Heterostructured Copper–Ceria and Iron–Ceria Nanorods: Role of Morphology, Redox, and Acid Properties in Catalytic Diesel Soot Combustion

This work reports the synthesis of heterostructured copper–ceria and iron–ceria nanorods and the role of their morphology, redox, and acid properties in catalytic diesel soot combustion. Microscopy images show the presence of nanocrystalline CuO (9.5 ± 0.5 nm) and Fe₂O₃ (7.3 ± 0.5 nm) particles on the surface of CeO₂ nanorods (diameter is 8.5 ± 2 nm and length within 16–89 nm). In addition to diffraction peaks of CuO and Fe₂O₃ nanocrystallites, X-ray diffraction (XRD) studies reveal doping of Cu²⁺ and Fe³⁺ ions into the fluorite lattice of CeO₂, hence abundant oxygen vacancies in the Cu/CeO₂ and Fe/CeO₂ nanorods, as evidenced by Raman spectroscopy studies. XRD and Raman spectroscopy studies further show substantial perturbations in Cu/CeO₂ rods, resulting in an improved reducibility of bulk cerium oxide and formation of abundant Lewis acid sites, as investigated by H₂-temperature-programmed reduction and pyridine-adsorbed Fourier transform infrared studies, respectively. The Cu/CeO₂ rods catalyze the soot oxidation reaction at the lowest temperatures under both tight contact (Cu/CeO₂; T50 = 358 °C, temperature at which 50% soot conversion is achieved, followed by Fe/CeO₂; T50 = 368 °C and CeO₂; T50 = 433 °C) and loose contact conditions (Cu/CeO₂; T50 = 419 °C and Fe/CeO₂; T50 = 435 °C). A possible mechanism based on the synergetic effect of redox and acid properties of Cu/CeO₂ nanorods was proposed: acid sites can activate soot particles to form reactive carbon species, which are oxidized by gaseous oxygen/lattice oxygen activated in the oxygen vacancies (redox sites) of ceria rods

New Insights into the Photocatalytic Properties of RuO₂/TiO₂ Mesoporous Heterostructures for Hydrogen Production and Organic Pollutant Photodecomposition

Photocatalytic activities of mesoporous RuO₂/TiO₂ heterojunction nanocomposites for organic dye decomposition and H₂ production by methanol photoreforming have been studied as a function of the RuO₂ loading in the 1–10 wt % range. An optimum RuO₂ loading was evidenced for both kinds of reaction, the corresponding nanocomposites showing much higher activities than pure TiO₂ and commercial reference P25. Thus, 1 wt % RuO₂/TiO₂ photocatalyst led to the highest rates for the degradation of cationic (methylene blue) and anionic (methyl orange) dyes under UV light illumination. To get a better understanding of the mechanisms involved, a comprehensive investigation on the photogenerated charge carriers, detected by electron spin resonance (ESR) spectroscopy in the form of O^–, Ti³⁺, and O₂^– trapping centers, was performed. Along with the key role of superoxide paramagnetic species in the photodecomposition of organic dyes, ESR measurements revealed a higher amount of trapped holes in the case of the 1 wt % RuO₂/TiO₂ photocatalyst that allowed rationalizing the trends observed. On the other hand, a maximum average hydrogen production rate of 618 μmol h^–1 was reached with 5 wt % RuO₂/TiO₂ photocatalyst to be compared with 29 μmol h^–1 found without RuO₂. Favorable band bending at the RuO₂/TiO₂ interface and the key role of photogenerated holes have been proposed to explain the highest activity of the RuO₂/TiO₂ photocatalysts for hydrogen production. These findings open new avenues for further design of RuO₂/TiO₂ nanostructures with a fine-tuning of the RuO₂ nanoparticle distribution in order to reach optimized vectorial charge distribution and enhanced photocatalytic hydrogen production rates