<i>In Situ</i> Raman Spectroscopy of Copper and Copper Oxide Surfaces during Electrochemical Oxygen Evolution Reaction: Identification of Cu^III Oxides as Catalytically Active Species

Scanning electron microscopy, X-ray diffraction, cyclic voltammetry, chronoamperometry, <i>in situ</i> Raman spectroscopy, and X-ray absorption near-edge structure spectroscopy (XANES) were used to investigate the electrochemical oxygen evolution reaction (OER) on Cu, Cu₂O, Cu­(OH)₂, and CuO catalysts. Aqueous 0.1 M KOH was used as the electrolyte. All four catalysts were oxidized or converted to CuO and Cu­(OH)₂ during a slow anodic sweep of cyclic voltammetry and exhibited similar activities for the OER. A Raman peak at 603 cm^–1 appeared for all the four samples at OER-relevant potentials, ≥1.62 V vs RHE. This peak was identified as the Cu–O stretching vibration band of a Cu^III oxide, a metastable species whose existence is dependent on the applied potential. Since this frequency matches well with that from a NaCu^IIIO₂ standard, we suggest that the chemical composition of the Cu^III oxide is CuO₂^–-like. The four catalysts, in stark contrast, did not oxidize the same way during direct chronoamperometry measurements at 1.7 V vs RHE. Cu^III oxide was observed only on the CuO and Cu­(OH)₂ electrodes. Interestingly, these two electrodes catalyzed the OER ∼10 times more efficiently than the Cu and Cu₂O catalysts. By correlating the intensity of the Raman band of Cu^III oxide and the extent of the OER activity, we propose that Cu^III species provides catalytically active sites for the electrochemical water oxidation. The formation of Cu^III oxides on CuO films during OER was also corroborated by <i>in situ</i> XANES measurements of the Cu K-edge. The catalytic role of Cu^III oxide in the O₂ evolution reaction is proposed and discussed

Enhanced Catalysis of the Electrochemical Oxygen Evolution Reaction by Iron(III) Ions Adsorbed on Amorphous Cobalt Oxide

The oxygen evolution reaction (OER) is the bottleneck in the efficient production of hydrogen gas fuel via the electrochemical splitting of water. In this work, we present and elucidate the workings of an OER catalytic system which consists of cobalt oxide (CoO_<i>x</i>) with adsorbed Fe³⁺ ions. The CoO_<i>x</i> was electrodeposited onto glassy-carbon-disk electrodes, while Fe³⁺ was added to the 1 M KOH electrolyte. Linear sweep voltammetry and chronopotentiometry were used to assess the system’s OER activity. The addition of Fe³⁺ significantly lowered the average overpotential (η) required by the cobalt oxide catalyst to produce 10 mA/cm² O₂ current from 378 to 309 mV. The Tafel slope of the CoO_<i>x</i> + Fe³⁺ catalyst also decreased from 59.5 (pure CoO_<i>x</i>) to 27.6 mV/dec, and its stability lasted ∼20 h for 10 mA/cm² O₂ evolution. Cyclic voltammetry showed that oxidation of the deposited CoO_<i>x</i>, from Co²⁺ to Co³⁺ occurred at a more positive potential when Fe³⁺ was added to the electrolyte. This could be attributed to interactions between the Co and Fe atoms. Comprehensive X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy were conducted. The in situ XANES spectra of Co sites in the CoO_<i>x</i>, CoO_<i>x</i> + Fe³⁺, and control Fe₄₈Co₅₂O_<i>x</i> catalysts were similar during the OER, which indicates that the improved OER performance of the CoO_<i>x</i> + Fe³⁺ catalyst could not be directly correlated to changes in the Co sites. The XANES spectra of Fe indicated that Fe³⁺ adsorbed on CoO_<i>x</i> did not further oxidize under OER conditions. However, Fe’s coordination number was notably reduced from 6 in pure FeO_<i>x</i> to 3.7 when it was adsorbed on CoO_<i>x</i>. No change in the Fe–O bond lengths/strengths was found. The nature and mechanistic role of Fe adsorbed on CoO_<i>x</i> are discussed. We propose that Fe sites with oxygen vacancies are responsible for the improved OER activity of CoO_<i>x</i> + Fe³⁺ catalyst

β‑FeOOH: An Earth-Abundant High-Capacity Negative Electrode Material for Sodium-Ion Batteries

Thanks to the great earth abundance and excellent energy density of sodium, sodium-ion batteries are promising alternative energy storage devices for large-scale applications. Developing cheap, safe, and high-capacity sodium-ion battery anode materials is one of the critical challenges in this field. Here, we show that β-FeOOH is a very promising low-cost anode material, with a high reversible capacity (>500 mAh g^–1 during initial cycles). The fundamental characteristics associated with the discharge/charge processes, in terms of the redox reactions, formation/deformation of the solid electrolyte interface (SEI) layers, and structural and morphological changes, are comprehensively investigated. In addition, a comparison study shows that the smaller-sized FeOOH has more serious kinetic restrictions, and thus lower capacities, while it shows better cyclability than the bigger one. Origins of the large overpotential are discussed, and it is suggested that the overpotential should be mainly due to the features of the surface-concentration-dependent potential and the slow diffusion of Na⁺; in addition, the presence of the SEI layers may also contribute to the overpotential

Degree of Geometric Tilting Determines the Activity of FeO₆ Octahedra for Water Oxidation

Fe oxides and (oxy)­hydroxides are promising cost-effective catalysts for scalable water electrolysis. For an improvement in the understanding of the structural factors required by the most active Fe sites, the role of geometric tilting in determining the activity of the FeO₆ octahedron for water oxidation was investigated. The catalytic performance of the FeO₆ octahedron in a series of crystalline structures, i.e., perovskites AFeO₃, spinel ZnFe₂O₄, and β-FeOOH, was found to be negatively correlated with their octahedral tilting degree. This correlation was rationalized through the Fe–O covalency, which is reflected by the O 2p band center as well as the charge-transfer energy obtained from ab initio calculations. Thus, it was disclosed that FeO₆ octahedral tilting alters the activity for water oxidation through changing the covalency degree of Fe–O bonds

Tailoring the Co 3d‑O 2p Covalency in LaCoO₃ by Fe Substitution To Promote Oxygen Evolution Reaction

LaCoO₃ is an active, stable catalyst in alkaline solution for oxygen evolution reaction (OER). With lower cost, it is a potential alternative to precious metal oxides like IrO₂ and RuO₂ in water electrolysis. However, room still remains for improving its activity according to recent understandings of OER on perovskite oxides. In this work, Fe substitution has been introduced in LaCoO₃ to boost its OER performance. Density function theory (DFT) calculation verified that the enhanced performance originates from the enhanced Co 3d-O 2p covalency with 10 at% Fe substitution in LaCoO₃. Both DFT calculations and Superconducting Quantum Design (SQUID) magnetometer (MPMS-XL) showed a Co³⁺ spin state transition from generally low spin state (LS: t_2g⁶ e_g⁰, S = 0) to a higher spin state with the effect of 10 at% Fe substitution. X-ray absorption near-edge structure (XANES) supports DFT calculations on an insulator to half-metal transition with 10 at% Fe substitution, induced by spin state transition. The half-metallic LaCo_0.9Fe_0.1O₃ possesses increased overlap between Co 3d and O 2p states, which results in enhanced covalency and promoted OER performance. This finding enlightens a new way of tuning the metal–oxygen covalency in oxide catalysts for OER

Hybrid Nanomaterials with Single-Site Catalysts by Spatially Controllable Immobilization of Nickel Complexes <i>via</i> Photoclick Chemistry for Alkene Epoxidation

Catalyst deactivation is a persistent problem not only for the scientific community but also in industry. Isolated single-site heterogeneous catalysts have shown great promise to overcome these problems. Here, a versatile anchoring strategy for molecular complex immobilization on a broad range of semiconducting or insulating metal oxide (<i>e</i>.<i>g</i>., titanium dioxide, mesoporous silica, cerium oxide, and tungsten oxide) nanoparticles to synthesize isolated single-site catalysts has been studied systematically. An oxidatively stable anchoring group, maleimide, is shown to form covalent linkages with surface hydroxyl functionalities of metal oxide nanoparticles by photoclick chemistry. The nanocomposites have been thoroughly characterized by techniques including UV–visible diffuse reflectance spectroscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy, and X-ray absorption spectroscopy (XAS). The IR spectroscopic studies confirm the covalent linkages between the maleimide group and surface hydroxyl functionalities of the oxide nanoparticles. The hybrid nanomaterials function as highly efficient catalysts for essentially quantitative oxidations of terminal and internal alkenes and show molecular catalyst product selectivities even in more eco-friendly solvents. XAS studies verify the robustness of the catalysts after several catalytic cycles. We have applied the photoclick anchoring methodology to precisely control the deposition of a luminescent variant of our catalyst on the metal oxide nanoparticles. Overall, we demonstrate a general approach to use irradiation to anchor molecular complexes on oxide nanoparticles to create recyclable, hybrid, single-site catalysts that function with high selectivity in a broad range of solvents. We have achieved a facile, spatially and temporally controllable photoclick method that can potentially be extended to other ligands, catalysts, functional molecules, and surfaces

Hybrid Nanomaterials with Single-Site Catalysts by Spatially Controllable Immobilization of Nickel Complexes <i>via</i> Photoclick Chemistry for Alkene Epoxidation

Catalyst deactivation is a persistent problem not only for the scientific community but also in industry. Isolated single-site heterogeneous catalysts have shown great promise to overcome these problems. Here, a versatile anchoring strategy for molecular complex immobilization on a broad range of semiconducting or insulating metal oxide (<i>e</i>.<i>g</i>., titanium dioxide, mesoporous silica, cerium oxide, and tungsten oxide) nanoparticles to synthesize isolated single-site catalysts has been studied systematically. An oxidatively stable anchoring group, maleimide, is shown to form covalent linkages with surface hydroxyl functionalities of metal oxide nanoparticles by photoclick chemistry. The nanocomposites have been thoroughly characterized by techniques including UV–visible diffuse reflectance spectroscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy, and X-ray absorption spectroscopy (XAS). The IR spectroscopic studies confirm the covalent linkages between the maleimide group and surface hydroxyl functionalities of the oxide nanoparticles. The hybrid nanomaterials function as highly efficient catalysts for essentially quantitative oxidations of terminal and internal alkenes and show molecular catalyst product selectivities even in more eco-friendly solvents. XAS studies verify the robustness of the catalysts after several catalytic cycles. We have applied the photoclick anchoring methodology to precisely control the deposition of a luminescent variant of our catalyst on the metal oxide nanoparticles. Overall, we demonstrate a general approach to use irradiation to anchor molecular complexes on oxide nanoparticles to create recyclable, hybrid, single-site catalysts that function with high selectivity in a broad range of solvents. We have achieved a facile, spatially and temporally controllable photoclick method that can potentially be extended to other ligands, catalysts, functional molecules, and surfaces

Operando Investigation of Mn₃O_4+δ Co-catalyst on Fe₂O₃ Photoanode: Manganese-Valency-Determined Enhancement at Varied Potentials

The development of efficient catalysts containing earth-abundant elements for the oxygen evolution reaction (OER) in photoelectrochemical (PEC) systems is highly desired for low-cost energy storage and conversion. In this work, mesoporous α-Fe₂O₃ thin film photoanodes coated with manganese oxide (Mn₃O_4+δ) co-catalysts are prepared by a dip-coating method. The co-catalyst coating significantly enhances PEC water oxidation performance as compared with the uncoated α-Fe₂O₃. To understand the origin of this enhancement, in situ X-ray absorption spectroscopy is employed to monitor the valence state of Mn in the Mn₃O_4+δ co-catalyst as a function of applied potential. It is found that the enhancement of the photocurrent is governed by the Mn valency, and the most prominent enhancement takes place at the valency of ∼3.4+, which is due to the optimal e_g electron filling in Mn cations as the electrocatalyst for OER. Our investigation indicates that the contribution of Mn₃O_4+δ co-catalyst to OER kinetics is variable at different applied potentials

Activation of Surface Oxygen Sites in a Cobalt-Based Perovskite Model Catalyst for CO Oxidation

Anionic redox chemistry is becoming increasingly important in explaining the intristic catalytic behavior in transition-metal oxides and improving catalytic activity. However, it is a great challenge to activate lattice oxygen in noble-metal-free perovskites for obtaining active peroxide species. Here, we take La_0.4Sr_0.6CoO_3‑δ as a model catalyst and develop an anionic redox activity regulation method to activate lattice oxygen by tuning charge transfer between Co⁴⁺ and O^2–. Advanced XAS and XPS demonstrate that our method can effectively decrease electron density of surface oxygen sites (O^2–) to form more reactive oxygen species (O^2‑<i>x</i>), which reduces the activation energy barriers of molecular O₂ and leads to a very high CO catalytic activity. The revealing of the activation mechanism for surface oxygen sites in perovskites in this work opens up a new avenue to design efficient solid catalysts. Furthermore, we also establish a correlation between anionic redox chemistry and CO catalytic activity

Per un PORTALE del NAUTICAL HERITAGE

Introduzione all'approccio disciplinare in seno alle attività accademiche. L'autore dell'opera di cui alla prefazione in oggetto, cultore della specifica disciplina di valorizzazione del patrimonio nautico storico, fin dai suoi primi studi accademici