Reactivity of Iron Nanostructures from Density Functional Theory

Marko Melander

    Research output: ThesisDoctoral ThesisCollection of Articles

    Abstract

    Most chemical processes rely on heterogeneous catalysis. The role of a catalyst is to provide an environment for more facile breaking and making of chemical bonds without being consumed itself. Catalytic performance is determined by the electronic structure, which can be modified by changing the atomic structure and composition. Conventional heterogeneous catalysis takes place on large crystal surfaces where the catalytic activity is determined mostly by the surface structure. However, catalyst particles smaller than 10 nm exhibit finite-size effects and the catalytic properties become very dependent on the size and geometry of the particle. The size- and geometry-dependent properties allow the modification of nanostructures towards specific catalytic applications. To reach this goal the relationships between catalytic performance and atomic and electronic factors must be understood. In this thesis a quantum mechanical approach within density functional theory is employed to obtain an atomic level understanding of CO dissociation on iron nanostructures. This is an important initial step for the total efficiency and selectivity in, e.g., Fischer-Tropsch synthesis for hydrocarbons and synthesis of carbon nanotubes. Results on 0.5-1.5 nm icosahedral, BCC, and amorphous iron nanoparticles show that the larger particles are more reactive towards CO dissociation. The presence of particle edges is essential for facile CO dissociation and on BCC particles very low dissociation barriers are observed even without atomic steps, which are the active sites on crystal surfaces. The reactivity of particle edges is explained by favorable orbital interactions between the metal surface and CO's LUMO orbital stabilizing the transition state. An optimal site for facile CO dissociation needs to fulfil several requirements: 1) CO is adsorbed at a BCC(100) like four-fold hollow site in the initial state,2) CO breaks over an atomic step, and 3) C is adsorbed in a fourfold site and O in a three-fold site in the final state. All properties of magnetic materials depend on their magnetic state but the effect of different magnetic states on catalytic activity has been largely ignored. In this thesis it is shown that the electron donation and reactivity of FCC(111) iron surface can be modified by changing the magnetic state. This gives a justification for magnetically controlled catalysis. Besides studies on reactivity of nanostructures, a general algorithm for improving transition state methods used in computational nanocatalysis was developed. The method removes external degrees of freedom using quaternion algebra and the new NEB-TR and DIMER-TR methods were shown to use fewer iterations to converge to a saddle point and describe minimum energy pathways more accurately in the case of NEB-TR.
    Translated title of the contributionRautananorakenteiden reaktiivisuus tiheysfunktionaaliteoriaan pohjautuen
    Original languageEnglish
    QualificationDoctor's degree
    Awarding Institution
    • Aalto University
    Supervisors/Advisors
    • Laasonen, Kari, Supervising Professor
    • Laasonen, Kari, Thesis Advisor
    Publisher
    Print ISBNs978-952-60-6267-9
    Electronic ISBNs978-952-60-6268-6
    Publication statusPublished - 2015
    MoE publication typeG5 Doctoral dissertation (article)

    Keywords

    • nanoparticle
    • catalysis
    • reactivity
    • density functional theory
    • iron

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