Detailed Surface Reaction Mechanism Research Articles

Detailed kinetic modeling of catalytic oxidative coupling of methane

The oxidative coupling of methane (OCM) at high temperature over monolithic Pt-based catalysts operated at short contact times is investigated as an attractive method for methane upgrading to higher value products like ethylene and acetylene. An experimental measurement campaign aiming at elucidating the effect of operation parameters on the catalyst performance revealed that lower N2 dilution, lower CH4/O2 ratio, and higher space velocities promote high C2 yields. A maximum C2 yield of 10 % with 94 % CH4 conversion was obtained at a CH4/O2 ratio of 1.1, 50 % N2 dilution, and a space velocity of 4.5 × 105 h−1. Since both heterogenous and homogenous gas-phase chemistry together are required for determining C2 formation pathways, a detailed OCM surface reaction mechanism over Pt is presented consisting of 26 species and 86 reactions that are consistent from the view of thermodynamics and micro-kinetic reversibility. The combination of this OCM surface mechanism with a detailed gas-phase mechanism allows a numerical micro-kinetic description of the experimental measurements. The simulations presented in this study suggest that C2 formation takes place in the gas-phase at temperature above 1200 K with both oxidative and pyrolytic pathways for methane dehydrogenation to form CH3 radicals, whose coupling results in C2H6, C2H4 and C2H2 formation. Furthermore, the simultaneous presence of sufficient oxygen content and heat are vital for high C2 species yields.

Investigation dry reforming of methane over nickel using a one‐dimensional model

AbstractIn the field of catalysis, dry reforming, that is, methane reforming with , is in the focus due to growing environmental concerns about oil depletion and global warming with a desire to produce synthesis gas. However, this process can lead to the formation of carbon, which can cause catalyst deactivation, especially at industrial conditions. Nevertheless, the key to develop a more coke‐resistant catalyst is a better comprehension of the reforming process at a molecular level. Regardless of all the investigations available in literature, the detailed path for the conversion of methane to syngas and carbon remains a controversial issue. Another problem in setting up a reaction mechanism is the difficulty to define the thermodynamic data for intermediate surface species and this leads to the development of thermodynamic consistent surface reaction mechanisms in literature where the thermodynamic data are not used to calculate the rate coefficients of the reverse reactions. Rather the Arrhenius parameters for the forward as well as backward reactions are explicitly given in the reaction mechanism to establish thermodynamic equilibrium. In this investigation, a kinetically consistent detailed surface reaction mechanism is developed which consists of 26 reversible reactions with the help of a one‐dimensional model, LOGEcat. Our previous work constructs the basis of the present investigation. Further, a detailed sensitivity analysis of reversible reactions and reaction pathways is performed to understand the mechanism better. The mechanism is validated for dry reforming of methane over nickel catalyst, however, it can also be used for other processes, such as, steam reforming and partial oxidation. The mechanism is tested by comparing the simulation results with the literature experiments and simulations in a wide range of temperature. The new developed kinetically consistent surface reaction mechanism is able to accurately express the dry reforming of methane over the nickel catalyst for complete range of temperature and also provide a useful insight into the key rate determining steps.

A detailed surface reaction mechanism to investigate oxidation of methane over nickel catalyst

AbstractWe have developed a kinetically consistent detailed surface reaction mechanism for modeling the oxidation of methane over a nickel‐based catalyst. A one‐dimensional model, LOGEcat based on the single‐channel 1D catalyst model, is used to perform the simulations. The original multi‐step reaction mechanism is thermodynamically consistent and consists of 52 reactions. By thermodynamic consistency, we mean that the equilibrium is achieved with the support of the Arrhenius parameters and does not depend on the thermochemistry of the species involved in the considered reactions. The detailed mechanism developed in this investigation contains 26 reversible reactions. These reactions are obtained with the use of the thermochemistry of the species. The study focuses on ensuring kinetic consistency and this is done with the help of thermodynamic analysis by bringing the thermochemistry of the species in play in order to develop a surface reaction mechanism. The new mechanism can be used to understand the other processes, for example, steam‐ and dry‐reforming of methane over nickel, however, the main focus of the paper is to check the performance of the detailed mechanism for catalytic partial oxidation of methane. The applicability of the mechanism is checked for various reactor conditions in terms of parameters such as temperature and pressure by comparing the results with the available reference data. The detailed mechanism developed in this study is able to accurately express oxidation of methane over the nickel catalyst for the considered reactor conditions.

Kinetically consistent detailed surface reaction mechanism for steam reforming of methane over nickel catalyst

A one-dimensional model, LOGEcat is used to develop a detailed surface reaction mechanism for modeling the steam reforming of methane over a nickel-based catalyst. The focus of the paper is to develop a kinetically consistent surface reaction mechanism. The two terms, kinetically and thermodynamically consistent mechanisms, will be used frequently in this article. Note that when the mechanism is thermodynamically consistent then the thermodynamic data or the thermochemistry of the species is not used and all the reactions in the mechanism are forward reactions (no backward reactions included) which need the Arrhenius parameters (pre-exponential factor, A_{r}; activation energy, E_{r}; temperature exponent, beta _r) explicitly for each reaction. The kinetically consistent mechanisms need Arrhenius parameters only for the forward reactions and does not need these parameters for backward reactions making the backward reactions independent of the kinetics. The rate for the backward reactions is calculated with the help of thermochemistry of the intermediate species involved in the reaction mechanism. Since the thermochemistry of the intermediate species are not easily available, this makes such studies much more complex. The original multi-step reaction mechanism consists of 42 reactions which are thermodynamically consistent. In this study we have modified this reaction mechanism from literature and used only 21 (reversible) direct reactions. This aspect is important because by using only 21 reactions, we use the thermochemistry of the species to achieve thermodynamic equilibrium instead of providing the Arrhenius parameters for forward and backward reactions. We use the same A_{r}, E_{r} and beta _r values as used in the reference paper for the considered 21 reactions which are in equilibrium. However, the value of A_{r}, E_{r} and beta _r for the inverse/backward reactions are not given in the mechanism and the reverse rates are calculated by using the equilibrium and forward rate. Therefore, a sensitivity analysis on thermochemistry of the species is performed for a better agreement with the literature data. The method is presented for ensuring kinetic consistency and bringing thermochemistry of species in play while developing a surface reaction mechanism. The applicability of the mechanism is tested for two different simulation set-ups available in literature considering various reactor conditions in terms of parameters given as temperature and steam-to-carbon (S/C) ratio. Several chemical reaction terms, such as, selectivity, conversion and mathrm {H_2}/{text{CO}} ratio are shown for different parameters in comparison with the available reference data. The detailed mechanism developed in this study is able to accurately express the steam reforming of methane over the nickel catalyst for complete ranges of temperature for both the set-ups and within acceptable limit for S/C ratio considered for the analysis.

Using Thermodynamic Analysis to Develop a Detailed Surface Reaction Mechanism for Steam-, CO2- Reforming and Oxidation of Methane Over Nickel Catalyst

Using Thermodynamic Analysis to Develop a Detailed Surface Reaction Mechanism for Steam-, CO2- Reforming and Oxidation of Methane Over Nickel Catalyst

The Simulation of Different Combustion Stages of Micron-sized Aluminum Particles

In this study, a quasi-steady combustion model of an aluminum particle is established, which is more accurate to simulate the physical combustion process. Detailed gas-phase reaction mechanism and surface reaction mechanism are considered. Moreover, the particle temperature is not constant in this work, which is calculated in different combustion stages. The judgement standard of each combustion stage is from observational data in the experiment and the simulation results of combustion durations of each stage, and distribution of ambient temperature and gas-phase species profiles are in good agreement with experimental results. The calculation results show that in the first stage, burning rate of the particle is the fastest, and in the second stage, particle temperature can drop to more than 100 K below the boiling point for the large particles, which is slightly below the boiling point for small ones. As the combustion stage changes, Da number is going to keep going down, which will lead to the transition of combustion method from diffusion-limited control to kinetic-limited control for an aluminum particle.

Detailed Surface Reaction Mechanism for TWC Based on Gaseous and Surface Species Analyses

Detailed Surface Reaction Mechanism for TWC Based on Gaseous and Surface Species Analyses

Development of Detailed Surface Reaction Mechanism of C2H4/C3H6 Oxidation on Pt/Al2O3 Monolith Catalyst Based on Gas Phase and Surface Species Analyses

ABSTRACT In this study, reaction characteristics of C2H4/O2 and C3H6/O2 on practical Pt/Al2O3 monolith catalyst are investigated, and a simplified microkinetic surface reaction mechanism is proposed by taking the observed surface species and their occupied active surface sites into account. Importance of more than one active PGM surface site occupations at the adsorption step of C2H4 or C3H6 and multiple-sited stable intermediate CH x species are considered. Gas phase experimental results show that the light-off temperature as well as complete conversion temperature of both C2H4/O2 and C3H6/O2 mixture shift to lower side with the increase in oxygen concentration. The surface species analyses show the presence of stable intermediate CH x species and linearly adsorbed CO on the platinum surface. Numerically simulated light-off performance with the proposed reaction mechanism shows good agreement with the experimental results of C2H4/O2 and C3H6/O2. Moreover, simulation results also agree well with the additional experimental results with multiple gas compositions and validate the proposed detailed reaction mechanism.

Heterogeneous and homogeneous combustion of fuel-lean C3H8/O2/N2 mixtures over rhodium at pressures up to 6 bar

The heterogeneous and homogeneous combustion of C3H8/O2/N2 mixtures over Rh was investigated at pressures 1-6 bar, catalyst surface temperatures 680-1100 K and C3H8-to-O2 equivalence ratios 0.25-0.52. Non-intrusive laser-based measurements were applied in a channel-flow catalytic reactor and involved 1-D Raman spectroscopy of major gas-phase species across the channel boundary layer for assessing the catalytic reactivity and planar laser induced fluorescence (PLIF) of the OH radical for monitoring homogeneous combustion. Simulations were carried out with a 2-D numerical code that included detailed hetero-/homogeneous chemical reaction mechanisms. By comparing the Raman-measured and predicted transverse profiles of the limiting C3H8 reactant, the suitability of a detailed surface reaction mechanism was initially evaluated and subsequently a one-step reaction was constructed, which was applicable for the C3H8 total oxidation over Rh at pressures 1 to 6 bar. The catalytic reactivity of C3H8 over Rh displayed a ∼p0.14 pressure dependence, which was substantially lower than a previously reported ∼p0.70 dependence over Pt. The weak pressure dependence of the C3H8 reactivity on Rh suggested caution when selecting catalysts for high-pressure power systems (recuperative microreactors, small-scale turbines) fueled with C3H8 or LPG (liquefied petroleum gas). Comparisons of PLIF-measured and predicted distributions of the OH radical indicated that the employed gas-phase reaction mechanism captured the onset of homogeneous ignition at pressures greater than or equal to 3 bar as well as the ensuing flame shapes. Predicted and measured homogeneous ignition distances agreed within 2.5% at 6 bar. With decreasing pressure, the predictions yielded gradually increasing but still modest underpredictions (up to 11.2% at 3 bar) of the homogeneous ignition distances. The key gas-phase reactions affecting homogeneous combustion at various pressures were finally identified.

Microscopic modeling and optimal operation of plasma enhanced atomic layer deposition

Plasma enhanced atomic layer deposition (PEALD) is one of the most widely adopted deposition methods used in the semiconductor industry. It is chosen largely due to its superior ability to produce ultra-thin high-k dielectric films, which are needed for the further miniaturization of microelectronic devices with the pace of Moore's Law. In contrast to the traditional thermal atomic layer deposition (ALD) method, PEALD allows for high deposition growth per cycle (GPC) under low operating temperature with the help of high energy plasma species. Despite the experimental effort in finding new precursors and plasmas, the detailed surface structures and reaction mechanisms in various PEALD processes remain hard to understand because of the limitation of current in-situ monitoring techniques and the deficiency of the first-principles based analysis. Therefore, in this work, an accurate, yet efficient kinetic Monte Carlo (kMC) model and an associated machine learning (ML) analysis are proposed to capture the surface deposition mechanism and to propose optimal operating conditions of the HfO2 thin-film PEALD using tetrakis-dimethylamino-Hafnium (TDMAHf) and oxygen plasma. Density Functional Theory (DFT) calculations are performed to obtain the key kinetic parameters and the structural details, subsequently employed in the kMC model. After the kMC model is validated by experimental data, a database is generated to explore a variety of precursor partial pressure and substrate temperature combinations using the kMC algorithm. A feed-forward Bayesian regularized artificial neural network (BRANN) is then constructed to characterize the input–output relationship and to investigate the optimal operating conditions.

Development of Surface Reaction Mechanisms of CO / O2 on Pt and Rh for Three Way Catalyst based on Gas Phase and Surface Species Analyses

ABSTRACT In this study, detailed surface reaction mechanisms for CO/O2 system on Pt/Al2O3 and Rh/Al2O3 have been developed based on the measurements of gaseous and surface species. The conventional conversion rate experiments for gaseous CO to CO2 were conducted with the monolith honeycomb catalyst, and furthermore, the adsorbed surface species were identified with the powdered catalyst by in-situ FTIR under the same conditions as conversion rate experiment. As a result, it was found that only linear CO adsorption was detected for Pt, while twin CO adsorption was detected for Rh. Based on the results, detailed surface reaction mechanisms have been developed for Pt/Al2O3 and Rh/Al2O3. As a result of numerical simulation, it was confirmed that the variations of CO to CO2 conversion rate with temperature were quantitatively reproduced for various O2 concentration conditions. Furthermore, using those reaction mechanisms, CO conversion rate on bimetal, Pt/Rh Al2O3 catalysts were examined for several Pt/Rh ratios. Numerical simulation results for bimetal catalysts were also quantitatively agreed with experimental results for various Pt/Rh ratios as well as CO/O2 ratios.

Investigation of HCHO Catalytic Oxidation over Platinum using Planar Laser-Induced Fluorescence

Planar laser-induced fluorescence is employed as noninvasive in situ technique to investigate the oxidation of formaldehyde over platinum. Two-dimensional absolute HCHO concentration distributions over a catalyst-coated plate are visualized in a parallel wall channel. The experimental results are compared with corresponding numerical simulations using ANSYS-Fluent with a detailed surface reaction mechanism. The influence of temperature, Pt-loading and mass flow rate on HCHO profiles are investigated experimentally and numerically in steady state. In the temperature range of 373 – 673 K, the HCHO distribution measured is well predicted by the model. A slow transition from a kinetically controlled regime to a mass transfer limited regime can be observed at 573 K in this study. It is found that the local HCHO concentration in the catalytic channel is governed by a combined effect of chemical kinetics, diffusive mass transfer, and flow field.

Formaldehyde Oxidation Over Platinum: On the Kinetics Relevant to Exhaust Conditions of Lean-Burn Natural Gas Engines

Kinetics of formaldehyde oxidation over Pt-based catalytic converters at oxygen-rich conditions related to the exhaust of lean-burn natural gas engines is investigated experimentally and numerically. The numerical model proposed in this work is based on a thermodynamically consistent detailed surface reaction mechanism consisting of 30 elementary-like-steps among six gas-phase and ten surface species. The surface mechanism is evaluated by comparison of the results of the numerical simulation with the experimental measurements. The experimental data are derived from isothermal end-of-pipe tests over a powdered Pt–TiO2–SiO2 catalyst as well as spatially resolved concentration measurements conducted with a catalytic monolith sample over a wide range of temperature. The measurements were performed in a gas mixture of 80 ppm HCHO, 10 vol% O2, 12 vol% H2O and 6 vol% CO2 balanced with N2, containing a formaldehyde concentration typical in the exhaust of lean-burn gas engines. The simulations are based on a one-dimensional approach for the description of a packed-bed reactor and a two-dimensional model for the calculation of flow field and chemical reactions inside a single channel of a monolith.

Heterogeneous reaction characteristics and its effects on homogeneous combustion of methane/air mixture in microchannels II. Chemical analysis

Catalytic combustion of methane/air mixture in various channels was numerically investigated to illuminate the chemical interaction mechanism between heterogeneous reactions (HTRs) and homogeneous reactions (HRs) with detailed gas-phase and surface reaction mechanisms. In the channels of different widths and with/without catalyst, the gaseous species, such as CH4, O2, CO, CO2, H, H2, and H2O and the reactions involving these species were monitored and the reaction rates along the streamwise direction were analyzed quantificationally. In the catalytic combustion, the HTR can be divided into prepare phase, weak HTR phase, violent HTR phase, second weak HTR phase, and completion phase according to the situation of gaseous species depletion on catalytic surface. The heat released by the homogeneous reaction and the consumption of O(s) can accelerate the absorption of CH4 on Pt surface in the violent HTR phase (phase III). Meanwhile, the stronger competition to CH4 of HR would inhibit the reactions of HTR involving CH4. The similar situation happens to O2, except the larger consumption of O2 by HTR than that of CH4 because of the larger stick coefficient of O2 on Pt surface. The additional reaction pathway of CO through HTR is favorable for the completeness of CO to CO2 during combustion. The consumption of H radical by HTR in the second weak HTR phase (phase IV) leads to the H2 release from Pt surface to the gas-phase and promotes the homogeneous reactions. The decrease of the width of the channel would suppress the HR intensity and enhance the HTR intensity. However, HR is still dominated in all the cases with a large amount of CH4 and O2 consumption and CO2 and H2O production.

Heterogeneous reaction characteristics and their effects on homogeneous combustion of methane/air mixture in micro channels I. Thermal analysis

Catalytic combustion of methane/air mixture in various channels was numerically investigated to illuminate the heat characteristics of heterogeneous reactions (HTRs) and their effects on homogeneous reactions (HRs). A two-dimensional (2D) simulation model of channels with a width (d) of 2.00–2.82 mm was employed, which included detailed gas-phase and surface reaction mechanisms. Further, the heat transfer mechanisms were elucidated. HRs always occupied the main part of over 80% of the total released heat in all the cases. HTR is relatively amplified in narrower channels, and S (the ratio of heat released by HTR to that of all reactions) rises by several percentage as d decreases (S = 17.61% when d = 2.10 mm), and it is more effective to use a catalyst in the microcombustor with a smaller gap size to enhance combustion reaction rate and stability. The onsite-S showed a trend of “stable-decrease–peak–rapid increase–slow increase” along x, corresponding to preheating, homogeneous ignition, heterogeneous ignition (conversion of CO to CO2 and H2 to H2O), intensive HTR stage (consumption of O, H, and OH and production of H2O and H2), and complete consumption of reactants in the gas phase. The analysis about the heat sources for homogeneous ignition shows that homogeneous ignition was dominated by the preheating heat from hot gas downstream, and affected by transverse preheating heat. As d decreased, the preheating heat obtained from the transverse preheating increased by several percentages, even reaching to a similar value to preheating heat from hot gas downstream when d = 2.10 mm. The heat released by weak HTR only takes a small part of preheating heat of inlet mixture and homogeneous ignition. After homogeneous ignition, HR promoted the HTR to the maximum. Generally, HTR would delay the homogeneous ignition, enlarge the flame thickness, increase the peak of flame temperature and the exit gas temperature, and improve the completeness of combustion.

Methane Oxidation over Palladium: On the Mechanism in Fuel-Rich Mixtures at High Temperatures

A kinetic modeling study on methane oxidation over reduced Pd for various fuel-rich conditions around the stoichiometric point of the partial oxidation at high temperatures (900–1100 K) is presented. A thermodynamically consistent detailed surface reaction mechanism is developed within the mean field approximation. The proposed kinetic model consists of 54 elementary-step based reactions including seven gas-phase species and 15 surface intermediates. Three different methane activation paths are implemented, comprising pyrolytic C–H bond dissociation steps, oxygen-assisted and dual-oxygen-assisted CH4 activation. In situ experimental measurements in a quasi-autothermally operated flow reactor, using the capillary sampling technique, are performed for model evaluation. The provided experimental data includes spatially resolved temperature and concentration profiles within a single catalytic channel of a Pd/Al2O3-coated monolith. Supplementary numerical simulations based on literature data for fuel-lean and fuel-rich conditions at high temperatures extend the model’s capability to predict a wide range of different experimental conditions.

Atomic Layer Deposition Process Modeling and Experimental Investigation for Sustainable Manufacturing of Nano Thin Films

This paper studies the adverse environmental impacts of atomic layer deposition (ALD) nanotechnology on manufacturing of Al2O3 nanoscale thin films. Numerical simulations with detailed ALD surface reaction mechanism developed based on density functional theory (DFT) and atomic-level calculations are performed to investigate the effects of four process parameters including process temperature, pulse time, purge time, and carrier gas flow rate on ALD film deposition rate, process emissions, and wastes. Full-cycle ALD simulations reveal that the depositions of nano thin films in ALD are in essence the chemisorption of the gaseous species and the conversion of surface species. Methane emissions are positively proportional to the film deposition process. The studies show that process temperature fundamentally affects the ALD chemical process by changing the energy states of the surface species. Pulse time is directly related to the precursor dosage. Purge time influences the ALD process by changing the gas–surface interaction time, and a higher carrier gas flow rate can alter the ALD flow field by accelerating the convective heat and mass transfer in ALD process.

Reaction class-based frameworks for heterogeneous catalytic systems

A systematic and self-consistent approach is applied to study the combustion of hydrogen and syngas over platinum using coupled detailed gas phase and surface reaction mechanisms. The surface chemistry is derived using a reaction class-based framework comprising variational transition state theory (VTST), two-dimensional collision theory and the unity bond index-quadratic exponential potential for barrier heights. The latter approach is augmented by the inclusion of more accurate data, such as the heat of adsorption of CO, and VTST is used to systematically remove the need for the surface sticking coefficients associated with adsorption and desorption processes. Transition-state theory estimates for several reaction classes are produced by combining the M06 family of density functionals with the Stuttgart/Dresden effective core potential for metal atoms. The developed method reproduces experimental data with an accuracy comparable or better than the previously used collision theory approach and without the reliance on experimental parameters. The presented framework is well-suited for the efficient generation of novel heterogeneous reaction mechanisms and also serves to identify key parameters where high accuracy ab initio methods may be required. The latter is exemplified via the sensitivity of selected results to the adsorption of carbon monoxide on platinum.

Hetero-/homogeneous combustion of fuel-lean methane/oxygen/nitrogen mixtures over rhodium at pressures up to 12 bar

The hetero-/homogeneous combustion of CH4/O2/N2 mixtures over rhodium was investigated experimentally and numerically at fuel-lean equivalence ratios φ=0.30–0.40, pressures 2–12bar, and catalyst temperatures 700–1250K. Experiments were performed in an optically accessible channel-flow reactor coated with rhodium and included in situ Raman measurements of major gas-phase species concentrations across the channel boundary layer for evaluating the catalytic processes, and planar laser induced fluorescence (LIF) of the OH radical for assessing homogeneous combustion. Computations were carried out with a 2-D elliptic code, using detailed heterogeneous and homogeneous chemical reaction mechanisms. Comparisons between Raman-measured and numerically-predicted transverse profiles of the limiting reactant (methane) mole fractions have led to the evaluation of the performance of detailed surface reaction mechanisms and to the construction of a one-step catalytic reaction suitable for pressures up to 12bar. For the investigated range 2–12bar, the catalytic reactivity of methane over rhodium exhibited an overall positive pressure dependence ∼p0.30, which was weaker compared to an earlier-reported pressure dependence of methane over platinum (∼p0.47). Comparisons of the planar OH-LIF measurements with numerical simulations employing different gas-phase reaction mechanisms demonstrated that the correct prediction of homogeneous ignition was particularly demanding at the present low equivalence ratios and moderate reactor temperatures. An elementary gaseous reaction mechanism was shown to reproduce the measured homogeneous ignition distances at all pressures, particularly when used in conjunction with the herein proposed one-step catalytic reaction.

Ammonia Decomposition for Hydrogen Production in Catalytic Microchannels with Slip/Jump Effects

The rarefaction effects on the catalytic decomposition of NH3 in ruthenium–coated planar microchannels is numerically simulated in the Knudsen number range 0.015-0.03. A colocated finite–volume method is used to solve the governing equations. A concentration jump model derived from the kinetic theory of gases is employed to account for the concentration discontinuity at the reactive walls. A detailed surface reaction mechanism for ammonia decomposition on ruthenium along with a multi-component species diffusion model are used to study the effects of concentration jump coupled with velocity slip and temperature jump on the walls. The velocity-slip, temperature-jump and concentration-jump boundary conditions have miscellaneous effects on flow, temperature and species concentration fields. The results suggest that the velocity-slip boundary condition only slightly influences the species distribution at the edge of the Knudsen layer as well as inside the channel, while the temperature-jump boundary condition affects the heat and mass transfer characteristics the most. The concentration-jump effect, on the other hand, can counter balance the temperature-jump effects in some cases.