Robert Schlögl

A unique oxygen ligand environment facilitates water oxidation in hole-doped IrNiOx core–shell electrocatalysts

AbstractThe electro-oxidation of water to oxygen is expected to play a major role in the development of future electrochemical energy conversion and storage technologies. However, the slow rate of the oxygen evolution reaction remains a key challenge that requires fundamental understanding to facilitate the design of more active and stable electrocatalysts. Here, we probe the local geometric ligand environment and electronic metal states of oxygen-coordinated iridium centres in nickel-leached IrNi@IrOx metal oxide core–shell nanoparticles under catalytic oxygen evolution conditions using operando X-ray absorption spectroscopy, resonant high-energy X-ray diffraction and differential atomic pair correlation analysis. Nickel leaching during catalyst activation generates lattice vacancies, which in turn produce uniquely shortened Ir–O metal ligand bonds and an unusually large number of d-band holes in the iridium oxide shell. Density functional theory calculations show that this increase in the formal iridium oxidation state drives the formation of holes on the oxygen ligands in direct proximity to lattice vacancies. We argue that their electrophilic character renders these oxygen ligands susceptible to nucleophilic acid–base-type O–O bond formation at reduced kinetic barriers, resulting in strongly enhanced reactivities.The precise understanding of the active phase under reaction conditions at the molecular level is crucial for the design of improved catalysts. Now, Strasser, Jones and colleagues correlate the high activity of IrNi@IrOx core–shell nanoparticles with the amount of lattice vacancies produced by the nickel leaching process that takes place before and during water oxidation, and elucidate the underlying structural-electronic effects.

Identification of reactive oxygen species in iridium-based OER catalysts by in situ photoemission and absorption spectroscopy

A change in our energy supply from fossil fuels to intermittent renewables requires energy storage capacity. Part of this capacity may be delivered from chemical energy conversion relying on H2 as basic fuel. Renewable H2 can be generated using proton exchange membrane (PEM) electrolyzers able to adapt to the varying voltage inputs of intermittent sources. Such PEM electrolyzers operate in acidic environments requiring corrosion-resistant catalysts for the H2 and O2 evolution reactions (HER & OER). Especially the OER is challenging and typical catalysts use rare and precious iridium oxides. Minimizing costs by reducing the iridium usage requires knowledge-based catalyst design: Favorable iridium oxide surface configurations need to be identified. X-ray photoemission and Near-edge X-ray absorption fine structure spectroscopy (XPS & NEXAFS) are powerful techniques to characterize surfaces. Therefore, having observed the increased catalytic OER activity of X-ray amorphous IrOx when compared to rutile-type IrO2, these techniques were combined with theory to identify respectively present iridium and oxygen species based on their signatures in the electronic structure. While rutile-type IrO2 was confirmed to consist only of Ir and OII−, the more active amorphous IrOx was observed to contain Ir and OI− in addition. The electron deficiency of the OI− species led to the suspicion that they may be good electrophiles and enhance the OER activity. Therefore, their character and reactivity were tested with the prototypical probe molecule CO. By monitoring both the gas phase composition and the spectroscopic fingerprint of OI− in the NEXAFS of the O Kedge, the spontaneous reaction between OI− and CO to form CO2 at room temperature was observed, confirming the electrophilic character and exceptional reactivity of OI−. To test the involvement of OI− species in OER catalysis, an electrochemical in situ cell was employed to monitor the electronic structure of an oxygen-evolving iridium surface by XPS and NEXAFS. These experiments confirmed the formation of a mixedvalent Ir matrix hosting both OII− and electrophilic OI− species during the OER. Measurements near the onset of iridium’s OER activity yielded a linear correlation between OI− concentration and OER activity. Further, major parts of the OI− contribution could be reversibly switched on and off when turning on and off the applied potential. These observations further indicated the intimate relationship between the presence of electrophilic OI− species and the OER activity of iridium-based catalysts. This connection may be understood by analogy with photosystem II: Electrophilic oxygen species can facilitate the nucleophilic attack of water during the O-O bond formation. This thesis demonstrates that the integration of electrophilic OI− species is a crucial design criterion for OER catalysts and explains why iridium is a good choice: It has the propensity to form electrophilic OI− species enhancing the O-O bond formation.

Iridium oxohydroxide electrocatalysts for the oxygen evolution reaction

................................................................................................................................................ vii Zusammenfassung ................................................................................................................................. ix Table of

Structure-function relationship of Strong Metal-Support Interaction studied on supported Pd reference catalysts

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Solid catalysts for methanol and ammonia synthesis investigated by in-situ neutron diffraction

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Spectroscopic studies towards the understanding of CeO2-based catalysts for chlorine production

The heterogeneously catalyzed gas-phase oxidation of HCl to Cl2 (Deacon reaction) is an energy-efficient and sustainable route to recycle chlorine from HCl-containing industrial waste streams. This work investigates HCl oxidation over CeO2 using in situ and ex situ characterization techniques, and provides molecular understanding of Deacon chemistry over ceria catalysts. The reactivity and stability of CeO2 in HCl oxidation were assessed by steady-state kinetic experiments, which indicate that CeO2 is an efficient catalyst for Cl2 recycling in the temperature range of 623-723 K. A structural-reactivity correlation obtained by the assessment of sample pretreatment effects, particularly calcination, on reactivity reveals that ceria samples calcined at mildly high temperature (1173 K) represent the best compromise between performance and stability. Calcination at this temperature optimizes both the number of vacancies and the structural stability of the catalyst. Because of the semiconductor nature of CeO2, HCl oxidation over CeO2 catalysts requires the redox chemistry of the catalyst as well, and thus O vacancies are expected to play a crucial role. X-ray diffraction and electron microscopy of samples exposed to reaction feeds with different O2:HCl ratios provide evidence that CeO2 does not suffer from bulk chlorination in O2-rich feeds (O2:HCl ≥ 0.75), while it does form chlorinated phases in stoichiometric or sub-stoichiometric feeds (O2:HCl ≤ 0.25). The quantitative analysis of the chlorine uptake by thermogravimetry and X-ray photoelectron spectroscopy indicates that chlorination under O2-rich conditions is confined to the surface and possibly one subsurface layer of CeO2 particles. Density functional theory simulations reveal that Cl activation from vacancy positions to surface Ce atoms is the most energy-demanding step, although chlorine-oxygen competition for the available active sites may render reoxidation as the rate-determining step. Since the surface of CeO2 is prone to chlorination upon exposure to the reaction gas mixture, and hence the active surface phase comprises Cl species in its structure, the stability and reactivity of CeOCl as a possible active phase for Deacon reaction were investigated using various feed compositions at 703 K. CeOCl was synthesized by the solid state reaction of cerium oxide and anhydrous cerium chloride. X-ray diffraction of post-reaction samples revealed that CeOCl is unstable, in both oxygen-rich and lean conditions. A complete transformation of CeOCl into CeO2 was observed by applying oxygen over-stoichiometric feeds. Considerable HCl conversions were obtained only after this transformation, which confirms the essential role of bulk cerium oxide in this catalytic system, probably by facilitating efficient O2 activation via bulk and surface O-vacancy dynamics. The impact of surface chlorination on the acid/base properties of ceria (fresh and after reaction) was investigated by probe molecule adsorption (CO2, NH3, CO) applying micro-calorimetry, FTIR, TPD and DFT calculations. Micro-calorimetric experiments with CO2 adsorption indicate that the basic character of CeO2 has been essentially eliminated upon reaction in HCl oxidation indicating that most of the basic lattice O sites are exchanged by chlorine and that the OH groups formed are rather acidic. Thus, HCl adsorption is certainly retarded by the loss of basic (O) sites required for H abstraction during dissociative HCl adsorption. Furthermore, FTIR and TPD adsorption experiments using NH3 and CO as probing molecules reveal that the density and the strength of surface acidic functions increased significantly upon reaction. EPR experiments were carried out on fresh and post-reaction samples using O2 as probing molecule to assess the effect of surface chlorination on the amount surface O vacancies required for O2 activation. The results strongly suggest that oxygen activation is inhibited by the high degree of surface chlorination. The coverage of most abundant surface intermediates, OH and Cl, were monitored by in situ infrared spectroscopy and in situ PGAA under various conditions. Higher temperature and p(O2) led to enhanced OH coverage, reduced Cl coverage and increased reactivity. Variation of p(HCl) gave rise to opposite correlations, while raising p(Cl2) did not induce any measurable increase in the Cl coverage, despite the strong inhibition of the reaction rate. The results indicate that only a small fraction of surface sites is actively involved in the reaction, and most of the surface species probed in the in situ observations are spectators. Kinetics of surface/bulk chlorination and dechlorination was investigated by means of in situ PGAA experiments. Using variable reaction conditions (T, pi) the in situ PGAA studies revealed that the chlorination rate is independent of the pre-chlorination degree but increases at lower oxygen over-stoichiometry, while dechlorination is effective in O2-rich feeds, and its rate is higher for a more extensively pre-chlorinated ceria. The role of trivalent (La, Sm, Gd, and Y) and tetravalent (Hf, Zr, and Ti) dopants in the catalytic, structural, and electronic properties of ceria was also investigated. Promoted ceria catalysts were synthesized by co-precipitation with ammonia and tested in HCl oxidation. The intrinsic reactivity of ceria was improved by a factor of 2 when doping with Hf and Zr in appropriate quantities, whereas trivalent dopants are detrimental. The effects of promoters on the electronic conductivity and the vacancy formation energy were studied by contactless conductivity experiments using the microwave cavity perturbation technique and by DFT calculations. In HCl oxidation, only the balanced reduction of both Cl and O vacancy formation energies allows for an enhanced reactivity. Promoters give rise to lattice contraction−expansion modifying vacancy formation energies, adsorption properties, and surface coverage.

Methane Oxidation on Platinum Catalysts Investigated by Spatially Resolved Methods

Catalytic partial oxidation (CPO) of methane is an attractive technology for industrial production of synthesis gas, an important precursor for the production of diverse basic chemicals, e.g. methanol, dimethyl ether, and formaldehyde. The exothermic reaction operates autothermally at temperatures around 1000 ◦C and gas hourly space velocity (GHSV) values up to 500000 h−1. On noble metal catalysts such as rhodium and platinum coated alumina foams, equilibrium synthesis gas yields are reached within millisecond contact time [1]. The CPO reaction proceeds in two steps along the catalyst bed. First a combination of direct methane partial oxidation coupled with methane deep oxidation is observed in the catalyst entrance section, where gas phase oxygen is present. After the oxygen is converted, product formation continues by a change in the reaction mechanism to steam reforming chemistry. Quantitative analysis reveals that the rates of oxidation and steam reforming are much lower on platinum than on rhodium coated foam catalysts. For rhodium catalysts sophisticated microkinetic models are available in literature, which can predict the reactant conversion and product formation with high accuracy [2–4]. These models allow a good understanding of the reaction mechanism and transport properties in rhodium coated foam monoliths. Within the last decade it became possible to validate the microkinetic models, due to the development of high resolution spatial profile measurement techniques, that can measure species and temperature gradients inside the catalyst foams. The pioneering work by Horn et al. [2,3,5–10], mainly focused on rhodium catalysts, is in this work extended to platinum catalysts. In a next generation reactor setup a set of reactor profiles was measured, systematically varying gas feed composition, contact time and reactor pressure. Besides foam monoliths, sphere beds and catalytic wall reactors have been tested. Microkinetic simulations applying a pseudo-2D heterogeneous reactor model that couples heat and mass transport limitations with detailed chemical kinetics of two different state-of-the-art microkinetic models taken from the literature have been used to simulate the experimentally measured reactor profiles through platinum coated foam monoliths. The reaction mechanisms predict species profiles considerably different from the measured profiles. By preand post-catalytic characterization of the catalyst by means of geometric, BET and platinum surface area, as well as metal dispersion and platinum crystallite size in combination with spatially resolved Raman spectroscopy and electron microscopy it was possible to identify significant metal redistribution and carbon formation on the catalyst surface as missing reaction pathways in the existing state-of-the-art microkinetic models. These findings are supported by in-situ Raman experiments on a polycrystalline platinum foil that follow the transition of the carbonaceous deposits with time on stream and reaction temperature [11]. The results presented in this thesis give new impulses for ongoing mechanism development.

Modifizierte Zirconiumdioxidträger für Kupferkatalysatoren in der Methanoldampfreformierung

..................................................................................................................V Danksagung...........................................................................................................VI 1 Einleitung..............................................................................................................1 1.1 Stand der Literatur..........................................................................................2 1.2 Aufgabenstellung und Gliederung der Arbeit..................................................3 2 Grundlagen...........................................................................................................5 2.1 Methanoldampfreformierung...........................................................................5 2.1.1 Katalyse............................................................................................................5 2.1.2 Methanol...........................................................................................................6 2.1.3 Wasserstoff.......................................................................................................8 2.1.4 Speicherung von Wasserstoff...........................................................................9 2.1.5 Vorgeschlagene Reaktionsmechanismen für die Methanoldampfreformierung ................................................................................................................................10 2.2 Strukturen.....................................................................................................13 2.2.1 Kupfer (Cu) und seine Oxide..........................................................................13 2.2.2 Zirconiumdioxid und Modifikationen im Anionenund Kationengitter..............16 2.2.2.1 Veränderungen im Kationenund Anionengitter.............................................18 2.2.2.1.1 CeO2 ZrO2..................................................................................................18 2.2.2.1.2 Y O 2 3 ZrO2...................................................................................................20 2.2.2.1.3 Stickstoffeinbau in das Gitter von Zirconiumdioxid.......................................20 2.3 Methoden......................................................................................................25 2.3.1 Strukturuntersuchungen.................................................................................25 2.3.1.1 Röntgenabsorptionsspektrokopie (XAS).........................................................25 2.3.1.1 Röntgenbeugung (XRD).................................................................................30 2.3.2 Transmissionselektronenmikroskopie (TEM)..................................................35 2.3.3 Gasphasenanalyse.........................................................................................37 2.3.4 Thermoanalytische Methoden.........................................................................38 2.3.5 Weitere Charakterisierungsmethoden.............................................................39 3 Experimenteller Teil...........................................................................................41 3.1 Präparation...................................................................................................41

Synthesis, Characterization and Catalysis of Nanostructured Vanadia Model Catalysts for Partial Oxidation of Propane

Nanostructured vanadia model catalysts supported by silica were synthesized in a multi-step procedure as well as through incipient wetness impregnation. Afterwards, the samples were characterized and tested in the oxidative dehydrogenation (ODH) of propane. Silica supports used were mesoporous SBA-15 and Aerosil 300. The multi-step synthesis includes a surface functionalization and an ion exchange with decavanadate ions. One aim of this study was the investigation of the influence between two support materials and between different synthesis method concerning the vanadia structure and to find a correlation to their catalytic behavior. Therefore, highly dispersed vanadia species with a similar vanadium density of 0.7 V atoms/nm2 were prepared. The samples were thoroughly characterized by nitrogen adsorption-desorption, small-angle XRD, TEM, XPS, Ramanand UV-vis spectroscopy. Furthermore, reactivity was tested with TPR besides catalytic test. It could be shown that the multi-step procedure has a stabilizing effect on the mesoporous material. After mechanical, thermal and hydrothermal treatment, the sample trated with surface functionalization shows a higher stability than blank SBA-15. The blank SBA-15 shows a significant decrease of the BET surface area already at a mechanical treatment already at 75 MPa, whereas a significant change of the surface area in the multi-step samples appears not until at 376 MPa. After pressure treatment at 752 MPa no mesoporous structure can be observed anymore for blank SBA-15, but for the multi-step sample it is in parts still observable. The impregnated samples show the same behavior as blank SBA-15. The enhanced stability has a positive influence on reaction behavior. The multi-step sample pressed at 752 MPa shows in the ODH of propane a higher selectivity towards propene than the impregnated sample treated in the same way. This can be explained by accessibility of active sites within the samples. In the impregnated sample are more active sites blocked than in the multi-step sample, due to the complete loss of mesoporous structure. When comparing both support materials and the different synthesis methods, an in-