Strukturna i katalitička svojstva sintetičkih, termički aktiviranih Mg-Al-Fe anjonskih glina

Abstract

Ispitivani su dvostruki slojeviti hidroksidi, LDH i mešoviti oksidi nastali njihovom razgradnjom, sa različitim sadržajem Mg-Al-Fe i proširenom supstitucijom aluminijuma. Koprecipitacione metode male i velike prezasićenosti odabrane su za sintezu LDH opšte formule [Mg1-x M(III)x (OH)2](CO3)x/2 ⋅ mH2O, gde M(III) pretstavlja Al i/ili Fe, a x udeo trovalentnih anjona, x = M(III) / [M(II) + M(III)], variran u intervalu 0,15 < x < 0,7. Katalizatori na bazi mešovitih oksida sa gvožđem pogodni za upotrebu u redoks reakcijama, formirani su nakon termičke aktivacije - razlaganja LDH. Izvršena je karakterizacija mešovitih oksida nastalih iz LDH, kao i netretiranih LDH, ispitivanjem kristalne strukture (XRD), termičke stabilnosti (TG-DTA), teksture (adsorpcija N2), redoks (H2 TPR) i kiselo-baznih karakteristika (NH3 TPD), a utvrđena je i priroda prisutnih vrsta gvožđa (Moessbauer spektroskopija). Pokazano je kako metoda pripreme i obim supstitucije M(III) jona utiču na strukturne, površinske i redoks karakteristike LDH i njihovih mešovitih oksida u korelaciji sa katalitičkim svojstvima u odabranim test reakcijama (razgradnja N2O i redukcija sa NH3). Utvrđeno je da na katalitičku aktivnost dominantno utiču redoks karakteristike i priroda vrste gvožđa u mešovitom oksidu, ali takođe i strukturne karakteristike polaznog LDH, pri čemu je površinska struktura aktivnih centara intervalentnog Fe2+-Fe3+ gvožđa ključna za redoks procese ispitivanih reakcija. Kod svih mešovitih oksida dobijenih iz LDH aktivni centri imaju relativno jednake jačine, dok razlike u katalitičkom ponašanju potiču od razlika u dostupnosti aktivnih centara. Najbolje performanse u katalitičkim reakcijama pokazali su uzorci sa najmanjom stabilnošću inicijalnih LDH kod kojih je količina supstituisanih M(III) jona bila blizu granice za inkorporaciju u LDH matricu (x = 0,5), a naročito uzorak sintetisan metodom velike prezasićenosti sa malom količinom dodatne Al(OH)3 faze.

Layered double hydroxides (LDHs) and derived mixed oxides with different Mg-Al-Fe content and extended aluminum substitution were investigated. High and low supersaturation precipitation methods were used for the synthesis of LDHs with general formula [Mg1-x M(III)x (OH)2](CO3)x/2 ⋅ mH2O where M(III) presents Al and/or Fe and x the content of trivalent ions, x = M(III) / [M(II) + M(III)], varied between 0.15 < x < 0.7. Iron-containing mixed oxides, suitable as catalysts for redox reactions, were formed after thermal activation - decomposition of LDH. Both, LDHs and derived mixed oxides were characterized with respect to crystalline structure (XRD), thermal stability (TG-DTA), textural (N2 adsorption), redox (H2 TPR) and acid-base properties (NH3 TPD) as well as the nature of the iron species (Moessbauer spectroscopy). It was demonstrated how preparation method and the extent of M(III) substitution influence the structure, surface and redox properties of LDHs and derived mixed oxides in correlation to catalytic properties in chosen test reactions (N2O decomposition and reduction with NH3). The catalytic behavior is mainly determined by the redox properties and the nature of the iron species in mixed oxides, but also by the structural properties of initial LDHs. It was confirmed that surface structure of intervalent Fe2+-Fe3+ iron active sites is crucial for the redox processes in chosen reactions. The strength of active sites is similar for all mixed oxides derived from LDH, but the differences in catalytic behavior arise from the differences in active sites accessibility. The best catalytic results were obtained when the stability of the initial LDH matrix was lowest e.g. when the amount of substituted M(III) was near the limit for the incorporation into LDH matrix (x = 0.5), especially for the sample synthesized with high superasaturation method containing a small amount of additional Al(OH)3 phase.