Washcoat Layer Research Articles

Control-Oriented Reduced-Order Modeling of Conversion Efficiency in Dual-Layer Washcoat Catalysts With Accumulation and Oxidation Functions

Abstract This work proposes a model for predicting conversion efficiency in multifunctional catalysts with dual-layer washcoat. The mass transfer is more relevant in these devices than in single-layer washcoats due to additional transport steps between the catalytic layers. In addition, the different reaction mechanisms between layers make the concentration of the chemical species differ in each layer. To deal with this boundary while considering the need for real-time computation, a reduced-order explicit solver for the convective diffusive reactive transport is presented for the case of dual-layer washcoats. Assuming one-dimensional quasi-steady flow, the solution procedure consisted of substituting the diffusive interfacial fluxes in the bulk gas and washcoat conservation equations by expressions that depend explicitly on the average concentration in the gas phase. The solution was then applied to model the performance of dual-layer oxidation catalysts with reductant accumulation in one washcoat layer, such as diesel oxidation catalyst (DOC) and ammonia slip catalyst (ASC) systems, during driving cycles. First, the response of these catalysts was analyzed by comparing them against experimental data and considering additional parameters provided by the model. Next, the importance of the mass transfer limitations was discussed to complete the analysis. The proposed model was compared with a simplified solver where the mass transfer steps were omitted, thus deteriorating the prediction capabilities in some driving cycle phases. Finally, a sensitivity study was performed to assess the impact of the mesh size on the prediction capabilities and computational requirements.

Towards FIB-SEM Based Simulation of Pore-Scale Diffusion in SCR Catalyst Layers

The diffusivity in the upper Cu-Chabazite layer of a dual layer ammonia oxidation catalyst with a lower Pt layer was investigated. In a first step, the pore structure of the upper Cu-Chabazite catalyst layer was determined by Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) slice&view tomography. From the FIB-SEM data the 3D pore structure of the catalyst was reconstructed and diffusion simulations were performed on the reconstructed pore geometry, resulting in an estimated effective diffusivity of Deff/Dgas = 0.31. To validate the FIB-SEM derived estimates of the diffusivity, measurements of CO oxidation on the dual layer catalyst were performed, where the CO was oxidized in the lower Pt-layer while the upper SCR layer served as an inactive diffusion barrier. In this way, the effective diffusivity can be determined from the measured CO conversion. An effective diffusion coefficient of Deff/Dgas = 0.11 was obtained from the CO oxidation measurements, three times lower than the value obtained from the FIB-SEM data, but in line with previous literature data for the effective diffusivity in monolith washcoat layers. Additional NH3 oxidation experiments were performed on the dual layer catalyst. The results were well reproduced by a reactor model applying the effective diffusion coefficient obtained by the CO oxidation experiments. The origin of this apparent inconsistency is currently not understood and requires further investigation.

A Novel Approach to Water Softening Based on Graphene Oxide-Activated Open Cell Foams

This work focuses on exploring a new configuration for the reduction of water hardness based on the surface modification of polyurethane (PU) open cell foams by the deposition of thin graphene oxide (GO) washcoat layers. GO was deposited by the dip–squeeze coating procedure and consolidated by thermal treatment. The final washcoat load was controlled by performing consecutive depositions, after three of which, a GO inventory up to 27 wt% was obtained onto PU foams of 60 pores per inch (PPI). The GO-coated PU foams were assembled into a filter, and the performance of the system was tested by continuously feeding water with hardness in the 190–270 mgCa2+,eq·L−1 range. Remarkable results were demonstrated in terms of total adsorbing capacity, which was evaluated by measuring the outlet total hardness by titration and exhibited values up to 63 mgCa2+,eq·gGO−1 at a specific filtered water volume of 650 mLH2O·gGO−1, outperforming the actual state-of-the-art adsorbing capacity of similar GO-based materials.

Gas diffusion in an automotive catalyst in an unsteady state

The influence of gas diffusion is often ignored because the washcoat layer is thin. Therefore, in addition to developing new catalytic materials, modifying the washcoat pores is important for improving effective gas diffusivity. The gas diffusion phenomena in an unsteady state in an experimentally simulated washcoat layer were evaluated by the pulse injection method. Wicke–Kallenbach diffusion cell was used. Effective diffusion coefficients obtained in an unsteady state were compared to those obtained in a steady state. The effective diffusion coefficients were similar to the values obtained under a steady state, except for low-molecular-weight gasses, due to the difference in mean molar speed between the evaluating gas and atmospheric gas.

Microreactor technology in experimental and modelling study of alcohol oxidation on nanogold

Selective oxidation of methanol, ethanol, 1-propanol and 1-butanol to corresponding aldehydes was performed in a microreactor in the presence of a Au/γ-Al2O3 coated catalyst. Nanoparticle size distribution, acidity, specific surface area and the average pore size as well as uniformity and thickness of the coating layer were evaluated with relevant characterization techniques. The experiments were designed to reveal the effect of temperature, residence time and oxygen-to-alcohol ratio on both alcohol conversion and product distribution. Stability and repeatability of the coating procedure was successfully demonstrated. To describe the reaction kinetics, plausible kinetic equations were implemented in a pseudo-homogeneous plug flow model, which turned out to be an adequate approximation to describe the flow pattern in the microreactor. Non-linear regression analysis enabled the determination of the rate and adsorption parameters included in the kinetic model. An advanced kinetic and mass transfer model was developed to reveal the impact of the diffusion inside the catalytic washcoat layer, confirming that molecular diffusion is not a limiting factor for alcohol oxidation in the microreactor.

Low-dimensional models for real time simulations of monolith reactors with dual washcoat layers

We present a novel and accurate procedure with local property dependent internal and external mass transfer coefficient matrices for real time simulations of monolith reactors with dual washcoat layers. Our approach extends the prior work based on the Thiele matrix method for accurately describing the washcoat diffusional effects in single layered systems. It combines the vector form of the multi-scale averaged model with calculation of the local property dependent internal, external and cross-Sherwood number matrices for multi-component systems. We illustrate the usefulness of the reduced order model using the example of a dual-layered model system with first washcoat layer of selective catalytic reduction (SCR) catalyst and second washcoat layer of lean NOx trap (LNT) catalyst. We determine the mesh size independent solutions of the full 1+1D detailed model and compare the same with that obtained with the reduced order model. We show that the proposed method has the same accuracy as the mesh size independent detailed solution while speeding up calculations by about three orders of magnitude for the example studied, and possibly by several more orders of magnitude for real systems with complex and detailed micro-kinetic models to describe the storage and reactions in the catalytic layers.

Non-thermal plasma in honeycomb catalyst for the high-throughput removal of dilute styrene from air

This study focused on the removal of styrene using a metal-coated monolith (MCM) catalyst integrated with non-thermal plasma. The MCM catalyst was prepared by depositing a ZSM-5 support impregnated with a combination of three metals (Co, Pd, La) on a practical-scale honeycomb monolith. A comparative study with a commercial metal-coated monolith (CMCM) of the same dimensions was conducted to evaluate the effectiveness of the MCM with respect to styrene removal. The plasma discharge in the monolith was characterized by measuring the voltage and current waveforms and optical emission spectroscopy (OES). The electric power of the plasma discharge largely depended on the metal loading, flow rate, and humidity of the feed gas. The removal efficiency of styrene (inlet concentration: 20 parts per million, volumetric (ppm)) was examined using both plasma catalytic oxidation (PCO) and thermal catalytic oxidation (TCO), to enable the effect of plasma to be clearly assessed. PCO with the MCM catalyst achieved styrene removal efficiency of nearly 100% at a specific energy input (SEI) of 40 J/L, whereas the CMCM catalyst achieved 92% removal for identical SEI. In comparison, TCO with the MCM and CMCM catalysts required high temperatures of 420 °C and 480 °C, respectively, for the complete removal of styrene. In addition, the CO2 selectivity of the MCM catalyst was higher at 83% compared with 75% for the CMCM at SEI of 40 J/L. The optimal silica binder content for the deposition of a robust washcoat layer was found to be approximately 5%.

Modelling of the interaction of kinetics and external transport phenomena in structured catalysts: The effect of reaction kinetics, mass transfer and channel size distribution in solid foams

The increasing use of structured reactors and catalysts, particularly solid open foams requires a new touch to the interaction of intrinsic chemical kinetics and mass transfer. In spite that the internal mass transfer resistance in the catalyst pores is efficiently suppressed because of thin washcoat layers on structured catalysts, the low flow velocities applied in many liquid-phase processes impair the mass transfer efficiency at the outer surface of the catalyst layer inside the channels of monoliths and open foams. The coupling of external mass transfer resistance and intrinsic kinetics was considered for various power-law and adsorption kinetics in tubular reactors filled with structural catalysts. The relevant mass balance equations were derived and solved numerically and the behaviors of single-channel systems were illustrated in the parameter spaces of Damköhler numbers. The single-channel model was successfully extended to a system of parallel channels with a distribution of channel sizes. The effect of the channel size distribution on the Reynolds and Sherwood numbers was illustrated by numerical simulations. The proposed approach is a potential tool for the design of structured catalysts.

In-situ observation of water evaporation in exhaust gas catalyst via sub-micron and sub-second X-ray tomography

Condensation of liquid water in the porous washcoat layer of exhaust gas catalysts limits their conversion efficiency during the cold-start of an internal combustion engine (ICE). The evaporation kinetics also determine the functionality of the catalyst. In this work, we study water evaporation in the washcoat layers using in-situ X-ray tomographic microscopy with high temporal and spatial resolution. A novel in-situ heating setup is developed, able to reproduce real-world conditions in terms of temperature and gas speed. It is demonstrated that with a voxel size of 0.4 μm, satisfactory data quality can be achieved with a total acquisition time for tomographic volumes of 0.5 s. Thanks to the high temporal and spatial resolution, the water evaporation in a Al2O3 washcoat layer can be described under representative conditions. We find that the time evolution of the evaporation follows an exponential-like behavior and water starts to evaporate from the macroscopic washcoat cracks.

Parametric study of methane catalytic combustion in a micro-channel reactor: Effects of porous washcoat properties

A washcoat resolved numerical model of catalytic micro combustor fueled by methane is developed. Detailed gas phase mechanism and alumina dioxide supported Pt-based surface mechanism are adopted. This model proves to capture flow, temperature and species fields precisely including impermeable flow and products accumulation in porous washcoat layer. The novelty and superiority of this research compared to traditional modeling method in which instantaneous diffusion onto catalytic wall is assumed lies in that: it is possible to investigate effects of porous washcoat properties with this full-scale numerical model. Computational results show that homogeneous reactions (HR) are impaired and heterogeneous reactions (HTR) are strengthened as channel confinement increases. Da number analysis reveals that the micro combustor works under a mixed controlled mode in which catalytic reaction rate and internal mass diffusion rate are comparable. When washcoat layer thickness increases from 15 μm to 75 μm, overall catalyst usage roughly decreases from 0.34 to 0.05. It is concluded washcoat thickness can’t be too thin in order to maintain HTR intensity and too thick since catalyst usage is extremely low. Porous washcoat properties impact combustion characteristics in a similar mechanism: modifying effective mass diffusion coefficient. In general, increasing porosity, average pore diameter and decreasing tortuosity will enhance HTR and impair HR. In real application, these parameters can be utilized to modify combustion temperature, emission performance and stability operation map.

Controlling the selectivity in the Fischer-Tropsch synthesis using foam catalysts: An integrated experimental and modeling approach

The Fischer-Tropsch synthesis (FTS) is widely applied to convert syngas to liquid fuels, being long-chain hydrocarbons (C5+) the preferred products. Combining experiments and first-principle simulations, this work analyzes the effect of intra and extra-particle mass transfer limitations on the FTS reaction rate and product selectivity using open-cell foams catalysts. Co/Al2O3 and Co/TiO2 catalysts were deposited on open-cell foam structures and tested for the FTS. A 1-D multi-scale first principle reactor model is developed in order to correlate the product distribution and the reactor performance with the system properties. Both experiments and modeling results demonstrate that an increase in the washcoat layer thickness leads to greater selectivity towards methane and that the reaction rate has a maximum at ca. 60 μm. The developed model is used to predict the foam-based reactor performance, under realistic industrial conditions, showing that the productivity to C5+ is severely affected by washcoat layers thicker than 50 μm.

Catalyst adhesion enhancement by porous TiO2 layer formed on anodized titanium honeycomb substrate

Titanium honeycomb substrates having TiO2 surface with nano- to micro-sized pores were prepared by electrochemical anodization and used to make Pt-based oxidation catalyst monoliths without conventional oxide-based washcoat layers for pre-turbocharger catalyst (PTC) applications exposed to harsh conditions under very high flow rates of exhaust gas and engine vibration. Adhesion strength and CO/C3H6 oxidation performance of the catalysts were compared with conventional washcoated FeCrAl monolith catalysts. The intermediate porous TiO2 layer formation by the anodization can dramatically enhance the catalyst adhesion strength without sacrificing the catalytic performance under GHSV (gas hourly space velocity) higher than 100,000 h−1 for PTC.

Production and characterization of copper periodic open cellular structures made by 3D printing-replica technique.

Additive manufacturing by 3D printing comprises a set of methods for production of 3D objects starting from a CAD file. Advantages of additive manufacturing combine high manufacturing resolution, a reduction of waste material, and the possibility of computer‐aided design (CAD). When applied to the manufacturing of structured catalyst substrates, the latter enables the optimization of transport properties of the catalyst support. Despite several methods have been introduced for a variety of materials, copper, well known for its high thermal conductivity, is still difficult to be handled. In this work, a novel approach for the additive manufacturing of copper periodic open cellular structures (POCS) is proposed and investigated. It consists in the use of the replica manufacturing procedure starting from resin supports produced by 3D printing stereolithography. Micrometric high purity copper powder was effectively dispersed using a liquid medium based on organic components; the resulting slurry was used for the washcoat deposition on the resin supports. Structures with diamond unit cell shape (cell size of 2.5 mm and void fractions in the 0.8‐0.9 range) were washcoated by dip‐spin coating. Homogeneous washcoat layers were obtained without occurrence of cell clogging phenomena. Optimized thermal treatment procedure was assessed for sintering the copper POCS. The resulting matrices preserved the morphology of the original structure, reaching a resolution in the range of 70 to 120 μm. These materials can eventually be used as catalyst supports for heat‐transfer limited applications (eg, steam reforming of methane), where copper‐based substrates were demonstrated to be an effective solution for process intensification.

Hyperpolarised xenon MRI and time-resolved X-ray computed tomography studies of structure-transport relationships in hierarchical porous media

Catalysed diesel particulate filter (DPF) monoliths are hierarchical porous solids, as demonstrated by mercury porosimetry. Establishing structure-transport relationships, including assessing the general accessibility of the catalyst, is challenging, and, thus, a comprehensive approach is necessary. Contributions, from each porosity level, to transport have been established using hyperpolarised (hp) xenon-129 magnetic resonance imaging (MRI) of gas dispersion within DPF monoliths at variable water saturation, since X-ray Computerised-Tomography, and 1H and 2H NMR methods, have shown that porosity levels dry out progressively. At high saturation, hp 129Xe MRI showed gas transport between the channels of the monolith is predominantly taking place at channel wall intersections with high macroporosity. The walls themselves make a relatively small contribution to through transport due to the distribution of the micro-/meso-porous washcoat layer away from intersections. Only at low saturation, when the smallest pores are opened, do hp 129Xe MR images became strongly affected by relaxation. This observation indicates accessibility of paramagnetic (catalytic) centres for gases arises only once the smallest pores are open.

Multi-scale coarse-grained continuum models for bifurcation and transient analysis of coupled homogeneous-catalytic reactions in monoliths

We use the Lyapunov-Schmidt (L-S) reduction technique of bifurcation theory to develop reduced order models for monolith reactors with catalytic reactions in the washcoat layer and homogeneous reactions in the flow channel, and with thermal effects. The L-S technique eliminates the two transverse coordinates and converts the detailed models expressed in terms of three spatial coordinates and time to multi-mode models in axial coordinate and time. The reduced order models are expressed in terms of phase averaged multiple concentration (temperature) modes and interfacial fluxes which are related through various transfer coefficients that are local scale, flow and property dependent. Accurate expressions are provided for estimating the local transfer coefficient matrices for multi-component systems. Various limiting cases of the multi-mode reduced order models are discussed and compared to other models in the literature. The usefulness of the reduced order models is illustrated by two examples: the first is the oxidative coupling of methane in a laboratory scale monolith reactor, the second is the determination of the ignition-extinction behavior of catalytically assisted combustion of propane. Other possible applications and extensions are also discussed.

Numerical Simulation of Methane and Propane Reforming Over a Porous Rh/Al2O3 Catalyst in Stagnation-Flows: Impact of Internal and External Mass Transfer Limitations on Species Profiles

Hydrogen production by catalytic partial oxidation and steam reforming of methane and propane towards synthesis gas are numerically investigated in stagnation-flow over a disc coated with a porous Rh/Al2O3 layer. A one-dimensional flow field is coupled with three models for internal diffusion and with a 62-step surface reaction mechanism. Numerical simulations are conducted with the recently developed computer code DETCHEMSTAG. Dusty-Gas model, a reaction-diffusion model and a simple effectiveness factor model, are alternatively used in simulations to study the internal mass transfer inside the 100 µm thick washcoat layer. Numerically predicted species profiles in the external boundary layer agree well with the recently published experimental data. All three models for internal diffusion exhibit strong species concentration gradients in the catalyst layer. In partial oxidation conditions, a thin total oxidation zone occurs close to the gas-washcoat interface, followed by a zone of steam and dry reforming of methane. Increasing the reactor pressure and decreasing the inlet flow velocity increases/decreases the external/internal mass transfer limitations. The comparison of reaction-diffusion and Dusty-Gas model results reveal the insignificance of convective flow on species transport inside the washcoat. Simulations, which additionally solve a heat transport equation, do not show any temperature gradients inside the washcoat.

Enhanced enzyme-based CO2 capture in countercurrent packed-bed column reactors

An enhanced CO2 hydration process with human carbonic anhydrase II (hCA II) enzyme immobilized on packing surface and micro-particles dispersed in the liquid phase was proposed. This innovative hybrid enzymatic process was simulated in a countercurrent packed-bed column reactor via an unsteady-state, isothermal, 3-D model including continuity, momentum and species (including the biocatalytic micro-particles) balance equations in the liquid and gas phases, filtration equation describing the accumulation of biocatalytic micro-particles in packed bed, chemical reaction and diffusion in hCA II enzyme washcoat and liquid film. In the hybrid enhanced immobilized enzyme CO2 capture system, hCA II enzyme supported on the surface of micro-particles works as a free solution-phase enzyme, which instantaneously catalyzes CO2 hydration reaction and upgrades significantly CO2 conversion, particularly in the region where the enzymatic process with immobilized hCA II enzyme on the packing surface is strongly limited by the diffusion. The impact of hCA II enzyme immobilized on the micro-particles surface is more important when the foremost amount of hCA II enzyme is immobilized on the packing via larger washcoat layers with enhanced mass transfer resistance, at larger feed buffer concentration and lower thermodynamic dissociation constant of the protonated acid form of buffer. Enhanced CO2 hydration process provide a reasonable CO2 conversion at a much lower liquid velocity and permits to significantly downgrade the height of the packed bed. These immobilized enzyme CO2 capture systems intend to maximize the mass transfer in packed-bed column reactors to expand the degree of utilization of the large hydration turnover of hCA II enzyme.

Open-cell foams as catalysts support: A systematic analysis of the mass transfer limitations

• Systematic analysis of the mass transfer limitation in wash-coated open-cell foams. • CFD simulations on Kelvin’s cell by DNS. • Mass transfer correlation for low Reynolds number. • Practical criteria in terms of dimensionless numbers for rate limiting regimes. In this work, the analysis of the mass transfer phenomena in catalytic open-cell foams is carried out through the combination of computational fluid dynamics (CFD) simulations and experiments, using the CO oxidation on Pt(1%)/γ-Al 2 O 3 /foam as a model reaction. The influence of the local hydrodynamic effects on the diffusion-reaction phenomena occurring at the gas-solid interface of the open-cell solid foams are investigated by Direct Numerical Simulations assuming an infinitely fast reaction. A correlation for the Sherwood number as function of Reynolds -for low Re- is proposed. To validate this experimentally, aluminum foams coated with Pt(1%)/γ-Al 2 O 3 are tested at different reaction conditions for the CO oxidation. The obtained reaction rates and apparent activation energy show the presence of external mass transfer limitations. An analysis of the diffusion-reaction phenomena taking place in the wash-coated layer is presented in terms of dimensionless numbers. A practical criterion is developed in terms of the Thiele modulus ( ϕ w ) and the Biot number (Bi m ) for the identification of the reaction regimes: kinetic control ( ϕ w 2 / B i m < 0.1 ) , internal and/or external mass transfer limitations ( 0.1 < ϕ w 2 / B i m < 10 ) , and full mass transfer control ( ϕ w 2 / B i m > 10 ) .

Effect of wash coat layers on the conversion efficiency of a catalytic converter in the SI engine

The present work deals with the effect of different wash coat layers such as micro alumina, nano alumina and yttria on the reduction of HC, CO emissions when compared to base line emissions. The morphology of various wash coat materials was characterised by means of a scanning electron microscope. The experimental investigations were carried out in a four-stroke spark ignition engine before and after ageing of catalytic converters. The decrease in HC and CO emissions was found to be in the order of nano alumina, yttria dual and micro alumina when compared to base line readings before ageing. The decreases in HC and CO emissions were found to be in the order of yttria dual, nano alumina and micro alumina when compared to base line readings after ageing. The maximum conversion of yttria dual can be attributed to the adhesion properties of the wash coat with the substrate.

Hybrid FEM and FDM Approach for Monolithic Catalytic Converters with Pore Diffusion

A transient, three-dimensional model is developed for monolith catalytic converters with washcoat pore diffusion. Due to the unique configuration of monolith converters, the flow within the channel can be considered fully developed laminar flow, and one-dimensional transport equations can be formulated for the flow and species. On the other hand, a three-dimensional equation is necessary for the monolith solid temperature to capture the effects of significant external heat loss or flow maldistribution. The proposed model employs a three-dimensional finite-element method (FEM) for the solid temperature equation and a one-dimensional finite-difference method (FDM) for the flow and species equations. A recently developed asymptotic approach (Bissett, Emiss. Control Sci. Technol. 1(1), 3–16, 2015, Bissett, Emiss. Control Sci. Technol. 5(1), 45–54, 2019) is utilized to solve the washcoat pore diffusion and surface coverages equations. The three-dimensional FEM meshes are chosen so the nodes also serve for the one-dimensional transport equations. All equations are solved fully coupled, without lagging or reliance upon external tools. The developed solver can efficiently simulate general cross-section geometries (circle, oval, rectangle, etc.) for a catalytic monolith under nonuniform inlet distribution and detailed washcoat pore diffusion. The solver is verified by comparing the simulation results to the one-dimensional simulation results for a simple geometry under uniform inlet conditions. The model can be applied to flow through general monolith catalytic reactors with single or dual washcoat layers and a wide range of kinetics.