Thermally activated iron containing layered double hydroxides as potential catalyst for N2O abatement

Abstract

Layered double hydroxides (LDHs) and derived mixed oxides with different Mg/Al/Fe contents were investigated. Two super-saturation precipitation methods were used for the synthesis of LDHs with general formula [Mg1−xM(III)x(OH)2](CO3)x/2⋅mH2O where M(III) presents Al and/or Fe. The content of trivalent ions x=M(III)/[M(II)+M(III)], was varied between 0.15<x<0.7. Such a wide range of trivalent ions was chosen with the aim to induce the formation of different multiphase mixed oxides. Iron was introduced as constituent metal in order to obtain redox properties. LDHs and their derived mixed oxides were characterized with respect to their crystalline structure (XRD), thermal stability (TG/DTA), textural (N2 adsorption), redox (H2 TPR) and acid properties (NH3 TPD) as well as the nature of the iron species (Mössbauer spectroscopy). Catalytic behavior was studied in two test reactions: N2O decomposition and reduction with NH3. It has been demonstrated that extended M(III) substitution influences the structure and surface properties of Mg–Al–Fe LDHs and derived mixed oxides, weakens Mg–Al–Fe–O interactions and improves catalytic behavior correlated with the presence of Fe–O–Fe–O–Fe entities providing possibility for facilitated extraction of oxygen with simultaneous redox Fe3+Fe2+ conversion. The catalytic behavior is mainly determined by redox properties, nature of iron species in mixed oxides and by structural properties of initial LDHs. The best catalytic results were obtained when the amount of M(III) was near the limit for the incorporation into LDH matrix.