Unmatched Redox Activity of the Palladium-Doped Indium Oxide Oxygen Carrier for Low-Temperature CO2 Splitting.

The chemical conversion of CO2 into value-added products is the key technology to realize a carbon-neutral society. One representative example of such conversion is the reverse water-gas shift reaction, which produces CO from CO2. However, the activity is insufficient at ambient pressure and lower temperatures (<600 °C), making it a highly energy-intensive and impractical process. Herein, we report indium oxide nanofibers modified with palladium catalysts that exhibit significantly potent redox activities toward the reduction of CO2 splitting via chemical looping. In particular, we uncover that the doped palladium cations are selectively reduced and precipitated onto the host oxide surface as metallic nanoparticles. These catalytic gems formed operando make In2O3 lattice oxygen more redox-active in H2 and CO2 environments. As a result, the composite nanofiber catalysts demonstrate the reverse water-gas shift reaction via chemical looping at record-low temperatures (≤350 °C), while also imparting high activities (CO2 conversion: 45%). Altogether, our findings expand the viability of CO2 splitting at lower temperatures and provide design principles for indium oxide-based catalysts for CO2 conversion.

Concurrent Amorphization and Nanocatalyst Formation in Cu-Substituted Perovskite Oxide Surface: Effects on Oxygen Reduction Reaction at Elevated Temperatures.

The activity and durability of chemical/electrochemical catalysts are significantly influenced by their surface environments, highlighting the importance of thoroughly examining the catalyst surface. Here, Cu-substituted La0.6Sr0.4Co0.2Fe0.8O3-δ is selected, a state-of-the-art material for oxygen reduction reaction (ORR), to explore the real-time evolution of surface morphology and chemistry under a reducing atmosphere at elevated temperatures. Remarkably, in a pioneering observation, it is discovered that the perovskite surface starts to amorphize at an unusually low temperature of approximately 100 °C and multicomponent metal nanocatalysts additionally form on the amorphous surface as the temperature raises to 400 °C. Moreover, this investigation into the stability of the resulting amorphous layer under oxidizing conditions reveals that the amorphous structure can withstand a high-temperature oxidizing atmosphere (≥650 °C) only when it has undergone sufficient reduction for an extended period. Therefore, the coexistence of the active nanocatalysts and defective amorphous surface leads to a nearly 100% enhancement in the electrode resistance for the ORR over 200 h without significant degradation. These observations provide a new catalytic design strategy for using redox-dynamic perovskite oxide host materials.

Improvement of Oxygen Mobility with the Formation of Defects in the Crystal Structure of Red Mud as an Oxygen Carrier for Chemical Looping Combustion.

Fe2O3 is the major component of red mud, which is a by-produced after eluting aluminum from bauxite in the Bayer process, and can be used as an oxygen carrier. On the other hand, red mud is unsuitable for using oxygen in the crystal lattice because of its low surface area. In this study the red-mud sample was sulfidated at high temperatures to improve the lattice oxygen mobility by forming lattice defects in the iron oxide crystals. To form crystal defects on red mud, iron oxide was converted to iron sulfide with hydrogen sulfide, and then re-oxidized by air to remove the sulfur components. In these processes, it was possible to generate defects could be generated in the crystal structure. Crystal defects are formed by the difference in the molar volume of oxygen and sulfur bound to the metal in the oxidation-sulfidation process. The surface area of the defective red mud increased from approximately 25.9 m2/g to 122.1 m2/g, and the pore volume increased from 0.1714cc/g to 0.2803 cc/g. In addition, the formation of crystal defects increased the oxygen transfer capacity of red mud from 1.75% to 2.25% at 15 vol.% hydrogen. This means that the amount of oxygen transported during the reduction process could be enhanced approximately 1.29 fold.