Experimental and Computational Insights into the Catalytic Mechanism of Y1-xBaxCoO3-δ Perovskite Oxides with a Controlled Crystal Structure.

The crystal structure of Co-based perovskite oxides (ACoO3) can be controlled by adjusting the A-site elements. In this study, we synthesized Y1-xBaxCoO3-δ (x = 0, 0.5, and 1.0) via a coprecipitation method and investigated their CO oxidation performances. YCoO3 (x = 0; cubic perovskite oxide; Pbnm) shows a higher catalytic performance than Y0.5Ba0.5CoO2.72 (x = 0.5; A-site-ordered double perovskite oxide; P4/nmm), which exhibits high oxygen nonstoichiometric properties, and BaCoO3 (x = 1.0; hexagonal perovskite oxide; P63/mmc), which contains high-valent Co4+ species. To elucidate the reaction mechanism, we conducted isotopic experiments with CO and 18O2. The CO oxidation reaction on YCoO3 proceeds via the Langmuir-Hinshelwood mechanism, which is a surface reaction of CO and O2 gas that does not utilize lattice oxygen. Because of the significantly smaller specific surface area of YCoO3 compared with that of the reference Pt/Al2O3, the bulk features of the crystal structures affect the catalytic reaction. When density functional theory is applied, YCoO3 clearly exhibits semiconducting properties in the ground state with the diamagnetic t2g6eg0 states, which can translate to a magnetic t2g5eg1 configuration upon excitation by a relatively low energy of 0.64 eV. We propose that the unique nature of YCoO3 activates oxygen in the gas phase, thereby enabling the smooth oxidation of CO. This study demonstrates that the bulk properties originating from the crystal structure contribute to the catalytic activity and reaction mechanism.

Roles of Carbon Nanotube Confinement in Modulating Regioselectivity of 1,3-Dipolar Cycloadditions.

DFT-based calculations were employed to investigate mechanisms of 1,3-dipolar cycloadditions between phenylacetylene and an azide (phenylazide or benzylazide) inside carbon nanotubes, whose diameters range from 10 to 14 Å, by obtaining their reaction species (reactant complex, transition state (TS), and product (Pro)). The reactions yield 1,4- and 1,5-triazoles, whose paths are denoted by 1,4- and 1,5-approaches, respectively. We found different geometrical features of reaction species between 1,4- and 1,5-approaches. Reflecting different reaction species, nanotube confinement has the power to enhance kinetically and thermodynamically controlled regioselectivity of 1,3-dipolar cycloadditions to form 1,4-triazoles. In inner 1,4-approaches, the reaction species have planar structures, being small relative to the cavity of tube hosts, and then, their activation energies are slightly lowered relative to those without tube surroundings, independent of the tube diameter. In inner 1,5-approaches, reaction species have phenyl groups overlapping each other, depending on the tube diameter: L-shaped and stacking fashions are found in thick and thin tubes, respectively. Particularly, the stacking fashion in thin tubes results in repulsive orbital interactions between two phenyl rings, destabilizing their TS and Pro. The presence of overlapping phenyl groups increases the activation energies in the 1,5-approaches with a decrease in the tube diameter, being larger than those without tube surroundings.

Theoretical understanding of stability of mechanically interlocked carbon nanotubes and their precursors.

Dispersion-corrected DFT calculations were performed on (a,a) nanotubes (a = 5-10) attached by a U-shaped functional group consisting of p-xylene-linked double 9,10-di(1,3-dithiol-2-ylidene)-9,10-dihydro anthracene terminated by CnH2n chains (n = 6, 8, and 9), and their ring-closing macrocycles containing tubes. The reactant precursors and macrocycles are denoted by UP-n-(a,a) and (a,a)@Cycle-n, respectively. We found that UP-n-(a,a) are energetically preferable relative to the dissociation limit toward a U-shaped functional group (UP-n) and a tube (initial state) due to the attractive CH-π and π-π interactions. The attractive interactions are enhanced by increasing the tube diameters and CnH2n chain lengths because UP-n structures can be easily adjusted to interact with the tubes. The stability of (a,a)@Cycle-n and related (a,b)@Cycle-n is sensitive to tube diameters due to the restriction of ring structures. When diameter differences between a Cycle-n and a tube (D-d) are larger than 5 Å, (a,a)@Cycle-n plus C2H4 are energetically preferable relative to the initial state. However, the (a,a)@Cycle-n plus C2H4 byproduct is always energetically unstable relative to UP-n-(a,a). The DFT calculations found that the energy differences were low at D-d values ranging from 7 to 8 Å, explaining the tube-diameter-selective formation of the mechanically-interlocked tubes, observed experimentally.

Ultralong Distance Hydrogen Spillover Enabled by Valence Changes in a Metal Oxide Surface.

Hydrogen spillover is a phenomenon in which hydrogen atoms generated on metal catalysts diffuse onto catalyst supports. This phenomenon offers reaction routes for functional materials. However, due to difficulties in visualizing hydrogen, the fundamental nature of the phenomenon, such as how far hydrogen diffuses, has not been well understood. Here, in this study, we fabricated catalytic model systems based on Pd-loaded SrFeOx (x ∼ 2.8) epitaxial films and investigated hydrogen spillover. We show that hydrogen spillover on the SrFeOx support extends over long distances (∼600 µm). Furthermore, the hydrogen-spillover-induced reduction of Fe4+ in the support yields large energies (as large as 200 kJ/mol), leading to the spontaneous hydrogen transfer and driving the surprisingly ultralong hydrogen diffusion. These results show that the valence changes in the supports' surfaces are the primary factor determining the hydrogen spillover distance. Our study leads to a deeper understanding of the long-debated issue of hydrogen spillover and provides insight into designing catalyst systems with enhanced properties.