Electron spin resonance and electron spin echo modulation studies of impregnated cupric ion in silica gel with different adsorbates

Abstract

Electron spin resonance (ESR) and electron spin echo modulation techniques were used to study the coordination of Cu2+/Si02 catalysts prepared by impregnation with various polar and nonpolar molecules. By high-temperature (900-1000 K) activation under vacuum a portion of the Cu2+ formed coordinatively unsaturated complexes with probable distorted tetrahedral coordination. Their interaction with CH3OH, (CH3)2SO, CH3N02, and CH3CN leads to the formation of octahedrally coordinated complexes. Square-planar complexes are formed with NH3 and C5H5N adsorbates and square-pyramidal complexes are formed with C2H4 and CO adsorbates. Adsorption of oxygen at 77 K leads to the formation of an oxygen-containing complex having a novel ESR spectrum which is attributed to copper atom clusters.

Catalytically important metal ion intermediates on zeolites and silica surfaces

“…Similar studies on impregnated cupric ion on silica show some general differences in the adsorbate geometries between these two preparation techniques [45,46]. For ionexchanged cupric ion intense well resolved ESR spectra are observed whereas for impregnated cupric ion much weaker, more poorly resolved ESR spectra are observed.…”

“…The most comprehensive studies have been carried out with cupric ion on silica surfaces [43][44][45][46]. Here the main focus is to determine differences in adsorbate geometry as the size and polarity of the adsorbate is changed.…”

“…The two types of preparation procedures that have been used involve ion exchange [43,44] and impregnation [45,46]. Both techniques are used to prepare metal ions on silica and other oxide surfaces for catalytic purposes.…”

Electron Spin Resonance and Electron Spin Echo Modulation Studies of the Incorporation of Macrocyclic-Complexed Cupric Ions into Siliceous MCM-41

“…The signal at higher field with g = 2.004 has been ascribed to an 0" framework defect that is formed on evacuation close to a cupric ion. 12 The Cu(II) ESR spectrum of calcined CuMCM-41 (Figure 4a) also differs from the as-synthesized material. The Cu(II) parameters gn = 2.383, g_ = 2.075, and An = 0.0145 cm-1 of calcined CuMCM-41 are exactly the same as for calcined (CESTO)CuMCM-41, indicating that the same copper species is formed in both samples on calcination.…”

Electron Spin Resonance and Electron Spin Echo Modulation Studies of Cupric Ion Ion-Exchanged into Siliceous MCM-41

“…Formation of Cu(II) pairs was only observed in impregnated silica gel where the bonding of the Cu(II) ions to the framework is weaker than in ion-exchanged silica gel. 23 Conclusions Cu(II) ions exchanged by liquid and solid state ion exchange into siliceous MCM-41 show different behavior under dehydra-tion of the MCM-41 depending on the ion exchange procedure. Cupric ions exchanged by a liquid state reaction are found to be four-coordinated to framework oxygen ions.…”