Relations between iron sites and performance of Fe/HBEA catalysts prepared by two different methods for NH3-SCR

A series of Fe/HBEA catalysts was prepared by conventional impregnation (IMP) and liquid ion exchange (LIE) methods with various levels of iron contents, all of which was used to identify and quantify iron species and study the roles of different iron species in NOx conversion efficiencies, catalytic selectivity, and temperature dependencies. The composition and distribution of the iron species strongly depended on iron contents and preparation methods. LIE method preferred to produce more iron ions at ion exchange sites than IMP method, and then obtained highly active and selective Fe/HBEA catalysts. Isolated iron species at ion exchange sites was active in the NOx reduction with ammonia reaction (including N2O reduction with ammonia reaction), and iron oxide particles mainly contributed to the ammonia oxidation at the whole temperature range. The results of DRIFTS showed that nitrate species, Fe2+ mononitrosyls [Fe2+(NO)], and Fe2+ dinitrosyls [Fe2+(NO)2] were coadsorbed on the Fe/HBEA samples, yet the LIE samples preferred to yield much more nitrate and nitrosyl species than the IMP samples. Fe2+ mononitrosyls was found to be the key intermediates in the SCR reaction at 200 °C, while the nitrates species, mainly generated on iron ion sites and adsorbed on zeolite support, did not take part in the SCR reaction. NH3-TPD and ammonia oxidation results showed zeolite acidity might not be critical for SCR reaction on Fe/HBEA catalysts, and would be inhibited with the introduction of iron.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The composition of the iron species strongly depend on iron contents and preparation methods. ► Liquid ion exchange method prefers to produce more active and selective Fe/HBEA catalysts. ► Fe2+ mononitrosyls are the key intermediates in the SCR reaction.