Role of Pentacoordinated Al³⁺ Ions in the High Temperature Phase Transformation of γ-Al₂O₃

Abstract

In this work, the structural stability of γ-alumina (γ-Al2O3) was investigated by a combination of XRD and high-resolution solid-state 27Al MAS NMR at an ultrahigh magnetic field of 21.1 T. XRD measurements show that γ-Al2O3 undergoes a phase transition to θ-Al2O3 during calcination at 1000 °C for 10 h. The formation of the θ-Al2O3 phase is further confirmed by 27Al MAS NMR; additional 27Al peaks centered at 10.5 and ∼78 ppm were observed in samples calcined at this high temperature. Both the XRD and NMR results indicate that, after calcination at 1000 °C for 10 h, the ratio of the θ-Al2O3 phase to the total alumina in samples modified by either BaO or La2O3 is significantly reduced in comparison with γ-Al2O3. 27Al MAS NMR spectra revealed that the reduction in the extent of θ-Al2O3 formation was highly correlated with the reduction in the amount of pentacoordinated aluminum ions, measured after 500 °C calcination, in both BaO- and La2O3-modified γ-Al2O3 samples. These results strongly suggest that the pentacoordinated aluminum ions, present exclusively on the surface of γ-Al2O3, play a critical role in the phase transformation of γ-Al2O3 to θ-Al2O3. The role of the modifiers, in our case BaO or La2O3, is to convert the pentacoordinated aluminum ions into octahedral ones, thereby improving the thermal stabilities of the samples. Oxide additives, however, seem to have little, if any beneficial effect on preventing reductions in specific surface areas that occurred during high-temperature (≤1000 °C) calcination.

The role of Ag O Al entities in adsorption of NCO species and reduction of NO

“…As shown in Fig. 1(A), the structure of ␥-Al 2 O 3 consists of octahedral and tetrahedral Al in proportions about 3:1, which is consistent with other works [28,29]. Based on this important result, we constructed the surface model of the support Al 2 O 3 .…”

“…1(A) shows the 27 Al MAS NMR spectra of ␥-Al 2 O 3 . The experimental NMR spectrum exhibited two main stable peaks at 7.5 and 65 ppm, which were assigned to AlO 6 (Al octahedral) and AlO 4 (Al tetrahedral) units in ␥-Al 2 O 3 respectively [28][29][30][31]. As shown in Fig.…”

“…It can be clearly seen in Fig. 1(C) that the Al penta site [28][29][30][31] (AlO 5 centered at 35 ppm) disappeared after silver loading and Al octa increased at the same time. Thus we can conclude that the Al penta site is the anchoring site for silver ions to form the Ag O Al octa entity.…”

Nanocomposite thin film of Ag nanoparticles embedded in amorphous Al 2 O 3 on optical sensors windows: Synthesis, characterization and targeted application towards transparency and anti-biofouling

“…In contrast, when supported on gamma alumina, γ-Al 2 O 3 , a synergistic effect occurs between Ag and γ-Al 2 O 3 , illuminating a bactericidal mechanism promoted by catalytic oxidation. It is well known that the catalytic activity of γ-Al 2 O 3 is due to penta-coordinated Al 3+ ions which are located on the surface of γ-Al 2 O 3 [12].…”

Adsorption states of typical intermediates on Ag/Al2O3 catalyst employed in the selective catalytic reduction of NOx by ethanol

“…Based on the 27 Al MAS NMR characterization of the Ag/Al2O3 catalyst, two types of stable Al atoms (Altetra and Alocta) could be observed on the Ag/Al2O3 catalyst [37][38][39][40]. Ag + formed Ag-O-Alocta entities upon interaction with Alocta sites (derived from Alpenta) that were the main anchoring sites [37,[39][40][41].…”