Bulk Metal Oxides Research Articles

Parahydrogen-induced polarization (PHIP) in heterogeneous hydrogenation over bulk metals and metal oxides

The results show that, in general, observation of PHIP effects on metals does not require the presence of a support. In addition, certain metal oxides can produce PHIP effects in the absence of a supported metal. The new promising catalysts for producing PHIP heterogeneously have been identified.

Photoinduced Topotactic Growth of Bismuth Nanoparticles from Bulk SrBi2Ta2O9

In situ growth of bismuth (Bi) nanoparticles on bulk SrBi2Ta2O9 (SBT) platelet is achieved by UV light illumination of SBT in aqueous glucose solutions. Interestingly, the as-produced Bi nanoparticles (NPs) are uniformly oriented with the epitaxial relationship of [001]Bi//[110]SBT, appearing as meso-single-crystal phase despite spatial separation between the NPs. Systematic investigations indicate that Bi(III) at the (110)SBT plane is topotactically reduced by photogenerated electrons, producing Bi atoms serving as nucleation seeds oriented from the (003) plane. Further growth of these oriented nuclei leads to formation of Bi NPs arranged laterally in mesocrystalline superstructure. The reported finding presents a simple, environmentally friendly approach toward preparation of highly organized, surface supported metal NPs through direct photoreduction of bulk metal oxides.

Highly ordered crystalline mesoporous metal oxides for hydrogen peroxide decomposition

Highly ordered mesoporous metal oxides (meso-MO x ) such as CeO2, Co3O4, Cr2O3, Mn2O3, NiO, RuO2, SnO2 and TiO2 were successfully synthesized using mesoporous silica KIT-6 as a hard template via the nano-replication method. The physicochemical properties of as-prepared meso-MO x were characterized by scanning electron microscopy, X-ray diffraction, N2 adsorption–desorption and temperature programmed techniques. Their catalytic behavior toward H2O2 decomposition was investigated and compared with the corresponding bulk metal oxides (bulk-MO x ) synthesized by the conventional precipitation method. These meso-MO x materials exhibited much higher catalytic activities than their bulk counterparts. In particular, meso-Mn2O3 and meso-RuO2 showed high activities, while meso-SnO2 resulted in no activity toward H2O2 decomposition. The overall conversion of H2O2 followed a general order: Mn2O3 > RuO2 > Co3O4 > CeO2 > NiO > Cr2O3 > TiO2 > SnO2.

A DFT-D study of structural and energetic properties of TiO2 modifications

The structures and relative energies of the three naturally occurring modifications of titanium dioxide (rutile, brookite and anatase) were investigated. For an accurate description, atom-pairwise dispersion-corrected density functional theory (DFT-D) was applied. The DFT-D3 scheme was extended non-empirically to improve the description of Ti atoms in bulk systems. New dispersion coefficients were derived from TDDFT calculations for electrostatically embedded TiO2 clusters. The dispersion coefficient is reduced by a factor of 18 compared to the free atom. The three TiO2 modifications were optimized in periodic plane-wave calculations with dispersion-corrected GGA (PBE, revPBE) and hybrid density functionals (PBE0, revPBE0). The calculated lattice parameters are in good agreement with experimental data, in particular the dispersion-corrected PBE0 and revPBE0 hybrid functionals. Although the observed relative stabilities could not be reproduced in all cases, dispersion corrections improve the results. For an accurate description of bulk metal oxides, London dispersion is a prominent force that should not be neglected when energies and structures are computed with DFT. Additionally, the influence of dispersion interactions on the relaxation of the TiO2(110) surface is investigated.

Transition metal-exchanged LTA zeolites as novel catalysts for methane combustion

The performance of different transition metal-exchanged zeolites for catalytic combustion of lean methane–air mixtures was studied in this work. Transition metals (Fe, Co, Mn) were incorporated to the parent LTA zeolite by ion exchange, using metal nitrates as precursors. Catalytic performance of these materials was compared to the corresponding to the bulk metal oxides in terms of activity and stability. In general terms, Co-exchanged zeolites present very similar behavior to the parent metal oxide, whereas the Mn-exchanged present very poor performance, and the Fe-exchanged zeolite present markedly better performance than the parent bulk oxide. Characterization of the fresh and used catalysts suggests that the superior behavior of the iron-exchanged zeolite is caused by a change in the reducibility properties of the metal species caused by the interaction with the zeolite framework (observed in TPR and O 2-TPD experiments).

Investigation on catalytic pyrolysis of pine sawdust: Catalyst screening by Py-GC-MIP-AED

On line Py-GC coupled to microwave induced plasma and atomic emission detector (Py-GC-MIP-AED) was used for screening the effect of various catalysts on the carbon yield of solid residue, gas and bio-oil from the pyrolysis at 500 °C of pine sawdust. Bio-oil was further divided into volatile, semi-volatile and non-volatile (i.e. heavy matter HM) fractions. The ability of Py-GC-MIP-AED for screening purposes was investigated by comparing the results with those obtained from preparative pyrolysis with a bench scale reactor. Catalyst screening was performed on 31 catalysts directly admixed with pine sawdust at 1/1 (w/w) ratio. Tested catalysts included mesoporous silica supported metal oxides, bulk metal oxides, clays, zeolites and catalysts for methanol synthesis from syngas (MS). A significant decrease in non-volatile fraction and slight decrease in bio-oil yield were obtained with ZnO, CuO, Fe 2O 3 and mixed oxide catalysts usually used for methanol synthesis from syngas. These materials actively transformed non-volatile fraction into GC detectable products with a net increase of gas, volatile or semi-volatile products.

On the mechanism of the transition from surface to bulk metal oxides (A perspective on the article “Mechanism of PdO thin film formation during the oxidation of Pd(1 1 1)”)

On the mechanism of the transition from surface to bulk metal oxides (A perspective on the article “Mechanism of PdO thin film formation during the oxidation of Pd(1 1 1)”)

Sulphur poisoning of transition metal oxides used as catalysts for methane combustion

Abstract Different bulk metal oxides (NiO, CuO, Mn 2 O 3 , Cr 2 O 3 and Co 3 O 4 ) were prepared and tested for the combustion of methane–air lean mixtures (5000 ppmV of CH 4 ) in presence of SO 2 (40 ppmV). Methane combustion experiments were carried out at ambient pressure, 425 and 625 °C and a space time of 93.3 g h mol CH4 −1 . Catalysts aged (60 h on stream) both in absence and in presence of SO 2 , were characterised by nitrogen physisorption (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM), temperature-programmed desorption (TPD-MS) and infrared spectroscopy (DRIFTS). It was observed that Cr 2 O 3 is not deactivated at the studied conditions, whereas all the other materials present fast deactivation in presence of SO 2 . Aged catalysts characterisation reveals that the outstanding behaviour of the Cr 2 O 3 catalyst is caused by the absence of formation of surface sulphates. By contrast, Mn 2 O 3 and Co 3 O 4 are more active than Cr 2 O 3 for methane oxidation in absence of sulphur species, but they are strongly deactivated in presence of SO 2 . Finally, the performance of the Cr 2 O 3 catalysts was compared to the corresponding to Pd/Al 2 O 3 catalyst and to a highly sulphur-tolerant perovskite (La 0.9 Ce 0.1 CoO 3 ) for the oxidation of methane in a real industrial emission from a coke oven, containing different inorganic gases (NH 3 , N 2 , H 2 , H 2 O, CO, CO 2 , SO 2 and H 2 S). Cr 2 O 3 catalyst shows to be also the most stable catalyst for the treatment of these emissions.

A strategy towards macroporous sponge-like networks of metal oxide-surfactant mesophases and bulk metal oxides

A methodology to prepare macroporous sponge-like networks of either mesostructured metal oxides or bulk metal oxides is reported. It uses bubbling oxygen gas released from hydrogen peroxide decomposition, through clear metal oxide gels with or without surfactant assemblies as mesostructure directors. The resulting inorganic sponge-like materials of various compositions such as SiO2, TiO2 and Nb2O5 consist of curved walls, 0.5–1.0 µm in thickness, of mesostructured or bulk metal oxides. This approach could possibly be extended to a wide variety of inorganic materials and should have significant benefits as the method is simple, inexpensive and amenable to scale-up and processing.

Surface Modification of Zinc Oxide Nanoparticles Influences the Electronic Memory Effects in ZnO−Polystyrene Diodes

Resistive switching effects in diode structures with a spin-coated active layer containing zinc oxide (ZnO) nanoparticles in a polystyrene matrix are studied. The switching effect can be influenced by modification of the surface of the nanoparticles with coordinating ligands (amines and thiols). Using n-propylamine as a ligand, memory effects are observed without the diodes having undergone the forming step that is usually required before switching effects can be observed in bulk metal oxides. Memory effects are characterized by impedance spectroscopy and temperature-dependent current-voltage measurements and involve a spontaneous, thermally activated gradual transition from a state with high frequency independent conduction to a state with lower conductivity.

Semi-ionic Model for Metal Oxides and Their Interfaces with Organic Molecules

A semi-ionic model is proposed for simulations of bulk metal oxides and interfaces between metal oxides and organic molecules. In this model, the electrostatic interactions are represented by partial charges on atoms; the covalent contributions are represented by a Morse function, and the van der Waals interactions are represented by a Lennard-Jones (LJ) function. Four potential functions are parametrized, using experimental data of representative binary oxides, by combining the Morse and electrostatic terms with LJ-12-6, LJ-9-6, and polarization shell model. The resulting potential functions are validated by predicting bulk properties of binary and ternary oxides. Coupled with classical force field parameters for organic molecules, one of the potentials is applied to study the adsorption energies, structures, and isothermal curves of methanol molecules on a MgO (100) surface.

Ab initio theory of magnetic interactions at surfaces

The low to high spin energy transition of Ni adsorbed on regular and defective sites ofMgO(100) and the relative strengths of bulk and surface magnetic coupling constants offirst row transition metal oxides (MnO, FeO, CoO, NiO and CuO) are taken as examples toillustrate some deficiencies of density functional theory (DFT). For these ionic systems acluster/periodic comparison within the same computational method (either DFT orHartree–Fock) is used to establish that embedded cluster models provide an adequaterepresentation. The cluster model approach is then used to obtain accurate values forthe magnetic properties of interest by using explicitly correlated wavefunctionmethods which handle the electronic open shell rigorously as spin eigenfunctions.

Tailor-made nanomaterials designed by electrochemical methods

We have developed different electrochemical procedures for the production of nanostructured bulk metals, catalyst particles and metal oxides with variable crystallite sizes. The crystallite size of the nanoparticles can be controlled by the variation of the physical and chemical process parameters. Efficient and inexpensive catalyst layers for polymer electrolyte fuel cells were prepared using pulsed electrodeposition (PED) and DC-plating procedures. The advantage of these techniques is the site-selective deposition of Pt and PtRu catalysts on the carbon support. Large amounts of nanostructured metal oxides with different crystallite sizes can be prepared with our EDOC process (electrochemical deposition under oxidizing conditions) which is based on the reduction of metal ions generated from the anodic dissolution of a sacrificial anode with subsequent oxidation of the formed metal clusters. For these nanomaterials we also discuss the physical properties (e.g. thermal stability, catalytic activity).

Listening to catalysis—a real time parallel method for high throughput product analysis

In this paper, we describe the development of two parallel real time detection systems for the analysis of the products of a heterogeneously catalyzed gas phase reaction based on the photoacoustic effect. We developed a process addressing the problem of automated catalyst preparation and prepared bulk metal oxides with high surface areas by an automated, parallelized synthesis based on activated carbon. To evaluate analytical systems and catalytic activity of our materials we performed catalytical tests in CO oxidation and ODH of ethane.

Quantitative Determination of the Catalytic Activity of Bulk Metal Oxides for Formic Acid Oxidation

The dissociative chemisorption of formic acid, HCOOH, to surface formate species, HCOO-M, on metal oxide catalysts was examined. The chemisorption studies reveal an average of ∼5–6 μmol/m 2 of active surface sites for many of the metal oxide catalysts. From TPD experiments, the decomposition temperatures of the surface formate ( T p) were found to vary from 88 to 313°C. The turnover frequency (TOF) values for formic acid oxidation were found to vary over 11 orders of magnitude (10 −3–10 8 s −1) at 250°C. No correlations were found between (i) H 2-TPR onset temperatures and the enthalpies of formation of bulk metal oxides (−Δ H f), (ii) TOFs and H 2-TPR onset temperatures, or (iii) TOFs and the isotopic dioxygen exchange rates with the metal oxide lattice oxygen. Weak inverse correlations were found for both TOFs and catalytic activities versus −Δ H f. A strong inverse relationship was found between the TOFs and the surface formate decomposition temperatures, supporting the assertion that the rate-determining step during formic acid oxidation is the decomposition of the surface formate intermediate.

Quantitative Determination of the Number of Surface Active Sites and the Turnover Frequencies for Methanol Oxidation over Metal Oxide Catalysts: Application to Bulk Metal Molybdates and Pure Metal Oxide Catalysts

The present work investigates the number and nature of the surface active sites and the catalytic activity of bulk metal molybdates and bulk metal oxides on methanol selective oxidation. Bulk metal molybdates were synthesized by coprecipitation and characterized by laser Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and specific surface area analysis. The number of surface active sites (Ns) was determined by measuring the amount of methoxy species produced by methanol chemisorption on the catalysts at 100°C. The specific activity values (TOFs) were calculated by normalizing the reaction rate by the number of surface active sites. The significant differences in catalytic behavior of bulk metal molybdates and bulk metal oxides, and the surface molybdenum enrichment observed on metal molybdates, gave evidences that the surface of bulk metal molybdates may be composed only by molybdenum oxide species in a two-dimensional overlayer. It was not possible to establish the structure of surface molybdenum oxide species; however, it can be concluded that the structure of surface active sites species of bulk metal molybdates and monolayer supported oxide catalysts are different according to the dispersion of the Ns values of these systems. This work shows, for the first time in the literature, that bulk metal molybdates and monolayer supported molybdenum oxide catalysts possess similar activity and TOF in methanol selective oxidation. The number of active surface sites and the specific activity toward selective oxidation products (TOF redox), CO2 (TOF basic), and dimethyl ether (TOF acid) of bulk metal oxides were also determined. In contrast to bulk metal molybdates, most bulk metal oxides catalyze the total combustion of methanol even at low methanol conversion.

Metal nanoclusters supported on metal oxide thin films: bridging the materials gap

Characterization and reactivity studies were performed on model catalysts comprised of metal clusters supported on metal oxide thin films. The thin films are prepared by vaporizing the parent metal onto a refractory metal substrate in an O2 environment. The oxide films are sufficiently conductive via defects and tunneling to the substrate that the use of charged particle spectroscopies does not lead to any adverse charging effects. Numerous characterization techniques demonstrated that both spectroscopically and chemically these thin films are comparable to the analogous bulk metal oxides. Model supported catalysts were subsequently prepared by vapor‐depositing catalytically‐interesting metals onto these thin film oxide supports. This deposition method realizes tight control over cluster size and, therefore, represents an ideal approach to studying size‐dependent chemical and physical properties. Reactivity studies established the validity of the supported systems as models of conventional catalysts. Furthermore, the use of these model catalysts provides a bridge between fundamental studies of single crystal reactivities and applied studies of high‐surface‐area catalyst activities.

Reaction-Induced Spreading of Metal Oxides onto Surfaces of Oxide Supports during Alcohol Oxidation: Phenomenon, Nature, and Mechanisms

Reaction-induced spreading of bulk metal oxides onto surfaces of oxide supports during alcohol oxidation, a new phenomenon occurring at temperatures much lower than that required for thermal spreading, has been extensively investigated with Raman spectroscopic and fixed-bed catalytic studies. The reaction-induced spreading kinetics were accelerated by reaction of gaseous components with metal oxides to form mobile complex compounds and found to depend on temperature, gaseous component, metal oxide, and oxide support. Increasing the reaction temperature increases the metal oxide spreading rate. The efficiencies of converting three-dimensional bulk metal oxides into two-dimensional surface metal oxide species by different gaseous components are methanol ≫ ethanol > 2-butanol, water ≫ oxygen. The high reaction-induced spreading efficiency of methanol is related to the high volatility and stability of its metal−methoxy complexes. Reaction-induced spreading of CrO3, MoO3, V2O5, Re2O7, and Cr2O3 during alcohol ...

New Approach to the Investigation of Mechanisms and Apparent Activation Energies for the Reduction of Metal Oxides Using Constant Reaction Rate Temperature-Programmed Reduction

A new system has been developed for the study of both bulk and surface metal oxides by temperature programmed reduction (TPR) under both conventional linear heating and constant rate thermal analysis (CRTA) conditions. It is shown that constant rate temperature-programmed reduction (CR-TPR) is capable of producing higher resolution of overlapping events, provides more insight into reduction mechanisms, and allows easier quantification of reduction processes than conventional TPR. The CR-TPR curves for both bulk and supported copper oxides confirmed that reduction followed a nucleation or autocatalytic mechanism. Bulk nickel oxide was found to reduce via a similar mechanism. Advantages of the CR-TPR “rate-jump” technique to determine reaction energetics are illustrated by investigation of the apparent activation energy (Ea) of CuO reduction, and the results are compared with those obtained under linear heating conditions. Both approaches yield reasonable values of Ea under the appropriate experimental cond...

Basicities of alumina-supported alkaline earth metal oxides

A series of alumina-supported alkaline earth metal oxide catalysts were prepared by incipient-wetness impregnation. These catalysts were characterized by nitrogen-sorption to determine their surface areas and pore size distributions. The basicities of these catalysts were characterized by temperature-programmed desorption of carbon dioxide. The TPD results demonstrate that all of the catalysts have one-peak profiles. The basicity increases with increasing atomic number of the alkaline earth metal. The alumina-supported alkaline earth oxides exhibit the same basic properties as bulk metal oxides. However, the presence of alumina can increase the mechanical strength of the catalyst, since the alkaline earth oxides have a weak mechanical strength. The basic properties of the catalysts are strongly influenced by the calcination temperature.