Einfluss des Stofftransports auf die Produktverteilung bei der elektrochemischen Reduktion von CO und CO2

Zusammenfassung in deutscher SpracheAbweichender Titel nach Übersetzung der Verfasserin/des Verfassers5

Deciphering the Structural and Chemical Transformations of Oxide Catalysts during Oxygen Evolution Reaction Using Quick X-ray Absorption Spectroscopy and Machine Learning

Bimetallic transition-metal oxides, such as spinel-like Co$_x$Fe$_{3–x}$O$_4$ materials, are known as attractive catalysts for the oxygen evolution reaction (OER) in alkaline electrolytes. Nonetheless, unveiling the real active species and active states in these catalysts remains a challenge. The coexistence of metal ions in different chemical states and in different chemical environments, including disordered X-ray amorphous phases that all evolve under reaction conditions, hinders the application of common operando techniques. Here, we address this issue by relying on operando quick X-ray absorption fine structure spectroscopy, coupled with unsupervised and supervised machine learning methods. We use principal component analysis to understand the subtle changes in the X-ray absorption near-edge structure spectra and develop an artificial neural network to decipher the extended X-ray absorption fine structure spectra. This allows us to separately track the evolution of tetrahedrally and octahedrally coordinated species and to disentangle the chemical changes and several phase transitions taking place in Co$_x$Fe$_{3–x}$O$_4$ catalysts and on their active surface, related to the conversion of disordered oxides into spinel-like structures, transformation of spinels into active oxyhydroxides, and changes in the degree of spinel inversion in the course of the activation treatment and under OER conditions. By correlating the revealed structural changes with the distinct catalytic activity for a series of Co$_x$Fe$_{3–x}$O$_4$ samples, we elucidate the active species and OER mechanism

Creation of exclusive artificial cluster defects by selective metal removal in the (Zn, Zr) mixed-metal UiO-66

The differentiation between missing linker defects and missing cluster defects in MOFs is difficult, thereby limiting the ability to correlate materials properties to a specific type of defects. Herein, we present a novel and easy synthesis strategy for the creation of solely "missing cluster defects" by preparing mixed-metal (Zn, Zr)-UiO-66 followed by a gentle acid wash to remove the Zn nodes. The resulting material has the reo UiO-66 structure, typical for well-defined missing cluster defects. The missing clusters are thoroughly characterized, including low-pressure Ar-sorption, iDPC-STEM at a low dose (1.5 pA), and XANES/EXAFS analysis. We show that the missing cluster UiO-66 has a negligible number of missing linkers. We show the performance of the missing cluster UiO-66 in CO2 sorption and heterogeneous catalysis

Efficient Electrochemical Nitrate Reduction to Ammonia with Copper Supported Rhodium Cluster and Single-Atom Catalysts

The electrochemical nitrate reduction reaction (NITRR) provides a promising solution for restoring the imbalance in the global nitrogen cycle while enabling a sustainable and decentralized route to source ammonia. Here, we demonstrate a novel electrocatalyst for NITRR consisting of Rh clusters and single-atoms dispersed onto Cu nanowires (NWs), which delivers a partial current density of 162 mA cm−2 for NH3 production and a Faradaic efficiency (FE) of 93% at −0.2 V vs. RHE. The highest ammonia yield rate reached a record value of 1.27 mmol h−1 cm−2. Detailed investigations by electron spin resonance, in-situ infrared spectroscopy, differential electrochemical mass spectrometry and density functional theory modeling suggest that the high activity originates from the synergistic catalytic cooperation between Rh and Cu sites, whereby adsorbed hydrogen on Rh sites transfer to vicinal *NO intermediate species adsorbed on Cu promoting the hydrogenation and ammonia formation.This work is supported by the Chinese Thousand Talents Program for Young Professionals, the startup funding from Nankai University, the “111” project (Grant No. B16027), the Spanish Ministry of Science & Innovation for the “Ramon y Cajal” Program (RYC2018-023888-I), and the Singapore Ministry of Education AcRF Tier 2 (2016-T2-2-153, 2016-T2-1-131), AcRF Tier 1 (RG7/18 and RG161/19). L. Bai acknowledges the Early Postdoc Research Fellowship (P2ELP2_199800) from the Swiss National Science Foundation. XAS experiments were performed at CLAESS beamline at ALBA Synchrotron with the collaboration of ALBA staff

Elucidating electrochemical nitrate and nitrite reduction over atomically-dispersed transition metal sites.

Electrocatalytic reduction of waste nitrates (NO3-) enables the synthesis of ammonia (NH3) in a carbon neutral and decentralized manner. Atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts demonstrate a high catalytic activity and uniquely favor mono-nitrogen products. However, the reaction fundamentals remain largely underexplored. Herein, we report a set of 14; 3d-, 4d-, 5d- and f-block M-N-C catalysts. The selectivity and activity of NO3- reduction to NH3 in neutral media, with a specific focus on deciphering the role of the NO2- intermediate in the reaction cascade, reveals strong correlations (R=0.9) between the NO2- reduction activity and NO3- reduction selectivity for NH3. Moreover, theoretical computations reveal the associative/dissociative adsorption pathways for NO2- evolution, over the normal M-N4 sites and their oxo-form (O-M-N4) for oxyphilic metals. This work provides a platform for designing multi-element NO3RR cascades with single-atom sites or their hybridization with extended catalytic surfaces

Elucidating electrochemical nitrate and nitrite reduction over atomically-dispersed transition metal sites

Abstract Electrocatalytic reduction of waste nitrates (NO3 −) enables the synthesis of ammonia (NH3) in a carbon neutral and decentralized manner. Atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts demonstrate a high catalytic activity and uniquely favor mono-nitrogen products. However, the reaction fundamentals remain largely underexplored. Herein, we report a set of 14; 3d-, 4d-, 5d- and f-block M-N-C catalysts. The selectivity and activity of NO3 − reduction to NH3 in neutral media, with a specific focus on deciphering the role of the NO2 − intermediate in the reaction cascade, reveals strong correlations (R=0.9) between the NO2 − reduction activity and NO3 − reduction selectivity for NH3. Moreover, theoretical computations reveal the associative/dissociative adsorption pathways for NO2 − evolution, over the normal M-N4 sites and their oxo-form (O-M-N4) for oxyphilic metals. This work provides a platform for designing multi-element NO3RR cascades with single-atom sites or their hybridization with extended catalytic surfaces