Recent progress in soft-templating of porous carbon materials

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Mesoporous carbons synthesized via a soft-templating approach have attracted much attention due to their easy synthesis and facile control over the derived pore structure. In analogy to soft-templating approaches for mesoporous metal oxides, their synthesis is based on a sequence of forming supramolecular arrangements of precursor molecules with the soft templates, stabilization of the precursor framework by polymerization and finally the removal of the templates. Using micelles of amphiphilic block-copolymers as templates, facile control over the morphology and size of mesopores can be achieved by e.g. controlling size, composition, and concentration of the template polymers or composition and degree of polymerization of the precursor. Moreover, soft templating approaches can be extended to obtain also carbon materials with hierarchical meso- and macroporosity. The additional macroporosity either can result from templating by polymer latex or is induced via macrophase separation. In this review, we describe recent progress and examples in the synthesis and application of mesoporous carbon materials based on soft-templating approaches. Moreover, we reiterate fundamental principles of self-aggregation, highlight proposed synthesis mechanisms and present means of controlling pore size, also in hierarchical meso–macroporous carbon materials.BMBF, 03X5517A, Rationales Design poröser Katalysatorfilme im Nanometerbereich - DEPOKATDFG, EXC 314, Unifying Concepts in Catalysi

Colloidal bimetallic platinum–ruthenium nanoparticles in ordered mesoporous carbon films as highly active electrocatalysts for the hydrogen evolution reaction

Hydrogen features a very high specific energy density and is therefore a promising candidate for clean fuel from renewable resources. Water electrolysis can convert electrical energy into storable and transportable hydrogen gas. Under acidic conditions, platinum is the most active and stable monometallic catalyst for the hydrogen evolution reaction (HER). Yet, platinum is rare and needs to be used efficiently. Here, we report a synthesis concept for colloidal bimetallic platinum–ruthenium and rhodium–ruthenium nanoparticles (PtRuNP, RhRuNP) and their incorporation into ordered mesoporous carbon (OMC) films. The films exhibit high surface area, good electrical conductivity and well-dispersed nanoparticles inside the mesopores. The nanoparticles retain their size, crystallinity and composition during carbonization. In the hydrogen evolution reaction (HER), PtRuNP/OMC catalyst films show up to five times higher activity per Pt than Pt/C/Nafion® and PtRu/C/Nafion® reference catalysts.TU Berlin, Open-Access-Mittel - 2020European Metrology Research Programme (EMRP), 16ENG0, Hybrid metrology for thin films in energy applications (HyMET)BMBF, 03VP05390, Nanostrukturierte Elektroden der nächsten Generation für eine energieeffiziente Produktion von Chlor - Next-Gen-ChlorBMBF, 03EK3009, Design hocheffizienter Elektrolysekatalysatore

Reliable palladium nanoparticle syntheses in aqueous solution: the importance of understanding precursor chemistry and growth mechanism

Reliable protocols for the synthesis of palladium nanoparticles (Pd-NPs) in aqueous solution are rarely found and the corresponding growth mechanisms often remain unknown. Furthermore, syntheses of Pd-NPs always demand the use of stabilizing agents which are often unfavorable for catalytic applications. In this contribution, the importance of the palladium precursor chemistry as a prerequisite for any reliable Pd-NP synthesis in aqueous solution is shown. This includes a detailed study of the influence of the precursor chemistry on the nanoparticle growth mechanism. The findings enable the controlled modification of a common synthetic protocol (i.e. the reduction of a palladium precursor with NaBH4) to obtain sub-5 nm Pd-NPs without the use of any stabilizing agent. In addition, it is also shown that such mechanistic studies are not only of great importance to the development of novel synthetic procedures. Exemplarily, the successful transfer of the synthesis from lab-to large-scale is demonstrated.BMBF, 03EK3009, Design hocheffizienter Elektrolysekatalysatore

Time-resolved in situ studies on the formation mechanism of iron oxide nanoparticles using combined fast-XANES and SAXS

The reaction of iron chlorides with an alkaline reagent is one of the most prominent methods for the synthesis of iron oxide nanoparticles. We studied the particle formation mechanism using triethanolamine as reactant and stabilizing agent. In situ fast-X-ray absorption near edge spectroscopy and small-angle X-ray scattering provide information on the oxidation state and the structural information at the same time. In situ data were complemented by ex situ transmission electron microscopy, wide-angle X-ray scattering and Raman analysis of the formed nanoparticles. The formation of maghemite nanoparticles (gamma-Fe2O3) from ferric and ferrous chloride was investigated. Prior to the formation of these nanoparticles, the formation and conversion of intermediate phases (akaganeite, iron(II, III) hydroxides) was observed which undergoes a morphological and structural collapse. The thus formed small magnetite nanoparticles (Fe3O4) grow further and convert to maghemite with increasing reaction time

Rationales Design poröser Katalysatorfilme im Nanometerbereich - DEPOKAT : Schlussbericht ; Berichtszeitraum: 01.06.2007 - 30.04.2009

[no abstract available

Die katalytische Ammoniakoxidation an polykristallinem Platin: Oberflächenmorphologie und Reaktionskinetik bei Normaldruck

Ziel dieser Arbeit war ein Verständnis der Reaktionskinetik und der durch die Reaktion induzierten Veränderungen des Katalysators für die Ammoniakoxidation an Pt im Druckbereich um 100 kPa. Primär- und Sekundärreaktionen der Ammoniakoxidation (NH3+O2 / N2O / NO; Zersetzung von NH3, N2O, NO) wurden bei Temperaturen zwischen 180 und 900 °C untersucht. Durch die Entwicklung eines mikrostrukturierten Quarzreaktors wurde eine kinetische Untersuchung der Ammoniakoxidation im Temperaturbereich von 286 bis 385°C ermöglicht. Die Platinmorphologie verändernde Effekte wurden identifiziert und zur Erklärung von beobachteten Aktivitätsänderungen herangezogen. Zur Untersuchung der durch Sauerstoff bedingten Änderungen der Pt-Morphologie wurde der Katalysator nach Behandlung in N2O, O2 and H2 mittels XPS, XRD, SEM und EDX charakterisiert. Die Behandelung des Platins im N2O-Strom bis 600°C erzeugt eine im Platingitter gelöste Sauerstoffspezies auf Zwischengitterplätzen (XPS, XRD). Bei höheren Temperaturen im N2O-Fluß (>600°C) oder in O2 (900°C) bildet sich eine stabile Platin-Sauerstoff-Phase, Pt-Ox (XPS, XRD). Die Sauerstoffspezies wird Pt-Plätzen im Pt-Gitter zugeordnet. Eine Wasserstoffbehandlung (900°C) stellt durch Entfernen der Sauerstoffspezies die ursprüngliche Pt-Struktur wieder her. Die Phasenumwandlung von Pt in Pt-Ox mindert drastisch dessen katalytische Aktivität bei der N2O und NO-Zersetzung, und für die Reaktion von Ammoniak mit O2, N2O oder NO. Im Gegensatz dazu ist nur Pt?Ox, nicht jedoch Pt, aktiv in der Ammoniakzersetzung bei 550 bis 850°C. Bei ausreichend hoher Temperatur aktiviert der Pt-Ox-Katalysator langsam (h) im Verlauf der Reaktion von Ammoniak mit O2, N2O oder NO, während Pt durch die N2O-Zersetzung innerhalb von Stunden desaktiviert wird (T>600°C). Die Aktivitätsänderungen deuten darauf hin, daß die Phasenumwandlung PtóPt?Ox reversibel verläuft und sich ihre Gleichgewichtslage den Reaktionsbedingungen entsprechend einstellt. Durch die Ammoniakoxidation an Pt-Folie wurde im gesamten untersuchten Temperaturbereich, d.h. von 286 bis 700°C, eine Facettierung der Katalysatoroberfläche hervorgerufen, deren Ausprägung stark von der Temperatur abhängt. Bis etwa 400°C bilden sich Reihen paralleler Facetten deren Größe mit steigender Temperatur von ca. 0.1 auf 0.5 µm zunimmt; oberhalb von ca. 500°C wachsen regelmäßig geformte Mikrokristalle auf der Oberfläche auf. Deren scharfe Kanten facettieren wiederum mit der Zeit, bis schließlich glatte und gerundete Kristalle die gesamte Oberfläche bedecken. Diese Umstrukturierung wird literaturgemäß bei niedrigen Temperaturen einer durch Adsorbate beschleunigten Oberflächendiffusion von Pt, bei hohen Temperaturen dem Transport als leicht-flüchtiges Platinoxid durch die Gasphase zugeschrieben. Die Kinetik der Ammoniakoxidation wurde im Temperaturbereich des kinetischen Regimes und langsamer Oberflächenrestrukturierungen untersucht (286-385°C). Die Bildungsgeschwindigkeit von NO, N2O und N2 wurde als Funktion der Partialdrücke (pNH3, pO2, pNO) und der Temperatur beschrieben; der Einfluß der Zugabe von N2O war vernachlässigbar. Ein kinetisches Modell wurde unter Annahme einer Pt(111)-Oberfläche, sowie eines gelumpten und 9 elementarer Reaktionsschritte erstellt. Das Model beschreibt die experimentellen Daten besser als die ebenfalls getesteten Modelle von Scheibe et al. (Surf.Sci. 576, p.131) und Rebrov et al. (Chem.Eng.R.&D., 81(7), p.744). Die Beschreibung des Temperatureinflusses konnte durch die Berücksichtigung der - mikroskopisch untersuchten - Temperaturabhängigkeit der reaktionsinduzierten Oberflächenaufrauhung deutlich verbessert werden.The aim of this work was to elucidate the reaction kinetics and reaction-induced changes of platinum catalyst for ammonia oxidation carried out at pressures in the kPa range. For this purpose, primary and secondary reactions of ammonia oxidation were studied on polycrystalline Pt at temperatures of 180 to 900°C (NH3+O2 / N2O / NO; decomposition of NH3, N2O, NO). Moreover, experimental conditions for a kinetic study were established in a novel micro-structured quartz-reactor, and the kinetics was determined for 286 to 385°C. Processes that induce modifications of the Pt morphology were identified, and related to activity changes. To elucidate oxygen-related changes of Pt morphology, catalysts treated in N2O, O2 and H2 were studied applying XPS, XRD, SEM and EDX. Heating Pt in N2O flow up to 600°C formed bulk-dissolved oxygen, Odiss, interpreted to reside in interstitial sites. This slightly expanded the fcc lattice of Pt. Upon heating to higher temperature in N2O flow (>600°C) and O2 (900°C), Pt transformed into a stable platinum-oxygen alloy, Pt?Ox, featuring a further expanded elementary cell of platinum. XPS indicated an additional oxygen species, interpreted as oxygen residing in substitutional position in the Pt lattice. H2 treatments (900°C) restored the Pt lattice by removing oxygen species. The transformation of Pt into the platinum-oxygen alloy Pt-Ox drastically reduced its ability to decompose N2O and NO, and to catalyse the reactions of ammonia with O2, N2O and NO. In contrast, Pt-Ox was active in ammonia decomposition at 550-850°C, while Pt was not. The oxygen-treated catalyst activated during reaction of ammonia with N2O, NO and O2 with time on stream at sufficiently high temperature, while Pt deactivated within hours in N2O decomposition (T>600°C). Activation and deactivation are interpreted as an adjusting of the oxygen content of Pt during reaction, i.e. a reversible phase transformation Pt ó Pt-Ox that equilibrates to reaction conditions. Facet formation was induced on the surface of Pt foil during ammonia oxidation in the whole investigated temperature range (286-700°C). The surface roughening depended strongly on temperature. At low temperatures ( 500°C) regular-shaped bulky micro-crystals grew. The crystal edges broke up with time-on-stream to form a surface covered with smooth and rounded Pt needles. The mechanism of surface reconstruction was interpreted as adsorbate-enhanced Pt diffusion at low temperatures, and gas phase transport via volatile PtO2 at high temperatures. The kinetics of ammonia oxidation was studied between 286 and 385°C, where reaction-induced surface reconstruction was slow, and the kinetic regime prevailed. Rates of NO, N2O and N2 formation were described as a function of pNH3, pO2, pNO and T. In contrast to NO, re-adsorption and reaction of N2O was negligible. A kinetic model with one lumped reaction (NH3?s + O?s) and 9 elementary reaction steps was developed using available mechanistic information, and assuming a Pt(111) model surface. The model fitted the experimental data better than the also tested steady-state kinetic models of Scheibe et al. (Surf. Sci. 576, p.131) and Rebrov et al. (Chem.Eng.R.&D., 81(7), p.744). The model was still deficient in the description of the temperature influence, which could be overcome by including a term for the temperature dependence of reaction-induced surface reconstruction

Versatile control over size and spacing of small mesopores in metal oxide films and catalytic coatings via templating with hyperbranched core-multishell polymers

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.BMBF, 03EK3009, Design hocheffizienter Elektrolysekatalysatore