Structural studies on iron-based exhaust gas catalysts by means of X-ray absorption spectroscopy

X-ray absorption near-edge structure (XANES) and pre-edge spectroscopy of supported iron oxide catalysts showed that the state of the catalytic sites varies between Fe2+ and Fe3+ without altering the number of nearest oxygen neighbours. The preferential location of Fe at the octahedral lattice sites of gamma-Al2O3 was verified by extended X-ray absorption fine structure (EXAFS). Valence-to-core X-ray emission spectroscopy (V2C-XES) gave new insight into intermediate species during catalytic deNOx

Phase- and Surface Composition-Dependent Electrochemical Stability of Ir-Ru Nanoparticles during Oxygen Evolution Reaction

The increasing scarcity of iridium (Ir) and its rutile-type oxide (IrO$_{2}$), the current state-of-the-art oxygen evolution reaction (OER) catalysts, is driving the transition toward the use of mixed Ir oxides with a highly active yet inexpensive metal (Ir$_{x}$M$_{1-x}$O$_{2}$). Ruthenium (Ru) has been commonly employed due to its high OER activity although its electrochemical stability in Ir-Ru mixed oxide nanoparticles (Ir$_{x}$Ru$_{1-x}$O$_{2}$ NPs), especially at high relative contents, is rarely evaluated for long-term application as water electrolyzers. In this work, we bridge the knowledge gap by performing a thorough study on the composition- and phase-dependent stability of well-defined Ir$_{x}$Ru$_{1-x}$O$_{2}$ NPs prepared by flame spray pyrolysis under dynamic operating conditions. As-prepared NPs (Ir$_{x}$Ru$_{1-x}$O$_{y}$) present an amorphous coral-like structure with a hydrous Ir-Ru oxide phase, which upon post-synthetic thermal treatment fully converts to a rutile-type structure followed by a selective Ir enrichment at the NP topmost surface. It was demonstrated that Ir incorporation into a RuO$_{2}$ matrix drastically reduced Ru dissolution by ca. 10-fold at the expense of worsening Ir inherent stability, regardless of the oxide phase present. Hydrous Ir$_{x}$Ru$_{1-x}$O$_{y}$ NPs, however, were shown to be 1000-fold less stable than rutile-type Ir$_{x}$Ru$_{1-x}$O$_{2}$, where the severe Ru leaching yielded a fast convergence toward the activity of monometallic hydrous IrO$_{y}$. For rutile-type Ir$_{x}$Ru$_{1-x}$O$_{2}$, the sequential start-up/shut-down OER protocol employed revealed a steady-state dissolution for both Ir and Ru, as well as the key role of surface Ru species in OER activity: minimal Ru surface losses (<1 at. %) yielded OER activities for tested Ir$_{0.2}$Ru$_{0.8}$O2 equivalent to those of untested Ir$_{0.8}$Ru$_{0.2}$O2. Ir enrichment at the NP topmost surface, which mitigates selective subsurface Ru dissolution, is identified as the origin of the NP stabilization. These results suggest Ru-rich Ir$_{x}$Ru$_{1-x}$O$_{2}$ NPs to be viable electrocatalysts for long-term water electrolysis, with significant repercussions in cost reduction

Microkinetic Analysis of the Oxygen Evolution Performance at Different Stages of Iridium Oxide Degradation

The microkinetics of the electrocatalytic oxygen evolution reaction substantially determines the performance in proton-exchange membrane water electrolysis. State-of-the-art nanoparticulated rutile IrO$_{2}$ electrocatalysts present an excellent trade-off between activity and stability due to the efficient formation of intermediate surface species. To reveal and analyze the interaction of individual surface processes, a detailed dynamic microkinetic model approach is established and validated using cyclic voltammetry. We show that the interaction of three different processes, which are the adsorption of water, one potential-driven deprotonation step, and the detachment of oxygen, limits the overall reaction turnover. During the reaction, the active IrO$_{2}$ surface is covered mainly by *O, *OOH, and *OO adsorbed species with a share dependent on the applied potential and of 44, 28, and 20% at an overpotential of 350 mV, respectively. In contrast to state-of-the-art calculations of ideal catalyst surfaces, this novel model-based methodology allows for experimental identification of the microkinetics as well as thermodynamic energy values of real pristine and degraded nanoparticles. We show that the loss in electrocatalytic activity during degradation is correlated to an increase in the activation energy of deprotonation processes, whereas reaction energies were marginally affected. As the effect of electrolyte-related parameters does not cause such a decrease, the model-based analysis demonstrates that material changes trigger the performance loss. These insights into the degradation of IrO$_{2}$ and its effect on the surface processes provide the basis for a deeper understanding of degrading active sites for the optimization of the oxygen evolution performance

Increased Ir–Ir Interaction in Iridium Oxide during the Oxygen Evolution Reaction at High Potentials Probed by Operando Spectroscopy

The structure of IrO$_{2}$ during the oxygen evolution reaction (OER) was studied by operando X-ray absorption spectroscopy (XAS) at the Ir L$_{3}$-edge to gain insight into the processes that occur during the electrocatalytic reaction at the anode during water electrolysis. For this purpose, calcined and uncalcined IrO$_{2}$ nanoparticles were tested in an operando spectroelectrochemical cell. In situ XAS under different applied potentials uncovered strong structural changes when changing the potential. Modulation excitation spectroscopy combined with XAS enhanced the information on the dynamic changes significantly. Principal component analysis (PCA) of the resulting spectra as well as FEFF9 calculations uncovered that both the Ir L$_{3}$-edge energy and the white line intensity changed due to the formation of oxygen vacancies and lower oxidation state of iridium at higher potentials, respectively. The deconvoluted spectra and their components lead to two different OER modes. It was observed that at higher OER potentials, the well-known OER mechanisms need to be modified, which is also associated with the stabilization of the catalyst, as confirmed by in situ inductively coupled plasma mass spectrometry (ICP-MS). At these elevated OER potentials above 1.5 V, stronger Ir–Ir interactions were observed. They were more dominant in the calcined IrO$_{2}$ samples than in the uncalcined ones. The stronger Ir–Ir interaction upon vacancy formation is also supported by theoretical studies. We propose that this may be a crucial factor in the increased dissolution stability of the IrO$_{2}$ catalyst after calcination. The results presented here provide additional insights into the OER in acid media and demonstrate a powerful technique for quantifying the differences in mechanisms on different OER electrocatalysts. Furthermore, insights into the OER at a fundamental level are provided, which will contribute to further understanding of the reaction mechanisms in water electrolysis

Unravelling the Zn‐Cu Interaction during Activation of a Zn‐promoted Cu/MgO Model Methanol Catalyst

We report on an inverse model Cu/MgO methanol catalyst modified with 5 % zinc oxide at the Cu surface to element-specifically probe the interplay of metallic copper and zinc oxide during reductive activation. The structure of copper and zinc was unraveled by in situ X-ray diffraction (XRD) and in situ X-ray absorption spectroscopy (XAS) supported by theoretical modelling of the extended X-ray absorption fine structure and X-ray absorption near-edge structure spectra. Temperature-programmed reduction in H2 during in situ XAS showed that copper was reduced starting at 145 °C. With increasing reduction temperature, zinc underwent first a geometrical change in its structure, followed by reduction. The reduced zinc species were identified as surface alloy sites, which coexisted from 200 °C to 340 °C with ZnO species at the copper surface. At 400 °C Zn−Cu bulk-alloyed particles were formed. According to in situ XRD and in situ XAS, about half of the ZnO was not fully reduced, which can be explained by a lack of contact with copper. Our experimental results were further substantiated by density functional theory calculations, which verified that ZnO with neighboring Cu atoms reduced more easily. By combining these results, the distribution, phase and oxidation state of Zn species on Cu were estimated for the activated state of this model catalyst. This insight into the interplay of Cu and Zn forms the basis for deeper understanding the active sites during methanol synthesis

Phase- and Surface Composition-Dependent Electrochemical Stability of Ir-Ru Nanoparticles during Oxygen Evolution Reaction

The increasing scarcity of iridium (Ir) and its rutile-type oxide (IrO2), the current state-of-the-art oxygen evolution reaction (OER) catalysts, is driving the transition toward the use of mixed Ir oxides with a highly active yet inexpensive metal (IrxM1–xO2). Ruthenium (Ru) has been commonly employed due to its high OER activity although its electrochemical stability in Ir-Ru mixed oxide nanoparticles (IrxRu1–xO2 NPs), especially at high relative contents, is rarely evaluated for long-term application as water electrolyzers. In this work, we bridge the knowledge gap by performing a thorough study on the composition- and phase-dependent stability of well-defined IrxRu1–xO2 NPs prepared by flame spray pyrolysis under dynamic operating conditions. As-prepared NPs (IrxRu1–xOy) present an amorphous coral-like structure with a hydrous Ir-Ru oxide phase, which upon post-synthetic thermal treatment fully converts to a rutile-type structure followed by a selective Ir enrichment at the NP topmost surface. It was demonstrated that Ir incorporation into a RuO2 matrix drastically reduced Ru dissolution by ca. 10-fold at the expense of worsening Ir inherent stability, regardless of the oxide phase present. Hydrous IrxRu1–xOy NPs, however, were shown to be 1000-fold less stable than rutile-type IrxRu1–xO2, where the severe Ru leaching yielded a fast convergence toward the activity of monometallic hydrous IrOy. For rutile-type IrxRu1–xO2, the sequential start-up/shut-down OER protocol employed revealed a steady-state dissolution for both Ir and Ru, as well as the key role of surface Ru species in OER activity: minimal Ru surface losses (<1 at. %) yielded OER activities for tested Ir0.2Ru0.8O2 equivalent to those of untested Ir0.8Ru0.2O2. Ir enrichment at the NP topmost surface, which mitigates selective subsurface Ru dissolution, is identified as the origin of the NP stabilization. These results suggest Ru-rich IrxRu1–xO2 NPs to be viable electrocatalysts for long-term water electrolysis, with significant repercussions in cost reduction