OH species activates molecular oxygen that reacts with propylene. In the case of the monooxo (SiO)3V=O and dioxo (SiO)2Cr=O2 species, the photoactivated lattice oxygen (OL-) directly reacts with propylene.

Alkenes↗
PubMed2000

Atomic-scale structure and catalytic reactivity of the RuO(2)(110) surface

The structure of RuO(2)(110) and the mechanism for catalytic carbon monoxide oxidation on this surface were studied by low-energy electron diffraction, scanning tunneling microscopy, and density-functional calculations. The RuO(2)(110) surface exposes bridging oxygen atoms and ruthenium atoms not capped by oxygen. The latter act as coordinatively unsaturated sites-a hypothesis introduced long ago to account for the catalytic activity of oxide surfaces-onto which carbon monoxide can chemisorb and from where it can react with neighboring lattice-oxygen to carbon dioxide. Under steady-state conditions, the consumed lattice-oxygen is continuously restored by oxygen uptake from the gas phase. The results provide atomic-scale verification of a general mechanism originally proposed by Mars and van Krevelen in 1954 and are likely to be of general relevance for the mechanism of catalytic reactions at oxide surfaces.

Journal Article↗
PubMed1999

Structures des phases paraélectrique et ferroélectrique de Pb2KNb5O15.

The paraelectric and the ferroelectric/ferroelastic structures of Pb(2)KNb(5)O(15) have been refined by the Rietveld method from neutron and X-ray powder diffraction data. The paraelectric phase is tetragonal (space group P4/mbm) with lattice parameters a(t) = 12.646 (1) and c(t) = 3.9551 (2) Å at 790 K. The Pb- and K-atom distribution has been determined. It was established that the Pb atoms situated on the 15-coordinate sites are not on the special position (2mm) but are distributed over two disordered positions as in the tetragonal phase of Pb(2)KTa(5)O(15). The ferroelectric/ferroelastic phase is orthorhombic. The space group of the average structure is Cm2m and the lattice parameters are a(o) = 17.779 (2), b(o) = 18.015 (2), c(o) = 3.9209 (4) Å at 294 K and a(o) = 17.756 (1), b(o) = 18.019 (1), c(o) = 3.9141 (2) Å at 81 K. It is isostructural with the average ferroelectric/ferroelastic structure of PbNb(2)O(6). The orthorhombic distortion is of the same order but the Curie temperature is lowered by the substitution 2K(+)-Pb(2+). The structures of PbNb(2)O(6), PbTa(2)O(6), Pb(2)KNb(5)O(15) and Pb(2)KTa(5)O(15) are compared. The ferroelectric polarization of the niobates is due to both the Pb off-site position and the Nb displacements, which induce a distortion of the oxygen lattice. In PbTa(2)O(6), the oxygen-lattice distortion is very weak. The polarization seems to be due only to the Pb atom. In the tantalates the substitution 2K(+)-Pb(2+) should weaken the correlations between the Pb atoms and consequently suppress the ferroelectric transition. Only a local polarization should take place; this might be the origin of the dielectric relaxation observed in this compound.

Journal Article↗
PubMed2005

Selective oxidation of methanol and ethanol on supported ruthenium oxide clusters at low temperatures.

RuO2 domains supported on SnO2, ZrO2, TiO2, Al2O3, and SiO2 catalyze the oxidative conversion of methanol to formaldehyde, methylformate, and dimethoxymethane with unprecedented rates and high combined selectivity (>99%) and yield at low temperatures (300-400 K). Supports influence turnover rates and the ability of RuO2 domains to undergo redox cycles required for oxidation turnovers. Oxidative dehydrogenation turnover rates and rates of stoichiometric reduction of RuO2 in H2 increased in parallel when RuO2 domains were dispersed on more reducible supports. These support effects, the kinetic effects of CH3OH and O2 on reaction rates, and the observed kinetic isotope effects with CH3OD and CD3OD reactants are consistent with a sequence of elementary steps involving kinetically relevant H-abstraction from adsorbed methoxide species using lattice oxygen atoms and with methoxide formation in quasi-equilibrated CH3OH dissociation on nearly stoichiometric RuO2 surfaces. Anaerobic transient experiments confirmed that CH3OH oxidation to HCHO requires lattice oxygen atoms and that selectivities are not influenced by the presence of O2. Residence time effects on selectivity indicate that secondary HCHO-CH3OH acetalization reactions lead to hemiacetal or methoxymethanol intermediates that convert to dimethoxymethane in reactions with CH3OH on support acid sites or dehydrogenate to form methylformate on RuO2 and support redox sites. These conclusions are consistent with the tendency of Al2O3 and SiO2 supports to favor dimethoxymethane formation, while SnO2, ZrO2, and TiO2 preferentially form methylformate. These support effects on secondary reactions were confirmed by measured CH3OH oxidation rates and selectivities on physical mixtures of supported RuO2 catalysts and pure supports. Ethanol also reacts on supported RuO2 domains to form predominately acetaldehyde and diethoxyethane at 300-400 K. The bifunctional nature of these reaction pathways and the remarkable ability of RuO2-based catalysts to oxidize CH3OH to HCHO at unprecedented low temperatures introduce significant opportunities for new routes to complex oxygenates, including some containing C-C bonds, using methanol or ethanol as intermediates derived from natural gas or biomass.

Journal Article↗
PubMed2006

Transient and steady state investigation of selective and non-selective reaction pathways in the oxidative dehydrogenation of propane over supported vanadia catalysts.

Mechanistic aspects of the formation of C3H6, CO and CO2 in the oxidative dehydrogenation of propane over VOx/gamma-Al2O3 materials have been investigated by means of steady state and transient isotopic tests. The materials possessed highly dispersed and polymerised VOx species as well as bulk-like V2O5. Propene was primarily formed via oxidative dehydrogenation of propane by lattice oxygen of VOx species. It was suggested that non-selective consecutive propene oxidation is initiated by the breaking of the C-C bond in the molecule by the lattice oxygen, forming formaldehyde as a side product, which is further oxidised to CO and CO2. The following order of initial steady state propene selectivity (at a zero degree of propane conversion) as a function of the nature of VOx species was established: a mixture of bulk-like V2O5 and polymerised VOx>polymerised VOx>highly dispersed VOx species. The low propene selectivity over highly dispersed VOx species was explained by the fact that these species do not fully cover the bare acidic surface of gamma-Al2O3 where propene adsorption and further oxidation take place. Thus, two different locations of COx formation were considered: (i) in the vicinity of acidic sites of the support and (ii) on VOx species. The propene selectivity over samples possessing polymerised VOx species and bulk-like V2O5 strongly decreased with an increasing degree of propane conversion. Contrarily, highly dispersed VOx species showed the lowest ability for consecutive propene oxidation.

Journal Article↗
PubMed2004

Electrochemical catalysis of styrene epoxidation with films of MnO(2) nanoparticles and H(2)O(2).

Films of polyions and octahedral layered manganese oxide (OL-1) nanoparticles on carbon electrodes made by layer-by-layer alternate electrostatic adsorption were active for electrochemical catalysis of styrene epoxidation in solution in the presence of hydrogen peroxide and oxygen. The highest catalytic turnover was obtained by using applied voltage -0.6 V vs SCE, O(2), and 100 mM H(2)O(2). (18)O isotope labeling experiments suggested oxygen incorporation from three different sources: molecular oxygen, hydrogen peroxide, and/or lattice oxygen from OL-1 depending on the potential applied and the oxygen and hydrogen peroxide concentrations. Oxygen and hydrogen peroxide activate the OL-1 catalyst for the epoxidation. The pathway for styrene epoxidation in the highest yields required oxygen, hydrogen peroxide, and a reducing voltage and may involve an activated oxygen species in the OL-1 matrix.

Catalysis↗
PubMed2002

Mechanism of interstitial oxygen diffusion in hafnia.

We have performed density functional calculations of oxygen incorporation and diffusion in monoclinic hafnia (HfO2) for a range of oxygen charge states. The calculations demonstrate that oxygen favors atomic incorporation and that O2- is the most stable species. We find that oxygen interstitials diffuse via exchange with lattice oxygen sites in hafnia, and that O- species have the smallest diffusion barrier.

Journal Article↗
PubMed2026

Decoupled Synthesis Pathway via Precursor Functionalization Stabilizes High-Voltage Nickel-Based Cathodes.

Nickel-based layered cathodes are promising candidates for high-performance, high-energy lithium-ion batteries, yet their high-voltage application is jointly limited by synthesis-inherited structural defects and an unstable lattice oxygen framework. Here, we show that both limitations can be overcome by decoupled synthesis pathway (DSP) via La/Nb oxalate functionalization of the Ni0.6Co0.1Mn0.3(OH)2 precursor. Unlike the conventional coupled synthesis pathway (CSP) where precursor dehydration and Li2CO3 decomposition overlap in temperature, the DSP introduces a low‑temperature decomposition of La/Nb oxalates at 200°C, which effectively avoids localized contact between the precursor and Li2CO3 and shifts Li2CO3-related reactions to high temperatures. This allows sequential precursor dehydroxylation, rock‑salt (RS) intermediate formation, and layered‑phase transformation over a broad temperature window. The resulting LiNi0.6Co0.1Mn0.3O2 cathode with La/Nb functionalization (NCM-LN) features a uniform surface LaNiO3 perovskite heterostructure and a Nb‑doped layered bulk with suppressed RS and spinel defects. Consequently, under 4.5 V operation (vs. Li+/Li), NCM-LN exhibits homogeneous Li+ (de)intercalation, and a stabilized oxygen framework. In graphite||NCM-LN full cells, NCM-LN retains 80.1% of its capacity after 2000 cycles at 1C, substantially outperforming the pristine cathode. This decoupling strategy is broadly effective across various Ni‑based systems, providing a generalizable route toward high‑energy, long‑life cathode materials.

decoupled synthesis pathway↗
PubMed2005

Role of surface defects in activation of O2 and N2O on ZrO2 and yttrium-stabilized ZrO2.

The relationship between the structure of both yttrium-stabilized zirconia (YSZ) and ZrO2 catalysts and their ability to activate N2O and O2 is studied by determination of catalytic properties and characterization with TPD, SEM, and XRD. Furthermore, the role of oxygen species formed via dissociation of either O2 or N2O in catalytic partial oxidation of methane (CPOM) is determined. N2O can be activated at both structural defects (e.g., Zr cations located at corners) and intrinsic oxygen vacancies (Zr'(Zr)-V(O)**Zr'(Zr)) and forms two types of oxygen species (alpha-O and beta-O) on the surface, respectively. In contrast, molecular oxygen gives rise to only one type of oxygen species (beta-O), that is, surface lattice oxygen. This type of oxygen species can be extracted by reaction with methane, forming the intrinsic oxygen vacancies again during CPOM. However, the structural defects are not active for oxygen activation during CPOM. Doping ZrO2 with Y2O3 significantly decreases the number of structural defects via replacement of Zr4+ cations by Y3+ cations, located at corners, steps, kinks, and edges of the crystallites. Calcination at higher temperatures results in less structural defects due to both increasing crystallite size as well as transformation to more regular shaped crystallites. High temperature calcinations also increase the activity of YSZ in CPOM. This is attributed to the increase in the exposition of low index planes, especially those (111) with the lowest surface energy and the highest coordination numbers, induced by the thermal treatment.

Journal Article↗
PubMed2004

Optical emission from C60-molecule-coupled Si nanocrystallites.

Using C60 molecule as a kind of surface-passivated agent to modify the electronic structure of Si nanocrystallites in porous silicon, we disclose that this kind of C60/nanocrystalline Si coupling system can show a strong blue emission at approximately 460 nm when stored in air for more than one year. After a full characterization of the photoluminescence properties, we propose a luminescent center in the SiOx layer at the surface of a Si nanocrystallite. It is a pair consisting of an oxygen vacancy and an interstitial oxygen. The interstitial oxygen also forms a peroxy linkage with a neighboring lattice oxygen. Radiative recombination of carriers photogenerated from Si nanocrystallite cores in the luminescent centers results in the observed blue photoluminescence. Neutron irradiation experiments support our assignment of the blue emission mechanism. This work improves the understanding of the origin of blue emission from silicon/oxygen-related nanostructured materials.

Journal Article↗
PubMed2005

Interfacial oxidation of ultrathin nickel and chromium films on yttria-stabilized zirconia.

The substrate-induced oxidation upon prolonged annealing in UHV of ultrathin films of Ni and Cr vapor deposited on yttria-stabilized zirconia YSZ(100) was studied by X-ray photoelectron spectroscopy (XPS) to obtain information about the oxidation mechanism, determine the available quantity of reactive oxygen in YSZ, and investigate the thermal stability of the thin oxide films. Up to about 0.8 ML of Ni deposited at room temperature was oxidized to NiO at a constant rate at 650 K via the substrate, whereas at slightly higher coverage, the oxidation rate under identical conditions was drastically reduced. In contrast to Ni, up to 4.8 ML of Cr deposited at 275 K could be oxidized via the substrate to Cr2O3 upon extensive UHV annealing at increasing temperature up to 820 K, indicating a reactive oxygen content of at least 4 x 10(-6) with respect to the lattice oxygen in the YSZ specimen. The Cr2O3 decomposed to metallic Cr above about 800 K, whereas NiO was stable up to the maximum temperature of 875 K. These results indicate that the oxidation via the substrate is kinetically analogous to the gas-phase oxidation of bulk Ni and Cr. The reactive oxygen content of the single-crystal YSZ is larger than expected, and part of it is accommodated at the surface of the substrate. The thermal stability of the thin oxide films is determined by the oxygen exchange with YSZ and not by the respective bulk oxide thermodynamic decomposition temperature.

Journal Article↗