IR studies of Fe modified ZSM-5 zeolites of diverse mesopore topologies in the terms of their catalytic performance in NH3-SCR and NH3-SCO processes

• Conventional microporous zeolite vs. hierarchical analogs.
• Superior activity of mesostructured Fe-ZSM-5 in the NH3-SCR and NH3-SCO processes.
• Isolated pseudotetrahedral Fe(III)-oxo sites as active species.

The desilication and direct synthesis route techniques were used to prepare mesostructured ZSM-5 zeolites. Iron was introduced to purely microporous and to hierarchical analogs by two-fold ion-exchange procedure using a Fe(NO3)3 solution. The results of the catalytic studies of the NH3-SCR process showed that the Fe-exchanged ZSM-5 sample prepared by direct synthesis route with amphiphilic organosilanes as a mesopore-directing agents presented the highest catalytic activity in the low temperature range comparing to the other catalyst studied. Moreover, the Fe-catalysts revealed high catalytic performance in the NH3-SCO process. High catalytic activity of the studied samples was related to high concentration of mononuclear Fe3+ cations with pseudo-Td or Oh coordination, guaranteed by the enhanced mesopore area of zeolite support, as well as with high acidity of the zeolite itself. The facility of the transport of the reactants to and from the active surface sites seems to be also ensured by highly developed system of mesopores. IR studies of adsorption forms both of nitrogen monoxide and ammonia as well as the products of their conversion gave insight into the NH3-SCR and NH3-SCO reaction mechanisms.

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