Thermodynamic consistency of surface reaction mechanisms and its consequences for steady-state solutions

Predictions of Relationships between Catalytic and Solid Phase Properties by Kinetic Models and Their Validation

We 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.

Photo-assisted dissolution of colloidal iron oxides by thiol-containing compounds: 2. Comparison of lepidocrocite (γ,-FeOOH) and hematite (α-Fe 2O 3) dissolution

For elucidating the mechanism of oxygen adsorption and its effect on selectivity in the oxidative coupling of methane (OCM) transient experiments were carried out in the TAP-2 reactor by pulsing oxygen over Na2O/CaO catalysts at temperatures between 623 and 873 K. The response signals were fitted to three different models for oxygen adsorption. Model discrimination showed that only a reversible and dissociative adsorption via the molecular precursor provides a good description of the transient oxygen responses over all catalysts studied. Rate constants and activation energies of the elementary reaction steps of oxygen adsorption, desorption, dissociation and association were estimated. Doping CaO with sodium oxide influenced the ratio of kads/kdis determining the coverage of the catalyst surface with molecular and atomic oxygen. The steady-state surface coverages with molecular and atomic adsorbed oxygen species were simulated for different partial oxygen pressures (0.5–15 kPa) using the kinetic parameters from transient experiments. These results may, however, be affected by extrapolating the pressure from 10-4 Pa to 15 kPa. It was derived that an increase of C2 selectivity in OCM on Na2O/CaO can be ascribed to a decrease in the coverages of adsorbed molecular oxygen, which appears to be a plausible interpretation confirming previous findings of the dependence of C2 selectivity on oxygen partial pressure.

Heterogeneous decomposition of nitrous oxide at pressures between 10−5 and 10−4 mm. on nickel oxide and doped nickel oxide catalysts has been studied in the temperature range 350° to 500 °C. Under most experimental conditions the reaction is first order in nitrous oxide. The observed activation energies are considerably smaller than those observed in previous work and are decreased on doping with Li2O and increased on adding Cr2O3. The results are explained by assuming that changes in the steady-state surface coverage with chemisorbed oxygen affect strongly the heats of chemisorption and, therefore, also the activation energies of the primary reactions.

Electrophoretic deposition of polymer chains flowing in a $(2+1)$-dimensional system is studied by computer simulations. Steady-state surface coverage $({\ensuremath{\theta}}_{j})$ is found to decay with the chain's length, i.e., ${\ensuremath{\theta}}_{j}\ensuremath{\sim}{L}_{c}^{\ensuremath{-}\ensuremath{\alpha}}$ with a nonuniversal exponent $\ensuremath{\alpha}\ensuremath{\simeq}0.0--0.9$ depending on the magnitude of driving field and temperature. Conformational crossover occurs for chains from a surface or wall to an adjacent bulk region with different scaling exponents for their longitudinal and transverse spread.

We present a density-functional theory based kinetic Monte Carlo study of CO oxidation at the (111) facet of RuO2. We compare the detailed insight into elementary processes, steady-state surface coverages, and catalytic activity to equivalent published simulation data for the frequently studied RuO2(110) facet. Qualitative differences are identified in virtually every aspect ranging from binding energetics over lateral interactions to the interplay of elementary processes at the different active sites. Nevertheless, particularly at technologically relevant elevated temperatures, near-ambient pressures and near-stoichiometric feeds both facets exhibit almost identical catalytic activity. These findings challenge the traditional definition of structure sensitivity based on macroscopically observable turnover frequencies and prompt scrutiny of the applicability of structure sensitivity classifications developed for metals to oxide catalysis.

When Ge is deposited epitaxially on Si, the strain energy from the lattice mismatch causes the Ge to form distinctive, three-dimensional islands. The shape of the islands is determined by the energies of the surface facets, facet edges, and interfaces. When phosphorus is added during chemical vapor deposition of Ge, the surface energies change, modifying the island shapes and sizes. Three different island shapes are found for doped layers, as for undoped layers; however, each doped island type is smaller than the corresponding undoped island type. The intermediate-size doped islands are of the same family as the undoped multifaceted “dome” structures, but are considerably smaller; they also have a narrow size distribution. The largest doped islands are related to the defective “superdomes” found for undoped islands, but are bounded by a smaller number of facets, creating pyramidal-shaped structures with their edges aligned along 〈110〉 directions. The distribution of Ge among the different island types depends on the phosphine partial pressure. Phosphorus appears to act as a mild surfactant, suppressing small islands at high PH3 partial pressures. Within the assumptions made, the segregation enthalpy is estimated to be −0.4 eV. Phosphine decreases the Ge deposition rate because of competitive adsorption; however, the steady-state surface coverage (as indicated by the Ge deposition rate) is not reached for thin layers.

A comprehensive surface reaction mechanism on Pt is presented that is capable of describing CO oxidation, H2 oxidation, water−gas shift (WGS), preferential oxidation (PROX) of CO, and the promoting role of H2O on CO oxidation reasonably well. This mechanism consists of a literature CO oxidation model, a surface reaction mechanism for H2 oxidation on Pt developed here, and coupling reactions between the CO and H2 chemistries included for the first time. Thermodynamic consistency, which is shown to be essential for WGS, is ensured in all steps of the entire mechanism. The CO−H2 coupling via the CO + OH reaction, which may involve direct CO2 formation, CO* + OH* ↔ CO2* + H*, as well as an indirect pathway via the carboxyl intermediate, is explored. It is shown that this coupling plays a significant role in capturing the promoting effect of H2O on the CO oxidation-temperature-programmed reaction experiments at low temperatures as well as the overall speed of the WGS and PROX reactions. With the parameters use...

Thermodynamic Consistency Test for Binary Gas + Water Equilibrium Data at Low and High Pressures

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.

Emission control devices such as selective catalytic reduction (SCR), exhaust gas recirculation (EGR), and scrubbers were installed in the compression ignition (CI) engine, and flow analysis of intake air and exhaust gas was required to predict the performance of the CI engine and emission control devices. In order to analyze such gas flow, it was inefficient to comprehensively analyze the engine’s cylinder and intake/exhaust systems because it takes a lot of computation time. Therefore, there is a need for a method that can quickly calculate the gas flow of the CI engine in order to shorten the development process of emission control devices. It can be efficient and quickly calculated if only the parts that require detailed observation among the intake/exhaust gas flow of the CI engine are analyzed in a 3D approach and the rest are analyzed in a 1D approach. In this study, an algorithm for gas flow analysis was developed by coupling 1D and 3D in the valve systems and comparing with experimental results for validation. Analyzing the intake/exhaust gas flow of the CI engine in a 3D approach took about 7 days for computation, but using the developed 1D–3D coupling algorithm, it could be computed within 30 min. Compared with the experimental results, the exhaust pipe pressure occurred an error within 1.80%, confirming the accuracy and it was possible to observe the detailed flow by showing the contour results for the part analyzed in the 3D zone. As a result, it was possible to accurately and quickly calculate the gas flow of the CI engine using the 1D–3D coupling algorithm applied to the valve system, and it was expected that it can be used to shorten the process for analyzing emission control devices, including predicting the performance of the CI engine.

<div class="htmlview paragraph">The paper deals with methodology for selection of Emission Control Devices (ECD), test matrix and evaluation procedure followed for vehicles, engines and analysis of test results.</div> <div class="htmlview paragraph">In the present scenario, it is always being discussed that how existing vehicles and engines manufactured in India complying to Euro I /Euro II standard will perform with different sulphur percentage fuel. The emission control device such as Oxidation Catalytic Converter (OCC) is not very commonly used on the Euro-II vehicle and heavy-duty commercial vehicles. The performance of diesel oxidation catalyst on different sulphur percentage is of vital importance in Indian scenario. The particulate matter is always a concern in diesel vehicles and engines and most of Indian vehicles are having very low margin for particulate matter vis-à-vis regulation.</div> <div class="htmlview paragraph">Considering above, a comprehensive study by ARAI is undertaken with the objective to compare emission characteristic of diesel 3 wheeler vehicle, utility vehicle, heavy duty engine and tractor engine with fuels having different sulphur content of 50 ppm, 150 ppm, 300 ppm and 500 ppm.</div> <div class="htmlview paragraph">The above engine and vehicles were installed with diesel oxidation catalyst, which is specially supplied by ECD manufacturer taking into account engine characteristics in terms of emission and performance. The set of engine and vehicles were subjected to evaluation with particulate trap specially designed and optimized to ascertain the performance with fuel having different sulphur content of 50 ppm, 150 ppm, 300 ppm and 500 ppm. Some vehicles were also subjected to evaluation with combination of oxidation catalyst and particulate trap.</div> <div class="htmlview paragraph">The study could establish the methodology and limitations of installation of ECD on vehicles. The parameters such as temperature and backpressure before and after ECD were monitored during test program, which gave technical inputs to ascertain the behaviour and effectiveness of ECD.</div>

<div class="section abstract"><div class="htmlview paragraph">Small impurities in the fuel can have a significant impact on the emissions control system performance over the lifetime of the vehicle. Of particular interest in recent studies has been the impact of sodium, potassium, and calcium that can be introduced either through fuel constituents, such as biodiesel, or as lubricant additives. In a collaboration between the National Renewable Energy Laboratory and the Oak Ridge National Laboratory, a series of accelerated aging studies have been performed to understand the potential impact of these metals on the emissions control system. This paper explores the effect of the rate of accelerated aging on the capture of fuel-borne metal impurities in the emission control devices and the subsequent impact on performance. Aging was accelerated by doping the fuel with high levels of the metals of interest. Three separate evaluations were performed, each with a different rate of accelerated aging. The aged emissions control systems were evaluated through vehicle testing and then dissected for a more complete analysis of the devices. Results from these experiments show that increasing the rate of acceleration impacts the amount of fuel-borne metals that are captured by the catalyst, which subsequently impacts the catalyst performance. Beyond a certain threshold, the acceleration rate creates an artificial mechanism for catalyst deactivation. In the range of acceleration rates that were examined in this study, these effects were primarily isolated to the inlet of the catalyst whereas performance further down the length of the catalyst was mostly unaffected.</div></div>

<div class="section abstract"><div class="htmlview paragraph">Modern light-duty vehicles require well-controlled engine-out feed-gas and very high catalyst efficiencies to meet the US Environmental Protection Agency (EPA) Tier 2 & 3 standards. When a vehicle with either a gasoline or diesel engine is operating within its controlled state-space the exhaust emissions present at the tailpipe are extremely low. When it is not operating within its controlled state-space the combustion process and therefore its exhaust emissions characteristics will be different. This may occur when an emission control device fails or if a defeat device is employed. Moreover, different control technologies each have unique characteristics or signatures that could assist in identifying either emission control device failure or an existing defeat device.</div><div class="htmlview paragraph">A simple exhaust extension apparatus equipped with a thermocouple for measuring exhaust temperature and a NO<sub>x</sub> / O<sub>2</sub> sensor to measure tailpipe NO<sub>x</sub> and O<sub>2</sub> concentrations can characterize this signature information for pattern recognition analysis. This device can be used both in a laboratory environment with conventional batch sampling systems or for on-road testing as a compact emission measurement system.</div><div class="htmlview paragraph">If this information was acquired during conventional laboratory emissions tests it would provide valuable dynamic system information. This information could characterize events such as cold start open loop operation, engine transient fuel compensation, and high-speed load enrichment and emission control device status with minimal cost.</div></div>