M. Schwidder

Active sites for NO reduction over Fe-ZSM-5 catalysts

A study of Fe-ZSM-5 catalysts with variable amounts of isolated, oligomeric and heavily aggregated Fe3+ oxo sites (as evidenced by UV-Vis and EPR spectroscopic data) and their catalytic properties in the selective catalytic reduction of NO by isobutane or by NH3 is presented, which allows development of a unified concept of the active Fe sites in these reactions, according to which isolated Fe sites catalyse both SCR reactions while oligomeric sites, though also involved in the selective reduction path, limit the catalyst performance by causing the total oxidation of the reductant.

The Identification of Different Active Sites on Mo/Al2O3 Metathesis Catalysts

Mo/Al2O3 catalysts prepared via fixation of Mo(η3-C3H5)4 on Al2O3 or by conventional impregnation (2.2 or 2.9 wt% Mo) have been compared with regard to their catalytic behavior in the metathesis of propene in different temperature ranges (293-323 K, 473 K). Different active sites have been distinguished. A site derived from a Mo(VI) precursor by thermal activation in inert gas exhibits stable activity, with a propene reaction order near 1. Other sites that are derived from a reduced Mo precursor, probably Mo(IV), are of higher activity but unstable with time-on-stream and also at elevated temperatures (>323 K). These sites support the metathesis at a propene reaction order of 0.5 and with activation energies between 10 and 25 kJ/mol depending on unknown structural details. Due to their instability, they cannot contribute to the high-temperature (T > 373 K) metathesis activity observed with Mo/Al2O3 catalysts. The latter is supported by Mo(VI)-derived sites or, at after reduction of catalysts with higher Mo contents, by Mo(IV)-derived sites that are different from those identified in the present study.

Fe-zsm-5 catalysts for the selective reduction of no: Influence of preparation route on structure and catalytic activity

Abstract Fe-ZSM-5 catalysts prepared via chemical vapour deposition (CVD) of FeCl3 and via mechanochemical treatment of a FeCl3/H-ZSM-5 mixture have been characterised by XAFS, EPR, and UV-Vis spectroscopy, and their activities in the selective catalytic reduction (SCR) of NO with ammonia and with different hydrocarbons have been studied. In the material prepared via the CVD route, extensive clustering was found by EPR and UV-vis, which was not detected by XAFS probably due to a disordered cluster structure. In the mechanochemically prepared material, the predominant iron species were mononuclear Fe3+ ions although a clustered minority phase was detected by UV-Vis and EPR. In the SCR with hydrocarbons (1000 ppm NO, 1000 ppm reductant, 2 % O2 at 30,000 h1), the catalyst prepared by the mechanochemical route was superior to the CVD-derived catalyst at almost all temperatures while it was inferior with the ammonia reductant (1000 ppm NO, 1000 ppm NH3, 2 % O2 at 750,000 h1). However, in terms of SCR rates related to Fe content, the mechanochemically prepared catalyst was superior in all cases. It is suggested that hydrocarbon SCR is favored by atomic or oligomeric dispersion of the Fe3+ ions while SCR with ammonia appears to be catalyzed also by FeOx aggregates.