| 1. |
- Bergman, Susanna, et al.
(author)
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In-situ studies of oxidation/reduction of copper in Cu-CHA SCR catalysts:comparison of fresh and SO2-poisoned catalysts
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Journal article (other academic/artistic)abstract
- SO2-poisoning results in deactivation of Cu-CHA SCR under standard SCR conditions; however regeneration at 700 ◦C completely restores the SCR performance. To understand the nature of these effects, Cu-species in the fresh and poisoned catalystswere characterized by in-situ temperature-dependent time-resolved Cu K-edge X-ray absorption spectroscopy using the multivariate curve resolution alternating least squares (MCR-ALS) approach and continuous Cauchy wavelet transforms. The extracted chemically-meaningful reference spectra of Cu-species were analyzed by DFT-assisted XANES calculations. Cu-bisulfates werefound as the most energetically favorable poisoned Cu-species. The response of Cu-species to a reducing environment differs inthe fresh and SO2-poisoned catalysts. Differences in reducibility are related to the formation of quasi-linear Cu-complexes in the SO2-poisoned catalyst formed during heating in H2/He. Heating in H2/He leads to partial desulfurization of the poisoned catalyst. Cooling in H2/He after heating results in more facile formation of Cu-metal clusters in fresh catalyst than in SO2-poisoned.
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| 2. |
- Bergman, Susanna L., et al.
(author)
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In-situ studies of oxidation/reduction of copper in Cu-CHA SCR catalysts: Comparison of fresh and SO2-poisoned catalysts
- 2020
-
In: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 269
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Journal article (peer-reviewed)abstract
- SO2-poisoning results in deactivation of Cu-CHA SCR under standard SCR conditions; however regeneration at 700 °C completely restores the SCR performance. To understand the nature of these effects, Cu-species in the fresh and poisoned catalysts were characterized by in-situ temperature-dependent time-resolved Cu K-edge X-ray absorption spectroscopy using the multivariate curve resolution alternating least squares (MCR-ALS) approach and continuous Cauchy wavelet transforms. The extracted chemically-meaningful reference spectra of Cu-species were analyzed by DFT-assisted XANES calculations. Cu-bisulfates were found as the most energetically favorable poisoned Cu-species. The response of Cu-species to a reducing environment differs in the fresh and SO2-poisoned catalysts. Differences in reducibility are related to the formation of quasi-linear Cu-complexes in the SO2-poisoned catalyst formed during heating in H2/He. Heating in H2/He leads to partial desulfurization of the poisoned catalyst. Cooling in H2/He after heating results in more facile formation of Cu-metal clusters in fresh catalyst than in SO2-poisoned.
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| 3. |
- Dahlin, Sandra, et al.
(author)
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Multivariate analysis of the effect of biodiesel-derived contaminants on V2O5-WO3/TiO2 SCR catalysts
- 2016
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In: Applied Catalysis B. - : Elsevier. - 0926-3373 .- 1873-3883. ; 183, s. 377-385
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Journal article (peer-reviewed)abstract
- This study investigates the effect of biodiesel-derived contaminants on vanadia-based NH3-SCR catalysts in heavy-duty exhaust aftertreatment. The aim was to study, not only the effect of single contaminants on the catalyst performance, but also of possible interaction effects between poisons.The effect of six potential catalyst poisons (Na, K, Mg, P, S and Zn) was evaluated using an experimental design and multivariate data analysis. Monolithic V2O5-WO3/TiO2 catalysts were subjected to accelerated laboratory-scale aging, where the six contaminants were fed simultaneously using a wet impregnation method. In addition to NOx conversion tests, the catalysts were characterized by means of ICP-OES, SEM-EDX, XPS, N2 physisorption and NH3-TPD. The lab-aged samples were compared to fresh and vehicle-aged catalysts.The accelerated aging method showed good reproducibility and gave rise to surface compounds similar to those found in vehicle-aged catalysts. Despite plausible differences regarding penetration depth of the contaminants into the walls of the catalyst, the aging method appears to be an efficient way to point out significant chemical poisons.The model obtained from the experimental design was found to correlate well with the experimental data and can therefore be used to predict effects of the various poisons and poison interactions. Significant effects on the NO. x conversion were found for P, S, Na, Mg and K as well as for the interactions P. ×. Na, P. ×. K and S. ×. Na. A poisoning effect was found for Mg, Na, K, P. ×. K, and P. ×. Na, where Na and K exhibited the strongest poisoning effect. The deactivating effect of alkali was lowered in the presence of phosphorus and sulfur, which is explained by the formation of phosphates and sulfates, preventing the interaction of the alkali metals with the vanadia active sites.
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| 4. |
- L. Bergman, Susanna, et al.
(author)
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In-situ characterization by Near-Ambient Pressure XPS of the catalytically active phase of Pt/Al2O3 during NO and CO oxidation
- 2018
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In: Applied Catalysis B. - Amsterdam : Elsevier. - 0926-3373 .- 1873-3883. ; 220, s. 506-511
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Journal article (peer-reviewed)abstract
- This study concerns near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) studies of a Pt/Al2O3 diesel oxidation catalyst used in exhaust aftertreatment. We apply the technique to an industrial-grade porous catalyst, thus bridging both the pressure and materials gap, and probe the shift in binding energy of Pt 4d under different atmospheres. We observe that oxidizing atmospheres induce a shift in binding energy, corresponding to changes in Pt oxidation state, especially pronounced under an atmosphere of NO and O2. Such changes in Pt oxidation state have previously been linked to dynamic changes in NO oxidation activity.
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| 5. |
- L. Bergman, Susanna, et al.
(author)
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Probing the Oxidation/Reduction Dynamics of Fresh and P-, Na-, and K-Contaminated Pt/Pd/Al2O3 Diesel Oxidation Catalysts by STEM, TPR, and in Situ XANES
- 2020
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In: The Journal of Physical Chemistry C. - Washington, D.C. : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 124:5, s. 2945-2952
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Journal article (peer-reviewed)abstract
- This study examines the oxidation and reduction behavior of Pt/Pd/Al2O3 diesel oxidation catalysts at temperatures and gas compositions that model the conditions of the working diesel oxidation catalyst in a truck exhaust system. In situ measurements using Pt L3 X-ray edge absorption spectroscopy are coupled with temperature-programmed reduction (TPR) and scanning transmission electron microscopy measurements to characterize the catalyst. The fresh catalyst is compared with chemically aged catalysts, exposed to Na, K, and P contaminants. Phosphorus exhibits strong, spatially localized interactions with the Pt/Pd catalyst nanoparticles that are reflected in a strong shift to lower energy of the Pt L3 edge under CO oxidation conditions. The Na and K poisons are spread more uniformly throughout the catalyst washcoat and do not strongly affect the edge spectra or the TPR-determined catalyst reducibility.
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| 6. |
- Mesilov, Vitaly, et al.
(author)
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Computational Study of Noble Metal CHA Zeolites : NO Adsorptionand Sulfur Resistance br
- 2022
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In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 126:16, s. 7022-7035
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Journal article (peer-reviewed)abstract
- Noble metal-exchanged small-pore molecular sieves withchabazite topology are promising materials for automotive cold-start NOxemission control applications. A combination offirst-principles thermody-namics and density functional theory was applied for the prediction ofmonomeric palladium, platinum, and ruthenium species formed in 1Al or 2Alsites of six-/eight-membered rings of the SSZ-13 framework in the presenceof SO2, NO, O2, and H2O at temperatures between 0 and 1100 K.Calculations using gradient-corrected Perdew-Burke-Ernzerhof (PBE)functional and hybrid Heyd-Scuseria-Ernzerhof (HSE06) functionalshowed that the binding energy of NO adsorbed on Pd, Pt, or Ru ions is astrong function of exchange-correlation functional. Use of the PBEfunctional overestimated the binding strengths of NO to Pd, Pt, or Ru ions compared to the HSE06 functional. While PBE ledto the adsorption of two NO per Pd, Pt, or Ru ion, HSE06 predicted the adsorption of a single NO. Isolated Pd, Pt, or Ru ions in 1Alsites tended to bind NO stronger than their counterparts in 2Al sites. Both functionals revealed that Pd and Pt ions have moresimilarities in terms of both NO adsorption and sulfur resistance compared to Ru ions. The results of this study are beneficial forfurther modeling of passive NOxadsorbers with improved properties to deliver cleaner tailpipe emissions during engine cold start
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| 7. |
- Mesilov, Vitaly, et al.
(author)
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First-Principles Calculations of Condition-Dependent Cu/Fe Speciation in Sulfur-Poisoned Cu- and Fe-SSZ-13 Catalysts
- 2021
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In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 125:8, s. 4632-4645
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Journal article (peer-reviewed)abstract
- A combination of first-principles thermodynamics and density functional theory (DFT) was applied for the prediction of sulfur-poisoned monomeric Cu/Fe species formed in the SSZ-13 catalyst framework under selective catalytic reduction (SCR)-relevant conditions in the presence of sulfur dioxide, ammonia, oxygen, and water. Differences in fresh and sulfur-poisoned species were found for Cu- and Fe-SSZ-13 catalysts containing one Al (1Al sites) or two Al (2Al sites) in 6-membered rings (6MRs) or 8-membered rings (8MRs). The impact of ammonia concentration during low-and high-temperature sulfur-poisoning on Cu- and Fe-speciation was also investigated. SCR-relevant concentrations of ammonia in the gas mixture led to the formation of ammonium sulfates over copper in 2Al and 1Al sites of Cu-SSZ-13, while bisulfate and sulfuric acid species were predicted at these copper sites either in the absence of ammonia or at negligible concentrations of ammonia during low- and high-temperature poisoning. The absence of ammonia in the gas mixture led to the formation of iron-bisulfates at 2Al sites of Fe-SSZ-13 during low- temperature poisoning, while the formation of ammonium sulfates was favorable under SCR-relevant conditions. In contrast to the facile formation of ammonium sulfates at copper sites of Cu-SSZ-13, only ammonium-free iron-sulfates formed at 1Al sites in Fe-SSZ-13 under realistic operational conditions. The regeneration of 2Al sites of Cu-SSZ-13 was predicted to occur at higher temperatures compared to 2Al sites in Fe-SSZ-13, whereas the opposite was predicted for 1Al sites. The analysis of fresh and regenerated Cu/Fe species was carried out as well. These theoretical results on model catalysts provide a first step in the understanding of sulfur-poisoning in Fe-SSZ-13 catalysts, supporting further experimental investigations to improve NH3-SCR catalysts for meeting future emission standards.
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| 8. |
- Mesilov, Vitaly, et al.
(author)
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Impact of Biodiesel-Based Phosphorus and Sulfur on Copper Speciation of Cu-SSZ-13 Catalysts : XAFS Scanning during H-2-TPR
- 2022
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In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 126:7, s. 3385-3396
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Journal article (peer-reviewed)abstract
- Biodiesel is a promising renewable fuel, which may help to limit our dependence on fossil fuels. However, the presence of contaminants in biodiesel can affect the Cu speciation of the Cu-SSZ-13 selective catalytic reduction (SCR) catalyst, resulting in its deactivation and decreased durability. In situ Cu K-edge X-ray absorption fine structure (XAFS) scanning during a temperature-programmed reduction in hydrogen (H-2-TPR) has been applied here for the analysis of Cu speciation in Cu-SSZ-13 catalysts aged using pure and contaminated biodiesel fuels. XAFS data were analyzed using the multivariate curve resolution alternating least-squares (MCR-ALS) method. While only reduction from Cu-II to Cu-I was observed at temperatures below 500 degrees C for the catalyst aged using pure biodiesel, a one-step reduction of Cu-II to Cu-0 at temperatures between 400 and 500 degrees C was found for the catalyst aged using P-doped biodiesel. The transformation of isolated CuII species to Cu-II clusters was suggested for the catalyst as a result of aging using P-doped biodiesel. The catalyst aged using S-doped biodiesel showed mainly the reduction of isolated Cu-II to Cu-I, which was inhibited as compared to that observed for the catalyst aged using pure biodiesel. The reduction of the catalyst aged using P+S-doped biodiesel led to the reduction of Cu-II to both Cu-I and Cu-0. The phosphorus was responsible for the formation of Cu-II clusters during aging of the catalyst using P+S-doped biodiesel. This study reveals that the presence of phosphorus in biofuels should be strictly regulated to avoid major changes in the Cu speciation of Cu-SSZ-13 catalysts.
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| 9. |
- Mesilov, Vitaly, et al.
(author)
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Regeneration of sulfur-poisoned Cu-SSZ-13 catalysts : Copper speciation and catalytic performance evaluation
- 2021
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In: Applied Catalysis B. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 299
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Journal article (peer-reviewed)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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| 10. |
- Mesilov, Vitaly V., et al.
(author)
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Differences in oxidation-reduction kinetics and mobility of Cu species in fresh and SO2-poisoned Cu-SSZ-13 catalysts
- 2021
-
In: Applied Catalysis B. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 284
-
Journal article (peer-reviewed)abstract
- Fresh and SO2-poisoned Cu-SSZ-13 selective catalytic reduction (SCR) catalysts were studied using near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) in the presence of NH3, O2, and NO, as well as by in situ X-ray absorption spectroscopy (XAS) using H2, He, and CO. In contrast to the fresh catalyst, inhibited reduction of Cu-species and the absence of Cu-metal nanoparticles was found in the SO2-poisoned catalyst during heating/cooling in H2 and CO. High structural disorder and differences in the formation of Cu-carbonyl species were seen for the SO2-poisoned catalyst compared to the fresh catalyst. Suppressed oxidation-reduction and low mobility of Cu-species during exposure to NH3-SCR-related gases were observed in the SO2-poisoned catalyst, unlike in the fresh catalyst. These observations help explain catalyst deactivation during the standard NH3-SCR reaction. The formation of Cu-metal nanoparticles in the fresh catalyst revealed another possible deactivation pathway for the SCR-catalyst in combined LNT-SCR systems during fuel-rich periods.
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