Knowledge on the mechanisms of oxygen reduction reaction (ORR) and descriptors linking the catalytic activity to the structural and electronic properties of transition metal oxides enable rational ...design of more efficient catalysts. In this work ORR electrocatalysis was studied on a set of single and complex Mn(III) oxides with a rotating disc electrode method and cyclic voltammetry. We discovered an exponential increase of the specific electrocatalytic activity with the potential of the surface Mn(IV)/Mn(III) red-ox couple, suggesting the latter as a new descriptor for the ORR electrocatalysis. The observed dependence is rationalized using a simple mean-field kinetic model considering availability of the Mn(III) centers and adsorbate-adsorbate interactions. We demonstrate an unprecedented activity of Mn2O3, ca. 40 times exceeding that of MnOOH and correlate the catalytic activity of Mn oxides to their crystal structure.
In this work we analyze the applicability limits of the rotating ring-disk electrode (RRDE) technique for quantifying the amount of oxygen produced during the oxygen evolution reaction (OER). We ...utilize a state-of-the art IrO2 oxide as a carbon-free OER catalyst and La1−xSrxMn0.5Co0.5O3−δ, x = 0.25 and 0.5 perovskites, which are studied in the presence of carbon. RRDE experiments are performed at different IrO2 loadings under both potentiodynamic and potentiostatic modes. The experimental data allow us to formulate the requirements to the experimental conditions necessary to avoid underestimation of the oxygen yield.
Hydrothermal crystallization pathways of amorphous ceric phosphate gels were found to be determined by the ammonia concentration in a reaction medium. This allows for highly selective hydrothermal ...synthesis of various finely crystalline ceric phosphates, including Ce(PO4)(HPO4)0.5(H2O)0.5, (NH4)2Ce(PO4)2(H2O), and previously unknown NH4Ce2(PO4)3. The structure of the latter compound was solved from powder X‐ray diffraction data. It appeared to be isostructural to ammonium thorium phosphate, NH4Th2(PO4)3; in this crystal structure, large channels (5.07 × 3.79 Å) located along the c‐axis are occupied by NH4+ ions.
A selective hydrothermal synthesis was proposed for crystalline ceric phosphates Ce(PO4)(HPO4)0.5(H2O)0.5, (NH4)2Ce(PO4)2(H2O), and previously unknown NH4Ce2(PO4)3.
Three different perovskite-related phases were isolated in the SrGa1–x Sc x O2.5 system: Sr2GaScO5, Sr10Ga6Sc4O25, and SrGa0.75Sc0.25O2.5. Sr2GaScO5 (x = 0.5) crystallizes in a brownmillerite-type ...structure space group (S.G.) Icmm, a = 5.91048(5) Å, b = 15.1594(1) Å, and c = 5.70926(4) Å with complete ordering of Sc3+ and Ga3+ over octahedral and tetrahedral positions, respectively. The crystal structure of Sr10Ga6Sc4O25 (x = 0.4) was determined by the Monte Carlo method and refined using a combination of X-ray, neutron, and electron diffraction data S.G. I41/a, a = 17.517(1) Å, c = 32.830(3) Å. It represents a novel type of ordering of the B cations and oxygen vacancies in perovskites. The crystal structure of Sr10Ga6Sc4O25 can be described as a stacking of eight perovskite layers along the c axis ...−(Sc/Ga)O1.6–SrO0.8–(Sc/Ga)O1.8–SrO0.8−2... Similar to Sr2GaScO5, this structure features a complete ordering of the Sc3+ and Ga3+ cations over octahedral and tetrahedral positions, respectively, within each layer. A specific feature of the crystal structure of Sr10Ga6Sc4O25 is that one-third of the tetrahedra have one vertex not connected with other Sc/Ga cations. Further partial replacement of Sc3+ by Ga3+ leads to the formation of the cubic perovskite phase SrGa0.75Sc0.25O2.5 (x = 0.25) with a = 3.9817(4) Å. This compound incorporates water molecules in the structure forming SrGa0.75Sc0.25O2.5·xH2O hydrate, which exhibits a proton conductivity of ∼2.0 × 10–6 S/cm at 673 K.
Compounds Sr3−xPrxFe1.25Ni0.75O7−δ with 0 ≤ x ≤ 0.4 and Ruddlesden–Popper n = 2 type structures were synthesized and investigated by X-ray and neutron powder diffraction, thermogravimetry, and ...Mössbauer spectroscopy. Both samples, prepared at 1300 °C under N2(g) flow and samples subsequently air-annealed at 900 °C, were studied. The structures contained oxygen vacancies in the perovskite layers, and the Fe/Ni cations had an average coordination number less than six. The oxygen content was considerably higher for air-annealed samples than for samples prepared under N2, 7 − δ = ~6.6 and ~5.6 per formula unit, respectively. Mössbauer data collected at 7 K, below magnetic ordering temperatures, were consistent with X-ray powder diffraction (XRD) and neutron powder diffraction (NPD) results. The electrical conductivity was considerably higher for the air-annealed samples and was for x = 0.1~30 S·cm−1 at 500 °C. The thermal expansion coefficients were measured in air between room temperature and 900 °C and was found to be 20–24 ppm·K−1 overall.
We consider compositional and structural factors which can affect the activity of bixbyite α-Mn2O3 towards the oxygen reduction reaction (ORR) and the stability of this oxide in alkaline solution. We ...compare electrochemistry of undoped, Fe and Al-doped α-Mn2O3 with bixbyite structure and braunite Mn7SiO12 having bixbyite-related crystal structure, using the rotating disk electrode (RDE), the rotating ring-disk electrode (RRDE), and cyclic voltammetry (CV) techniques. All manganese oxides under study are stable in the potential range between the ORR onset and ca. 0.7 V vs. Reversible Hydrogen Electrode (RHE). It is found that any changes introduced in the bixbyite structure and/or composition of α-Mn2O3 lead to an activity drop in both the oxygen reduction and hydrogen peroxide reactions in this potential interval. For the hydrogen peroxide reduction reaction these modifications also result in a change in the nature of the rate-determining step. The obtained results confirm that due to its unique crystalline structure undoped α-Mn2O3 is the most ORR active (among currently available) Mn oxide catalyst and favor the assumption of the key role of the (111) surface of α-Mn2O3 in the very high activity of this material towards the ORR.
We report the voltammetric and soft X-ray absorption spectroscopy (XAS) data for two series of perovskite oxides, namely La1−ySryMn0.5Co0.5O3−δ with a constant Mn:Co ratio and variable extent of Sr ...doping, and Sr0.5Gd0.5MnxCo1−xO3−δ perovskite oxides with a constant Sr content but variable extent of the Co doping. The essential differences in the values and scan rate dependences of the total charges observed between the studied materials in alkaline solutions in the potential regions of their stability are interpreted on the basis of separation of the ‘fast’ and ‘slow’ recharging components. Our analysis of the total charge is based on two approaches, the first proposed by Trasatti, the second by Conway. The ‘slow’ charge is assigned to the oxygen intercalation in sub-surface oxide layers. We demonstrate correlations of the ‘fast’ and the ‘slow’ recharging components with the electronic state of Mn, Co and O determined from spectroscopic data. We also present the arguments in favor of possible contribution of the interfacial oxygen vacancies to the ‘fast’ interfacial recharging. These findings may be useful for understanding correlations of the composition and stoichiometry of the oxides on the one hand, and their electrocatalytic activity on the other.
Transition metal oxides are attractive noble metal-free catalysts of the oxygen reduction for application at the cathode of alkaline membrane fuel cells or metal-air batteries. However, despite of a ...rapidly increasing number of publications devoted to the oxygen electrocatalysis on transition metal oxides, a clear picture regarding the relations between their structure and composition on the one hand and electrocatalytic activity on the other hand is lacking. This short review discusses challenges facing researchers seeking to understand electrocatalysis of the oxygen reduction reaction on transition metal oxides.
•Factors affecting ORR activities of transition metal oxides are briefly reviewed.•Guidelines are provided on how to avoid pitfalls in establishing structure–activity relations.•Influence of carbon on the electrocatalytic activity is highlighted.•Oxygen reduction pathways and mechanisms on transition metal oxides are analyzed.
The influence of different synthesis routes on the structure and luminescent properties of KTb(MoO4)2 (KTMO) was studied. KTMO samples were prepared by solid-state, hydrothermal, and Czochralski ...techniques. These methods lead to the following different crystal structures: a triclinic scheelite-type α-phase is the result for the solid-state method, and an orthorhombic KY(MoO4)2-type γ-phase is the result for the hydrothermal and Czochralski techniques. The triclinic α-KTMO phase transforms into the orthorhombic γ-phase when heated at 1273 K above the melting point, while KTMO prepared by the hydrothermal method does not show phase transitions. The influence of treatment conditions on the average crystallite size of orthorhombic KTMO was revealed by X-ray diffraction line broadening measurements. The electrical conductivity was measured on KTMO single crystals. The orthorhombic structure of KTMO that was prepared by the hydrothermal method was refined using synchrotron powder X-ray diffraction data. K+ cations are located in extensive two-dimensional channels along the c-axis and the a-axis. The possibility of K+ migration inside these channels was confirmed by electrical conductivity measurements, where strong anisotropy was observed in different crystallographic directions. The evolution of luminescent properties as a result of synthesis routes and heating and cooling conditions was studied and compared with data for the average crystallite size calculation and the grain size determination. All samples’ emission spectra exhibit a strong green emission at 545 nm due to the 5D4 → 7F5 Tb3+ transition. The maximum of the integral intensity emission for the 5D4 → 7F5 emission under λex = 380 nm excitation was found for the KTMO crashed single crystal.
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