Bulk tungsten-substituted vanadium oxide for low-temperature NOx removal in the presence of water

Zeolites with different framework structures (SSZ-13, ZSM-5, BEA) but similar Si/Al ratios and Pd loading (~1 wt%) were synthesized and evaluated as low temperature passive NOx adsorbers (PNA). These materials exhibit high NOx adsorption efficiency with atomically dispersed Pd the active adsorption site. Hydrothermal aging at 750 ºC for 16 hours in the presence of 10% water vapor in air resulted in the formation of PdO nanoparticles in all three samples as evidenced by high energy XRD. Hydrothermal aging of the small-pore Pd/SSZ-13 (Si/Al = 6), which contain ~100-90% atomically dispersed palladium ions, decreases its PNA performance only by ~10-20%, indicating agglomeration of only ~10-20% of all atomically dispersed Pd into PdO. High-field solid state 27Al NMR studies on the fresh and aged samples substantiate dealumination and significant changes in the distribution of Al (and thus, Brönsted acid) sites after hydrothermal aging. FTIR measurements with NO probe molecule and titration of Brönsted acid sites with nitrosyl (NO+) ions further corroborate the 27Al NMR data. Because framework aluminum atoms are the anchoring sites for atomically dispersed Pd ions, their elution from the framework causes the loss of active atomically dispersed Pd species. With the aid of HAADF-STEM imaging and synchrotron XRD studies, we further confirm and visualize the fate of these Pd species – they agglomerate into PdO nanoparticles on the external surface of zeolite. Consequently, these changes lead to the decrease in PNA performance of these materials after hydrothermal aging. The thus formed agglomerates cannot be re-dispersed back to their ionic state due to the loss of framework Al T-sites and/or inherent stability of such large PdO particles. Our study demonstrates that, unlike in previous studies that found increased PNA performance upon HTA, high temperatures hydrothermal aging of PNA materials, that contain atomically dispersed Pd initially, results in a decrease in NOx storage efficiency due to the formation of PdO agglomerates. However, we also highlight the high hydrothermal stability of predominantly atomically dispersed 1-3 wt% Pd/SSZ-13 (Si/Al = 6), whose performance decreases only marginally after prolonged hydrothermal aging at 750 ºC. This study shows that hydrothermally stable passive NOx materials can be prepared using small-pore SSZ-13 zeolite.

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Palladium/ Zeolite Low Temperature Passive NOx Adsorbers (PNA): Structure-Adsorption Property Relationships for Hydrothermally Aged PNA Materials

10.26434/chemrxiv.8067020 ◽

2019 ◽

Author(s):

Konstantin Khivantsev ◽

Nicholas R. Jaegers ◽

Libor Kovarik ◽

Jian Zhi Hu ◽

Yong Wang ◽

...

Keyword(s):

Low Temperature ◽

Acid Sites ◽

Bronsted Acid ◽

Brønsted Acid ◽

Hydrothermal Aging ◽

Brønsted Acid Sites ◽

Brönsted Acid ◽

Small Pore ◽

Brönsted Acid Sites ◽

Bronsted Acid Sites

Zeolites with different framework structures (SSZ-13, ZSM-5, BEA) but similar Si/Al ratios and Pd loading (~1 wt%) were synthesized and evaluated as low temperature passive NOx adsorbers (PNA). These materials exhibit high NOx adsorption efficiency with atomically dispersed Pd the active adsorption site. Hydrothermal aging at 750 ºC for 16 hours in the presence of 10% water vapor in air resulted in the formation of PdO nanoparticles in all three samples as evidenced by high energy XRD. Hydrothermal aging of the small-pore Pd/SSZ-13 (Si/Al = 6), which contain ~100-90% atomically dispersed palladium ions, decreases its PNA performance only by ~10-20%, indicating agglomeration of only ~10-20% of all atomically dispersed Pd into PdO. High-field solid state 27Al NMR studies on the fresh and aged samples substantiate dealumination and significant changes in the distribution of Al (and thus, Brönsted acid) sites after hydrothermal aging. FTIR measurements with NO probe molecule and titration of Brönsted acid sites with nitrosyl (NO+) ions further corroborate the 27Al NMR data. Because framework aluminum atoms are the anchoring sites for atomically dispersed Pd ions, their elution from the framework causes the loss of active atomically dispersed Pd species. With the aid of HAADF-STEM imaging and synchrotron XRD studies, we further confirm and visualize the fate of these Pd species – they agglomerate into PdO nanoparticles on the external surface of zeolite. Consequently, these changes lead to the decrease in PNA performance of these materials after hydrothermal aging. The thus formed agglomerates cannot be re-dispersed back to their ionic state due to the loss of framework Al T-sites and/or inherent stability of such large PdO particles. Our study demonstrates that, unlike in previous studies that found increased PNA performance upon HTA, high temperatures hydrothermal aging of PNA materials, that contain atomically dispersed Pd initially, results in a decrease in NOx storage efficiency due to the formation of PdO agglomerates. However, we also highlight the high hydrothermal stability of predominantly atomically dispersed 1-3 wt% Pd/SSZ-13 (Si/Al = 6), whose performance decreases only marginally after prolonged hydrothermal aging at 750 ºC. This study shows that hydrothermally stable passive NOx materials can be prepared using small-pore SSZ-13 zeolite.

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Probing brønsted acid sites in zeolite HY with low temperature 17O MAS NMR spectroscopy

From Zeolites to Porous MOF Materials - The 40th Anniversary of International Zeolite Conference, Proceedings of the 15th International Zeolite Conference - Studies in Surface Science and Catalysis ◽

10.1016/s0167-2991(07)80921-9 ◽

2007 ◽

pp. 783-789 ◽

Cited By ~ 2

Author(s):

Hua Huo ◽

Luming Peng ◽

Clare P. Grey

Keyword(s):

Nmr Spectroscopy ◽

Low Temperature ◽

Acid Sites ◽

Mas Nmr ◽

Bronsted Acid ◽

Brønsted Acid ◽

Brønsted Acid Sites ◽

Brönsted Acid ◽

Brönsted Acid Sites ◽

Bronsted Acid Sites

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Effective co-sensitization using D–π–A dyes with a pyridyl group adsorbing at Brønsted acid sites and Lewis acid sites on a TiO2 surface for dye-sensitized solar cells

RSC Advances ◽

10.1039/c4ra14190c ◽

2015 ◽

Vol 5 (4) ◽

pp. 2531-2535 ◽

Cited By ~ 18

Author(s):

Yousuke Ooyama ◽

Koji Uenaka ◽

Takafumi Sato ◽

Naoyuki Shibayama ◽

Joji Ohshita

Keyword(s):

Lewis Acid ◽

Acid Sites ◽

Bronsted Acid ◽

Brønsted Acid ◽

Lewis Acid Sites ◽

Brønsted Acid Sites ◽

Pyridyl Group ◽

Brönsted Acid ◽

Brönsted Acid Sites ◽

Bronsted Acid Sites

Effective and convenient co-sensitization method for DSSC have been newly developed by employing two kinds of D–π–A dyes with pyridyl group capable of adsorbing at the Brønsted acid sites and the Lewis acid sites on TiO2 surface.

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IR Studies of Acid Properties of H-Boralites Activated at Various Temperatures

Collection of Czechoslovak Chemical Communications ◽

10.1135/cccc19920745 ◽

1992 ◽

Vol 57 (4) ◽

pp. 745-749 ◽

Cited By ~ 8

Author(s):

Jerzy Datka ◽

Marek Kawałek

Keyword(s):

Acid Sites ◽

Bronsted Acid ◽

Brønsted Acid ◽

Lewis Acid Sites ◽

Brønsted Acid Sites ◽

Activation Temperature ◽

Ir Studies ◽

Brönsted Acid ◽

Brönsted Acid Sites ◽

Bronsted Acid Sites

Four kinds of hydroxyls are present in H-boralite: 3450 cm-1 (Si-OH...O), 3670 cm-1 (B-OH), 3700-3720 cm-1 (Si-OH...B) and 3740 cm-1 (Si-OH). Only 3700-3720 cm-1 (Si-OH...B) hydroxyls are Bronsted acid sites. Their concentration, as well as the concentration of Lewis acid sites, was found to be independent of the activation temperature (in the range 473-873 K). On the other hand, both 3450 and 3670 cm-1 hydroxyls are very prone to dehydroxylation which starts at as low temperature as 473 K. This is due to a reaction: Si-OH + O(H)-B → SiOB + H2O. It has been found that the stretching frequency of Si-OH···B groups (Bronsted acid sites) depends on the activation temperature (3700 cm-1 at 573 K and 3720 cm-1 at 773 K) suggesting a variation of the properties of Bronsted sites with temperature. We explain it by the loss of Si-OHO and B-OH hydroxyls situated in close vicinity of Si-OH...B ones.

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On the Role of Protonic Acid Sites in Cu Loaded FAU31 Zeolite as a Catalyst for the Catalytic Transformation of Furfural to Furan

Molecules ◽

10.3390/molecules26072015 ◽

2021 ◽

Vol 26 (7) ◽

pp. 2015

Author(s):

Łukasz Kuterasiński ◽

Małgorzata Smoliło-Utrata ◽

Joanna Kaim ◽

Wojciech Rojek ◽

Jerzy Podobiński ◽

...

Keyword(s):

Active Sites ◽

Acid Sites ◽

Bronsted Acid ◽

Brønsted Acid ◽

Brønsted Acid Sites ◽

Protonic Acid ◽

Brönsted Acid ◽

Brönsted Acid Sites ◽

Bronsted Acid Sites

The aim of the present paper is to study the speciation and the role of different active site types (copper species and Brønsted acid sites) in the direct synthesis of furan from furfural catalyzed by copper-exchanged FAU31 zeolite. Four series of samples were prepared by using different conditions of post-synthesis treatment, which exhibit none, one or two types of active sites. The catalysts were characterized by XRD, low-temperature sorption of nitrogen, SEM, H2-TPR, NMR and by means of IR spectroscopy with ammonia and CO sorption as probe molecules to assess the types of active sites. All catalyst underwent catalytic tests. The performed experiments allowed to propose the relation between the kind of active centers (Cu or Brønsted acid sites) and the type of detected products (2-metylfuran and furan) obtained in the studied reaction. It was found that the production of 2-methylfuran (in trace amounts) is determined by the presence of the redox-type centers, while the protonic acid sites are mainly responsible for the furan production and catalytic activity in the whole temperature range. All studied catalysts revealed very high susceptibility to coking due to polymerization of furfural.

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