Selective catalytic reduction of NO by ethanol: Speciation of iron and “structure–properties” relationship in FeSiBEA zeolite

The speciation of iron in FeSiBEA zeolites is investigated in order to evidence the “structure–properties” relationship in the selective catalytic reduction (SCR) of NO by ethanol. FexSiBEA zeolites are prepared in acidic (pH 2.5) (x = 0.3, 0.9 and 4.2 Fe wt%) or basic (pH 10) (x = 3.6 Fe wt%) conditions by a two-step postsynthesis method which allows to incorporate Fe into zeolite, as evidenced by XRD. For low Fe content (Fe0.3SiBEA, Fe0.9SiBEA), iron incorporated as Fe(III) ions generates Brønsted acidic sites as shown by FTIR of pyridine. Framework tetrahedral Fe(III) ions are evidenced by diffuse reflectance UV–vis, XANES and EXAFS. For higher Fe content (Fe4.2SiBEA), beside tetrahedral Fe(III) ions which are dominant, octahedral Fe(III) species are also present as shown by DR UV–vis, XPS and EXAFS. In contrast, for Fe3.6SiBEA prepared in basic condition (pH 10), an extra-framework Fe(III) oxide phase is mainly observed.The catalytic activity of FexSiBEA in the SCR of NO by ethanol strongly depends on the speciation of iron and a structure–properties relationship has been evidenced. Fe0.3SiBEA and Fe0.9SiBEA which mainly contain framework tetrahedral Fe(III) ions are active, with selectivity toward N2 exceeding 90% for NO conversion from 25% to 55%. When additional octahedral Fe(III) species are present (Fe4.2SiBEA), the full oxidation of ethanol and NO by O2 becomes important, with CO2 and NO2, respectively, appearing at the expenses of N2. The NO conversion linearly depends on Fe concentration assuming a first order reaction, suggesting that tetrahedral and octahedral Fe(III) species are well dispersed, as confirmed by XRD. The iron oxide phase is quite inactive, the activity and selectivity of Fe3.6SiBEA being governed by the small amount of tetrahedral Fe(III) ions.