Hydrothermal Aging Conditions Research Articles

Development and Validation of a Two-Site Kinetic Model for NH3-SCR over Cu-SSZ-13. Part 2. Full-Scale Model Validation, ASC Model Development, and SCR-ASC Model Application

We present herein the final part in the development and validation of a two-site kinetic model for NH3-SCR over Cu-SSZ-13. To predict tailpipe emissions accurately, it was necessary to combine the kinetic model of the SCR catalyst developed in part I (Daya et al. 2018), with a reaction-diffusion model of the dual-layer ammonia slip catalyst (ASC). This dual-layer ASC model was developed following a three-step process, including development of the kinetic models of the individual layers, followed by parameterization of a parallel pore diffusion model of the dual-layer ASC. Reactor-scale validation of the dual-layer ASC model confirmed the kinetic model accuracy and highlighted the significance of intra-porous diffusion. Following this, the SCR model developed in part I of this paper was validated on an engine dynamometer through comprehensive steady-state experiments with inlet NH3 to NOx ratio (ANR) sweeps. The final SCR and ASC models were then evaluated on cold and hot heavy-duty transient (HDT) cycles, to examine the capability of predicting tailpipe NOx, NH3 slip as well as storage-based dynamics. Overall cycle-averaged NOx conversion was predicted within 3% using these models. Validated models have significant application in model-based control as well as improving catalyst design through improved functional understanding. The present Cu-SSZ-13 SCR model was simulated using the four-step SCR protocol (Kamasamudram et al. Catal. Today 151(3):212–222) to calculate the intra-catalyst dynamic capacity. These numerical experiments showed that the dynamic capacity decreases upon hydrothermal aging but leads to higher NOx conversion under standard and fast SCR conditions at 250 °C. This increase in NOx conversion is due to more uniform NH3 storage along the length of the catalyst, leading to higher NH3 utilization near the rear of the aged catalyst. Similar numerical experiments on the dual-layer ASC model demonstrated intra-layer washcoat distributions causing NO slip during transient drive cycles for both hydrothermal aging conditions.

Effect of various activation conditions on the low temperature NO adsorption performance of Pd/SSZ-13 passive NOx adsorber

Various activation conditions such as temperature, time and the concentration of H2O during hydrothermal aging (HTA) treatment were extensively investigated to identify the proper ion-exchange conditions of Pd/SSZ-13 catalyst for low temperature NO adsorption performance. In addition, we examined how the cation in SSZ-13 (i.e. NH4-, Na- or K-SSZ-13) affects the low temperature NO adsorption performance of Pd/SSZ-13 after HTA treatment. It is found that NO adsorption ability begins to increase substantially when Pd/SSZ-13 is hydrothermally aged above 650°C. Pd/SSZ-13 demonstrated the maximum NO adsorption performance after hydrothermal aging treatment in the range of 700–750°C, followed by the severe decline above 850°C. Such deactivation is attributed to the agglomeration of Pd species due to the dealumination of SSZ-13, as indicated by XRD and NMR analysis. At least 15h of HTA treatment at 750°C is required to provide the optimum activation of Pd/SSZ-13. Water vapor is essential component to activate Pd/SSZ-13 during the hydrothermal aging treatment. Unlike NH4-SSZ-13, Na- or K-SSZ-13 cannot be activated even if the optimum hydrothermal aging condition (750°C for 25h with 10% H2O) is applied. EXAFS and XRD analysis provides the evidence about the sintering of Pd species of the Pd/Na- or Pd/K-SSZ-13 catalysts rather than Pd ion exchange, indicating that the pre-existing Na or K ion inhibits the ion exchange during hydrothermal aging treatment. It can be concluded that the selection of proper hydrothermal aging conditions and zeolite form is essentially required to obtain the excellent low temperature NO adsorption performance of Pd/SSZ-13 as a passive NOx adsorber.

Hydrothermal aging effects on Cu-zeolite NH3-SCR catalyst

After-treatment systems for diesel vehicles face a challenge for upcoming emission regulations. Regarding the level of nitrogen oxides (NOx) and soot required, the global catalytic activity of the exhaust line needs to be improved. Hydrothermal aging conditions explored during particulate filter regeneration are very stringent for the deNOx catalyst and can strongly reduce its durability. Ammonia selective catalytic reduction (NH3-SCR) systems mainly based on copper or iron exchanged zeolite catalysts have emerged as effective technologies to reduce NOx emissions with their durability being continuously improved. In this work, we evaluated the impact of different hydrothermal treatments on a commercial copper-exchanged zeolite catalyst. The goal was to link the catalytic material property changes with catalytic performance changes as a function of both aging duration and temperature. The evolution of the zeolite structure as well as the changes of the copper state were experimentally investigated. Based on a detailed material properties analysis of Cu-zeolite, a semi-detailed kinetic model of has been built to quantitatively predict adsorption and desorption of NH3 as a function of hydrothermal aging conditions. This work is a first step to bridge the gap between lab analysis conclusions and global reactivity observed in real conditions.

Commercialization research of a metal DOC based on Fe-Cr-Ni substrate

Abstract The aim of this study is to investigate the possibility of commercialization through the evaluation of hydrothermal aging durability of diesel oxidation catalyst (DOC) catalysts based on a metal substrate that has higher thermal conductivity, larger physical strength and better performance at a high space velocity compared to a ceramic substrate. In the case of hydrothermal aging for 45 h at 800 °C, Pt had a maximum size of 200 nm and higher activation energy was required; eventually, there was a decrease in the activation point of the catalyst, leading to a decline in the catalyst performance. The harmful gases reached the light-off temperature (LOT 50) in the order: CO, NO, and THC respectively (ranked according to ascending temperature), while CO with low activation energy did not show a significant temperature difference upon reaching LOT 50 depending on hydrothermal aging conditions. A additionally, it was possible to approximate the a trend in performance decline by monitoring the aging temperature and time needed to reach LOT 50 and the maximum NO maximum conversion. The harmful gas reduction characteristics of the DOC based on a Fe-Cr-Ni substrate were well activated at a low temperature, and no substantial performance decline was observed upon excessive hydrothermal aging, leading the way to future commercialization of the catalyst.

Uniform nanoparticles of hydrotalcite-like materials and their textural properties at optimized conditions of urea hydrothermal treatment

The efficient urea hydrothermal synthetic technique for hydrotalcite (HTlc) was optimized to prepare uniform HTlc nanoparticles that exhibit high performance in variety of applications. Experimentally, a series of single-phase Mg–Al HTlc(s) with good textural properties was successfully obtained under optimized urea/metals molar ratio and hydrothermal aging conditions. The X-ray analysis confirmed the purity and the high crystallinity of the prepared HTlc(s). The chemical composition, crystallinity, and textural properties of the optimized HTlc(s) were investigated and compared to those of the conventionally prepared HTlc(s). The increase in the urea concentration decreased the crystallite size of the HTlc(s). The reduction in the hydrothermal aging time led to a narrow particle size distribution. The greatest uniformity of particle size was achieved at a urea/metals molar ratio of 3.0 and at a hydrothermal aging time of 12h. An increase in the urea/metals molar ratio to 3.0 increased the micropore volume available for adsorption, and the BET surface area consequently increased. The hydrothermal aging of the HTlc(s) significantly decreased the amount of nitrogen the samples adsorbed.

Effective additives to improve hydrothermal aging of DME reforming catalyst in terms of hydrogen yield and de-NOx performance of RC + LNT combined system

The purpose of this study is to observe the effects of H2 generation by a reforming catalyst (Cu30/γ-Al2O3) with various additives, so as to improve the durability of the combined system of reforming catalyst (RC)+LNT. Under both fresh and hydrothermal aging conditions, the H2 yields of the Cu29CeO21/γ-Al2O3 catalyst were the highest among the reforming catalysts prepared by the sol–gel method. Even though the H2 yield obtained using the DME reforming reaction of the exhaust gases of DME vehicles has the advantage of using the heat source and water in the exhaust gas, the existence of NO, CO2 and O2 in the exhaust gases is disadvantageous to the DME reforming reaction. The NOx conversion of the combined system of RC (Cu29CeO21/γ-Al2O3)+LNT was higher than that of the LNT alone. This shows that the H2 generated by the reforming reaction using the exhaust gases of the DME engine is a more effective reductant. Therefore, in order to effectively improve the NOx conversion, the combined system of RC+LNT is needed as an after-treatment system for DME vehicles.

Possible origin of improved high temperature performance of hydrothermally aged Cu/beta zeolite catalysts

The hydrothermal stability of Cu/beta NH3 SCR catalysts are explored here. In particular, this paper focuses on the interesting ability of this catalyst to maintain and even enhance high-temperature performance for the “standard” SCR reaction after modest (900°C, 2h) hydrothermal aging. Characterization of the fresh and aged catalysts was performed with an aim to identify possible catalytic phases responsible for the enhanced high temperature performance. XRD, TEM and 27Al NMR all showed that the hydrothermal aging conditions used here resulted in almost complete loss of the beta zeolite structure between 1 and 2h aging. While the 27Al NMR spectra of 2 and 10h hydrothermally aged catalysts showed significant loss of a peak associated with tetrahedrally coordinated Al species, no new spectral features were evident. Two model catalysts, suggested by these characterization data as possible mimics of the catalytic phase formed during hydrothermal aging of Cu/beta, were prepared and tested for their performance in the “standard” SCR and NH3 oxidation reactions. The similarity in their reactivity compared to the 2h hydrothermally aged Cu/beta catalyst suggests possible routes for preparing multi-component catalysts that may have wider temperature windows for optimum performance than those provided by current Cu/zeolite catalysts.

Influence of Ionic Strength on Brookite Content in Sol-Gel Synthesized Titania before and after Hydrothermal Aging

This work investigates the effect of increasing ionic strength on the phase composition of hydrothermally aged titania. Rietveld refinements were used to systematically track the brookite content and both anatase and brookite particle size. Results demonstrate that increasing ionic strength during hydrothermal aging has no effect on brookite content. The presence of chloride is concluded to have the most influence over the brookite content during the sol gel synthesis and that the addition of chloride to chloride-free, sol gel synthesized titania has no effect during hydrothermal aging. Photocatalysis results using particles of similar size but different phase composition suggest that particles with higher brookite content are less photocatalytically active than those with lower brookite content. Results suggest that substantial control over photocatalytic activity of titania nanoparticles can be achieved by modifying sol-gel synthesis and hydrothermal aging conditions