The oxygen reduction reaction (ORR) is the rate-limiting reaction in the cathode side of fuel cells. In the quest for alternatives to Pt-electrodes as cathodes in ORR, appropriate transition metal oxide-based electrocatalysts are needed. In the present work, we have synthesized Co3O4 and CoO/Co3O4 nanostructures using flame spray pyrolysis (FSP), as electrocatalysts for ORR in acidic and alkaline media. A detailed study of the effect of (Co-oxide)/Pt ratio on ORR efficiency shows that the present FSP-made Co-oxides are able to perform ORR at very low-Pt loading, 0.4% of total metal content. In acid medium, an electrode with (5.2% Pt + 4.8% Co3O4), achieved the highest ORR performance (Jmax = 8.31 mA/cm2, E1/2 = 0.66 V). In alkaline medium, superior performance and stability have been achieved by an electrode with (0.4%Pt + 9.6% (CoO/Co3O4)) with ORR activity (Jmax = 3.5 mA/cm2, E1/2 = 0.08 V). Using XRD, XPS, Raman and TEM data, we discuss the structural and electronic aspects of the FSP-made Co-oxide catalysts in relation to the ORR performance. Cyclic voltammetry data indicate that the ORR process involves active sites associated with Co3+ cations at the cobalt oxide surface. Technology-wise, the present work demonstrates that the developed FSP-protocols, constitutes a novel scalable process for production of co-oxides appropriate for oxygen reduction reaction electrodes.

Currently, plasmonic nanoparticles (PNPs) are considered highly efficient enhancers of catalytic processes. Herein, we report a concept where plasmonic Ag0@SiO2 nanoparticles can reversibly switch-off an oxidation catalytic process under light-excitation. The catalytic process recommences when illumination is stopped. The catalytic system under study is a well-characterized molecular LMnII catalyst that performs alkene oxidation, with H2O2 as the oxidant. Three types of plasmonic core–shell Ag0@SiO2 nanoparticles, with a SiO2 shell of varying thickness (0.1–5 nm), were utilized in this study. Using electron paramagnetic resonance spectroscopy, we have identified the reversible inhibition of the transient LMnIVO intermediate formation, to be the key-step of the photoinduced pause of the catalytic process by the Ag0@SiO2 PNPs. Surface-enhanced Raman spectroscopy (SERS) and redox potential data show that the plasmonic Ag0@SiO2 NPs exert a moderate SERS effect on the LMnII catalyst, and a considerable lowering of the solution redox potential E h. Our data show that near-field generation is not the sole origin of inhibition of LMnIVO formation, while plasmonic heating was insignificant. We suggest that the generation of hot electrons by the Ag0@SiO2 PNPs is implicated, along with near-field generation, in the reversible switch-off of the catalytic process.

Palladium is among the most versatile noble-metal atoms that, when dispersed on solid supports, can be stabilized in 0, +1, +2, +3 redox states. Moreover, despite its noble-metal character, Pd shows a considerable degree of chemical reactivity. In Pd Nanoparticles (NPs), atomic {Pdn+-X} states, where n = 0, 1, 2, 3, and X = atom or hydride, can play key roles in catalytic processes. Pd-oxygen moieties can be stabilized at nanointerfaces of Pd in contact with metal-oxides. These {Pdn+-X}s can be either isolated Pd atoms dispersed on the support, or, more interestingly, atomic states of Pd occurring on the Pd NPs. The present review focuses on the role of such {Pdn+-X} states in catalytic processes related to energy storage or energy conversion, with specific focus on photocatalysis, H2 production reaction (HRR), oxygen reduction reaction (ORR), and water-splitting. Synthesis of atomic {Pdn+-X} states and their detection methodology is among the current challenges. Herein, the chemistry of {Pdn+-X} states on Pd- [metal oxide] interfaces, methods of detection, and identification are discussed. The implication of {Pdn+-X} in transient catalytic intermediates is reviewed. Finally, the role of {Pdn+-X} in photo electrocatalytic processes is critically discussed.

Atomic palladium states formed on Pd nanoparticles are recognized as key components for many catalytic processes. Herein, we have generated a library of atomic Pd states (Pd 1+ , Pd 2+ −O 2 − , and Pd 3+ ) stabilized on TiO 2 support using flame spray pyrolysis. Using electron paramagnetic resonance (EPR) spectroscopy, we have identified the g-tensors and molecular orbital configuration of these Pd 1+ , {Pd 2+ -O 2 − }, and Pd 3+ states formed at the Pd/TiO 2 interface. Their evolution was studied under conditions pertinent to a wide range of catalytic processes, that is, such as H 2 , BH 4 treatment, solar-light irradiation, or catalytic HCOOH dehydrogenation. Under these conditions, the Pd 1+ and Pd 3+ states can act as electron acceptors and hydride acceptors. The unconventional {Pd 2+ −O 2 − } state is formed as a transient intermediate from a Pd 1+ precursor. Our data reveal a dynamic landscape regarding these multipotent Pd n+ states formed at the interface of Pd nanoparticles with TiO 2 , in relation to catalytic processes.

Cu-oxide nanophases (CuO, Cu2O, Cu0) constitute highly potent nanoplatforms for the development of efficient Artificial Photosynthesis catalysts. The highly reducing conduction band edge of the d-electrons in Cu2O dictates its efficiency towards CO2 reduction under sunlight excitation. In the present review, we discuss aspects interlinking the stability under photocorrosion of the (CuO/Cu2O/Cu0) nanophase equilibria, and performance in H2-production/CO2-reduction. Converging literature evidence shows that, because of photocorrosion, single-phase Cu-oxides would not be favorable to be used as a standalone cathodic catalyst/electrode; however, their heterojunctions and the coupling with proper partner materials is an encouraging approach. Distinction between the role of various factors is required to protect the material from photocorrosion, e.g., use of hole scavengers/electron acceptors, band-gap engineering, nano-facet engineering, and selectivity of CO2-reduction pathways, to name a few possible solutions. In this context, herein we discuss examples and synthesis efforts that aim to clarify the role of interfaces, faces, and phase stability under photocatalytic conditions.