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Instruments / Technical Area	Publication Details	Published	Actions
I20-EDE-Energy Dispersive EXAFS (EDE)	Dynamics of palladium single-atoms on graphitic carbon nitride during ethylene hydrogenation Maurice Vennewald , Nina Michelle Sackers , Andree Iemhoff , Isabella Kappel , Claudia Weidenthaler , Ansgar Meise , Marc Heggen , Rafal E. Dunin-Borkowski , Luke Keenan , Regina Palkovits Journal Of Catalysis 118 DOI: 10.1016/j.jcat.2023.03.011 Diamond Proposal Number(s): [29667] Show Abstract ▼ Abstract: Single-atoms on carbon-nitrogen supports are considered catalysts for a multitude of reactions. However, doubts remain whether really these species or subnanometer clusters formed under reaction conditions are the active species. In this work, we investigate the dynamics of palladium single-atoms on graphitic carbon nitride during ethylene hydrogenation and H2-D2 exchange. By employing aberration-corrected scanning transmission electron microscopy, x-ray photoelectron spectroscopy and x-ray absorption spectroscopy, we will show that palladium, originally present as single-atoms, agglomerates to clusters at 100 °C in a gas atmosphere that contains both ethylene and hydrogen. This agglomeration goes in hand with the emergence of catalytic activity in both ethylene hydrogenation and H2-D2 exchange, suggesting that clusters, rather than single-atoms, are the active species. The results presented herein highlight the potential of analytics over the course of reaction to identify the active species and provide new insights into the influence of gas atmosphere on metal speciation.	Mar 2023
B07-C-Versatile Soft X-ray beamline: Ambient Pressure XPS and NEXAFS I20-Scanning-X-ray spectroscopy (XAS/XES)	On the stability of isolated iridium sites in N‐rich frameworks against agglomeration under reducing conditions Andree Iemhoff , Maurice Vennewald , Jens Artz , Chalachew Mebrahtu , Alexander Meledin , Thomas E. Weirich , Heinrich Hartmann , Astrid Besmehn , Matteo Aramini , Federica Venturini , Fred Mosselmans , Georg Held , Rosa Arrigo , Regina Palkovits Chemcatchem DOI: 10.1002/cctc.202200179 Diamond Proposal Number(s): [26053, 26030] Show Abstract ▼ Abstract: Stabilization of single metal atoms is a persistent challenge in heterogeneous catalysis. Especially supported late transitions metals are prone to undergo agglomeration to nanoparticles under reducing conditions. In this study, nitrogen-rich covalent triazine frameworks (CTFs) are used to immobilize iridium complexes. Upon reduction at 400°C, immobilized Ir(acac)(COD) on CTF does not form nanoparticles but transforms into a highly active Ir single atom catalyst. The resulting catalyst systems outperforms both the immobilized complex and supported nanoparticles in the dehydrogenation of formic acid as probe reaction. This superior performance could be traced back to decisive changes of the coordination geometry positively influencing activity, selectivity and stability. Spectroscopic analysis reveals an increase of electron density on the cationic iridium site by donation from the CTF macroligand after removal of the organic ligand sphere from the Ir(acac)(COD) precursor complex upon reductive treatment. This work demonstrates the ability of nitrogen moieties to stabilize molecular metal species against agglomeration and opens avenues for catalysts design using isolated sites in high-temperature applications under reducing atmosphere.	Mar 2022

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