Sulfur poisoning of Cu-SSZ-13 is a function of the catalyst's oxidation ability.

We describe a combined experimental kinetics and modeling study for NH3 oxidation on Pt/Al2O3 and development of a predictive microkinetic model valid over a range of conditions. The NH3 oxidation rate (TOF) dependence is reported for NH3 concentrations between 10 and 25,000 ppm and temperatures below 250 degrees C for Pt/Al2O3 powder and washcoated monoliths. The data reveal a shift from positive- to negative-order rate dependence on NH3 with the rate maximum dependent on the processing of Pt/Al2O3; unmilled Pt/Al2O3 powder exhibits a rate maximum at 500 ppm NH3, while ball-milled Pt/Al2O3 has a maximum at similar to 10,000 ppm. This unexpected enhancement of Pt/Al2O3 activity from milling results in a lowering of the light-off temperature (T-50) by up to 100 degrees C. Further examination rules out the extent of pore diffusion limitations as the root cause, but rather an increase in the fraction of stepped crystalline planes and destabilization of Pt oxide shown from XRD and H-2-TPR characterization. The dependence of a shift in the rate maximum to higher NH3 concentrations with milling extent is shown to require single- and dual-site reaction pathways. The kinetic scheme also captures a subtle shift in the apparent NH3 reaction order for both unmilled and milled Pt/Al2O3. The microkinetic scheme is incorporated into fixed-bed and monolith reactor models which show excellent agreement with the NH3 conversion and product distribution data.

Modeling and optimization is presented of the Pt/Al2O3@Cu/ZSM-5 core-shell (CS) catalyst introduced in our recent study in which we successfully demonstrated the synthesis and application of the CS catalyst as an Ammonia Slip Catalyst (ASC). The commercial ASC converts NH3 to N-2 using a dual-layer architecture comprising a bottom oxidation catalyst layer (supported Pt) and a top selective catalytic reduction (SCR) of NOx layer (Fe- or Cu-exchanged zeolite). The Pt/Al2O3@Cu/ZSM-5 particle, which emulates the dual-layer architecture, was shown to have superior performance due in part to an enhanced Pt activity and dense zeolite shell. A 1 + 1 D heterogeneous fixed-bed reactor containing the core-shell catalyst is developed in this study to predict the CS catalyst performance data, advance the understanding, and enable optimization. Incorporating independent kinetic models for Pt-catalyzed NH3 oxidation and Cu/ZSM-5 catalyzed NOx reduction, the reactor model predicts all of the experimental trends. The Cu/ZSM-5 kinetics are tuned using independently measured data for NH3 uptake/desorption, NH3 and NO oxidation, and standard, fast, and NO2 SCR. The particle-scale model segregates the SCR reactions from the parallel process of intracrystalline diffusion of reacting species using independently-estimated species diffusivities from diffusion-limited NH3 oxidation on a Pt/Al2O3@Na/ZSM-5. The reactor model is validated for CS catalysts having a 0.5 and 1.2 mu m thick Cu/ZSM-5 shell. The tuned model is used to determine the minimum Pt loading and Cu/ZSM-5 shell thickness that achieves prescribed NH3 conversion and N-2 selectivity targets. A Pt loading as low as 0.02 wt% along with 0.83 mu m thick Cu/ZSM-5 is predicted to give a NH3 conversion of 70% at 250 degrees C and a N-2 selectivity at 500 degrees C of 90%.

The state-of-the-art Ammonia Slip Catalyst (ASC) has a dual-layer washcoat architecture with a bottom layer of Pt/Al2O3 and a top layer of Cu/SSZ-13. A trade-off between the NH3 conversion and N-2 selectivity presents a challenge in the ASC design. While a sufficiently thick and active zeolitic top layer increases the N-2 selectivity, it also imposes a diffusion barrier to the reacting species in reaching the bottom Pt layer, lowering NH3 conversion. Here we describe a systematic study to identify the ASC architecture and composition that optimizes the tradeoff. The in-house synthesized ASC samples span the single layer Pt/Al2O3, conventional dual-layer Pt/Al2O3 + Cu/SSZ-13, uniform single layer of mixed Pt/Al2O3 + Cu/SSZ-13, and a hybrid design comprising a bottom layer of mixed Pt/Al2O3 + Cu/SSZ-13 and a thin top layer of Cu/SSZ-13. The overall Pt and Cu loadings are fixed across the series of samples with the Cu distributed between the two layers. The best results are obtained with the combination of a base mixed layer that provides for effective coupling between Pt and Cu active sites and a top Cu/SSZ-13 layer of an intermediate thickness and nominally half of the total Cu loading. This design has sufficient oxidation activity to convert the NH3 and reduction activity to limit NOx slippage. A 1 + 1 dimensional model which follows from our recent work [3] is effective in predicting most of the data and assists in converging on the best composition and architecture. The hybrid design exhibits a linearly decreasing dependence of the NH3 conversion and logarithmically increasing dependence of the N-2 selectivity on the top layer Cu loading. The intersection of the two functions is shown to provide a good balance between the two opposing performance variables. The model is used to identify the combination of Pt loading and Cu loading distribution giving the maximum N-2 yield for a specified temperature and space velocity.

The effects of oscillations in gas composition, known as lean/rich dithering, on the performance of a commercial Pd-based three-way catalyst (TWC) for stoichiometric natural gas (NG) engines were evaluated using synthetic exhaust flow reactor experiments. Under simulated NG exhaust conditions, NO conversion was intimately correlated to CH4 conversion at slightly fuel-rich operating conditions. CH4 conversion significantly varied with lambda (or O-2 concentration) and depended on the direction of the lambda change. The dynamic CH4 conversion is likely related to the change of catalyst oxidation state and structures of Pd active sites. The CH4-NO cross-over point was found at a rich-biased lambda rather than stoichiometry. Compared to static operation, catalyst performance was much higher under a realistic dithering condition. The impacts of dithering parameters including amplitude and frequency on CH4 and NO conversions were explored. O-2 dosage is proposed as an effective descriptor of the TWC dithering performance. In the O-2-dosage space, the performance of distinct dithering parameters collapse, helping to elucidate the fundamental influence of dithering parameters on TWC performance. With the help of O-2 dosage, an optimal operating window leading to high CH4 and NO conversions was identified. A higher dithering amplitude is required to keep the catalyst in a reduced state for higher CH4 and NO conversions, while an appropriate O-2 dosage that does not exceed the breakthrough OSC is necessary to avoid excess O-2 that results in NO slip. The dithering amplitude and O-2-dosage-metric maps provide a new method for studying dynamic TWC performance and identifying optimum operation strategies.

The ammonia slip catalyst (ASC) is an essential final step in the emission control system and involves the selective oxidation of NH3 to N-2. The state-of-the-art ASC has a dual-layer architecture composed of a Pt/Al2O3 (PGM) bottom layer and a metal (Fe, Cu)-exchanged zeolite (M-Z) top layer. The PGM layer provides high NH3 oxidation activity; however, the desired N-2 product is achieved over a narrow temperature range just above light-off, whereas the reaction byproducts N2O and NOx (i.e., NO and NO2) appear at intermediate and high temperatures, respectively. An advantage of the M-Z catalyst is the selective lean reduction of NO via conversion of NH3 to N-2 over a broad temperature range. Although recent studies demonstrate the effectiveness of the dual-layer design, further advances are needed to reduce the PGM loading and ASC volume while enhancing low-temperature activity. In this study, the dual-layer concept is scaled down to the level of a single core-shell (CS) catalyst particle, Pt/Al2O3@Cu/ZSM-5, composed of a PGM core and a M-Z shell, with the intent to meet the aforementioned challenges. The CS catalyst was realized by rational design of key synthesis steps, the most critical being the initial growth of an intermediate silicalite-1 layer to prevent Al leaching during the secondary growth of the ZSM-5 shell. Characterization of the CS spherical catalyst reveals a mesoporous PGM core (ca. 40 mu m diameter) that is active and a nearly dense zeolitic shell (ca. 1 mu m thick). Evaluation of the CS catalyst in a fixed-bed reactor shows excellent NH3 oxidation activity and N-2 selectivity. In addition, we obtained an unanticipated enhancement of the Pt/Al2O3 performance within the CS configuration that gives an exceptional light-off of the NH3 oxidation. Our findings reveal that the CS catalyst has an equivalent activity to that of a conventional Pt/Al2O3 catalyst containing 3 times higher Pt loading. Further, a dual-layer ASC composed of a bottom layer containing the seeded core Pt/Al2O3 and a Cu-SSZ-13 top layer achieves the same performance as a dual-layer ASC having 3 times higher Pt loading but with unmodified Pt/Al2O3. The enhanced activity of the Pt/Al2O3 catalyst is attributed to a modification of the reducibility of oxides of Pt crystallites owing to the overgrowth of silicalite-1 and ZSM-5 layers in the CS configuration. Finally, the separate impacts of H2O in the feed and of hydrothermal aging (HTA) on catalyst performance are reported. H2O in the feed is shown to have a negligible impact on conversion and product distribution. The silicalite-modified Pt/Al2O3 catalyst is more resilient to HTA treatment than conventional Pt/Al2O3.

This study investigated the impact of low temperature sulfur exposure prior to hydrothermal aging on the catalytic structure and performance of Cu-SSZ-13 catalyst toward NH3-SCR reaction. The sulfur exposure temperature, duration and the sequence of sulfur exposure vs. hydrothermal aging are investigated. All samples hydrothermally aged in presence of SOx (SO3/SOx = 0.7) have showed decreased activity compared to the sample aged hydrothermally only, but the extent of deactivation varies with different sulfur exposure conditions. The sulfur exposure at 200 degrees C for 100 hours prior to HTA has the most pronounced impact on the population of active site (Cu) and the SCR performance at low temperatures. And the sample of equivalent thermal and sulfur exposure but with the HTA prior to the sulfur exposure, has less decreases in the active Cu sites and less sulfur stored. In contrast, sulfur exposure at high temperature, e.g. 650 degrees C, has the least impact on the Cu-SSZ-13 catalyst. The molar ratio of stored sulfur to the decrease amount of active Cu sites is close to 1:1 ratio, which implies the deactivation of sulfur exposed catalyst is mainly due to the decrease of active Cu population. Furthermore, DRIFTs demonstrated that both Z(2)Cu and ZCuOH sites decrease in most cases and ZCuOH is more impacted compared to Z(2)Cu. With these findings, we concluded that low temperature sulfur exposure prior to HTA treatment leads to accelerated aging. Further characterization reveals that the aging is mainly due to the decrease in the population of active Cu sites. The finding suggests that a hydrothermal pretreatment before sulfur exposure of Cu-SCR catalyst can retard the deactivation of catalyst to a limited extent in the real-world application.

The impact of Pd oxidation state on CH4 oxidation kinetics over a supported Pd-based TWC was studied using combination of experimental and kinetic modeling approaches. Two distinct oxidation states were generated using different pre-treatment protocols that developed in this paper: pre-oxidized surface that was dominated by PdO and pre-reduced surface that was dominated by metallic Pd. The CH4 kinetics was investigated on the catalysts with these two different Pd oxidation states. It was discovered that metallic Pd played a vital role in low temperature CH4 oxidation activity, which was indicated by a higher reaction rate on the pre-reduced catalyst as compared to the pre-oxidized catalyst. The apparent activation energies were estimated to be 94.0 and 82.2 KJ/mole on PdO dominated and metallic Pd dominated surfaces, respectively. At higher temperature, CH4 conversions on pre-reduced catalysts presented a "bend" shape, which could not be merely explained by the CH4 kinetics built upon the pre-reduced catalyst. This "unusual" change in CH4 kinetics was explained by the transition of metallic Pd to PdO with increasing temperature. CH4 oxidation kinetics followed two distinct pathways on metallic Pd and PdO dominated surfaces: the activation of CH4 on PdO dominated surfaces requires a pair of site including a PdO site and a vacant site while activation of CH4 on metallic Pd dominated surfaces involves a pair of site consisting of a metallic Pd and an oxygen lattice from support. Additionally, it was revealed that CH4 activation was much faster on metallic Pd as compared to PdO. A CH4 oxidation kinetic model was developed to simulate the dynamic change of Pd active site pairs so as to accurately predict the CH4 oxidation with different Pd oxidation states under reaction conditions.

Modeling and analysis of washcoated single- and dual-layer monolith catalysts is presented for selective catalytic reduction (SCR) on Cu-SSZ-13 and ammonia oxidation on Cu-SSZ-13 + Pt/Al2O3 ammonia slip catalyst (ASC).

A multi-site kinetic model was developed capable of predicting the sulfation and desulfation of Cu-SSZ-13 for the NH3selective catalytic reduction (NH3-SCR) of NOx.

Washcoated monolithic reactors are extensively used in vehicle emission control systems to reduce pollutants from engine exhaust. Selective catalytic reduction (SCR) with the downstream ammonia slip catalyst (ASC) are deployed in diesel emission control to remove NOx and prevent ammonia release. Earlier works have shown that washcoat diffusion can be rate controlling for both SCR and ASC. Here we use sacrificial agents (polymer, yeast) to enhance the conversion without compromising the selectivity for SCR and ASC. The agents are removed during calcination, creating inter-crystallite pores that increase the washcoat macroporosity. Steady state reaction studies of SCR on a Cu-SSZ-13 coated monolith show a noted increase in NOx conversion during standard SCR (NH3 + NO + O-2). The improved performance is sustained for hydrothermally aged samples. Similar conversion enhancement is observed for NH3 oxidation over the single-layer Cu-SSZ-13 washcoat. Conversion increases are also encountered for the dual-layer ASC (top Cu-SSZ-13 and base Pt/Al2O3 layer) during the selective oxidation of NH3 to N-2. For both SCR and ASC, the N-2 product selectivity is negligibly impacted on the modified catalysts. Sharper NH3 desorption profiles are obtained during NH3 temperature programmed desorption (TPD) experiments with Cu-SSZ-13. The results show that modified washcoat may reduce the amount of catalyst to maintain a desired conversion. For the ASC up to a 70% reduction in the Pt loading is demonstrated by using the sacrificial agents to achieve the same conversion. The implication for both SCR and ASC catalyst design and performance is discussed.

The effect(s) of SO2 on the two types of active sites on Cu-SSZ-13 NH3-SCR catalysts, Z2Cu and ZCuOH, were investigated. Two Cu-SSZ-13 catalysts with Si:Al ratios of 6 and 30 were synthesized, and they provide very different distributions of these two active sites. Inductively coupled plasma optical emission spectroscopy (ICP-OES), H-2 temperature-programmed reduction (H-2-TPR), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were utilized to characterize catalyst samples and quantify the amounts of total Cu, Z2Cu and ZCuOH. In situ DRIFTS results show that Z2Cu and ZCuOH responses to low-temperature (<200 degrees C) SO2 poisoning were site-dependent. Results of SO2 and SO2 + NH3 temperature programmed desorption (TPD) and DRIFTS experiments, supplemented with DFT calculations, revealed that the different observed responses correspond to different sulfur intermediates that form. On Z2Cu sites, SO2 only adsorbs when it is cofed with NH3 via formation of ammonium sulfate, with its fingerprint TPD feature at 380 degrees C. However, low-temperature interaction between SO2 and ZCuOH leads to copper bisulfite species formation, which can be further oxidized to form copper bisulfate with increasing temperature. In terms of low-temperature SCR functionality, the activity of both Cu-SSZ-13 samples were found to be significantly inhibited by SO2. However, in terms of regeneration (i.e., desulfation) behavior, Cu-SSZ-13 with a Si:Al = 30 (higher ZCuOH compared to Z2Cu) seemed to require higher desulfation temperatures (>550 degrees C). Therefore, compared with Z2Cu, ZCuOH sites are more susceptible to severe low-temperature SO2 poisoning because of the formation of more stable bisulfite and ultimately bisulfate species.

Emission control catalysts experience degradation under the severe operating conditions which can negatively impact the product distribution and conversion. Understanding and quantifying this impact is necessary to properly design the catalyst. This study elucidates the effect of hydrothermal aging on the performance of a Pt/gamma Al2O3 washcoated monolith used for oxidation of ammonia and of propylene, two reaction systems pertinent to the Ammonia Slip Catalyst (ASC) and Diesel Oxidation Catalyst (DOC), respectively. The Pt/gamma-Al2O3 catalyst was subjected to a feed stream containing H2O at 550 degrees C for over 250 h. The performance of the catalyst was measured at discrete intervals as it was progressively aged. The catalyst performance results reveal for ammonia oxidation that hydrothermal aging has a negligible impact on Pt/Al2O3 activity for temperatures below 250 degrees C but a significant detrimental impact for temperatures exceeding 300 degrees C. A (1D + 1D) reactor model was developed to predict the decline in the observed activity with aging time. Using kinetics and catalyst characterization data, the reactor model predicts that changes in washcoat morphology during aging is likely the underlying mechanism for the decline in activity of the catalyst. The results are corroborated with testing and modeling of propylene oxidation on the Pt/Al2O3 catalyst.

The aging impact on oxygen storage capacity (OSC) of three-way catalyst was investigated through experiments and modeling. OSC measurements were conducted on two commercial TWCs (one fresh and one aged) over the temperature window of 100-600 degrees C. The amount of OSC decreased by 44% at 400 degrees C and the kinetic restrictions of OSC reductions extended towards higher temperatures after standard bench cycle (SBC) aging at 955 degrees C for 57 h. The relationship between OSC and temperature was modified. Dynamic OSCs (breakthrough OSC and subsurface OSC) on the fresh and aged TWCs were predicted with a dual-site OSC model. From the modeling results, we found that not only the quantities of oxygen storage sites decreased but also the qualities of storage sites (rate constants) declined after aging. The PGM and surface ceria interface (site S1) declined more severely compared to the sub-surface ceria (site S2). The change of the amount of oxygen storage sites as well as kinetics on the aged TWC sample are reported. From thermodynamic analysis, the alteration of the temperature dependence of OSC is attributed to the change of the thermodynamic properties of ceria.