Mechanism of formation of framework Fe3+ in bimetallic Ag-Fe mordenites - Effective catalytic centers for deNOx reaction

As it was reported in our previous study (P. Sánchez-López et al., 2019), mono- and bimetallic systems of Ag, Fe, and Ag-Fe supported on mordenites exhibit a nontrivial dependence of deNOx activity from the preparation procedure. In this work we report on the results of our comprehensive study of the sample composition, structure and morphology to reveal issues of the observed synergetic effect. It was found that both the order of the ion exchange reactions and processing temperature affect the distribution and state of silver and iron species in the mordenite matrix. In the Ag-containing samples silver is accumulated on the surface in the form of X-ray amorphous nanoparticles of 2–10 nm in size; however, charge compensating Ag+ cations are also present. The Mössbauer spectroscopy study of the iron-containing samples evidences the isomorphous substitution of Al3+ by Fe3+ in the mordenite framework. The portion of this framework Fe3+ correlates with catalytic deNOx activity of the studied materials. Hydrated extra-framework Fe2+ cations are Mössbauer silent at room temperature, but their presence is evident from the increasing of the a/b ratio caused by the contraction of the main mordenite channel upon iron cations loading. Based on the reported results we propose a mechanism of the isomorphous substitution of the framework Al3+ by Fe3+ promoted by Ag+. This mechanism leads to creation of new effective catalytic centers for deNOx reaction, which are (i) framework Fe3+ activated by nearby Ag+ and (ii) a very few nanosized Ag particles on the Fe-mordenite surface.