Heterogeneous catalysis performed in wall-coated reactors and electrocatalysis require homogeneous catalytic coatings with high surface area and good accessibility of the active sites. Conventional coating methods necessitate the use of binder components that often block pores and active sites, which limits catalytic efficiency, and utilization of expensive active metals. We report an approach for the direct and binder-free synthesis of chemically, mechanically, and thermally stable catalytic coatings based on ordered mesoporous carbon films employed as catalyst support. The synthesis relies on the codeposition of a structure-directing agent and small clusters of polymeric carbon precursors along with ionic metal species on a substrate. A sequence of thermal treatments converts the polymer into partly graphitized carbon, decomposes the structure-directing agent, and converts the metal precursor into highly active nanoparticles. Syntheses and catalytic applications are exemplarily demonstrated for palladium on carbon, a system widely used in heterogeneous catalysis and electrocatalysis. The obtained catalysts provide significantly higher space–time yields in the selective gas-phase hydrogenation of butadiene than all reported Pd/C catalysts while at the same time retaining isothermal reactor conditions. Moreover, when they were tested in the electrocatalytic hydrogen evolution reaction (HER), the catalysts outperformed reported Pd/C catalysts by a factor of 3, which underlines the benefits of the developed binder-free catalyst system.Keywords: butadiene hydrogenation; carbon; catalyst synthesis; HER; Nafion; palladium;

Iron can form numerous oxides, hydroxides, and oxide−hydroxides. Despite their relevance, many of the transformation processes between these phases are still poorly understood. In particular the crystallization of quasi-amorphous hydroxides and oxide−hydroxides is difficult to assess, since typical diffraction and scattering methods provide only sample-averaged information about the crystallized phases. We report a new approach for the investigation of the crystallization of oxide−hydroxides. The approach relies on model-type films that comprise a defined homogeneous nanostructure. The nanostructure allows quantitative linking of information obtained by bulk-averaging diffraction techniques (XRD, SAXS) with locally resolved information, i.e., domain sizes (SEM, TEM, LEEM) and phase composition (SAED). Using time-resolved imaging and diffraction we deduce mechanism and kinetics for the crystallization of ferrihydrite into hematite. Hematite forms via nucleation of hematite domains and subsequent domain growth that terminates only upon complete transformation. A Johnson–Mehl–Avrami–Kolmogorov model describes the kinetics over a wide temperature range. The derived understanding enables the first synthesis of ferrihydrite films with ordered mesoporosity and quantitative control over the films’ hematite and ferrihydrite content.

•New synthesis route to bimetallic nanoparticles in mesoporous carbon films proposed.•Films are homogeneous, conductive also mechanically, chemically and thermally stable.•Films with RuPt nanoparticles show the highest Pt-mass based activity in the HER.Platinum is the most active catalyst for the electrolytic hydrogen evolution reaction (HER). Yet, it is expensive as well as scarce and therefore needs to be used most efficiently. In order to reduce the amount of necessary Pt we propose a synthesis approach to alloyed RuPt nanoparticles which retain the enhanced catalytic activity of Pt. These particles are dispersed on an electrically conductive, highly accessible and stable carbon coating. The synthesis route relies on the co-deposition of molecular Ru and Pt precursors together with a carbon precursor with micelles as structure directing pore templates. The films adhere without the use of binding agents like Nafion, which can decrease catalytic activity by blocking of pores and active surface sites. Carbonization in reducing atmosphere (H2/Ar) leads to ordered mesoporous carbon films with a controlled morphology and a very high electrical conductivity. The films exhibit well-dispersed small nanoparticles.The presented RuPt catalyst films show a four times higher activity per Pt in the hydrogen evolution reaction compared to conventional Nafion-based Pt/C reference catalysts. Moreover, the synthesis approach represents a generic approach to ordered mesoporous carbon coatings containing metal nanoparticles.Download high-res image (97KB)Download full-size image

•We investigate the porosity and optical properties of porous TiO2 layers.•With a new preparation technique, the layer porosity, pore size, and pore number is controllable.•By a combination of spectroscopic ellipsometry and EPMA, the porosity can be independently determined.•A new method for determining model uncertainties for spectroscopic ellipsometry is used.•The goal is a better understanding of metrology of complex material parameters with ellipsometry.The practical performance of surface coatings in applications like catalysis, water splitting or batteries depends critically on the coating materials’ porosity. Determining the porosity in a fast and non-destructive way is still an unsolved problem for industrial thin-films technology. As a contribution to calibrated, non-destructive, optical layer characterisation, we present a multi-method comparison study on porous TiO2 films deposited by sol-gel synthesis on Si wafers. The ellipsometric data were collected on a range of samples with different TiO2 layer thickness and different porosity values. These samples were produced by templated sol-gel synthesis resulting in layers with a well-defined pore size and pore density. The ellipsometry measurement data were analysed by means of a Bruggeman effective medium approximation (BEMA), with the aim to determine the mixture ratio of void and matrix material by a multi-sample analysis strategy. This analysis yielded porosities and layer thicknesses for all samples as well as the dielectric function for the matrix material. Following the idea of multi-method techniques in metrology, the data was referenced to imaging by electron microscopy (SEM) and to a new EPMA (electron probe microanalysis) porosity approach for thin film analysis. This work might lead to a better metrological understanding of optical porosimetry and also to better-qualified characterisation methods for nano-porous layer systems.Download high-res image (315KB)Download full-size image

The nature of the small iron-oxo oligomers in iron-(III) aqueous solutions has a determining effect on the chemical processes that govern the formation of nanoparticles in aqueous phase. Here we report on a liquid-jet photoelectron-spectroscopy experiment for the investigation of the electronic structure of the occurring iron-oxo oligomers in FeCl3 aqueous solutions. The only iron species in the as-prepared 0.75 M solution are Fe3+ monomers. Addition of NaOH initiates Fe3+ hydrolysis which is followed by the formation of iron-oxo oligomers. At small enough NaOH concentrations, corresponding to approximately [OH]/[Fe] = 0.2–0.25 ratio, the iron oligomers can be stabilized for several hours without engaging in further aggregation. Here, we apply a combination of non-resonant as well as iron 2p and oxygen 1s resonant photoelectron spectroscopy from a liquid microjet to detect the electronic structure of the occurring species. Specifically, the oxygen 1s partial electron yield X-ray absorption (PEY-XA) spectra are found to exhibit a peak well below the onset of liquid water and OH− (aq) absorption. The iron 2p absorption gives rise to signal centered between the main absorption bands typical for aqueous Fe3+. Absorption bands in both PEY-XA spectra are found to correlate with an enhanced photoelectron peak near 20 eV binding energy, which demonstrates the sensitivity of resonant photoelectron (RPE) spectroscopy to mixing between iron and ligand orbitals. These various signals from the iron-oxo oligomers exhibit maximum intensity at [OH]/[Fe] = 0.25 ratio. For the same ratio, we observe changes in the pH as well as in complementary Raman spectra, which can be assigned to the transition from monomeric to oligomeric species. At approximately [OH]/[Fe] = 0.3 we begin to observe particles larger than 1 nm in radius, detected by small-angle X-ray scattering.

Micro structured reactors are attractive candidates for further process intensification in heterogeneous catalysis. However, they require catalytic coatings with significantly improved space-time yields compared to traditional supported catalysts. We report the facile synthesis of homogeneous nanocrystalline Pt coatings with hierarchical pore structure by electrochemical dealloying of amorphous sputter-deposited platinum silicide layers. Thickness, porosity and surface composition of the catalysts can be controlled by the dealloying procedure. XPS analysis indicates that the catalyst surface is primarily composed of metallic Pt. Catalytic tests in gas-phase hydrogenation of butadiene reveal the typical activity, selectivity and activation energy of nanocrystalline platinum. However, space time yields are about 13 to 200 times higher than values reported for Pt-based catalysts in literature. The highly open metallic pore structure prevents heat and mass transport limitations allowing for very fast reactions and reasonable stability at elevated temperatures.

Controlling the pore structure of metal oxide films and supported catalysts is an essential requirement for tuning their functionality and long-term stability. Typical synthesis concepts such as “Evaporation Induced Self Assembly” rely on micelle formation and self assembly. These processes are dynamic in nature and therefore strongly influenced by even slight variations in the synthesis conditions. Moreover, the synthesis of very small mesopores (2–5 nm) and independent control over the thickness of pore walls are very difficult to realize with micelle-based approaches. In this contribution, we present a novel approach for the synthesis of mesoporous metal oxide films and catalytic coatings with ordered porosity that decouples template formation and film deposition by use of hyperbranched core–multishell polymers as templates. The approach enables independent control of pore size, wall thickness and the content of catalytically active metal particles. Moreover, dual templating with a combination of hyperbranched core–multishell polymers and micelles provides facile access to hierarchical bimodal porosity. The developed approach is illustrated by synthesizing one of the most common metal oxides (TiO2) and a typical supported catalyst (PdNP/TiO2). Superior catalyst performance is shown for the gas-phase hydrogenation of butadiene. The concept provides a versatile and general platform for the rational optimization of catalysts based e.g. on computational prediction of optimal pore structures and catalyst compositions.

Minimizing efficiency losses caused by unwanted light reflection at the interface between lenses, optical instruments and solar cells with the surrounding medium requires antireflective coatings with adequate refractive index and coating thickness. We describe a new type of antireflective coating material with easily and independently tailorable refractive index and coating thickness based on the deposition of colloidal MgF2 nanoparticles. The material synthesis employs micelles of amphiphilic block copolymers as structure directing agent to introduce controlled mesoporosity into MgF2 film. The coatings thickness can be easily adjusted by the applied coating conditions. The coatings refractive index is determined by the materials porosity, which is controlled by the amount of employed pore template. The refractive index can be precisely tuned between 1.23 and 1.11, i.e., in a range that is not accessible to nonporous inorganic materials. Hence, zero reflectance conditions can be established for a wide range of substrate materials.Keywords: antireflective coating; mesoporous material; micelle; sol−gel; thin film

Catalysis, energy storage, and light harvesting require functional materials with tailored porosity and nanostructure. However, common synthesis methods that employ polymer micelles as structure-directing agents fail for zinc oxide, for cobalt oxide, and for metal carbonates in general. We report the synthesis of the oxides and carbonates of zinc, cobalt, and aluminum with micelle-templated structure. The synthesis relies on poly(ethylene oxide)-block-poly(butadiene)-block-poly(ethylene oxide) triblock copolymers and a new type of precursor formed by chemical complexation of a metal nitrate with citric acid. A general synthesis mechanism is deduced. Mechanistic insights allow for the prediction of optimal processing conditions for different oxides and carbonates based on simple thermogravimetric analysis. Employing this synthesis, films of ZnO and Co3O4 with micelle-controlled mesoporosity become accessible for the first time. It is the only soft-templating method reported so far that also yields mesoporous metal carbonates. The developed synthesis is generic in nature and can be applied to many other metal oxides and carbonates.Keywords: cobalt oxide; EISA; metal carbonate; metal oxide; pore templating; zinc oxide;

Chlorine evolution is one of the most important electrochemical reactions applied in industry. We present a method for the synthesis of chlorine evolution catalysts with improved performance. The performance increase results from the introduction of controlled mesoporosity into the pore system of Ru- and Ir-containing TiO2 catalysts by pore templating with micelles of amphiphilic block-copolymers. Micelle-templated TiO2-based catalysts were synthesized with loadings up to 15 wt % of either Ru, Ir, or a combination of both active metals. The catalysts’ walls are composed of nanocrystalline mixed oxides with rutile structure. The templated mesopores are about 10 nm in size and form an ordered cubic pore system with good pore connectivity. All studied catalysts are active in chlorine evolution. Adding templated mesoporosity doubles the catalyst performance at identical catalyst composition. The influences of film thickness, composition, and porosity of the developed catalytic coatings on the catalytic performance are discussed.Keywords: chlorine evolution; iridium oxide; mesoporous films; ruthenium oxide; titanium oxide

Control over the size of active metal particles and the structure of catalysts pore system is an essential requirement for the design of supported catalysts. Polymeric templates combined with a suitable metal-oxide precursor enable the synthesis of defined pore systems, whereas colloidal metal particles can provide access to the particle-size control. However, pore template, metal-oxide precursor, and colloidal metal particles combined in one synthesis solution are often not compatible with each other due to aggregation, precipitation, and dissolution processes. We present a new approach to the preparation of supported catalysts that permits the controlled coassembly of preformed colloidal metal nanoparticles, polymeric pore templates, and a metal-oxide precursor from a water-based solution. The synthesis is enabled by establishing under pH-neutral conditions the templating of defined pores using titanium(IV) bis(ammonium lactato) dihydroxide as an unconventional metal-oxide precursor. The presented approach provides a modular strategy for the precise control of the catalysts nanostructure. This is illustrated for the synthesis of mesoporous as well as hierarchically porous Pd/TiO2 catalysts prepared from colloidal solutions of palladium nanoparticles. The catalysts show high activity and selectivity in the gas-phase hydrogenation of 1,3-butadiene.Keywords: hierarchical porous; hydrogenation of 1,3-butadiene; mesoporous materials; palladium nanoparticle; Pd-TiO2 catalyst; titanium oxide films;

Synthesis of mesoporous iridium oxide films via soft templating and evaporation-induced self-assembly is demonstrated employing an amphiphilic triblock-copolymer PEO-PB-PEO. Films possess nanocrystalline walls and feature locally ordered pores of about 16 nm diameter. Analysis of the film properties by SEM, TEM, EDX, XPS, SAXS, XRD, and BET along the thermal treatment that succeeds dipcoating shows that the polymer template is removed by calcination between 200 and 300 °C, accompanied by uniaxial shrinkage of film and pore system perpendicular to the substrate. Treating the film in excess of 450 °C leads to further growth of crystallite size and loss of surface area progressing gradually with increasing calcination temperature. Templated IrO2 films conditioned at 450 °C show substantially reduced electrocatalytic overpotentials (efficiency increases) for the oxygen evolution reaction (OER) compared to those of untemplated coatings. Pore templating thus enables direct control over surface catalytic properties of iridium oxide.Keywords: electro catalyst; iridium oxide films; mesoporous materials; OER; PEO-PB-PEO template;

A novel film coating technique, template-assisted electrostatic spray deposition (TAESD), was developed for the synthesis of porous metal oxide films and tested on TiO2. Organic templates are codeposited with the titania precursor by electrostatic spray deposition and then removed during calcination. Resultant films are highly porous with pores casted by uniformly sized templates, which introduced a new level of control over the pore morphology for the ESD method. Employing the amphiphilic block copolymer Pluronic P123, PMMA latex spheres, or a combination of the two, mesoporous, macroporous, and hierarchically porous TiO2 films are obtained. Decoupled from other coating parameters, film thickness can be controlled by deposition time or depositing multiple layers while maintaining the coating’s structure and integrity.

A new setup for fast in situ SAXS studies of early stages in the nucleation and growth of colloidal nanoparticles is presented. Evading the disturbing influence of container walls and minimizing the possibility of beam-induced reactions, the benefits of the setup are demonstrated exemplarily for the well-known synthesis of gold nanoparticles via the Turkevich method. Analysis with the new experimental setup reveals the initial rate of particle formation, and enables analysis of particle growth rates.

Mesoporous titania films were prepared by template-assisted dip coating on 1.4301 stainless-steel substrates processed by grinding and spark erosion to different degrees of roughness. The influence of substrate roughness on the morphology and mesostructure of deposited films was studied. Textures produced by grinding with roughness Ra ranging from 0.10 to 0.78 μm did not noticeably affect the pore structure as confirmed by similar pore size and a single cubic mesophase formed on grinded steel. Grinding had a modest effect on the film integrity which manifested in fractures developed in the texture depressions. Greater roughness of the steel produced by spark erosion affected the micelle self-assembly process yielding two different mesophases on a substrate of 1.08 μm roughness, and resulting in a predominant loss of templated mesostructure on a rougher (Ra = 2.69 μm) substrate surface. Film surface area expressed as m2 BET per m2 of the substrate planar dimensions increased with substrate roughness. Higher roughness resulted in higher photocatalytic activity of crystalline films when tested in methylene blue decomposition. Given that a moderate surface texture had a negligible effect on the film mesostructure, introducing controlled substrate roughness may serve as a technique to enhance the total film surface area.

Although gold nanoparticles (GNP) are among the most intensely studied nanoscale materials, the actual mechanisms of GNP formation often remain unclear due to limited accessibility to in situ-derived time-resolved information about precursor conversion and particle size distribution. Overcoming such limitations, a method is presented that analyzes the formation of nanoparticles via in situ SAXS and XANES using synchrotron radiation. The method is applied to study the classical GNP synthesis route via the reduction of tetrachloroauric acid by trisodium citrate at different temperatures and reactant concentrations. A mechanism of nanoparticle formation is proposed comprising different steps of particle growth via both coalescence of nuclei and further monomer attachment. The coalescence behavior of small nuclei was identified as one essential factor in obtaining a narrow size distribution of formed particles.

Controlling the pore structure of metal oxide films and supported catalysts is an essential requirement for tuning their functionality and long-term stability. Typical synthesis concepts such as “Evaporation Induced Self Assembly” rely on micelle formation and self assembly. These processes are dynamic in nature and therefore strongly influenced by even slight variations in the synthesis conditions. Moreover, the synthesis of very small mesopores (2–5 nm) and independent control over the thickness of pore walls are very difficult to realize with micelle-based approaches. In this contribution, we present a novel approach for the synthesis of mesoporous metal oxide films and catalytic coatings with ordered porosity that decouples template formation and film deposition by use of hyperbranched core–multishell polymers as templates. The approach enables independent control of pore size, wall thickness and the content of catalytically active metal particles. Moreover, dual templating with a combination of hyperbranched core–multishell polymers and micelles provides facile access to hierarchical bimodal porosity. The developed approach is illustrated by synthesizing one of the most common metal oxides (TiO2) and a typical supported catalyst (PdNP/TiO2). Superior catalyst performance is shown for the gas-phase hydrogenation of butadiene. The concept provides a versatile and general platform for the rational optimization of catalysts based e.g. on computational prediction of optimal pore structures and catalyst compositions.

Micro structured reactors are attractive candidates for further process intensification in heterogeneous catalysis. However, they require catalytic coatings with significantly improved space-time yields compared to traditional supported catalysts. We report the facile synthesis of homogeneous nanocrystalline Pt coatings with hierarchical pore structure by electrochemical dealloying of amorphous sputter-deposited platinum silicide layers. Thickness, porosity and surface composition of the catalysts can be controlled by the dealloying procedure. XPS analysis indicates that the catalyst surface is primarily composed of metallic Pt. Catalytic tests in gas-phase hydrogenation of butadiene reveal the typical activity, selectivity and activation energy of nanocrystalline platinum. However, space time yields are about 13 to 200 times higher than values reported for Pt-based catalysts in literature. The highly open metallic pore structure prevents heat and mass transport limitations allowing for very fast reactions and reasonable stability at elevated temperatures.

A new setup for fast in situ SAXS studies of early stages in the nucleation and growth of colloidal nanoparticles is presented. Evading the disturbing influence of container walls and minimizing the possibility of beam-induced reactions, the benefits of the setup are demonstrated exemplarily for the well-known synthesis of gold nanoparticles via the Turkevich method. Analysis with the new experimental setup reveals the initial rate of particle formation, and enables analysis of particle growth rates.