Co2+/Co0 redox couple revealed by EPR spectroscopy triggers preferential coordination of reactants during SCR of NOx with propene over cobalt-exchanged zeolites

Abstract

Catalytic reduction of NOx with propene over Co2+ -exchanged beta and ZSM-5 zeolites occurs with formation of zero-valent cobalt; NOx preferentially adsorbed on Co2+ plays the role of a metal reducing agent while ligation of propene is favored for Co(0) centers.

The Reduction and Oxidation of Co Species in CoZSM-5 Zeolites Studied by IR Spectroscopy

“…-NO band at 1940 cm -1 still remains. According to our earlier results[21], the Co 3? -NO band shows three submaxima at 1940, 1950 and 1970 cm -1 , suggesting the presence of three kinds of Co 3?…”

“…in reduced CoZSM-5. Pietrzyk et al [21], who studied EPR spectra of Co 2? -CO adducts, have also recently reported the reduction of Co ions to Co-metal by NO at elevated temperatures.…”

Chapter 1. In situ electron paramagnetic resonance (EPR) – a unique tool for analysing structure and reaction behaviour of paramagnetic sites in model and real catalysts

“…The triplet structure arises from the shfs coupling of the single electron with the nuclear spins of two equivalent protons in the species [H 2 Fe 2 (CO) 7 ] À . The observed shfs constant A H ¼ 22.3 G agrees very well with that observed for the radical [HFe 2 (CO) 8 ] . 95 This radical, however, comprises only B3% of the total iron content.…”

“…12). These signals are assigned to the radicals [Fe 3 (CO) 12 ] À , [Fe 3 (CO) 11 ] À , and [Fe 2 (CO) 8 ] À , which are formed upon one-electron from the reduced IrPS. The assignment of the EPR signals is based on comparison of their g values with literature g values of known iron carbonyl anions.…”

“…Many of such catalysts have been used to remove nitrogen oxides (NO x ) from exhaust gases. Relevant examples comprise the use of Co-BEA 8 and Fe-ZSM-5 9 for selective catalytic reduction of NO x , but also Cu-ZSM-5 has been widely used for the same purpose by Kucherov et al, whose work has been reviewed in ref. 1.…”