xO2 Oxides Research Articles

Mixed RuxIr1−xO2 Supported on Rutile TiO2: Catalytic Methane Combustion, a Model Study

AbstractWith a modified Pechini synthesis, mixed RuxIr1−xO2 is grown on rutile‐TiO2 with full control of the composition x, where the preformed TiO2 particles serve as nucleation sites for the active component. Catalytic and kinetic data of the methane combustion over RuxIr1−xO2@TiO2 and unsupported RuxIr1−xO2 catalysts reveal that the least active catalyst is RuO2@TiO2 (onset temperature: 270 °C), while the most active catalyst is Ru0.25Ir0.75O2 with an onset temperature below 220 °C. Surprisingly, even Ru0.75Ir0.25O2@TiO2 is remarkably active in methane combustion (onset temperature: 230 °C), indicating that little iridium in the mixed RuxIr1−xO2 oxide component already improves the activity of the methane combustion considerably. We conclude that iridium in the mixed RuxIr1−xO2 oxide enables efficient methane activation, while ruthenium promotes the subsequent oxidation steps of the methyl group to produce CO2. Kinetic data provide a reaction order in O2 of zero, while that of methane is close to one, indicating that the methane activation is rate limiting. The apparent activation energy varies among RuxIr1−xO2 from 110 (x=0) to 80 kJ ⋅ mol−1 (x=1). This variation in the apparent activation energy may be explained by the variation in adsorption energy of oxygen. Under the given reaction conditions the catalyst's surface is saturated with adsorbed oxygen and only if oxygen desorbs, methane can be activated and the methyl group can be accommodated at the liberated surface metal sites.

Mixed RuxIr1–xO2 Oxide Catalyst with Well-Defined and Varying Composition Applied to CO Oxidation

The Pechini method allows for compositional and structural control of mixed ruthenium–iridium powder samples. Extensive characterization reveals that the calcination step leads to agglomerates with a metallic core that is encapsulated by the mixed RuxIr1–xO2 oxides. In the composition range of 21 up to 74 mol % ruthenium, the metal core reveals a miscibility gap with an Ir-rich fcc structure and a Ru-enriched hcp structure. Quite in contrast, the mixed oxide particles form a well-defined RuxIr1–xO2 solid solution throughout the entire composition range. Catalytic activity tests of the mixed RuxIr1–xO2 oxide catalysts are conducted with the prototypical CO oxidation reaction under oxidizing and under both stoichiometric reaction conditions; note that the metallic core is buried and does therefore not contribute to the activity. The least active catalyst is pure IrO2. At moderate reaction temperatures above 100 °C, the most active oxidation catalyst is identified with the Ir0.125Ru0.875O2 mixed oxide. All mixed RuxIr1–xO2 oxide catalysts are bulk-stable under the reaction conditions considered. However, upon CO oxidation reaction, the Ir4+ concentration at the mixed oxide surface is significantly enhanced with respect to its bulk composition. This information may be important for other catalytic oxidation reactions as well, such as the anodic oxygen evolution reaction in the electrocatalytic water splitting.

Unravelling local environments in mixed TiO2–SiO2 thin films by XPS and ab initio calculations

Mixed TixSi1−xO2 oxide can exhibit a partial phase separation of the TiO2 and SiO2 phases at the atomic level. The quantification of TiO2–SiO2 mixing in the amorphous material is complicated and was so far done mostly by infrared spectroscopy. We developed a new approach to the fitting of X-ray photoelectron spectroscopy data for the quantification of partial phase separation in amorphous TixSi1−xO2 thin films deposited by plasma enhanced chemical vapour deposition. Several fitting constraints reducing the total number of degrees of freedom in the fits and thus the fit uncertainty were obtained by using core electron binding energies predicted by density functional theory calculations on TixSi1−xO2 amorphous supercells. Consequently, a decomposition of the O1s peak into TiO2, SiO2 and mixed components was possible. The component areas ratios were compared with the ratios predicted by older theoretical models based on the atomic environment statistics and we also developed several new models corresponding to more realistic atomic structure and partial mixing. Based on the comparison we conclude that the studied films are mostly disordered, with only a moderate phase separation.

Sulfated CexZr1−xO2 oxides. Surface properties and performance for methane oxidation under fuel-rich conditions

Two series of sulfated zirconia-ceria based catalysts have been prepared by coprecipitation of the Zr4+ and Ce3+ salts, which were then sulfated by impregnation using H2SO4 and [(NH4)2S2O8] and calcined at 650 °C for 4 h. The sulfated and persulfated Zr-Ce-O samples were synthesized with Zr/Ce ratios ranging from 1 to 15. The textural, structural and surface characteristics of the resulting modified catalysts were determined by N2 adsorption-desorption, XRD, XPS spectroscopy and TPD of NH3, and results were related to their catalytic behaviour. The catalysts were tested in the direct conversion of methane at high temperature (650 °C) under fuel-rich conditions to obtain C1 oxygenates (CH3OH and HCHO). Experiments were conducted using low O2/CH4 ratio mixtures and high space velocity. It was shown that O2/CH4 feeds resulted in practically total combustion of CH4 when sulfated/persulfated ceria-zirconia catalysts are used, while the nature of the different phases modifies the surface acidity, which plays a crucial role on the extent of deep oxidation reactions.

Iron modified titanium–hafnium binary oxides as catalysts in total oxidation of ethyl acetate

Multicomponent iron–titanium–hafnium oxide materials with different compositions were prepared by combination of homogeneous precipitation with urea and incipient wetness impregnation techniques and tested as catalysts for ethyl acetate oxidation as representative VOCs. Nitrogen physisorption, XRD, Raman, UV–Vis, XPS, Mössbauer spectroscopy and TPR analyses reveal co-existence of substituted FexTi1−xO2 oxide, finely dispersed iron oxide species with supper paramagnetic behavior and well crystallized α-Fe2O3 particles, which relative part depends on hafnium content in titania lattice. The effect of phase composition on the catalytic behavior of these materials in ethyl acetate oxidation was discussed.

Chromium substitution effect on structural and electrochemical behavior of Li-Cr-Ni-O oxides

The chromium-substituted LiCr x Ni1−x O2 oxides were prepared from lithium hydroxide monohydrate (LiOH·H2O), NiO, and Cr2O3. The structure of Li1.5Cr0.5NiO3 phase has been investigated by means of X-ray single crystal and powder diffraction (space group P-3m1, a = 2.9256(1), c = 7.1539(2) A). The relations between Li1.5Cr0.5NiO3 structure, rhombohedral LiNiO2, and NiO are described by a group-subgroup scheme on the basis of Barnighausen formalism. Electronic structures were calculated applying the TB-LMTO-ASA 4.7 program. The maximum of electron localization function (ELF) was obtained around the oxygen atoms. Features of the crystal structure of the trigonal Li1.5Cr0.5NiO3 phase cause lower electrode capacity compared to rhombohedral LiNiO2.

HCl Oxidation for Sustainable Cl2 Recycle over the CexZr1–xO2 Catalysts: Effects of Ce/Zr Ratio on Activity and Stability

CexZr1–xO2 oxides prepared by a soft reactive grinding (SRG) procedure have been investigated as the catalysts for HCl oxidation. The results show that the catalytic activities and stabilities of CexZr1–xO2 oxides are remarkably dependent on the Ce/Zr ratio. A correlation between the physicochemical properties and catalytic performances of these samples reveals that the cubic phase in CexZr1–xO2 with higher oxygen storage capacity (OSC) is more active for HCl oxidation, but the tetragonal phase is crucial to the stability of the catalysts. The Ce0.5Zr0.5O2 catalyst with mixed phases and the highest OSC (80 μmolO2·gcat–1) exhibits an excellent activity (1.89 gCl2·gcat–1·h–1 at 430 °C) and stability (300 h) in the target reaction. Kinetic studies show that both O2 and HCl competed for the active sites rendering desorption of surface Cl as the rate-determining step. Accelerating the replacement of surface Cl by O2 is essential for improving the activity of Ce0.5Zr0.5O2.

Electrochemical behavior of IrxRu1−xO2 oxides as anodic electrocatalyst for electrosynthesis of dinitrogen pentoxide

The IrxRu1−xO2/Ti electrodes (x=0–1) were prepared by thermal decomposition of non-aqueous solutions of H2IrCl6 and RuCl3. The surface morphology and microstructure of the IrxRu1−xO2 coatings were characterized by XRD and SEM. The electrochemical characterization of the surface was carried out in a 0.5molL−1 H2SO4 solution. The IrxRu1−xO2 coating with x=0.5 showed a minimal nanocrystalline particle size and a maximum surface charge (q*). The electrochemical behaviors of the IrxRu1−xO2 coatings for oxidation of N2O4 were determined by cyclic voltammetry (CV), the open circuit potential (OCP) and Tafel plot in 1.7molL−1N2O4/HNO3 solutions. The open-circuit potential (Eocp) in N2O4/HNO3 solution was about 1.18V and it was almost independent of the surface composition. Cyclic voltammetry showed a broad wave feature at 1.4–1.8V in the anodic scan, which was attributed to the oxidation of NO2 derived from N2O4 to NO2+. The redox couple of NO2/NO2+ showed a quasi-reversible behavior and the process was under diffusion control. The electrocatalytic activities of various IrxRu1−xO2 coatings for oxidation of N2O4 were evaluated by potentiostatic polarization curves under the quasi-steady state conditions. The Ru0.5Ir0.5O2 coating showed the lowest Tafel slope, 35.7mV, in the low current densities, indicating that this coating had the highest electrocatalytic activity. The experiments of electrochemical synthesis of N2O5 from N2O4 in nitric acid also indicated that the Ru0.5Ir0.5O2 coating had the best electrocatalytic performance.

Heat capacity of Li x Ni2 − x O2 solid solutions

We have synthesized LixNi2 − xO2 oxides in the range x = 0.1–0.84 and showed that the solid-solution system contains a two-phase region. The heat capacity of LixNi2 − xO2 has been determined by differential scanning calorimetry.

Study of IrxRu1−xO2 oxides as anodic electrocatalysts for solid polymer electrolyte water electrolysis

Abstract Electrocatalysts of the general formula IrxRu1−xO2 were prepared using Adams’ fusion method. The crystallite characterization was examined via XRD, and the electrochemical properties were examined via cyclic voltammetry (CV) in, linear sweep voltammetry (LSV) and chronopotentiometry measurements in 0.5 M H2SO4. The electrocatalysts were applied to a membrane electrode assembly (MEA) and studied in situ in an electrolysis cell through electrochemical impedance spectroscopy (EIS) and stationary current density–potential relations were investigated. The IrxRu1−xO2 (x = 0.2, 0.4, 0.6) compounds were found to be more active than pure IrO2 and more stable than pure RuO2. The most active electrocatalyst obtained had a composition of Ir0.2Ru0.8O2. With an Ir0.2Ru0.8O2 anode, a 28.4% Pt/C cathode and the total noble metal loading of 1.7 mg cm−2, the potential of water electrolysis was 1.622 V at 1 A cm−2 and 80 °C.

Structural and transport properties of layered Li1+x(Mn1/3Co1/3Ni1/3)1−xO2 oxides prepared by a soft chemistry method

Abstract In this work structural and transport properties of layered Li 1+ x (Mn 1/3 Co 1/3 Ni 1/3 ) 1− x O 2 oxides ( x = 0; 0.03; 0.06) prepared by a “soft chemistry” method are presented. The excessive lithium was found to significantly improve transport properties of the materials, a corresponding linear decrease of the unit cell parameters was observed. The electrical conductivity of Li 1.03 (Mn 1/3 Co 1/3 Ni 1/3 ) 0.97 O 2 composition was high enough to use this material in a form of a pellet, without any additives, in lithium batteries and characterize structural and transport properties of deintercalated Li 1.03− y (Mn 1/3 Co 1/3 Ni 1/3 ) 0.97 O 2 compounds. For deintercalated samples a linear increase of the lattice parameter c together with a linear decrease of the parameter a with the increasing deintercalation degree occurred, but only up to 0.4–0.5 mol of extracted lithium. Further deintercalation showed a reversal of the trend. Electrical conductivity measurements performed of Li 1.03− y (Mn 1/3 Co 1/3 Ni 1/3 ) 0.97 O 2 samples ( y = 0.1; 0.3; 0.5; 0.6) showed an ongoing improvement, almost two orders of magnitude, in relation to the starting composition. Additionally, OCV measurements, discharge characteristics and lithium diffusion coefficient measurements were performed for Li/Li + /Li 1.03− y (Mn 1/3 Co 1/3 Ni 1/3 ) 0.97 O 2 cells.

Characterizing Hf X Zr1−X O2 by EXAFS: Relationship Between Bulk and Surface Composition, and Impact on Catalytic Selectivity for Alcohol Conversion

A series of mixed Hf X Zr1−X O2 oxide catalysts was prepared according to a recipe that yields the monoclinic structure. The samples were examined by EXAFS spectroscopy at the Zr K and Hf LIII edges. A fitting model was used that simultaneously fits data from both edges, and makes use of an interdependent mixing parameter X mix to take into account substitution of the complementary atom in the nearest metal-metal shell. For XPS analysis, Scofield factors were applied to estimate the relative atomic surface concentrations of Zr and Hf. EXAFS results suggested that a solid bulk solution was formed over a wide range of X for Hf X Zr1−X O2 binary oxides, and that the relative ratio was retained in the surface shell (i.e., including some subsurface layers by XPS) and the surface (e.g., by ISS). The increase in selectivity for the 1-alkene from dehydration of alcohols at high Zr content does not correlate smoothly with the tuned relative atomic concentration of Hf to Zr. The step change at high Zr content appears to be due to other indirect factors (e.g., surface defects, oxygen vacancies).

Highly Dispersed Ce x Zr1−x O2 Nano-Oxides Over Alumina, Silica and Titania Supports for Catalytic Applications

We have been exploring the utilization of supported ceria and ceria–zirconia nano-oxides for different catalytic applications. In this comprehensive investigation, a series of Ce x Zr1−x O2/Al2O3, Ce x Zr1−x O2/SiO2 and Ce x Zr1−x O2/TiO2 composite oxide catalysts were synthesized and subjected to thermal treatments from 773 to 1073 K to examine the influence of support on thermal stability, textural properties and catalytic activity of the ceria–zirconia solid solutions. The physicochemical characterization studies were performed using X-ray diffraction (XRD), Raman spectroscopy (RS), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HREM), thermogravimetry and BET surface area methods. To evaluate the catalytic properties, oxygen storage/release capacity (OSC) and CO oxidation activity measurements were carried out. The XRD analyses revealed the formation of Ce0.75Zr0.25O2, Ce0.6Zr0.4O2, Ce0.16Zr0.84O2 and Ce0.5Zr0.5O2 phases depending on the nature of support and calcination temperature employed. Raman spectroscopy measurements in corroboration with XRD results suggested enrichment of zirconium in the Ce x Zr1−x O2 solid solutions with increasing calcination temperature thereby resulting in the formation of oxygen vacancies, lattice defects and oxygen ion displacement from the ideal cubic lattice positions. The HREM results indicated a well-dispersed cubic Ce x Zr1−x O2 phase of the size around 5 nm over all supports at 773 K and there was no appreciable increase in the size after treatment at 1073 K. The XPS studies revealed the presence of cerium in both Ce4+ and Ce3+ oxidation states in different proportions depending on the nature of support and the treatment temperature applied. All characterization techniques indicated absence of pure ZrO2 and crystalline inactive phases between Ce–Al, Ce–Si and Ce–Ti oxides. Among the three supports employed, silica was found to stabilize more effectively the nanosized Ce x Zr1−x O2 oxides by retarding the sintering phenomenon during high temperature treatments, followed by alumina and titania. Interestingly, the alumina supported samples exhibited highest OSC and CO oxidation activity followed by titania and silica. Details of these findings are consolidated in this review.

The electrical properties and stability of the hafnium silicate/Si0.8Ge0.2(100) interface

The effect of post-oxidation N2 annealing and post-metallization forming-gas annealing on the electrical properties of Pt/Hf-silicate (3 nm)/Si0.8Ge0.2(100)/n-type Si(100) metal-oxide semiconductor (MOS) capacitors is reported. Capacitance-voltage (C-V) and current density-voltage (J-V) measurements of asgrown, 3-nm-thick, hafnium-silicate films containing ∼12at.%Hf indicate a large number of bulk and interface traps with a current density of ∼10−2 A/cm2 at VFB+1 V. Post-ultraviolet (UV)/O3 oxidation annealing in N2 at 350°C for 30 min leads to a significant improvement in the electrical characteristics of the film. A post-metallization anneal (PMA) at 450°C for 30 min in forming gas (90% N2:10% H2), however, degraded the electrical properties of the films. X-ray photoelectron spectroscopy (XPS) analyses of the forming-gas-annealed films indicate that a possible cause for the degradation in electrical properties is the hydrogen-induced reduction of GeO2 in the interfacial SixGe1−xO2 oxide layer to elemental germanium. Implications for the introduction of hafnium silicate as a viable gate dielectric for SiGe-based devices are discussed.

Acid-Delithiated Li1-x(NiyCo1-y)1+xO2 as Insertion Electrodes in Lithium Batteries

Monophase Li1-x(NiyCo1-y)1+xO2 oxides (0 ≤ y ≤ 0.4 and 0.8 ≤ y ≤ 0.9) with a layered structure (R3̄m space group) have been prepared by interaction of (NiyCo1-y)3O4 (0 ≤ y ≤ 0.4) and NiyCo1-yO (0.8 ≤ y ≤ 0.9) with LiOH at low temperature (450°c). As in the case of high-temperature analogues, cobalt substitution for nickel stabilizes the Ni3+ ions and improves the two-dimensionality of the crystal lattice. Acid treatment of LT and HT oxides leads to a partial dissolution of the solid and to removal of lithium and "impurity" cobalt and/or nickel from LiO2 layers of the trigonal framework. For cobalt-rich oxides, the ion extraction is accompanied by partial Li+/H+ exchange, especially for LT samples. Acid-delithiated oxides are studied as cathode materials in lithium batteries. The resulting cells show a high value of the initial voltage (4 V), which allows a direct discharge of the cell without previous charge. For LT samples, the discharge and charge processes occur at lower voltage (especially for Co-rich oxides) and the reversibility of the cell is lower as compared to that of HT analogues. The better reversibility of the cell in the 4.0-3.2 V range is found for HT Ni-rich oxide. The composition of the acid-treated oxides can be recovered without an enhancement polarization of the cell. XRD analysis and IR spectroscopy show good reversibility of lithium insertion and extraction reactions by chemical and electrochemical procedures.

Kinetics and mechanism of wet oxidation of GexSi1−x alloys

The thermal oxidation of polycrystalline GexSi1−x (0.10≤x≤0.47) and pseudomorphic Ge0.2Si0.8 has been studied in wet ambient at 550 to 900 °C. A uniform GexSi1−xO2 oxide is observed by backscattering spectrometry for a high Ge content at low oxidation temperatures; a SiO2 oxide is obtained for a low Ge content at high temperatures; a GeySi1−yO2 oxide with reduced Ge content (y<x) is found in between. Ge piles up behind the oxide when SiO2 or GeySi1−yO2 form. The transition between these three types of oxides also depends on the crystallinity of the GeSi alloy. When a uniform GexSi1−xO2 oxide grows, its thickness is proportional to the square root of the oxidation duration, which indicates that the rate-limiting process is the diffusive transport across the oxide of, most probably, the oxidant. The rate increases with the Ge content in the alloys. The proportionality constant, B, for this process is B(T)=[(1.0±0.2)×1011 nm2/h]exp[(−1.1±0.2 eV)/kT] for Ge0.47Si0.53. It is proposed that, in general, the oxidation behavior is determined by the competition between the speed of the diffusive process in the unoxidized GeSi alloy and the velocity at which the oxidation front progresses. The controlling factors are the oxidation temperature, the composition, and the structure of the GexSi1−x alloy. A model is proposed that is based on these three factors. Analogies with this system exist where all three elements are solid.

Wet oxidation of GeSi at 700 °C

About 500-nm-thick films of Ge0.36Si0.64 and Ge0.28Si0.72 grown epitaxially on (100)Si have been oxidized at 700 °C in wet ambient. A uniform GexSi1−xO2 oxide layer forms with a smooth interface between it and the unoxidized GexSi1−x layer below. The composition and structure of that layer remains unchanged as monitored by backscattering spectrometry or cross-sectional transmission electronic microscopy. The oxide of both samples grows as square root of oxidation duration. The parabolic rate constant increases with the Ge content and is larger than that for wet oxidation of pure Si at the same temperature. The absence of a regime of linear growth at this relatively low temperature indicates a much enhanced linear rate constant.