Constructing an oxygen vacancy- and hydroxyl-rich TiO2-supported Pd catalyst with improved Pd dispersion and catalytic stability

Surface property modification of catalyst support is a straightforward approach to optimize the performance of supported noble metal catalysts. In particular, oxygen vacancies and hydroxyl groups play significant roles in promoting noble metal dispersion on catalysts as well as catalytic stability. In this study, we developed a nanoflower-like TiO2-supported Pd catalyst that has a higher concentration of oxygen vacancies and surface hydroxyl groups compared to that of commercial anatase and P25 support. Notably, due to the distinctive structure of the nanoflower-like TiO2, our catalyst exhibited improved dispersion and stabilization of Pd species and the formation of abundant reactive oxygen species, thereby facilitating the activation of CO and O2 molecules. As a result, the catalyst showed remarkable efficiency in catalyzing the low-temperature CO oxidation reaction with a complete CO conversion at 80 °C and stability for over 100 h.

Effect of Water Vapor on Oxidation Processes of the Cu(111) Surface and Sublayer

Copper-based catalysts have different catalytic properties depending on the oxidation states of Cu. We report operando observations of the Cu(111) oxidation processes using near-ambient pressure scanning tunneling microscopy (NAP-STM) and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). The Cu(111) surface was chemically inactive to water vapor, but only physisorption of water molecules was observed by NAP-STM. Under O₂ environments, dry oxidation started at the step edges and proceeded to the terraces as a Cu₂O phase. Humid oxidation of the H₂O/O₂ gas mixture was also promoted at the step edges to the terraces. After the Cu₂O covered the surface under humid conditions, hydroxides and adsorbed water layers formed. NAP-STM observations showed that Cu₂O was generated at lower steps in dry oxidation with independent terrace oxidations, whereas Cu₂O was generated at upper steps in humid oxidation. The difference in the oxidation mechanisms was caused by water molecules. When the surface was entirely oxidized, the diffusion of Cu and O atoms with a reconstruction of the Cu₂O structures induced additional subsurface oxidation. NAP-XPS measurements showed that the Cu₂O thickness in dry oxidation was greater than that in humid oxidation under all pressure conditions.

Pt-Based Nanostructures for Electrochemical Oxidation of CO: Unveiling the Effect of Shapes and Electrolytes

Direct alcohol fuel cells are deemed as green and sustainable energy resources; however, CO-poisoning of Pt-based catalysts is a critical barrier to their commercialization. Thus, investigation of the electrochemical CO oxidation activity (COOxid) of Pt-based catalyst over pH ranges as a function of Pt-shape is necessary and is not yet reported. Herein, porous Pt nanodendrites (Pt NDs) were synthesized via the ultrasonic irradiation method, and its CO oxidation performance was benchmarked in different electrolytes relative to 1-D Pt chains nanostructure (Pt NCs) and commercial Pt/C catalyst under the same condition. This is a trial to confirm the effect of the size and shape of Pt as well as the pH of electrolytes on the COOxid. The COOxid activity and durability of Pt NDs are substantially superior to Pt NCs and Pt/C in HClO4, KOH, and NaHCO3 electrolytes, respectively, owing to the porous branched structure with a high surface area, which maximizes Pt utilization. Notably, the COOxid performance of Pt NPs in HClO4 is higher than that in NaHCO3, and KOH under the same reaction conditions. This study may open the way for understanding the COOxid activities of Pt-based catalysts and avoiding CO-poisoning in fuel cells.

Mechanistic and thermodynamic insights into the SO2 oxidation on MnO2 catalysts: A combined theoretical and experimental study

Manganese oxide (especially manganese dioxide [MnO₂]) is an excellent catalytic material for SO₂ removal in flue gas desulfurization. In this study, the effect of crystalline structure of MnO₂ (α-MnO₂, β-MnO₂, γ-MnO₂ and δ-MnO₂) on their activity for SO₂ oxidation was studied based on density functional theory with Hubbard U corrections (DFT + U). The calculated results showed that α-MnO₂ has mild energy barriers of 0.69 eV and 0.46 eV, and β-MnO₂ has poor redox performance on SO₂ molecules, which has the highest energy barrier of 2.17 eV and the largest oxygen formation energy of 1.74 eV, making it difficult for the oxygen atom to remove from the surface lattice to form reactive sites. Thermodynamic calculations showed that α-MnO₂ is suitable for SO₂ oxidation for its low energy barriers, reaction energy close to zero in the first half, and relatively high spontaneity in the whole reaction. Experimental tests showed that α-MnO₂ had the best catalytic oxidation effect, with the highest sulfur capacity (304.11 mg/g), but β-MnO₂ had poor catalytic oxidation performance, with a sulfur capacity of 41.59 mg/g. This work studies the catalytic performance and mechanism of SO₂ removal and proposes a strategy to improve the catalytic activity by phase structure.

Synergistic Correlation in the Colloidal Properties of TiO2 Nanoparticles and Its Impact on the Photocatalytic Activity

In this work, the relationship between the photodegradation rate of methylene blue (MB) and the effective surface charge of titania nanoparticles (TiO2 NPs) in an aqueous solution is addressed. Colloidal dispersions were prepared from TiO2 NPs (4–10 nm) for the heterogenous photocatalysis test. The dispersion properties such as pH, hydrodynamic diameter, zeta potential, and isoelectric point were studied. Acidic TiO2 dispersions (pH = 3.6–4.0) with a positive zeta potential and smaller hydrodynamic diameter exhibit larger colloidal stability and pseudo-first-order kinetics for the degradation of MB. The largest rate constant (5 × 10−2 min−1) corresponded to a conversion of 98% within 75 min under UV light. This enhanced rate is a synergic effect between the surface area, charge, and optimal hydrodynamic diameter of TiO2 NPs. A linear correlation between the calculated values for the absorption cross-section and normalized rate constant was found for the systems under study. It was observed that an eventual increase in the pH (4–5.5) reduces the effective surface charge and dispersion stability, causing a decrease in the rate constants of one order of magnitude (10−3 min−1) for TiO2 agglomerates with a larger hydrodynamic diameter (300–850 nm).

Phase-selective active sites on ordered/disordered titanium dioxide enable exceptional photocatalytic ammonia synthesis

Photocatalytic N2 fixation to NH3 via defect creation on TiO2 to activate ultra-stable N[triple bond, length as m-dash]N has drawn enormous scientific attention, but poor selectivity and low yield rate are the major bottlenecks. Additionally, whether N2 preferentially adsorbs on phase-selective defect sites on TiO2 in correlation with appropriate band alignment has yet to be explored. Herein, theoretical predictions reveal that the defect sites on disordered anatase (Ad) preferentially exhibit higher N2 adsorption ability with a reduced energy barrier for a potential-determining-step (*N2 to NNH*) than the disordered rutile (Rd) phase of TiO2. Motivated by theoretical simulations, we synthesize a phase-selective disordered-anatase/ordered-rutile TiO2 photocatalyst (Na-Ad/Ro) by sodium-amine treatment of P25-TiO2 under ambient conditions, which exhibits an efficient NH3 formation rate of 432 μmol g-1 h-1, which is superior to that of any other defect-rich disordered TiO2 under solar illumination with a high apparent quantum efficiency of 13.6% at 340 nm. The multi-synergistic effects including selective N2 chemisorption on the defect sites of Na-Ad with enhanced visible-light absorption, suitable band alignment, and rapid interfacial charge separation with Ro enable substantially enhanced N2 fixation.

Cu oxide deposited on shape-controlled ceria nanocrystals for CO oxidation: influence of interface-driven oxidation states on catalytic activity

The polar CeO2 (100) surface facets contribute considerably to the formation of surface hydroxyl groups, which are necessary for selective, stable Cu1+ state loading and enhancement of CO oxidation activity.

Remote controlled optical manipulation of bimetallic nanoparticle catalysts using peptides

Remote optical manipulation of peptide ligands on bimetallic nanoparticle surfaces allows for tunable catalytic reactivity.