Restructuring and redispersion of silver on SiO2 under oxidizing/reducing atmospheres and its activity toward CO oxidation

The effects of oxygen-hydrogen pretreatments of nanosilver catalysts in cycle mode on the structure and particle size of silver particles, and subsequently the activity of the catalyst toward CO oxidation (or CO selective oxidation in the presence of H2), are reported in this paper. Ag/SiO2 catalyst with silver particle sizes of ca. 6 approximately 8 nm shows relatively high activity in the present reaction system. The adopting of a cycle of oxidation/reduction pretreatment has a marked influence on the activity of the catalyst. Oxygen pretreatment at 500 degrees C results in the formation of subsurface oxygen and activates the catalyst. As evidenced by in-situ XRD and TEM, the following H2 treatment at low temperatures (100 approximately 300 degrees C) causes surface faceting and redispersing of the silver particles without destroying the subsurface oxygen species. The subsequent in-situ FTIR and catalytic reaction results show that CO oxidation occurs at -75 degrees C and complete CO conversion can be obtained at 40 degrees C over such a nanosilver catalyst pretreated with oxygen at 500 degrees C followed by H2 at 100 degrees C. However, prolonged hydrogen treatment at high temperatures (>300 degrees C) after oxygen pretreatment at 500 degrees C induces the aggregation of silver particles and also depletes so much subsurface oxygen species that the pathway of CO oxidation by the subsurface oxygen species is inhibited. Meanwhile, the ability of the catalyst to adsorb reactants is greatly depressed, resulting in a 20 approximately 30% decrease in the activity toward CO oxidation. However, the activity of the catalyst pretreated with oxygen at 500 degrees C followed by hydrogen treatment at high temperatures (>300 degrees C) is still higher than that directly pretreated with H2. This kind of catalytic behavior of silver catalyst is associated with physical changes in the silver crystallites because of surface restructuring and crystallite redispersion during the course of oxygen-hydrogen pretreatment steps.     

Surface and structural properties of pt/ceo2catalyst under preferential co oxidation in hydrogen (PROX)

Summary Preferential oxidation of CO in the presence of excess hydrogen was studied on Pt/CeO₂with 5% metal loading. Catalytic data were similar to those observed on 1% Pt/CeO₂earlier [16]. The optimum temperature region is T￡373 K; conversion and selectivity of CO oxidation strongly decreased at higher temperatures. High-pressure XPS indicated CO adsorbed on platinum particles and significant amount of water on the ceria surface. The top-most ceria surface re-oxidized as small amount of oxygen (3%) was introduced into the H₂/CO feed. Despite this surface re-oxidation, high-resolution TEM after reaction indicated oxygen deficient ceria bulk structure, in which the defects formed a super-cell, with CeO_1.695structure. The defective ceria is suggested to play an important role stabilizing the hydrogen bonded surface water, which (i) suppresses further hydrogen oxidation and (ii) reacts at the metal/support interface with linearly adsorbed CO in a low temperature water-gas-shift type reaction to produce CO₂.</o:p>     

Catalytic oxidation of no to NO2 over Cr/TiO2 and Cu/TiO2 under oxidizing atmosphere

The catalytic activity of Cr/TiO₂ and Cu/TiO₂ for the oxidation of NO under an oxidizing atmosphere has been examined. Both catalysts had excellent ability for the oxidation of NO to NO₂ in the temperature range of 350–400°C.     

Effect of cation-radical activation and catalysis on SO2 oxidation in mild conditions

SO₂ activation by cation-radicals (CR) of tri-p-tolylamine, tri-p-anisylamine and N,N,N,N-tetramethyl-p-phenylenediamine has revealed that stable CR oxidize SO₂ to SO₄ ^2–. The dependence of the reaction rate on the oxidative properties of CR and the reaction medium has been studied. The possibility of SO₂ oxidation by oxygen with CR acting as catalysts is shown.

---

- () --, -- N, N, N, N —. , SO₂ SO₂ ^4–. . SO₂ , .

     

Surface oxidation of Ni-cermet electrodes by CO2 and H2O and how to moderate it

The oxidation of porous Ni-yttria-stabilized zirconia (YSZ) and Ni-gadolinia-doped ceria (GDC) ceramicmetal (cermet) electrodes in H₂O and CO₂atmospheres was studied by near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS).We show that the oxidation of nickel by the two gases is not similar,as is commonly believed,but it depends on the ceramic type.Nickel is vulnerable to oxidation in H₂O but it resists to CO₂in Ni-GDC,as compared to the Ni-...     

Substrate temperature effects on the structural,compositional, and electrical properties of VO2 thin films deposited by pulsed laser deposition

The vanadium dioxide (VO₂) thin films were deposited on silicon (100) substrate using the pulsed laser deposition technique. The thin films were deposited at different substrate temperatures (500°C, 600°C, 700°C, and 800°C) while keeping all the other parameters constant. X‐ray diffraction confirmed the crystalline VO₂ (B) and VO₂ (M) phase formation at different substrate temperatures. X‐ray photoelectron spectroscopy analysis showed the presence of V⁴⁺ and V⁵⁺ charge states in all the deposited thin films which confirms that the deposited films mainly consist of VO₂ and V₂O₅. An increase in the VO₂/V₂O₅ ratio has been observed in the films deposited at higher substrate temperatures (700°C and 800°C). Scanning electron microscope micrographs revealed different surface morphologies of the thin films deposited at different substrate temperatures. The electrical properties showed the sharp semiconductor to metal transition behavior with approximately 2 orders of magnitude for the VO₂ thin film deposited at 800°C. The transition temperature for heating and cooling cycles as low as 46.2°C and 42°C, respectively, has been observed which is related to the smaller difference in the interplanar spacing between the as‐deposited thin film and the standard rutile VO₂ as well as to the lattice strain of approximately −1.2%.     

Electronic,mechanistic, and structural factors that influence the performance of molecular water oxidation catalysts anchored on electrode surfaces

To harness solar energy to generate fuels powerful anodes for the water oxidation reaction need to be developed. During the last decade, an extensive number of molecular water oxidation catalysts based on transition metals have been reported, and in some cases, these molecular catalysts have been anchored on conductive surfaces generating molecular anodes. In this review, we analyze the factors that influence the performance of these molecular anodes, which are largely related to their mechanism of O–O bond formation and the nature of the anchoring functionality.     

Effect of thermal ageing in oxidizing or reducing atmosphere on the texture and structure of palladium, copper and palladium-copper on alumina catalysts

A series of palladium, copper and palladium-copper supported on alumina catalysts were aged at high temperature (100°C) in oxidizing (oxygen) or reducing (hydrogen) atmosphere. The effect of this treatment on the texture and structure of these samples was studied. Oxidizing atmosphere gives large particles of palladium oxide, whgereas reducing atmosphere gives large particles of copper or of palladium-copper on alumina catalysts when agend in reducing conditions while the hydrogenating activity is drastically decreased.     

Combination of NH2OH·HCl and NaIO4: a new and mild oxidizing agent for selective oxidation of alcohols to carbonyl compounds

A new method for oxidation of alcohols to carbonyl compounds has been developed using NH₂OH·HCl and NaIO₄ under mild reaction conditions at room temperature. Application of the method for the synthesis of diiodo compound from α,β-unsaturated carbonyl compound is also described.     

Sorbent-based sampling methods for volatile and semi-volatile organic compounds in air. Part 2. Sorbent selection and other aspects of optimizing air monitoring methods

Sorbent tubes/traps are widely used in combination with gas chromatographic (GC) analytical methods to monitor the vapour-phase fraction of organic compounds in air. Applications range from atmospheric research and ambient air monitoring (indoor and outdoor) to occupational hygiene (personal exposure assessment) and measuring chemical emission levels. Part 1 of this paper reviewed the main sorbent-based air sampling strategies including active (pumped) tube monitoring, diffusive (passive) sampling onto sorbent tubes/cartridges plus sorbent trapping/focusing of whole air samples that are either collected in containers (such as canisters or bags) or monitored online. Options for subsequent extraction and transfer to GC(MS) analysis were also summarised and the trend to thermal desorption (TD)-based methods and away from solvent extraction was explained. As a result of this trend, demand for TD-compatible sorbents (alternatives to traditional charcoal) is growing. Part 2 of this paper therefore continues with a summary of TD-compatible sorbents, their respective advantages and limitations and considerations for sorbent selection. Other analytical considerations for optimizing sorbent-based air monitoring methods are also discussed together with recent technical developments and sampling accessories which have extended the application range of sorbent trapping technology generally.     

Effects of deposition of different contents of Ag nanoparticles on thickness and structural,morphological, compositional,optical, and photocatalytic properties of 1D TiO2 nanoarrays

Research on Chemical Intermediates - One-dimensional (1D) TiO2 nanoarrays (NAs) were synthesized by a hydrothermal process, and Ag nanoparticles (NPs) were deposited on the arrays using a...     

Plasma SNMS investigations on powder metallurgical Cr and Ti-48Al-2Cr after oxidation in air and 15N2/18O2 atmosphere

Oxidation experiments at 800°C and 900°C, partly in atmospheres enriched in isotope tracers, with subsequent SNMS depth profiling confirmed that two well-known oxidation mechanisms are also valid for the title materials: In powder metallurgical Cr, Y addition shows the reactive element effect, most completely by means of Y implantation and to a lesser extent by the addition of Y₂O₃ dispersion, classical fast Cr outward transport dominated kinetics is replaced by kinetics that is governed by slow inward diffusion of oxygen. In γ-TiAl with 2 at-% Cr, N is identified as the “frontline” oxidizing element being able to penetrate existing oxide and nitride layers inward towards the bulk metal. These results exemplify that the combination of plasma SNMS depth profiling with oxidation experiments, especially in ¹⁵N₂/¹⁸O₂ tracer atmosphere, and with reactive element implantation is an excellent method to obtain basic insights into corrosion mechanisms. Especially advantageous is the use of SNMS because of its much lesser matrix dependence compared to SIMS which has more frequently been used for this kind of experiments.     

Air oxidation of hydrazine. 1. Reaction kinetics on natural kaolinites, halloysites, and model substituent layers with varying iron and titanium oxide and O- center contents

Air oxidation of hydrazine was studied by using a group of kaolinites, halloysites, and substituent oxides as models for the tetrahedral and octahedral sheets. The rate was found to be linear with oxygen. The stoichiometry showed that oxygen was the primary oxidant and that dinitrogen was the only important nitrogen-containing product. The rates on kaolinites were strongly inhibited by water. Those on three-dimensional silica and gibbsite appeared not to be. That on a supposedly layered silica formed from a natural kaolinite by acid leaching showed transitional behavior--slowed relative to that expected from a second-order reaction relative to that on the gibbsite and silica but faster than those on the kaolinites. The most striking result of the reaction was the marked increase in the rate of reaction of a constant amount of hydrazine as the amount of clay was increased. The increase was apparent (in spite of the water inhibition at high conversions) over a 2 order of magnitude variation of the clay weight. The weight dependence was taken to indicate that the role of the clay is very important, that the number of reactive centers is very small, or that they may be deactivated over the course of the reaction. In contrast to the strong dependence on overall amount of clay, the variation of amounts of putative oxidizing centers, such as structural Fe(III), admixed TiO2 or Fe2O3, or O- centers, did not result in alteration of the rate commensurate with the degree of variation of the entity in question. Surface iron does play some role, however, as samples that were pretreated with a reducing agent were less active as catalysts than the parent material. These results were taken to indicate either that the various centers interact to such a degree that they cannot be considered independently or that the reaction might proceed by way of surface complexation, rather than single electron transfers.     

Monolayer oxide catalysts from alkoxide precursors, part IV. Vanadia, titania and stibia on SiO2, SnO2, and TiO2 in 2-propanol oxidation

Vanadia monolayer catalysts supported on SnO₂, ZrO₂, TiO₂, and SiO₂, similarly as titania and stibia monolayers deposited on SiO₂, have been synthesized by reacting the corresponding metal alkoxides with hydroxyls on the carrier surfaces. The metal ions loads in monolayer systems were determined. The catalysts activity was tested in 2-propanol transformations. The nature of carrier has a strong influence on the dehydrating to dehydrogenating activity ratio of vanadia monolayer.     

Oxidative cleavage reactions of catechol and phenol to monoester of cis,cis,-muconic acid with the oxidizing systems of O2/CuCl,KOH/CuCl2

Oxidative cleavage reactions of catechol with CuCI to give monoester of cis,cis-muconic acid in pyridine containing alcohol was investigated under various conditions. The same oxidation was carried out also with the systems of KO₂/CuCl₂ and KOH/CuCl₂ in pyridine containing alcohol in the absence of oxygen. Phenol was oxidized with the same oxidizing systems to give the same monoester of muconic acid.     

Surface species and gas phase products in the preferential oxidation of CO on TiO2-supported Au-Rh bimetallic catalysts

The oxidation of CO in the presence of hydrogen (PROX process) was investigated on bimetallic Au-Rh catalysts at 300–373 K by Fourier transform infrared spectroscopy and mass spectroscopy. The effects of catalyst composition, reaction temperature and composition of the reacting gas mixtures have been studied. The IR studies revealed the formation of bi- and monodentate carbonates, bicarbonates and hydrocarbonates on the catalysts surfaces; these surface species proved to be not involved in the surface reactions. The formation of adsorbed formaldehyde was observed on all surfaces, except 1% (0.25Au+0.75Rh)/TiO₂. Adsorbed CO₂ (as the surface product of CO oxidation) was not detected on any surface. The presence of both O₂ and H₂ reduced the surface concentration of CO adsorbed on the metallic sites. Mass spectroscopic analysis of the gas phase showed that gaseous CO₂ was formed in the highest amount in the CO+O₂ mixture, the presence of H₂ suppressed the amount of CO₂ produced. This negative effect of H₂ was the lowest on the 1% Rh/TiO₂ and 1% (0.25Au+0.75Rh)/TiO₂ catalysts.     

Influence of the activation temperature on structural and textural properties of NiMo/Al2O3 hydrodesulfurization catalysts

Investigations of a commercial NiMo/Al₂O₃ hydrodesulfurization (HDS) catalyst are directed towards optimization of the activation procedure of HDS catalyst concerning active phase formation and thermal stability. Structural and textural data obtained with XRD, IR-FTIR, XPS and LTNA reveal that the optimal temperature for the formation of active species on the catalyst surface and an appropriate pore structure is 300°C.     

Photoinduced and chemical oxidation of coordinated imine to amide in isomeric osmium(II) complexes of N-arylpyridine-2-carboxaldimines. Synthesis,characterization, electron transfer properties,and structural studies

The reaction of N-arylpyridine-2-carboxaldimine [C(5)H(4)NC(H)NC(6)H(4)R] (HL) with ammonium hexabromoosmate (NH(4))(2)[OsBr(6)] in boiling 2-methoxyethanol afforded a violet solution from which two geometrical isomers of [OsBr(2)(HL)(2)] (1 and 2) were isolated. These are characterized by analytical and spectroscopic data. (1)H NMR spectral data were used for the identification of the isomers. The blue-violet isomer, 1 (designated as ctc), has a 2-fold symmetry axis and gave rise to resonances for only one coordinated HL. The geometry of the ctc-isomer was, however, revealed from the X-ray structure determination of a representative example. The red-violet isomer (2, designated as ccc), on the other hand, is unsymmetrical and gave rise to a large number of proton resonances. The isomeric complexes, [OsBr(2)(HL)(2)], showed intense MLCT transitions in the visible region. This transition, in the ccc-isomer, is slightly (10 nm) red shifted in comparison to the ctc-isomer. These diimine complexes showed one metal based reversible oxidation assignable to the Os(III)/Os(II) process followed by two irreversible oxidations at more anodic potentials (>1.4 V). In addition to these, the complexes also showed two irreversible ligand reductions at high cathodic potentials (<-1.4 V). An unusual type of photochemical transformation of the azomethine function of coordinated HL in osmium compounds 1 is studied. When an air equilibrated acetonitrile solution of 1 was exposed to a xenon lamp, it underwent oxidation affording the mixed ligand, amido complexes of general formula [OsBr(2)(HL)(LO)], 3 (LO = C(5)H(4)NC(O)-N-C(6)H(4)R), in an excellent yield (>95%). This transformation (1 --> 3) was achieved chemically when H(2)O(2) was used as an oxidant. Notably, the chemical oxidation with H(2)O(2) also led to the formation of a tetravalent complex, [OsBr(2)(LO)(2)], 4, as a minor product. Compound 3 was characterized by various spectroscopic and analytical techniques. The room temperature magnetic moment of 3 corresponds to a t(2)(5) configuration for the osmium(III) center. EPR spectra of the amido complexes were recorded at 77 K in 1:1 dichloromethane-toluene glass, and they were anisotropic in nature. FAB mass spectra of 3 displayed intense peaks due to parent molecular ions. For example, the complex [OsBr(2)(HL(1))(L(1)O)], 3a, showed a strong peak at m/z 729 amu. The electronic spectrum of compound 3 consisted of a broad LMCT transition (ca. 525 nm; epsilon, 3000 M(-1) cm(-1)). The cyclic voltammogram of compound 3 consisted of two responses, one each on the positive and negative side of SCE, corresponding to Os(IV)/Os(III) (ca. 0.8V) and Os(III)/Os(II) (ca. -0.3V) couples, respectively. There has been a large cathodic shift of potential for the Os(III)/Os(II) couple in 3 in comparison to that in the parent complex, 1. The diamido compound [OsBr(2)(LO)(2)], 4, is diamagnetic and insoluble in common solvents. The X-ray structure determination of a representative sample, 4a, is reported. The molecule contains a C(2)-symmetry axis with bromide ions in relative cis positions. The Os-N(amide) bond lengths are considerably shorter than the Os-N(pyridine) lengths. All other bond lengths and angles fall within the expected range.     

Control of sp2/sp3 carbon ratio and surface chemistry of nanodiamond powders by selective oxidation in air

The presence of large amounts of nondiamond carbon in detonation-synthesized nanodiamond (ND) severely limits applications of this exciting nanomaterial. We report on a simple and environmentally friendly route involving oxidation in air to selectively remove sp(2)-bonded carbon from ND. Thermogravimetric analysis and in situ Raman spectroscopy shows that sp(2) and sp(3) carbon species oxidize with different rates at 375-450 degrees C and reveals a narrow temperature range of 400-430 degrees C in which the oxidation of sp(2)-bonded carbon occurs with no or minimal loss of diamond. X-ray absorption near-edge structure spectroscopy detects an increase of up to 2 orders of magnitude in the sp(3)/sp(2) ratio after oxidation. The content of up to 96% of sp(3)-bonded carbon in the oxidized samples is comparable to that found in microcrystalline diamond and is unprecedented for ND powders. Transmission electron microscopy and Fourier transform infrared spectroscopy studies show high purity 5-nm ND particles covered by oxygen-containing surface functional groups. The surface functionalization can be controlled by subsequent treatments (e.g., hydrogenization). In contrast to current purification techniques, the air oxidation process does not require the use of toxic or aggressive chemicals, catalysts, or inhibitors and opens avenues for numerous new applications of nanodiamond.