Wyniki wyszukiwania - BazTech

Ograniczanie wyników

1 Materials Science Poland

1 2005

Znaleziono wyników: 1

Liczba wyników na stronie

Wyniki wyszukiwania

Sortuj według:

Ogranicz wyniki do:

EPR study of chromium centres in CrAPO-5 molecular sieves

Padylak B., Kornatowski J., Zadrożna G., Fabisiak K., Gutsze A.

Materials Science Poland

2005

Vol. 23, No. 4

1035--1046

The X-band EPR spectra of CrAPO-5 molecular sieves exhibiting high or low sorption capacities for nitrogen and benzene, signed HS and LS samples, respectively, were studied at 300 and 77 K. The EPR spectra of the as-synthesized HS and LS samples consist of a broad intense signal (geff = 1.971 ± 0.001, ΔHpp ≅ 500 G) with a positive lobe at geff = 5.15±0.01, assigned to Cr3+ (3d3, 4F3/2) ions with a rhombic distorted octahedral coordination. Calcined samples show an additional narrow EPR signal of axial symmetry with g|| = 1.971 ± 0.001 and g⊥ = 1.959 ± 0.001, also assigned to Cr5+ (3d1, 2D3/2) ions in an octahedral coordination. HS samples heated under vacuum exhibit a decrease in the intensity of the Cr3+ lines as well as several new signals. Two of them are characterized by an axially symmetric g factor and belong to Cr5+ centres in square pyramidal and tetrahedral coordination. The third signal may be assigned to Cr+ (3d5, 6S5/2) ions. The EPR spectra of LS samples heated under vacuum show a strong decrease in the intensity of Cr3+ lines and only the two signals of Cr5+ centres in the same coordination. Their g factors are similar to those in the HS samples. Thermal treatment under oxidative (O2) and reductive (H2) atmospheres reveal the reproducible redox properties of Cr5+ ions and a high stability of Cr3+ ions, especially in the HS samples.