Deactivation of Cu/SSZ-13 NH3-SCR catalyst by exposure to CO, H2, and C3H6

Lean nitric oxide (NOx)-trap (LNT) and selective catalytic reduction (SCR) are efficient systems for the abatement of NOx. The combination of LNT and SCR catalysts improves overall NOx removal, but there is a risk that the SCR catalyst will be exposed to high temperatures and rich exhaust during the LNTs sulfur regeneration. Therefore, the effect of exposure to various rich conditions and temperatures on the subsequent SCR activity of a Cu-exchanged chabazite catalyst was studied. CO, H2, C3H6, and the combination of CO + H2 were used to simulate rich conditions. Aging was performed at 800 \ub0C, 700 \ub0C, and, in the case of CO, 600 \ub0C, in a plug-flow reactor. Investigation of the nature of Cu sites was performed with NH3-temperature-programed desorption (TPD) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) of probe molecules (NH3 and NO). The combination of CO and H2 was especially detrimental to SCR activity and to NH3 oxidation. Rich aging with low reductant concentrations resulted in a significantly larger deactivation compared to lean conditions. Aging in CO at 800 \ub0C caused SCR deactivation but promoted high-temperature NH3 oxidation. Rich conditions greatly enhanced the loss of Br\uf8nsted and Lewis acid sites at 800 \ub0C, indicating dealumination and Cu migration. However, at 700 \ub0C, mainly Br\uf8nsted sites disappeared during aging. DRIFT spectroscopy analysis revealed that CO aging modified the Cu2+/CuOH+ ratio in favor of the monovalent CuOH+ species, as opposed to lean aging. To summarize, we propose that the reason for the increased deactivation observed for mild rich conditions is the transformation of the Cu species from Z2Cu to ZCuOH, possibly in combination with the formation of Cu clusters.\ua0\ua9 2019, MDPI AG. All rights reserved

Lean and rich aging of a Cu/SSZ-13 catalyst for combined lean NO x trap (LNT) and selective catalytic reduction (SCR) concept

\ua9 2019 The Royal Society of Chemistry. In the combined lean NO x trap (LNT) and selective catalytic reduction (SCR) concept, the SCR catalyst can be exposed to rich conditions during deSO x of the LNT. Aging of Cu/SSZ-13 SCR catalysts, deposited on a cordierite monolith, was therefore studied in rich, lean and cycling lean/rich operations at 800 \ub0C (lean condition: 500 ppm NO, 8% O 2 , 10% H 2 O and 10% CO 2 ; rich condition: 500 ppm NO, 1% H 2 , 10% H 2 O and 10% CO 2 ). The structure of the catalyst was investigated by X-ray diffraction (XRD), surface area measurements and scanning transmission electron microscopy (STEM). In general, aging decreased the SCR activity and NH 3 oxidation. However, rich conditions showed a very rapid and intense deactivation, while lean aging led to only a small low-temperature activity decrease. The XRD results showed no sign of structure collapse, but the number of active sites, as titrated by NH 3 temperature-programed desorption (NH 3 -TPD) and in situ DRIFTS, revealed an important loss of acid sites. NH 3 storage was significantly more depleted after rich aging than after lean aging. The Lewis sites, corresponding to exchange Cu 2+ , were preserved to some extent in lean conditions. Lean aging also decreased the enthalpy of NH 3 adsorption from -158 kJ mol -1 to -136 kJ mol -1 . Moreover, a comparison of aging in lean-rich cycling conditions with aging only in rich conditions revealed that adding lean events did not hinder or reverse the deactivation, and it was mainly the time in rich conditions that determined the extent of the deactivation. The STEM images coupled with elemental analysis revealed the formation of large Cu particles during rich aging. Conversely, Cu remained well dispersed after lean aging. These results suggest that the copper migration and agglomeration in large extra-framework particles, accelerated by the action of hydrogen, caused the observed severe deactivation

Modelling of non uniform washcoat in catalytic monolith reactors

For advances in the design of exhaust aftertreatment systems, modelling can be a valuable tool. There have been various efforts in modelling the diesel oxidation catalyst (DOC) with varying degree of complexity. The simplest 1D models discretize the monolithic channel axially and use an effectiveness factor to account for different washcoat geometries. The more complex 1+1D models also resolve the catalyst washcoat which makes them able to better predict efficiency of e.g. layered catalysts. However, the vast majority of these 1+1D models assume the washcoat to be a uniform slab with homogeneous properties. Thus, they cannot identify tangential washcoat variations which have been found to have substantial effects on washcoat diffusivity (1). In this work a new parallel 1+1D reactor model has been developed. Similar to the sectionalizing method presented by Papadias et al (2), the washcoat is sliced into multiple tangential sections based on an evenly distributed angle. The model then solves each section of the washcoat independently – assuming that no mass is transferred between each section. Preliminary results (see figure 1) compare NO light-off simulations using the original 1+1D model and the parallel model using 3 sections. If the washcoat properties for each section are kept constant, then the conversion efficiency of the parallel model is slightly decreased because of the increased diffusive resistance of the thicker corners. However, if including the local porosity of each section (where the corners show a 13% increase in local porosity based on SEM images), the conversion efficiency of the parallel model is significantly higher because of the increased pore diffusivity in the corners. The proposed methodology enables analysis of local washcoat properties. The results are important for high performance modelling towards zero emission vehicles

Efficient Experimental Approach to Evaluate Mass Transfer Limitations for Monolithic DOCs

The diesel oxidation catalyst (DOC, Pt/γ-Al2O3) was used in a synthetic-gas catalyst test bench to study internal mass transfer limitations during NO oxidation. A simple and fast experimental methodology, by varying the washcoat thickness in monolithic DOCs was developed and the results were evaluated using various experimental time scales. The ratio between the reaction time constant and the washcoat diffusion time constant was useful in identifying temperatures where the DOCs tested transitioned between a kinetically controlled region and an internal mass transfer controlled region. The NO conversion was shown to be significantly limited by internal mass transfer already at 175\ua0\ub0C for an average washcoat thickness of 110\ua0\ub5m

Use of 3D-printed mixers in laboratory reactor design for modelling of heterogeneous catalytic converters

A method for identifying radial concentration maldistribution in synthetic catalyst activity test (SCAT) benches, is presented, where spatially resolved concentration measurements are not available. The developed methodology was successfully tested for an injection-based SCAT. To resolve the radial concentration maldistribution a static mixer was designed, 3D-printed and inserted upstream the test sample. The methodology could also prove the effectiveness of the mixer, which did not only resolve the concentration maldistribution but also avoided causing reaction disturbances. The resulting increased axial dispersion from the turbulence created by the static mixer was evaluated using a 3D CFD model in Ansys Fluent 19. The axial dispersion of the injection-based SCAT bench was compared to a premixed SCAT bench through classical Aris-Taylor calculations. The results from the axial dispersion calculations show that the injection-based design with the use of a static mixer is far superior to the premixed design – both with regards to pulse broadening but also time delay. This is highly desirable for modelling studies towards zero emission exhaust aftertreatment

Aspects of Efficient Parameter Estimation for Diesel Oxidation Catalysts

In this thesis the objective was to tune the model parameters of diesel oxidation catalysts (DOC) to measurement data from engine rig experiments in an efficient manner. The scope was however not limited to the algorithm of parameter search alone but instead included the whole process starting with the design of experiments. Different aspects of efficient parameter estimation of a full scale DOC were evaluated. This included different kinetic models, mass transfer resistance evaluation, experiments at both engine rig and lab scale, and parameter estimation algorithms. A specially developed detailed kinetic model, a method for parameter estimation using Multivariate Data Analysis, and a method for full scale engine rig experiments were all important products. In addition to these outputs some relevant conclusions were made based on the studies• Including internal mass transport parameter in the tuning improved the possibilities of achieving a good fit for the catalyst model• If internal mass transport is to be modeled the kinetic model cannot include parameters mimicking the effects of transport resistance• A detailed kinetic model improves the conditions for separating kinetics and mass transport but also increases the need for experimen-tal complexity at the same time as model instability may increas

Parameter Estimation of a DOC from Engine Rig Experiments

In this thesis methods of parameter estimation of a Diesel Oxidation Catalyst (DOC) from engine rig experiments were investigated. The investigation did not only include methods of parameter fitting to experimental data but a large effort was also put into catalyst modeling and experimental design.Several different catalyst configurations were used with varying Pt loading, washcoat thickness and volume. To further expand the experimental space, engine operating points were chosen with a wide variation in variables (inlet conditions) and both transient and stationary operation was used. A catalyst model was developed where the catalyst washcoat was discretized as tanks in series both radially and axially and for parameter estimation a traditional gradient search method was used. Four different modeling approaches were used for parameter tuning where the most successful one tuned kinetic parameters as well as internal mass transfer parameters. It was also shown that it is of high importance that the kinetic model used has an intrinsic structure when the catalyst model separates mass transport and kinetics and when several catalyst configurations are used.A new method was evaluated where sensitivity analysis and data selection was used as a part of the parameter estimation. This methodology renders better statistical properties and should improve the parameter tuning when using gradient search methods. Furthermore, a reduced computational cost could be achieved by using only the most relevant data points during parameter tuning. The evaluated method did neither result in an improved fit to measurement data nor reduce the time for parameter tuning compared to a reference case. Adjustment of an unbalanced weighting of the residuals for the different components was identified as the most important factor for a future improvement of the method but more transient experimental data was also suggested as a possible improvement. A method of creating fast transients in concentration for a full scale engine rig system was presented and evaluated. The method included an engine rig where an SCR with urea injection and a DOC with bypass possibility were situated between the test object, which also was a DOC, and the engine. By controlling urea injection and DOC bypass a wide range of exhaust compositions, not possible by only controlling the engine, could be achieved which will improve the possibilities for parameter estimation for the modeling of the DOC in future studies

Parameter Estimation of a DOC from Engine Rig Experiments

In this thesis methods of parameter estimation of a Diesel Oxidation Catalyst (DOC) from engine rig experiments were investigated. The investigation did not only include methods of parameter fitting to experimental data but a large effort was also put into catalyst modeling and experimental design.Several different catalyst configurations were used with varying Pt loading, washcoat thickness and volume. To further expand the experimental space, engine operating points were chosen with a wide variation in variables (inlet conditions) and both transient and stationary operation was used. A catalyst model was developed where the catalyst washcoat was discretized as tanks in series both radially and axially and for parameter estimation a traditional gradient search method was used. Four different modeling approaches were used for parameter tuning where the most successful one tuned kinetic parameters as well as internal mass transfer parameters. It was also shown that it is of high importance that the kinetic model used has an intrinsic structure when the catalyst model separates mass transport and kinetics and when several catalyst configurations are used.A new method was evaluated where sensitivity analysis and data selection was used as a part of the parameter estimation. This methodology renders better statistical properties and should improve the parameter tuning when using gradient search methods. Furthermore, a reduced computational cost could be achieved by using only the most relevant data points during parameter tuning. The evaluated method did neither result in an improved fit to measurement data nor reduce the time for parameter tuning compared to a reference case. Adjustment of an unbalanced weighting of the residuals for the different components was identified as the most important factor for a future improvement of the method but more transient experimental data was also suggested as a possible improvement. A method of creating fast transients in concentration for a full scale engine rig system was presented and evaluated. The method included an engine rig where an SCR with urea injection and a DOC with bypass possibility were situated between the test object, which also was a DOC, and the engine. By controlling urea injection and DOC bypass a wide range of exhaust compositions, not possible by only controlling the engine, could be achieved which will improve the possibilities for parameter estimation for the modeling of the DOC in future studies

Aspects of Efficient Parameter Estimation for Diesel Oxidation Catalysts

In this thesis the objective was to tune the model parameters of diesel oxidation catalysts (DOC) to measurement data from engine rig experiments in an efficient manner. The scope was however not limited to the algorithm of parameter search alone but instead included the whole process starting with the design of experiments. Different aspects of efficient parameter estimation of a full scale DOC were evaluated. This included different kinetic models, mass transfer resistance evaluation, experiments at both engine rig and lab scale, and parameter estimation algorithms. A specially developed detailed kinetic model, a method for parameter estimation using Multivariate Data Analysis, and a method for full scale engine rig experiments were all important products. In addition to these outputs some relevant conclusions were made based on the studies• Including internal mass transport parameter in the tuning improved the possibilities of achieving a good fit for the catalyst model• If internal mass transport is to be modeled the kinetic model cannot include parameters mimicking the effects of transport resistance• A detailed kinetic model improves the conditions for separating kinetics and mass transport but also increases the need for experimen-tal complexity at the same time as model instability may increas

Parameter Estimation of a DOC from Engine Rig Experiments with a Discretized Catalyst Washcoat Model

Parameter tuning was performed against data from a full scale engine rig with a Diesel Oxidation Catalysts (DOC). Several different catalyst configurations were used with varying Pt loading, washcoat thickness and volume. To illustrate the interplay between kinetics and mass transport, engine operating points were chosen with a wide variation in variables (inlet conditions) and both transient and stationary operation was used. A catalyst model was developed where the catalyst washcoat was discretized as tanks in series both radially and axially. Three different model configurations were used for parameter tuning, evaluating three different approaches to modeling of internal transport resistance. It was concluded that for a catalyst model with internal transport resistance the best fit could be achieved if some parameters affecting the internal mass transport were tuned in addition to the kinetic parameters. However it was also shown that a model with negligible internal transport resistance still could obtain a good fit since kinetic parameters could compensate for transport limitations. This highlighted the inherent difficulties using kinetic models with high parameter correlation and also showed the importance of using a kinetic model with a structure that is capable of describing exclusively intrinsic kinetics