Iron-Induced Activation of Ordered Mesoporous Nickel Cobalt Oxide Electrocatalyst for the Oxygen Evolution Reaction

Herein, ordered mesoporous nickel cobalt oxides prepared by the nanocasting route are reported as highly active oxygen evolution reaction (OER) catalysts. By using the ordered mesoporous structure as a model system and afterward elevating the optimal catalysts composition, it is shown that, with a simple electrochemical activation step, the performance of nickel cobalt oxide can be significantly enhanced. The electrochemical impedance spectroscopy results indicated that charge transfer resistance increases for Co₃O₄ spinel after an activation process, while this value drops for NiO and especially for CoNi mixed oxide significantly, which confirms the improvement of oxygen evolution kinetics. The catalyst with the optimal composition (Co/Ni 4/1) reaches a current density of 10 mA/cm² with an overpotential of a mere 336 mV and a Tafel slope of 36 mV/dec, outperforming benchmarked and other reported Ni/Co-based OER electrocatalysts. The catalyst also demonstrates outstanding durability for 14 h and maintained the ordered mesoporous structure. The cyclic voltammograms along with the electrochemical measurements in Fe-free KOH electrolyte suggest that the activity boost is attributed to the generation of surface Ni­(OH)₂ species that incorporate Fe impurities from the electrolyte. The incorporation of Fe into the structure is also confirmed by inductively coupled plasma optical emission spectrometry

Synthesis, Crystal Structures, and Hydrogen-Storage Properties of Eu(AlH₄)₂ and Sr(AlH₄)₂ and of Their Decomposition Intermediates, EuAlH₅ and SrAlH₅

Complex Eu­(AlH₄)₂ and Sr­(AlH₄)₂ hydrides have been prepared by a mechanochemical metathesis reaction from NaAlH₄ and europium or strontium chlorides. The crystal structures were solved from powder X-ray diffraction data in combination with solid-state ²⁷Al NMR spectroscopy. The thermolysis pathway was analyzed in detail, allowing identification of new intermediate EuAlH₅/SrAlH₅ compounds. Rehydrogenation experiments indicate that the second decomposition step is reversible

Design of Ordered Mesoporous Composite Materials and Their Electrocatalytic Activities for Water Oxidation

The controlled synthesis of a series of ordered mesoporous composite materials via solid–solid reaction of ordered mesoporous Co₃O₄ with various transition metal precursors is reported. This versatile methodology allows preparation of a range of composites with precisely controllable material compositions. The textural parameters of the heterostructured compounds are highly dependent on the oxidation state of the dopant. Electrocatalytic activities of the prepared materials were investigated as oxygen evolution catalysts for the electrolysis of water. Among the ordered mesoporous composite materials, Co₃O₄–CuCo₂O₄ shows a significant enhancement for electro-catalytic water splitting with a lower onset potential and higher current density. Following these results, a series of ordered mesoporous composite materials based on cobalt and copper oxides with different atomic ratios were prepared through a nanocasting route. Enhanced electrocatalytic performance was obtained for all composite samples in comparison with Co₃O₄

The Mechanism and Pathway of Selective Partial Oxidation of <i>n</i>‑Butane to Maleic Anhydride Studied on Titanium Phosphate Catalysts

The partial selective oxidation of n-butane to maleic anhydride with molecular oxygen is commercially well-established and strongly associated with the vanadium phosphorus oxide (VPO) catalyst. We report that also titanium phosphate (TiPO) exhibits the rare feature of accomplishing the most demanding complex selective oxidation reaction industrially applied. A facile molten salt method was used to prepare TiPO catalysts from mixtures of (NH4)2HPO4 and TiO2 (P25). In a continuous flow process under industrially relevant conditions with TiPO, conversions above 50% resulted in 20% overall selectivity for maleic anhydride with 90% oxygenate selectivity. Due to a high tendency to total oxidation (>60%), the performance of TiPO catalysts cannot yet compete with the industrial VPO catalyst. However, herein we want to highlight our studies on the reaction pathway and mechanism for the complex multistep conversion of n-butane to maleic anhydride, which is still under debate for the VPO catalyst after more than four decades of research. A complete chain of reaction intermediates was identified via online mass spectroscopy, under industrially relevant conditions, and in pulse experiments, including the consecutive formation of 2-butene, 1,3-butadiene, furan, and 2-furanone as C4 intermediates. Cyclic pulse experiments complemented with EPR measurements revealed a combined mechanism involving carbocation chemistry via Brønsted acid sites and a redox mechanism according to Mars van Krevelen

Size-Controlled Synthesis and Microstructure Investigation of Co₃O₄ Nanoparticles for Low-Temperature CO Oxidation

Noble-metal-free functional oxides are active catalysts for CO oxidation at low temperatures. Spinel-type cobalt oxide (Co₃O₄) nanoparticles can be easily synthesized by impregnation of activated carbon with concentrated cobalt nitrate and successive carbon burn off. Mean size and particle size distribution can be tuned by adding small amounts of silica to the carbon precursor, as witnessed by whole powder pattern modeling of the X-ray powder diffraction data. The catalytic tests performed after silica removal show a significant influence of the mean domain size and of size distribution on the CO oxidation activity of the individual Co₃O₄ specimens, whereas defects play a less important role in the present case

Highly Ordered Mesoporous Cobalt-Containing Oxides: Structure, Catalytic Properties, and Active Sites in Oxidation of Carbon Monoxide

Co₃O₄ with a spinel structure is a very active oxide catalyst for the oxidation of CO. In such catalysts, octahedrally coordinated Co³⁺ is considered to be the active site, while tetrahedrally coordinated Co²⁺ is assumed to be basically inactive. In this study, a highly ordered mesoporous CoO has been prepared by H₂ reduction of nanocast Co₃O₄ at low temperature (250 °C). The as-prepared CoO material, which has a rock-salt structure with a single Co²⁺ octahedrally coordinated by lattice oxygen in <i>Fm</i>3̅<i>m</i> symmetry, exhibited unexpectedly high activity for CO oxidation. Careful investigation of the catalytic behavior of mesoporous CoO catalyst led to the conclusion that the oxidation of surface Co²⁺ to Co³⁺ causes the high activity. Other mesoporous spinels (CuCo₂O₄, CoCr₂O₄, and CoFe₂O₄) with different Co species substituted with non/low-active metal ions were also synthesized to investigate the catalytically active site of cobalt-based catalysts. The results show that not only is the octahedrally coordinated Co³⁺ highly active but also the octahedrally coordinated Co²⁺ species in CoFe₂O₄ with an inverse spinel structure shows some activity. These results suggest that the octahedrally coordinated Co²⁺ species is easily oxidized and shows high catalytic activity for CO oxidation

Doping of Nanostructured Co₃O₄ with Cr, Mn, Fe, Ni, and Cu for the Selective Oxidation of 2‑Propanol

A series of transition-metal-substituted (M = Cr, Mn, Fe, Ni, Cu) ordered mesoporous cobalt oxide catalysts were synthesized via nanocasting method using KIT-6 silica as a hard template. While the pristine Co3O4 formed as a perfect replication of KIT-6, metal substitution resulted in less ordered and smaller domains of the replica oxides. The catalysts were applied in the selective oxidation of 2-propanol in the gas phase to reveal the role of the systematic metal substitution. Cu and Ni substitutions were found to be beneficial for the catalytic activity, while Cr, Mn, and Fe substitutions were detrimental. Cofeeding water vapor shifted the onset temperature of 2-propanol conversion to higher temperatures (ΔT = 10–20 K), while a beneficial effect was observed at high temperatures (>260 °C) decreasing deactivation by slowing the reduction of active Co3+ and/or reducing coke deposition. The activity scaled with the reducibility of the catalysts probed by H2 temperature-programmed reduction with the positive effect of a higher reducibility, indicating the crucial role of oxygen activation during 2-propanol oxidation at the gas–solid interface. 2-Propanol activation probed by adsorption/desorption experiments monitored by diffuse reflectance infrared Fourier transform spectroscopy showed a weakening of the interaction and changing of the adsorption mode from dissociative to molecular adsorption following the periodic table from Cr to Cu, suggesting that the activation of 2-propanol plays a minor role compared with oxygen activation. Fe-substituted Co3O4 was the least active catalyst due to the decrease of the number of active Co3+ sites

Ultrastructure and Surface Composition of Glutathione-Terminated Ultrasmall Silver, Gold, Platinum, and Alloyed Silver–Platinum Nanoparticles (2 nm)

Alloyed ultrasmall silver–platinum nanoparticles (molar ratio Ag:Pt = 50:50) were prepared and compared to pure silver, platinum, and gold nanoparticles, all with a metallic core diameter of 2 nm. They were surface-stabilized by a layer of glutathione (GSH). A comprehensive characterization by high-resolution transmission electron microscopy (HRTEM), electron diffraction (ED), X-ray diffraction (XRD), small-angle X-ray scattering (SAXS), differential centrifugal sedimentation (DCS), and UV spectroscopy showed their size both in the dry and in the water-dispersed state (hydrodynamic diameter). Solution NMR spectroscopy (1H, 13C, COSY, HSQC, HMBC, and DOSY) showed the nature of the glutathione shell including the number of GSH ligands on each nanoparticle (about 200 with a molecular footprint of 0.063 nm2 each). It furthermore showed that there are at least two different positions for the GSH ligand on the gold nanoparticle surface. Platinum strongly reduced the resolution of the NMR spectra compared to silver and gold, also in the alloyed nanoparticles. X-ray photoelectron spectroscopy (XPS) showed that silver, platinum, and silver–platinum particles were at least partially oxidized to Ag(+I) and Pt(+II), whereas the gold nanoparticles showed no sign of oxidation. Platinum and gold nanoparticles were well crystalline but twinned (fcc lattice) despite the small particle size. Silver was crystalline in electron diffraction but not in X-ray diffraction. Alloyed silver–platinum nanoparticles were almost fully amorphous by both methods, indicating a considerable internal disorder

<i>In Situ</i> X‑ray Diffraction Study of Co–Al Nanocomposites as Catalysts for Ammonia Decomposition

Co–Al nanocomposite materials as active and stable catalysts for ammonia decomposition have been synthesized by a one-pot evaporation-induced self-assembly method. The catalysts were characterized by various techniques including powder X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), X-ray photoelectron spectroscopy (XPS), N₂ adsorption/desorption, and transmission/scanning electron microscopy (TEM/SEM). Especially, <i>in situ</i> XRD under catalytic reaction conditions was performed, and metallic Co with a cubic structure was identified to be most probably the active crystalline phase for the decomposition of ammonia; also, contribution of CoO to the catalytic activity cannot be excluded. Most importantly, the introduction of alumina can significantly suppress the agglomeration of the active metallic Co phase and thus maintain the high activity of the cobalt catalyst