Crystalline Co 3 O 4 nanoparticles with a uniform size of 9 nm as shown by Xray diffraction (XRD) and transmission electron microscopy (TEM) were synthesized by thermal decomposition of cobalt acetylacetonate in oleyl amine and applied in the oxidation of 2-propanol after calcination. The catalytic properties were derived under continuous flow conditions as function of temperature up to 573 K in a fixed-bed reactor at atmospheric pressure. Temperature-programmed oxidation, desorption (TPD), surface reaction (TPSR) and 2-propanol decomposition experiments were performed to study the interaction of 2-propanol and O 2 with the exposed spinel surfaces. Co 3 O 4 selectively catalyzes the oxidative dehydrogenation of 2-propanol yielding acetone and H 2 O and only to a minor extent the total oxidation to CO 2 and H 2 O at higher temperatures. The high catalytic activity of Co 3 O 4 reaching nearly full conversion with 100% selectivity to acetone at 440 K is attributed to the high amount of active Co 3+ species at the catalyst surface as well as surface-bound reactive oxygen species observed in the O 2 TPD, 2-propanol TPD, TPSR, and 2-propanol decomposition experiments. Density functional theory calculations with a Hubbard U term support the identification of fivefold coordinated octahedral surface as the active site, Co 3 + 5c and oxidative dehydrogenation involving adsorbed atomic oxygen was found to be the energetically most favored pathway. The consumption of surface oxygen and reduction of

Using density functional theory calculations with an on-site Hubbard term (DFT+U), we explore the effect of surface termination and cation substitution on the performance of the Co x Ni1–x Fe2O4(001) surface (x = 0.0, 0.5, 1.0) as an anode material in the oxygen evolution reaction (OER). Different reaction sites (Fe, Co, Ni, and an oxygen vacancy) were investigated at three terminations: the B-layer with octahedrally coordinated Co/Ni and with an additional half and full monolayer of Fe (0.5A and A-layer, respectively). Ni substitution with an equal concentration of Co and Ni (x = 0.5) reduces the overpotential over the end members for the majority of reaction sites. Surface Co cations are identified as the active sites and the ones at the A-layer termination for x = 0.5 exhibit one of the lowest theoretically reported overpotentials of 0.26 V. The effect of the additional iron layer on the active site modification is 2-fold: analysis of the electronic properties and spin densities indicates that the additional Fe layer stabilizes a bulk-like oxidation state of +2 for Co and Ni at the A-layer termination, whereas at the B-layer termination, they are oxidized to 3+. Moreover, the unusual relaxation pattern enables the formation of a hydrogen bond of the OOH intermediate to a neighboring surface oxygen that lowers the reaction free energy of this formerly rate-limiting step, leading to a deviation from the scaling relationship and almost equidistant reaction free-energy steps of intermediates. This renders an example of how a selective surface modification can result in a significant improvement of OER performance.

Sub-10 nm CoFe₂O₄ nanoparticles with different sizes and various compositions obtained by (partial) substitution of Co with Ni cations have been synthesized by using a one-pot method from organic solutions by the decomposition of metal acetylacetonates in the presence of oleylamine. The electrocatalytic activity of CoFe₂O₄ towards the oxygen evolution reaction (OER) is clearly enhanced with a smaller size (3.1 nm) of the CoFe₂O₄ nanoparticles (compared with 4.5 and 5.9 nm). In addition, the catalytic activity is improved by partial substitution of Co with Ni, which also leads to a higher degree of inversion of the spinel structure. Theoretical calculations attribute the positive catalytic effect of Ni owing to the lower binding energy differences between adsorbed O and OH compared with pure cobalt or nickel ferrites, resulting in higher OER activity. Co_0.5Ni_0.5Fe₂O₄ exhibited a low overpotential of approximately 340 mV at 10 mAcm^-2, a smaller Tafel slope of 51 mVdec^-1, and stability over 30 h. The unique tunability of these CoFe₂O₄ nanocrystals provides great potential for their application as an efficient and competitive anode material in the field of electrochemical water splitting as well as for systematic fundamental studies aiming at understanding the correlation of composition and structure with performance in electrocatalysis

Using density functional theory calculations with an on-site Hubbard U term, we study the oxygen evolution reaction (OER) at the Co 3 O 4 (001) surface. The stability of different surface terminations as a function of oxygen partial pressure as well as pH and applied voltage is compiled in a Pourbaix diagram. The termination with octahedral Co and O ions (B-layer) is found to have the lowest overpotential of 0.46 V for an octahedral Co reaction site, associated with its p-type conducting character and the higher oxidation state of the active site (+4) during OER. Furthermore, we systematically investigated the effect of Fe and Ni doping on the overpotential. Our results indicate that Ni doping at an octahedral site in the surface layer reduces the overpotential from 0.46 to 0.34 V. Likewise, Fe doping at an octahedral site at the tetrahedral Co termination (A-layer) lowers η from 0.63 to 0.37 V with octahedral Co remaining in the active site. We note that the potential determining step changes from *OH (B-layer) to *OOH formation (A-layer). While implicit solvation increases the overpotential by 0.2 V (B-layer) and 0.4 V (A-layer), which is attributed to enhanced binding energies of the intermediates, the general trends with respect to doping remain unchanged. The scaling relationship of the binding energies of *OOH and *OH is overall satisfied, with the doped systems lying close to the top of the volcano plot of the overpotential versus (ΔG *O b − ΔG *OH b). A further insight into the origin of this behavior is gained by analyzing the changes in oxidation states of surface ions and, in particular, the Co active site during OER.

Cation substitution in transition metal oxides is an important approach to improve electrocatalysts by the optimization of their composition. Herein, we report on phase-pure spinel-type CoV2-xFexO4 nanoparticles with 0 ≤ x ≤ 2 as a new class of bi-functional catalysts for the oxygen evolution (OER) and oxygen reduction reactions (ORR). The mixed-metal oxide catalysts exhibit high catalytic activity for both OER and ORR that strongly depends on the V and Fe content. CoV2O4 is known to exhibit a high conductivity, while in CoFe2O4 the cobalt cation distribution is expected to change due to the inversion of the spinel structure. The optimised catalyst, CoV1.5Fe0.5O4, shows an overpotential for OER of ~300 mV for 10 mA cm-2 with a Tafel slope of 38 mV dec-1 in alkaline electrolyte. DFT+U+SOC calculations on cation ordering confirm the tendency towards the inverse spinel structure with increasing Fe concentration in CoV2-xFexO4 that starts to dominate already at low Fe contents. The theoretical results also show that the variation of oxidation states are related to the surface region, where the redox activity was found experimentally to be manifested in the transformation of V3+ → V2+. The high catalytic activity, facile synthesis, and low cost of the CoV2-xFexO4 nanoparticles render them very promising for application in bifunctional electrocatalysis

The influence of nanoscale on the formation of metastable phases is an important aspect of nanostructuring that can lead to the discovery of unusual material compositions. Here, the synthesis, structural characterization, and electrochemical performance of Ni/Co mixed oxide nanocrystals in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is reported and the influence of nanoscaling on their composition and solubility range is investigated. Using a solvothermal synthesis in tert-butanol ultrasmall crystalline and highly dispersible Ni x Co 1−x O nanoparticles with rock salt type structure are obtained. The mixed oxides feature non-equilibrium phases with unusual miscibility in the whole composition range, which is attributed to a stabilizing effect of the nanoscale combined with kinetic control of particle formation. Substitutional incorporation of Co and Ni atoms into the rock salt lattice has a remarkable effect on the formal potentials of NiO oxidation that shift continuously to lower values with increasing Co content. This can be related to a monotonic reduction of the work function of (001) and (111)-oriented surfaces with an increase in Co content, as obtained from density functional theory (DFT+U) calculations. Furthermore, the electrocatalytic performance of the Ni x Co 1−x O nanoparticles in water splitting changes significantly. OER activity continuously increases with increasing Ni contents, while HER activity shows an opposite trend, increasing for higher Co contents. The high electrocatalytic activity and tunable performance of the nonequilibrium Ni x Co 1−x O nanoparticles in HER and OER demonstrate great potential in the design of electrocatalysts for overall water splitting.

The beneficial effects of Sn(IV) as a dopant in ultrathin hematite (α‐Fe2O3) photoanodes for water oxidation are examined. Different Sn concentration profiles are prepared by alternating atomic layer deposition of Fe2O3 and SnO x . Combined data from spectrophotometry and intensity‐modulated photocurrent spectroscopy yields the individual process efficiencies for light harvesting, charge separation, and charge transfer. The best performing photoanodes are Sn‐doped both on the surface and in the subsurface region and benefit from enhanced charge separation and transfer. Sn‐doping throughout the bulk of the hematite photoanode causes segregation at the grain boundaries and hence a lower overall efficiency. Fe2O3 (0001) and terminations, shown to be dominant by microstructural analysis, are investigated by density functional theory (DFT) calculations. The energetics of surface intermediates during the oxygen evolution reaction (OER) reveal that while Sn‐doping decreases the overpotential on the (0001) surface, the Fe2O3 orientation shows one of the lowest overpotentials reported for hematite so far. Electronic structure calculations demonstrate that Sn‐doping on the surface also enhances the charge transfer efficiency by elimination of surface hole trap states (passivation) and that subsurface Sn‐doping introduces a gradient of the band edges that reinforces the band bending at the semiconductor/electrolyte interface and thus boosts charge separation.

Co3O4 nanospheres with a mean diameter of 19 nm were applied in the selective oxidation of 2-propanol to acetone in the gas phase. Compared with 9 nm spheres, the 19...