Investigating the Influence of Fe Speciation on N2O Decomposition Over Fe-ZSM-5 Catalysts

Nia Richards; Ewa Nowicka; James H Carter; David J Morgan; Nicholas F Dummer; Stanislaw Golunski; Graham J Hutchings

doi:10.1007/s11244-018-1024-0

Investigating the Influence of Fe Speciation on N₂O Decomposition Over Fe-ZSM-5 Catalysts

Top Catal. 2018;61(18):1983-1992. doi: 10.1007/s11244-018-1024-0. Epub 2018 Jul 24.

Authors

Nia Richards¹, Ewa Nowicka¹, James H Carter¹, David J Morgan¹, Nicholas F Dummer¹, Stanislaw Golunski¹, Graham J Hutchings¹

Affiliation

¹ Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Cardiff, CF10 3AT UK.

Abstract

The influence of Fe speciation on the decomposition rates of N₂O over Fe-ZSM-5 catalysts prepared by Chemical Vapour Impregnation were investigated. Various weight loadings of Fe-ZSM-5 catalysts were prepared from the parent zeolite H-ZSM-5 with a Si:Al ratio of 23 or 30. The effect of Si:Al ratio and Fe weight loading was initially investigated before focussing on a single weight loading and the effects of acid washing on catalyst activity and iron speciation. UV/Vis spectroscopy, surface area analysis, XPS and ICP-OES of the acid washed catalysts indicated a reduction of ca. 60% of Fe loading when compared to the parent catalyst with a 0.4 wt% Fe loading. The TOF of N₂O decomposition at 600 °C improved to 3.99 × 10³ s^-1 over the acid washed catalyst which had a weight loading of 0.16%, in contrast, the parent catalyst had a TOF of 1.60 × 10³ s^-1. Propane was added to the gas stream to act as a reductant and remove any inhibiting oxygen species that remain on the surface of the catalyst. Comparison of catalysts with relatively high and low Fe loadings achieved comparable levels of N₂O decomposition when propane is present. When only N₂O is present, low metal loading Fe-ZSM-5 catalysts are not capable of achieving high conversions due to the low proximity of active framework Fe³⁺ ions and extra-framework ɑ-Fe species, which limits oxygen desorption. Acid washing extracts Fe from these active sites and deposits it on the surface of the catalyst as Fe_xO_y, leading to a drop in activity. The Fe species present in the catalyst were identified using UV/Vis spectroscopy and speculate on the active species. We consider high loadings of Fe do not lead to an active catalyst when propane is present due to the formation of Fe_xO_y nanoparticles and clusters during catalyst preparation. These are inactive species which lead to a decrease in overall efficiency of the Fe ions and consequentially a lower TOF.

Keywords: Acid washing; Chemical vapour impregnation; Fe–ZSM-5; Iron species; Iron zeolites; N2O decomposition; Nitrous oxide; UV/Vis; XPS.