Number Of Metal Oxides Research Articles

Carbon dioxide decomposition through gas exchange in barium calcium iron niobates

A number of metal oxides and perovskites are capable of being reduced at high temperatures and then re-oxidised in the presence of CO2 or H2O to form CO or H2. Barium calcium iron niobates have been found to be redox-active in this way. The redox activity of these perovskites was explored, and the chemical and physical stability was investigated using EDX and SEM imaging, respectively. The most promising, Ba2Ca0.66Nb0.34FeO6-δ (BCNF1), showed mass changes of 0.45 % after five cycles of reduction with N2 and re-oxidation with 10 % CO2. BCNF1 is chemically stable as it shows no changes in XRD and shows no evidence of sintering, although cracking of the pellets was observed after re-oxidation. The low enthalpy of re-oxidation of BCNF1 coupled with the high and sustained mass change makes this perovskite suitable for chemical looping use for energy storage and conversion systems.

Unexpected ground-state crystal structures and mechanical properties of transition metal pernitrides MN2 (M= Ti, Zr, and Hf)

Titanium pernitride (TiN2) with CuAl2-type structure (I4/mcm, Z = 4), the first reported member to the family of early transition metal pernitride was recently synthesized via direct chemical reactions between titanium mononitride and nitrogen molecules at 73 GPa and 2400 K. Using an unbiased structure searching method combined with first principles calculations, we here have fully investigated the energy landscape of MN2 (M = Ti, Zr, and Hf) at high pressure up to 100 GPa and suggested that the synthesized I4/mcm structure is a metastable form for TiN2 at ambient conditions. A novel tetragonal I4/mmm (Z = 4) structure composed of the MN6 octahedrons connected by peculiar double NN bonded dimers, was identified as the universal thermodynamic ground-state structure for three MN2. Under high pressure, the I4/mmm structure undergoes a first-order phase transition to the discovered I4/mcm structure with a large volume drop, accompanying the increase of coordination and oxidation number of metal M atoms in MN2. The configuration of M+2 [N2]−2 for MN2 in the I4/mmm structure was demonstrated by electronic structure and chemical bonding analyses. The mechanical properties including elastic parameters and elastic anisotropy behaviors of MN2 within I4/mmm structure were systematically studied. Results on the elastic anisotropy and ideal tensile strengths indicate that the (001) planes may be viewed as their cleavage planes.

Functionalization of wood/plant-based natural cellulose fibers with nanomaterials: a review

Being the most abundant natural biopolymer on earth, cellulose has been vastly exploited in a range of applications, from writing paper to high-end biosensors. Natural cellulose fibers can be isolated from wood or non-woody plants such as hemp, jute, flax, and bamboo by chemical or mechanical treatments. To make it suitable for targeted applications, cellulose fibers are modified with functional moieties in the nanometer scale. Cellulose has been functionalized with noble metals such as silver and gold nanoparticles for catalysis and antimicrobial applications. A number of metal oxides, such as zinc oxide, titanium dioxide, and tin dioxide have been incorporated into cellulose. The porosity, hydrophilicity, and roughness of cellulose surface makes it an ideal substrate for a plethora of sensing applications. Further, it can be made into a lightweight, portable, foldable, and disposable device, which provides an excellent platform for various point-of-care purposes. Cellulose fibers have also been immobilized with carbon nanomaterials, including carbon nanotubes and graphene oxide. For optical applications, [Fe(hptrz)3](OTs)2 spin-crossover nanoparticles have also been immobilized on cellulose fibers. Likewise, many enzymes, macromolecules, and some polymers have been used to modify natural cellulose for specific end uses. This review focuses on recent developments in the modification or immobilization of functional materials on cellulose fibers, in macro-scale only, obtained from wood or plant sources.

Electrochemical Doping as a Way to Enhance Water Photooxidation on Nanostructured Nickel Titanate and Anatase Electrodes

AbstractA number of metal oxides have been proposed as useful materials for the photoelectrochemical (PEC) production of hydrogen from water. However, up to now, an ideal standalone material has not been found. We have investigated the possible use of nickel titanate (NiTiO3) nanorods as a photoanode. Although these electrodes absorb visible light, they show a modest PEC behavior. Interestingly, the photocurrent for water oxidation undergoes a 30‐fold enhancement after an optimized reductive electrochemical pretreatment. Here, the induced doping is studied and compared with the corresponding for anatase nanoporous electrodes. The results reveal the key role of the electrolyte pH as well as the size of the electrode building blocks. The photocurrent promotion upon the electrochemical pretreatment can be ascribed to an enhanced charge transport linked to the ability of proton insertion in the crystal structure.

(Invited) Photocatalytic and Photoelectrochemical Water Splitting Under Visible Light Irradiation

Photocatalytic water splitting into H2 and O2 using a semiconductor photocatalyst has received much attention recently due to the potential of this method for the clean production of H2 from water utilizing solar energy. Although a number of metal oxides have been reported to be active photocatalysts for the water-splitting reaction, most only function under UV light owing to the large band gap energy of the materials. Since almost half of all incident solar energy at the Earth’s surface falls in the visible region, the efficient utilization of visible light remains indispensable for realizing practical H2 production. However, it had been noted that it is intrinsically difficult to develop an oxide semiconductor photocatalyst that has both a sufficiently negative conduction band for H2 production and a sufficiently narrow band gap (i.e., <3.0 eV) for visible light absorption because of the highly positive valence band (at ca. +3.0 V vs. NHE) formed by the O 2p orbital. Although some non-oxide semiconductors, such as (oxy)sulfides, (oxy)nitrides, and organic dyes (including metal complexes) posses appropriate band levels for water splitting under visible light, they are generally unstable and readily become deactivated through photocorrosion or self-oxidation, rather than evolving O2. In order to split water under visible light, we have developed new type of water splitting system based on two-step photoexcitation between two different photocatalyst materials. Over a H2 evolution photocatalyst, the photoexcited electrons reduce water to H2 and holes oxidize a reductant (Red) to an oxidant (Ox). The Ox is reduced back to the Red by photoexcited electrons generated over an O2 evolution photocatalyst, where the holes oxidize water to O2. Based on the strategy, we have achieved overall water splitting using various visible light responsive photocatalysts, such as SrTiO3 doped with Cr, tantalum oxynitrides (TaON or BaTaO2N), and organic dyes, which work as a H2 evolution photocatalyst, combined with WO3 for O2 evolution in the presence of a shuttle redox mediator such as iodate/iodide (IO3 –/I–). The use of BaTaO2N or coumarin organic dye was demonstrating to be photoactive at wavelength up to ca. 700 nm, indicating the potential of a two-step water-splitting system for utilizing a broader band of visible spectrum. We also demonstrated that some oxy-halides such as Bi4NbO8Cl can stably oxidize water in the presence of iron Fe3+/Fe2+ redox under visible light, also affording Z-scheme water spliting under visible light. The introduction of two-step photoexcitation mechanism (Z-scheme) also enabled us to generate H2 separately from O2, which will be quite important in the practical application to avoid the explosion of the mixture, by employing simple porous glass filters that divide the two different photocatalysts but permit the transportation of redox couple. Some metal oxynitride materials possess appropriate band levels for water splitting as well as a narrow band gap that permits visible light absorption; are thus a promising candidate as photoanode materials for efficient water splitting under visible light with low applied bias. However, the introduction of N 2p orbitals in the valence band generates a new problem in stability that need to be solved. Most oxynitrides undergo self-oxidative deactivation some degree, in which photogenerated holes oxidize nitrogen anions to N2. Recently, we have developed a simple method for fabricating stable and efficient photoanodes of oxynitrides (TaON and BaTaO2N), in which highly dispersed nanoparticles of water-oxidizing catalyst (e.g., CoOx) loaded efficiently scavenge the photogenerated holes and effectively suppress self-oxidative deactivation of the surface. It was also demonstrated that co-loading of RhOx with CoOx significantly improved both the stability and density of photocurrent on the oxynitride photoanodes. These modifications enabled us to stably split water into H2 and O2 efficiently under visible light irradiation at a relatively low applied bias.

Effects of nanoparticle and porous metal oxide supports on the activity of palladium catalysts in the oxidative coupling of 4-methylpyridine

The oxidative coupling of 4-methylpyridine to 4,4′-dimethyl-2,2′-bipyridine over palladium oxide is a simple, environmentally friendly, one-step process to produce bipyridines, which are commonly used with transition metal ions to form complexes with interesting properties. However, the reaction is slow and the palladium catalyst deactivates during reaction, which means that catalyst improvements are needed for large-scale production of more economically viable bipyridine products. In this study, a number of metal oxides were investigated as catalyst supports and compared to the best performing catalysts to date, i.e. Pd/C and Pd/n-Al 2O 3(+). Catalysts supported on several nanoparticle oxides with varying properties as well as some conventional supports were prepared and characterized in an attempt to determine properties that lead to high catalytic activities in the oxidative coupling of 4-methylpyridine. It was found that two general categories of active catalysts can be prepared; (1) palladium supported on very high surface area materials, such as Pd/n-Al 2O 3(+) and Pd/MgO, and (2) palladium supported on metal oxides known to induce strong palladium-support interactions, e.g. Pd/ZrO 2, Pd/(n-ZrO 2 + n-CeO 2) and Pd/n-ZnO. While there is no simple correlation between the palladium surface area and the catalytic activity, higher palladium dispersions generally gave higher yields compared to lower dispersion catalysts. The results indicate that the reaction is structure sensitive, i.e. not all the palladium on the surface is equivalent and some palladium species are more active than others. The acidic and basic properties of the supports were determined via chemisorption of ammonia and carbon dioxide, respectively. The data indicate that there is no correlation between the acidic or basic sites of the supports and the palladium dispersion or the catalytic activity, although highly acidic or highly basic supports should be avoided as they resulted in lower dispersions than expected from their corresponding surface areas. In terms of economic viability the porous TiO 2 support was determined to be the most competitive with the nanoparticle alumina support as it results in a catalyst with comparable yields and is less expensive compared with nanoparticle alumina. The palladium supported on nanoparticle ZrO 2 and MgO are also promising catalysts.

Thermodynamics of Hydrogen Bonding between CO and the Supercage Brønsted Acid Sites of the H-Y Zeolite − Studies from Variable Temperature IR Spectrometry

A procedure was developed for studying the thermodynamics involved in the interaction of carbon monoxide with Brønsted acid sites in zeolites, without the need for microcalorimetric measurements. The observables are the integrated infrared intensity of the C−O stretching mode of the OH···CO adducts, temperature, and equilibrium pressure. These quantities were measured by means of variable-temperature IR spectroscopy, using a home made IR cell equipped with a capacitance pressure gauge. Interaction of CO with Brønsted acid sites at the supercage of the faujasite-type H-Y zeolite involves a standard enthalpy change, ΔH° of −25.6 kJ mol−1, and a standard entropy change, ΔS° of −161 J mol−1 K−1. These values were compared with the corresponding figures obtained from the calorimetric determination of CO adsorption on a number of metal oxides and zeolites.

Inference of Ga Ion TOF-SIMS Fragments of Metal-chlorides and -oxides.

Gallium ion TOF-SIMS fragment patterns from some metal-chlorides and -oxides can be qualitatively inferred. Considering the electron affinities of Cl and O, comparatively higher than those of other metal elements, rules of nx ≥ (y + 1) for positive ions and nx ≤ (y + 1) for negative ions were introduced and applied to infer the fragment patterns of metal-chlorides, MxCly, where the oxidation number of metal M is n. For oxides, MxOy (oxidation number of metal M is n), the rules used to infer the fragment patterns nx ≥ (2y + 1) for positive ions and nx ≤ (2y + 1) for negative ions. Further, the effect of water on the surfaces of chlorides and oxides, and the fragment patterns of some metal-nitrates are discussed.

Catalytic effects of metal oxides on the thermal decomposition of sodium chlorate

The thermal decomposition of NaClO 3 in the presence of metal oxides was studied using thermogravimetric analysis. A number of metal oxides were prepared to have comparable surface areas, and their catalytic effects on NaClO 3 decomposition were compared. It was found that the catalytic activity depends on the electron configurations of the metal cations. Oxides containing metal cations with partially filled d-orbitals are all active catalysts for NaClO 3 decomposition. Transition metal cations with no d-electrons are moderately active. Metal oxides containing metal cations with completely filled d-orbitals or noble gas configurations have very little activity.

Nitrous Oxide Formation and Destruction by Industrial No Abatement Techniques Including Scr

Systematic investigations on N2O emission from full scale stationary combustion units, equipped with primary or secondary NO control techniques, are scarce or inexistent. Recent results obtained from laboratory scale studies are presented, from which it appears that fuel staging, selective non catalytic NO reduction with ammonia and selective catalytic NO reduction in the presence of ammonia, should be considered as potential sources of nitrous oxide emission enhancement. For the two mentioned gas phase NO abatement techniques (fuel staging and NCSR), this N2O emission enhancement is clearly linked with a decrease of the temperature, a result that might have been expected from the known gas phase reactions of N2O formation and destruction. Production of N2O from NCSR is more important than from fuel staging, and increases with ammonia concentration; this probably is related to the fact that ammonia yields N2O precursors (NH, NH2) readily by its decomposition. Separate injection of pure NO or NH3 suggests that N2O is a product of the interaction of those two reactants, whereas NO also is formed as a primary ammonia decomposition product in the presence of oxygen. A kinetic investigation of N2O formation from SCR has been made. It is shown that catalytic decomposition of neat ammonia yields both NO and N2O, the former as a primary product (from adsorbed ammonia and solid bound oxygen), the latter as a secondary product (from NO and adsorbed ammonia). Both NO and N2O subsequently undergo catalytic decomposition. In the presence of molecular oxygen, another NO formation (from O2 and adsorbed ammonia) manifests itself at somewhat higher temperatures, creating the well known optimum temperature window . Comparative tests conducted on a number of metal oxides, tend to show that high efficiency for NO decomposition is often related to high production of N2O within the temperature window .

Thorium as a carbon oxidation catalyst

A large number of metal oxides are known to catalyze the gasification of carbon by molecular oxygen [1,2] and very small concentrations of the most active materials are capable of reducing the temperature at which carbon begins to oxidize in air by several hundred degrees [3]. Although active catalytic species come from every Group of the Periodic Table, the majority of catalysts appear to function via a redox cycle, being reduced to lower oxidation states by reaction with the carbon substrate and then reoxidizing back to the original state by reaction with ambient oxygen [3,4]. A few catalysts, such as Cr,O,, silver metal and possibly transition metal carbides [5], appear to accelerate the oxidation reaction by providing sites for the dissociation of molecular oxygen to give absorbed oxygen atoms, which then diffuse across the carbon surface to active sites where the carbon is gasified. In some cases the catalytic effect may be enhanced by the formation of a molten phase, followed by spreading and wetting of the carbon surface [6]. The actinide elements and their compounds have not been studied as catalysts for this reaction, although their behavior is somewhat predictable based on the above considerations. Uranium, which forms a series of oxides, has been found to be active catalytically, when present as UO, or U,O, [7]. Sampath et al. [8] have, however, recently reported that thorium oxide, ThO,. is an active catalyst for the oxidation of graphite in air in the temperature range 600-900°C. As the single oxide of thorium is very stable (AH&R = -293 kcal mol-‘), it would not be reduced to metal by heating with carbon in this temperature range and hence it would not be expected to participate in oxidation-reduction cycles on the graphite surface. ThO, is also much more stable in the presence of oxygen than the known carbides, ThC and ThC,, and hence carbide formation during carbon gasification would not be expected. In addition, the very high melting point (3220 o C) of ThO, would preclude the formation of molten phases and the wetting of the graphite substrate. For all these reasons a demonstrated catalytic activity for thoria would disqualify the usual redox process as the likely mechanism of the catalyzed gasification of carbon in this case, and as ThO, is only an indifferent catalyst for gas phase oxidation reactions (e.g., oxidation of CO) [9], it is also improbable that this oxide would bring about the dissociation

Oxidation of methane over supported precious metal catalysts

Studies of the kinetics of methane oxidation over palladium and platinum catalysts supported on a number of metal oxides were made using a pulse-flow microreactor technique. The reaction was investigated at temperatures in the range 500–800 K using reactant mixtures with CH 4:O 2 ratios varying from 1:10 to 10:1. The supports themselves, with the exception of tin(IV) oxide, did not appreciably catalyze the oxidation reaction. Nevertheless, the activity of both palladium and platinum catalysts was enhanced by the use of metal oxides of large surface area, such as γ-aluminum(III) oxide. The support also influenced the ability of palladium to adsorb oxygen and there was a correlation between the adsorption capacity of the supported precious metal and its catalytic activity for methane oxidation. The presence of adsorbed oxygen was confirmed using X-ray photoelectron spectroscopy but oxygen adsorbed at high temperatures proved detrimental towards methane oxidation. Prolonged exposure of the precious metals to oxygen at high temperatures caused structural changes in palladium, although this did not occur with platinum. The different catalytic behaviour of palladium and platinum was attributed to the differing abilities of these precious metals to adsorb oxygen prior to reaction. Measurement by transmission electron microscopy of the diameters of palladium particles dispersed on a γ-aluminum(III) oxide support showed that the catalytic activity for methane oxidation was independent of particle size.

Anomalous Polarization in Ferroelectrics and other Oxides

The presence of a large relaxation polarization in a number of metal oxides is reported. It is shown that this phenomenon cannot be explained in terms of ferroelectricity or in terms of the Maxwell-Wagner effect. A series of experiments are described from the results of which it is deduced that the materials behave as solid state secondary cells. It is suggested that oxygen vacancies act as charge carriers and interact with the metal electrodes deposited on the specimen.