• [Show abstract] [Hide abstract]
    ABSTRACT: Rare earth and alkaline earth metal perovskites with general formula ABO3 have attracted much attention as electrocatalysts for state-of-the-art fuel cells, and catalysts for hydrogen generation and hydrocarbons oxidation. Tuning the ion conductivity through doping A and B and subsequent formation of oxygen vacancies is essential for the performance of perovskites materials. To provide insights into factors that affect stability of oxygen vacancies and understand the origin of the activity of doped perovskite materials, we investigate the structure and energetics of cubic ABO3 perovskites (A = La and/or Be, Mg, Ca, Sr, Ba; B = Ti, V, Cr, Mn, Fe, Co, and Ni) using density functional theory calculations. It is found that the lattice constant of ABO3 generally increases as the ionic radius of A and B; the bulk formation energy of ABO3 is decomposed into the ionization energy and lattice energy, which depend on the ionic radius and valence. The trend of bulk formation energy corresponds to that of ionization energy at a given ionic valence, while corresponds to that of lattice energy as doping La by alkali earth metals with lower valence. There exists a good linear relationship between the bulk formation energy and oxygen vacancy formation energy. This work provides an understanding toward the origin of the activity of perovskites at the atomic level.
    Journal of Materials Science 12/2014; 50(4). DOI:10.1007/s10853-014-8731-0 · 2.31 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Abstract Trends in hydrodesulfurization (HDS) activity are investigated on the basis of surface properties calculated by density functional theory for a series of HDS catalysts. It is shown that approximately linear correlations exist between HS group binding energies and activation barriers of key elementary reactions in HDS of thiophene. These linear correlations are used to develop a simple kinetic model, which qualitatively describes experimental trends in activity. The kinetic model identifies the HS-binding energy as a descriptor of HDS activity. This insight contributes to understanding the effect of promotion and structure–activity relationships. Graphical Abstract
    Catalysis Letters 08/2014; 144(8):1425-1432. DOI:10.1007/s10562-014-1279-4 · 2.29 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The role of Cu-ion doping in alpha-MnO2 electrocatalysts for the oxygen reduction reaction in alkaline electrolyte was investigated. Cu-doped alpha-MnO2 nanowires (Cu-alpha-MnO2) were prepared with varying amounts (up to similar to 3%) of Cu2+ using a hydrothermal method. The electrocatalytic data indicate that Cu-alpha-MnO2 nanowires have up to 74% higher terminal current densities, 2.5 times enhanced kinetic rate constants, and 66% lower charge transfer resistances that trend with Cu content, exceeding values attained by alpha-MnO2 alone. The observed improvement in catalytic behavior correlates with an increase in Mn3+ content at the surface of the Cu-alpha-MnO2 nanowires. The Mn3+/Mn4+ couple is the mediator for the rate-limiting redox-driven O-2/OH- exchange. O-2 adsorbs via an axial site (the e(g) orbital on the Mn3+ d(4) ion) at the surface or at edge defects of the nanowire, and the increase in covalent nature of the nanowire with Cu-ion doping leads to stabilization of O-2 adsorbates and faster rates of reduction. A smaller crystallite size (roughly half) for Cu-alpha-MnO2 leading to a higher density of (catalytic) edge defect sites was also observed. This work is applicable to other manganese oxide electrocatalysts and shows for the first time there is a correlation for manganese oxides between electrocatalytic activity for the oxygen reduction reaction (ORR) in alkaline electrolyte and an increase in Mn3+ character at the surface of the oxide.
    The Journal of Physical Chemistry C 08/2014; 118(31):17342-17350. DOI:10.1021/jp5039865 · 4.84 Impact Factor