EPR and NMR studies of amorphous aluminium borates

Abstract

Amorphous aluminium borates, Al,,,-x,B,,O, with 0 d x d 0.5, prepared from mixtures of aluminium nitrate, boric acid and glycerol, have been studied by EPR and MASNMR as a function of composition and heattreatment temperature (Tt < 860 "C). EPR studies showed t h e presence of physisorbed NO,, NO and 0, molecules, produced by decomposition reactions during t h e thermal treatment. The 0, molecules in t h e gaseous state were observed in a narrow temperature interval around 60 K and in t h e condensed phase at low temperature (t20 K). The D value for condensed 0, amounts to 109 GHz, significantly lower than the value for 'free' O, , which is 119 GHz. Above 20 K the NO, molecules in all samples rotate rapidly (2 lo7 Hz) about an axis parallel to the interatomic oxygen-xygen direction ; this mobility decreases with increasing heat-treatment temperature. Some EPR lines were tentatively ascribed to pairs or clusters of t h e abovementioned paramagnetic molecules. ,' At MASNMR studies showed t h e presence of six-, five-and four-coordinate Al atoms, their relative concentrations being strongly dependent on the thermal history and composition of the samples. The fractions of tetraand penta-coordinated Al atoms were maximum at heat-treatment temperatures between 300 and 600 "C and decreased considerably after t h e samples were exposed to air. Therefore t h e low coordinated Al atoms are predominantly located at the surface. The decreased mobility of NO, molecules, at high treatment temperatures, indicates that NO, interacts strongly with t h e pore surface when it contains a large fraction of four-and five-coordinate Al ions. Aluminas are extensively used as the supporting material in catalytic reactions. They are acid-base catalysts with a high surface area.' Successful efforts have been made to prepare amorphous aluminas that exhibit a zeolite-type porosity. The pore configuration and dimensions depend on composition, preparation method and heat treatment.'., The addition of typical glass-forming components such as

Terahertz Emission from Tubular Pb(Zr,Ti)O3Nanostructures

“…It is unambiguously molecular oxygen trapped inside a microcavity. Our spectra match closely that identified from the oxygen center in microcavities of amorphous aluminum borate, 36 in which case the signals have quite different values than for free O 2 . Therefore we interpret the center as oxygen molecules trapped inside the PZT nanotubes and not adsorbed onto the outer surfaces.…”

X-ray Diffraction, EPR, and 6Li and 27Al MAS NMR Study of LiAlO2−LiCoO2 Solid Solutions

“…The analysis of 27 Al NMR data was carried out to obtain complementary information about the local environment of aluminum atoms. The presence of AlO 4 and AlO 6 , and more recently aluminum pentacoordinated by oxygen, has been resolved in crystalline and amorphous materials and minerals. − However, the interpretation of 27 Al NMR spectra is complicated by the quadrupolar interaction resulting from the spin I = 5 / 2 . MAS measurements can only average efficiently first-order broadening effects.…”

The aluminum chemistry and corrosion in alkaline solutions

“…Finally, it should be noted that aluminum borates have been synthesized by a precipitation process in which dilute solutions of aluminum nitrate nonahydrate Al(NO 3 ) 3 :9H 2 O and boric acid are precipitated into a basic solution of ammonium carbonate. It was found that until excessive heating was applied, the material was largely amorphous in nature [43][44][45].…”