Environment-Driven Variability in Absolute Band Edge Positions and Work Functions of Reduced Ceria.

The absolute band edge positions and work function (Φ) are the key electronic properties of metal oxides that determine their performance in electronic devices and photocatalysis. However, experimental measurements of these properties often show notable variations, and the mechanisms underlying these discrepancies remain inadequately understood. In this work, we focus on ceria (CeO2), a material renowned for its outstanding oxygen storage capacity, and combine theoretical and experimental techniques to demonstrate environmental modifications of its ionization potential (IP) and Φ. Under O-deficient conditions, reduced ceria exhibits a decreased IP and Φ with significant sensitivity to defect distributions. In contrast, the IP and Φ are elevated in O-rich conditions due to the formation of surface peroxide species. Surface adsorbates and impurities can further augment these variabilities under realistic conditions. We rationalize the shifts in energy levels by separating the individual contributions from bulk and surface factors, using hybrid quantum mechanical/molecular mechanical (QM/MM) embedded-cluster and periodic density functional theory (DFT) calculations supported by interatomic-potential-based electrostatic analyses. Our results highlight the critical role of on-site electrostatic potentials in determining the absolute energy levels in metal oxides, implying a dynamic evolution of band edges under catalytic conditions.

Multiscale Investigation of the Mechanism and Selectivity of CO2 Hydrogenation over Rh(111).

CO2 hydrogenation over Rh catalysts comprises multiple reaction pathways, presenting a wide range of possible intermediates and end products, with selectivity toward either CO or methane being of particular interest. We investigate in detail the reaction mechanism of CO2 hydrogenation to the single-carbon (C1) products on the Rh(111) facet by performing periodic density functional theory (DFT) calculations and kinetic Monte Carlo (kMC) simulations, which account for the adsorbate interactions through a cluster expansion approach. We observe that Rh readily facilitates the dissociation of hydrogen, thus contributing to the subsequent hydrogenation processes. The reverse water-gas shift (RWGS) reaction occurs via three different reaction pathways, with CO hydrogenation to the COH intermediate being a key step for CO2 methanation. The effects of temperature, pressure, and the composition ratio of the gas reactant feed are considered. Temperature plays a pivotal role in determining the surface coverage and adsorbate composition, with competitive adsorption between CO and H species influencing the product distribution. The observed adlayer configurations indicate that the adsorbed CO species are separated by adsorbed H atoms, with a high ratio of H to CO coverage on the Rh(111) surface being essential to promote CO2 methanation.

Metastable gallium hydride mediates propane dehydrogenation on H2 co-feeding.

In heterogeneous catalysis, the catalytic dehydrogenation reactions of hydrocarbons often exhibit a negative pressure dependence on hydrogen due to the competitive chemisorption of hydrocarbons and hydrogen. However, some catalysts show a positive pressure dependence for propane dehydrogenation, an important reaction for propylene production. Here we show that the positive activity dependence on H2 partial pressure of gallium oxide-based catalysts arises from metastable hydride mediation. Through in situ spectroscopic, kinetic and computational analyses, we demonstrate that under reaction conditions with H2 co-feeding, the dissociative adsorption of H2 on a partially reduced gallium oxide surface produces H atoms chemically bonded to coordinatively unsaturated Ga atoms. These metastable gallium hydride species promote C-H bond activation while inhibiting deep dehydrogenation. We found that the surface coverage of gallium hydride determines the catalytic performance. Accordingly, benefiting from proper H2 co-feeding, the alumina-supported, trace additive-modified gallium oxide catalyst GaOx-Ir-K/Al2O3 exhibited high activity and selectivity at high propane concentrations.

Synergistic Mechanism of Platinum-GaOx Catalysts for Propane Dehydrogenation.

The synergy between metals and metal oxides can effectively improve the heterogeneous catalytic process. This paper describes the intrinsic effect of Pt modification over GaOx (Pt-GaOx ) on propane dehydrogenation. The presence of Pt promotes H2 dissociation and surface coverage of hydrogen species, which is beneficial for the activation of C-H in propane. With excessive Pt, Gaδ+ can be further reduced to form Pt-Ga alloy with less surface hydrogen species. Consequently, the relative propylene formation rate between Pt-GaOx and the summed contribution of individual Pt and GaOx increases linearly with the content of hydrogen species. Optimally, the relative propylene formation rate of Pt-GaOx with 0.03 wt % Pt exceeds 25 % of the summed contribution of individual components.

SESAM mode-locked Yb:Sr3Y2(BO3)4 laser.

We demonstrate the first sub-40 fs soliton pulse generation from a diode-pumped Yb:Sr3Y2(BO3)4 laser passively mode-locked by a semiconductor saturable absorber mirror. Pulses as short as 38 fs at a central wavelength of 1051.7 nm were achieved with an average output power of 115 mW and a pulse repetition rate of 67.7 MHz. The maximum average output power reached 303 mW at 1057.8 nm with a slightly longer pulse duration of 52 fs, which corresponded to a peak power of 76.9 kW and an optical efficiency of 25.3%.

Extending Unique 1D Borate Chains to 3D Frameworks by Introducing Metallic Nodes.

Two novel alkali/alkaline-earth borates, Ba6 [B6 O9 (OH)6 ]2 (H3 BO3 ) (1) and Li7 MAlB12 O24 (M=Ba, Sr, Ca) (2 a-c), with 1D and 3D structures have been made hydrothermally and characterized. 1 features a rare 1D anionic chain built by hexaborate clusters of B6 O11 (OH)6 ; each made of six BO4 /BO2 (OH)2 tetrahedra. The anionic chains are embedded in the channels of a Ba6 -based wheel-cluster open-framework. On the basis of the structural analyses of 1, by incorporating Al atoms as the linkers and tuning the reaction conditions, the novel anhydrous 3D aluminoborates (ABOs) 2 a-c have been successfully obtained, constructed from B6 O14 -based cluster chains and AlO6 octahedra. The 3D ABO framework and 3D Ba-O-Li network are interpenetrated to give a final dense structure. The study not only realized the expansion of the structure from the 1D chain of 1 to the 3D dense ABOs 2 a-c, but also obtained the first 3D AlO6 -containing ABOs made under hydrothermal conditions. Different from the previously known 4-connected zeolite-type ABOs, alternately arranged from AlO4 tetrahedra and oxo-boron clusters, the AlO6 octahedra in 2 a-c as the linkers join to six 1D B6 O14 -based cluster chains to produce 3D ABOs. The optical diffuse reflectance spectra reveal that 2 a-c have wide range transparency. In addition, the thermal property analysis proves that 2 a-c are congruently melting compounds and possess high thermostability.

Thermal, Spectral and Laser Properties of Er3+:Yb3+:GdMgB5O10: A New Crystal for 1.5 µm Lasers.

A novel laser crystal of Er3+:Yb3+:GdMgB5O10 with dimension of 26 × 16 × 12 mm³ was grown successfully from K2Mo3O10 flux by the top seeded solution growth method. The thermal diffusivity and specific heat capacity were measured to calculate the thermal conductivity of the crystal. The absorption and fluorescence properties of the crystal at room temperature were investigated in detail. The Judd-Ofelt method was used to analyze the polarized absorption spectra. The emission cross-section of the 4I13/2→4I15/2 transition was calculated by the Füchtbauer-Ladenburg formula and the relevant gain cross-sections were estimated. Continuous-wave laser output of 140 mW at 1569 nm with the slope efficiency of 17.8% was demonstrated in a plano-concave resonator. The results reveal that Er3+:Yb3+:GdMgB5O10 crystal is a promising material for 1.5 µm lasers.