Surface Reaction Model Research Articles

Kinetics of solid reactions with gas intermediates in chemical looping combustion

• Reaction route of the CLC reactions with gas intermediates were identified. • The effect of solid particle size ranges was analyzed. • The sensitivity of reaction temperature was determined. • The kinetics models could be used in the solid reactions with gas intermediates. Chemical looping combustion (CLC) is an advanced technology for converting solid fuel with in-situ CO 2 capture in a low-cost and sustainable way. However, unlike gaseous fuel, the CLC process of solid fuel is lack of the reaction understanding between fuel particles and OC particles (solid-to-solid), and this result directly influences the design of the reactor. In this context, the study focused on the solid reactions with gas intermediates taking place in the CLC reactor. Well-mixed solid particles of the coal char and the hematite were investigated on the thermogravimetric and mass spectrometry (TG-MS) system. The effects of different particle size ranges and different reaction temperatures were determined. The kinetics on this kind of solid reactions was calculated and discussed. The result showed that the carbon gasification was identified as the control step in reactions. The diffusional resistance of 45–75 µm char particles could be ignored. Two models of kinetic reactions were developed based on the contracting sphere rate law and the first order rate law. The Activation Energy E a of the surface reaction model and the first order reaction model was 208.75 kJ/mol and 224.75 kJ/mol, respectively. Overall, the experiments and the kinetic models carried out in the present work improved the reaction understanding on the field of the solid reactions with gas intermediates in CLC.

Role of chemical reactions in the stagnation point heat flux of rarefied hypersonic cylinder flows

This work investigates the variations of the stagnation point heat flux (SPHF) in hypersonic cylinder flows using the direct simulation Monte Carlo method, with the consideration of a constant freestream Knudsen number but different cylinder diameters. Four different freestream Mach numbers and the accompanying chemical reactions are considered. The result reveals a high-density effect in chemical reactions inside the thermal boundary layer, which induces an increasingly rising SPHF with a decreased cylinder diameter for all the cases. The cases at Ma∞ = 30 exhibit a characteristic of peculiarity that the value of SPHF increases the fastest, which is strongly correlated with the different high-density effects at different Ma∞. Further analysis demonstrates that the NO dissociation and recombination reactions always play a vitally important role in the high-density effect. A secondary NO dissociation reaction was observed inside the thermal boundary layer when Ma∞ > 30. This observation is the result of the shift of chemical equilibrium induced by violent recombination reaction and sufficiently high flow temperature. Subsequently, the newly emerging secondary dissociation reaction weakens the influence of recombination reaction; thus, the growth of SPHF at a high Mach number is not so strong as that with Ma∞ ≤ 30. Furthermore, in order to provide more reliable results, additional simulations are discussed by employing the widely accepted total collision energy and catalytic surface reaction models.

Synthesis, characterization and application of cross-linked chitosan/oxalic acid hydrogels to improve azo dye (Reactive Red 195) adsorption

Cross-linked oxalic acid/chitosan hydrogels (ChOxb) were synthesized, characterized and tested for the adsorption of azo-dyes (Reactive Red 195 RR195) in contaminated wastewater; this novel biodegradable material was highly efficient. SEM-EDS and N2 adsorption/desorption were used to analyze specific area; FTIR-ATR spectra evidenced electrostatic interactions between protonated amino groups of chitosan and oxalate ions (intensity decrease and shift of amide-II band). Adsorption performance in batch assays showed a maximum percentage of removal of 90.6%, at pH = 4 (initial dye concentration of 300 mg.L−1). The maximum adsorption capacity (Qm) at pH = 4 was 110.7 mg.g−1, positioning ChOxb as one of the most efficient adsorption materials. Oxalic acid (natural organic acid) is an eco-friendly cross-linking agent that reduced swelling and improved chemical stability of ChOxb at low pH = 2.5. Redlich-Peterson adsorption isotherm presented the best fit; adsorption thermodynamic parameters were evaluated. Mixed surface reaction and diffusion-controlled kinetic model was the equation that best fitted kinetic data. FTIR-ATR, SEM-EDS and Z-potential showed electrostatic interactions between dye and amino groups of ChOxb; ChOxd can be reused without losing adsorption capacity and competitive electrostatic interactions between ChOxb-nitrate were observed. This new bio-material is an excellent alternative for the removal of RR195, improving Qm with better structural and functional properties.

Catalytic mechanism of activated carbon-assisted bioleaching of enargite concentrate

The catalytic mechanism of activated carbon-assisted bioleaching of enargite concentrate (enargite 37.4%; pyrite 47.3%) was investigated by employing microbiological, electrochemical and kinetic studies. By using moderately thermophilic microorganisms at 45 °C, the final Cu dissolution was improved from 36% to 53% at 0.2% (w/v) activated carbon. An excess activated carbon addition showed an adverse effect. The enargite mineral itself favored higher solution redox potential (Eh) for solubilization. However, the dissolution of co-existing pyrite, which also favors high Eh, immediately hindered enargite dissolution through the passivation effect. The surface of activated carbon functioned as an electron mediator to couple RISCs oxidation and Fe3+ reduction, so that elevation of the Eh level was controlled by offsetting microbial Fe3+ regeneration. As long as the Eh level was suppressed at <700 mV, the dissolution of pyrite was largely avoided, enabling a steady and continuous dissolution of the enargite mineral through the surface chemical reaction model. When the Eh-control by activated carbon becomes no longer sustainable and the Eh hits 700 mV, rapid pyrite dissolution was initiated and the surface chemical reaction of enargite dissolution came to an end. Arsenic species dissolved from enargite was constantly immobilized with an efficiency of 75–90% as amorphous ferric arsenate. However, the sudden initiation of pyrite dissolution also triggered the re-solubilization of ferric arsenate. Therefore, the sustainable Eh-controlling effect was shown to be critical to enable longer Cu dissolution from enargite as well as stabilization of As precipitates.

Universal surface reaction model of plasma oxide etching

We propose a universal surface reaction model without any ad-hoc assumptions for fluorocarbon (FC) plasma oxide etching. A self-consistent numerical algorithm was developed to predict the deposition and etch yields simultaneously from our model considering the passivation layer and mixed layer. The internal model variables such as surface coverages showed consistent results under a wide range of FC plasma conditions. This model predicts the transition conditions between deposition and etch yield and the FC passivation layer thickness during the etching process. Finally, quantitative verification of the proposed model was performed through comparison to various FC plasma experimental data.

Comparative study of thermal and radical-enhanced methods for growing boron nitride films from diborane and ammonia

This work studies the deposition of boron/boron nitride (B/BN) composite films at low substrate temperature (275–375 °C) by alternating pulses of diborane (B2H6) and ammonia (NH3) with argon purging in between to avoid gas-phase reactions of the precursors. This process is similar to atomic layer deposition in which the dominance of surface reactions simplifies the growth mechanism. However, non-self-limiting decomposition of B2H6 and incomplete nitridation lead to the incorporation of pure boron (pure-B), causing deviation from the desired 1:1 B:N stoichiometry. Using the pure-B fraction as a measure of incomplete nitridation, this article describes consecutive experiments to control this effect and ultimately understand it in the context of a surface reaction model. First, it is demonstrated that, in a purely thermal mode, the growth of the layers and their composition strongly depend on the total gas pressure. The pure-B content (not to be confused with the total boron content) could thus be varied in the range of ∼6–70 vol. %. Next, enhancement of nitridation by the dissociation of NH3 into reactive radicals using a hot-wire was found to be insufficient to produce stoichiometric BN. Finally, plasma-assisted deposition at 310 °C resulted in nearly stoichiometric polycrystalline BN with an interplane distance matching that of hexagonal BN; the material was stable in air for at least six months. The pressure dependence in the purely thermal mode is consistent with a growth model of BN from B2H6 and NH3 via the so-called surface-adduct mechanism. The effects of the radical-enhanced methods on nitridation are explained using this model.

A unified semi-global surface reaction model of polymer deposition and SiO2 etching in fluorocarbon plasma

We propose a semi-global surface reaction model to capture simultaneous polymer deposition and oxide etching in fluorocarbon plasma. The critical parameters of this model within reasonable ranges can be determined using predesigned experimental data with plasma diagnostics in inductively coupled fluorocarbon plasma. This model can describe the transition behavior from polymer deposition to oxide etch self-consistently without ad-hoc assumptions, providing better insight into abnormal etching behaviors in high aspect ratio contact hole etching, such as sidewall bowing and twisting toward next generation of memory devices in semiconductor industries.

Multiscale plasma and feature profile simulations of plasma-enhanced chemical vapor deposition and atomic layer deposition processes for titanium thin film fabrication

Mechanisms of titanium (Ti) thin films deposited in plasma-enhanced (PE) chemical vapor deposition (CVD) and atomic layer deposition (ALD) processes have been elucidated via multiscale plasma and feature profile simulations. Firstly, by iterating a 2D reactor-scale plasma simulation and a feature-scale deposition profile simulation, a shared surface reaction model has been determined for PECVD processes. Ti film thicknesses and profiles consistently computed both along a wafer surface and inside a test structure agreed very well with the experimental data. Then, another multiscale simulation, with the same plasma model and the surface reaction model to which the Eley–Rideal surface kinetics is added, has been applied to a PEALD process. The simulation succeeded again in reproducing and explaining the non-conformal Ti film deposition observed in experiments, while conformal Ti films are obtained in PECVD processes. Through the simulation work, comprehensive mechanisms of Ti film deposition, which cover both PECVD and PEALD processes, have been discussed and proposed in this study.

Modeling Hydrogen Chloride and Aluminum Surface Interactions for Spacecraft Fire Safety Applications

Experiments and modeling were performed to determine the surface kinetics of gaseous hydrogen chloride (HCl) with aluminum surfaces subjected to various treatments. HCl and other acid gases are a spacecraft fire safety concern because they are commonly found in products from electrical wire pyrolysis. Three types of aluminum surfaces were considered: surface with chromate conversion coating (iridite), anodized, and untreated. A test cell made of polytetrafluoroethylene was used to measure the difference in the HCl concentration from the inlet to the outlet. A simple one-step global surface reaction model that accounts for active surface sites was proposed. The kinetic constants were calibrated using the measured data. The calibrated model was validated against experiments with different flow rates and HCl inlet concentrations. The results showed that anodized aluminum had the most HCl uptake, followed by the iridite, and then the untreated aluminum. The amount of HCl uptake appeared to correlate well with the thickness of the oxide layer on aluminum. The relevance of these findings is discussed with respect to the design of large-scale fire safety experiments in space and various fire safety application scenarios.

Solvability theorem for a surface reaction type model

The purpose of this paper is to investigate the existence and uniqueness of classical solutions to a model of surface reactions between two polyatomic reactants. The model is described by a coupled system of four quasilinear parabolic equations where two of them are determined in the domain and the other two are considered on a part of its surface. The elliptic operators of the parabolic equations determined on the surface are allowed to be degenerate in the sense that the density-dependent diffusion coefficients pi(θi) may have the property pi(1)=0, i=1,2.

Understanding surface charge regulation in silica nanopores.

Nanoporous silica is used in a wide variety of applications, ranging from bioanalytical tools and materials for energy storage and conversion as well as separation devices. The surface charge density of nanopores is not easily measured by experiment yet plays a vital role in the performance and functioning of silica nanopores. Herein, we report a theoretical model to describe charge regulation in silica nanopores by combining the surface-reaction model and the classical density functional theory (CDFT). The theoretical predictions provide quantitative insights into the effects of pH, electrolyte concentration, and pore size on the surface charge density and electric double layer structure. With a fixed pore size, the surface charge density increases with both pH and the bulk salt concentration similar to that for an open surface. At fixed pH and salt concentration, the surface charge density rises with the pore size until it reaches the bulk asymptotic value when the surface interactions become negligible. At high pH, the surface charge density is mainly determined by the ratio of the Debye screening length to the pore size (λD/D).

Modelling of ethanol pyrolysis in a commercial CVD reactor for growing carbon layers on alumina substrates

Chemical vapour deposition (CVD) is widely used for preparation of pyrolytic carbons from various precursors. The prediction of deposition kinetics requires deep understanding of all transport phenomena involved. In this work, we perform the computational modelling of ethanol pyrolysis in a commercial CVD reactor (a tube furnace). The reactor is employed for growing carbon layers on alumina substrates. The inlet gas flow is produced by evaporating azeotrope ethanol/water mixture and mixing it with inert gas (argon). The modelling is performed in 3D and 2D statements using Marinov mechanism with 57 species participating in 383 reactions. The heat and species transport is taken into account with temperature dependent physical properties. It is shown that the inlet gas velocity in the 2D statement should be corrected for a meaningful comparison with the 3D case. A good agreement is found between species mole fractions at the substrate position for 3D and 2D statements at low volume flow rates, while at high flow rates some deviations are observed. The temperature at the substrate position is found to be lower than at the reactor wall due to inflow of a colder gas. The main pyrolysis products at moderate temperatures (around 900 °C) are water, ethylene, hydrogen, carbon monoxide, and methane. With increasing temperature, the mole fractions of hydrogen, acetylene, and carbon monoxide increase, while those of water and methane become smaller. With increasing ethanol/water volume flow rate, the mole fractions of ethanol and pyrolysis products saturate at some constant values due to incomplete thermal decomposition of ethanol in the reactor volume. The rise of argon flow rate leads to the decrease of pyrolysis products mole fractions due to decrease of residence time. The obtained results can be employed for simulating and analyzing pyrolysis processes in realistic CVD reactors with complex geometry as well as for the development of coupled gas phase and surface reaction model of carbon layer deposition on nanoporous substrates.

Integrated Experimental-Theoretical Approach To Determine Reliable Molecular Reaction Mechanisms on Transition-Metal Oxide Surfaces.

By combining experimental and theoretical approaches, we investigate the quantitative relationship between molecular desorption temperature and binding energy on d and f metal oxide surfaces. We demonstrate how temperature-programmed desorption can be used to quantitatively correlate the theoretical surface chemistry of metal oxides (via on-site Hubbard U correction) to gas surface interactions for catalytic reactions. For this purpose, both CO and NO oxidation mechanisms are studied in a step-by-step reaction process for perovskite and mullite-type oxides, respectively. Additionally, we show solutions for over-binding issues found in COx, NOx, SOx, and other covalently bonded molecules that must be considered during surface reaction modeling. This work shows the high reliability of using TPD and density functional theory in conjunction to create accurate surface chemistry information for a variety of correlated metal oxide materials.

Control of Selectivity through a New Hydrogen-Transfer Mechanism in Photocatalytic Reduction Reactions: Electronically Relaxed Neutral H and the Role of Electron-Phonon Coupling.

Controlling the fate of hydrogen in photocatalytic synthesis reactions has been an ongoing challenge in CO2 reduction by H2O and nitrogen fixation efforts. Our studies have identified catalysts (SiC) that exhibit dramatically improved selectivity toward hydrogenation and a photocatalytically active ground-state neutral H that is transferred via vibrational excitation through electronic-vibrational coupling with excited states. This new species and mechanism have been directly connected to the fate of H by comparing GaN and SiC and purposefully manipulated over a single catalyst (SiC) to illustrate generality. Studies included surface reaction modeling using density functional theory (DFT), experimental performance, 1H NMR spectroscopy, and deuterium kinetic isotope effect. The discovery of this mechanism may have considerable impact on the direction of photocatalytic synthesis, the understanding of the coupling of thermal and photoelectrochemical reaction steps, and electronic-vibrational spectrum coupling in energy sequestration.

Dissolution Kinetics of Ilmenite Ore in A Binary Solution

Leaching of iron from ilmenite ore using a binary solution (HCl-NaNO3) was investigated. The raw ilmenite ore sample was characterized using Scanning Electron Microscopy (SEM), X-ray diffraction spectroscopy (XRD) and X-ray Flourescence (XRF) techniques. The influence of acid concentration, oxidant concentration, particle size, solution temperature, stirring speed and liquid-to-solid ratios on the extent of dissolution was examined. The experimental data obtained at various process parameter conditions were tested in six kinetics models: shrinking core model’s diffusion through liquid film model(DTLF), diffusion through product layer model (DTPL), surface chemical reaction model (SCR)); mixed kinetics model (MKM), Jander (three dimensional) model and Kröger and Ziegler model. The crystalline morphology of the sample was displayed by the SEM micrograph. XRF result revealed the dominance of titanium and iron in ilmenite while XRD confirmed that ilmenite exist mainly as FeTiO2. The results of the leaching studies showed that ilmenite dissolution in the binary solution increases with increasing acid concentration, oxidant concentration, reaction temperature, stirring speed and liquid-to-solid ratio; while it decreases with particle size. The study showed that 94.77% iron was dissolved by 1MHCl-0.6M NaNO3 at 75μm particle size, 75˚C reaction temperature, 300rpm stirring speed and 30L/g liquid-to-solid ratio. The kinetics of the leaching process was best described by Kröger and Ziegler model with diffusion through the product layer as rate controlling step. The activation energy, Ea, was calculated to be 6.42kJ/mol. The results indicate that HCl-NaNO3 binary solution can be used as an effective lixiviant for extracting iron from ilmenite ores.

Dissolution Kinetics of Ilmenite Ore in A Binary Solution

Leaching of iron from ilmenite ore using a binary solution (HCl-NaNO3) was investigated. The raw ilmenite ore sample was characterized using Scanning Electron Microscopy (SEM), X-ray diffraction spectroscopy (XRD) and X-ray Flourescence (XRF) techniques. The influence of acid concentration, oxidant concentration, particle size, solution temperature, stirring speed and liquid-to-solid ratios on the extent of dissolution was examined. The experimental data obtained at various process parameter conditions were tested in six kinetics models: shrinking core model’s diffusion through liquid film model(DTLF), diffusion through product layer model (DTPL), surface chemical reaction model (SCR)); mixed kinetics model (MKM), Jander (three dimensional) model and Kröger and Ziegler model. The crystalline morphology of the sample was displayed by the SEM micrograph. XRF result revealed the dominance of titanium and iron in ilmenite while XRD confirmed that ilmenite exist mainly as FeTiO2. The results of the leaching studies showed that ilmenite dissolution in the binary solution increases with increasing acid concentration, oxidant concentration, reaction temperature, stirring speed and liquid-to-solid ratio; while it decreases with particle size. The study showed that 94.77% iron was dissolved by 1MHCl-0.6M NaNO3 at 75?m particle size, 75?C reaction temperature, 300rpm stirring speed and 30L/g liquid-to-solid ratio. The kinetics of the leaching process was best described by Kröger and Ziegler model with diffusion through the product layer as rate controlling step. The activation energy, Ea, was calculated to be 6.42kJ/mol. The results indicate that HCl-NaNO3 binary solution can be used as an effective lixiviant for extracting iron from ilmenite ores.

Computational study on silicon oxide plasma enhanced chemical vapor deposition (PECVD) process using tetraethoxysilane/oxygen/argon/ helium

Plasma enhanced chemical vapor deposition (PECVD) of silicon oxide (SiO2) using tetraethoxysilane (TEOS) was investigated theoretically by developing an unprecedented plasma chemistry model in TEOS/O2/Ar/He gas mixture. In the gas phase reactions, a TEOS molecule is decomposed by the electron impact reaction and/or chemically oxidative reaction, forming intermediate TEOS fragments, i.e., silicon complexes. In this study, we assume that SiO is the main precursor that contributes to SiO2 film growth under a particular process or simulation condition. The surface reaction was also investigated using quantum mechanical simulations with density functional theory. Based on the gas and surface reaction models, we constructed a computational plasma model for SiO2 film deposition in a PECVD process. The simulation results using CHEMKIN pro and CFD-ACE + have shown that the neutral atomic O and SiO as well as the charged O2+ are the dominant species to obtain a high deposition rate and uniformity. The spatial distributions of various species in the TEOS/O2/Ar/He gas mixture plasma were shown in the study. The uniformity of deposited film due to the change in the plasma bulk property was also discussed.

Experimental and numerical study of the ignition of hydrogen-air mixtures by a localized stationary hot surface

The ignition of hydrogen-air mixtures by a stationary hot glow plug has been experimentally investigated using two-color pyrometry and interferometry. The ignition process was characterized by the surface temperature at ignition, as well as by the location where the initial flame kernel was formed. The experimental results indicate that: (i) the ignition temperature threshold is a function of equivalence ratio; (ii) the ignition location is a function of the rate at which the glow plug is heated because high heating rates favor non-uniform heating. As a result, ignition occurs on the side rather than near the top face of the glow plug. Comparison with two-dimensional numerical simulations exhibits discrepancies in terms of the temperature threshold value and dependence on equivalence ratio. Simulations performed imposing a non-uniform surface temperature show that a temperature difference between the side and the top of the glow plug as low as 12.5 to 25 K resulted in side ignition for hydrogen-air mixtures. The effect of surface chemistry was estimated numerically by imposing a boundary condition of zero species concentration for intermediate species, H and HO2, at the hot surface, which increased the ignition threshold by up to 50 K for an initial H2 concentration of 70%. The present study shows that surface temperature non-uniformity, heterogeneous chemistry and reaction model used, could influence the experimentally reported and numerically predicted ignition threshold as well as the location of ignition.

Development of the Effective Reaction Model for Diamond Deposition Simulation within the Continuous Media Approach

Three models for diamond growth process by the chemical vapor deposition of methane are proposed. They differ in the degree of detail of the surface reaction description. The most complete model contains the reactions of deposition, etching and insertion. Gas-dynamic simulations have been performed for all those models. The species delivery to the substrate and the contribution from different species to the growth process is analysed. It is shown that different surface reaction models lead to different profiles of the species concentrations in the immediate vicinity of the substrate, thus, the experimental data on the growth rate may give information on the growth mechanism.

New form for reduced modeling of soot oxidation: Accounting for multi-site kinetics and surface reactivity

New formulation is introduced to model surface oxidation of soot particles. In the new development, the surface is represented by an arbitrary number of reactive sites and their physically-founded transformations. The latter are combined and integrated with gas-phase and particle-dynamics models. The surface reaction model defines two state properties and establishes a structural relationship between them that guides evolution of the surface. This new model form for the surface-chemistry led to close reproduction of shock-initiated oxidation of soot: CO profiles in two experiments performed at substantially different temperatures, 1990 and 2780 K, as well as CO production rates over a wide range of temperatures, 1652–3130 K, all without employing the parameter-α empiricism of the previous model formulation.