35 research outputs found
First principles studies of the potential-induced lifting of the Au(100) surface reconstruction
The potential-induced surface reconstruction of Au(100) has been studied by a combination of density functional theory and thermodynamic considerations. Surface free energies of reconstructed-(5 x 1) and unreconstructed-(1 x 1) surfaces were calculated as function of an external electric field using the extended ab initio atomistic thermodynamics approach. After relating electric field and electrode potential by using capacitance measurements, we calculate lifting of the reconstruction to occur at 0.58 V in 0.01 M HClO4 and 0.27 V in 0.01 M H2SO4, being in agreement with the experimental values of 0.60 and 0.27 V (vs. SCE). Finally, the consequences of using experimental capacitance measurements for calculating surface free energies are discussed
Two-Scale Kirchhoff Theory: Comparison of Experimental Observations With Theoretical Prediction
We introduce a non-perturbative two scale Kirchhoff theory, in the context of
light scattering by a rough surface. This is a two scale theory which considers
the roughness both in the wavelength scale (small scale) and in the scales much
larger than the wavelength of the incident light (large scale). The theory can
precisely explain the small peaks which appear at certain scattering angles.
These peaks can not be explained by one scale theories. The theory was assessed
by calculating the light scattering profiles using the Atomic Force Microscope
(AFM) images, as well as surface profilometer scans of a rough surface, and
comparing the results with experiments. The theory is in good agreement with
the experimental results.Comment: 6 pages, 8 figure
Roughening of Pt nanoparticles induced by surface-oxide formation
Using density functional theory (DFT) and thermodynamic considerations we studied the equilibrium shape of Pt nanoparticles (NPs) under electrochemical conditions. We found that at very high oxygen coverage, obtained at high electrode potentials, the experimentally-observed tetrahexahedral (THH) NPs consist of high-index (520) faces. Since high-index surfaces often show higher (electro-)chemical activity in comparison to their close-packed counterparts, the THH NPs can be promising candidates for various (electro-) catalytic applications
Modeling of packed absorption tower for volatile organic compounds emission control
Development of chemical industry and high vapor pressure of volatile
organic compounds (VOCs) have been caused that these materials recently
be considered as a source of air pollution. These are different methods
for separation of VOC from air that absorption by suitable and
selective solvent is an efficient method. In this research,
mathematical modeling of a packed absorption tower for separation of
VOCs from air has been presented. Then, acetone vapors separation in
this tower and with Intalox saddle packing has been investigated.
Concentration of acetone in inlet air stream was 1.5 mol % which has
reduced to 150 ppm (0.015 mol %) in effluent air stream. The results
show that the packed absorption tower with this type of packing can
separate 99% of acetone vapors. Comparison of the results obtained by
Intalox saddle packing with the results obtained by another type of
packing (with trade mark of Kerapak patented by Sulzer Company) shows
that the tower gives the same efficiency for acetone separation. Thus,
the efficiency of absorption process, mainly, depends on solvent type,
composition of VOC in feed air, desired composition of VOC in outlet
air (or percentage of VOC separation) and pressure drop
Partial catalytic hydrogenation of diolefins formed during PACOL process (Paraffin conversion to olefin) to monoolefins
Nanoscale surface chemistry over faceted substrates: structure, reactivity and nanotemplates
Faceting is a form of self-assembly at the nanometre-scale on adsorbate-covered single-crystal surfaces, occurring when an initially planar surface converts to a "hill and valley" structure, exposing new crystal faces of nanometre-scale dimensions. Planar metal surfaces that are rough on the atomic scale, such as bcc W(111), fcc Ir(210) and hcp Re(1231), are morphologically unstable when covered by monolayer films of oxygen, or by certain other gases or metals, becoming "nanotextured" when heated to temperatures above ~ 700 K. Faceting is driven by surface thermodynamics (anisotropy of surface free energy) but controlled by kinetics (diffusion, nucleation). Surfaces can spontaneously rearrange to minimize their total surface energy (by developing facets), even if this involves an increase in surface area. In this critical review, we discuss the structural and electronic properties of such surfaces, and first principles calculations are compared with experimental observations. The utility of faceted surfaces in studies of structure sensitive reactions (e.g., CO oxidation, ammonia decomposition) and as templates for growth of metallic nanostructures is explored (122 references)
First-principles studies on oxygen-induced faceting of Ir(210)
Density functional theory calculations were performed to obtain an atomistic understanding of facet formation on Ir(210). We determined geometries and energetics of clean and oxygen-covered surfaces of planar Ir(210) as well as Ir(311) and two types of Ir(110) surfaces, which are involved in faceting by forming three-sided nanopyramids. Using the energies together with the ab initio atomistic thermodynamics approach, we studied the stability of substrate and facets in the presence of an oxygen environment. Our results show that facets are stable over the entire temperature range at which oxygen is adsorbed on the surface at coverages: 0.45 physical ML, supporting the picture of a thermodynamic driving force. We also investigated the dependence of the phase diagram on the choice of the exchange-correlation functional and obtained qualitatively the same behavior. Finally, this work helps to better understand reactivity and selectivity of 0-covered planar and faceted Ir surfaces in catalysis
Bridging the gap between nanoparticles and single crystal surfaces
Using density functional theory calculations and the extended ab initio atomistic thermodynamics approach, we studied the adsorption of oxygen on the different surface faces, which are involved in the faceting of Ir(210). Constructing the (p,T)-surface phase diagrams of the corresponding surfaces in contact with an oxygen atmosphere, we find that at high temperatures the planar surfaces are stable, while lowering the temperature stabilizes those nano-facets found experimentally. Afterwards, we constructed the (a,T,Δ,Φ)-phase diagram for Ir(210) in contact with an aqueous electrolyte and found that the same nano-facets should be stable under electrochemical conditions. Motivated by this prediction from theory, experiments were performed using cyclic voltammetry and in-situ scanning tunneling microscopy. The presence of nano-facets for Ir(210) gives rise to a characteristic current-peak in the hydrogen adsorption region for sulfuric acid solution. Furthermore, first results on the electrocatalytic behavior of nano-faceted Ir(210) are presented
Structure of palladium nanoparticles under oxidative conditions
Using density functional theory (DFT) and thermodynamic considerations we study the shape and stability of Pd nanoparticles in oxygen-lean and oxygen-rich atmospheres. We find that at very high oxygen coverage cubes exposing (100) faces will form, which are stabilized due to the formation of a ... overlayer. The shape of oxygen-covered Pd and Pt nanoparticles is compared in this study