Surface Diffusion Rate Research Articles

Controlled ligament coarsening in nanoporous gold by annealing in vacuum versus nitrogen

The structural evolution of nanoporous gold during thermal treatment was studied by annealing samples in vacuum and in flowing nitrogen. As expected, ligament thickness generally increased in both environments. However, ligaments annealed at high temperature in vacuum remained relatively narrow, undergoing much less coarsening than nitrogen-annealed samples, albeit with some ligament agglomeration. When annealed in flowing nitrogen, gold ligaments coarsened significantly at temperatures above 300 °C. This discrepancy is attributed to different surface diffusion rates of gold in the two annealing environments. The current results suggest that diffusion on the surfaces of nanoporous gold ligaments proceeds more quickly in nitrogen than in vacuum.

Surface Self-Diffusion of Organic Glasses

Surface self-diffusion coefficients have been determined for the organic glass Nifedipine using the method of surface grating decay. The flattening of 1000 nm surface gratings occurs by viscous flow at 12 K or more above the glass transition temperature and by surface diffusion at lower temperatures. Surface diffusion is at least 10(7) times faster than bulk diffusion, indicating a highly mobile surface. Nifedipine glasses have faster surface diffusion than the previously studied Indomethacin glasses, despite their similar bulk relaxation times. Both glasses exhibit fast surface crystal growth, and its rate scales with surface diffusivity. The observed rate of surface diffusion implies substantial surface rearrangement during the preparation of low-energy glasses by vapor deposition. The Random First Order Transition Theory and the Coupling Model successfully predict the large surface-enhancement of mobility and its increase on cooling, but disagree with the experimental observation of the faster surface diffusion of Nifedipine.

Product desorption rate influence on catalytic reactivity of spatially inhomogeneous surfaces

We investigate numerically a phenomenological mathematical model of unimolecular reactions proceeding on inhomogeneous planar surfaces in the two-dimensional space case taking into account: the bulk diffusion of the reactant from the bounded vessel toward the adsorbent and the product bulk one from the adsorbent into the same vessel, the adsorption and desorption of reactant particles, long-range surface diffusion of the adsorbate, and a slow product desorption from the adsorbent. Simulations were performed using the finite difference technique. The influence of the long-range surface diffusion and product desorption rate on the kinetics of processes catalysed by inhomogeneous surfaces with different arrangements of reactive and nonreactive adsorption sites are studied.

Modeling adsorption rate of organic micropollutants present in landfill leachates onto granular activated carbon

The overall adsorption rate of single micropollutants present in landfill leachates such as phthalic acid (PA), bisphenol A (BPA), diphenolic acid (DPA), 2,4-dichlorophenoxy-acetic acid (2,4-D), and 4-chloro-2-methylphenoxyacetic acid (MCPA) on two commercial activated carbons was studied. The experimental data obtained were interpreted by using a diffusional model (PVSDM) that considers external mass transport, intraparticle diffusion, and adsorption on an active site. Furthermore, the concentration decay data were interpreted by using kinetics models. Results revealed that PVSDM model satisfactorily fitted the experimental data of adsorption rate on activated carbon. The tortuosity factor of the activated carbons used ranged from 2 to 4. The contribution of pore volume diffusion represented more than 92% of intraparticle diffusion confirming that pore volume diffusion is the controlling mechanism of the overall rate of adsorption and surface diffusion can be neglected. The experimental data were satisfactorily fitted the kinetic models. The second-order kinetic model was better fitted the experimental adsorption data than the first-order model.

Normal versus anomalous roughening in electrodeposited Prussian Blue layers

Electrochemical deposition of Prussian Blue shows a time increase of adsorption rate and produces films with faceted surfaces and smooth morphology at short length scales. Roughness scaling suggests anomalous roughening. However, an interpretation of normal Family–Vicsek scaling provides exponents consistent with roughening dominated by surface diffusion and time-dependent deposition rate characteristic of electrodeposition.

ChemInform Abstract: A Study of Iron Catalyst for Ammonia Synthesis by Electron Spectroscopy Methods

AbstractReview: 63 refs.

Adsorbate localization versus diffusion: a coherent time dependent theory of coupling to gravitons in hidden large dimensions

A long standing unsolved puzzle is the very small rate of surface diffusion of ad-sorbates in the low temperature STM in the regime of quantum diffusion, differing by orders of magnitude from the results of modern quantum theories of surface diffusion (QTSD) based on ab initio potential energy surfaces (refs. [1]-[5]). We investigate whether decoherence mechanisms, neglected in QTSD, can shed some light on these discrepancies. A quantum-mechanical theory is presented treating the entanglement of adsorbates with their environment (phonons, electron-hole pairs, gravitons) [6]. Time dependent wave packet evolution including the environmental modes indicates possible paths towards an explanation of the observed discrepancies and gives an insight into phenomenological descriptions like adsorbate localization, quantum Zeno effect (permenant "measurement" by specific environmental excitations) and wave function collapse.

The growth of Al-doped ZnO nanorods on c-axis sapphire by pulsed laser deposition

Transmission and scanning electron microscopy are used to compare the growth of nanorod arrays in ZnO and Al-doped ZnO (2%Al) films grown by pulsed laser deposition. For a laser pulse energy of 10 mJ/pulse, nanorod arrays were formed at temperatures 575–625 °C for ZnO films and 650–675 °C for Al-doped ZnO films. For higher laser pulse energies, up to 30 mJ/pulse, nanorod growth in both cases moved to lower temperature regimes. By comparing nanorod growth temperature, morphology and density for ZnO and Al-doped ZnO growth, it is concluded that the differences in growth are due to lower surface diffusion rates in Al-doped films. The electrical resistivities of the Al doped ZnO films were in the range 5–7 × 10 − 3 Ω∙cm.

A Monte Carlo investigation of Gallium and Arsenic migration on GaAs(100) surface

We perform an atomic scale simulation of GaAs/GaAs(100) growth, using the Kinetic Monte Carlo (KMC) technique, to investigate some aspects of Gallium and Arsenic surface migration. We show that the interlayer migration rate is smaller for Arsenic than for Gallium. Results suggest that upward diffusion is thermally activated while downward diffusion is kinetically controlled. We also find an oscillatory behavior of the surface diffusion rates during the growth, in close relation to the roughening of the substrate. The surface migration rates are governed by the temperature, but mostly by the Arsenic to Gallium flux ratio. The Gallium average diffusion lengths are estimated to be ~440 nm at 620°C and ~130 nm at 530°C, in agreement with experimental data.

Comparative analysis for three different immobilisation strategies in the hexavalent chromium biosorption process using Bacillus sphaericus S‐layer

AbstractHexavalent chromium constitutes an important water pollutant due to its cancerogenic properties. In countries such as Colombia it is widely used as a preservative in the leather industry. Bacillus sphaericus S‐layer exhibits high metal‐binding capacity hence the use of this microorganism to remove metals such as chromium from polluted regions based on immobilised cells systems is suggested to be an interesting proposal. The paper describes thermodynamic adsorption process of Cr(VI) on immobilised biomass of B. sphaericus cells on polyurethane foam, sol–gel, and sawdust. The adsorption isotherms describe different immobilisation mechanisms regarding the support utilised. For example, polyurethane and sawdust immobilisation process were described with the classical Langmuir model while sol–gel followed Generalised Freundlich–Kiselev behaviour. Moreover, through determining breakthrough curves in packed columns with each medium we found surface diffusion and mass bulk transfer rate as crucial factors when scaling‐up the process.

Investigation of aluminosilicate as a solid oxide fuel cell refractory

Aluminosilicate represents a potential low cost alternative to alumina for solid oxide fuel cell (SOFC) refractory applications. The objectives of this investigation are to study: (1) changes of aluminosilicate chemistry and morphology under SOFC conditions, (2) deposition of aluminosilicate vapors on yttria stabilized zirconia (YSZ) and nickel, and (3) effects of aluminosilicate vapors on SOFC electrochemical performance. Thermal treatment of aluminosilicate under high temperature SOFC conditions is shown to result in increased mullite concentrations at the surface due to diffusion of silicon from the bulk. Water vapor accelerates the rate of surface diffusion resulting in a more uniform distribution of silicon. The high temperature condensation of volatile gases released from aluminosilicate preferentially deposit on YSZ rather than nickel. Silicon vapor deposited on YSZ consists primarily of aluminum rich clusters enclosed in an amorphous siliceous layer. Increased concentrations of silicon are observed in enlarged grain boundaries indicating separation of YSZ grains by insulating glassy phase. The presence of aluminosilicate powder in the hot zone of a fuel line supplying humidified hydrogen to an SOFC anode impeded peak performance and accelerated degradation. Energy dispersive X-ray spectroscopy detected concentrations of silicon at the interface between the electrolyte and anode interlayer above impurity levels.

Mechanisms of nanodot formation under focused ion beam irradiation in compound semiconductors

We have examined the responses of GaAs, InP, InAs, and AlAs to 30 keV focused ion beam (FIB) irradiation and applied a unified model that consistently explains the observed effects. Nanodots were observed to form on GaAs, InP, and InAs under irradiation at normal incidence, while nanodots are not observed on AlAs. The FIB response and nanodot formation behavior of each material is discussed with regard to a few basic material properties and a model for nanodot creation and growth by the action of preferential sputtering and Ostwald ripening. The model predicts the development of a stable average nanodot size with increasing ion dose, with the average nanodot size depending on the excess group III adatom yield, adatom surface diffusion rate, and surface tension. These predictions qualitatively agree with the experimentally observed trends for GaAs and InP. They also agree for the initial nanodot formation on InAs, but this material system exhibits a sudden transition in the nanodot size distribution. The model predicts that nanodots will have difficulty forming and growing on AlAs, which is also in agreement with our experimental results.

Ge instability and the growth of Ge epitaxial layers in nanochannels on patterned Si (001) substrates

We report the Ge thermal instability against surface diffusion in extremely narrow channels isolated by SiO2 and demonstrate the selective growth of strain-relaxed Ge epitaxial layers in channels as narrow as 10 nm on patterned Si (001) substrates by a combination of low temperature growth and selective recrystallization using Ge melt and regrowth during a millisecond laser anneal. Filling narrow channels at high growth temperatures is prohibited by Ge outdiffusion due to the high Ge chemical potential in such narrow channels. The Ge thermal instability depends on the channel width and a critical channel width of 50 nm is extracted from our calculation and experimental results. The Ge surface diffusion rate is dominated by the surface diffusion activation energy barrier that is significantly enhanced by the adsorbed surface hydrides. At a low temperature below 450 °C, a hydride-terminated surface is maintained which increases the Ge surface diffusion barrier and retards the outdiffusion of the Ge adatoms, providing excellent channel filling. The resulting low crystalline quality can be restored with a millisecond laser melt and regrowth.

The Role of Electrostatic Interactions in Protease Surface Diffusion and the Consequence for Interfacial Biocatalysis

This study examines the influence of electrostatic interactions on enzyme surface diffusion and the contribution of diffusion to interfacial biocatalysis. Surface diffusion, adsorption, and reaction were investigated on an immobilized bovine serum albumin (BSA) multilayer substrate over a range of solution ionic strength values. Interfacial charge of the enzyme and substrate surface was maintained by performing the measurements at a fixed pH; therefore, electrostatic interactions were manipulated by changing the ionic strength. The interfacial processes were investigated using a combination of techniques: fluorescence recovery after photobleaching, surface plasmon resonance, and surface plasmon fluorescence spectroscopy. We used an enzyme charge ladder with a net charge ranging from -2 to +4 with respect to the parent to systematically probe the contribution of electrostatics in interfacial enzyme biocatalysis on a charged substrate. The correlation between reaction rate and adsorption was determined for each charge variant within the ladder, each of which displayed a maximum rate at an intermediate surface concentration. Both the maximum reaction rate and adsorption value at which this maximum rate occurs increased in magnitude for the more positive variants. In addition, the specific enzyme activity increased as the level of adsorption decreased, and for the lowest adsorption values, the specific enzyme activity was enhanced compared to the trend at higher surface concentrations. At a fixed level of adsorption, the specific enzyme activity increased with positive enzyme charge; however, this effect offers diminishing returns as the enzyme becomes more highly charged. We examined the effect of electrostatic interactions on surface diffusion. As the binding affinity was reduced by increasing the solution ionic strength, thus weakening electrostatic interaction, the rate of surface diffusion increased considerably. The enhancement in specific activity achieved at the lowest adsorption values is explained by the substantial rise in surface diffusion at high ionic strength due to decreased interactions with the surface. Overall, knowledge of the electrostatic interactions can be used to control surface parameters such as surface concentration and surface diffusion, which intimately correlate with surface biocatalysis. We propose that the maximum reaction rate results from a balance between adsorption and surface diffusion. The above finding suggests enzyme engineering and process design strategies for improving interfacial biocatalysis in industrial, pharmaceutical, and food applications.

ChemInform Abstract: Mechanisms of Catalyst Poisoning by Sulfur Species

Summary The term ‘catalyst deactivation’ or ‘catalyst poisoning' generally refers to a decline in the rate of catalytic conversion and/or a variation in product selectivity as a result of physically or chemically induced changes of the catalyst. In terms of physical changes the deactivation process may involve crystallite growth, reconstruction accompanied by the formation of new surface structures and coordination sites, and surface enrichment or depletion with respect to bulk components. In terms of chemical changes the adsorption of gaseous impurities from the feed is the most common source of catalyst poisoning. The adspecies of major concern contain sulfur, phosphorous, halogens, and carbon. Thermodynamic measurements of surface-vapor equilibria demonstrate the magnitude of the binding energies involved. Segregation studies of different components from the bulk phase to the surface have provided considerable insight into the mechanism and kinetics of adsorbate-induced surface modifications. Recently the surface transport of adspecies involving reaction intermediates and catalyst poisons has received attention. Surface diffusion rates are greater by several orders of magnitude than those for bulk diffusion. For Ni(111)-S the variation of the surface diffusion coefficient with sulfur coverage has been correlated with changes in binding energy.

Stress relaxation of thin film due to coupled surface and grain boundary diffusion

A numerical scheme, based on an energy statement, is developed to simulate thin film morphology evolution and stress relaxation due to concurrent surface diffusion and grain boundary diffusion. Different from previously published works, this paper also explores the effects of mobility ratio of the two processes and the dihedral angle at the surface–grain boundary triple junction. The range of mobility ratio, in which the stress relaxation process is limited by either surface diffusion or grain boundary diffusion, is determined. It is found that, when the stress relaxation is limited by the rate of surface diffusion, the dihedral angle at the surface–grain boundary junction plays a significant role. A scheme of using both experimental and numerical results to determine grain boundary diffusivity is also presented. As an example, we obtain the activation energy and diffusivity for grain boundary diffusion in Cu thin film.

A study of iron catalyst for ammonia synthesis by electron spectroscopy methods

A concise review of former studies into iron catalyst using electron spectroscopy methods is presented. A model of iron catalyst’s surface, the so-called double layer model is presented. On the basis of this model both a method of determining surface content of a real iron catalyst and a method of determining the rate of potassium surface diffusion were developed. A double layer model was used to interpret thermal stability loss of potassium atoms on iron surface subjected to sulphur compounds’ exposure.

A novel process to treat spent petroleum catalyst using sulfur-oxidizing lithotrophs

A novel process was developed using sulfur-oxidizing bacteria to extract metal values like Ni, V and Mo from spent petroleum catalyst. Bacteria were grown in elemental sulfur media for five day and after filtering, the filtrate was used for leaching purpose. Effect of different parameters such as contact time, particle size, pulp density and lixiviant composition were studied to find out the extent of metal leaching during the leaching process. XRD analysis proved the existence of V in oxide form, Ni in sulfide form, Mo both in oxide as well as sulfide forms, and sulfur in elemental state only. In all the cases studied Ni and V showed higher leaching efficiency compared to Mo. The low Mo leaching rate may be either due to formation of impervious sulfur layer or refractoriness of sulfides or both. Leaching kinetics followed dual rate, initial faster followed by slower. Dissolution mechanism was explained on the basis of both surface and pore diffusion rate. The leaching kinetics followed 1st order reaction rate. Finally, multiple linear regression analysis was carried out to compare the observed and calculated leaching percentage values for three metals.

Catalytic Cracking of 1-Hexene to Propylene Using SAPO-34 Catalysts with Different Bulk Topologies

Abstract Three SAPO-34 catalysts, 100% SAPO-34, 30% SAPO-34, and meso-SAPO-34, with different bulk topologies were prepared. The catalysts were characterized by N 2 adsorption, scanning electron microscopy, X-ray diffraction, and infrared spectroscopy techniques. The pore size, total acidity, and internal cage structure of the catalysts were almost identical, but they had different bulk appearances. The role of the bulk topology/structure of the catalysts was studied using 1-hexene cracking. On 30% SAPO-34, the surface acidity and diffusion rate decreased due to blocking by binder, which adversely affected catalytic activity. 100% SAPO-34 gave better cracking ability and higher propylene selectivity because of suitable acid sites and effective shape selectivity, respectively. In order to study the effect of diffusion, meso-SAPO-34 was used. The different bulk structure gave different feed conversion and selectivity profiles. A superior control of the stereochemistry was observed in the cracking by the meso-SAPO-34 and 100% SAPO-34 catalysts, in which enhanced diffusion mass transport played an appreciable role. Most of the propylene was produced by the direct cracking pathway by the β-scission carbenium ion mechanism. Hydrogen transfer reactions became significant at higher conversions. Decreasing the residence time to a certain extend is an appropriate way to obtain high propylene yield and selectivity. Activity and selectivity patterns for 1-hexene cracking to propylene were compared to justify superior SAPO-34 topology for 1-hexene cracking to propylene.

Influence of migration anisotropy on the growth mechanism

The kinetics Monte Carlo (KMC) simulation and the rate equation method were used to study the influence of the anisotropic surface migration on the submonolayer growth mechanism. Simulations show that the influence of the migration anisotropy tends to saturate when the migration anisotropy exceeds 100. There is good agreement between the KMC simulation and the adjusted rate-equation. The researches show that it is significant to define a new parameter: effective diffusion rate. The effective diffusion rate is not only determined by the temperature and the hopping barrier, but is also by the anisotropy. The effective surface diffusion rate will decrease with the increase of the surface migration anisotropy.