Microkinetic Mechanism Research Articles

Low-Temperature Oxidation of Methane on Rutile TiO2(110): Identifying the Role of Surface Oxygen Species.

Understanding the microkinetic mechanism underlying photocatalytic oxidative methane (CH4) conversion is of significant importance for the successful design of efficient catalysts. Herein, CH4 photooxidation has been systematically investigated on oxidized rutile(R)-TiO2(110) at 60 K. Under 355 nm irradiation, the C-H bond activation of CH4 is accomplished by the hole-trapped dangling OTi- center rather than the hole-trapped Ob- center via the Eley-Rideal reaction pathway, producing movable CH3• radicals. Subsequently, movable CH3• radicals encounter an O/OH species to form CH3O/CH3OH species, which could further dissociate into CH2O under irradiation. However, the majority of the CH3• radical intermediate is ejected into a vacuum, which may induce radical-mediated reactions under ambient conditions. The result not only advances our knowledge about inert C-H bond activation but also provides a deep insight into the mechanism of photocatalytic CH4 conversion, which will be helpful for the successful design of efficient catalysts.

The oscillating Fischer-Tropsch reaction.

The mechanistic steps that underlie the formation of higher hydrocarbons in catalytic carbon monoxide (CO) hydrogenation at atmospheric pressure over cobalt-based catalysts (Fischer-Tropsch synthesis) have remained poorly understood. We reveal nonisothermal rate-and-selectivity oscillations that are self-sustained over extended periods of time (>24 hours) for a cobalt/cerium oxide catalyst with an atomic ratio of cobalt to cerium of 2:1 (Co2Ce1) at 220°C and equal partial pressures of the reactants. A microkinetic mechanism was used to generate rate-and-selectivity oscillations through forced temperature oscillations. Experimental and theoretical oscillations were in good agreement over an extended range of reactant pressure ratios. Additionally, phase portraits for hydrocarbon production were constructed that support the thermokinetic origin of our rate-and-selectivity oscillations.

Automated Generation of Microkinetics for Heterogeneously Catalyzed Reactions Considering Correlated Uncertainties.

The study presents an ab-initio based framework for the automated construction of microkinetic mechanisms considering correlated uncertainties in all energetic parameters and estimation routines. 2000 unique microkinetic models were generated within the uncertainty space of the BEEF-vdW functional for the oxidation reactions of representative exhaust gas emissions from stoichiometric combustion engines over Pt(111) and compared to experiments through multiscale modeling. The ensemble of simulations stresses the importance of considering uncertainties. Within this set of first-principles-based models, it is possible to identify a microkinetic mechanism that agrees with experimental data. This mechanism can be traced back to a single exchange-correlation functional, and it suggests that Pt(111) could be the active site for the oxidation of light hydrocarbons. The study provides a universal framework for the automated construction of reaction mechanisms with correlated uncertainty quantification, enabling a DFT-constrained microkinetic model optimization for other heterogeneously catalyzed systems.

Automatisierte Generierung von Mikrokinetiken für heterogen katalysierte Reaktionen unter Berücksichtigung korrelierter Unsicherheiten**

AbstractDie Studie stellt eine ab initio basierte Methode für die automatische Konstruktion von detaillierten Mikrokinetiken vor, welche korrelierte Unsicherheiten in allen energetischen Parametern und Schätzroutinen berücksichtigt. Hierfür wurden innerhalb des Unsicherheitsraums des BEEF‐vdW Funktionals 2000 Mikrokinetiken für die Oxidationsreaktionen repräsentativer Abgasemissionen von stöchiometrischen Verbrennungsmotoren auf Pt(111) generiert und durch Multiskalenmodellierung mit Experimenten verglichen. Das Ensemble der Simulationsergebnisse unterstreicht die Bedeutung der Berücksichtigung von Unsicherheiten. Innerhalb dieses Ensembles ist es möglich, eine Mikrokinetik zu identifizieren, welche mit den experimentellen Daten übereinstimmt. Diese Mikrokinetik lässt sich auf ein einziges Austausch‐Korrelations‐Funktional zurückführen und legt nahe, dass Pt(111) das aktive Zentrum für die Oxidation leichter Kohlenwasserstoffe sein könnte. Die Studie bietet einen universellen Rahmen für die automatisierte Konstruktion von Reaktionsmechanismen unter Berücksichtigung korrelierter Unsicherheiten und ermöglicht eine DFT‐basierte Optimierung der Mikrokinetik auch für andere heterogen katalysierte Systeme.

Efficient catalyst screening using graph neural networks to predict strain effects on adsorption energy.

Small-molecule adsorption energies correlate with energy barriers of catalyzed intermediate reaction steps, determining the dominant microkinetic mechanism. Straining the catalyst can alter adsorption energies and break scaling relationships that inhibit reaction engineering, but identifying desirable strain patterns using density functional theory is intractable because of the high-dimensional search space. We train a graph neural network to predict the adsorption energy response of a catalyst/adsorbate system under a proposed surface strain pattern. The training data are generated by randomly straining and relaxing Cu-based binary alloy catalyst complexes taken from the Open Catalyst Project. The trained model successfully predicts the adsorption energy response for 85% of strains in unseen test data, outperforming ensemble linear baselines. Using ammonia synthesis as an example, we identify Cu-S alloy catalysts as promising candidates for strain engineering. Our approach can locate strain patterns that break adsorption energy scaling relations to improve catalyst performance.

Molecular simulation of alkyl glycoside surfactants with different concentrations inhibiting methane diffusion in coal

The injection of surfactant solutions into coal seams is one of the most critical technical means of preventing coal mine disasters. It can effectively prevent coal and gas outburst disasters by reducing the gas concentration at the coal mining face. The Wiser model of coal was utilized to establish the initial CH4-containing coal. Different concentrations (0%, 0.2%, 0.4%, 0.8%, 1.2%, and 1.6%) of alkyl poly glucoside surfactant (APG) were injected into CH4-containing coal to investigate the microkinetic mechanism of inhibiting gas desorption. The diffusion parameters of CH4 (i.e., diffusion coefficient, relative concentration, interaction radius), the gyration radius of APG, and the interaction energy between surfactant solution and CH4-containing coal were analyzed on the basis of the kinetic results. Results showed that with the increase in APG concentration, the diffusion coefficient of CH4 decreased gradually. The distribution of CH4 in coal molecules and the change rate of diffusion coefficient increased remarkably. When the concentration of APG was 0%, the diffusion coefficient was 12.00 × 10−7 m2 s−1, and that of 1.6% was 5.65 × 10−7 m2 s−1, a reduction of 52.91% compared with the concentration of 0%. With the increasing concentration of APG, the gyration radius increased, and the interaction energy between APG and CH4-containing coal molecules decreased. The CH4 diffusion coefficient was negatively correlated with the APG gyration radius, and the number of hydrogen bonds was positively correlated with the interaction energy. With the increase in the gyration radius of APG molecule, more hydrogen bonds were formed between APG surfactant and coal molecules, resulting in a decrease in the interaction energy. The system gradually stabilized to effectively inhibit the diffusion of CH4 in coal, which can provide a basis to prevent coal and gas outburst disasters.

The Stoichiometric Uniqueness of Multiple Chemical Reaction Systems in Chemical Thermodynamics, Kinetics and Catalysis – Contributions of Professor Ilie Fishtik

The main scientific achievements of great significance accomplished by Professor Ilie Fishtik at the University of Iowa and the Worcester Polytechnic Institute, in several fields of the physical chemistry as chemical thermodynamics, kinetics and heterogeneous catalysis were revealed and briefly analysed. Fundamental equations of chemical thermodynamics within the De Donder (stoichiometric) approach were reformulated in terms of a special class of chemical reactions, called as response reactions. Using this approach, the unusual behaviour of chemical equilibrium systems, to interpret the apparent contradictions to Le Chatelier principle and to discover hitherto unnoticed thermodynamic identities, was rationalised. The stabilities of chemical species were formulated in terms of a certain class of stoichiometrically unique chemical reactions and their thermochemical characteristics. A completely new approach for the generation and simplification of kinetic mechanisms for complex reaction systems was developed and applied. Based on a new type of reaction networks, referred to as reaction route graphs, a systematic method of analysis and reduction of a microkinetic mechanism was established and employed.

Computer-Generated Kinetics for Coupled Heterogeneous/Homogeneous Systems: A Case Study in Catalytic Combustion of Methane on Platinum

The automatic microkinetic mechanism generator for heterogeneous catalysis, RMG-Cat, has been extensively updated. Density functional theory calculations were performed for 69 adsorbates on Pt(111), and the resulting thermodynamic properties were added to RMG-Cat. The thermo database is significantly more accurate; it includes nitrogen-containing adsorbates for the first time as well as better capabilities for predicting the thermochemistry of novel adsorbates. Additionally, RMG-Cat can now simultaneously pursue a mechanism expansion both on the surface and in the gas phase. This heterogeneous/homogeneously coupled capability is tested on the catalytic combustion of methane on platinum. The results confirm that under some conditions the catalyst is capable of inducing thermal ignition in the gas phase.

Insight into the mechanism of acetonitrile hydrogenation in liquid phase on Pt/Al2O3 by ATR-FTIR

Abstract The mechanism of the acetonitrile hydrogenation to amines over a platinum/alumina catalyst was investigated by in situ infrared spectroscopy. The reaction was studied under realistic conditions -liquid phase using toluene as solvent- using a flow-through cell-microreactor in Attenuated Total Reflection (ATR) mode. Acetonitrile linearly chemisorbed on platinum sites is stepwise hydrogenated to form an imine surface intermediate (CH3CH = NH), which in turn is hydrogenated to ethylamine. Using deuterium, the sequential formation of C D and N D bonds was observed. Then, imine intermediate can condense producing diethylamine and triethylamine, giving ammonia as a by-product. The temporal evolution of the IR signals was modeled using a proposed microkinetic mechanism and intrinsic kinetic constants were obtained under chemical control.

Constrained Chemical Dynamics of CO Dissociation/Hydrogenation on Rh Surfaces.

Among noble metal catalysts, rhodium (Rh) is unique in its ability to perform a one-step synthesis of ethanol from syngas. The first steps following the adsorption of syngas on Rh surfaces are assumed to be responsible for the conversion of CO and the selectivity effects between C1 , C2 , and oxygenated species. In the current work, constrained ab initio molecular dynamics are applied to investigate the kinetics of CO dissociation and hydrogenation over flat and stepped Rh surfaces. The obtained barriers for the Rh(111) surface are in good agreement with the literature data. On the stepped Rh(211) surface, a large site-dependent variation in barrier height is shown, with the upper terrace exhibiting behavior comparable to the Rh(111) surface, whereas the barriers over the lower terrace site are generally significantly lower. The rate constants are calculated using transition state theory for both surfaces, and are applied successfully in a microkinetic model, confirming the predicted impact on CO conversion and CH4 /C1 -oxygenate/C2 Hn selectivity. In addition to the high-accuracy energetics and rate constants reported for CO dissociation/hydrogenation and the presentation of an updated microkinetic mechanism for Rh catalysts, the applicability of constrained molecular dynamics for reaction barrier calculation is confirmed, and sensitive pathways affecting the selectivity between formaldehyde/methanol over Rh catalysts are highlighted.

Automatic Generation of Microkinetic Mechanisms for Heterogeneous Catalysis

A novel approach is presented for generating microkinetic mechanisms in heterogeneous catalysis. The open-source software RMG-Cat automatically develops a detailed list of elementary surface reactions, including thermodynamic properties for the adsorbates and parametrized rate coefficients for the reactions. The software proposes numerous possible surface intermediates and reactions, but it only retains those species that have a sufficiently high rate of formation. RMG-Cat was tested on the dry reforming of methane on nickel. The software correctly found the same set of elementary reactions as in a previously compiled microkinetic mechanism, as well as a few missing reactions. These results demonstrate the potential of this approach for predicting the dominant pathways in heterogeneous catalysis.

Autothermal reforming of methane on rhodium catalysts: Microkinetic analysis for model reduction

Methane autothermal reforming has been studied using comprehensive, detailed microkinetic mechanisms, and a hierarchically reduced rate expression has been derived without apriori assumptions. The microkinetic mechanism is adapted from literature and has been validated with reported experimental results. Rate determining steps are elicited by reaction path analysis, partial equilibrium analysis and sensitivity analysis. Results show that methane activation occurs via dissociative adsorption to pyrolysis, while oxidation of the carbon occurs by O(s). Further, the mechanism is reduced through information obtained from the reaction path analysis, which is further substantiated by principal component analysis. A 33% reduction from the full microkinetic mechanism is obtained. One-step rate equation is further derived from the reduced microkinetic mechanism. The results show that this rate equation accurately predicts conversions as well as outlet mole fraction for a wide range of inlet compositions.

REACTOR OF CATALYTIC METHANE OXIDATION FROM VENTILATION AIR — FROM LABORATORY TO QUARTER-TECHNICAL SCALE PROTOTYPE

The most research efforts in heterogeneous catalysis are directed at understanding the correlations between the catalytic activity of studied contact and its physical-structural properties. One way to address this issue is through examination of specific reaction mechanism which has been of particular interest to many researchers. The most important from these basic studies focus on the effect of variables such as temperature and/or concentration of substrate or product on the reaction rate. In many cases, significant differences in mechanisms may lead to similar microscopic observations, and thus, to similar expressions for the reaction rate. This means that the experimentally determined kinetic parameters usually do not allow firm conclusions on microkinetic mechanism. This problem is especially important for multi-step processes such as the transformation reactions of hydrocarbons, in particular catalytic oxidation. However, it should be noted that any proposed mechanism must be compatible with the evaluated kinetics.

Experimental Characterization of Thin-film Ceria Solid Oxide Fuel Cell Anodes

This study presents an experimental effort to elucidate the role of CeO2-x in composite SOFC anodes, by characterizing electrochemical oxidation of small fuel molecules (H2, CO) on thin-film ceria anodes. Thin ceria films were sputter-deposited onto thick YSZ-electrolyte-supported button cells with high-surface-area, porous LSM/YSZ cathodes. The mixed ionic-electronic conductivity of ceria provides multiple reaction pathways for electrochemical oxidation and electrochemical characterization of the ceria films over a range of gas compositions at temperatures from 650 to 850 {degree sign}C provide a basis for elucidating the importance of competing mechanisms for fuel oxidation on the ceria. Analysis of the experiment results provides a basis for formulating a micro-kinetic mechanism for the oxidation of small fuel species on ceria anodes which can be the implemented in SOFC modeling efforts for optimizing ceria-based composite anodes.

An improved microkinetic model for the water gas shift reaction on copper

A previously developed 13 elementary step microkinetic mechanism for the WGSR on Cu(1 1 1) [Surf. Sci. 512 (2002) 229] is extended by including four new elementary steps. Employing a systematic reaction route analysis, the microkinetic model is further analyzed, simplified and reduced. It is shown that from the total of 70 possible overall reaction routes (RRs), the kinetics of the WGS reaction on Cu is still dominated by only three RRs. Two of them, namely, the formate and associative RRs, are dominant at lower temperatures. The third one, referred to as a modified redox RR, is dominant at higher temperatures. Utilizing the quasi-equilibrium and steady-state assumptions, the microkinetic model is further reduced to a two-step mechanism similar to that of Temkin. The corresponding explicit rate expression with all parameters predicted a priori provides excellent agreement with the complete microkinetic model as well as with new experimental data provided here.

Multiwall carbon nanotubes: Self-organization and inhibition of step-flow growth kinetics

In this article the results of a numerical study on the self-organization and inhibition of step-flow growth of carbon nanotubes viewed within the framework of the continuum surface diffusion equation are given. Incorporation constants of C atoms, which differ depending on what side of the step the atom has been chemisorbed prior to incorporation, are considered. These differences can lead to the onset of surface multi-island nucleation in front of a propagating step with decrease in the growth temperature. This effect is able to cause formation of defects in the growing layer and even to inhibit stable step-flow modes of nanotube growth, leading to the formation of misoriented surface nuclei which may be likened to amorphous matter. A diagram distinguishing three characteristic temperature regions for nanotube formation is given: (i) the region where there is no secondary layer nucleation because the surface concentration of adsorbate on the surface of the first layer is not sufficiently high, (ii) the region of successive nucleation and propagation of one layer after another, i.e., stable step-flow growth, and (iii) the region where the nanotube surface is prone to multi-island nucleation, which inhibits stable step-flow growth and causes “amorphization” of the external surface of the nanotube. The simultaneous propagation of multilayer steps coupled by lip–lip interaction is shown to be feasible only if a microkinetic mechanism exists, which effectively redistributes to the edges of internal layers the atoms arriving initially by surface diffusion at the edge of the external layer