In this work, we apply for the first time ambient pressure operando soft X-ray absorption spectroscopy (XAS) to investigate the location, structural properties, and reactivity of the defective sites present in the prototypical metal–organic framework HKUST-1. We obtained direct evidence that Cu⁺ defective sites form upon temperature treatment of the powdered form of HKUST-1 at 160 °C and that they are largely distributed on the material surface. Further, a thorough structural characterization of the Cu⁺/Cu²⁺ dimeric complexes arising from the temperature-induced dehydration/decarboxylation of the pristine Cu²⁺/Cu²⁺ paddlewheel units is reported. In addition to characterizing the surface defects, we demonstrate that CO₂ may be reversibly adsorbed and desorbed from the surface defective Cu⁺/Cu²⁺ sites. These findings show that ambient pressure soft-XAS, combined with state-of-the-art theoretical calculations, allowed us to shed light on the mechanism involving the decarboxylation of the paddlewheel units on the surface to yield Cu⁺/Cu²⁺ complexes and their reversible restoration upon exposure to gaseous CO_2.

Recently, several researchers have claimed the existence of superior catalytic activity associated with topological materials belonging to the class of Dirac/Weyl semimetals, owing to the high electron conductivity and charge carrier mobility in these topological materials. By means of X-ray photoelectron spectroscopy, electrocatalytic tests, and density functional theory, we have investigated the chemical reactivity (chemisorption of ambient gases), ambient stability, and catalytic properties of PdSn₄, a topological semimetal showing Dirac node arcs. We find a Tafel slope of 83 mV in the hydrogen evolution reaction (HER) dec^–1 with an overpotential of 50 mV, with performances resembling those of pure Pd, regardless of its limited amount in the alloy, with a subsequent reduction in the cost of raw materials by ∼80%. Remarkably, the PdSn₄-based electrode shows superior robustness to CO compared to pure Pd and Pt and high stability in water media, although the PdSn₄ surface is prone to oxidation with the formation of a sub-nanometric SnO₂ skin. Moreover, we also assessed the significance of the role of topological electronic states in the observed catalytic properties. Actually, the peculiar atomic structure of oxidized PdSn₄ enables the migration of hydrogen atoms through the Sn–O layer with a barrier comparable with the energy cost of the Heyrovsky step of HER over Pt(111) in acidic media (0.1 eV). On the other hand, the topological properties play a minor role, if existing, contrarily to the recent reports overestimating their contribution in catalytic properties.

We employed operando soft X-ray absorption spectroscopy (XAS) to monitor the changes in the valence states and spin properties of LaMn_1-x Co _x O₃ catalysts subjected to a mixture of CO and O₂ at ambient pressure. Guided by simulations based on charge transfer multiplet theory, we quantitatively analyze the Mn and Co 2p XAS as well as the oxygen K-edge XAS spectra during the reaction process. The Mn sites are particularly sensitive to the catalytic reaction, displaying dynamics in their oxidation state. When Co doping is introduced (x ≤ 0.5), Mn oxidizes from Mn²⁺ to Mn³⁺ and Mn⁴⁺, while Co largely maintains a valence state of Co²⁺. In the case of LaCoO₃, we identify high-spin and low-spin Co³⁺ species combined with Co²⁺. Our investigation underscores the importance to consider the spin and valence states of catalyst materials under operando conditions.

Interfaces between water and materials are ubiquitous and are crucial in materials sciences and in biology, where investigating the interaction of water with the surface under ambient conditions is key to shedding light on the main processes occurring at the interface. Magnesium oxide is a popular model system to study the metal oxide-water interface, where, for sufficient water loadings, theoretical models have suggested that reconstructed surfaces involving hydrated Mg²⁺ metal ions may be energetically favored. In this work, by combining experimental and theoretical surface-selective ambient pressure X-ray absorption spectroscopy with multivariate curve resolution and molecular dynamics, we evidence in real time the occurrence of Mg²⁺ solvation at the interphase between MgO and solvating media such as water and methanol (MeOH). Further, we show that the Mg²⁺ surface ions undergo a reversible solvation process, we prove the dissolution/redeposition of the Mg²⁺ ions belonging to the MgO surface, and we demonstrate the formation of octahedral [Mg(H₂O)₆]²⁺ and [Mg(MeOH)₆]²⁺ intermediate solvated species. The unique surface, electronic, and structural sensitivity of the developed technique may be beneficial to access often elusive properties of low-Z metal ion intermediates involved in interfacial processes of chemical and biological interest.