Engineering Active Sites of Gold-Cuprous Oxide Catalysts for Electrocatalytic Oxygen Reduction Reaction.

Understanding how the catalyst morphology influences surface sites is crucial for designing active and stable catalysts and electrocatalysts. We here report a new approach to this understanding by decorating gold (Au) nanoparticles on the surface of cuprous oxides (Cu2O) with three different shape morphologies (spheres, cubes, and petals). The Au-Cu2O particles are dispersed onto carbon nanotube (CNT) matrix with high surface area, stability, and conductivity for oxygen reduction reaction. A clear morphology-dependent enhancement of the electrocatalytic activity is revealed. Oxygenated gold species (AuO-) are found to coexist with Au0 on the cube and petal catalysts, whereas only Au0 species are present on the sphere catalyst. The AuO- species function effectively as active sites, resulting in the improved catalytic performance by changing the reaction mechanism. The enhanced catalytic performance of the petal-shaped catalyst in terms of onset potential, half-wave potential, diffusion-limited current density, and stability is closely associated with the presence of the most abundant AuO- species on its surface. Highly active AuO- species are identified on the surface of the catalysts as a result of the unique structural characteristics, which is attributed to the structural origin of high activity and stability. This insight constitutes the basis for assessing the detailed correlation between the morphology and the electrocatalytic properties of the nanocomposite catalysts, which has implications for the design of surface-active sites on metal/metal oxide electrocatalysts.

Active Sites and Interfacial Reducibility of CuxO/CeO2 Catalysts Induced by Reducing Media and O2/H2 Activation.

The development of a highly efficient and stable catalyst for preferential oxidation of CO for the commercialization of proton-exchange membrane fuel cells has been a result of continuous effort. The main challenge is the simultaneous control of abundant active sites and interfacial reducibility over the catalyst CuxO/CeO2. Here, we report a strategy to modulate porosity, active sites, and O-vacancy sites (OV) by reducing media and O2/H2 activation. O2-pretreated CeO2-supported Cu catalysts unequivocally demonstrate the low-temperature activity owing to the excess concentrations of Cu+ and Cu2+ as well as the relative population of Ce3+ and O-vacancy sites at the surface. O2 activation improves the Cu2+ diffusion into the CeO2 lattice to generate the synergistic effect and induces the formation of electron-enriched Cu2+-OV-Ce3+ sites, which accelerate the activation and dissociation of CO/O2 and the formation of reactive oxygen species during catalysis. Density function theory (DFT) calculations reveal that CO adsorbs on Cu2O {110} and CuO {111} with relatively optimal adsorption energy and longer C-Cu lengths in contrast to that on Cu {111}, favoring the adsorption and desorption of CO. These are crucial for ongoing CO oxidation, producing CO2 by the Mars-van Krevelen mechanism.

Probing Heteroatomic Dopant-Activity Synergy over Co3O4/Doped Carbon Nanotube Electrocatalysts for Oxygen Reduction Reaction.

Understanding and predicting how heteroatomic dopants of carbon nanotubes (CNTs)-based catalysts alter their catalytic performance at nanoscale is essential to design superior electrocatalysts for oxygen reduction reaction (ORR). This report describes findings of an investigation of the heteroatomic dopant-activity relationship for Co3O4/doped CNTs catalysts with different heteroatoms including N, O, and P atoms in ORR. By using an array of techniques to probe the structure and elementary valence of the catalysts, the incorporation of the Co3O4 nanoparticles can introduce defects into the doped CNTs, especially the N-CNTs, which should contribute to the generation of active sites. The Co3O4/N-CNTs are shown to exhibit both the highest ORR activity and stability compared with Co3O4/O-CNTs, Co3O4/P-CNTs, and Co3O4/CNTs, manifesting the synergistic correlation of Co3O4 nanoparticles, heteroatoms, and CNTs. This kind of synergy is assessed by density functional theory calculations based on the electronic properties and molecular orbitals. It is found that N, O, or P atoms can tune the charge distribution of CNTs by decreasing the lowest unoccupied molecular orbital-highest occupied molecular orbital energy gap, thus activating the adjacent C atoms. And the addition of Co3O4 will further redistribute the charge of CNTs from CNTs to Co3O4 toward enhanced ORR activity. Moreover, the Co3O4/N-CNTs catalyst exhibits a maximum structural stability due to the strong electronic interaction between Co2+ ions and N atoms, which is believed to result in its high ORR stability. Analysis of the results, along with a combined theoretical and experimental study, has provided implications for the design of catalysts with controlled activity and stability for ORR.