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

Abstract

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.

Experimental techniques for investigating the surface oxygen formation in the N2O decomposition on Fe-MFI zeolites

“…(1)), where an atomic, surface bound oxygen species is formed during the interaction of N 2 O with active iron sites in the zeolite Z [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]. This step initiates not only the stoichiometric N 2 O decomposition to nitrogen and oxygen, but also provides a selective oxygen species called "␣-oxygen" by Panov and co-workers [3,4,[9][10][11][12].…”

“…They quantify the amount of surface oxygen in a closed, static vacuum set-up through N 2 O decomposition in a temperature-programmed or an isothermal experiment at low temperatures (≤523 K). Subsequently, the exchange rate of surface oxygen with 18 O 2 isotopes is determined. Results obtained with MFI zeolites containing different iron amounts indicate that the surface oxygen is stable up to 573 K, but that it can be completely exchanged with 18 O 2 already at room temperature.…”

“…Subsequently, the exchange rate of surface oxygen with 18 O 2 isotopes is determined. Results obtained with MFI zeolites containing different iron amounts indicate that the surface oxygen is stable up to 573 K, but that it can be completely exchanged with 18 O 2 already at room temperature. Kinetic studies revealed that the activation energy is about 42 kJ mol −1 for the surface oxygen formation [3,9].…”

“…It is generally questionable if these results can be used for kinetic and mechanistic interpretations of results obtained under technically relevant conditions slightly above ambient pressure. Therefore, several transient response techniques have also been applied under atmospheric pressure [16][17][18][19][20][21][22][23][24][25], sometimes combined with subsequent temperature-programmed desorption of the surface bound species as well as with isotopic labelling experiments [21,22].…”

“…Step experiments with N 2 O were carried out at temperatures lower than 573 K to exclusively study the surface oxygen formation on ZSM5 zeolites with low iron contents (≤1000 ppm Fe, w/w [16][17][18] and 0.38% Fe, w/w [19]). The same experiments were also performed at 673 K, above the lightoff temperature of the total N 2 O decomposition, with zeolites containing high iron amounts (3.5-4.5% Fe, w/w) [20][21][22][23].…”