Heterogeneous Surface Reactions Research Articles

PtRu colloidal catalysts: Characterisation and determination of kinetics for methanol oxidation

The activities of three 30 wt.% PtRu/C catalysts of the same nominal composition (Pt:Ru, 1:1 a/o) were investigated with regard to methanol oxidation. The catalysts were synthesized by Bönnemann's colloidal precursor method using different metal salt precursors and reducing agents. The catalysts were characterized ex situ by energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD) and in situ CO stripping and cyclic voltammetry. The activity towards methanol electro-oxidation was checked in steady-state experiments at 22 and 60 °C. The experimental rate data can be described well by a kinetic model, which includes methanol adsorption on the Pt-sites, formation of C-containing adsorbed species, OH ads formation on the Ru sites and heterogeneous surface reaction between C-adsorbate and OH ads. The kinetic model parameters were identified from the experimental data and were used to explain the differences in catalytic activity.

Element partitioning and potential mobility within surface dusts on buildings in a polluted urban environment, Budapest

A voluminous literature exists on the analysis of water-soluble ions extracted from gypsum crusts and patinas formed on building surfaces. However, less data is available on the intermediate dust layer and the important role its complex matrix and constituents play in crust/patina formation. To address this issue, surface dust samples were collected from two buildings in the city of Budapest. Substrate properties, different pollution levels and environmental variations were considered by collecting samples from a city centre granite building exposed to intense traffic conditions and from an oolitic limestone church situated in a pedestrian area outside and high above the main pollution zone. Selective extraction examines both water-soluble ions (Ca 2+, Mg 2+, Na +, K +, Cl −, NO 3 −, SO 4 2−) and selected elements (Fe, Mn, Zn, Cu, Cr, Pb, Ni) from the water-soluble, exchangeable/carbonate, amorphous Mn, amorphous Fe/Mn, crystalline Fe/Mn, organic and residual phases, their mobility and potential to catalyse heterogeneous surface reactions. Salt weathering processes are highlighted by high concentrations of water-soluble Ca 2+, Na +, Cl − and SO 4 2− at both sites. Manganese, Zn and Cu and to a lesser extent Pb and Ni, are very mobile in the city centre dust, where 30%, 54%, 38%, 11% and 11% of their totals are bound by the water-soluble phase, respectively. Church dust shows a sharp contrast for Mn, Zn, Cu and Pb with only 3%, 1%, 12% and 3% of their totals being bound by the water-soluble phase, respectively. This may be due to (a) different environmental conditions at the church e.g. lower humidity (b) continuous replenishment of salts under intensive city centre traffic conditions (c) enrichment in oxidisable organic carbon by a factor of 4.5 and a tenfold increase in acidity in the city centre dust.

마이크로 연소기에서 발생하는 열 소염과 화학 소염 현상 (II)- SiOx(x≤2) 플레이트의 물리, 화학적 성질이 소염에 미치는 영향 -

In order to realize a stably propagating flame in a narrow channel, flame instabilities resulting from flame-wall interaction should be avoided. In particular flame quenching is a significant issue in micro combustion devices; quenching is caused either by excessive heat loss or by active radical adsorptions at the wall. In this paper, the relative significance of thermal and chemical effects on flame quenching is examined by means of quenching distance measurement. Emphasis is placed on the effects of surface defect density on flame quenching. To investigate chemical quenching phenomenon, thermally grown silicon oxide plates with well-defined defect distribution were prepared. ion implantation technique was used to control defect density, i.e. the number of oxygen vacancies. It has been found that when the surface temperature is under , the quenching distance is decreased on account of reduced heat loss; as the surface temperature is increased over , however, quenching distance is increased despite reduced heat loss effect. Such abberant behavior is caused by heterogeneous surface reactions between active radicals and surface defects. The higher defect density, the larger quenching distance. This result means that chemical quenching is governed by radical adsorption that can be parameterized by oxygen vacancy density on the surface.

Deposition Temperature Effect on the Structure and Optical Property of RF‐PACVD‐Derived Hydrogenated SiCNO Film

AbstractAmorphous hydrogenated silicon oxocarbonitride (SiCNO:H) films have been deposited by plasma‐assisted chemical vapour deposition (PACVD) using bis(trimethylsilyl)carbodiimide (BTSC) as a single source precursor in a argon (Ar) radio‐frequency plasma. In this work the SiCNO:H films deposited at different deposition temperatures were studied in terms of deposition rate, refractive index, surface roughness, microstructure, and chemical composition including bonding state. The results showed that a higher deposition temperature enhanced the formation of Si‐N bonds, and disfavoured the formation of N=C=N, Si‐NCN, C‐H and Si‐CH3 bonds. A higher deposition temperature also decreased the deposition rate and increased the refractive index of the resulting SiCNO:H film. With increasing temperature a denser film was formed, indicating a change of the deposition mechanism, i.e., transformation from particle precipitation to heterogeneous surface reaction. Except for the coatings deposited at room temperature, the surface of the films was smooth with a roughness of around 4 nm at the centre in the range of 5 μm x 5 μm area. Moreover, the films contained 8 ∼ 16 at.% oxygen bonded to Si, which originated from the remnant H2O in the deposition chamber.

OXIDATION REACTIONS OF SOLID CARBON IN SUPERCRITICAL WATER

ABSTRACT Compared to the research for homogeneous phase supercritical water oxidation (SCWO), kinetic understanding of SCWO progress on solid materials is less developed. We studied the reaction progress of carbon particles under SCWO conditions, where mass transfer processes and heterogeneous reactions are involved with mutual interactions. Reaction progress under mass-transfer limited conditions was observed for solid activated carbon at 400–500°C. The overall reaction of synthetic graphite observed at 500–600°C and that of activated carbon below 400°C were controlled by the internal surface heterogeneous reaction. The reaction rate at a unit internal surface area was proportional to the oxygen concentration and the specific reaction rate was described as 1.7 × exp (− 117 kJ mol−1/RT) m s−1. The reaction product was exclusively CO2 with a small amount of CO in the case of activated carbon.

Adaptation of the in situ adaptive tabulation (ISAT) procedure for efficient computation of surface reactions

In this article, the in situ adaptive tabulation (ISAT) procedure, originally developed [Pope, S. (1997). Computationally efficient implementation of combustion chemistry using in situ adaptive tabulation. Combustion Theory and Modeling, 1, 41–63] for the efficient computation of homogeneous reactions in chemically reacting flows, is adapted for the efficient computation of heterogeneous surface reactions. The main difference between the treatment of homogeneous and heterogeneous reactions is that heterogeneous reaction calculations require solution of a set of nonlinear algebraic equations at boundary faces/nodes, as opposed to the solution of an initial value problem, for which the original ISAT procedure was developed. The article first explains how the ISAT procedure is adapted for repetitive solution of a single nonlinear equation (scalar case), and subsequently presents the mathematical formulation for a system of nonlinear equations with multiple inputs and outputs (vector case). The new procedure is tested for catalytic combustion of a methane–hydrogen mixture on platinum using a reaction mechanism that involves 24 steps and 19 species (8 gas-phase and 11 surface-adsorbed). For this particular test case, the computational speed-up factor was found to be close to two orders of magnitude. Strategies to minimize memory requirements, while retaining optimum efficiency and accuracy are also outlined in the article.

Impact of heterogeneous sulfate formation at mineral dust surfaces on aerosol loads and radiative forcing in the Goddard Institute for Space Studies general circulation model

Heterogeneous chemical reactions between sulfate precursors on the surface of mineral dust aerosols affect the atmospheric aerosol cycle and the Earth radiation budget. Heterogeneous reactions of sulfur dioxide with mineral dust particles enhance sulfate formation by producing internally mixed sulfate dust aerosols. The anthropogenic sulfate forcing is estimated to be reduced to −0.18 W/m2 because of the reduced load of externally mixed sulfate aerosols, compared to −0.25 W/m2 when heterogeneous surface reactions are excluded. Sulfate coating on mineral dust particles increases wet deposition of dust, causing a positive anthropogenic forcing due to less sulfate coating at preindustrial times. However, heterogeneous reaction pathways are highly uncertain, which is reflected in the wide spread of reaction pathways and uptake probability coefficients in the literature. We undertake a series of sensitivity experiments with the Goddard Institute for Space Studies climate model to investigate the impact of the uncertainty in uptake mechanisms and dust aerosol size distributions on the simulated sulfate cycle. The results of this study are very sensitive to both tested variables. For example, doubling the clay emissions (particles whose radii are less than 1 μm) leads to a sevenfold increase in heterogeneous sulfate production.

Mechanistic Model for Aluminum Particle Ignition andCombustion in Air

A mechanistic model for the ignition and combustion of an isolated aluminum particle burning in air is presented. The model consists of two stages, ignition and combustion. In the ignition stage, melting and heterogeneous surface reactions (HSR) are assumed to occur until a predefined transition temperature of the oxide is attained. In the combustion stage, a quasi-steady state diffusion flame is assumed, and a new conserved scalar formulation is presented that accounts for the deposition of metal oxide on the surface of the molten aluminum. A system of nonlinear ordinary differential equations that describes each stage self-consistently with the gas-phase analysis is developed. Representative results are presented for a range of ambient temperature conditions and compared to experimental measurements. Predictions of overall burn rates, particle velocity, and flame radius show good agreement with experimental data. Also discussed is the extension of the conserved scalar approach to include a more generalized oxidizing environment as well as HSR from nitride reactions during the quasi-steady burning stage.

Ground-Based DOAS Measurements of Stratospheric Trace Gases at Two Antarctic Stations during the 2002 Ozone Hole Period

Abstract Compared to recent years, the development of the Antarctic ozone hole in 2002 showed very unusual dynamical features. The midwinter polar vortex was one of the smallest observed during the past decade. Driven by planetary waves, the vortex showed a strong asymmetry in early spring. A large air mass separated in late September, leaving what was previously a small vortex even smaller. Furthermore, stratospheric temperatures exceeded the polar stratospheric cloud (PSC) threshold earlier than in previous years, leading to a decrease in halogen activation by heterogeneous surface reactions. Ground-based observation of stratospheric trace gases in austral spring of 2001 and 2002 using passive Differential Optical Absorption Spectroscopy (DOAS) observations of zenith-scattered sunlight in the UV and visible wavelength region (320–650 nm) are presented. Using DOAS measurements of ozone, NO2, BrO, and OClO at two different Antarctic sites, Neumayer Station (70°S, 8°W) and Arrival Heights (78°S, 167°E), the chemical composition of the stratosphere is investigated under the unusual conditions of the 2002 ozone hole period and compared to the more typical observations of the previous year (2001).

Slow Adsorption Reaction between Arsenic Species and Goethite (α-FeOOH): Diffusion or Heterogeneous Surface Reaction Control

The slow stage of phosphate or arsenate adsorption on hydrous metal oxides frequently follows an Elovich equation. The equation can be derived by assuming kinetic control by either a diffusion process (either interparticle or intraparticle) or a heterogeneous surface reaction. The aim of this study is to determine whether the slow stage of arsenic adsorption on goethite is more consistent with diffusion or heterogeneous surface reaction control. Adsorption kinetics of arsenate and dimethylarsinate (DMA) on goethite (alpha-FeOOH) were investigated at different pH values and inert electrolyte concentrations. Their adsorption kinetics was described and compared using Elovich (Gamma vs ln time) plots. Desorption of arsenate and DMA was studied by increasing the pH of the suspension from pH 4.0 to pH 10.0 or 12.0. The effective particle sizes and zeta-potential of goethite were also determined. Effective particle size increased rapidly as the pH approached pH(IEP), both in the absence and presence of arsenic. Inert electrolyte concentrations and pH had no effect on the slow stage of arsenate adsorption on goethite, while the kinetics of DMA adsorption on goethite was influenced by both parameters. The slow stage of arsenate adsorption on goethite follows an Elovich equation. Since effective particle size changes with both pH and inert electrolyte concentrations, and effective particle size influences interparticle diffusion, the arsenate adsorption kinetics indicate that the slow adsorption step is not due to interparticle diffusion. DMA also has complex adsorption kinetics with a slow adsorption stage. DMA desorbed completely and rapidly when the pH was raised, in contrast to the slow adsorption kinetics, indicating that the slow adsorption step is not due to intraparticle diffusion. The slow adsorption is not the result of diffusion, but rather is due either to the heterogeneity of the surface site bonding energy or to other reactions controlling arsenic removal from solution.

A free boundary problem describing reaction–diffusion problems in chemical vapor infiltration of pyrolytic carbon

In this paper we consider chemical vapor deposition of pyrolytic carbon from methane in hot wall reactors. Especially, we deal with the interaction of homogeneous gas-phase and heterogeneous surface reactions. The resulting mathematical model is composed of a system of reaction–diffusion equations in a corner domain supplied with the Gibbs–Thomson law, which describes the movement of the free boundary, arising from the carbon deposition. We prove a short time existence and uniqueness result in Hölder spaces. We achieve this by contraction arguments and transforming the Gibbs–Thomson law to local coordinates to obtain a nonlinear parabolic equation on a manifold.

Particle-beam experiment to study heterogeneous surface reactions relevant to plasma-assisted thin film growth and etching

An ultrahigh-vacuum-based particle-beam experiment to study heterogeneous surface reactions relevant to plasma–surface interaction processes is presented. The experiment comprises two radical beam sources and a source for low energy ions. As diagnostic tools real-time in situ ellipsometry and infrared spectroscopy are implemented. The infrared sensitivity for thin films is enhanced through application of an optical cavity substrate. The fluxes of the radical beam sources are quantified absolutely for the production of hydrogen atoms and methyl radicals. The ion source is also quantified for a wide variety of ionic species, e.g., He+, Ar+, H+, H2+, H3+, and CH3+. Ion energies from above 1 keV down to 1 eV are achievable. The setup allows one to investigate heterogeneous surface processes of one single species or simultaneous interaction of up to three different, individually controllable species with a surface of interest. By running the radical sources to produce hydrogen and methyl radicals and the ion source with the ions mentioned, microscopic surface processes relevant to deposition and erosion of hydrocarbon layers in low-pressure gas discharges were studied in great detail. Sample results for the hydrogen CH3 growth synergism, ion-assisted film growth, and chemical sputtering are presented to demonstrate the capability of the setup.

Effect of heat and mass transfer coefficients on the performance of automotive catalytic converters

This paper numerically investigates the role of heat and mass transfer coefficients on the performance of automotive catalytic converters, which are employed to reduce engine exhaust emissions. The pollutant conversion performance of a converter is influenced by a number of physical and chemical processes that take place in gaseous and solid phases as the exhaust gases flow through the catalyst. A quantitative predictive understanding of these complex catalyst processes involving flow dynamics, heterogeneous surface reactions and heat and mass transport mechanisms is important in improving the converter design. The role of convective transport phenomena becomes important at high temperature when the mass transfer becomes rate-limiting to an increasing extent. The objective of the present study is to elucidate the influence of convective heat and mass transfer coefficients (mechanisms). The mathematical model considers the conservation of mass, momentum and energy in both gaseous and solid phases. In addition to the heterogeneous surface reactions, the model also takes into account the adsorption/desorption of oxygen in the catalyst during non-stoichiometric composition of air/fuel mixtures. The governing equations are solved by an implicit scheme using a successive line under a relaxation method. The converter performance under the transient conditions as simulated by the US Federal Test Procedure (US-FTP) is analysed.

Kinetics of surface reactions in carbon deposition from light hydrocarbons

Carbon deposition from ethene, ethine and propene as a function of pressure was studied at various temperatures and two different surface area/volume ratios. Deposition rates as a function of pressure of all hydrocarbons indicate Langmuir–Hinshelwood kinetics which suggests that the deposition process is controlled by the heterogeneous surface reactions (growth mechanism). These kinetics are favored at decreasing reactivity (C 3H 6>C 2H 2>C 2H 4), decreasing temperature and residence time as well as increasing surface area/volume ratio. A linear rate increase at high pressures suggests that carbon is additionally or preferentially deposited by aromatic condensation reactions between polycyclic aromatic hydrocarbons large enough to be physisorbed or condensed on the substrate surface (nucleation mechanism). The results completely agree with earlier results obtained with methane.

Coarse bifurcation analysis of kinetic Monte Carlo simulations: A lattice-gas model with lateral interactions

We present a computer-assisted study of “coarse” stability/bifurcation calculations for kinetic Monte Carlo simulators using the so-called coarse timestepper approach presented in A. G. Makeev, D. Maroudas, and I. G. Kevrekidis, J. Chem. Phys. 116, 10083 (2002). Our illustrative example is a model of a heterogeneous catalytic surface reaction with repulsive adsorbate–adsorbate interactions and fast diffusion. Through numerical continuation and stability analysis, we construct one- and two-parameter coarse bifurcation diagrams. We also discuss several computational issues that arise in the process, the most important of which is the “lifting” of coarse, macroscopic initial conditions (moments of adsorbate distributions) to fine, microscopic initial conditions (distributions conditioned on these moments).

Low reynolds number combustion of a spherical carbon particle

A theory is presented for the steady combustion of a spherical carbon particle in the slow viscous flow of an oxidizing ambient. Heterogeneous surface reactions produce radial convection and diffusion from the particle, which couples with the ambient convection to affect the rate of particle mass loss. In the theory, a small Reynolds number characterizes the ambient convection. However, the radial convection, associated with the particle mass loss, is not necessarily small. The gas stream function and chemical species are described. The emphasis is upon explicit analytical relationships for particle mass loss rates that can be used to interpret experimental observations.

Kinetics of Heterogeneous Catalytic Ozone Decomposition in Water on an Activated Carbon

Ozone decomposition in water in the presence of an activated carbon has been studied. Variables investigated were agitation speed, carbon particle size, temperature and pH. In all cases, the presence of activated carbon improved the ozone decomposition rate. Between pH 2 and 7 the ozone decomposition rate due to both the homogeneous and heterogeneous mechanisms hardly varied while a significant increase was noticed with increasing pH. A kinetic study based on a Langmuir-Hinselwood type mechanism for the heterogeneous surface reaction was undertaken. According to this mechanism the heterogeneous ozone decomposition kinetics can be simplified to follow a first order process. Fit of experimental results to the kinetic equations derived from the mechanism allowed for the determination of the apparent first order rate constants of the ozone surface heterogeneous reaction and adsorption equilibrium constants.

Application of Aerosol Techniques to Study the Catalytic Formation of Methane on Gasborne Nickel Nanoparticles

“Aerosol catalysis” is shown to be a powerful tool for investigating the catalytic properties of freshly formed nanoparticles in situ and without substrate interference. The method is first outlined conceptually, followed by an illustrative application to the catalytic formation of methane on a nickel nanoaerosol. Reaction order and activation energy were found conform with generally accepted values from supported Ni catalysts. The TOR decreases strongly during the first 10 s as the reaction proceeeds toward a steady value. The decrease correlates with a buildup of about 0.3 monolayer equivalents of carbon on the particle surface measured by TGA and a decline in particle photoelectric activity observed via measurement by aerosol photoemission spectroscopy (APES). APES is shown to be capable of detecting the progressive degradation of the freshly formed particle surface due to a heterogeneous surface reaction on a millisecond time scale. Furthermore, it was possible to induce order-of-magnitude changes in ...

Nonlocal nature of the electron energy spectrum in a glow-discharge in pure O2: II. Actinometry of O(3 P) atoms in a plasma at low gas pressures

Results are presented from measurements of the density of oxygen atoms in the positive column of a dc discharge in pure oxygen by the actinometric technique using Ar atoms. Based on the excitation rate constants calculated using two different approaches (namely, the two-term approximation and the Monte Carlo method) to solving the Boltzmann equation for a spatially inhomogeneous electron distribution function, the applicability of the actinometric technique is analyzed. The effects of the discharge kinetics and the nonlocal character of the electron energy spectrum on the accuracy of actinometric measurements are studied. It is shown that the results of measurements depend only slightly on the accuracy with which the electron energy distribution function is described. Over a wide range of the reduced electric field E/N≈40–250 Td, the oxygen atom density calculated using the spatially homogeneous distribution function differs by several percent from that calculated accurately, taking into account the nonlocal character of the electron energy spectrum. It is shown that using the actinometric technique to measure the absolute concentration of oxygen atoms in a plasma requires a detailed description of the discharge plasmochemical kinetics, including a thorough analysis of all possible processes (particularly, surface heterogeneous reactions) that determine the density of active particles at low pressures. At the same time, the use of the actinometric technique for monitoring the behavior of the density of oxygen atoms in a plasma is justified over a wide range of reduced electric fields up to ∼200 Td when the O(3p3P-3p3S) transition (λ=844.6 nm) is used and the degree of dissociation is [O]/[O2]>0.02.

Mobilization of iron from coal fly ash was dependent upon the particle size and source of coal: analysis of rates and mechanisms.

The observed iron mobilization rate from size-fractionated coal fly ash is consistent with the model predictions for a limiting case of mass transfer where the dominant resistance is diffusion through a layer of depleted solid between the surface of spherical particles and a shrinking core of unreacted material. The rate of mobilization of iron from coal fly ash under physiologically relevant conditions in vitro was previously shown to depend on the size of the ash particles and on the source of the coal, and these in vitro measurements have been shown to correlate with indirect measurements of excess iron in cultured cells. Existing iron mobilization data were compared to mathematical models for mass transfer and chemical reaction in solid-liquid heterogeneous systems. Liquid-phase diffusion resistance can be ruled out as the rate-limiting mechanism for iron mobilization as the model predictions for this case are clearly inconsistent with the measurements. Other plausible hypotheses, such as a rate limited by a heterogeneous surface reaction, cannot be conclusively ruled out by the available data. These mathematical analysis methods are applicable to the design of future experiments to determine the rate-limiting mechanism for the mobilization of iron and of other transition metals from both ambient air samples and surrogates for major sources of particulate air pollution.