Andre Heel

One-step hydrothermal coating approach to photocatalytically active oxide composites

Three-dimensional Bi(2)WO(6)/TiO(2) hierarchical heterostructures with secondary anatase TiO(2) nanoparticles grown on primary Bi(2)WO(6) microspheres have been successfully obtained by a low temperature hydrothermal method. The hierarchical Bi(2)WO(6)/TiO(2) structures displayed significantly enhanced visible-light-driven photocatalytic activity in comparison to isolated Bi(2)WO(6) microspheres and TiO(2) nanoparticles. Furthermore, this methodology is of general interest, because it can be used to fabricate other oxide heterostructures, such as BiVO(4)/TiO(2) and Bi(2)MoO(6)/TiO(2).

Production and Characterization of Pd/SiO2 Catalyst Nanoparticles from a Continuous MOCVS/MOCVD Aerosol Process at Atmospheric Pressure

A continuous 2-step aerosol process is described for the generation of SiO2 supported palladium (Pd/SiO 2 ) catalyst particles from metal-organic (MO) precursors. In a first flow reactor, submicron SiO 2 support particles are generated by chemical vapor synthesis (CVS) from TEOS [tetraethyl(ortho)silicate]. These silica particles are then coated with palladium in a second flow reactor by chemical vapor deposition (CVD) from ( η 3 -allyl)( η 5 -cyclopentadienyl)palladium [Pd(allyl)Cp]. The sublimation and decomposition behavior of both metal-organic precursors was measured by thermo-gravimetric analysis (TGA) and FTIR; the vapor concentration of Pd(allyl)Cp was determined for the range of process conditions used. Each process step was characterized both with regard to aerosol properties as well as morphology and composition of individual particles. This was done with a variety of on-line and off-line techniques including electrical mobility analysis, TEM, energy-dispersive X-ray analysis, and physisorption methods like BET. The coating thickness was also measured on line by a high-resolution single-stage impactor (SS-LPI) technique. It is shown that the continuous CVS process can be set to generate constant concentrations and sizes of silica support particles with a specific surface area of 350 m 2 g −1 , which are carbon free and non-porous. The silica particles can be restructured to spheres if desired. The continuous palladium CVD process was able to generate variable and defined coatings of narrowly distributed Pd nanoparticles with mean sizes between 0.75 and about 3.5 nm. The on-line measurements by SS-LPI showed equivalent coating thicknesses from 0.3 nm up to 3 nm, which were stable over several hours.

Photocatalytic Activity of W-Doped TiO2 Nanopowders

Abstract Polycrystalline tungsten-doped titanium dioxide nanopowders within a dopant concentration of 0-1 at.% were prepared by a one-step flame spray synthesis (FSS). Mixtures of titanium tetra-isopropoxide dissolved in ethanol and tungsten hexacarbonyl solubilized in tetrahydrofuran were used as precursors. The specific surface area (SSA) of the powders was between 40 and 130 m2/g depending on the process parameters, namely the flame combustion stoichiometry and their chemical composition. Irrespective of their composition the powders consisted of spherical particles, as visible by transmission electron microscopy (TEM). They are highly crystallized with anatase as the predominant phase, according to X-ray diffraction (XRD). The band gap energy was determined by spectrophotometric measurement and was between 3.29 eV and 3.34 eV. The photocatalytic performance of the powders was studied by the degradation of methylene blue (MB) and methyl orange (MO) in aqueous suspensions under UV-A (365 nm) as well as visible (400-500 nm) irradiation. The catalytic activity was determined as a function of the tungsten concentration, the specific surface area and the flame combustion stoichiometry. It was found that the TiO2-0.7 at %W particles with high SSA have the highest photoactivity for degradation of MB and MO. This can be explained by the incorporation of the tungsten into the TiO2 crystal lattice as well as modification surface properties.

Determination of Coating Thickness of DEHS on Submicron Particles by Means of Low Pressure Impaction

The increasing use of submicron and nano-scaled particles in process engineering sets a demand for new methods for the characterization of particle properties. In the present work low pressure impaction as a very sensitive method for the characterization of the coating of nanoparticles is introduced. The determination of the coating thickness is based on the change of effective density due to the differing densities of particle and coating material, As model systems the coating of spherical silver nanoparticles and agglomerates of silver nanoparticles with DEHS (di(2-ethylhexyl)sebacate) were used. A tandem differential modelling analyzer (DMA) setup served as a reference for the measurement results obtained with the low pressure impactor (LPI). The LPI showed a very high sensitivity for the coating of spherical particles. Changes of the particle radius below 1 nm could be detected. In contrast to spherical particles, agglomerated particles did not show a clear correlation between coating thickness and the change of effective density for coated agglomerates.

Smart material concept: reversible microstructural self-regeneration for catalytic applications

This paper presents a proof-of-concept study and demonstrates the next generation of a “smart” catalyst material, applicable to high temperature catalysis and electro-catalysis such as gas processing and as a catalyst for solid oxide cells. A modified citrate-gel technique was developed for the synthesis of LaxSr1−1.5xTi1−yNiyO3−δ. This method allowed the synthesis of single phase materials with a high specific surface area, after the first calcination step at temperatures as low as 650 °C. Up to 5 at% of nickel could be incorporated into the perovskite structure at this low calcination temperature. X-ray powder diffraction and microscopy techniques have proven the exsolution of nickel nanoclusters under low oxygen partial pressure. The amount of exsoluted nickel nanoparticles was sensitive to surface finishing, whereby much more exsoluted nanoparticles were observed on pre-treated and polished surfaces as compared to original ones. Increasing A-site deficiency leads to a larger number of nickel particles on the surface, indicating a destabilizing influence of the A-site vacancies on the B-site metal cations. Repetitive redox cycles prove that the nickel exsolution and re-integration is a fully reversible process. These materials working in a cyclic and repetitive way may overcome the drawbacks of currently used conventional catalysts used for high temperature systems and overcome major degradation issues related to catalyst poisoning and coarsening-induced aging.

Structural Reversibility and Nickel Particle stability in Lanthanum Iron Nickel Perovskite-Type Catalysts

Perovskite-type oxides have shown the ability to reversibly segregate precious metals from their structure. This reversible segregation behavior was explored for a commonly used catalyst metal, Ni, to prevent Ni sintering, which is observed on most catalyst support materials. Temperature-programmed reduction, X-ray diffraction, X-ray absorption spectroscopy, electron microscopy, and catalytic activity tests were used to follow the extent of reversible Ni segregation. LaFe1-x Nix O3±δ (0≤x≤0.2) was synthesized using a citrate-based solution process. After reduction at 600 °C, metallic Ni particles were displayed on the perovskite surfaces, which were active towards the hydrogenation of CO2 . The overall Ni reducibility was proportional to the Ni content and increased from 35 % for x=0.05 to 50 % for x=0.2. Furthermore, Ni could be reincorporated reversibly into the perovskite lattice during reoxidation at 650 °C. This could be exploited for catalyst regeneration under conditions under which impregnated materials such as Ni/LaFeO3±δ and Ni/Al2 O3 suffer from sintering.

A Cost Estimation for CO2 Reduction and Reuse by Methanation from Cement Industry Sources in Switzerland

The Swiss government has signed the Paris Climate Agreement and various measures need to be implemented in order to reach the target of a 50 % reduction in CO2 emissions in Switzerland by 2030 compared with the value for 1990. Considering the fact that the production of cement in Switzerland accounts around 2.5 million tonnes for CO2 emissions of which corresponds to roughly 7 % of the country’s total CO2 emissions, the following article examines how this amount could be put to meaningful use in order to create a new value-added chain through CO2 methanation, and thus reduce the consumption and import of fossil fuels in Switzerland. With power-to-gas technology, this CO2, along with regenerative hydrogen from photovoltaics, can be converted into methane, which can then be fed into the existing natural-gas grid. This economic case study shows a cost prediction for conversion of all the CO2 from the cement industry into methane by using the technologies available today in order to replacing fossil methane imports.

Evolution of Water Diffusion in a Sorption-Enhanced Methanation Catalyst

Sorption-enhanced methanation has consequent advantages compared to conventional methanation approaches; namely, the production of pure methane and enhanced kinetics thanks to the application of Le Châtelier’s principle. In this paper, we address the question of the long-term stability of a sorption-enhanced methanation catalyst-support couple: Ni nanoparticles on zeolite 5A. Compared to most conventional methanation processes the operational conditions of sorption-enhanced methanation are relatively mild, which allow for stable catalyst activity on the long term. Indeed, we show here that neither coking nor thermal degradation come into play under such conditions. However, a degradation mechanism specific to the sorption catalysis was observed under cyclic methanation/drying periods. This severely affects water diffusion kinetics in the zeolite support, as shown here by a decrease of the water-diffusion coefficient during multiple cycling. Water diffusion is a central mechanism in the sorption-enhanced methanation process, since it is rate-limiting for both methanation and drying.