A comprehensive microscopic description of thermally induced distortions in lead halide perovskites is crucial for their realistic applications, yet still unclear. Here, we quantify the effects of thermal activation in CsPbBr 3 nanocrystals across length scales with atomic-level precision, and we provide a framework for the description of phase transitions therein, beyond the simplistic picture of unit-cell symmetry increase upon heating. The temperature increase significantly enhances the short-range structural distortions of the lead halide framework as a consequence of the phonon anharmonicity, which causes the excess free energy surface to change as a function of temperature. As a result, phase transitions can be rationalized via the soft-mode model, which also describes displacive thermal phase transitions in oxide perovskites. Our findings allow to reconcile temperature-dependent modifications of physical properties, such as changes in the optical band gap, that are incompatible with the perovskite time- and space-average structures.

Reducible oxides are effective aerobic oxidation catalysts being able to activate molecular oxygen. This ability is generally attributed to the high concentration of oxygen vacancies serving as oxygen activation sites. At the same time, the mechanism of oxygen activation remains unclear since surface oxygen activation sites cannot be easily detected using conventional methods. In this work, we unraveled the mechanism of oxygen activation over iron sites of Pt-FeO x /Al 2 O 3 during carbon monoxide oxidation using a combination of in situ and operando methods. In situ/operando XAS at the Pt L 3 and Fe K-edges, in situ Fourier transform infrared (FTIR) spectroscopy, and carbon monoxide chemisorption showed that carbon monoxide activation takes place at metallic platinum sites and is not affected by the presence of cationic iron species. Operando time-resolved Fe K-edge X-ray absorption spectroscopy (XAS) demonstrated that the Fe 2+ /Fe 3+ redox pair is directly involved in the mechanism of oxygen activation of Pt-FeO x /Al 2 O 3 . The detailed analysis of oxygen cutoff experiments demonstrated that after switching off oxygen, approximately one carbon dioxide molecule was formed for each Fe 3+ ion reduced to produce Fe 2+ . At the same time, the steady-state carbon dioxide formation rate was about twice higher than the initial rate of Fe 2+ formation after cutoff of oxygen from the catalytic feed. These experiments allude to a catalytic cycle involving electrophilic oxygen species adsorbed on iron centers as reaction intermediates. Similar mechanisms might be expected for other catalytic oxidation reactions over cationic iron of both chemical and biological importance.

The effective preparation method of epitaxial VO2 films on the r-Al2O3 substrates based on the MOCVD technique and postdeposition annealing is described. The composition, orientation and morphology of the films obtained were investigated by Raman spectroscopy, XRD, EBSD, XPS, SEM and AFM methods. The samples obtained demonstrate high crystal quality and excellent physical properties: sharp metal-insulator (>10 4 resistance change) and intensive optical reflectivity (IR and THz regions) transitions. The model of VO2 films recrystallization based on the peritectic decomposition of intergrain vanadium oxide phases is proposed.The new effective chemical synthesis of the epitaxial VO 2 films with record electrical and optical switch properties is presented.

Catalytic systems based on supported noble metals are extensively studied because of their widespread application. Discussions remain about the nature of the active species, whether they are atomically dispersed or nanoparticles, and their reactivity. In this work, combining in situ/operando spectroscopy with theoretical modeling, we propose a phase diagram of atomically dispersed platinum on ceria, demonstrating that it reversibly changes from PtIVO2 to PtIIO as a function of temperature and oxygen partial pressure. The phase diagram helps identify the stability domain of each species, while spectroscopies provide a quantitative evaluation depending on the reaction conditions. Finally, our results show that high-temperature activation in the presence of steam of supported atomically dispersed platinum enhances the activity toward low-temperature carbon monoxide oxidation because it promotes aggregation into nanoparticles. This work highlights the structure–activity relationship in supported metal catalysts and proposes a suitable approach to determine the amount of each species before the investigation of the reaction mechanism.

The dissociation of H 2 is an essential elementary step in many industrial chemical transformations, typically requiring precious metals. Here, we report a hierarchical nanoporous Cu catalyst doped with small amounts of Ti (npTiCu) that increases the rate of H 2 −D 2 exchange by approximately one order of magnitude compared to the undoped nanoporous Cu (npCu) catalyst. The promotional effect of Ti was measured via steady-state H 2 −D 2 exchange reaction experiments under atmospheric pressure flow conditions in the temperature range of 300−573 K. Pretreatment with flowing H 2 is required for stable catalytic performance, and two temperatures, 523 and 673 K, were investigated. The experimentally determined H 2 −D 2 exchange rate is 5−7 times greater for npTiCu vs the undoped Cu material under optimized pretreatment and reaction temperatures. The H 2 pretreatment leads to full reduction of Cu oxide and partial reduction of surface Ti oxide species present in the as-prepared catalyst as demonstrated using in situ ambient pressure X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. The apparent activation energies and pre-exponential factors measured for H 2 −D 2 exchange are substantially different for Ti-doped vs undoped npCu catalysts. Density functional theory calculations suggest that isolated, metallic Ti atoms on the surface of the Cu host can act as the active surface sites for hydrogen recombination. The increase in the rate of exchange above that of pure Cu is caused primarily by a shift in the rate-determining step from dissociative adsorption on Cu to H/D atom recombination on Ti-doped Cu, with the corresponding decrease in activation entropy that it produces.

Many important catalytic reactions take place at metal−oxide interfaces. However, their mechanisms are typically difficult to probe due to the low concentration of active sites and the lack of highly sensitive spectroscopic methods. In this work, we analyze the impact of oxide reducibility on the mechanism of low-temperature CO oxidation over platinum nanoparticles supported on ceriabased solid solutions. We demonstrate that the easier reducibility of Ce 4+ at the Pt/ Ce 0.5 Sn 0.5 O 2 interface (in comparison to that for Pt/CeO 2 ) increases the catalytic CO oxidation rate, lowers the apparent activation energy, and increases the reaction order in oxygen. Operando time-resolved X-ray absorption spectroscopy suggests that the Ce 4+ reduction rate at the Pt/Ce 0.5 Sn 0.5 O 2 interface is accelerated, while the Ce 3+ oxidation rate becomes rate-limiting. Importantly, no reduction of Sn 4+ in the ceria−tin solid solution and no formation of Pt/Sn alloys were detected under relevant reaction conditions using in situ X-ray absorption spectroscopy, ambient-pressure X-ray photoelectron spectroscopy, and infrared spectroscopy. This work provides a better understanding on the reactivity of interfaces. It demonstrates that the reducibility of the oxide close to a metal strongly influences catalytic rates, which provides ideas for the design of better catalysts.

We report a synthesis method for highly monodisperse Cu−Pt alloy nanoparticles. Small and large Cu−Pt particles with a Cu/Pt ratio of 1:1 can be obtained through colloidal synthesis at 300 °C. The fresh particles have a Pt-rich surface and a Cu-rich core and can be converted into an intermetallic phase after annealing at 800 °C under H 2 . First, we demonstrated the stability of fresh particles under redox conditions at 400 °C, as the Pt-rich surface prevents substantial oxidation of Cu. Then, a combination of in situ scanning transmission electron microscopy, in situ X-ray absorption spectroscopy, and CO oxidation measurements of the intermetallic CuPt phase before and after redox treatments at 800 °C showed promising activity and stability for CO oxidation. Full oxidation of Cu was prevented after exposure to O 2 at 800 °C. The activity and structure of the particles were only slightly changed after exposure to O 2 at 800 °C and were recovered after re-reduction at 800 °C. Additionally, the intermetallic CuPt phase showed enhanced catalytic properties compared to the fresh particles with a Pt-rich surface or pure Pt particles of the same size. Thus, the incorporation of Pt with Cu does not lead to a rapid deactivation and degradation of the material, as seen with other bimetallic systems. This work provides a synthesis route to control the design of Cu−Pt nanostructures and underlines the promising properties of these alloys (intermetallic and non-intermetallic) for heterogeneous catalysis.

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