Carbon Encapsulation of Supported Metallic Iridium Nanoparticles: An in Situ Transmission Electron Microscopy Study and Implications for Hydrogen Evolution Reaction.

Carbon-supported metal nanoparticles (NPs) comprise an important class of heterogeneous catalysts. The interaction between the metal and carbon support influences the overall material properties, viz., the catalytic performance. Herein we use in situ and ex situ transmission electron microscopy (TEM) in combination with in situ X-ray spectroscopy (XPS) to investigate the encapsulation of metallic iridium NPs by carbon in an Ir/C catalyst. Real-time atomic-scale imaging visualizes particle reshaping and increased graphitization of the carbon support upon heating of Ir/C in vacuum. According to in situ TEM results, carbon overcoating grows over Ir NPs during the heating process, starting from ca. 550 °C. With the carbon overlayers formed, no sintering and migration of Ir NPs is observed at 800 °C, yet the initial Ir NPs sinter at or below 550 °C, i.e., at a temperature associated with an incomplete particle encapsulation. The carbon overlayer corrugates when the temperature is decreased from 800 to 200 °C and this process is associated with the particle surface reconstruction and is reversible, such that the corrugated carbon overlayer can be smoothed out by increasing the temperature back to 800 °C. The catalytic performance (activity and stability) of the encapsulated Ir NPs in the hydrogen evolution reaction (HER) is higher than that of the initial (nonencapsulated) state of Ir/C. Overall, this work highlights microscopic details of the currently understudied phenomenon of the carbon encapsulation of supported noble metal NPs and demonstrates additionally that the encapsulation by carbon is an effective measure for tuning the catalytic performance.

Modulating the strong metal-support interaction of single-atom catalysts via vicinal structure decoration.

Metal-support interaction predominately determines the electronic structure of metal atoms in single-atom catalysts (SACs), largely affecting their catalytic performance. However, directly tuning the metal-support interaction in oxide supported SACs remains challenging. Here, we report a new strategy to subtly regulate the strong covalent metal-support interaction (CMSI) of Pt/CoFe2O4 SACs by a simple water soaking treatment. Detailed studies reveal that the CMSI is weakened by the bonding of H+, generated from water dissociation, onto the interface of Pt-O-Fe, resulting in reduced charge transfer from metal to support and leading to an increase of C-H bond activation in CH4 combustion by more than 50 folds. This strategy is general and can be extended to other CMSI-existed metal-supported catalysts, providing a powerful tool to modulating the catalytic performance of SACs.

Atomic reconfiguration among tri-state transition at ferroelectric/antiferroelectric phase boundaries in Pb(Zr,Ti)O3.

Phase boundary provides a fertile ground for exploring emergent phenomena and understanding order parameters couplings in condensed-matter physics. In Pb(Zr1-xTix)O3, there are two types of composition-dependent phase boundary with both technological and scientific importance, i.e. morphotropic phase boundary (MPB) separating polar regimes into different symmetry and ferroelectric/antiferroelectric (FE/AFE) phase boundary dividing polar and antipolar dipole configurations. In contrast with extensive studies on MPB, FE/AFE phase boundary is far less explored. Here, we apply atomic-scale imaging and Rietveld refinement to directly demonstrate the intermediate phase at FE/AFE phase boundary exhibits a rare multipolar Pb-cations ordering, i.e. coexistence of antipolar or polar displacement, which manifests itself in both periodically gradient lattice spacing and anomalous initial hysteresis loop. In-situ electron/neutron diffraction reveals that the same parent intermediate phase can transform into either FE or AFE state depending on suppression of antipolar or polar displacement, coupling with the evolution of long-/short-range oxygen octahedra tilts. First-principle calculations further show that the transition between AFE and FE phase can occur in a low-energy pathway via the intermediate phase. These findings enrich the structural understanding of FE/AFE phase boundary in perovskite oxides.

Promoting exsolution of RuFe alloy nanoparticles on Sr2Fe1.4Ru0.1Mo0.5O6-δ via repeated redox manipulations for CO2 electrolysis.

Metal nanoparticles anchored on perovskite through in situ exsolution under reducing atmosphere provide catalytically active metal/oxide interfaces for CO2 electrolysis in solid oxide electrolysis cell. However, there are critical challenges to obtain abundant metal/oxide interfaces due to the sluggish diffusion process of dopant cations inside the bulk perovskite. Herein, we propose a strategy to promote exsolution of RuFe alloy nanoparticles on Sr2Fe1.4Ru0.1Mo0.5O6-δ perovskite by enriching the active Ru underneath the perovskite surface via repeated redox manipulations. In situ scanning transmission electron microscopy demonstrates the dynamic structure evolution of Sr2Fe1.4Ru0.1Mo0.5O6-δ perovskite under reducing and oxidizing atmosphere, as well as the facilitated CO2 adsorption at RuFe@Sr2Fe1.4Ru0.1Mo0.5O6-δ interfaces. Solid oxide electrolysis cell with RuFe@Sr2Fe1.4Ru0.1Mo0.5O6-δ interfaces shows over 74.6% enhancement in current density of CO2 electrolysis compared to that with Sr2Fe1.4Ru0.1Mo0.5O6-δ counterpart as well as impressive stability for 1000 h at 1.2 V and 800 °C.

Atomic-Scale Insight into Exsolution of CoFe Alloy Nanoparticles in La0.4 Sr0.6 Co0.2 Fe0.7 Mo0.1 O3-δ with Efficient CO2 Electrolysis.

In situ exsolution of metal nanoparticles in perovskite under reducing atmosphere is employed to generate a highly active metal-oxide interface for CO2 electrolysis in a solid oxide electrolysis cell. Atomic-scale insight is provided into the exsolution of CoFe alloy nanoparticles in La0.4 Sr0.6 Co0.2 Fe0.7 Mo0.1 O3-δ (LSCFM) by in situ scanning transmission electron microscopy (STEM) with energy-dispersive X-ray spectroscopy and DFT calculations. The doped Mo atoms occupy B sites of LSCFM, which increases the segregation energy of Co and Fe ions at B sites and improves the structural stability of LSCFM under a reducing atmosphere. In situ STEM measurements visualized sequential exsolution of Co and Fe ions, formation of CoFe alloy nanoparticles, and reversible exsolution and dissolution of CoFe alloy nanoparticles in LSCFM. The metal-oxide interface improves CO2 adsorption and activation, showing a higher CO2 electrolysis performance than the LSCFM counterparts.

A highly active Rh1/CeO2 single-atom catalyst for low-temperature CO oxidation.

Rh1/CeO2 single-atom catalyst is highly active for CO oxidation and has the potential to serve as a multifunctional catalyst to save the usage of other noble metals in three-way catalysts. The high activity is achieved on single-atom active sites via the Mars-van Krevelen mechanism, thus avoiding CO poisoning at low temperatures.

Strong metal-support interaction promoted scalable production of thermally stable single-atom catalysts.

Single-atom catalysts (SACs) have demonstrated superior catalytic performance in numerous heterogeneous reactions. However, producing thermally stable SACs, especially in a simple and scalable way, remains a formidable challenge. Here, we report the synthesis of Ru SACs from commercial RuO2 powders by physical mixing of sub-micron RuO2 aggregates with a MgAl1.2Fe0.8O4 spinel. Atomically dispersed Ru is confirmed by aberration-corrected scanning transmission electron microscopy and X-ray absorption spectroscopy. Detailed studies reveal that the dispersion process does not arise from a gas atom trapping mechanism, but rather from anti-Ostwald ripening promoted by a strong covalent metal-support interaction. This synthetic strategy is simple and amenable to the large-scale manufacture of thermally stable SACs for industrial applications.

In Situ Investigation of Reversible Exsolution/Dissolution of CoFe Alloy Nanoparticles in a Co-Doped Sr2 Fe1.5 Mo0.5 O6- δ Cathode for CO2 Electrolysis.

Reversible exsolution and dissolution of metal nanoparticles in perovskite has been investigated as an efficient strategy to improve CO2 electrolysis performance. However, fundamental understanding with regard to the reversible exsolution and dissolution of metal nanoparticles in perovskite is still scarce. Herein, in situ exsolution and dissolution of CoFe alloy nanoparticles in Co-doped Sr2 Fe1.5 Mo0.5 O6-δ (SFMC) revealed by in situ X-ray diffraction, scanning transmission electron microscopy, environmental scanning electron microscopy, and density functional theory calculations are reported. Under a reducing atmosphere, facile exsolution of Co promotes reduction of the Fe cation to generate CoFe alloy nanoparticles in SFMC, accompanied by structure transformation from double perovskite to layered perovskite at 800 °C. Under an oxidizing atmosphere, spherical CoFe alloy nanoparticles are first oxidized to flat CoFeOx nanosheets, and then dissolved into the bulk with structure evolution from layered perovskite back to double perovskite. Electrochemically, CO2 electrolysis performance can be retrieved during 12 redox cycles due to the regenerative ability of the CoFe alloy nanoparticles. The anchoring of the CoFe alloy nanoparticles in SFMC perovskite via reduction shows enhanced CO2 electrolysis performance and stability compared with the parent SFMC perovskite.

Ti-O-O coordination bond caused visible light photocatalytic property of layered titanium oxide.

The layered titanium oxide is a useful and unique precursor for the facile and rapid preparation of the peroxide layered titanium oxide H1.07Ti1.73O4·nH2O (HTO) crystal with enhanced visible light photoactivity. The H2O2 molecules as peroxide chemicals rapidly enter into the interlayers of HTO crystal, and coordinate with Ti within TiO6 octahedron to form a mass of Ti-O-O coordination bond in the interlayers. The introduction of these Ti-O-O coordination bonds result in lowering the band gap of HTO, and promoting the separation efficiency of the photo induced electron-hole pairs. Meanwhile, the photocatalytic investigation indicates that such peroxide HTO crystal has the enhanced photocatalytic performance for RhB degradation and water splitting to generate oxygen under visible light irradiating.