Spin-polarized density functional theory study of reactivity of diatomic molecule on bimetallic system: the case of O2 dissociative adsorption on Pt monolayer on Fe(001).

The energetics of O(2) adsorption and dissociation are discussed in terms of 6D potential energy surface based on spin-polarized density functional theory calculations that predict O(2) access to both molecular and dissociative chemisorption wells with no obvious barriers. Specifically, a molecularly chemisorbed state in a top-bridge-top (t-b-t) configuration is identified, and a "no barrier" dissociative adsorption over hollow site with the O-O axis spanning toward the bridge sites (b-h-b) is noted. Both the translation of O(2) from the molecular state (t-b-t) to the dissociated state on bridge and the direct nonactivated dissociative adsorption over the hollow sites (b-h-b) are likely pathways for O(2) dissociation. Interestingly, such O(2) reaction pathways are consistent with the density functional theory calculations and molecular beam experiments on O(2) dissociative adsorption on Pt(100)-(1 x 1). Modification of the electronic structure of the Pt surface due to the Fe substrate relevant for O(2) reactivity is discussed in an effort to provide insight into the experimentally discovered significant enhancement in electrocatalytic activity of Pt-Fe alloys for fuel cell applications.

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

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.

Electrocatalysis of borohydride oxidation: a review of density functional theory approach combined with experimental validation.

The electrocatalysis of borohydride oxidation is a complex, up-to-eight-electron transfer process, which is essential for development of efficient direct borohydride fuel cells. Here we review the progress achieved by density functional theory (DFT) calculations in explaining the adsorption of BH4(-) on various catalyst surfaces, with implications for electrocatalyst screening and selection. Wherever possible, we correlate the theoretical predictions with experimental findings, in order to validate the proposed models and to identify potential directions for further advancements.

Structure and stability of borohydride on Au(111) and Au3M(111) (M = Cr, Mn, Fe, Co, Ni) surfaces.

We study the adsorption of borohydride on Au and Au-based alloys (Au(3)M with M = Cr, Mn, Fe, Co, and Ni) using first-principles calculations based on spin-polarized density functional theory. Favorable molecular adsorption and greater adsorption stability compared to pure Au are achieved on Au(3)M alloys. For these alloys, there is an emergence of unoccupied states in the surface d band around the Fermi level with respect to the fully occupied d band of pure Au. Thus, the derived antibonding state of the sp-d interaction is upshifted and becomes unoccupied compared to pure Au. The B-H bond elongation of the adsorbed borohydride on these alloy surfaces points to the role of surface-parallel (d(xy) and d(x(2)-y(2)) states) components of the d-band of the alloying metal M, most pronouncedly in the cases of M = Co or Ni. On the alloy surfaces, B binds directly with the alloying metal, unlike in the case of pure Au where the surface bonding is through the H atoms. These results pose relevant insights into the design of Au-based anode catalysts for the direct borohydride fuel cell.

Nitric oxide adsorption effects on metal phthalocyanines.

The adsorption of nitric oxide (NO) on various metal phthalocyanines (MPc, M = Mn, Fe, Co) has been studied using first-principles calculations based on density functional theory (DFT). In this study, we investigated the fully optimized geometries and electronic structure of MPc. We found that the electronic structures of metal atoms are essential in shaping the ground-state electronic structure near the Fermi level. These states are defined mostly by the d orbitals of the transition-metal atoms and, to some degree, by the states of nitrogen and carbon atoms of the inner rings. The numerical calculations showed that NO strongly chemisorbs to the metal atom with an end-on configuration and results in a change in geometric and electronic structures of MPc. The N-O bond lengths are slightly longer than that of the isolated NO molecule. The orbital energy levels are shifted with respect to the Fermi level. The HOMO-LUMO gap widens as compared to bare MPc. These changes are attributed to the hybridization of the pi* orbital of NO and the d orbitals of the transition metal. Specifically, the interaction between dpi and the pi* orbital is significant for MnPc-NO, while the hybridization of d(z(2)) and the pi* orbital plays an important role in CoPc-NO.

A study on the stability of O2 on oxometalloporphyrins by the first principles calculations.

The authors investigated the interaction of oxometalloporphyrins (MO(por))--specifically, MoO(por), WO(por), TiO(por), VO(por), and CrO(por)--with O(2) by using first principles calculations. MoO(por) and WO(por) undergo reactions with O(2); on the other hand, TiO(por), VO(por), and CrO(por) do not. Next, they compared the interaction of MoO(por) and WO(por) with O(2). Activation barriers for the reactions of MoO(por) and WO(por) with a side-on O(2) are small. For MoO(por)(O(2)), the activation barrier for the reverse reaction that liberates O(2) is also small; however, that for WO(por)(O(2)) is large. The experimental results that photoirradiation with visible light or heating of Mo (VI)O(tmp)(O(2)) regenerates Mo (VI)O(tmp) by liberating O(2) while W (VI)O(tmp)(O(2)) does not [J. Tachibana, T. Imamura, and Y. Sasaki, Bull. Chem. Soc. Jpn. 71, 363 (1998)] are explained by the difference in activation barriers of the reverse reactions. This means that bonds formed between the W atom and O(2) are stronger than those between the Mo atom and O(2). The bond strengths can be explained by differences in the energy levels between the highest occupied molecular orbital of MoO(por) and WO(por), which are mainly formed from the a orbitals of the central metal atom and pi(*) orbitals of O(2).