Strong metal support interaction state in the Fe/TiO2 system- an XPS study

Abstract: Magnetic photocatalysts were synthesized by coating titanium dioxide particles onto colloidal magnetite and nano-magnetite particles. The photoactivity of the prepared coated particles was lower than that of single-phase TiO2 and was found to decrease with an increase in the heat treatment. These observations were explained in terms of an unfavorable heterojunction between the titanium dioxide and the iron oxide core, leading to an increase in electron−hole recombination. Interactions between the iron oxide core and the titanium dioxide matrix upon heat treatment were also seen as a possible cause of the observed low activities of these samples. Other issues considered include the physical and chemical characteristics of the samples, such as surface area and the presence of surface hydroxyl groups. Depending on the calcination conditions, these photocatalysts were found to suffer from varying degrees of photodissolution. Photodissolution tests revealed the greater the extent of the heat treatment, the low...

Abstract: An exhaustive review on the photochemical properties of iron-doped TiO2 semiconductors is presented. Photocatalytic reactions (reductions and oxidations) using Fe-containing TiO2 on different organic and inorganic substrates are reported. Different aspects relating to structural, surface and photophysical properties of these photocatalysts are extensively discussed. The origin of the photoactivity of this kind of mixed oxides is considered with regards to previously proposed physical and chemical processes and on the role of the iron content.

Abstract: A stable magnetic photocatalyst was prepared by coating a magnetic core with a layer of photoactive titanium dioxide. A direct deposition of titanium dioxide onto the surface of magnetic iron oxide particles proved ineffective in producing a stable magnetic photocatalyst, with high levels of photodissolution being observed with these samples. This observed photodissolution is believed to be due to the dissolution of the iron oxide phase, induced by the photoactive the titanium dioxide layer due to electronic interactions at the phase junction in these magnetic photocatalysts. The introduction of an intermediate passive SiO2 layer between the titanium dioxide phase and the iron oxide phase inhibited the direct electrical contact and hence prevented the photodissolution of the iron oxide phase. The magnetic photocatalyst is for use in slurry-type reactors from which the catalyst can be easily recovered by the application of an external magnetic field.

Abstract: Magnetic iron oxide–titania photocatalysts (Fe 3 O 4 –TiO 2 ) were prepared using a coating technique in which the photoactive titanium dioxide was deposited onto the surface of a magnetic iron oxide core. These particles had a core–shell structure. The prepared particles were heat treated at high temperature in order to transform the amorphous titanium dioxide into a photoactive crystalline phase. The heat treatment temperature and duration were varied, and the correlation between the heat treatment and the observed photoactivities was investigated. An increase in the applied heat treatment, either by increasing the temperature or increasing the heat treatment duration, led to a decrease in the activities of the catalyst particles. A decrease in surface area due to sintering, along with the diffusion of Fe ions into the titanium dioxide coating are seen as contributing factors to the decline in photoactivity which accompanied an increase in the heat treatment. Differential scanning calorimetry analysis (DSC) results confirmed that the presence of the iron oxide core did not have an effect on the phase transformation of titania under the experimental conditions investigated. In this study we also present surface charge measurements which show that the surface of the particles became more positive as the heat treatment was increased. This is an indication of changing surface properties as heat treatment is applied. For single-phase TiO 2 powders, this is postulated to be due to a decrease in the surface hydroxyl (OH) groups and/or residual organics (OR) groups. For the Fe 3 O 4 –TiO 2 powders, in addition to the loss of OH and OR groups, the diffusion of the Fe into the titania shell is postulated to also play a role in the changing surface properties with applied heat treatment. Experiments aimed at reducing the duration of the heat treatment revealed that a heat treatment duration of 20 min at 450 °C was sufficient to transform amorphous titanium dioxide into a photoactive crystalline phase. This does not only minimise loss of surface hydroxyl groups but it also has the potential to limit the oxidation of the magnetic core, which occurs due to the porosity of the coating. This has practical implications in terms of separating the magnetic particles from the treated waste waters under the application of an external magnetic field. It also presents an opportunity to produce photoactive composite particles while limiting the interactions between the core and the shell during the heat treatment.

Abstract: Interactions between metals and supports are of fundamental interest in heterogeneous catalysis. The electronic, geometric and bifunctional effects originating from Strong Metal-Support Interactions (SMSI) that are responsible for the catalyst's activity, selectivity, and stability are key factors that determine performance. Research into SMSI is fast-growing with many revolutionary systems being developed to enhance our understanding of its nature and effects. This review starts with a brief overview of heterogeneous catalysis and SMSI; then three major mechanisms involving electronic, geometric and bifunctional effects are summarized to introduce the fundamental concepts, recent progress and disagreement remained. Subsequently, advanced analytical techniques are introduced as contemporary approaches to the investigation and understanding of SMSI. In addition, the effects of SMSI on the catalytic activity, selectivity and stability of various reaction systems, such as heterogeneous catalysis and electrocatalysis are examined. Additionally, a brief review of various protocols used for the manipulation of interactions between metals and supports is given. Lastly, the future of SMSI with respect to further developments and ongoing challenging issues is addressed.

Abstract: The core and valence level XPS spectra of FexO (x ~ 0.90–0.95); Fe2O3 (α and γ); Fe3O4; and FeOOH have been studied under a variety of sample surface conditions. The oxides may be characterized by a combination of valence level differences and core-level effects (chemical shifts, multiplet splittings, and shake-up structure). FeII and FeIII states are distinguishable, but octahedral and tetrahedral sites are not. The O 1 s BE cannot be used to distinghuish between the oxides since it has a nearly constant value. Fe 3d valence level structure spreads some 10 eV below EF, much broader than suggested by previous UPS and photoelectron-spin-polarization (ESP) measurements for FexO and Fe3O4. Fe surfaces (films, foils, (100) face) yield predominantly FeIII species when exposed to high exposures of oxygen or air, though there is evidence for some FeII also. At low exposures the FeII/FeIII ratio increases.

Abstract: Metallic iron particles supported on titania were prepared by aqueous incipient wetness impregnation, nonaqueous impregnation, and thermal decomposition of iron pentacarbonyl. For iron loadings between 1 and 5 wt%, only the last technique produced metallic iron particles smaller than 10 nm, as judged by X-ray line-broadening and CO chemisorption measurements. During the decomposition of iron pentacarbonyl at 380 K, Mossbauer spectroscopy showed that Fe2+ and zero-valent iron (e.g., subcarbonyl species) were formed on titania, presumably due to the interaction of iron carbonyl with hydroxyl groups on the support surface; however, these iron species were reduced to metallic iron particles in hydrogen at ca. 700 K. The ammonia synthesis apparent activation energy, EA, and ammonia partial pressure dependence, m, for all Fe TiO 2 catalysts reduced at ca. 700 K were similar to those values for metallic iron supported on silica or magnesia. Upon increasing the reduction temperature to ca. 800 K, the values of EA and m increased for Fe TiO 2 catalysts with iron particles are large as 20 nm. Room temperature Mossbauer spectra of the Fe TiO 2 catalysts after reduction at either 700 or 800 K indicated that the majority or the iron was present as α-Fe, with Mossbauer parameters essentially identical to bulk metallic iron. Accordingly, the metal-support interaction between iron and titania, which is initiated by reduction at 800 K and is responsible for the changes in ammonia synthesis kinetics, is due to titanium species at the surface of the metallic iron particles. This interaction, which also suppresses CO chemisorption, can be partially destroyed by exposure of the sample to air at room temperature. Possible modes of transport of titanium species during catalyst preparation, reduction, and oxidation are discussed.

Abstract: Recently, a TiO2-based catalyst has been commercially used on a large scale; this first application was the selective catalytic reduction of NOxin air pollution control. In this review, the application and physico-chemical properties of TiO2-based catalysts are discussed. A TiO2-based catalyst is defined as a catalyst whose major component is TiO2. Physico-chemical properties of TiO2 as a support are compared with those of conventional catalyst supports such as Al2O3 and SiO2. The preparation procedure of a high surface area TiO2 support is briefly summarized. The activity and durability of TiO2-based catalysts have been studied in the selective catalytic reduction of NOx with NH3, partial oxidation of hydrocarbons, hydrodesulphurization of hydrocarbon oils, coal liquefaction and adsorption of H2S.

Abstract: Surface compositional changes were observed for TiO2 single crystal electrodes used for photoelectrolysis of water. Surface stoichiometries of several types of TiO2, SrTiO3 and BaTiO3 electrodes were characterized by XPS and compared with a variety of titanium, titanium oxide and titanium hydride standard materials. Reduction of the electrode surface in a hydrogen atmosphere results in an oxygen deficient surface composition. Photoelectrolysis at current densities of 10–15 mAcm2 for periods up to 8 h appears to return the electrode surface to a nearly stoichiometric oxygen-to-metal ratio. Reduction of the titanium oxide surfaces was also observed by exposure to an argon ion beam. Analysis of the electrode surface by a combination of XPS and ion-sputter profiling was still possible by simultaneous analysis of standard materials.

Abstract: The particle size and crystallite size of anatase increase markedly in the region of the crystal structure transformation. The unit cell of anatase seems to expand prior to the transformation to rutile. This expansion has been attributed to a displacive transformation of the type defined by Buerger. Smaller particle size and larger surface area seem to favour the transformation. The kinetics of the transformation of anatase prepared by the hydrolysis of titanium sulphate have been studied at different temperatures and are found to be considerably different from the kinetics of the transformation of pure anatase. The transformation becomes immeasurably slow below ~695±10°C compared to ~610°C for pure anatase. An induction period is observed in the transformation of anatase obtained from sulphate hydrolysis and the duration decreases with increase in temperature. The activation energy is ~120 kcal/mole, a value higher than that for the pure anatase-rutile transformation. The results have been interpreted in terms of the relative rates of nucleation and propagation processes. The activation energy for the nucleation process seems to be much larger than for the propagation process. The kinetics of the transformation of anatase samples doped with different amounts of sulphate ion impurity have also been studied and the transformation is found to be progressively decelerated with increase in the impurity concentration. The energy of activation for the transformation appears to increase progressively with increase in impurity concentration.