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Density Functional Study of Adsorption of Small Molecules on Nano-Clusters

Nahali, Masoud | 2012

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 43051 (03)
  4. University: Sharif University of Technology
  5. Department: Chemistry
  6. Advisor(s): Gobal, Fereydoon
  7. Abstract:
  8. This work investigates adsorption and possible dissociation of a number of small molecules on small clusters and carbonic surface. Theoretical study of carbon monoxide adsorption on SixGe4_x(x=0–4) nano-clusters has been carried out using MPW1B95 density functional. The geometry, adsorption energy, charge distribution, and vibrational frequency of CO adsorption on all possible structures were investigated. The maximum vibrational frequency changes occur in the bridge structures while the most stable structures occur when CO adsorbs on one silicon atom in a flat surface. Adsorption energies of CO on one Si atom are by far more negative than the corresponding value for on Ge atom. Theoretical study of nitrogen monoxide adsorption on small Six (x=3-5) clusters has been carried out using the MPW1B95. The geometry, adsorption energy, natural bond orbital charge, natural population analysis (NPA)-derived spin density and vibrational frequency of NO adsorption on all optimized nanoclusters were investigated. Also using the NPA, we have investigated the change of bond orders through adsorption. It has been found that NO is capable of making n-centre bonds (n=1-4) from the nitrogen side but bonds to one site from the oxygen end. The adsorption and dissociation of H2O2 on small PdxM3-x (M=Pt, Cu; x=1–3) clusters is investigated using the B3PW91 hybrid density functional method. Generally, H2O2 adsorbs on top positions through one of its oxygen atoms and only in a few cases reacts with the cluster through both oxygen and hydrogen sides. Interestingly, on the triplet Pd2Pt cluster, H2O2 dissociates spontaneously to two OH that adsorb strongly on the Pd sites. In this chemical bond the oxygen p orbitals interact with the Pd d orbitals. We believe that not only the electric field of the positively charged Pd atoms but also the Pd unpaired d electrons in the triplet Pd2Pt cluster make H2O2 dissociative adsorption to be thermodynamically and kinetically favoured. Density functional theory is used in a spin-polarized plane wave pseudopotential implementation to investigate molecular oxygen adsorption and dissociation on graphite and nickel-doped graphite surfaces. It is found that substituting a carbon atom of the graphite surface by a single doping nickel atom (2.8% content) makes the surface active for oxygen chemisorption. The large electric field near the doping nickel atom along with the excess electrons on the neighbouring carbon atoms, which are bound to the nickel induce molecular oxygen to adsorb and dissociate favourably
  9. Keywords:
  10. Chemical Absorption ; Catalyst ; Density Functional Theory (DFT) ; Cluster ; Nano-Cluster

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