Calorimetric And Spectroscopic Investigation Of Acidity

Calorimetric And Spectroscopic Investigation Of Acidity

Introduction

Noble metal-exchanged zeolites are active catalysts in economically important processes. In particular, Cu(I)-ZSM-5 zeolite is of greatest interest for the direct conversion of NO into N2 and O2 while Ag(I)-ZSM-5 shows high activity in several catalytic and photocatalytic processes. Among them the most important is the photochemical dissociation of H2O in H2 and O2. In both copper and silver cases, the metal-exchanged zeolites are much more active than the other supported noble metal based catalysts, suggesting that the unique catalytic properties of these systems are probably related to the high coordinative unsaturation of the extra-framework cations hosted in the MFI structure.

Beside their interest as catalytic systems, zeolites are also of greatest interest in that their three-dimensionally organised porous structure allows stable species like small metal clusters and coordination complexes to form in the zeolite cavities. Cu(I) cations highly dispersed in the zeolite framework are able to form at room temperature (RT) relatively stable adducts such as the “end-on” [Cu(N2)]+ species as well as mono-carbonyl [Cu(CO)]+ and di-carbonyl [Cu(CO)2]+ complexes. These species evolve to tri-carbonyl [Cu(CO)3]+ complexes at low temperature (˜100 K). In contrast, the extra-framework Ag(I) cations do not form at all adducts at RT with N2, [Ag(N2)]+ complexes being observed only at T˜100 K , and the only carbonyl-like species they form are the mono-carbonyl [Ag(CO)]+ ones, that evolve to the di-carbonyl [Ag(CO)2]+ species only at low temperature . These data suggest that the adducts the two cations do form are characterised by a lower coordination number for Ag(I) with respect to Cu(I), in agreement with the homogeneous chemistry behaviour: the typical coordination number for Cu(I) is four (tetrahedral geometry) whereas is two for Ag(I) (linear geometry). The two group-11 Cu(I) and Ag(I) cations have indeed electronic similarities (3d10 and 4d10 valence shell electronic configuration, respectively) but different size (rCu(I)=0.96 Å and rAg(I)=1.26 Å).

The complexity of the interaction between transition-metal-exchanged zeolites and reactants molecules in the catalytic processes of interest is not well understood yet, and this hinders a deep understanding of the processes. The aim of the present work is to investigate the RT interaction of NH3 with coordinatively unsaturated Cu(I) and Ag(I) cations highly dispersed in ZSM-5 zeolites. The energetics of the formation of the amino-complexes, their stoichiometry and their stability upon outgassing was investigated by adsorption microcalorimetry. In the case of Cu(I)-ZSM-5, the formation of mixed amino-carbonyl species at the metallic sites was also studied. The nature of the species formed in the zeolite cavities was monitored by FT-IR spectroscopy. EXAFS data allowed to check the local environment of noble metal cations in the zeolite cavities as well as to determine structural parameters needed for a better comprehension of the species formed in the adsorption processes under study.

Experimental

Materials

Cu(I)-ZSM-5 and Ag(I)-ZSM-5

The noble metals exchanged zeolites were prepared starting from the same NH4-ZSM-5 precursor (Si/Al=14, EniChem SpA, Centro Ricerche di Novara). The exchange procedure was done in two steps. First, a thermal treatment in vacuo (p=10-3 Torr; ...