In Situ Preparation of Pr1-xCaxMnO3 and La1-xSrxMnO3 Catalysts Surface for High-Resolution Environmental Transmission Electron Microscopy

Operando Electron Microscopy of Catalysts: The Missing Cornerstone in Heterogeneous Catalysis Research?

Heterogeneous catalysis in thermal gas-phase and electrochemical liquid-phase chemical conversion plays an important role in our modern energy landscape. However, many of the structural features that drive efficient chemical energy conversion are still unknown. These features are, in general, highly distinct on the local scale and lack translational symmetry, and thus, they are difficult to capture without the required spatial and temporal resolution. Correlating these structures to their function will, conversely, allow us to disentangle irrelevant and relevant features, explore the entanglement of different local structures, and provide us with the necessary understanding to tailor novel catalyst systems with improved productivity. This critical review provides a summary of the still immature field of operando electron microscopy for thermal gas-phase and electrochemical liquid-phase reactions. It focuses on the complexity of investigating catalytic reactions and catalysts, progress in the field, and analysis. The forthcoming advances are discussed in view of correlative techniques, artificial intelligence in analysis, and novel reactor designs.

Advancements in Liquid Jet Technology and X-ray Spectroscopy for Understanding Energy Conversion Materials during Operation

ConspectusWater splitting is intensively studied for sustainable and effective energy storage in green/alternative energy harvesting-storage-release cycles. In this work, we present our recent developments for combining liquid jet microtechnology with different types of soft X-ray spectroscopy at high-flux X-ray sources, in particular developed for studying the oxygen evolution reaction (OER). We are particularly interested in the development of in situ photon-in/photon-out techniques, such as in situ resonant inelastic X-ray scattering (RIXS) techniques at high-repetition-frequency X-ray sources, pointing toward operando capabilities. The pilot catalytic systems we use are perovskites having the general structure ABO₃ with lanthanides or group II elements at the A sites and transition metals at the B sites. Depending on the chemical substitutions of ABO₃, their catalytic activity for OER can be tuned by varying the composition.In this work, we present our in situ RIXS studies of the manganese L-edge of perovskites during OER. We have developed various X-ray spectroscopy approaches like transmission zone plate-, reflection zone plate-, and grating-based emission spectroscopy techniques. Combined with tunable incident X-ray energies, we yield complementary information about changing (inverse) X-ray absorption features of the perovskites, allowing us to deduce element- and oxidation-state-specific chemical monitoring of the catalyst. Adding liquid jet technology, we monitor element- and oxidation-state-specific interactions of the catalyst with water adsorbate during OER. By comparing the different technical spectroscopy approaches combined with high-repetition-frequency experiments at synchrotrons and free-electron lasers, we conclude that the combination of liquid jet with low-resolution zone-plate-based X-ray spectroscopy is sufficient for element- and oxidation-state-specific chemical monitoring during OER and easy to handle.For an in-depth study of OER mechanisms, however, including the characterization of catalyst-water adsorbate in terms of their charge transfer properties and especially valence intermediates formed during OER, high-resolution spectroscopy tools based on a combination of liquid jets with gratings bear bigger potential since they allow resolution of otherwise-overlapping X-ray spectroscopy transitions. Common for all of these experimental approaches is the conclusion that without the versatile developments of liquid jets and liquid beam technologies, elaborate experiments such as high-repetition experiments at high-flux X-ray sources (like synchrotrons or free-electron lasers) would hardly be possible. Such experiments allow sample refreshment for every single X-ray shot for repetition frequencies of up to 5 MHz, so that it is possible (a) to study X-ray-radiation-sensitive samples and also (b) to utilize novel types of flux-hungry X-ray spectroscopy tools like photon-in/photon-out X-ray spectroscopy to study the OER.

Atomistic Insights into Activation and Degradation of La0.6Sr0.4CoO3-δ Electrocatalysts under Oxygen Evolution Conditions

The stability of perovskite oxide catalysts for the oxygen evolution reaction (OER) plays a critical role in their applicability in water splitting concepts. Decomposition of perovskite oxides under applied potential is typically linked to cation leaching and amorphization of the material. However, structural changes and phase transformations at the catalyst surface were also shown to govern the activity of several perovskite electrocatalysts under applied potential. Hence, it is crucial for the rational design of durable perovskite catalysts to understand the interplay between the formation of active surface phases and stability limitations under OER conditions. In the present study, we reveal a surface-dominated activation and deactivation mechanism of the prominent electrocatalyst La_0.6Sr_0.4CoO_3−δ under steady-state OER conditions. Using a multiscale microscopy and spectroscopy approach, we identify the evolving Co-oxyhydroxide as catalytically active surface species and La-hydroxide as inactive species involved in the transient degradation behavior of the catalyst. While the leaching of Sr results in the formation of mixed surface phases, which can be considered as a part of the active surface, the gradual depletion of Co from a self-assembled active CoO(OH) phase and the relative enrichment of passivating La(OH)₃ at the electrode surface result in the failure of the perovskite catalyst under applied potential.

Site-specific plan-view TEM lamella preparation of pristine surfaces with a large field of view

Transmission electron microscopy has become a major characterization tool with an ever increasing variety of methods being applied in a wide range of scientific fields. However, the probably most famous pitfall in related workflows is the preparation of high-quality electron-transparent lamellae enabling for extraction of valuable information. Particularly in the field of solid state physics and materials science, it often required to study the surface of a macroscopic specimen with plan-view orientation. Nevertheless, despite tremendous advances in instrumentation, i.e. focused ion beam, the yield of existing plan-view lamellae preparation techniques is relatively low compared to cross-sectional extraction methods. Furthermore, techniques relying on mechanical treatments, i.e. conventional preparation, compromise site-specifity. In this paper, we demonstrate that by combining a mechanical grinding step prior to backside lift-out in the focused ion beam plan-view lamellae preparation becomes increasingly easy. The suggested strategy combines site-specifity with micrometer precision as well as possible investigation of pristine surfaces with a field of view of several hundred square micrometers.