Electron Spin Resonance of High-Spin Cobalt in Microporous Crystalline Cobalt-Containing Aluminophosphates

Abstract

Four highly crystalline, cobalt-containing microporous aluminophosphates have been investigated by using liquid He X-and/or Q-band electron spin resonance (ESR) spectroscopy in order to investigate the coordination of high-spin cobalt before and after calcination. The ESR spectra of the four zeolite structures are characterized by an axial signal with an effective g ⊥ ≈ 5.80-5.44 and g || ≈ 2.00. Quantitative temperature dependence measurements of this axial signal in the temperature range 4-30 K reveal a Curie-Weiss behavior for both as-synthesized and calcined samples confirming (a) the m s ) ( 1/2 ground state of magnetically isolated high-spin cobalt and (b) a zero field splitting ∆ > 0 cm -1 . Quantitation of the ESR signals indicated that most of the Co 2+ is ESR active and that only about 30% of this Co 2+ can be oxidized to the ESR-inactive Co 3+ after calcination. The spin Hamiltonian parameters of as-synthesized and calcined CoAPO-5 material, as determined by spectrum simulation and the microwave power saturation technique, support the presence of framework Co 2+ in a flattened or elongated tetrahedron (D 2d ).

Novel Flexible Frameworks of Porous Cobalt(II) Coordination Polymers That Show Selective Guest Adsorption Based on the Switching of Hydrogen-Bond Pairs of Amide Groups

“…Computer simulation has yielded g' 1 g' 2 5.0, g' 3 2.4 and peak-topeak line widths W 1 W 2 27 mT and W 3 90 mT. [67] Several types of high-spin Co II complexes have so far been studied by EPR spectroscopy, [68] and the differences in the EPR pattern between tetrahedral and octahedral Co II have been well documented. [69±72] In tetrahedral Co II , the ground orbital state is 4 A 2 , and thus the orbital angular momentum is well quenched; this makes the spin Hamiltonian approach appropriate.…”

Solvent effects in the synthesis of CoAPO-5, -11 and -34 molecular sieves

“…One of the most fascinating researches on microporous AlPO 4 -n materials is the process of isomorphous substitution, in which framework Al 3+ and/or P 5+ ions are replaced by e.g. transition metal ions, such as Mn 2+ [3][4][5][6], Fe 3+ [7,8], V 4+ [9][10][11][12][13], Ti 4+ [14,15], Zn 2+ [16,17], Nb 5+ [18], Cr 3+ [19][20][21] and Co 2+ [22][23][24][25][26]. This process can generate Brö nsted acid and redox sites, leading to intriguing shape-selective catalytic performances.…”

“…The incorporation of Co 2+ (d 7 ) can be considered as a showcase for these substituted materials since this transition metal ion not only gives the solid an intense blue color and interesting oxidation properties, but also provides convincing spectroscopic evidence for the framework substitution of Co 2+ as obtained with e.g. diffuse reflectance UV-Vis-NIR spectroscopy, electron spin resonance and X-ray absorption spectroscopy [1, 3,[22][23][24][25][26].…”

Co doped ZnO semiconductor materials: structural, morphological and magnetic properties

“…5. These spectra are typical for the isolated Co 2+ ion in powders [26]. The cobalt atoms substitute the zinc atoms and the neutral charge state is Co 2+ (a 3d 7 configuration).…”