Thema der Dissertation:
Electronic Structure and Chemical Composition Study of MnOx Catalysts for Water Oxidation using Synchrotron Spectroscopic Techniques
Electronic Structure and Chemical Composition Study of MnOx Catalysts for Water Oxidation using Synchrotron Spectroscopic Techniques
Abstract: Understanding the electronic structure and the chemical composition of a catalyst is crucial not only for the scientific fields but also for future technology to develop catalysts for (photo)electrochemical cells to conquer the steadily increasing problems facing the world, like global warming, energy demand, and pollution. Manganese oxide (MnOx) catalysts are promising candidates taking advantage of being abundant, non-toxic, and inexpensive for economically competitive future technology. This thesis reports two main topics of the manganese oxide system, used as electrocatalyst or as co-catalyst deposited on a semiconducting photoanode in conjunction with X-ray absorption and photoemission spectroscopy, respectively. The first topic addresses studies with soft X-ray absorption and resonant inelastic X-ray scattering measured at the manganese L2,3-edge and the oxygen K-edge to gain electronic structural insights of MnOx applied as an electrocatalyst for water oxidation. Two main questions were addressed: The first is to elucidate the origin of catalytic activity changes in MnOx films prepared under different pH conditions. The answer is found in the X-ray absorption spectra of seven samples prepared from highly acidic to (near-)neutral to highly basic electrolytes. Spectra showed a remarkable change in the electronic structures of the pH-dependent MnOx catalysts. All catalysts include mixtures of Mn2O3, Mn3O4, and birnessite in different proportions. The highest catalytic active catalyst prepared under neutral conditions is composed of birnessite and Mn2O3. In contrast, other films are predominantly composed of either one of these phases with a low content of Mn3O4. It reveals that the interplay of MnIII and MnIV species can drastically enhance the catalytic activity, but not a single contribution of any of them alone. The study also showed that creating efficient and active catalysts in desired ratios of MnOx phases is possible by controlling the preparation parameters. This led to the second significant question in the first part of the thesis: What are the active oxidation states of manganese present in the MnOx catalysts during the water oxidation reaction and whether there is a charge transfer between metal-ligand during OER? Are the phases continuously increasing with increasing oxidizing potentials? The answer has been found after conducting an in situ XAS/ RIXS spectroscopic study tracking the transformations happening in the electrodeposited MnOx catalyst under real reaction conditions (0.75-2.25 VRHE). The in situ XAS reveals a full conversion of the MnOx film into a birnessite state at ca. 1.45 VRHE just before the water oxidation. However, the in situ RIXS analysis showed continuous changes in the electronic state of MnOx up to a potential of ca. 1.75 VRHE. More precisely, by applying a more positive potential to the MnOx films, the Mn 3d - O 2p hybridization degree increases up to 1.75 VRHE. Furthermore, water oxidation catalysis by MnOx is facilitated by an O-to-Mn charge transfer, achieved at ca 1.75 V, which is believed to be of crucial importance for efficient electrocatalytic water oxidation. The second part of the thesis discusses how the MnOx can enhance the water-oxidation activity of a tantalum-oxynitride (TaON) photoanode when it is coupled as an overlaying co-catalyst to the photoanode. The chemical composition and electronic structure changes at the MnOx/TaON interface were addressed and studied with hard X-rays photoemission spectroscopy changing the thickness of the MnOx overlayer from 2, 5, 7 to finally 26 nm. By depositing MnOx on the TaON photoabsorber, the photocurrent enhances ca. 5 times, whereas the amount of Ta2O5 at the interface MnOx/TaON was reduced with increasing MnOx film thickness, which implies that the MnOx co-catalyst reduces the unfavorable Ta2O5 at the TaON surface and as a consequence facilitates hole transfer. Simultaneously, MnOx at the interface oxidizes to Mn2O3 and a birnessite phase. Our interpretation suggests that MnOx at the interface consumes oxygen from Ta2O5, leading to a reduction of this oxide and oxidation of MnII to MnIII and MnIV. SESSA simulations offered quantitative analysis, supporting the experimental results. In the present dissertation, some of the manganese oxide characteristics as (co)catalysts for oxygen evolution from water could be highlighted. These results are of interest for artificial photosynthesis devices and provide some of the missing pieces of the puzzle toward elucidating the challenging water oxidation reaction.
Zeit & Ort
03.02.2021 | 16:00