| 1. |
- Bjerregaard, Joachim, 1996, et al.
(författare)
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Interpretation of H 2 -TPR from Cu-CHA Using First-Principles Calculations
- 2024
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Ingår i: Journal of Physical Chemistry C. - 1932-7447 .- 1932-7455. ; 128:11, s. 4525-4534
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Tidskriftsartikel (refereegranskat)abstract
- Temperature-programmed reduction and oxidation are used to obtain information on the presence and abundance of different species in complex catalytic materials. The interpretation of the temperature-programmed reaction profiles is, however, often challenging. One example is H2 temperature-programmed reduction (H2-TPR) of Cu-chabazite (Cu-CHA), which is a material used for ammonia assisted selective catalytic reduction of NOx (NH3-SCR). The TPR profiles of Cu-CHA consist generally of three main peaks. A peak at 220 °C is commonly assigned to ZCuOH, whereas peaks at 360 and 500 °C generally are assigned to Z2Cu, where Z represents an Al site. Here, we analyze H2-TPR over Cu-CHA by density functional theory calculations, microkinetic modeling, and TPR measurements of samples pretreated to have a dominant Cu species. We find that H2 can react with Cu ions in oxidation state +2, whereas adsorption on Cu ions in +1 is endothermic. Kinetic modeling of the TPR profiles suggests that the 220 °C peak can be assigned to Z2CuOCu and ZCuOH, whereas the peaks at higher temperatures can be assigned to paired Z2Cu and Z2CuHOOHCu species (360 °C) or paired Z2Cu and Z2CuOOCu (500 °C). The results are in good agreement with the experiments and facilitate the interpretation of future TPR experiments.
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| 2. |
- Han, Joonsoo, 1990, et al.
(författare)
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Effect of SO 2 and SO 3 Exposure to Cu-CHA on Surface Nitrate and N 2 O Formation for NH 3 -SCR
- 2024
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Ingår i: ACS ENGINEERING AU. - 2694-2488. ; In Press
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Tidskriftsartikel (refereegranskat)abstract
- We report effects of SO2 and SO3 exposure on ammonium nitrate (AN) and N2O formation in Cu-CHA used for NH3-SCR. First-principles calculations and several characterizations (ICP, BET, XRD, UV-vis-DRS) were applied to characterize the Cu-CHA material and speciation of sulfur species. The first-principles calculations demonstrate that the SO2 exposure results in both (bi)sulfite and (bi)sulfate whereas the SO3 exposure yields only (bi)sulfate. Furthermore, SOx adsorption on framework-bound dicopper species is shown to be favored with respect to adsorption onto framework-bound monocopper species. Temperature-programmed reduction with H-2 shows two clear reduction states and larger sulfur uptake for the SO3-exposed Cu-CHA compared to the SO2-exposed counterpart. Temperature-programmed desorption of formed ammonium nitrate (AN) highlights a significant decrease in nitrate storage due to sulfur species interacting with copper sites in the form of ammonium/copper (bi)bisulfite/sulfate. Especially, highly stable sulfur species from SO3 exposure influence the NO2-SCR chemistry by decreasing the N2O selectivity during NH3-SCR whereas an increased N2O selectivity was observed for the SO2-exposed Cu-CHA sample. This study provides fundamental insights into how SO2 and SO3 affect the N2O formation during ammonium nitrate decomposition in NH3-SCR applications, which is a very important topic for practical applications.
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| 3. |
- Han, Joonsoo, 1990
(författare)
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Fundamentals of NH3-SCR and SOx chemistry over copper zeolite for the control of NOx and greenhouse gas (N2O) emissions
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The release of N2O over Cu-zeolite was investigated due to the critical contribution it makes to the atmosphere as one of the greenhouse gases (GHGs). An exhaust after-treatment system (EATS) for heavy-duty vehicles that uses Cu/SSZ-13 as the selective catalyst reduction (SCR) catalyst experiences not only a variety of flow, temperature and chemical conditions but is also exposed to various resulting contaminants. Among these, sulfur has been the focus of studies due to its critical deactivation of DeNOx performance, even at low concentration levels. Adsorption/desorption of sulfur species over Cu/SSZ-13 is connected closely to the lifetime of the SCR catalyst. The task of unravelling the synergistic effect of sulfur has on the formation of N2O during the NH3-SCR of NOx, while also reflecting realistic application conditions, is challenging. The ambition here is to gain a comprehensive understanding of the formation of N2O in the presence of sulfur species during the NH3-SCR of NOx at low temperature for NOx emissions control. Model catalysts were prepared to investigate a variety of copper species and ammonium nitrate (AN) within the CHA cage. It was found that the CHA structure promotes surface nitrate species in NO2-rich conditions, and an environment with strong polarity within the CHA cage was proposed as a feasible reason for the largest formation of ammonium nitrate compared to the MFI and BEA structures with medium and large pore/cage sizes. In addition, zeolite acidity was more a viable reason as to why ammonium nitrate is thermally stable in Cu/CHA compared to Cu/MFI and Cu/BEA than the pore-confinement effect, which has been used to explain the thermal stability of AN over Cu/zeolites. The IR signature of the ammonium nitrate was also obtained; it is believed that ammonium nitrate exists mainly by interacting with copper ions to form copper-ammonium nitrate within the CHA cage. Copper species and the adsorption of sulfur oxides (SO2/SO3) were investigated and their synergistic effects on the formation of N2O assessed. The SO3-poisoning effect was highlighted which, from an experimental aspect, is very challenging. We found two different states of sulfur species. SO2 and SO3 interaction with copper-monomers (Z1CuOH and Z2Cu) and copper-dimer (Z2Cu2O2) are possible. Sulfated Cu-dimer is lower in energy compared to its copper-monomers counterparts. SO3 interacts with both Z1CuOH and Z2Cu but SO2 mainly interacts with Z1CuOH. SO3-poisoning only causes (bi)sulfate (Z1HSO4), whereas SO2-poisoning can cause both (bi)sulfite (Z1HSO3) and (bi)sulfate (Z1HSO4). Taken together, out results well demonstrates why the SO3 exposure results in more critical chemical poisoning (irreversible deactivation) compared to the SO2 exposure to Cu/CHA. Finally, synergistical effect of sulfur, copper, and Brønsted sites were found for N2O formation under NO2-rich condition. Consequently, we found that sulfur promotes N2O intermediate formation but increasing Brønsted site density increases N2 selectivity, thereby, reducing N2O formation from Cu/CHA.
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| 4. |
- Han, Joonsoo, 1990, et al.
(författare)
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N 2 O Formation during NH 3 -SCR over Different Zeolite Frameworks: Effect of Framework Structure, Copper Species, and Water
- 2021
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Ingår i: Industrial & Engineering Chemistry Research. - : American Chemical Society (ACS). - 1520-5045 .- 0888-5885. ; 60:49, s. 17826-17839
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Tidskriftsartikel (refereegranskat)abstract
- The formation characteristics of N2O were investigated with respect to copper-functionalized zeolites, i.e., Cu/SSZ-13 (CHA), Cu/ZSM-5 (MFI), and Cu/BEA (BEA) and compared with the corresponding zeolites in the H form as references to elucidate the effect of the framework structure, copper addition, and water. Temperature-programmed reduction with hydrogen showed that the CHA framework has a higher concentration of Cu2+ (Z2Cu) compared to MFI and BEA. The characterizations and catalyst activity results highlight that CHA has a framework structure that favors high formation of ammonium nitrate (AN) in comparison with MFI and BEA. Moreover, AN formation and decomposition were found to be promoted in the presence of Cu species. On the contrary, lower N2O formation was observed from Cu/CHA during standard and fast SCR reactions, which is proposed to be due to highly stabilized AN inside the zeolite cages. On the other hand, significant amounts of N2O were released during heating due to decomposition of AN, implying pros and cons of AN stability for Cu/CHA with possible uncontrolled N2O formation during transient conditions. Additionally, important effects of water were found, where water hinders AN formation and increases the selectivity for decomposition to NO2 instead of N2O. Thus, less available AN forming N2O was observed in the presence of water. This was also observed in fast SCR conditions where all Cu/zeolites exhibited lower continuous N2O formation in the presence of water.
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| 5. |
- Han, Joonsoo, 1990
(författare)
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NH3-SCR chemistry for NOx abatement: Influence of zeolite support on N2O formation & phosphorus species addition.
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Different lattice arrangements of Si, Al, and O atoms result in a variety of pore dimensions and zeolite channel network systems, giving rise to different physicochemical environments inside the catalyst cage. The objective of the thesis work was to understand the influence of zeolite topology on NH3-assisted SCR chemistry over CHA, MFI, and BEA frameworks, on the formation of N2O and the addition of phosphorus species to attain fundamental insight into NOx emission control. A variety of characterization techniques and mechanistic experimental protocols were used to examine Cu2+ ions coordinated with 1Al and 2Al in frameworks, denoted as Z2Cu and ZCuOH, respectively. Catalytic activity tests were performed to investigate the influence of ammonium nitrate (AN)-forming N2O and phosphorus species on DeNOx performance over the Cu-impregnated CHA, MFI, and BEA. In Paper I, ammonium nitrate (AN) formation and decomposition were thoroughly investigated to see the effect of zeolite topology, copper species, and water vapor on the N2O formation in NH3 SCR of NOx at low temperatures. Three different Cu/zeolites (CHA, MFI, and BEA) were used, and these were compared with H/zeolites as the reference. H2 temperature programmed reduction and in-situ IR spectroscopy suggest that the CHA framework structure is more favorable than MFI and BEA to form AN inside the catalyst cage. AN formation was enhanced in the presence of Cu ions over the Cu/zeolites. Catalyst activity tests demonstrated that Cu/CHA has a potential for uncontrolled N2O emission in transient conditions despite it shows lower N2O formation over standard and fast SCR reaction due to a highly stabilized AN inside the catalyst cage at low temperatures. This indicates pros and cons of AN stability over CHA in NH3-SCR systems. The critical effect of water vapor on AN formation and decomposition was found. The water vapor causes the cleavage of Cu dimers into Cu2+-OH groups, which are responsible for NO oxidation forming NO+ and surface nitrates. Thus, it results in the formation of fewer surface nitrates, leading to less AN and less N2O formation in wet conditions. In Paper II, phosphorus poisoning of different zeolite topologies was investigated. Fresh Cu/zeolites (CHA, MFI, and BEA) were used as the reference, and these were compared with P-poisoned Cu/zeolites. X-ray powder diffraction (XRD) revealed that the MFI framework was vulnerable to phosphorus species attack, resulting in the deformation of the framework structure. A variety of phosphorus species, such as PO-3, PO3- 4, and P2O5 populations, were observed over Cu/zeolites with Xray photoelectron spectroscopy. Transient response methodologies suggest that ZCuOH sites were significantly poisoned by PO-3 and PO3- 4 over Cu/CHA. Accordingly, a promoted or deteriorated redox feature of Cu ions (i.e., Z2Cu and ZCuOH) was suggested. A significant drop in catalytic activity was demonstrated over Cu/MFI in catalytic activity tests. It is suggested that MFI framework deformation, such as pore-blockage, local expansion, and cracking, impedes the mobility of Cu+(NH3)2 complexes at low temperatures under standard SCR conditions. Consequently, we hypothesize that MFI framework degradation hinders the formation of NH3-solvated Cu dimer complexes, which are responsible for O2 activation.
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| 6. |
- Ho, Hoang Phuoc, 1983, et al.
(författare)
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The role of Pd-Pt Interactions in the Oxidation and Sulfur Resistance of Bimetallic Pd-Pt/γ-Al2O3 Diesel Oxidation Catalysts
- 2021
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Ingår i: Industrial & Engineering Chemistry Research. - : American Chemical Society (ACS). - 1520-5045 .- 0888-5885. ; 60:18, s. 6596-6612
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Tidskriftsartikel (refereegranskat)abstract
- Diesel oxidation catalysts (DOC) were investigated for oxidation activity, NO conversion stability, and sulfur poisoning/regeneration on Pd/Al2O3, Pt/Al2O3, and Pd-Pt/Al2O3 catalysts. The Pd/Al2O3 catalyst was more active for CO and hydrocarbon (C3H6 and C3H8) oxidation, while the Pt/Al2O3 catalyst efficiently oxidized NO. The formation of a Pd-Pt alloy in the Pd-Pt/Al2O3 catalyst maintained Pd in a more reduced phase, resulting in the superior activity of this catalyst for the oxidation of CO, C3H6, and NO in comparison with its monometallic counterparts. The Pd-Pt alloy not only provided more low-temperature activity but also retained the stability of NO oxidation. The Pd-Pt alloy also favored the spillover of SO2 to the alumina support, resulting in significantly higher adsorption capacity of the Pd-Pt/Al2O3 catalyst, extensively prolonging its lifetime. However, the stable sulfates on Pd-Pt/Al2O3 made it difficult to completely regenerate the catalyst. The bimetallic sample showed higher activity for CO, C3H8, and C3H6 after sulfur poisoning and regeneration.
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| 7. |
- Isapour Toutizad, Ghodsieh, 1986, et al.
(författare)
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In situ DRIFT studies on N2O formation over Cu-functionalized zeolites during ammonia-SCR
- 2022
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Ingår i: Catalysis Science and Technology. - : Royal Society of Chemistry (RSC). - 2044-4753 .- 2044-4761. ; 12:12, s. 3921-3936
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Tidskriftsartikel (refereegranskat)abstract
- The influence of the zeolite framework structure on the formation of N2O during ammonia-SCR of NOx was studied for three different copper-functionalized zeolite samples, namely Cu-SSZ-13 (CHA), Cu-ZSM-5 (MFI), and Cu-BEA (BEA). The evolution of surface species during the SCR reaction at different temperatures was monitored with step-response experiments using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) at different reaction conditions. Also, density functional theory (DFT) calculations were performed to assist the interpretation of the experimental results. The DRIFTS results indicate that NO+ and nitrate species are the main products formed during NO oxidation, and NO appears to adsorb on both Cu-Lewis and Al-Lewis acid sites. The DFT calculations for NO adsorption on the SSZ-13 sample reveal adsorption at Bronsted acid sites with similar adsorption energies but with a slight difference in NO+ stretching vibrations in the DRIFT spectra. Within the standard SCR reaction, in the O-H stretching region, the number of NH3 molecules adsorbed on the Bronsted acid sites is higher for the small-pore size sample compared to the medium- and large-pore zeolites. The obtained DRIFTS results for nitrate species are supported by DFT calculations by simulating the IR spectra of mobile and framework bound nitrate species, which both have a signature at 1604 cm(-1) associated with the O-N bond on NO3-. It is revealed that N2O is produced in a higher amount at lower temperatures for all three samples irrespective of the NO/NO2 ratio. Furthermore, the obtained results from both DRIFTS studies and flow reactor experiments show the higher formation of N2O for the large-pore zeolite compared to the medium- and small-pore zeolite.
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| 8. |
- Mesilov, Vitaly, et al.
(författare)
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Regeneration of sulfur-poisoned Cu-SSZ-13 catalysts : Copper speciation and catalytic performance evaluation
- 2021
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Ingår i: Applied Catalysis B. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 299
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Tidskriftsartikel (refereegranskat)abstract
- Regeneration of a sulfur-poisoned Cu-SSZ-13 catalyst via a temperature ramp in an inert atmosphere with subsequent holding under oxidizing conditions at 500 degrees C restores significant activity for NOx conversion under standard, fast, and NO2-rich SCR conditions. The N2O selectivity of the regenerated catalyst is higher than for the fresh catalyst under NO2-rich SCR conditions at 280 degrees C, while the opposite was observed for the standard and fast SCR conditions. Analysis of copper speciation showed that sulfur-free Cu species have different conditiondependent behavior in the fresh and regenerated catalysts. Heating the poisoned catalyst in an oxidizing atmosphere transforms a portion of ammonium sulfates into stable metal sulfates, while heating under inert or reducing conditions leads to more effective desulfation without the formation of stable metal sulfates. Reducing conditions result in desulfation at lower temperatures compared to inert conditions. These results contribute to the further development of regeneration strategies for Cu-SSZ-13 catalysts.
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| 9. |
- Wang, Aiyong, 1989, et al.
(författare)
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A deactivation mechanism study of phosphorus-poisoned diesel oxidation catalysts: Model and supplier catalysts
- 2020
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Ingår i: Catalysis Science and Technology. - : Royal Society of Chemistry (RSC). - 2044-4753 .- 2044-4761. ; 10:16, s. 5602-5617
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Tidskriftsartikel (refereegranskat)abstract
- The effect of phosphorus poisoning on the catalytic behavior of diesel oxidation catalysts was investigated over model and supplier monolith catalysts, i.e., Pd-Pt/Al2O3. The results of ICP and XPS from the vapor-phase poisoning over model catalysts suggested that the temperature of phosphorus poisoning affects both the overall content of phosphorus and the dispersion of phosphorus (i.e., inlet/outlet and surface/bulk). Phosphorus oxide (P2O5), metaphosphate (PO3-), and phosphate (PO43-) were identified in the poisoned model and supplier catalysts. The distribution of these species on poisoned model catalysts was highly dependent on the poisoning temperature, i.e., a higher temperature resulted in a higher concentration of PO43-. The outlets of the monoliths contained more PO43- and less P2O5 than the inlets. Both active sites and surface OH groups on model and supplier catalysts were contaminated upon phosphorus poisoning. It is found that PO43- had a stronger influence on the active sites than P2O5. One significant finding in this study is that the vapor-phase phosphorus poisoning could be a practical and cost efficient approach to simulate an accelerated aging/poisoning process.
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| 10. |
- Wang, Aiyong, 1989, et al.
(författare)
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Insight into the effect of phosphorus poisoning of Cu/zeolites with different framework towards NH 3 -SCR
- 2023
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Ingår i: Chemical Engineering Journal. - : Elsevier BV. - 1385-8947. ; 454
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Tidskriftsartikel (refereegranskat)abstract
- Cu/zeolites were prepared to elucidate the effect of phosphorus poisoning on different zeolite framework structures for NH3-SCR. The results show that there are significant differences in phosphorus poisoning depending on the zeolite framework structure. The PO3−/PO43− species gradually decreased along with an increase in P2O5 in the following order: Cu/SSZ-13, Cu/ZSM-5, and Cu/BEA. One possible reason could be the increased pore size of these zeolites, which results in less steric hindrance for larger P2O5 species. P2O5 is suggested to enhance the redox ability of Cu ions, which results in an increase in low-temperature activity in NH3-SCR, whereas Cu ions were significantly poisoned by PO3−/PO43−, resulting in low-temperature deactivation. Furthermore, the effect of phosphorus poisoning on the structure of Cu/ZSM-5 was found to be much greater than that of Cu/BEA and Cu/SSZ-13, possibly due to phosphorus attacked the surface defects of the zeolite, causing local expansion and cracking.
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| 11. |
- Yao, Dawei, 1991, et al.
(författare)
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Insight into CO induced degradation mode of Pd/SSZ-13 in NOx adsorption and release: Experiment and modeling
- 2022
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Ingår i: Chemical Engineering Journal. - : Elsevier BV. - 1385-8947. ; 439
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Tidskriftsartikel (refereegranskat)abstract
- Passive NOx adsorption (PNA) on Pd zeolites is an important technique to remove NOx during the cold start of the engine. However, the stability of Pd zeolites under high concentrations of CO is still challenging in multiple cold starts of an engine. Herein, we illustrate the CO-induced degradation mechanism of Pd zeolite by combining experiments and kinetic models. Pd/SSZ-13 has been used in multicycle processes containing NOx adsorption at low temperature and temperature programmed desorption, which represents the PNA degradation in multiple cold start periods. A kinetic model was developed to describe the NOx storage and degradation behavior of Pd/SSZ-13. Both experimental and modelling observations suggested that two Pd sintering modes are occurring under high CO concentration (4000 ppm), namely Ostwald ripening and particle migration. Apart from the degradation behavior, this model is also adequate for describing multi-cycle NOx storage and release behavior under low CO concentration.
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| 12. |
- Yao, Dawei, 1991, et al.
(författare)
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Kinetic modeling of CO assisted passive NOx adsorption on Pd/SSZ-13
- 2022
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Ingår i: Chemical Engineering Journal. - : Elsevier BV. - 1385-8947. ; 428
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Tidskriftsartikel (refereegranskat)abstract
- Passive NOx adsorption (PNA) has been recently developed as a promising technology for controlling the NOx emissions during the cold start period. In this work, we illustrate a CO-assisted mechanism by combining experimental and kinetic modeling studies. Pd/SSZ-13 has been synthesized, characterized and evaluated as a PNA in low-temperature NOx adsorption and temperature program desorption cycles, to represent multiple cold start periods. The gas compositions were also systemically changed, where both the effect of varying NOx and CO feed was evaluated in the presence of high water and oxygen contents. A kinetic model was developed to simulate the profiles of NO and NO2, including three initial Pd sites (Z-Pd(II)Z-, Z-[Pd(II)OH]+ and PdO). It is concluded from XPS and in situ DRIFTS experiments, flow reactor measurements and modelling observations that CO reduces Pd(II) species to Pd(I)/Pd(0) species, which increases the stability of the stored NOx species, resulting in a release above the urea dosing temperature. The model could well describe the experimental features, including the effect of CO. In addition, the model was used for full-scale catalytic converter simulations.
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