A series of gas‐phase reactants is used to treat a Cu‐exchanged mordenite zeolite with the aim of studying the influence of the reaction environment on the formation of Cu pairs. The rearrangement of Cu ions to form multimeric sites as a function of their oxidation state was probed by X‐ray absorption spectroscopy (XAS) and also by applying advanced analysis through wavelet transform, a method able to specifically locate Cu–Cu interactions also in the presence of overlapping contributions from other scattering paths. The nature of the Cu‐oxo species formed upon oxidation was further crosschecked by DFT‐assisted fitting of the EXAFS data and by resonant Raman spectroscopy. Altogether, the Cu^I/Cu^II speciation clearly correlates with Cu proximity, with metal ion pairs quantitatively forming under an oxidative environment.

Nickel and nitrogen co-doped carbon (Ni-N-C) catalysts are attracting attention due to their exceptionally high performance in the electrocatalytic reduction of CO 2 ( CO 2 RR ) to CO. However, the direct experimental insight into the working mechanism of these catalysts is missing, hindering our fundamental understanding and their further improvement. This work sheds light on the nature of adsorbates forming under CO 2 RR at singly dispersed Ni sites. In particular, high energy resolution fluorescence detected x-ray absorption near edge structure (HERFD-XANES) at the Ni K -edge together with valence-to-core x-ray emission spectroscopy (vtc-XES) and x-ray absorption (XAS) at the Ni L 3 -edge were employed to unveil the structure and electronic properties of the reaction intermediates. These techniques, coupled with unsupervised and supervised machine learning methodologies and density functional theory, enabled a comprehensive characterization of the local atomistic and electronic structure of the working Ni-N-C catalysts. Specifically, we were able to distinguish between the structural and electronic changes of the Ni sites associated with the CO 2 RR functionality from the effect of radiation-induced damage, providing direct insight into the bond formation between the Ni centers and CO 2 RR intermediates such as CO adsorbates. Published by the American Physical Society 2024

The speciation of the Cu^II sites in Cu-CHA catalysts for NH₃-SCR is studied, to investigate the influence of the Al content on the structure and reactivity of these sites in a NO/O₂ mixture.

Five molecules at the energy-chemistry nexus, their position in the planetary boundary framework and Anthropocene-spurred interdisciplinary connections between chemistry, social sciences, and humanities to help define sustainability-driven research.

The direct activation of methane to methanol (MTM) proceeds through a chemical-looping process over Cu-oxo sites in zeolites. Herein, we extend the overall understanding of oxidation reactions over metal-oxo sites and C–H activation reactions by pinpointing the evolution of Cu species during reduction. To do so, a set of temperature-programmed reduction experiments were performed with CH₄, C₂H_6, and CO. With a temperature ramp, the Cu reduction could be accelerated to detect changes in Cu speciation that are normally not detected due to the slow CH₄ adsorption/interaction during MTM (∼200 °C). To follow the Cu-speciation with the three reductants, X-ray absorption spectroscopy (XAS), UV-vis and FT-IR spectroscopy were applied. Multivariate curve resolution alternating least-square (MCR-ALS) analysis was used to resolve the time-dependent concentration profiles of pure Cu components in the X-ray absorption near edge structure (XANES) spectra. Within the large datasets, as many as six different Cu^II and Cu^I components were found. Close correlations were found between the XANES-derived Cu^II to Cu^I reduction, CH₄ consumption, and CO₂ production. A reducibility–activity relationship was also observed for the Cu-MOR zeolites. Extended X-ray absorption fine structure (EXAFS) spectra for the pure Cu components were furthermore obtained with MCR-ALS analysis. With wavelet transform (WT) analysis of the EXAFS spectra, we were able to resolve the atomic speciation at different radial distances from Cu (up to about 4 Å). These results indicate that all the Cu^II components consist of multimeric Cu^II-oxo sites, albeit with different Cu–Cu distances.

The development of Ce-based materials is directly dependent on the catalyst surface defects, which is caused by the calcination steps required to increase structural stability. At the same time, the evaluation of cerium’s redox properties under reaction conditions is of increasing relevant importance. The synthesis of Ce-UiO-66 and CeZr-UiO-66 and their subsequent calcination are presented here as a simple and inexpensive approach for achieving homogeneous and stable CeO₂ and CeZrO_x nanocrystals. The resulting materials constitute an ideal case study to thoroughly understand cerium redox properties. The Ce³⁺/Ce⁴⁺ redox properties are investigated by H₂-TPR experiments exploited by in situ FT-IR and Ce M₅-edge AP-NEXAFS spectroscopy. In the latter case, Ce³⁺ formation is quantified using the MCR-ALS protocol. FT-IR is then presented as a high potential/easily accessible technique for extracting valuable information about the cerium oxidation state under operating conditions. The dependence of the OH stretching vibration frequency on temperature and Ce reduction is described, providing a novel tool for qualitative monitoring of surface oxygen vacancy formation. Based on the reported results, the molecular absorption coefficient of the Ce³⁺ characteristic IR transition is tentatively evaluated, thus providing a basis for future Ce³⁺ quantification through FT-IR spectroscopy. Finally, the FT-IR limitations for Ce³⁺ quantification are discussed.