The impact of steam on the electronic structure of the selective propane oxidation catalyst MoVTeNb oxide (orthorhombic M1 phase)

The selective propane oxidation catalyst MoVTeNb oxide M1 was investigated by microwave conductivity, synchrotron X-ray photoelectron, soft X-ray absorption and resonant photoelectron spectroscopy under reaction conditions to identify the influence of steam on the electronic bulk and surface properties. Steam significantly increases both the conversion of propane and the selectivity to the target product acrylic acid. The increased catalytic performance comes along with a decreased conductivity, a modification of the surface chemical and electronic structure with an enrichment of covalently bonded V5+ species to the extent of Mo6+, a decreased work function and hence polarity of the surface and a modified valence band structure. The higher degree of covalency in metal oxide bonds affects the mobility of the free charge carriers, and hence explains the decrease of the conductivity with steam. Furthermore we could prove that a subsurface space charge region depleted in electrons and thus an upward bending of the electronic band structure are induced by the reaction mixture, which is however not dependent on the steam content

The mechanism of interfacial CO2 activation on Al doped Cu/ZnO

We report on a combined quantitative charge carrier and catalytic activity analysis of Cu/ZnO(:Al) model catalysts. The promoting effect of Al3+ on the ZnO support for CO2 activation via the reverse water–gas-shift reaction has been investigated. The contact-free and operando microwave Hall Effect technique is applied to measure charge carriers in Cu/ZnO(:Al) based model catalysts under reverse water–gas shift reaction conditions. This method allows us to monitor the electrical conductivity, charge carrier mobility, and absolute number of charge carriers. An increase in charge carrier concentration with increasing Al3+ content and its direct correlation with the catalytic activity for CO formation is found. We conclude that the increased availability of charge carriers plays a key role in CO2 activation and CO formation, which finds additional support in a concurrent decrease of the apparent activation energy and increase in the reaction order of CO2. In combination with comprehensive DFT calculations, the impact of the interfacial charge transfer, coupled to oxygen defect sites in ZnO and CO2 adsorption properties, is elucidated and highlighted. In conclusion, the results from this operando investigation combined with DFT calculations demonstrate the importance of charge transfer processes as decisive descriptors for understanding and explaining catalytic properties

Synchrotron- und laseraktiviertes Wachstum von Edelmetallpartikeln in Gläsern

Im Rahmen dieser Arbeit wurde die Methode der Synchrotronröntgenaktivierung entwickelt, um Edelmetallpartikel in Natron-Kalk-Silicatgläsern zu erzeugen. Diese Photoaktivierung ermöglichte es, den Nukleations- vom Wachstumsprozess der Metallteilchen zu trennen und somit sehr kleine Partikel und die Frühstadien des Wachstumsprozesses zu identifizieren. Die Gläser wurden mit Photolumineszenz-, UV-Vis-Absorptions-, EPR-, SAXS- sowie XANES-Spektroskopie untersucht. So konnten in synchrotronaktivierten Gold-dotierten Gläsern Golddimere nachgewiesen werden. Die thermische Behandlung bei 550°C führte über eine heterogene Nukleation zum Wachstum von Goldnanopartikeln. Als Modell für das Nukleationszentrum wurde das an eine Silanolateinheit gebundene Golddimer entwickelt. In synchrotronaktivierten Silber-dotierten Gläsern bildeten sich zunächst Silberatome. Während das Tempern bei 300°C zur Entstehung kleiner lumineszierender Silberteilchen führte, induzierte die thermische Behandlung bei 500°C das Wachstum von Silbernanopartikeln. Die Frühstadien des Gold- und Silberpartikelwachstums wurden als effiziente Donoren in Photolumineszenz-Energietransferprozessen identifiziert. Durch die Anwendung eines Sol-Gel-Spin-Coating-Verfahrens konnten Gold-dotierte Silicat-Titanat-Glasschichten mit einer Goldkonzentration von bis zu 20 mol% hergestellt werden. Die Goldnanopartikel, welche durch Tempern bei 300°C erzeugt wurden, waren durch eine dreiphotonisch induzierte nichtlineare Lumineszenz charakterisiert. Weiterhin konnte die Methode der Titan:Saphir-Laseraktivierung entwickelt und damit Goldnanopartikel-haltige, nanometergroße Strukturen in die Glasschichten geschrieben werden. TEM- und REM-Studien haben gezeigt, dass die Goldnanopartikel nicht nur in die Glasschicht eingebettet sind, sondern sich auch auf der Oberfläche befinden. Die Zugänglichkeit gegenüber Biomolekülen konnte durch den Nachweis der Oberflächen-verstärkten Ramanstreuung adsorbierter Adeninmoleküle bewiesen werden.Within this work, the technique of synchrotron X-ray activation has been developed to generate noble metal particles in soda-lime silicate glasses. The photoactivation has enabled the separation of nucleation and growth of noble metal particles. Thus, very small particles and the early stages of the cluster growth process could be identified. The glasses have been characterized by photoluminescence, UV-Vis absorption, EPR, SAXS and XANES spectroscopy. As a result, gold dimers could be identified in synchrotron activated gold-doped glasses. Thermal annealing at 550°C has induced the growth of gold nanoparticles by heterogeneous nucleation. As a model for the nucleation center a gold dimer bound to a silanolate center has been suggested. In contrast to gold, in synchrotron activated silver-doped glasses non-luminescent silver atoms have been generated. Annealing at 300°C has induced the growth of small luminescent silver particles. After a thermal treatment at a higher temperature of 500°C silver nanoparticles have been generated. The early stages of the gold and silver cluster growth process have been identified as efficient donators in photoluminescence energy transfer processes. An unprecedented increase of the gold amount of up to 20 mol% has been achieved by the preparation of ultrathin silicate-titanate layers with a sol-gel spin-coating approach. The nonlinear photoluminescence of the gold nanoparticles generated by annealing at 300°C could be excited by a three-photon induced process. Furthermore, the technique of titanium:sapphire laser activation has been developed to write nanometer-sized patterns containing gold nanoparticles into the glassy layers. TEM and SEM studies have shown that the gold nanoparticles are not only embedded within the thin films, but are also located on top of the glassy layers. Their accessibility to biological molecules has been proven by determining the surface-enhanced Raman scattering of adsorbed adenine molecules

Preparation and Real World Applications of Titania Composite Materials for Photocatalytic Surface, Air, and Water Purification: State of the Art

The semiconducting transition metal oxide TiO2 is a rather cheap and non-toxic material with superior photocatalytic properties. TiO2 thin films and nanoparticles are known to have antibacterial, antiviral, antifungal, antialgal, self, water, and air-cleaning properties under UV or sun light irradiation. Based on these excellent qualities, titania holds great promises in various fields of applications. The vast majority of published field and pilot scale studies are dealing with the modification of building materials or generally focus on air purification. Based on the reviewed papers, for the coating of glass, walls, ceilings, streets, tunnels, and other large surfaces, titania is usually applied by spray-coating due to the scalibility and cost-efficiency of this method compared to alternative coating procedures. In contrast, commercialized applications of titania in medical fields or in water purification are rarely found. Moreover, in many realistic test scenarios it becomes evident that the photocatalytic activity is often significantly lower than in laboratory settings. In this review, we will give an overview on the most relevant real world applications and commonly applied preparation methods for these purposes. We will also look at the relevant bottlenecks such as visible light photocatalytic activity and long-term stability and will make suggestions to overcome these hurdles for a widespread usage of titania as photocalyst

Preparation and Real World Applications of Titania Composite Materials for Photocatalytic Surface, Air, and Water Purification: State of the Art

The semiconducting transition metal oxide TiO2 is a rather cheap and non-toxic material with superior photocatalytic properties. TiO2 thin films and nanoparticles are known to have antibacterial, antiviral, antifungal, antialgal, self, water, and air-cleaning properties under UV or sun light irradiation. Based on these excellent qualities, titania holds great promises in various fields of applications. The vast majority of published field and pilot scale studies are dealing with the modification of building materials or generally focus on air purification. Based on the reviewed papers, for the coating of glass, walls, ceilings, streets, tunnels, and other large surfaces, titania is usually applied by spray-coating due to the scalibility and cost-efficiency of this method compared to alternative coating procedures. In contrast, commercialized applications of titania in medical fields or in water purification are rarely found. Moreover, in many realistic test scenarios it becomes evident that the photocatalytic activity is often significantly lower than in laboratory settings. In this review, we will give an overview on the most relevant real world applications and commonly applied preparation methods for these purposes. We will also look at the relevant bottlenecks such as visible light photocatalytic activity and long-term stability and will make suggestions to overcome these hurdles for a widespread usage of titania as photocalyst

Promoting strong metal support interaction: Doping ZnO for enhanced activity of Cu/ZnO:M (M = Al, Ga, Mg) catalysts

The promoting effect of Al, Ga, and Mg on the support in Cu/ZnO catalysts for methanol synthesis has been investigated. Different unpromoted and promoted ZnO supports were synthesized and impregnated with Cu metal in a subsequent step. All materials, supports, and calcined and activated catalysts were characterized by various methods, including contactless (microwave) conductivity measurements under different gas atmospheres. Small amounts of promoters were found to exhibit a significant influence on the properties of the oxide support, concerning textural as well as electronic properties. We found correlations between the conductivity of the ZnO support and the activity of the catalyst in the reverse water-gas shift reaction (rWGS) as well as in methanol synthesis. In rWGS the activation energy and reaction order in H2 are decreased upon promotion of the ZnO support with the trivalent promoters Al3+ and Ga3+, indicating an electronic promotion. In methanol synthesis, results point to a structural promotion by Al3+ and Ga3+. A detrimental effect of Mg2+ doping was observed in both reactions. This effect is discussed in the context of the reducibility of ZnO under reaction conditions, which can be tuned by the promoter in different ways. The reducibility is seen as a critical property for the dynamic metal support interaction of the Cu/ZnO system.Peer Reviewe

Promoting Strong Metal Support Interaction: Doping ZnO for Enhanced Activity of Cu/ZnO:M (M = Al, Ga, Mg) Catalysts

The promoting effect of Al, Ga, and Mg on the support in Cu/ZnO catalysts for methanol synthesis has been investigated. Different unpromoted and promoted ZnO supports were synthesized and impregnated with Cu metal in a subsequent step. All materials, supports, and calcined and activated catalysts were characterized by various methods, including contactless (microwave) conductivity measurements under different gas atmospheres. Small amounts of promoters were found to exhibit a significant influence on the properties of the oxide support, concerning textural as well as electronic properties. We found correlations between the conductivity of the ZnO support and the activity of the catalyst in the reverse water-gas shift reaction (rWGS) as well as in methanol synthesis. In rWGS the activation energy and reaction order in H₂ are decreased upon promotion of the ZnO support with the trivalent promoters Al³⁺ and Ga³⁺, indicating an electronic promotion. In methanol synthesis, results point to a structural promotion by Al³⁺ and Ga³⁺. A detrimental effect of Mg²⁺ doping was observed in both reactions. This effect is discussed in the context of the reducibility of ZnO under reaction conditions, which can be tuned by the promoter in different ways. The reducibility is seen as a critical property for the dynamic metal support interaction of the Cu/ZnO system

Ambient-Pressure Soft X‑ray Absorption Spectroscopy of a Catalyst Surface in Action: Closing the Pressure Gap in the Selective <i>n</i>‑Butane Oxidation over Vanadyl Pyrophosphate

In order to close the pressure gap in the investigation of catalyst surfaces under real operation conditions we have developed a variable-pressure soft X-ray (<i>h</i>ν ≤1.5 keV) absorption cell coupled to a gas analysis system to study the pressure dependency of the electronic and catalytic properties of catalyst surfaces in reactive atmospheres at elevated temperatures. With this setup we investigated the vanadium L₃-edge and catalytic performance of polycrystalline vanadyl pyrophosphate in the selective oxidation of <i>n</i>-butane to maleic anhydride between 10 and 1000 mbar at 400 °C. As a result, major gas phase and pressure dependent spectral changes are observed at energies attributed to V 2p-3d_<i>z</i>² excitations assigned to vanadium atoms square-pyramidally coordinated to oxygen atoms. This can be interpreted in terms of a shortened vanadyl bond (VO) and an increased vanadium oxidation state with higher pressures. Since this is accompanied by an increasing catalytic activity and selectivity, it indicates that vanadyl oxygen is actively involved in the selective oxidation of the alkane

Promoted Ceria: A Structural, Catalytic, and Computational Study

The role of trivalent (La, Sm, Gd, and Y) and tetravalent (Hf, Zr, and Ti) dopants in the catalytic, structural, and electronic properties of ceria was investigated. Promoted ceria catalysts were synthesized by coprecipitation with ammonia and tested in HCl and CO oxidation. Ceria catalysts exhibit a medium high reactivity and excellent stability in HCl oxidation. The intrinsic reactivity of ceria in HCl oxidation can be improved by a factor of 2 when doping with Hf and Zr in appropriate quantities, whereas trivalent dopants are detrimental. Although both oxidation reactions rely on the existence of oxygen vacancies, the order of reactivity in HCl and CO oxidation is not completely parallel. The effects of promoters on the electronic conductivity and the vacancy formation energy were studied by contactless conductivity experiments using the microwave cavity perturbation technique and by density functional theory calculations. Furthermore, transport properties were also assessed on the basis of theoretical calculations. The order of oxygen vacancy formation energy follows well the order of conductivity (polaron mobility) (trivalent > tetravalent > undoped) observed under inert and oxidizing conditions. This implies that none of these properties correlates with the reactivity. On the other hand, reducing conditions strongly enhanced the conductivity of Hf- and Zr-doped ceria. In HCl oxidation, only the balanced reduction of both Cl and O vacancy formation energies allows for an enhanced reactivity. Promoters give rise to lattice contraction–expansion modifying vacancy formation energies, adsorption properties, and surface coverages