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Now showing items 1 - 16 of 30

  • Adsorption of CH4 and SO2 on Unsupported Pd1-xMxO(101) Surface

    Arevalo, Ryan Lacdao   Aspera, Susan Menez   Otadoy, Roland Emerito   Nakanishi, Hiroshi   Kasai, Hideaki  

    PdO is known to efficiently catalyze the oxidation of methane but suffers tremendously from sulfur poisoning that lowers its catalytic activity. In this paper, dispersion-corrected density functional theory based first principles calculations were performed to systematically screen the metal impurities M (where M is a transition metal) on a Pd1-xMxO catalyst that promote the desired adsorption energies for CH4 and SO2 to gain insights into the design of sulfation-resistant PdO-based methane oxidation catalysts. Specific Pd1-xMxO(101) catalyst was identified to thermodynamically avoid surface sulfation while maintaining the active sites for methane activation at typical experimental conditions. Results indicate a potential route of tuning the catalytic property of PdO by the introduction of a surface metal impurity. Graphic
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  • DFT-Guided Design of Catalysts for Methane Activation

    Arevalo, Ryan Lacdao   Aspera, Susan Menez   Nakanishi, Hiroshi   Kasai, Hideaki  

    Methane activation is one of the most important industrial processes in a modern-day society as it plays a key role in the production of syngas that is used to make a wide spectrum of hydrocarbons and alcohols that sustain the energy and chemical needs of humankind. Using density functional theory calculations, we identified the electronic properties governing the high stability of atomic carbon on stepped Ni surface to address the carbon formation or "coking" reaction that deactivates the catalyst. Results show that atomic carbon adsorption is uniquely characterized by a 5-coordinated bonding with Ni atoms from both the surface and subsurface layers of stepped Ni surface. Interestingly, we found that substituting the specific subsurface Ni atoms with other elements can dramatically change the reaction mechanism of methane decomposition on the surface, suggesting a new approach to catalyst design for hydrocarbon reforming applications. In this proceeding, the other possible models for modifying Ni surface that utilizes this idea are presented.
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  • Adsorption of Carbohydrazide on Au(111) and Au3Ni(111) Surfaces

    Arevalo, Ryan Lacdao   Aspera, Susan Me?ez   Nakanishi, Hiroshi   Kasai, Hideaki   Yamaguchi, Susumu   Asazawa, Koichiro  

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  • A Theoretical Study on the Adsorption of CO2 on CuO(110) Surface

    Moreno, Joaquin Lorenzo Valmoria   Arevalo, Ryan Lacdao   Escano, Mary Clare Sison   Padama, Allan Abraham Bustria   Kasai, Hideaki  

    The adsorption of CO2 on CuO(110) was investigated using density functional theory calculations. The CO2 molecule adsorbs on top of an unsaturated Cu atom with a titled configuration. The low adsorption energy and minimal charge transfer confirm the physisorption character of the adsorption process. Unlike pure copper, the more reactive behavior towards CO2 of copper oxides makes them useful for applications such as the photocatalytic reduction of CO2.
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  • Reactivity Descriptors for Borohydride Interaction with Metal Surfaces

    Escano, Mary Clare Sison   Gyenge, Elod   Arevalo, Ryan Lacdao   Kasai, Hideaki  

    First-principles density functional theory calculations were performed to study the adsorption of borohydride (BH(4)(-)) on close-packed transition-metal surfaces, M(111) (M = Au, Pt, Ir, Os, Ag, Pd, Rh, Ru). A correlation between the relative adsorption energies of BH(4ad) and the d-band center of the metals is established. In terms of the adsorbate configuration, both molecular (BH(4ad)) and dissociated (BH(4y,ad) + yH(ad), y = 1,...,3) structures are possible regardless of the adsorption energy value. On Os, Rh, and Ru surfaces, molecular (i.e., undissociative) adsorption is preferred despite the strong surface binding energy of BH(4ad). Orbital-specific analysis of the bonding, points to the role of the d(zz) and d(yz) states of the surface metal atoms in determining the final BH(4ad) configuration on all metals. However, in the presence of H(2)O molecules, the preference for strong molecular adsorption may be lost because of BH(4ad)-H(2)O(ad) interaction. Using the coadsorption on Os(111) of BH(4ad) and H(2)O(ad) with and without the presence of Had (generated either by electrosorption of OH(-) or dissociative water adsorption), the origins of the adsorbate adsorbate and adsorbate metal interactions are discussed. Electronic factors to predict the BH(4ad) conformation on metal catalysts in water environment are proposed.
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  • Pt(111)-Alloy Surfaces for Non-Activated OOH Dissociation

    Cahyanto, Wahyu Tri   Escaño, Mary Clare   Kasai, Hideaki   Arevalo, Ryan Lacdao  

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  • Ru-Catalyzed Steam Methane Reforming:Mechanistic Study from First Principles Calculations

    Arevalo, Ryan Lacdao   Aspera, Susan Menez   Escano, Mary Clare Sison   Nakanishi, Hiroshi   Kasai, Hideaki  

    Elucidating the reaction mechanism of steam methane reforming (SMR) is imperative for the rational design of catalysts for efficient hydrogen production. In this paper, we provide mechanistic insights into SMR on Ru surface using first principles calculations based on dispersion-corrected density functional theory. Methane activation (i. e., C-H bond cleavage) was found to proceed via a thermodynamically exothermic dissociative adsorption process, resulting in (CHy + zH)* species ("*" denotes a surface-bound state, and y + z =3D 4), with C* and CH* being the most stable adsorbates. The calculation of activation barriers suggests that the conversion of C* into O-containing species via C-O bond formation is kinetically slow, indicating that the surface reaction of carbon intermediates with oxygen is a possible rate-determining step. The results suggest the importance of subsequent elementary reactions following methane activation in determining the formation of stable carbon structures on the surface that deactivates the catalyst or the conversion of carbon into O-containing species.
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  • Computational Mechanistic Study of Borohydride Electrochemical Oxidation on Au3Ni(111)

    Arevalo, Ryan Lacdao   Escano, Mary Clare Sison   Kasai, Hideaki  

    Au-3d metal alloys have recently gained interest for use as anode catalysts for direct borohydride fuel cells since these are less expensive than pure Au and exhibit desirable properties for borohydride oxidation. In this paper, a mechanistic study on the electrochemical oxidation of borohydride on Au3Ni(111) using first principles calculations based on spin-polarized density functional theory is presented. A reaction energy diagram showing the free energies of possible elementary surface-bound species on Au3Ni(111) as a function of electrode potential is constructed to show the favorable reaction path for a complete eight-electron oxidation of borohydride. As compared to pure Au, the adsorption of borohydride is favorable on Au3Ni(111) at lower potential due to the greater stability of borohydride on this surface, which is attributed to the upshift of the derived antibonding states of the BH4-sp and Au3Ni-d interaction with respect to pure Au. At a potential of -0.44 V vs NHE and T = 300 K, all subsequent elementary reaction steps for the complete oxidation of borohydride are downhill in energy with B(OH)(3,ads) as the highly favored final adsorbed species. The overall oxidation is limited by the initial adsorption of borohydride on the surface since it requires the highest electrode potential requirement among all electrochemical steps considered. Calculation of the dehydrogenation barrier of borohydride provides significant evidence that a more favorable elementary reaction on Au3Ni(111) as compared to Au(111) also leads to a lower reaction barrier.
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  • Ru-Catalyzed Steam Methane Reforming: Mechanistic Study from First Principles Calculations

    Arevalo, Ryan Lacdao   Aspera, Susan Me?ez   Sison Esca?o, Mary Clare   Nakanishi, Hiroshi   Kasai, Hideaki  

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  • Branching Reaction in Melanogenesis:The Effect of Intramolecular Cyclization on Thiol Binding

    Kishida, Ryo   Kasai, Hideaki   Aspera, Susan Menez   Arevalo, Ryan Lacdao   Nakanishi, Hiroshi  

    With the aid of density functional theory-based first principles calculations, we investigated energetics and electronic structure changes in reactions involving dopaquinone to give insights into the branching behaviors in melanogenesis. The reactions we investigated are the intramolecular cyclization and thiol binding, which are competing with each other. It was found that, in order to accomplish thiol binding, charge transfer of around one electron from thiol to dopaquinone occurs. Furthermore, intramolecular cyclization of dopaquinone increases the lowest unnoccupied molecular orbital level substantially. This result clearly shows prevention of the binding of thiol by intramolecular cyclization.
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  • First principles study of methane decomposition on B5 step-edge type site of Ru surface

    Arevalo, Ryan Lacdao   Aspera, Susan Me?ez   Sison Esca?o, Mary Clare   Nakanishi, Hiroshi   Kasai, Hideaki  

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  • A theoretical study of the structure and stability of borohydride on 3d transition metals

    Arevalo, Ryan Lacdao   Escano, Mary Clare Sison   Gyenge, Elod   Kasai, Hideaki  

    The adsorption of borohydride on 3d transition metals (Cr, Mn, Fe, Co, Ni and Cu) was studied using first principles calculations within spin-polarized density functional theory. Magnetic effect on the stability of borohydride is noted. Molecular adsorption is favorable on Co, Ni and Cu, which is characterized by the strong s-d(zz) hybridization of the adsorbate-substrate states. Dissociated adsorption structure yielding one or two H adatom fragments on the surface is observed for Cr, Mn and Fe. (c) 2012 Elsevier B.V. All rights reserved.
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  • A theoretical study of the reactivity of Cu2O(111) surfaces: the case of NO dissociation

    Kishi, Hirofumi   Padama, Allan Abraham Bustria   Arevalo, Ryan Lacdao   Moreno, Joaquin Lorenzo Valmoria   Kasai, Hideaki   Taniguchi, Masashi   Uenishi, Mari   Tanaka, Hirohisa   Nishihata, Yasuo  

    We compare the electronic properties of Cu(111) and Cu2O(111) surfaces in relation to the dissociation of NO using first principles calculations within density functional theory. We note a well-defined three-fold site on both O- and Cu-terminated Cu2O surfaces which is verified as the active site for the adsorption and dissociation of NO. The interaction of Cu with O atoms results in the forward shifting of the local density of states and formation of unoccupied states above the Fermi level, compared to the fully occupied d band of pure Cu. These results give valuable insights in the realization of a catalyst without precious metal for the dissociation of NO.
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  • Sulfation of a PdO(101) methane oxidation catalyst:mechanism revealed by first principles calculations

    Arevalo, Ryan Lacdao   Aspera, Susan Menez   Nakanishi, Hiroshi  

    PdO efficiently catalyzes the oxidation of methane but suffers tremendously from sulfur poisoning that lowers its catalytic activity. In this work, first principles calculations were performed to reveal the mechanism of PdO(101) sulfation and how the active sites for methane activation are altered upon the formation of SOy (y =3D 2 to 4) species on the surface. The results suggest that under typical experimental conditions with a high O-2/SO2 gas ratio, the formation of SO4-decorated PdO(101) is favored and contributes significantly to the poisoning of PdO(101) as it blocks the coordinatively unsaturated Pd atoms that were identified to play a crucial role in the activation of methane. At a low temperature regime, SO2 oxidation forming SO3 and SO4 species is highly exothermic via the Eley-Rideal and Langmuir-Hinshelwood mechanisms but is limited by the high activation barrier for O-2 dissociation. On the other hand, the Mars-van Krevelen mechanism has low exothermicity but provides facile elementary steps. From these results, insights into the design of PdO-based sulfur poisoning-resistant methane oxidation catalysts were drawn.
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  • Sulfation of a PdO(101) methane oxidation catalyst: mechanism revealed by first principles calculations

    Arevalo, Ryan Lacdao   Aspera, Susan Meñez   Nakanishi, Hiroshi  

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  • Tuning methane decomposition on stepped Ni surface:The role of subsurface atoms in catalyst design

    Arevalo, Ryan Lacdao   Aspera, Susan Menez   Escano, Mary Clare Sison   Nakanishi, Hiroshi   Kasai, Hideaki  

    The decomposition of methane (CH4) is a catalytically important reaction in the production of syngas that is used to make a wide spectrum of hydrocarbons and alcohols, and a principal carbon deposition pathway in methane reforming. Literatures suggest that stepped Ni surface is uniquely selective toward methane decomposition to atomic C, contrary to other catalysts that favor the CH fragment. In this paper, we used dispersion-corrected density functional theory-based first principles calculations to identify the electronic factors that govern this interesting property of stepped Ni surface. We found that the adsorption of atomic C on this surface is uniquely characterized by a 5-coordinated bonding of C with Ni atoms from both the surface and subsurface layers. Comparison with Ru surface indicates the importance of the subsurface atoms of stepped Ni surface on its selectivity toward methane decomposition to atomic C. Interestingly, we found that substituting these subsurface atoms with other elements can dramatically change the reaction mechanism of methane decomposition, suggesting a new approach to catalyst design for hydrocarbon reforming applications.
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