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

ZnO Coatings with Controlled Pore Size, Crystallinity and Electrical Conductivity

Zinc oxide is a wide bandgap semiconductor with unique optical, electrical and catalytic properties. Many of its practical applications rely on the materials pore structure, crystallinity and electrical conductivity. We report a synthesis method for ZnO films with ordered mesopore structure and tuneable crystallinity and electrical conductivity. The synthesis relies on dip-coating of solutions containing micelles of an amphiphilic block copolymer and complexes of Zn2+ ions with aliphatic ligands. A subsequent calcination at 400 °C removes the template and induces crystallization of the pore walls. The pore structure is controlled by the template polymer, whereas the aliphatic ligands control the crystallinity of the pore walls. Complexes with a higher thermal stability result in ZnO films with a higher content of residual carbon, smaller ZnO crystals and therefore lower electrical conductivity. The paper discusses the ability of different types of ligands to assist in the synthesis of mesoporous ZnO and relates the structure and thermal stability of the precursor complexes to the crystallinity and electrical conductivity of the zinc oxide

An efficient bifunctional two-component catalyst for oxygen reduction and oxygen evolution in reversible fuel cells, electrolyzers and rechargeable air electrodes

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.We report on a non-precious, two-phase bifunctional oxygen reduction and evolution (ORR and OER) electrocatalyst with previously unachieved combined roundtrip catalytic reactivity and stability for use in oxygen electrodes of unitized reversible fuel cell/electrolyzers or rechargeable metal-air batteries. The combined OER and ORR overpotential, total, at 10 mA cm(-2) was a record low value of 0.747 V. Rotating Ring Disk Electrode (RRDE) measurements revealed a high faradaic selectivity for the 4 electron pathways, while subsequent continuous MEA tests in reversible electrolyzer cells confirmed the excellent catalyst reactivity rivaling the state-of-the-art combination of iridium (OER) and platinum (ORR).BMBF, 03SF0433A, Effiziente edelmetallfreie Katalysatorsysteme basierend auf Mangan und Eisen für flexible MeerwasserelektrolyseureBMBF, 03SF0527A, LoPlaKa

A meta-analysis of catalytic literature data reveals property-performance correlations for the OCM reaction

Decades of catalysis research have created vast amounts of experimental data. Within these data, new insights into property-performance correlations are hidden. However, the incomplete nature and undefined structure of the data has so far prevented comprehensive knowledge extraction. We propose a meta-analysis method that identifies correlations between a catalyst’s physico-chemical properties and its performance in a particular reaction. The method unites literature data with textbook knowledge and statistical tools. Starting from a researcher’s chemical intuition, a hypothesis is formulated and tested against the data for statistical significance. Iterative hypothesis refinement yields simple, robust and interpretable chemical models. The derived insights can guide new fundamental research and the discovery of improved catalysts. We demonstrate and validate the method for the oxidative coupling of methane (OCM). The final model indicates that only well-performing catalysts provide under reaction conditions two independent functionalities, i.e. a thermodynamically stable carbonate and a thermally stable oxide support

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

Eigenschafts-Aktivitäts-Beziehungen von Katalysatoren in der oxidativen Kupplung von Methan : ein kombinierter statistischer und experimenteller Ansatz

The catalytic oxidative coupling of methane (OCM) is a „dream reaction“ of heterogeneous catalysis, as it provides a direct conversion route of methane to the value-added and easily transportable C2 coupling products ethane and ethene. Since its first description in the early 1980s, a multitude of catalysts containing nearly any element have been tested, but so far the development of an economically viable process has not been successful due to insufficient yield. An important reason for this is the lack of understanding of relationships between catalyst properties and catalytic performance, which is valid for catalysts with widely varying compositions. The present work uses the available experimental data of historical catalyst testings in a new way, in order to reveal property-performance relationships for a large variety of catalysts by using a statistical meta-analysis. Furthermore, property-performance relationships were also studied by systematic experimental studies of catalyst series with specific properties, and results were compared with the statistical analysis. For the statistical evaluation, a new meta-analysis method was developed, which allows extraction of specific knowledge from datasets of heterogeneously catalysed reactions. The method assigns properties to each catalyst described by objective descriptors. The descriptors utilize simple physico-chemical properties of the elements contained in the catalyst, as well as the individually applied reaction temperature. After assignment of catalyst properties, hypotheses regarding the effect of specific properties on catalytic performance are tested. To this end, groups of catalysts with distinct properties were compared using a multiple variable regression and the effect size as well as its statistical significance were quantified. In application of this method to an OCM dataset, a hierarchical classification based on catalyst properties could be constructed using four of the tested hypotheses. According to the results, a well-performing OCM catalyst contains at least two components, of which one functions as a support under reaction conditions. Moreover, the ability to form carbonates and the related basicity are important properties. It is beneficial for catalytic performance if the support component of an OCM catalyst can also form a carbonate and therefore exhibits at least weak basicity. Furthermore there is a positive effect on performance if a catalyst contains a component with strong basicity that can form a carbonate which is thermodynamically stable. Such carbonates are likely to be stable under reaction conditions and can directly contribute to the OCM Performance beyond the effect of catalyst basicity. For the experimental study of property-performance relationships, series of simple carbonate catalysts without and with an Al2O3 support were tested. Important catalyst properties such as the support function and the carbonate stability were determined by characterization methods and are in accordance to the descriptors employed in the statistical evaluation. Furthermore, differences between unsupported and supported catalysts regarding the important property carbonate stability were determined. The combination of the measured catalytic performance with determined catalyst properties allows the deduction of property-performance relationships. For a series of catalysts, these relationships are in accordance with the results of the statistical evaluation. Furthermore, in the experimental studies it was observed that a further stabilization of the carbonate species (important for OCM performance) by addition of CO2 to the feed had a positive effect on C2 selectivity. The property-performance relationships determined in this thesis represent an important step towards understanding the OCM reaction, and can be used as guidelines for the development of improved catalysts and processes. Beyond that, the new method of statistical evaluation has the potential to be applied also to other heterogeneous catalysed reactions as a valuable tool for knowledge extraction, as well as for directed planning of efficient experimental studies.Die katalytische oxidative Kupplung von Methan (OCM) ist eine “dream reaction” der heterogenen Katalyse, da sie einen direkten Weg der Umwandlung von Methan zu den wertvollen und leicht transportablen C2 Kupplungsprodukten Ethan und Ethen darstellt. Seit der ersten Beschreibung in den frühen 1980er Jahren wurden verschiedenste Katalysatoren mit nahezu jedem Element getestet, jedoch gelang bisher keine Entwicklung eines wirtschaftlichen Prozesses wegen zu geringer Ausbeuten. Ein wichtiger Grund dafür ist, dass es an einem Verständnis für Zusammenhänge zwischen Katalysatoreigenschaften und katalytischer Performance mangelt, welches für Katalysatoren mit verschiedensten Zusammensetzungen gültig ist. Die vorliegende Arbeit nutzt die vorhandenen experimentellen Daten bisheriger OCM Katalysatortestungen auf neue Weise, um mit Hilfe einer statistischen Meta-Analyse Eigenschafts-Performance Beziehungen für eine große Bandbreite an Katalysatoren herzustellen. Zudem wurden Eigenschafts-Performance Beziehungen auch anhand systematischer experimenteller Studien von Katalysatorserien mit bestimmten Eigenschaften untersucht und mit den Ergebnissen der statistischen Analyse verglichen. Zur statistischen Auswertung wurde eine neue Methode der Meta-Analyse entwickelt, die der gezielten Wissensextraktion aus Datensätzen heterogen katalysierter Reaktionen dient. Die Methode ordnet jedem Katalysator Eigenschaften zu, welche durch objektive Deskriptoren beschrieben werden. Die Deskriptoren beruhen auf einfachen physiko-chemischen Eigenschaften der im Katalysator enthaltenen Elemente, sowie der individuell angewendeten Reaktionstemperatur. Nach Zuordnung der Katalysatoreigenschaften wurden Hypothesen hinsichtlich des Effekts bestimmter Eigenschaften auf die katalytische Performance getestet. Dazu wurden Gruppen von Katalysatoren mit bestimmten Eigenschaften mit Hilfe einer multivariablen Regression verglichen und die Effektgröße sowie seine statistische Signifikanz quantifiziert. In Anwendung dieser Methode auf einen OCM Datensatz konnte mit Hilfe von vier der getesteten Hypothesen eine hierarchische Klassifizierung nach Katalysatoreigenschaften konstruiert werden. Ein guter OCM Katalysator besitzt demnach mindestens zwei Komponenten, von denen eine unter Reaktionsbedingungen eine Trägerfunktion übernimmt. Zudem ist die Fähigkeit der Bildung von Carbonaten und die damit verbundene Basizität eine wichtige Eigenschaft. So ist es günstig für die katalytische Performance, wenn die Träger-Komponente eines OCM Katalysators ein Carbonat bilden kann und zumindest geringfügige Basizität aufweist. Weiterhin gibt es einen positiven Effekt auf die Performance, wenn ein Katalysator eine Komponente besitzt die aufgrund starker Basizität ein thermodynamisch stabiles Carbonat bilden kann. Solche Carbonate sind wahrscheinlich auch unter Reaktionsbedingungen stabil und können einen direkten Beitrag zur OCM Performance leisten, der über den Effekt der Basizität hinaus geht. Zur experimentellen Untersuchung von Eigenschafts-Performance Beziehungen wurden Serien einfacher Katalysatorsysteme aus Carbonaten sowohl ohne Träger als auch mit einem Al2O3 Träger getestet. Wichtige Katalysatoreigenschaften wie die Trägerfunktion und die Carbonatstabilität wurden durch Charakterisierungsmethoden bestimmt und stehen im Einklang mit den in der statistischen Auswertung verwendeten Deskriptoren. Zudem wurden Unterschiede zwischen den Katalysatoren mit und ohne Al2O3 Träger hinsichtlich der wichtigen Eigenschaft Carbonatstabilität festgestellt. Die Verbindung der gemessenen katalytischen Performance mit den ermittelten Katalysatoreigenschaften ermöglicht die Ableitung von Eigenschafts-Performance Beziehungen. Für eine Reihe von Katalysatoren stehen diese Beziehungen im Einklang mit den Ergebnissen der statistischen Auswertung. Zudem wurde in den experimentellen Untersuchungen der Effekt beobachtet, dass eine weitere Stabilisierung der für die OCM Performance wichtigen Carbonatspezies durch Zugabe von CO2 in den Reaktandenstrom einen positiven Effekt auf die C2 Selektivität hatte. Die in dieser Arbeit ermittelten Eigenschafts-Performance Beziehungen stellen einen wichtigen Schritt zum Verständnis der OCM Reaktion dar, und können als Richtlinie für die Entwicklung verbesserter Katalysatoren und Prozesse dienen. Darüber hinaus hat die neuartige Methode der statistischen Auswertung das Potential, auch für andere heterogen katalysierte Reaktionen als wertvolles Werkzeug zum Erkenntnisgewinn, sowie zur gezielten Planung effizienter experimenteller Studien genutzt zu werden

Oxide-supported carbonates reveal a unique descriptor for catalytic performance in the oxidative coupling of methane (OCM)

The oxidative coupling of methane (OCM) is a promising reaction for direct conversion of methane to higher hydrocarbons. The reaction can be performed over oxide-based catalysts with very diverse elemental composition. Yet, despite decades of research, no general common structure-activity relationship has been deduced. Our recent statistical meta-analysis across a wide range of catalyst compositions reported in the literature suggested that only the catalysts combining thermodynamically stable (under reaction conditions) carbonate and thermally stable oxide support exhibit good catalytic performance. Guided by these findings we explore now experimentally correlations between descriptors for structure, stability and decomposition behavior of supported metal carbonates vs. the materials’ respective performance in OCM catalysis. In this study, carbonates of Rb, Cs and Mg were supported on oxides of Sm, Y, Gd, Ce, Sr and Ba, tested in OCM and studied by IR spectroscopy and thermal analysis. From the evaluation of six proposed property-descriptors we derive a statistically robust volcano-type correlation between the onset temperature of carbonate decomposition and the C2 yield, indicating the importance of CO2 adsorption and surface carbonates in selective methane conversion. Moreover, we discuss mechanisms that can account for the observed property-performance correlation across a wide range of OCM catalysts. Carbonate species are suggested to block highly reactive sites during OCM catalysis, which reduces overoxidation and enables the formation of C2 products

Stabilization of Mesoporous Iron Oxide Films against Sintering and Phase Transformations via Atomic Layer Deposition of Alumina and Silica

International audienceThe stabilization of crystal phases and nanostructured morphologies is an essential topic in application-driven design of mesoporous materials. Many applications, e.g. catalysis, require high temperature and humidity. Typical metal oxides transform under such conditions from a metastable, low crystal-line material into a thermodynamically more favorable form, i.e. from ferrihy-drite into hematite in the case of iron oxide. The harsh conditions induce also a growth of the crystallites constituting pore walls, which results in sintering and finally collapse of the porous network. Herein, a new method to stabi-lize mesoporous templated metal oxides against sintering and pore collapse is reported. The method employs atomic layer deposition (ALD) to coat the internal mesopore surface with thin layers of either alumina or silica. The authors demonstrate that silica exerts a very strong influence: It shifts hematite formation from 400 to 600 °C and sintering of hematite from 600 to 900 °C. Differences between the stabilization via alumina and silica are rationalized by a different interaction strength between the ALD material and the ferrihydrite film. The presented approach allows to stabilize mesoporous thin films that require a high crystallization temperature, with submonolayer quantity of an ALD mate-rial, and to apply mesoporous materials for high temperature applications