Ab initio study of changing the oxygen reduction activity of Co-Fe-based perovskites by tuning the B-site composition.

Perovskite oxides are promising low-cost and stable alternative electrocatalysts for the oxygen reduction reaction (ORR), relative to the precious metal-based electrocatalysts. Despite the experimental research on substituting various transition metals into the B-site of perovskite catalysts to improve the ORR performance, the detailed ORR mechanism due to the substitution process is rarely studied. In this paper, the ORR activity of La0.5Sr0.5CoxFe1-xO3 perovskites (x = 0, 0.25, 0.5, 0.75, and 1) is studied by density functional theory (DFT). The ORR mechanism in alkaline solution is theoretically examined as a function of the Co/Fe composition at different potentials. The substitution of Co for Fe at the B-site of the perovskites dramatically changes the theoretical overpotential and enhances the activity. The HOO* formation is the potential-determining step for all the Co/Fe compositions. In comparison with the other compositions, the Co0.5/Fe0.5 composition exhibits the lowest overpotential and bonding with the reaction intermediates moderately. Furthermore, the oxygen binding energy is correlated with the bulk oxygen p-band center relative to the Fermi level. Among all the Co/Fe compositions, the Co0.5/Fe0.5 composition shows neither too low nor too high oxygen p-band center value. These results provide deep insights into the ORR mechanism on B-site substituted perovskites and guidelines for the design of cost-effective and Pt-free electrocatalysts for oxygen reduction.

Disordered-Layer-Mediated Reverse Metal-Oxide Interactions for Enhanced Photocatalytic Water Splitting.

In heterogeneous catalysts, metal-oxide interactions occur spontaneously but often in an undesired way leading to the oxidation of metal nanoparticles. Manipulating such interactions to produce highly active surface of metal nanoparticles can warrant the optimal catalytic activity but has not been established to date. Here we report that a prior reduced TiO2 support can reverse the interaction with Pt nanoparticles and augment the metallic state of Pt, exhibiting a 3-fold increase in hydrogen production rate compared to that of conventional Pt/TiO2. Spatially resolved electron energy loss spectroscopy of the Ti valence state and the electron density distribution within Pt nanoparticles provide direct evidence supporting that the Pt/TiO2/H2O triple junctions are the most active catalytic sites for water reduction. Our reverse metal-oxide interaction scheme provides a breakthrough in the stagnated hydrogen production efficiency and can be applied to other heterogeneous catalyst systems composed of metal nanoparticles with reducible oxide supports.

Electronic structure, magnetic properties, and mixed valence character of Ce2 Ni3 Si5 from first principles calculations.

Cerium intermetallic compounds exhibit anomalous physical properties such as heavy fermion and Kondo behaviors. Here, an ab initio study of the electronic structure, magnetic properties, and mixed valence character of Ce2 Ni3 Si5 using density functional theory (DFT) is presented. Two theoretical methods, including pure Perdew-Burke-Ernzerhof (PBE) and PBE + U, are used. In this study, Ce3+ and Ce4+ are considered as two different constituents in the unit cell. The formation energy calculations on the DFT level propose that Ce is in a stable mixed valence of 3.379 at 0 K. The calculated electronic structure shows that Ce2 Ni3 Si5 is a metallic compound with a contribution at the Fermi level from Ce 4f and Ni 3d states. With the inclusion of the effective Hubbard parameter (Ueff ), the five valence electrons of 5 Ce3+ ions are distributed only on Ce3+ 4f orbitals. Therefore, the occupied Ce3+ 4f band is located in the valence band (VB) while Ce4+ 4f orbitals are empty and Located at the Fermi level. The calculated magnetic moment in Ce2 Ni3 Si5 is only due to cerium (Ce3+ ) in good agreement with the experimental results. The Ueff value of 5.4 eV provides a reasonable magnetic moment of 0.981 µB for the unpaired electron per Ce3+ ion. These results may serve as a guide for studying present mixed valence cerium-based compounds. © 2017 Wiley Periodicals, Inc.

Simple physical mixing of zeolite prevents sulfur deactivation of vanadia catalysts for NOx removal.

NOx abatement has been an indispensable part of environmental catalysis for decades. Selective catalytic reduction with ammonia using V2O5/TiO2 is an important technology for removing NOx emitted from industrial facilities. However, it has been a huge challenge for the catalyst to operate at low temperatures, because ammonium bisulfate (ABS) forms and causes deactivation by blocking the pores of the catalyst. Here, we report that physically mixed H-Y zeolite effectively protects vanadium active sites by trapping ABS in micropores. The mixed catalysts operate stably at a low temperature of 220 °C, which is below the dew point of ABS. The sulfur resistance of this system is fully maintained during repeated aging/regeneration cycles because the trapped ABS easily decomposes at 350 °C. Further investigations reveal that the pore structure and the amount of framework Al determined the trapping ability of various zeolites.

Origin of the enhanced photocatalytic activity of (Ni, Se, and B) mono- and co-doped anatase TiO2 materials under visible light: a hybrid DFT study.

The characteristic properties of TiO2 (anatase) make doping necessary to enhance its photocatalytic activity. Herein, a density functional theory (DFT) study using the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional was performed to precisely investigate the effect of mono- and co-doping (Ni, Se and B) on the structural, electronic and optical properties of anatase TiO2. Notably, the origin of the enhanced photocatalytic activity of the modified systems was determined. The response to visible light was enhanced for all the mono- and co-doped materials except for Bint, and the highest absorption coefficient was observed for Se4+ mono-doping and Se/Bint+sub and Ni/Bsub co-doping. The decrease in bandgap is associated with a red shift in the absorption edges with the smallest bandgap calculated for Ni/Bsub (2.49 eV). Additionally, the Ni, Se4+ and Se2- mono-doped systems and Ni/Se4+ co-doped systems are proposed as promising photocatalysts for water splitting applications and further experimental validation. Moreover, the Ni/Bint+sub and Se/Bint+sub co-doped materials can also be valuable photocatalysts for other energy applications due to their enhanced visible light activity and the prolonged lifetime of their produced charge carriers.

Engineering of Charged Defects at Perovskite Oxide Surfaces for Exceptionally Stable Solid Oxide Fuel Cell Electrodes.

Cation segregation, particularly Sr segregation, toward a perovskite surface has a significant effect on the performance degradation of a solid oxide cell (solid oxide electrolysis/fuel cell). Among the number of key reasons generating the instability of perovskite oxide, surface-accumulated positively charged defects (oxygen vacancy, Vo··) have been considered as the most crucial drivers in strongly attracting negatively charged defects (SrA - site') toward the surface. Herein, we demonstrate the effects of a heterointerface on the redistribution of both positively and negatively charged defects for a reduction of Vo·· at a perovskite surface. We took Sm0.5Sr0.5CoO3-δ (SSC) as a model perovskite film and coated Gd0.1Ce0.9O2-δ (GDC) additionally onto the SSC film to create a heterointerface (GDC/SSC), resulting in an ∼11-fold reduction in a degradation rate of ∼8% at 650 °C and ∼10-fold higher surface exchange (kq) than a bare SSC film after 150 h at 650 °C. Using X-ray photoelectron spectroscopy and electron energy loss spectroscopy, we revealed a decrease in positively charged defects of Vo·· and transferred electrons in an SSC film at the GDC/SSC heterointerface, resulting in a suppression of negatively charged Sr (SrSm') segregation. Finally, the energetic behavior, including the charge transfer phenomenon, O p-band center, and oxygen vacancy formation energy calculated using the density functional theory, verified the effects of the heterointerface on the redistribution of the charged defects, resulting in a remarkable impact on the stability of perovskite oxide at elevated temperatures.