Surface Reaction Model Research Articles

Alternative method to characterize continuous and discontinuous phase transitions in surface reaction models

In this paper we revisited the Ziff-Gulari-Barshad model to study its phase transitions and critical exponents through time-dependent Monte Carlo simulations. We use a method proposed recently to locate the nonequilibrium second-order phase transitions and that has been successfully used in systems with defined Hamiltonians and with absorbing states. This method, which is based on optimization of the coefficient of determination of the order parameter, was able to characterize the continuous phase transition of the model, as well as its upper spinodal point, a pseudocritical point located near the discontinuous phase transition. The static critical exponents β,ν_{∥}, and ν_{⊥}, as well as the dynamic critical exponents θ and z for the continuous transition point, were also estimated and are in excellent agreement with results found in literature.

Mesoporous InN/In2O3 heterojunction with improved sensitivity and selectivity for room temperature NO2 gas sensing

Establishing heterostructures is a good strategy to improve gas sensing performance, and has been studied extensively. In this work, mesoporous InN/In2O3 composite (InNOCs) heterostructures were prepared through a simple two-step strategy involving hydrothermal synthesis of In2O3 and subsequent nitriding into InN-composite In2O3 heterostructures. We found that the InN content has great influence on the resistance of InNOCs, and thus, the gas sensing performance. In particular, InNOC-36.9 (with InN content of 36.9% in the composites) shows an excellent sensing response towards different concentrations of NO2, as well as good stability after one week of exposure to 200 ppb NO2 at room temperature. The highest sensing response (ΔR/R0) is up to 1.8 for the low NO2 concentration of 5 ppb. Even more significantly, the theoretical limit of detection (LOD) of the InNOC-36.9 sensor is 31.7 ppt based on a signal-to-noise ratio of 3 (the measured LOD is 5 ppb), which is far below the US NAAQS value (NO2: 53 ppb). In addition, a rational band structure model combined with a surface reaction model is proposed to explain the sensing mechanism.

An investigation on dissolution kinetics of single sodium carbonate particle with image analysis method

Dissolution kinetics of sodium carbonate is investigated with the image analysis method at the approach of single particle. The dissolution experiments are carried out in an aqueous solution under a series of controlled temperature and pH. The selected sodium carbonate particles are all spherical with the same mass and diameter. The dissolution process is quantified with the measurement of particle diameter from dissolution images. The concentration of dissolved sodium carbonate in solvent is calculated with the measured diameter of particle. Both surface reaction model and mass transport model are implemented to determine the dissolution mechanism and quantify the dissolution rate constant at each experimental condition. According to the fitting results with both two models, it is clarified that the dissolution process at the increasing temperature is controlled by the mass transport of dissolved sodium carbonate travelling from particle surface into solvent. The dissolution process at the increasing pH is controlled by the chemical reaction on particle surface. Furthermore, the dissolution rate constant for each single spherical sodium carbonate particle is quantified and the results show that the dissolution rate constant of single spherical sodium carbonate increases significantly with the rising of temperature, but decreases with the increasing of pH conversely.

Adsorbent layer for adsorption heat pump prepared with the surface-modified ferroaluminophosphate particles and inorganic silica binder

In this study, we prepared an adsorbent layer for adsorption heat pump by using ferroaluminophosphate (FAPO) particles and inorganic silica binder and characterized the physical properties thereof. We tried two ideas in preparing the adsorbent layer: dual precursor for binder and surface modification of the FAPO particles. The inorganic silica binder was prepared through a sol–gel process by using tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) as the dual precursor. The surface modification by TEOS molecules on the FAPO particles was done to promote the reactivity between particles and binder. The surface modification to form monolayer of silane was carefully done in a hydrophobic solvent, cyclohexane, preventing the supply of water molecules into the solution. As a result, both the use of dual precursor binder and the surface modification effectively improved the mechanical stability of the layer as shown by higher resistance to mechanical vibration. It was analyzed that the addition of MTES served to enhance the mechanical stability of the adsorbent layer by providing the flexible element, Si–C bonding, in the rigid inorganic matrix. To explain the microstructure analysis by SEM and TEM, and the zeta potential measurement, we proposed a surface reaction model comprising the formation of very thin layer of surface silanol on FAPO particles enhancing the reactivity between the particle and the binder. Both the dual precursor and the surface modification produced no noticeable detrimental effect on the key properties such as water vapor adsorption and thermal conductivity.

How theory and simulation can drive fuel cell electrocatalysis

Over the last decade, theory and modeling have become essential tools to navigate the parameter space that governs activity and stability of electrocatalyst systems for polymer electrolyte fuel cells. This perspective covers essential phenomena from atomic scale to nanoscale and discusses the impact of the key parameters at play. It is centered around the development of first-principles electrochemical methods as a foremost goal in the field. The general modeling framework entails at its core a self-consistency problem that must be solved to relate the metal phase potential to descriptors of catalyst activity and stability. Density functional theory has captured a central role in this rapidly evolving field. The article puts more than usual emphasis on aspects of the multifaceted challenges in fuel cell electrocatalysis that at present lie beyond the capabilities of density functional theory; they include metal charging and solvent effects. Following the general discussion of the theoretical-computational framework, an approach for “deciphering” the oxygen reduction reaction is demonstrated; it reconciles reaction pathways and free energy profiles obtained from density functional theory simulations with kinetic modeling of surface reactions and effective kinetic parameters. Another section dwells on the importance of metal charging phenomena that are especially important for the catalytic function of nanoporous media. The penultimate section exposes the ambivalent role of Pt oxide formation in modulating catalytic properties for the oxygen reduction reaction as well as for catalyst corrosion.

(Invited) Atomically Controlled Processing for Si and Ge CVD Epitaxial Growth

Atomically controlled processing has become indispensable for the fabrication of Si-based ultrasmall devices and heterodevices for ultralarge scale integration. Our concept of atomically controlled processing is based on atomic-order surface reaction control [1-3]. The final goal is the generalization of atomic-order surface reaction processes and the creation of new properties in Si-based ultimate small structures. In the stage of nanometer-scale device fabrication, preparing atomic-order controlled surfaces and interfaces and the introduction of Ge into the Si process are required. In this work, we describe thesegregation of dopants to the surface and surface roughness generation observed during in-situ doping and their suppression by applying an atomic layer doping approach. Furthermore results of nm-order thick Ge epitaxial growth on Si(100) using an ultraclean low-pressure CVD system are presented. Figure 1 shows the P depth profile in a Si epitaxial film. Even after switching off the PH3 gas supply, P atoms are incorporated into the Si layer during Si epitaxial growth. In the steady state, it is estimated that P concentration saturates at around 3x1019cm-3 and P coverage on the Si surface is about 0.1 monolayer (6.8x1013cm-2). These values are calculated from the PH3 partial pressure and Si growth rate shown in Fig. 1 based on the surface reaction model shown in Ref. 3. The P incorporation rate, that is the product of the Si growth rate (0.8nm/min) and P concentration (about 1x1019cm-3), is about 8x1011cm-2/min. If it is assumed that the desorption of P atoms on the Si surface can be neglected [4], 6.8x1013cm-2 P atoms on the surface are incorporated into the Si epitaxial layer with 8x1011cm-2/min even after the PH3 supply has been stopped. The increase of surface roughness from RMS 0.1nm to 0.8nm during in-situ doping can be explained by the suppression of the SiH4reaction caused by the P atoms which are present on the surface. Suppression of P segregation and surface roughening are achieved using P atomic layer doping in Si. This includes the optimization of the capping Si layer deposition conditions and the P atomic amount [5] as shown in Fig. 2. This may result from low temperature Si epitaxial growth on P atoms with high growth rate. That could be demonstrated for P atomic layer doping during Ge epitaxial growth too [6]. In a second part of the paper results for nm-order-thick Ge high quality epitaxial growth on Si (100) are described. Because pure Ge and Si oxide are formed on Si substrate even at 300oC by the reaction between vaporized Ge oxide and Si substrate [7], the suppression of Ge oxide vaporization in the CVD reactor is very important. It was proposed that Ge island growth in an initial growth stage is caused by the suppression of adsorption and/or decomposition of GeH4 on the H-terminated Si surface [8]. Even on the initially H-free Si surface, Si atoms are H-terminated by the decomposition of GeH4. In order to decrease the hydrogen desorption temperature from the Si site, nm-thick Si1-xGex buffer layer on Si (100) was proposed [9,10]. The suppression of surface roughening was improved by nm-order-thick Ge epitaxial growth on nm-order-thick Si1-xGex/Si(100). The surface segregation of Si through nm-order-thick Ge layer from the substrate was observed at Ge growth temperature of 350oC. These results demonstrate the capability of atomically controlled processing for nm-order thick Si and Ge CVD epitaxial growth for ultralarge scale integration. This work is partially supported by the JSPS Core-to-Core Program, “International Collaborative Research Center on Atomically Controlled Processing for Ultralarge Scale Integration”.

Calcium isotopic fractionation during travertine deposition under different hydrodynamic conditions: Examples from Baishuitai (Yunnan, SW China)

Revealing the potential of Ca isotopes in the research of travertine formation processes is the principal goal of this study. To achieve this, the hydrochemistry, travertine precipitation rates, and variations in Ca isotopic compositions (δ44/40Ca) of present-day endogenic (thermogenic) travertine in two different hydrodynamic systems (canal and pool) are studied at Baishuitai (Yunnan, SW China). In the canal, where the travertine precipitation is faster, δ44/40Ca values of both solution and travertine show a downstream increase, with Ca isotopic fractionation (Δ44/40CaCaCO3-aq) equal to ~−1.6‰. Compared to the canal, Ca isotopic fractionation between travertine and aqueous Ca2+ is smaller (Δ44/40CaCaCO3-aq≈−1.2‰) in the pools where travertine precipitation rates are lower. The spatial variations in δ44/40Ca values of solution and travertine are related to the amount of Ca removed from the solution when δ44/40Ca values of spring water remain stable. In addition, the results confirm the control of precipitation rates on Ca isotopic fractionation between calcite and parent aqueous Ca2+ using natural samples. The observed negative correlation between Δ44/40CaCaCO3-aq and travertine precipitation rates can be explained by the steady-state kinetic surface reaction model proposed by DePaolo (2011), with equilibrium and kinetic fractionation factors of αeq=1.0000±0.0001 and αf=0.9983±0.0002, respectively. An important consequence of this study is that Ca isotopes in travertine systems are good tools to have access to variations of δ44/40Caaq, which are directly linked to the amount of precipitated Ca. This could then be useful for reconstructing past hydrodynamic conditions when applied to travertine core data.

Study on the roasting and leaching behavior of high-sulfur bauxite using ammonium bisulfate

High sulfur content is one of the many challenges in processing the refractory bauxites of China. A new method to prepare Al2O3 from high-sulfur bauxite with the NH4HSO4 roasting process was studied. The paper offers a possible alternative method for desulfurization of bauxite that may improve overall process economics and environmental impact. The roasting mechanism and various parameters including the amount of the ammonium bisulfate addition, roasting temperature and roasting time were investigated. The results showed that the control step of the roasting process was chemical control and the apparent activation energy was 65.74kJ/mol over the reaction temperature range from 623 to 648K. The leaching kinetics of the sintered clinker by water was investigated, which was affected by temperature, mass ratio of liquid to solid and time. Analysis of the kinetic data indicates that the dissolution of bauxite ore was controlled by the shrinking core with surface reaction model. The apparent activation energy of the leaching process was found to be 37.94kJ/mol from 323 to 363K. The maximum aluminum dissolution ratio reaches about 93% under the optimal condition. This paper aims at giving some revelation and instruction for the further research and practice on high sulfur bauxites development and application.

Development of a Nonequilibrium Finite-Rate Ablation Model for Radiating Earth Reentry Flows

Thermal protection system design for atmospheric reentry vehicles remains a challenging and complex problem. Recent advances in computational modeling of air–carbon interactions consider competing finite-rate reactions on a limited number of available surface sites. One of the most advanced kinetic models is due to Zhluktov and Abe. However, the Zhluktov and Abe model only describes the oxidation and sublimation of carbon and has no nitridation mechanism. The following study develops several modifications to the Zhluktov and Abe air–carbon model that account for all three reaction mechanisms with the goal of improving cyanogen shock-layer radiation predictions to recent experimental results. First, the study examines two possible paths for carbon nitridation and then assesses the augmented surface reaction model in a representative blunt-body reentry flow. Second, a sensitivity analysis is performed to determine which surface reactions have the most impact on altering cyanogen radiance predictions. It was found that modifications to the oxidation mechanisms significantly improved the comparisons and the nitridation mechanism had a minor effect. Specifically, it was determined that carbon monoxide must be the principal oxidation product at high surface temperatures instead of carbon dioxide, as the Zhluktov and Abe model had originally predicted. More detailed measurements of both the carbon oxidation and nitridation reactions at higher surface temperatures are required to further validate and improve the rate parameters derived in this study.

Removal of Zn from Contaminated Sediment by FeCl3 in HCl Solution

Harbor sediments contaminated with ZnS concentrate were treated by ferric chloride in HCl solution to remove Zn. The sediments were evaluated using Tessier’s sequential extraction method to determine the different metal phase associations of Zn. Leaching tests were performed to investigate the effects of experimental factors, such as agitation speed, ferric ion concentration, temperature, and pulp density, on the removal of Zn. The sequential extraction procedure revealed that about 17.7% of Zn in the sediment was associated with soluble carbonate and oxide phases. The results of the leaching tests indicated that higher ferric concentration and temperature increased the leaching efficiencies significantly, while the agitation speed has a negligible effect on the removal of Zn. The removal ratio increased to more than 99% within 120 min of treatment at 1 kmol·m−3 HCl solution with 1 kmol·m−3 Fe3+, 10% pulp density, and 400 rpm at 90 °C. The dissolution kinetics of Zn were discussed by comparing the two shrinking core models. It was determined that the kinetic data followed the diffusion controlled model well compared to the surface chemical reaction model. The activation energies were calculated to be 76.9 kJ/mol, 69.6 kJ/mol, and 58.5 kJ/mol for 0.25 kmol·m−3, 0.5 kmol·m−3, and 1 kmol·m−3 Fe3+, respectively.

A coupled numerical modelling and experimental approach in chemical vapour infiltration (CVI) process of SiC/SiC composites

Fabrication of silicon carbide fibres reinforced silicon carbide composite (SiC/SiC) by chemical vapour infiltration (CVI) process was investigated in this research with the help of both simulation and experimental set-up. CVI of silicon carbide preform was carried out through the pyrolysis of methyltrichlorosilane (MTS) over a broad temperature range at atmospheric pressure. The overall aim was the morphological description of the matrix (co-deposition of Si, SiC and C) during pyrolysis of MTS by CVI process and its state-of-the-art numerical calculation. Phase-field model was developed and deployed to predict the evolution of the microstructures and to describe the influence of the infiltration conditions on the properties of the composite during CVI process in conjunction with the implication of finite element method. Both mass transport and fluid motion in gas phase were considered. Experimental results exhibit three deposition regimes at different temperature ranges as predicted by the numerical simulation results. This also implies different deposition kinetics involved as investigated in the present research. The great difference of the steady-state deposition rate exceeding three orders of magnitude was explained in terms of a multiple steady-state surface reaction model of co-deposition of SiC, Si and C. Corresponding gas-phase compositions, over the temperature region covered in the present experiments, were calculated with a detailed pyrolysis reaction mechanism of MTS. (Less)

Dynamic order reduction of thin-film deposition kinetics models: A reaction factorization approach

A set of numerical tools for the analysis and dynamic dimension reduction of chemical vapor and atomic layer deposition (ALD) surface reaction models is developed in this work. The approach is based on a two-step process where in the first, the chemical species surface balance dynamic equations are factored to effectively decouple the (nonlinear) reaction rates, a process that eliminates redundant dynamic modes and that identifies conserved quantities. If successful, the second phase is implemented to factor out redundant dynamic modes when species relatively minor in concentration are omitted; if unsuccessful, the technique points to potential model structural problems. An alumina ALD process is used for an example consisting of 19 reactions and 23 surface and gas-phase species. Using the approach developed, the model is reduced by nineteen modes to a four-dimensional dynamic system without any knowledge of the reaction rate values. Results are interpreted in the context of potential model validation studies.

Protein Cross-Linking and Oligomerization through Dityrosine Formation upon Exposure to Ozone.

Air pollution is a potential driver for the increasing prevalence of allergic disease, and post-translational modification by air pollutants can enhance the allergenic potential of proteins. Here, the kinetics and mechanism of protein oligomerization upon ozone (O3) exposure were studied in coated-wall flow tube experiments at environmentally relevant O3 concentrations, relative humidities and protein phase states (amorphous solid, semisolid, and liquid). We observed the formation of protein dimers, trimers, and higher oligomers, and attribute the cross-linking to the formation of covalent intermolecular dityrosine species. The oligomerization proceeds fast on the surface of protein films. In the bulk material, reaction rates are limited by diffusion depending on phase state and humidity. From the experimental data, we derive a chemical mechanism and rate equations for a kinetic multilayer model of surface and bulk reaction enabling the prediction of oligomer formation. Increasing levels of tropospheric O3 in the Anthropocene may promote the formation of protein oligomers with enhanced allergenicity and may thus contribute to the increasing prevalence of allergies.

Extracting B2O3 from calcined boron mud using molten sodium hydroxide

Extracting B2O3 from calcined boron mud (CBM) was studied. The effect of factors such as reaction temperature and NaOH-to-CBM mass ratio on B2O3 extraction efficiency was investigated. The results show that increasing reaction temperature and NaOH-to-CBM mass ratio increases B2O3 extraction efficiency. There are two stages for the B2O3 extracting process: 0–20 min is the first stage, which is rapid; 20–50 min is the second stage, which is slower than the first stage. The overall extracting process follows the shrinking core model, and the first and second stages are determined to obey the surface chemical reaction model and the diffusion through the products layer model, respectively. The activation energies of the first and second stages are calculated to be 41.74 and 15.43 kJ·mol−1, respectively. The B2O3 extracting kinetics equations of the first and second stages are also obtained.

Theory and Modeling of Platinum Surface Reactions

The formation and reduction of surface oxide species determine both the electrocatalytic activity of Pt towards the oxygen reduction reaction as well as the rate of corrosive Pt dissolution [1]. We perform theory and modeling work to rationalize the various stages of oxide formation and reduction at Pt. Our mechanistic models establish relations between metal phase potential and surface oxidation state that govern the transient current response of the electrode, as probed for instance in cyclic voltammetry. In the first part, we will discuss a recently developed kinetic model for oxide formation and reduction at Pt in the voltage range of 0.65–1.15 V [2]. The model is evaluated against electrochemical [3], spectroscopic [4] and computational studies [5]. In the second part, we will present a kinetic model of oxide growth on platinum in the high voltage regime, above 1.15 V. The governing equations of the oxide growth model account for mass and charge conservation, species migration, and electric field effects. As the outcome, we will obtain a generalized oxide growth law of platinum. The results will be compared to experimental cyclic voltammetry data to extract rates of kinetic and transport processes. Moreover, the model incorporates a mechanism of platinum dissolution. Using this model, we strive to explain the dramatically enhanced rate of Pt dissolution at extended surfaces and in nanoparticle systems, observed in experimental studies that involved voltage cycling through the high voltage regime [6,7,8]. Knowledge of the mechanisms of growth and reduction of oxides on platinum will allow us to refine our theory of platinum dissolution in polymer electrolyte fuel cells [9].

Kinetics of the heterogeneous photo oxidation of the pesticide bupirimate by OH-radicals and ozone under atmospheric conditions

This article is concerned with the study of the photochemical degradation of bupirimate adsorbed on a quartz surface by atmospheric oxidants, namely ozone and OH radicals. OH oxidation experiments were conducted relative to two reference compounds, terbuthylazine and (4-chlorophenyl)(3,4-dimethoxyphenyl) methanone. Meanwhile, ozone oxidation experiments were performed in the absolute mode and were interpreted by both, the Surface Layer Reaction and the Gas Surface Reaction models of heterogeneous reactions. The obtained results show that the rate constants for the reactions between bupirimate and OH radicals and ozone are (cm3molecule−1s−1): (1.06±0.87)×10−12 and (5.4±0.3)×10−20, respectively. As a consequence, for the experimental conditions used in this study, the lifetime of bupirimate at quartz like surface/atmosphere interfaces is several months against ozone and a tenth of days against OH-radical.

Surface modification and dissolution behavior of Mg–Al hydrotalcite particles

The dissolution behavior of Mg–Al–CO32– hydrotalcite (AHT) in aqueous solution was studied. The AHT dissolution strongly depended on pH and temperature, and the dissolution of AHT fitted the surface chemical reaction model well. The apparent activation energy during the dissolution process was 34.11 kJ/mol, which further confirmed that the dissolution was controlled by the surface chemical reaction. During the SO42– modification, the exchange process between SO42– and CO32– in interlayer spaces of the AHT took place along with Mg2+ partial dissolution from edges of the brucite sheets. On the other hand, the modification mechanism of the AHT was discussed based on the dissolution–reprecipitation (D–R) mechanism. The modification process was suggested to occur on the solid/water interface, i.e., dissolution of Al-ions and Mg2+, adsorption and reprecipitation of the Al aqua ions on the surface of the solids.

Comparison of CF4 and C4F8 gas etching profiles by multiscale simulation

The CF4 and C4F8 gas etching profiles of oxide films were compared by multiscale simulation that comprises gas reaction, sheath, and surface reaction models. The densities of CF3, CF2, and CF radicals in CF4/Ar or C4F8/Ar gas were measured and compared with those obtained by simulation using the gas reaction model. From this comparison, the electron temperatures were determined to be 2.8 and 4.5 eV for CF4 and C4F8 gases, respectively. In the sheath model, the behavior of ions in a sheath was simulated for sheath lengths calculated from these electron temperatures. In the surface reaction model, we simulated the formation of a polymer and active layers by CF2 radicals, determined the depth of etching resulting from ion bombardment, and obtained the etching profiles of the oxide films. The profile of a contact hole with a depth of 820 nm and an aperture diameter of 160 nm was simulated. The results showed that the photoresist height was approximately 70 nm greater and the bowing diameter was approximately 10 nm smaller in the case of using C4F8 gas than in the case of using CF4 gas. This is because the CF2 density in the C4F8 gas is approximately 30 times higher than that in the CF4 gas and the polymer layer more strongly protects the underlying film. When the etching profiles were simulated with a fixed density of positive ions but with various CF2 density, the bottom diameter was constant but the bowing diameter changed for CF2 densities between 1013 and 1014 cm−3.

Dissolution characteristics of calcium‐based alkaline industrial derived wastes

Accurately evaluating the rate at which calcium‐based alkaline industrial derived wastes (CAIDW) dissolve is very important in the design and development of CAIDW‐type wet flue gas desulfurization plants. The dissolution characteristics of alkaline slag (AS) and salt mud (SM) have been studied using pH stat method, and the effects of various parameters such as reaction temperature, sorbent particle size and solution acidity are studied to determine the dissolution kinetics of AS and SM. The production process of CAIDW presents high influence on its dissolution performance in acidic conditions. The kinetics analysis based on the modified shrinking core model (MSCM) indicates that AS dissolution kinetics, which is accompanied by surface reaction control, follows the film diffusion, and the activation energies of the two steps are 4.62 ± 0.8 and 36.34 ± 3.1 kJ/mol, respectively. Meanwhile, the SM dissolution obeys the surface reaction model with the activation energy of 9.69 ± 1.9 kJ/mol.

Acoustic behavior of porous ceiling absorbers based on local and extended reaction.

The acoustic behavior of ceiling absorbers can be predicted under different surface reaction assumptions: Local and extended reaction. This study aims to experimentally validate acoustic transfer functions near a ceiling absorber in an anechoic chamber based on the two surface reaction models. First, a ceiling absorber with two mounting conditions is modeled by equivalent fluid models, such as Delany-Bazley's, Miki's, and Komatsu's model, in various ways: (1) Local vs extended reaction and (2) plane-wave vs spherical-wave incidence. For a single absorber under anechoic conditions, the acoustic transfer functions for four source-receiver pairs are simulated using a pressure-based image source model, and then compared with measurements. For a rigid backing condition, both the local and extended reaction models agree well with the measurement. For an absorber backed by an air cavity, the extended reaction model agrees better at larger incidence angles at lower frequencies than the local reaction model.