Evolution of Iron States and Formation of α-Sites upon Activation of FeZSM-5 Zeolites

Дубков, К. А.; Ovanesyan, N. S.; Shteinman, A. A.; Starokon, E. V.; Panov, G. I.

doi:10.1006/jcat.2002.3552

Cited by 347 publications

(349 citation statements)

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“…The first step corresponds to surface atomic oxygen formation. Extraframework Fe 2+ sites created by autoreduction of Fe 3+ to Fe 2+ during thermal treatment of the zeolite in He or vacuum [13,[21][22][23][24][25] are suggested as the active sites for this reaction. NO is formed in the second step and accumulates slowly on the zeolite surface.…”

Section: Discussionmentioning

confidence: 99%

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Dynamics of N2O decomposition over HZSM-5 with low Fe content

Bulushev

Kiwi‐Minsker

Renken

2004

Journal of Catalysis

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The dynamics of N 2 O decomposition to gaseous nitrogen and oxygen over HZSM-5 catalysts with a low iron content (200 and 1000 ppm) was studied by the transient response method in the temperature range 523-653 K. The active catalysts were prepared from HZSM-5 with Fe in the framework on its steaming at 823 K followed by thermal activation in He at 1323 K. Two main steps were distinguished in the dynamics of N 2 O decomposition. The first step represents N 2 O decomposition forming gaseous nitrogen and surface atomic oxygen. The second step is associated with surface oxygen recombination and desorption. At 523-553 K only the first step is observed. Above 573 K the decomposition of N 2 O to O 2 and N 2 in stoichiometric amounts starts at a rate increasing with time until a steady-state value is reached. This increase was assigned to the catalysis by adsorbed NO formed slowly on the catalyst surface from N 2 O, as indicated by temperature-programmed desorption. The catalytic effect of the adsorbed NO was also confirmed by transient experiments with forced addition of NO in the stream of N 2 O during its decomposition. A simplified kinetic model is proposed to explain the autocatalytic reaction. Catalyst pretreatment in O 2 did not affect N 2 O decomposition, but irreversible water vapor adsorption at 603 K resulted in a twofold decrease in surface oxygen loading from N 2 O and complete inhibition of the oxygen desorption.

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Section: Discussionmentioning

confidence: 99%

“…Step 4 is considered in the literature as completing the catalytic cycle during N 2 O decomposition [21]. However, the increase in N 2 O decomposition rate and reaction dynamics (Figs.…”

Section: Discussionmentioning

confidence: 99%

Dynamics of N2O decomposition over HZSM-5 with low Fe content

Bulushev

Kiwi‐Minsker

Renken

2004

Journal of Catalysis

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“…There is a wide consensus concerning the Fe-ions activity, but the specific nature of the active sites, especially the Fe nucleation, remains an open question. Activity was assigned by Panov and co-workers [1] to binuclear iron clusters in the zeolite extra-framework, where each Fe 2+ -site would be able to generate one active O-atom from N 2 O, called ␣-oxygen. This ␣-oxygen was claimed to have high oxidative power converting CO to CO 2 , or forming phenol from benzene at low temperatures [1,2].…”

Section: Introductionmentioning

confidence: 99%

“…Activity was assigned by Panov and co-workers [1] to binuclear iron clusters in the zeolite extra-framework, where each Fe 2+ -site would be able to generate one active O-atom from N 2 O, called ␣-oxygen. This ␣-oxygen was claimed to have high oxidative power converting CO to CO 2 , or forming phenol from benzene at low temperatures [1,2]. But the experimental evidence for the stabilization of isolated iron cations after zeolite steaming has also to be taken into account [3,4].…”

Section: Introductionmentioning

confidence: 99%

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Transient response method for characterization of active sites in HZSM-5 with low content of iron during N2O decomposition

2004

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The surface active Fe-sites in HZSM-5 with low content of iron (<1000 ppm) activated by steaming and high temperature (up to 1323 K) calcination in inert lead to the formation of surface oxygen (O) ad species from N 2 O and were characterized quantitatively by transient response method. Only a part of (O) ad deposited on zeolite by decomposing N 2 O was active in CO oxidation at 523 K. A binuclear Fe-center is suggested as an active center, featuring a "diamond core" structure, similar to that of the monooxygenase enzyme. The active O-atoms were assigned to the paired terminal oxygen atoms each bound to one Fe-site in the binuclear [Fe 2 O 2 H] + -cluster.Zeolite pre-saturated by water vapor at 473-523 K generates (O) ad species from N 2 O completely inactive in the CO oxidation. The total amount of the oxygen, (O) ad , deposited on the pre-saturated by water zeolite corresponds to a half of stoichiometric amount of the surface Fe-atoms and suggests that water blocks a half of the binuclear [Fe 2 O 2 H] + -center, the remaining acting as a single Fe-site.

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