Formation of the Surface NO during N₂O Interaction at Low Temperature with Iron-Containing ZSM-5

Abstract

Interaction of N2O at low temperatures (473-603 K) with Fe-ZSM-5 zeolites (Fe, 0.01-2.1 wt %) activated by steaming and/or thermal treatment in He at 1323 K was studied by the transient response method and temperature-programmed desorption (TPD). Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) of NO adsorbed at room temperature as a probe molecule indicated heterogeneity of surface Fe(II) sites. The most intensive bands were found at 1878 and 1891 cm(-1), characteristic of two types mononitrosyl species assigned to Fe2+(NO) involved in bi- and oligonuclear species. Fast loading of atomic oxygen from N2O on the surface and slower formation of adsorbed NO species were observed. The initial rate of adsorbed NO formation was linearly dependent on the concentration of active Fe sites assigned to bi- and oligonuclear species, evolving oxygen in the TPD at around 630-670 K. The maximal coverage of a zeolite surface by NO was estimated from the TPD of NO at approximately 700 K. This allowed the simulation of the dynamics of the adsorbed NO formation at 523 K, which was consistent with the experiments. The adsorbed NO facilitated the atomic oxygen recombination/desorption, the rate determining step during N2O decomposition to O2 and N2, taking place at temperatures > or =563 K.

NO – oxygen scavenger or reaction intermediate in the decomposition of N2O?

“…It thus follows that NOx surface species must be formed. The tailing of dinitrogen formation reported by Bulushev et al [12] during the decomposition of nitrous oxide over Fe-MFI also points to a role of mechanism (2) in addition to the simple reaction (1) (2) formation of surface NO (3) decomposition of surface NO Active sites ( ) Fe for further N 2 O decomposition are thus liberated and more dinitrogen is formed.…”

“…However, the TPD of surface species left after the N 2 O decomposition was found to yield NO 2 in addition to NO [7][8][9][10][11][12]. It thus follows that NOx surface species must be formed.…”

N2O Decomposition over Fe‐ZSM‐5 Studied by Transient Techniques

“…Following the results of Bulushev et al [16], a simplified scheme of the N 2 O interaction with Fe-ZSM-5 at low temperatures can be suggested: …”

“…The addition of gaseous NO enhances the activity of Fe-containing zeolites especially at low temperatures < 623 K [9,[11][12][13][14][15]. Slow NO formation from N 2 O and accumulation on the catalyst surface is observed over Fe-and Cu-zeolites [16][17][18][19][20]. Nitro/nitrous [11,14,21] as well as nitrite/nitrate [22,23] redox cycles are proposed for the active sites regeneration.…”

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

“…After correcting the amount of the formed N 2 by subtracting twice the formed O 2 amount (Table 3, 4th column), the calculated surface oxygen contents are still higher than those obtained from the multipulse experiments. A reasonable explanation could be the formation of NO adsorbed in the zeolite which is formed at high N 2 O gas phase concentrations during the step, as shown by other authors [17,36,37]. Due to the reaction equation of the NO formation, N 2 O + Z → NO-Z + 0.5N 2 , this process would falsify the determination of surface oxygen from the formed N 2 or the converted N 2 O amount using the step technique.…”