Tuning Strong Metal–Support Interaction Kinetics on Pt-Loaded TiO₂(110) by Choosing the Pressure: A Combined Ultrahigh Vacuum/Near-Ambient Pressure XPS Study

Pt catalyst particles on reducible oxide supports often change their activity significantly at elevated temperatures due to the strong metal–support interaction (SMSI), which induces the formation of an encapsulation layer around the noble metal particles. However, the impact of oxidizing and reducing treatments at elevated pressures on this encapsulation layer remains controversial, partly due to the “pressure gap” between surface science studies and applied catalysis. In the present work, we employ synchrotron-based near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to study the effect of O2 and H2 on the SMSI-state of well-defined Pt/TiO2(110) catalysts at pressures of up to 0.1 Torr. By tuning the O2 pressure, we can either selectively oxidize the TiO2 support or both the support and the Pt particles. Catalyzed by metallic Pt, the encapsulating oxide overlayer grows rapidly in 1 × 10–5 Torr O2, but orders of magnitude less effectively at higher O2 pressures, where Pt is in an oxidic state. While the oxidation/reduction of Pt particles is reversible, they remain embedded in the support once encapsulation has occurred

Ion-Solvation-Induced Molecular Reorganization in Liquid Water Probed by Resonant Inelastic Soft X‑ray Scattering

The molecular structure of liquid water is susceptible to changes upon admixture of salts due to ionic solvation, which provides the basis of many chemical and biochemical processes. Here we demonstrate how the local electronic structure of aqueous potassium chloride (KCl) solutions can be studied by resonant inelastic soft X-ray scattering (RIXS) to monitor the effects of the ion solvation on the hydrogen-bond (HB) network of liquid water. Significant changes in the oxygen <i>K-edge</i> emission spectra are observed with increasing KCl concentration. These changes can be attributed to modifications in the proton dynamics, caused by a specific coordination structure around the salt ions. Analysis of the spectator decay spectra reveals a spectral signature that could be characteristic of this structure

Direct Observation of Reactant, Intermediate, and Product Species for Nitrogen Oxide-Selective Catalytic Reduction on Cu-SSZ-13 Using <i>In Situ</i> Soft X‑ray Spectroscopy

Catalytic processes have supported the development of myriad beneficial technologies, yet our fundamental understanding of the complex interactions between reaction intermediates and catalyst surfaces is still largely undefined for many reactions. Experimental analyses have generally been limited to investigation of catalyst materials or a subset of functional groups as indirect probes of the critical surface-bound intermediate species and reaction mechanisms. A more direct approach is to probe the intermediate species themselves, but this requires direct study of the local chemical environment of light elements. In this work, we use soft X-ray emission spectroscopy (XES) and a custom-designed in situ reactor cell to directly observe and characterize the electronic structure of reactant, intermediate, and product species under reaction conditions. Specifically, we employ N K XES to probe the interaction of various nitrogen species with a Cu-SSZ-13 catalyst during selective catalytic reduction of nitrogen oxides (NO and NO2) by ammonia (NH3-SCR), a reaction that is critical for the removal of NOx pollutants formed in combustion reactions. This work reveals a novel spectral feature for all spectra measured with flowing NO gas present, which we attribute to the interaction of NO with the catalyst. We find that introducing both NO and O2 gases (compared to only NO) increases the interaction of NO with Cu-SSZ-13. Adsorption of NH3 leads to a more pronounced spectral signal compared to NO adsorption. For the standard NH3-SCR reaction, we observe a strong N2 signal, comprising 30% of the total spectral intensity. These results demonstrate the vast potential of this technique to provide direct, novel insights into the complex interactions between reaction intermediates and the active sites of catalysts, which may guide advanced knowledge-based optimization of these processes

Formation of a KInSe Surface Species by NaF/KF Postdeposition Treatment of Cu(In,Ga)Se₂ Thin-Film Solar Cell Absorbers

A NaF/KF postdeposition treatment (PDT) has recently been employed to achieve new record efficiencies of Cu­(In,Ga)­Se₂ (CIGSe) thin film solar cells. We have used a combination of depth-dependent soft and hard X-ray photoelectron spectroscopy as well as soft X-ray absorption and emission spectroscopy to gain detailed insight into the chemical structure of the CIGSe surface and how it is changed by different PDTs. Alkali-free CIGSe, NaF-PDT CIGSe, and NaF/KF-PDT CIGSe absorbers grown by low-temperature coevaporation have been interrogated. We find that the alkali-free and NaF-PDT CIGSe surfaces both display the well-known Cu-poor CIGSe chemical surface structure. The NaF/KF-PDT, however, leads to the formation of bilayer structure in which a KInSe species covers the CIGSe compound that in composition is identical to the chalcopyrite structure of the alkali-free and NaF-PDT absorber