Geometrical Structure of the Gold–Iron(III) Oxide Interfacial Perimeter for CO Oxidation

A Water-Promoted Mars-van Krevelen Reaction Dominates Low-Temperature CO Oxidation over Au-Fe2O3 but Not over Au-TiO2

We provide experimental evidence that is inconsistent with often proposed Langmuir-Hinshelwood (LH) mechanistic hypotheses for water-promoted CO oxidation over Au-Fe2O3. Passing CO and H2O, but no O2, over Au-γ-Fe2O3 at 25 °C, we observe significant CO2 production, inconsistent with LH mechanistic hypotheses. Experiments with H218O further show that previous LH mechanistic proposals cannot account for water-promoted CO oxidation over Au-γ-Fe2O3. Guided by density functional theory, we instead postulate a water-promoted Mars-van Krevelen (w-MvK) reaction. Our proposed w-MvK mechanism is consistent both with observed CO2 production in the absence of O2 and with CO oxidation in the presence of H218O and 16O2. In contrast, for Au-TiO2, our data is consistent with previous LH mechanistic hypotheses.

Size-dependent phase transitions boost catalytic activity of sub-nanometer gold clusters

The characterization and identification of the dynamics of cluster catalysis are crucial to unraveling the origin of catalytic activity. However, the dynamical catalytic effects during the reaction process remain unclear. Herein, we investigate the dynamic coupling effect of elementary reactions with the structural fluctuations of sub-nanometer Au clusters with different sizes using ab initio molecular dynamics and the free energy calculation method. It was found that the adsorption-induced solid-to-liquid phase transitions of the cluster catalysts give rise to abnormal entropy increase, facilitating the proceeding of reaction, and this phase transition catalysis exists in a range of clusters with different sizes. Moreover, clusters with different sizes show different transition temperatures, resulting in a non-trivial size effect. These results unveil the dynamic effect of catalysts and help understand cluster catalysis to design better catalysts rationally.

MXene-Supported, Atomic-Layered Iridium Catalysts Created by Nanoparticle Re-Dispersion for Efficient Alkaline Hydrogen Evolution

Tailoring the structure of metal components and interaction with their anchored substrates is essential for improving the catalytic performance of supported metal catalysts; the ideal catalytic configuration, especially down to the range of atomic layers, clusters, and even single atoms, remains a subject under intensive study. Here, an Ir-on-MXene (Mo₂ TiC₂ T_x ) catalyst with controlled morphology changing from nanoparticles down to flattened atomic layers, and finally ultrathin layers and single atoms dispersed on MXene nanosheets at elevated temperature, is presented. The intermediate structure, consisting of mostly Ir atomic layers, shows the highest activity toward the hydrogen evolution reaction (HER) under industry-compatible alkaline conditions. In addition, the better HER activity of Ir atomic layers than that of single atoms suggests that the former serves as the main active sites. Detailed mechanism analysis reveals that the nanoparticle re-dispersion process and Ir atomic layers with a moderate interaction to the substrate associate with unconventional electron transfer from MXene to Ir, leading to suitable H* adsorption. The results indicate that the structural design is important for the development of highly efficient catalysts.

Pd Nanoparticles Supported on Triangle-Shaped La2 O2 CO3 Nanosheets: A New Highly Efficient and Durable Catalyst for Selective Hydrogenation of Cinnamaldehyde to Hydrocinnamaldehyde

A catalyst in which Pd nanoparticles are supported on triangle-shaped La₂ O₂ CO₃ nanosheets exposing predominantly the (001) planes (Pd/La₂ O₂ CO₃ -TNS; where TNS denotes triangular nanosheets) was prepared by a facile solvothermal method. The Pd/La₂ O₂ CO₃ -TNS catalysts exhibited excellent catalytic activity and recycling stability for hydrogenation of cinnamaldehyde to hydrocinnamaldehyde with turnover frequency of up to 41 238 h^-1 . This enhanced activity of Pd/La₂ O₂ CO₃ -TNS results from strong metal-support interactions. Structure analysis and characterization demonstrated that surface-oxygen-enriched La₂ O₂ CO₃ -TNS supports exposing (001) planes are beneficial to charge transfer between the Pd nanoparticles and triangle-shaped La₂ O₂ CO₃ nanosheets and increase the electron density of Pd. Moreover, the modulated electronic states of the Pd/La₂ O₂ CO₃ -TNS catalysts can enhance the adsorption and activation of hydrogen to enhance the hydrogenation activity.

Structural Evolution of α-Fe2O3(0001) Surfaces Under Reduction Conditions Monitored by Infrared Spectroscopy

The precise determination of the surface structure of iron oxides (hematite and magnetite) is a vital prerequisite to understand their unique chemical and physical properties under different conditions. Here, the atomic structure evolution of the hematite (0001) surface under reducing conditions was tracked by polarization-resolved infrared reflection absorption spectroscopy (IRRAS) using carbon monoxide (CO) as a probe molecule. The frequency and intensity of the CO stretch vibration is extremely sensitive to the valence state and electronic environments of surface iron cations. Our comprehensive IRRAS results provide direct evidence that the monocrystalline, stoichiometric α-Fe₂O₃(0001) surface is single Fe-terminated. The initial reduction induced by annealing at elevated temperatures produces surface oxygen vacancies, where the excess electrons are localized at adjacent subsurface iron ions (5-fold coordinated). A massive surface restructuring occurs upon further reduction by exposing to atomic hydrogen followed by Ar⁺-sputtering and annealing under oxygen poor conditions. The restructured surface is identified as a Fe₃O₄(111)/Fe_1−xO(111)-biphase exposing both, Fe³⁺ and Fe²⁺ surface species. Here the well-defined surface domains of Fe₃O₄(111) exhibit a Fe_oct2-termination, while the reduced Fe_1−xO(111) is Fe²⁺(oct)-terminated. These findings are supported by reference IRRAS data acquired for CO adsorption on magnetite (111) and (001) monocrystalline surfaces.