Nature of Active Sites of Fe-Beta Catalyst for NOx-SCR by NH3

Abstract

A correlation of the reaction rate over Fe-Beta catalysts with a concentration of specific isolated Fe 3? sites represented by an ESR signal at g & 4.3 and by UV adsorption bands with k max \ 300 nm was observed. Blocking zeolite cationic positions with Ca 2? and investigation of ''Fe-free'' Beta-zeolites confirmed the idea of isolated Fe complexes in cationic positions being active sites for NH 3 -deNO x .

Combined catalytic systems for enhanced low-temperature NO abatement

“…It is evident that only Fe-Beta zeolite demonstrates significant activity in Standard SCR. Commercial H-Beta shows minor activity at T react > 330 • C attributable to NO x conversion over residual Fe [12]. "Fe-free" Beta, USY, and SiO 2 -Al 2 O 3 do not exhibit noticeable conversion in Standard SCR.…”

“…NH 4 -Beta was converted into the H-form by calcination in airflow at 550 • C for 4 h. SiO 2 -Al 2 O 3 (Si/Al ≈ 2.1) was provided by "Sasol". "Fe-free" Beta (Si/Al ≈ 12.5) was synthesized via hydrothermal synthesis described in detail by Doronkin et al [12]. All zeolites and SiO 2 -Al 2 O 3 powders were calcined at 550 • C for 4 h in airflow.…”

“…For this purpose a series of Beta with various Fe loading including "Fe-free" sample was used for the CombiCat preparation. This approach was based on the observation that Fe cations play an important role both in Standard SCR [12] and in Fast SCR [13,14]. However, our recent study indicated that even residual amount of Fe (<0.02 wt%) is sufficient for Fast SCR reaction over BETA zeolite [15], while its Standard SCR activity is essentially ceased.…”

Fe-Beta zeolite for selective catalytic reduction of NOx with NH3: Influence of Fe content

“…5 shows UV-Vis spectra of the Fe-Beta catalysts. It is widely accepted that based on the UV-Vis spectra, the Fe species in a Fe-doped zeolite can be distinguished as isolated Fe 3+ in tetrahedral sites (<250 nm) [41,42] and higher coordination octahedral sites (250-300 nm) [8,43], small oligonuclear FexOy (between 300 and 400 nm) [44], and FexOy nanoparticles on the zeolite surface (above 400 nm) [27,45,46]. In order to calculate the relative amounts of the different Fe species, the UV-Vis spectra of the Fe-Beta catalysts were subject to peak resolution and fitting according to the references [47][48][49].…”

Study on the kinetics of the adsorption and desorption of NH3on Fe/HBEA zeolite

“…12,19,20 In contrast, a lower SCR performance was shown for oligomeric and particulate Fe species. 6,21,22 The adsorption and desorption of NH 3 occur in multiple cycles on the zeolites 23 being much faster than the SCR kinetics. It is known that NH 3 adsorbs on Brønsted acid sites to form ammonium-like species, while molecular adsorption on Lewis acid/basic sites also occurs.…”