Roland Marschall

Preparation of porous composite ion-exchange membranes for desalination application

Via a two-step phase inversion technique, composite membranes with controllable porosity and a significant improvement of electrochemical properties were successfully prepared. The presence of surface functionalized mesoporous silica (SS) as inorganic fillers in the sulfonated polyethersulfone (sPES) polymer matrix was proved to have a great impact on the resultant membrane structure, which subsequently led to significantly enhanced ionic conductivity of the membranes. The correlation among inorganic fillers, composite structures, electrochemical properties and desalination performance by electrodialysis (ED) was discussed in detail. The optimal membrane was the composite with 0.2 wt% SS loading, which possessed a good ionic conductivity of 5.554 mS cm−1, a high selectivity with 0.95 transport number while maintaining good mechanical strength and thermal stability. Moreover, the performance of this membrane in ED was comparable to a commercial membrane (FKE), exhibiting a current efficiency of 0.84 and 3.82 kW h kg−1 of salt removed.

Nanoparticles of Mesoporous SO3H-Functionalized Si-MCM-41 with Superior Proton Conductivity

Nanometer-sized mesoporous silica particles of around 100-nm diameter functionalized with a large amount of sulfonic acid groups are prepared using a simple and fast in situ co-condensation procedure. A highly ordered hexagonal pore structure is established by applying a pre-hydrolysis step in a high-dilution synthesis approach, followed by adding the functionalization agent to the reaction mixture. The high-dilution approach is advantageous for the in situ functionalization since no secondary reagents for an effective particle and framework formation are needed. Structural data are determined via electron microscopy, nitrogen adsorption, and X-ray diffraction, proton conductivity values of the functionalized samples are measured via impedance spectroscopy. The obtained mesoporous SO(3)H-MCM-41 nanoparticles demonstrate superior proton conductivity than their equally loaded micrometer-sized counterparts, up to 5 x 10(-2) S cm(-1). The mesoporosity of the particles turns out to be very important for effective proton transport since non-porous silica nanoparticles exhibit worse efficient proton transport, and the obtained particle size dependence might open up a new route in rational design of highly proton conductive materials.

Improved overall water splitting with barium tantalate mixed oxide composites

The combination of effective charge carrier separation and improved electron transfer in highly crystalline barium tantalate composites modified with Rh–Cr2O3 core–shell co-catalyst systems induces enhanced activity for overall water splitting (OWS) with stoichiometric amounts of H2 and O2 (2 : 1). A sol–gel route employing complexing reagents was investigated to prepare selectively defined mixed oxide materials with improved surface areas and smaller particle sizes compared to the conventional solid state reaction (SSR). The catalytic activities of the materials are investigated in photocatalytic test reactions for hydrogen production and overall water splitting. The formation of Rh–Cr2O3 core–shell co-catalyst systems for water splitting is evidenced by transmission electron microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS). Moreover, we developed new and highly active barium tantalate composites for hydrogen generation from aqueous methanol solutions under UV-light, which show the highest hydrogen evolution rate for a three-component composite consisting of Ba5Ta4O15/Ba3Ta5O15/BaTa2O6. Hydrogen rates of more than 6 mmol h−1 can be achieved without any co-catalyst. Using Rh–Cr2O3 core–shell co-catalysts on these three-component composites simultaneous generation of H2 and O2 from pure water splitting reaches rates up to 70% higher than for the pure Ba5Ta4O15.

Layered Perovskite Nanofibers via Electrospinning for Overall Water Splitting

The (111)-layered perovskite materials Ba5 Ta4 O15 , Ba5 Ta2 Nb2 O15 and Ba5 Nb4 O15 are prepared with nanofiber morphology via electrospinning for the first time. The nanofibers are built up from small single crystals, with up to several micrometers length even after calcination. The formation mechanism is investigated in detail, revealing an intermediate formation of amorphous barium carbonate strengthening the nanofiber morphology for high temperature treatment. All nanofiber compounds are able to generate hydrogen without any co-catalyst in photocatalytic reformation of methanol. After photodeposition of Rh-Cr2 O3 co-catalysts, the nanofibers show better activity in overall water splitting compared to sol-gel-derived powders.

Tetragonal tungsten bronze-type nanorod photocatalysts with tunnel structures: Ta substitution for Nb and overall water splitting

Tetragonal tungsten bronze-type tantalum (Ta) substituted Sr2KNb5O15 nanorod photocatalysts with tunnel structures were prepared by a facile and low-cost molten salt method using potassium chloride (KCl) at 850 °C for only 2 h. Although all native photocatalysts did not possess any detectable activity in pure water splitting, after deposition of NiOx (double-layered Ni/NiO) as co-catalysts, samples of Sr2KNb5O15 and Sr2KTa5O15 can split pure water into H2 and O2 in a stoichiometric amount (≈2 : 1), which can be ascribed to the improved charge carrier separation and transfer in the presence of NiOx. Furthermore, Ta substitution effects on the photocatalytic behaviour were systematically investigated for hydrogen production by aqueous methanol reforming. The average H2 formation rates of Sr2KNb5−xTaxO15 first decrease with tantalum substitution for x 2.5) and especially the fully substituted Sr2KTa5O15. This can be explained by a stronger driving force for photogenerated conduction band electrons to reduce water.

Sol–gel synthesis of defect-pyrochlore structured CsTaWO6 and the tribochemical influences on photocatalytic activity

The semiconductor mixed oxide photocatalyst CsTaWO6 was prepared via an aqueous sol–gel citrate route for the first time. The mild reaction conditions yield smaller primary particle sizes and larger specific surface areas than the conventional solid state reaction. The photocatalytic properties are determined using both photocatalytic terephthalic acid hydroxylation and photocatalytic hydrogen generation. All of the materials described herein generate hydrogen without the addition of a co-catalyst. Due to the initially agglomerated but porous morphology of the sol–gel-derived CsTaWO6, tribochemical treatment via short term ball milling has a strong effect on the photocatalytic activities of these materials. Ball milling increases the surface area of the materials, leading to strongly improved activity for the generation of ˙OH radicals, but also generates surface defects in the materials. The defective sites act as electron traps, which depress the photocatalytic hydrogen evolution activity. However, by combining ball milling and photodeposition of Rh, highly improved hydrogen generation rates for CsTaWO6 are achieved. The sol–gel citrate route finally leads to more active materials than the solid state reaction.

Detection of Homogeneous Distribution of Functional Groups in Mesoporous Silica by Small Angle Neutron Scattering and in Situ Adsorption of Nitrogen or Water

The distribution of SO(3)H-functional groups attached to the ordered inner pore walls of mesoporous Si-MCM-41 materials based on SiO(2) was investigated by gas adsorption combined with in situ small angle neutron scattering (SANS). The functionalization was performed by two different methods, (i) grafting and (ii) co-condensation. The adsorbates N(2) at 77 K or a H(2)O/D(2)O mixture of 42:58 at 298 K possess neutron scattering length densities (SLD) similar to that of SiO(2) and therefore quench the diffraction signals of the nonmodified silica. SANS measurements show that N(2) matches completely not only with the pristine mesoporous Si-MCM-41 but also with Si-MCM-41-SO(3)H functionalized by grafting. Thus, full access of adsorbate into the entire length of the pores is proven. For the analysis of the distribution of functional groups within the pores in dependence on the used functionalization method, grafting or co-condensation, however, the more specific adsorbate H(2)O/D(2)O (42:58) is necessary, because it reacts more sensitively toward small changes in the SLD of the host material. For grafted Si-MCM-41-SO(3)H materials, an incomplete quenching was observed, indicating that only some regions, probably the pore mouths, have been modified. For a sample functionalized by co-condensation, almost no quenching of the neutron diffraction was found, indicating a very homogeneous distribution of the functional groups along the entire pores.

Hollow α-Fe2O3 nanofibres for solar water oxidation: improving the photoelectrochemical performance by formation of α-Fe2O3/ITO-composite photoanodes

We demonstrate the synthesis and photoelectrochemical performance of high-aspect ratio dense and hollow α-Fe2O3 nanofibres, and the formation of core–shell-like α-Fe2O3/indium-tin oxide (ITO) nanocomposites utilised as a photoanode for solar water splitting. α-Fe2O3 nanofibres were prepared via a single-nozzle electrospinning technique using iron chloride (FeCl3) and poly(vinylpyrrolidone) (PVP) as precursors, followed by calcination. A new synthetic formation mechanism has been proposed taking into account the significance of three control parameters: (i) the iron precursor, (ii) the role of a co-solvent and (iii) the influence of the humidity on the tube evolution of α-Fe2O3 nanotubes. Hollow α-Fe2O3 fibres showed enhanced photocurrents and incident photon-to-current efficiency (IPCE) values compared to dense fibres, which are ascribed to the superior surface area of hollow fibres offering a good accessibility for the electrolyte and thus leading to improved mass transport. The photoelectrochemical properties of the α-Fe2O3 nanofibres could be further enhanced by the combination with highly crystalline, uniform ITO nanocrystals (O 10 nm), thus forming a core–shell-like α-Fe2O3/ITO fibre nanocomposite. The doubled photocurrent of the α-Fe2O3/ITO nanocomposite can most likely be attributed to the fast interfacial charge carrier exchange between the highly conductive ITO nanoparticles and α-Fe2O3, thus inhibiting the recombination of the electron–hole pairs in the semiconductor by spatial separation.

Active Sites for Light Driven Proton Reduction in Y2Ti2O7 and CsTaWO6 Pyrochlore Catalysts Detected by In Situ EPR

Abstract In situ EPR spectroscopy proved to be a versatile tool to identify active sites for photocatalytic hydrogen generation in modified Y2Ti2O7 and CsTaWO6 catalysts of pyrochlore structure, in which the metal cations are located in two different positions A and B. It was found that the B-sites exclusively occupied by titanium (Y2Ti2O7) and tantalum/tungsten (CsTaWO6) act as electron traps on the surface. From these sites, electron transfer to the co-catalysts proceeds. Thus, the B-sites are responsible for photocatalytic water reduction.Graphical Abstract

Highly mesoporous CsTaWO6 via hard-templating for photocatalytic hydrogen production

The quaternary photocatalyst CsTaWO6 was for the first time prepared with a high surface area of 115 m2 g−1 via a hard-templating approach. The highly crystalline and phase-pure material was applied in photocatalytic hydrogen production experiments to demonstrate the influence of porosity, surface area and crystallinity on charge carrier transfer.