| 1. |
|
|
| 2. |
|
|
| 3. |
|
|
| 4. |
|
|
| 5. |
- Bergman, Susanna L., et al.
(författare)
-
In-situ studies of oxidation/reduction of copper in Cu-CHA SCR catalysts: Comparison of fresh and SO2-poisoned catalysts
- 2020
-
Ingår i: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 269
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 6. |
- L. Bergman, Susanna, et al.
(författare)
-
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
-
Ingår i: The Journal of Physical Chemistry C. - Washington, D.C. : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 124:5, s. 2945-2952
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 7. |
- Mesilov, Vitaly, et al.
(författare)
-
First-Principles Calculations of Condition-Dependent Cu/Fe Speciation in Sulfur-Poisoned Cu- and Fe-SSZ-13 Catalysts
- 2021
-
Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 125:8, s. 4632-4645
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 8. |
- Mesilov, V., et al.
(författare)
-
Insights into sulfur poisoning and regeneration of Cu-SSZ-13 catalysts: : In situ Cu and S K-edge XAS studies
- 2021
-
Ingår i: Catalysis Science & Technology. - : Royal Society of Chemistry (RSC). - 2044-4753 .- 2044-4761. ; 11:16, s. 5619-5632
-
Tidskriftsartikel (refereegranskat)abstract
- In situ Cu and S K-edge X-ray absorption spectroscopy (XAS) was used for the investigation of sulfur-poisoned and regenerated Cu-SSZ-13 selective catalytic reduction (SCR) catalysts. Highly dispersed sulfur in the oxidation state +6 was found in the catalysts. Even though a similar amount of sulfur was deposited in the catalysts poisoned at both 200 and 500 °C, a higher fraction of sulfur-free Cu species was seen for the catalyst poisoned at the higher temperature. Regeneration at 550 °C resulted in more sulfur-free Cu species in the catalyst poisoned at 500 °C, even though a higher amount of sulfur was detected in this catalyst compared to the regenerated catalyst poisoned at 200 °C. Inconsistencies between the amount of sulfur and the fraction of sulfur-free Cu species were attributed to the additional sulfur storage at sites that do not involve Cu. It was suggested that increased temperature of poisoning may facilitate the formation of Al sulfates along with Cu sulfates. These results provide the next step in detailed understanding of sulfur poisoning and regeneration of Cu-SSZ-13 catalysts.
|
|
| 9. |
- Mesilov, Vitaly, et al.
(författare)
-
Regeneration of sulfur-poisoned Cu-SSZ-13 catalysts : Copper speciation and catalytic performance evaluation
- 2021
-
Ingår i: Applied Catalysis B. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 299
-
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.
|
|
| 10. |
- Mesilov, Vitaly V., et al.
(författare)
-
Differences in oxidation-reduction kinetics and mobility of Cu species in fresh and SO2-poisoned Cu-SSZ-13 catalysts
- 2021
-
Ingår i: Applied Catalysis B. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 284
-
Tidskriftsartikel (refereegranskat)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.
|
|
