Session U27: Focus Session: Computational Nanoscience VII-Reactions on Surfaces and Nanostructures

Effects of Rh coordination and surface strain on NO dissociation

The Rh(100) surface is known to be a good catalyst for the reduction of NO. We try to understand the effect of Rh coordination and in plane strain on NO adsorption and its dissociation. To distinguish between the strain effects and charge transfer we look at the adsorption and dissociation of NO on Rh(100) and stretched Rh(100), in addition to NO adsorption on 1 ML of Rh/MgO at MgO and Rh lattice constants. As expected, we find that the adsorption energy of NO increases with reduction in effective coordination of Rh, i.e., while going from Rh(100), to stretched Rh(100), to 1 ML Rh/MgO, to 0.25 ML of Rh/MgO. In the case of NO on Rh(100) and on 1 ML Rh/MgO at Rh lattice constant, we find that NO adsorbs vertically at the bridge site between two Rh atoms; whereas, in the case of stretched Rh(100) and 1 ML of Rh/MgO at MgO lattice constant, NO adsorbs horizontally at the hollow site. Hence, as the effective Rh coordination is changed, both adsorption geometries and adsorption energies change in a systematic way. This has an influence on the energetic barriers for the rate-limiting step in the reduciton of NO on these surfaces.   

Bridging the materials gap in catalytic kinetics via first principles coarse-grained kinetic Monte Carlo simulations

We describe a first principles coarse-grained kinetic Monte Carlo technique enabling us to simulate catalysis under different technologically relevant conditions. By implementing coarse-grained KMC, various simulations are performed efficiently at much larger time and length scales. Using this approach, we studied catalytic oxidation of CO on different metal surfaces. Especially, we tried to understand the role of defects in catalytic kinetics such as steps, kinks, and multiple facets. These studies led us to some interesting results bridging the material gap in catalysis.   

Formation of Pt nano-islands on Ru(0001) surface: insights from ab initio calculations .

As reported [1], Ru nanoparticles with sub-monolayer of Pt are much more efficient catalysts for hydrogen oxidation than anodes used in standard fuel cells. Since this effect apparently depends on the size of Pt islands, we have performed density functional theory based calculations of energetics of Pt islands of varying size on the Ru(0001) surface. We find the formation energy of the island per atom to decrease monotonically from -5.1 eV to -6.1 eV, as the number of Pt atoms in the 2D island increases from one atom to one monolayer. Propensity of the systems to increase the number of interatomic bonds thus overpowers the effect of the stress caused by the 2.6{\%} misfit of Ru and Pd interatomic bonds. As a result, Pt atoms prefer to coalesce on Ru(0001) increasing the cluster size up to one monolayer, in agreement with experimental data. This raises the question whether the key role of nano-sized Ru clusters [1] is to prevent coalescence of Pt islands by restricting their diffusion through facet edges. To address this issue we consider the effect of surface steps on Ru(0001) on the Pt island formation. 1. S. R. Brankovic, et al. , Electrochem. Sol.-St. Lett. \textbf{4} (12) A217-A220 (2001).   

First principles studies of CO adsorption and diffusion over Pt nano-islands on Ru(0001) surface

Proton exchange fuel cells are promising tools for hydrogen economy. However, CO, present in hydrogen, blocks active Pt sites of anode that poisons its reactivity. As reported [1], small coverage of Pt on Ru nanoparticles is much less sensitive to CO than commercial catalysts. To understand this effect, we have performed density functional theory based calculations of the energetics of adsorption and diffusion of CO on the 7Pt-atom islands and on the Ru(0001) substrate. We find that CO adsorption energy increases as it moves from the center of the Pt island to its edge and further onto substrate. CO thus tends to move from the Pt island to the Ru substrate. Diffusion barriers are found to be lower than 0.3 eV suggesting this process to be fast. This finding suggests that this hydrogen oxidation catalyst is CO tolerant because of the propensity of CO to move from active Pt island site to the Ru substrate. We present the rationale for this effect using insights from detailed electronic structure calculations. [1] S. R. Brankovic, et al., Electrochem. Sol.-St. Lett. \textbf{4} (12) A217-A220 (2001).   

Mechanism of the Energy Barrier Formation during Dissociation of Hydrogen Molecule on Mg(0001)

There has been no clear explanation on the mechanism of the energy barrier for the dissociation of the hydrogen molecules on the metal surfaces. When the hydrogen molecule approaches the surface of metals, the molecule dissociates and forms a new bond with the atoms on the surfaces. During the process the energy barrier appears on the Au or Cu or Mg surface, although no barrier appears on the Pt or Ni surface. Although the stability of hydrogen atoms has been clarified by use of the density of stare,[1] there has been no clear explanation on the mechanism of the barrier formation. In the present paper we use a density functional calculation method to evaluate the charges that belong to the hydrogen atoms during the dissociation process of the hydrogen molecule on the Mg(0001) using Bader analysis. During the barrier formation, the charge transfers from the Mg substrate to the dissociating hydrogen atoms. We will discuss how this energy barrier can be explained with the sum of the independent systems of the electron donated Mg surface and the electron received hydrogen molecule. [1] B.Hammer and J.K.N{\o }rskov, Nature 376, 238(1995).   

Theoretical study of tunneling spectra of tetramantane on Au(111) surface

Diamondoids are a class of carbon-based molecules in which the carbon atoms are structured in a diamond-like fashion with all dangling bonds saturated with hydrogen atoms. Recently, scanning tunneling microscopy experiments were carried out to study the tunneling spectra of tetramantane on Au (111) surface. The elastic tunneling images have a LUMO-like character in a broad energy range around the Fermi level, while the inelastic signal shows spatial localization. We use ab-initio density functional theory calculations to study the molecule-surface system and discuss the observed elastic and inelastic tunneling spectra. This work was supported by National Science Foundation Grant No. DMR04-39768, by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, and by an NSF Graduate Research Fellowship. Computational resources have been provided by NERSC and NPACI.   

Motion of clusters on complex surfaces

Polymer and organic molecule assemblies have been investigated intensely in the past decade, due to their vast range of applications in nano-molecular electronics and as bio-sensors. In particular, self-assembled monolyers (SAMs) of alkanethiol on the Au(111) surface are used widely in surface studies because they are simple structurally, stable thermodynamically and have well-defined order. In this project, inspired by recent experiments, we use classical molecular dynamics simulations to study motions of Ag$_{n}$ clusters with various sizes on the alkanthiol SAMs. We report detailed results on dynamics, diffusion, and sintering processes of these nano-clusters.   

Effects of water on the reactivity and stability of SiO2 nanostructure.

To investigate effects of water on the reactivity and stability of SiO2 nanostructure, we have performed first-principles molecular dynamics simulations of SiO2 nano-chain and nano-rod. The SiO2 nanostructures, which have stimulated many current research endeavors, can react with water strongly under internal or external stress. In our study, water monolayer films that cover the entire system are used to study the collective motion of protons. Structure, charge separation, stress dependent bond breaking and formation, and proton conduction are discussed based on results obtained at room temperature. Finally, we extend our effort to carbon nanotubes.   

Enantiospecific adsorption of chiral molecules on chiral Au clusters

Enantioselectivity in gold clusters is investigated by studying the adsorption of a chiral amino acid (cysteine) on a chiral Au$_{55}$ cluster using density functional calculations. The highest adsorption energies were found when the amino and thiolate functional groups of cysteine are bonded to the lowest coordinated edges of the chiral cluster. Enantiospecific adsorption is primarily obtained from the different bond location and strength, at the cluster edge, of the carboxyl groups forming the left- and right-handed enantiomers. These results provide theoretical support to convey enantioselectivity in asymmetric nanocatalysts using chiral gold clusters.   

Spin-dependent electronic structure of transition-metal atomic chains adsorbed on single-wall carbon nanotubes

We present a systematic study of the electronic and magnetic properties of transition-metal (TM) atomic chains adsorbed on the zigzag single-wall carbon nanotubes (SWNTs). We examined the effect of the TM coverage and geometry on the binding energy and the spin polarization at the Fermi level. All those adsorbed chains studied have ferromagnetic ground state, but only their specific types and geometries demonstrated high spin polarization near the Fermi level. Their magnetic moment and binding energy in the ground state display interesting variation with the number of d electrons of the TM atom. Spin-dependent electronic structure becomes discretized when TM atoms are adsorbed on finite segments of SWNTs. Once coupled with nonmagnetic metal electrodes, these magnetic needles or nanomagnets can perform as spin-dependent resonant tunneling devices. Our study is performed by using first-principles pseudopotential plane wave method within spin-polarized density functional method. \textbf{Reference:} E. Durgun and S. Ciraci Phys. Rev B 74, 125404 (2006).   

\textit{Ab inition }study of adenine and thymine adsorption on carbon nanotubes.

The adsorption properties of the DNA bases, adenine and thymine, on the surface of single walled metallic and semiconducting carbon nanotubes are calculated from first principle density functional theory calculations using the VASP code (Vienna Ab initio Simulation Package). Calculations for the adsorption of the charged adenine and thymine molecules are also presented. The equilibrium positions and their energies are determined. The adsorption process is analyzed in terms of the electronic structure of the various configurations. The effects of charging of the DNA bases in the adsorption on the nanotubes are also determined. The results for the density of states, band structure and charge transfer analysis are used to establish a model of the interaction of these biological molecules with carbon nanotubes.   

First-Principle Calculation on Hydrogen Dissociation on Pd-doped CNT

We have calculated hydrogen dissociation on Pd-doped (5,5) CNT using density functional theory (DFT), and examined its influence on the conductance of CNT using a non-equilibrium green's function (NEGF) method. Our results show that the hydrogen dissociation is barrier-less on edge of Pd cluster, while on the top of Pd cluster the molecule would not automatically dissociate. Calculations also show that a dense doping with Pd cluster would modify the band structure of CNT substantially such that the doped tube becomes a semi-metal. The dissociation of hydrogen molecule will further change it into a semiconductor. Our NEGF calculations confirmed the band structure calculation, and suggested that Pd-doped CNT could be used as a hydrogen sensor device by measuring the conductance change of the device induced by hydrogen dissociation.   

Dynamics and shape fluctuations of nanosized water pools in reverse micelles

Reverse micelles are surfactant assemblies containing nanosized water reservoirs which can serve as confined media for studying chemical reactions as well as for nanoparticle synthesis. Molecular dynamics simulations are performed for reverse micelles formed by the surfactant CTAB (Cetyl Trimethyl Ammonium Bromide) in cyclohexane with the cosurfactant pentanol. We present results for the dynamics of confined water and for shape fluctuations of the reverse micelles which can be probed experimentally using terahertz time-domain spectroscopy.   

First-Principles Studies of sila-Diamondoids

While there has been recent progress in the isolation of carbon-based diamondoids from petroleum oil by the Chevron-Texaco group, there is still no experimental evidence for the existence of the silicon analogue of adamantane, sila-adamantane (Si$_{10}$H$_{16}$). As adamantane forms the central building block for diamondoids, we speculate that sila-adamantane could serve as the template for a novel class of materials known as sila-diamondoids. We predict that because sila-diamondoids are nanostructures derived from bulk crystalline silicon they will have important applications in molecular electronics and nanotechnology. Recently, Fischer, Baumgartner, and Marschner (Science {\bf 310,} 825 (2005)) have made an important contribution to the field of sila-diamondoids with the synthesis of a four-fold silylated molecule, C$_{24}$H$_{72}$Si$_{14}$. In this work, we show that density-functional theory is capable of calculating the structural, electronic, and vibrational properties of C$_{24}$H$_{72}$Si$_{14}$ that compare very well with the recent experimental data.