Model Of Heterogeneous Catalysis Research Articles

Multilevel on-the-fly sparse grids for coupling coarse-grained and high fidelity models in heterogeneous catalysis

Coupling microscopic high-fidelity models, such as microkinetic models, into continuum scale simulations can easily become intractable in practice due to the costs of the high-fidelity model evaluation. To lift this burden, we present a novel multilevel self-consistent on-the-fly sparse grid approach, which integrates the construction of surrogates of the high-fidelity model in a multilevel fashion into the continuum solution process. Besides its efficiency, an appealing feature of the approach is its simplicity and robustness. A single hyperparameter controls the whole workflow, from training set design to the accuracy of the reactor model. We demonstrate the methodology on a recent microkinetic model for catalytic combustion in a fixed-bed reactor model as a representative example. Already with modest numbers of data, the approach achieves sufficient accuracy, reducing the effort by orders of magnitude compared to a direct coupling.

New Models of Heterogeneous Catalysis for Numerical Study of Flows and Heat Transfer in an Induction HF Plasmatron

New Models of Heterogeneous Catalysis for Numerical Study of Flows and Heat Transfer in an Induction HF Plasmatron

Elusive supported surface M2Ox dimer active site (M = Re, W, Mo, Cr, V, Nb, and Ta)

Supported transition metal oxide catalysts are extensively used as heterogeneous catalysts for various energy, chemical, and environmental applications. The molecular structures of dehydrated surface metal oxide phases are crucial for understanding structure-activity/selectivity relationships that guide the design of enhanced catalysts. Some early studies suggested that dimeric (aka binuclear) surface metal oxide sites were more active/selective than monomeric (aka mononuclear) sites, prompting interest in synthesizing catalysts with supported dimeric metal oxide structures. This review examines the literature on dehydrated silica-based supported group 7-5 MOx catalysts (ReOx, WOx, MoOx, CrOx, VOx, NbOx, and TaOx on SiO2, MCM-41, AlOx/SiO2, and H-ZSM-5) for their surface metal oxide structures. In situ Raman, extended x-ray absorption fine structure, and ultraviolet-visible spectroscopy indicate that monomeric surface MOx structures predominate in all such catalysts. Therefore, the cursory use of dimeric surface M2Ox sites in catalytic mechanisms and reaction models in heterogeneous catalysis by supported metal oxides is questionable, and moving forward, the invoking of supporting dimeric surface M2Ox sites should be critically examined and backed up with direct spectroscopic methods.

Analysis of bulk-surface reaction-sorption-diffusion systems with Langmuir-type adsorption

We consider a class of bulk-surface reaction-adsorption-diffusion systems, i.e. a coupled system of reaction-diffusion systems on a bounded domain Ω⊆Rd (bulk phase) and its boundary Σ=∂Ω (surface phase), which are coupled via nonlinear normal flux boundary conditions. In particular, this class includes a heterogeneous catalysis model with Fickian bulk and surface diffusion and nonlinear adsorption of Langmuir type, i.e. transport from the bulk phase to the active surface, and desorption. For this model, we obtain well-posedness, positivity and global-in-time existence of solutions under some realistic structural conditions on the chemical reaction network and the sorption model. We work in appropriate Sobolev-Slobodetskii settings, where we aim for a wide range for the integrability index, including in particular values p<d.

Hydrothermally Synthesized ZnCr- and NiCr-Layered Double Hydroxides as Hydrogen Evolution Photocatalysts.

The layered double hydroxides (LDHs) of transition metals are of great interest as building blocks for the creation of composite photocatalytic materials for hydrogen production, environmental remediation and other applications. However, the synthesis of most LDHs is reported only by the conventional coprecipitation method, which makes it difficult to control the catalyst's crystallinity. In the present study, ZnCr- and NiCr-LDHs have been successfully prepared using a facile hydrothermal approach. Varying the hydrothermal synthesis conditions allowed us to obtain target products with a controllable crystallite size in the range of 2-26 nm and a specific surface area of 45-83 m2∙g-1. The LDHs synthesized were investigated as photocatalysts of hydrogen generation from aqueous methanol. It was revealed that the photocatalytic activity of ZnCr-LDH samples grows monotonically with the increase in their average crystallite size, while that of NiCr-LDH ones reaches a maximum with intermediate-sized crystallites and then decreases due to the specific surface area reduction. The concentration dependence of the hydrogen evolution activity is generally consistent with the standard Langmuir-Hinshelwood model for heterogeneous catalysis. At a methanol content of 50 mol. %, the rate of hydrogen generation over ZnCr- and NiCr-LDHs reaches 88 and 41 μmol∙h-1∙g-1, respectively. The hydrothermally synthesized LDHs with enhanced crystallinity may be of interest for further fabrication of their nanosheets being promising components of new composite photocatalysts.

The influence of heterogeneous catalytic processes on the heat flux to the surface and the chemical composition of the shock layer at high-speed flow around blunt bodies

Numerical modeling of the high-speed flow of a multicomponent dissociated gas around a blunt body was carried out for the flow regime corresponding to the thermally stressed point of the gliding trajectory of entry into the Earth's atmosphere in the surface temperature range of 800–2000 K. As boundary conditions, a stage-by-stage model of heterogeneous catalysis of the interaction of dissociated air with the thermal protection material SiO2 was used, previously developed by the authors on the basis of transition state theory and quantum mechanical calculations. The dependence of the modes of heterogeneous chemical reactions on the surface temperature and the density of surface adsorption centers and their influence on the chemical composition of the shock layer and the convective heat flux to the streamlined surface are analyzed. The contributions of individual mechanisms of heterogeneous recombination to the total rate of formation of molecules on the surface are estimated. The possibility of using the density of adsorption sites as an effective parameter when setting boundary conditions on a real thermal protection surface in flow problems is shown.

Infinitely Fast Heterogeneous Catalysis Model for Premixed Hydrogen Flame-Wall Interaction

In most computations of reacting flows, walls are considered as inert. This approach can produce non-physical results that lead to very large values of heat release at the wall for hydrogen Flame-Wall Interaction (FWI). This paper confirms this unexpected behavior at walls for H2 flames. It also explores the possibility that the problem does not come from a deficiency of the chemical scheme at low temperatures but from the wall boundary condition: due to the large amount of radical species produced by H2 flames, the “inert” assumption may not be valid anymore. The purpose of this study is twofold: (1) to propose a simplified model named Infinitely Fast Heterogeneous Catalysis (IFHC) that mimics surface reactions and (2) to test it on the canonical Head-On Quenching (HOQ) configuration. A large set of parameters is investigated: a skeletal (San Diego) and a detailed (Burke) chemical schemes are tested, the grid-dependence and the influence of operating conditions (ϕ, Tw and P) are assessed. Results show that the IFHC model can inhibit the chemical pathways that are responsible for the heat generation peak observed on the wall in hydrogen FWI with inert surfaces. The IFHC model also allows computations to converge when the mesh is refined, a property that cannot be reached for inert wall treatments. Using IFHC, neither the predicted quenching distance nor the flame structure are altered. However, the maximum wall heat flux is significantly decreased when compared to inert walls. Over the range of operating conditions studied, the maximum wall heat flux is reached at high pressure, low wall-temperature and equivalence ratios close to the maximum of reactivity of H2 flames (ϕ≈1.7). This study shows that, for H2 FWI, walls should be at least modeled with simple surface reactions – such as IFHC – and that assuming inert walls is not possible.Novelty and significance statementThe work presented in this paper is new, original and of interest at the present time as many groups simulate near-walls H2/Air flames. The IFHC model is relevant for all premixed hydrogen combustion close to walls and is, thus, relevant for most realistic premixed hydrogen combustion scenarios. Besides being conservative in terms of mass, momentum, energy and atom balance, IFHC allows to reach mesh convergence during quenching contrary to simplistic inert wall treatments. It also allows to describe walls at an affordable computational cost compared to a full set of heterogeneous reactions. This work also opens the discussion with experimentalists since, to the best of the authors’ knowledge, the dimensional maximum wall heat fluxes are not yet available in the literature and are, thus, provided here for future comparison.

XPK: Toward Accurate and Efficient Microkinetic Modeling in Heterogeneous Catalysis

XPK: Toward Accurate and Efficient Microkinetic Modeling in Heterogeneous Catalysis

Fast evaluation of the adsorption energy of organic molecules on metals via graph neural networks

Modeling in heterogeneous catalysis requires the extensive evaluation of the energy of molecules adsorbed on surfaces. This is done via density functional theory but for large organic molecules it requires enormous computational time, compromising the viability of the approach. Here we present GAME-Net, a graph neural network to quickly evaluate the adsorption energy. GAME-Net is trained on a well-balanced chemically diverse dataset with C1–4 molecules with functional groups including N, O, S and C6–10 aromatic rings. The model yields a mean absolute error of 0.18 eV on the test set and is 6 orders of magnitude faster than density functional theory. Applied to biomass and plastics (up to 30 heteroatoms), adsorption energies are predicted with a mean absolute error of 0.016 eV per atom. The framework represents a tool for the fast screening of catalytic materials, particularly for systems that cannot be simulated by traditional methods.

Postplasma Catalytic Model for NO Production: Revealing the Underlying Mechanisms to Improve the Process Efficiency

Plasma catalysis is emerging for plasma-assisted gas conversion processes. However, the underlying mechanisms of plasma catalysis are poorly understood. In this work, we present a 1D heterogeneous catalysis model with axial dispersion (i.e., accounting for back-mixing and molecular diffusion of fluid elements in the process stream in the axial direction), for plasma-catalytic NO production from N2/O2 mixtures. We investigate the concentration and reaction rates of each species formed as a function of time and position across the catalyst, in order to determine the underlying mechanisms. To obtain insights into how the performance of the process can be further improved, we also study how changes in the postplasma gas flow composition entering the catalyst bed and in the operation conditions of the catalytic stage affect the performance of NO production.

Kinetic Monte Carlo modeling of the interface heterogeneous catalysis effect based on machine learning

<p indent="0mm">Heterogeneous catalysis in a high-enthalpy environment has a remarkable influence on the aerodynamic heating of hypersonic aircraft, and modeling this multiscale effect is an important prerequisite for the lightweight and low-redundancy design of an aircraft thermal protection system. A mesoscale interface heterogeneous catalysis model applicable to macroscale flow field calculation was established by the Monte Carlo method. By the radial basis function neural network algorithm, a machine learning-based reaction kinetics analytical surrogate model was constructed, by which fast and accurate prediction of the catalytic recombination coefficient was realized. The results showed that the machine learning model predicted a recombination coefficient similar to the kinetic Monte Carlo method while reducing the magnitude of the calculation time to that of the traditional macroscale method. Coupled with the macroscale CFD solver, the proposed model can efficiently predict surface heat flux with a multiscale heterogeneous catalysis effect. Modeling of microscale results by the machine learning method and its application in macroscale simulation not only covers the heterogeneous catalytic kinetics but also improves computational efficiency. This modeling provides a theoretical basis for multiscale modeling of complicated high-temperature effects on hypersonic aircraft surfaces.

Achieving Theory-Experiment Parity for Activity and Selectivity in Heterogeneous Catalysis Using Microkinetic Modeling.

ConspectusMicrokinetic modeling based on density functional theory (DFT) energies plays an essential role in heterogeneous catalysis because it reveals the fundamental chemistry for catalytic reactions and bridges the microscopic understanding from theoretical calculations and experimental observations. Microkinetic modeling requires building a set of ordinary differential equations (ODEs) based on the calculation results of thermodynamic properties of adsorbates and kinetic parameters for the reaction elementary steps. Solving a microkinetic model can extract information on catalytic chemistry, including critical reaction intermediates, reaction pathways, the surface species distribution, activity, and selectivity, thus providing vital guidelines for altering catalysts.However, the quantitative reliability of traditional microkinetic models is often insufficient to conclusively extrapolate the mechanistic details of complex reaction systems. This can be attributed to several factors, the most important of which is the limitation of obtaining an accurate estimation of the energy inputs via traditional calculation methods. These limitations include the difficulty of using static DFT methods to calculate reaction energies of adsorption/desorption processes, often rate-controlling or selectivity-determining steps, and the inadequate consideration of surface coverage effects. In addition, the robust microkinetic software is rare, which also complicates the resolution of complex catalytic systems.In this Account, we review our recent works toward refining the predictions of microkinetic modeling in heterogeneous catalysis and achieving theory–experiment parity for activity and selectivity. First, we introduce CATKINAS, a microkinetic software developed in our group, and show how it disentangles the problem that traditional microkinetic software has and how it can now be applied to obtain kinetic results for more sophisticated reaction systems. Second, we describe a molecular dynamics method developed recently to obtain the free-energy changes for the adsorption/desorption process to fill in the missing energy inputs. Third, we show that a rigorous consideration of surface coverage effects is pivotal for building more realistic models and obtaining accurate kinetic results. Following a series of studies on acetylene hydrogenation reactions on Pd catalysts, we demonstrate how this new approach can provide an improved quantitative understanding of the mechanism, active site, and intrinsic structural sensitivity. Finally, we conclude with a brief outlook and the remaining challenges in this field.

Reaction mechanism development and investigation on the convergence influence in a 1D catalyst model for a γ-alumina stabilized three-way catalyst

Accurate and computational cost-effective modeling tools for the optimization of processes and devices of all kinds are needed in nearly all scientific fields. While experimental optimization entails high expenses in terms of cost and time virtual optimization may be a promising alternative. In this work, the suitability and accuracy of a 1D heterogeneous catalytic model is investigated. First, the influence of cell discretization and residence time on the convergence in a 1D catalyst model are investigated. Second, the catalyst model is investigated and validated with use of a stoichiometric steady state three-way catalyst experiment. With the help of these investigations the reaction mechanism is further developed and new reaction rates for two reactions are presented. The modeling results are compared to a 2D simulation approach in terms of computational time and catalyst conversion behavior. The presented model is capable to capture the experimental results with a drastically reduced computational time in comparison to the 2D simulation presented in literature.

Photoelectrochemical performance of tubewall‐separated titanium dioxide nanotube array photoelectrode

AbstractTitanium dioxide nanotube array (TiO2 NTA) grown on titanium substrate has been fabricated as active photoelectrode for organic compound degradation. TiO2 NTA shows the tubewall‐separated nanotube arrangement and anatase crystal structure. Transient photocurrent of 1.86 mA and open‐circuit photovoltage of 0.26 V are determined to keep stable level under ultraviolet (UV) light irradiation, indicating its high photoelectrochemical activity. The cathodic polarization process causes more feasible electron transfer than the equilibrium process and anodic polarization process, which is consistent with the declining current response at an increasing positive potential. The photocatalytic activity of TiO2 NTA is fully evaluated through UV light‐induced photocatalysis and photoelectrocatalysis degradation of organic dye X‐3B as a reactant pollutant. The pseudo‐first‐order reaction rate constant is enhanced from 0.00346 min−1 for photocatalysis to 0.00521 min−1 for photoelectrocatalysis using Langmuir–Hinshelwood kinetic model of heterogeneous catalysis. The tubewall‐separated nanotube structure could induce photoelectron directional transport and provide extra interspace for reactant organic molecule diffusion at accessible surface area. The positive potential applied on TiO2 NTA could further promote photoelectron–hole pair separation. Two strategies of photoelectron generation and photoelectron–hole separation are accordingly adopted to improve its electro‐assisted photocatalytic activity. Therefore, the tubewall‐separated TiO2 NTA could present the promising photoelectrocatalysis application.

Recent implementations in LASP 3.0: Global neural network potential with multiple elements and better long-range description

LASP (large-scale atomistic simulation with neural network potential) software developed by our group since 2018 is a powerful platform (www.lasphub.com) for performing atomic simulation of complex materials. The software integrates the neural network (NN) potential technique with the global potential energy surface exploration method, and thus can be utilized widely for structure prediction and reaction mechanism exploration. Here we introduce our recent update on the LASP program version 3.0, focusing on the new functionalities including the advanced neural network training based on the multi-network framework, the newly-introduced S7 and S8 power type structure descriptor (PTSD). These new functionalities are designed to further improve the accuracy of potentials and accelerate the neural network training for multiple-element systems. Taking Cu-C-H-O neural network potential and a heterogeneous catalytic model as the example, we show that these new functionalities can accelerate the training of multi-element neural network potential by using the existing single-network potential as the input. The obtained double-network potential CuCHO is robust in simulation and the introduction of S7 and S8 PTSDs can reduce the root-mean-square errors of energy by a factor of two.

Near-Surface Imaging of the Multicomponent Gas Phase above a Silver Catalyst during Partial Oxidation of Methanol

Fundamental chemistry in heterogeneous catalysis is increasingly explored using operando techniques in order to address the pressure gap between ultrahigh vacuum studies and practical operating pressures. Because most operando experiments focus on the surface and surface-bound species, there is a knowledge gap of the near-surface gas phase and the fundamental information the properties of this region convey about catalytic mechanisms. We demonstrate in situ visualization and measurement of gas-phase species and temperature distributions in operando catalysis experiments using complementary near-surface optical and mass spectrometry techniques. The partial oxidation of methanol over a silver catalyst demonstrates the value of these diagnostic techniques at 600 Torr (800 mbar) pressure and temperatures from 150 to 410 °C. Planar laser-induced fluorescence provides two-dimensional images of the formaldehyde product distribution that show the development of the boundary layer above the catalyst under different flow conditions. Raman scattering imaging provides measurements of a wide range of major species, such as methanol, oxygen, nitrogen, formaldehyde, and water vapor. Near-surface molecular beam mass spectrometry enables simultaneous detection of all species using a gas sampling probe. Detection of gas-phase free radicals, such as CH3 and CH3O, and of minor products, such as acetaldehyde, dimethyl ether, and methyl formate, provides insights into catalytic mechanisms of the partial oxidation of methanol. The combination of these techniques provides a detailed picture of the coupling between the gas phase and surface in heterogeneous catalysis and enables parametric studies under different operating conditions, which will enhance our ability to constrain microkinetic models of heterogeneous catalysis. (Less)

Removal of Nonylphenol Polyethylene Glycol (NPEG) with Au-TiO2 Catalysts: Kinetic and Initial Transformation Path

The purpose of this study was to evaluate the efficiency of the Au-TiO2 catalyst in the degradation of nonylphenol polyethylene glycol (NPEG). In the first part of the study, the catalyst was synthesized and characterized. Initially, the catalyst (TiO2 Degussa P-25) was doped with gold precursor salts (HAuCl4) at different concentrations (5, 10, and 15%) and the photodeposition method with UV light. It was determined by diffuse reflectance (DF) and scanning electron microscopy (SEM) that the photodeposition method was effective for the inclusion of gold particles on the surface. The catalyst band gap showed a reduction to 2.9 e.v (compared to TiO2 Degussa P-25), and it was observed that the gold-doped catalyst showed absorption in the visible light range 500 to 600 nm. The percentage of deposited gold was determined by energy dispersive spectroscopy (EDS). In the second part of the study, various NPEG degradation experiments were performed; with the catalyst that showed the best conversion percentages of NPEG, the experimental data were analyzed using UV-Vis spectrophotometry and TOC (total organic carbon). With these results, a carbon-based mass balance and reaction kinetics were generated using the Langmuir–Hinshelwood (L–H) heterogeneous catalysis model. For the estimation of the kinetic constants, the non-linear regression of the Levenger–Marquardt algorithm was used. With these results, kinetic models of the degradation of the molecule and the generation and consumption of organic intermediate products (OIPs) were generated.

The fast-sorption and fast-surface-reaction limit of a heterogeneous catalysis model

<p style="text-indent:20px;">Within this paper, we consider a heterogeneous catalysis system consisting of a bulk phase <inline-formula><tex-math id="M1">\begin{document}$ \Omega $\end{document}</tex-math></inline-formula> (chemical reactor) and an active surface <inline-formula><tex-math id="M2">\begin{document}$ \Sigma = \partial \Omega $\end{document}</tex-math></inline-formula> (catalytic surface), between which chemical substances are exchanged via adsorption (transport of mass from the bulk boundary layer adjacent to the surface, leading to surface-accumulation by a transformation into an adsorbed form) and desorption (the reverse process). Quite typically, as is the purpose of catalysis, chemical reactions on the surface occur several orders of magnitude faster than, say, chemical reactions within the bulk phase, and sorption processes are often quite fast as well. Starting from the non-dimensional version, different limit models, especially for fast surface chemistry and fast sorption at the surface, are considered. For a particular model problem, questions of local-in-time existence of strong and classical solutions and positivity of solutions are addressed.

Modeling Morphology and Catalytic Activity of Nanoparticle Ensembles Under Reaction Conditions

Although nanoparticle catalysts obtain different sizes and shapes under reaction conditions, computational modeling in heterogeneous catalysis is usually based on well-defined crystallographic plan...

Progress in Accurate Chemical Kinetic Modeling, Simulations, and Parameter Estimation for Heterogeneous Catalysis

Chemical kinetic modeling in heterogeneous catalysis is advancing in its ability to provide qualitatively or even quantitatively accurate prediction of real-world behavior because of new advances i...